| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
State of the Art |
1 From the Department of Radiology, Hospital Calmette, Boulevard Jules Leclerc, 59037 Lille, France. Received October 6, 1998; revision requested November 24; revision received January 5, 1999; accepted April 6. Address reprint requests to M.R.J. (e-mail: mremy-jardin@chru-lille.fr).
Abstract
Spiral computed tomographic (CT) angiography of the pulmonary circulation has emerged recently as a potential useful diagnostic method for the evaluation of the pulmonary circulation. As a minimally invasive examination, this technique is becoming widely available and has progressively replaced conventional and digital pulmonary angiography as the standard diagnostic imaging modality of the pulmonary circulation. The purpose of this review is to capture the current state of the art of the technical aspects of spiral CT angiography, with special emphasis on the postprocessing techniques currently available. As CT is responsible for a considerable part of the medical radiation dose applied to the population, the current trends for dose saving will also be emphasized. With regard to the clinical applications of spiral CT angiography, its introduction into the diagnostic work-up of pulmonary embolism has considerably modified the diagnostic algorithms. Our 8-year review of the literature presented herein is expected to provide the readers with a basis for formulating an informed opinion on this topic. In addition, a large number of congenital and acquired disorders of the pulmonary circulation are also relevant to this technique, and these indications are discussed in the context of the corresponding therapeutic options. Owing to the multiple possibilities inherent to this technique, spiral CT has the potential for cost savings without reduction in image quality or diagnostic accuracy.
Index terms: Computed tomography (CT), angiography, 944.12916, 945.12916 • Computed tomography (CT), helical, 944.12915, 945.12915 • Computed tomography (CT), volume rendering, 944.12917, 945.12917 • Embolism, pulmonary, 60.72, 944.77 • Lung neoplasms, CT, 60.30 • Pulmonary arteries, CT, 944.12916 • Pulmonary veins, 945.12915 • State-of-art reviews
Several less-invasive modern technologies have largely replaced invasive procedures such as angiography for imaging of the pulmonary arteries. These techniques have the advantage of short exposure times and the ability to create three-dimensional (3D) data sets that have greater diagnostic possibilities than do standard projection angiographic images. Contrast material–enhanced spiral computed tomography (CT) provides these advantages. Since its introduction in the early 1990s, spiral CT angiography of the pulmonary circulation has progressively gained widespread acceptance, enabling one not only to obtain uniform and nearly constant opacification of pulmonary vessels down to 2–3 mm in diameter, but also to analyze the peripheral pulmonary vasculature with more precise anatomic details than those available with conventional angiographic studies.
The purpose of this article is to review the current clinical applications of spiral CT in the evaluation of a variety of congenital and acquired disorders of the pulmonary circulation, except the systemic arterial supply of the lung parenchyma. Although these disorders represent an increasing indication for spiral CT angiography in clinical practice, to our knowledge each specific indication has not been systematically evaluated in the literature. Therefore, in addition to a review of the available literature, several specific indications of this imaging modality will be suggested in the context of their expected clinical interest.
TECHNICAL APPROACH
Nonenhanced CT Examination
As reported for the evaluation of many vessels, spiral CT angiography of the pulmonary arteries results from a focal spiral CT examination that is usually preceded by nonenhanced CT scanning over the entire thorax. The value of this preliminary CT examination is threefold. First, it enables analysis of the lung parenchyma, pleura, and chest wall for associated abnormalities. Second, nonenhanced scans are helpful for identifying calcified lesions such as hilar lymph nodes (which could hamper analysis of contrast-enhanced CT scans) or calcified thrombi (which could be missed on contrast-enhanced images). Third, the preliminary CT examination enables an accurate localization of the anatomic volume of interest before spiral CT scanning. This preliminary CT examination can be performed with a conventional or spiral acquisition. When using the spiral mode, scanning with a low-dose technique is strongly recommended; this technique includes the selection of a pitch of 2 (especially when a subsecond scanning time is available), use of the lowest milliamperage available, and use of breast protection for young female patients. Although 180° interpolation and narrow beam collimation is ideal for a subsecond unit with the latest x-ray tube generating higher heat capacity, it should be emphasized that image noise in old spiral units with these parameters may substantially degrade the image.
Spiral CT Protocol
As recently reported by Fishman (1), the current challenges regarding CT angiography can be divided into three categories: data acquisition, data processing, and data display. For each step, several parameters have to be considered by the radiologist at the console to optimize the final display, as further detailed.
Acquisition Parameters
Scanning a distance of 10–12 cm (from the aortic arch to 2 cm below the level of the inferior pulmonary veins), enables imaging of central and peripheral pulmonary arteries of the upper, middle, and lower lobes during the CT sequence in every patient. This region-of-interest may be covered by scanning the patient in the craniocaudal direction or scanning in the caudocranial direction. In clinical practice, the direction of the table feed is not a major issue, as it does not influence the overall quality of the spiral CT examination. More important is the selection of the region to be surveyed in order to precisely include the target vasculature within the volume scanned.
The radiologist must select the collimation and the speed of the table feed with two imperatives in mind. The first is to find the optimal compromise between the patient's respiratory status and the breath-holding duration necessary for evaluation of the pulmonary vasculature in a single CT acquisition. The second objective is to scan the patient with the thinnest collimation possible, to reduce partial volume effects. This second point is of great concern for an accurate evaluation of the peripheral pulmonary arteries. During the past few years, modification of the interpolation algorithms and introduction of subsecond scanning times have considerably modified the spatial resolution of spiral CT. The older technique of scanning with 5-mm collimation and a pitch of 1 and reconstructing the images with a 360° linear interpolation algorithm resulted in an effective section thickness of 6.57 mm. Consequently, the main drawback was partial volume effects at the level of small vessels, thus limiting the evaluation of peripheral pulmonary abnormalities (2–5). With selection of a 180° linear interpolation algorithm and a subsecond scanning time, the effective section thickness is of 3.86 mm when scanning with 3-mm collimation and a pitch of 1.7; the effective section thickness is reduced to 2.65 mm with the use of 2-mm collimation and a pitch of 2. Applied to the evaluation of the pulmonary circulation, use of 3-mm collimation was found to improve spatial resolution at the level of segmental arteries as suggested by a lower number of peripheral arterial branches coded as nonanalyzable (6). The recent availability of subsecond scanning times has made it possible to select a thinner collimation and to scan with a pitch of 2 without any modification in the breath-hold duration or the anatomic coverage. This protocol was shown to allow marked improvement in the analysis of segmental and subsegmental pulmonary arteries (7).
It is strongly recommended that patients be scanned while they maintain strict apnea. Interpretable images might possibly be obtained while the patient is gently breathing, but this should be reserved for only those patients with severe lung impairment. However, this latter acquisition condition is associated with two main limitations: a confident evaluation of the pulmonary circulation limited to central vessels and the inability to generate high-quality multiplanar and/or 3D reformations. Whenever possible, patients should be scanned at total lung capacity owing to the direct relationship between the degree of arterial opacification and the level of pulmonary arterial resistance. Physiologic studies have demonstrated that pulmonary resistance increases at high states of lung inflation because lung inflation causes the small alveolar vessels to be compressed (8,9). The end result is an increase in the resistance to flow in all the vessels exposed to alveolar pressure, which facilitates high-quality arterial opacification. Obtaining a deep inspiration may be difficult for patients suspected of having PE. In such situations, several physiologic considerations suggest that the patient should stop breathing near end-expiration, which can also achieve high levels of pulmonary resistance and thus good arterial opacification (8,9). Intubated patients referred from intensive care units can benefit from a spiral CT evaluation of the pulmonary circulation. The overall quality of these examinations may be excellent as it is possible to manually suspend ventilation for 15–20 seconds in deep inspiration while the patient is sedated, thus avoiding motion artifacts.
Contrast Material Administration
The objective of the spiral protocol is to start scanning while the target vascular structures are opacified and to ensure a constant degree of pulmonary arterial opacification during the entire spiral sequence. An adequate examination of the pulmonary arteries requires the radiologist to select the most appropriate contrast agent and timing of injection, while carefully monitoring venous access. A power injection is mandatory for obtaining a homogeneous and constant level of arterial enhancement through the entire spiral examination. However, because of the rapid administration of contrast material, care must be taken to carefully adapt the rate of injection to the site of administration, especially when selecting high-flow protocols. In most spiral CT examinations, an 18- or 20-gauge catheter is inserted into a medially located antecubital vein, through which the bolus of contrast material is safely injected at a rate varying between 2 and 5 mL/sec. When an antecubital vein is not accessible, a more peripheral venous access can be used but care must be taken to avoid extravasation from a small and fragile peripheral vein. This requires reducing the rate of injection and increasing the start delay by 4–6 seconds. When a central venous access is available as frequently observed for patients referred from intensive care units, verifying the maximum rate of injection acceptable for a given catheter (often 4–5 mL/sec) is recommended. Because of the central location of the tip of the catheter, usually at the level of the superior vena cava, a shorter scanning delay is selected, which results in a shorter injection period.
The combination of an automated injection of contrast medium with the use of spiral CT enables an excellent quality of arterial opacification while using a smaller amount of iodine than previously recommended with sequential CT. Several approaches have been reported for the evaluation of pulmonary vessels with spiral CT, either selecting a low concentration–high flow protocol or a high concentration–low flow protocol. The low concentration–high flow protocol consists in injecting 150–240 mg/mL contrast agent at a rate of 4–5 mL/sec, whereas a high concentration–low flow protocol uses 300–350 mg/mL iodinated contrast agent administered at 2–3 mL/sec. In patients with normal right ventricular outflow, a 12–15-second scanning delay usually provides sufficient latitude so that the pulmonary arteries are almost always well opacified. For patients with right ventricular failure and patients with pulmonary hypertension, a longer scanning delay is required, varying from 15 to 18 seconds. In most cases, an empiric determination of the scanning delay is sufficient to obtain excellent arterial enhancement. An alternative is to monitor contrast enhancement visually or with densitometry during the early stages of contrast material injection. This method provides a mechanism by which the time of scanning initiation can be individualized on the basis of actual enhancement of anatomic structures. When a desired level of enhancement is reached for a particular structure, transition is made to a routine diagnostic spiral imaging series. This technique is mainly indicated for patients with pulmonary hypertension and/or right ventricular failure. Although back-to-back helical scans are available, the interval of time between the two acquisitions is not compatible with adequate intravascular levels of enhancement within pulmonary branches over time.
Image Reconstruction
A brief summary of the technical peculiarities of the postprocessing techniques currently applicable to the pulmonary circulation is presented below.
Transverse CT scans.—From each data set, transverse CT scans are systematically reconstructed. Creation of overlapping reconstructions is of major importance when scanning with a 5-mm collimation to avoid interpretive difficulties due to volume averaging effects. Owing to the considerable improvement in spatial resolution when scanning with a 2-mm collimation, reconstruction of contiguous sections is now widely accepted. An economic parameter (ie, the number of hard-copy films necessary to photograph overlapping images in a 2-mm collimation protocol) is the second major reason for generating contiguous images. Cost-reduction efforts regarding use of film by laser printers connected to spiral CT scanners are a challenge facing many medical centers. Consequently, reconstruction of overlapping sections from data sets acquired with thin collimation should be limited to the anatomic levels showing equivocal findings, namely, helping to distinguish an endoluminal clot from a partial volume effect with the arterial wall.
Multiplanar reformation.—This technique is most commonly used to obtain sagittal or coronal views of transverse CT data sets, but it also can be valuable to provide oblique or curvilinear views of CT data sets. Multiplanar reformation (MPR) images completely eliminate superimposition of pixels lying outside the selected plane, and all the attenuation values included in the reformatted plane are visualized. With most available software programs, reformation planes are defined by projecting a line on one of the transverse sections so that the resultant MPR images will be perpendicular to the transverse plane and parallel to the projected line. MPR may also be planned on frontal or profile scout views. These planar reformations, having a single obliquity relative to the x, y, and z axes, do not take into account the orientation of the more cranial and caudal parts of the structure to be imaged. Whenever the structure of interest is not strictly parallel to the chosen plane, its changing orientations will not be considered in the final image. Consequently, inaccurately selected MPR may lead to erroneous interpretations, either due to the omission of part of the lesion or to the creation of a pseudoabnormality. When a structure of interest requires optimal double oblique planar views, the adequate plane of reformation of this structure can be chosen on a 3D image. Another alternative is to determine the plane of reformation by a direct selection of the voxels located at the center of the vascular lumen.
Three-dimensional images.—Acquisition of the entire thorax within a single breath hold must be considered as a volumetric acquisition that offers multiple visualization techniques, which can be classified either according to their rendering, their projectional, or their computer-based image processing capabilities. As pointed out by Brink (10), a quality 3D image requires a combination of narrow section collimation and small intersection data reconstruction. As section thickness is a key factor, one should systematically envision the utility of a 3D reconstruction before selection of the acquisition protocol to be able to generate high-quality 3D images.
Until now, maximum intensity projection (MIP) has been the most popular method of obtaining 3D CT angiographic images (11,12). The MIP technique depicts the highest pixel value along the observer's line of sight and is particularly well suited for displaying small subvolumes in which superimposition of structures does not occur or is eliminated by editing procedures. However, several drawbacks have to be mentioned. The most important one is the inability to provide images of diagnostic quality when evaluating complex anatomic situations with superimposition of structures of interest. The depth information is absent unless a cine mode display with slightly varying viewing angles is used. According to the specificity of this technique for selection of voxels of interest, it is important to keep in mind that MIP does not enable the visualization of hypoattenuating intraluminal filling defects such as pulmonary emboli unless they are in close contact with the vessel wall.
The most frequently used 3D reconstruction of the pulmonary circulation is obtained with surface-rendering techniques. The shaded surface display (SSD) depicts the outer contour of the column of contrast material as an opaque surface. The main hallmark of SSD images is the thresholding segmentation that results in a binary classification of the voxels. Two categories of voxels are taken into account for the reformatted image: voxels higher than a user-selected single threshold or voxels included within a double-thresholding segmentation. For 3D SSD images of the pulmonary vasculature generated from nonenhanced data sets, the threshold values range from -600 to -700 HU. Should contrast medium be used, the thresholding segmentation has to be changed from -700 HU to +100 to 150 HU. Four categories of pitfalls can be encountered with this technique of reconstruction (13). The first group concerns the pitfalls related to the thresholding range, explaining why the stenosed part of a tubular structure can be artificially narrowed with too high a threshold or artificially minimized with too low a threshold. In such cases, an additional 3D display should be systematically performed with a lower or a higher thresholding segmentation, respectively, to avoid interpretive errors. However, low threshold values create a complex vascular background that may prevent optimal viewing of the vessel of interest, explaining why this kind of selection is usually obtained at second intention. The second group of pitfalls is related to the section thickness, either responsible for partial volume artifacts or stair-step artifacts. The third category of pitfalls may be caused by motion during data acquisition, either because of the patient's inability to maintain strict apnea or because of organ pulsation. The last group of pitfalls is related to contrast enhancement, which may be variable along the z axis.
The most recently introduced technique for creating angiographic-like images of the pulmonary vasculature is the volume-rendering technique. In this technique, classification of the voxels estimates the probability of a tissue type being homogeneously present in a voxel and attributes to this voxel a percentage corresponding to the amount of this tissue. This classification mode is called percentage classification or probabilistic classification, and a trapezoid is attributed to each tissue type (14). Several parameters have to be selected to generate volume-rendering technique reconstructions: (a) position of the trapezoid, which determines the attenuation of voxels incorporated into the image (this selection consists of defining a window width and a window level [15]); (b) shape of the trapezoid, corresponding to the intersections between the sides of the trapezoid and the top; (c) opacity value, which determines the relative transparency of the voxels incorporated into the image, and thus the degree of vascular transparency; and (d) shading option (not systematically available on volume-rendering programs), which modifies the vessel wall smoothness. Preliminary evaluation of the volume-rendering technique at the level of the pulmonary circulation has led to the selection of the following parameters: triangular trapezoid shape, selection of voxels from -750 to -450 HU for nonenhanced vessels and from 50 to 200 HU for enhanced vessels, 80% opacity value, and unshaded algorithm (authors' unpublished data, 1997). This selection was found to create diagnostically useful images of the pulmonary circulation, easily applicable in clinical practice. With continued advances in computer technology, interactive volume-rendering techniques soon will be widely available, an imperative for further evaluation of this technique at the level of the pulmonary circulation and rigorous comparison with alternative reconstruction techniques (16).
Slabs.—The delineation of peripheral pulmonary vessels is facilitated by the low attenuation of the surrounding lung parenchyma, explaining why their analysis does not systematically require injection of contrast medium. With regard to the technical particularities of a spiral CT evaluation of the peripheral vascular bed, one should mention the usefulness of generating slabs from nonenhanced focal spiral CT examinations. On cross-sectional images, a linear or tubular structure obliquely oriented or perpendicular to the plane of the CT section is segmented proportionally to the section thickness. Conversely, the stacking of several contiguous transverse images allows one to benefit from the advantages of the summation effect, thus enabling an easy identification of a pulmonary vessel in its entirety from the hilum to the periphery. Further anatomic details can be obtained when thin-slab MIP with or without automatic sliding in the acquired volume as well as targeted SSD images are obtained. The most frequently used technique is called sliding thin-slab (STS) MIP, which consists of an automatic sliding through the volume of interest (17). Because the selection of a narrow collimation is mandatory for high-quality reconstructions, care should be taken to precisely determine the volume to be scanned, usually limited to 2–3 cm.
Current Trends for Dose Saving
Because CT is responsible for a considerable part of the medical radiation dose applied to the population, concerted efforts are being made to minimize the dose required for a spiral CT examination without making compromises in image quality. To reach this goal at the level of the thorax, several recommendations can be made. First, a decreased kilovoltage can be easily applied to routine spiral CT angiography of the pulmonary circulation and the milliamperage setting can be individually selected. The use of subsecond scanning and the selection of a pitch of 2 are two additional efficient means of reducing the radiation dose. Moreover, dose modulation methods have been recently developed by manufacturers; these methods enable the tube current and hence the dose applied to be adjusted to the individual patient's geometry and patient's absorption during the acquisition. From preliminary evaluations, dose savings of up to 40% are obtained, depending on the patient and the examined region of the thorax (18). As recently proposed by Hopper et al (19), the use of thin layers of radiation absorbent material such as bismuth can provide breast radiation protection to women without affecting the quality of diagnostic CT images. These authors emphasized the seriousness of the dose administered to such a radiosensitive organ, reminding us that a woman's breast receives 2.0–3.5 rad (0.020–0.035 Gy) of radiation from a thoracic CT examination. To emphasize the importance of this radiation dose, one should keep in mind that delivery of 1 rad (0.01 Gy) of radiation to a woman's breasts before age 35 years fractionally increases her risk of breast cancer by 13.6% over the expected spontaneous rate for the general population.
CLINICAL INDICATIONS: ACQUIRED DISEASES
Pulmonary Embolism
Diagnosis of Acute Pulmonary Embolism
CT features and pitfalls.—Until the early 1990s, the standard diagnostic imaging procedures for evaluation of pulmonary embolism (PE) were ventilation-perfusion (V-P) scintigraphy and pulmonary angiography. During the past few years, there has been a progressive replacement of pulmonary angiography with spiral CT angiography, as the latter imaging procedure allows a noninvasive identification of intravascular clots. The CT criteria for acute PE are the tomographic equivalent of the classic angiographic signs of PE (ie, partial or complete filling defects and the "railway track" signs) (Figs 1, 2). In terms of contrast material administration, the drawbacks of the high concentration–low flow protocol argue for the use of the low concentration–high flow protocol in the evaluation of PE. The injection of a highly concentrated material usually results in streak artifacts at the level of the superior vena cava, which can substantially degrade quality in the adjacent right main pulmonary artery but also the right upper lobe pulmonary artery and its segmental branches. At the level of these branches, this may hamper detection of intraluminal changes. Although the low rate of injection recommended with this protocol is usually sufficient for depicting large emboli, it is not always compatible with the identification of emboli at the level of segmental arteries and precludes any detection of subsegmental emboli. The ideal concentration of contrast material for low concentration–high flow protocols has not yet been definitely determined, to our knowledge, but many users agree to consider a 150–200 mg/mL concentration.
|
|
A number of interpretive pitfalls exist in assessing enhanced spiral CT images, but their recognition is less and less problematic as the radiologist gains experience with spiral CT of the pulmonary vasculature (23). Four main categories of technical or pathophysiologic factors may be responsible for pseudo–filling defects. In severely tachypneic patients, breathing artifacts may be observed at the level of obliquely oriented arteries due to the variable position of the vessel in the section width in two successive scans. Consequently, any abrupt decreased attenuation at the level of a given arterial section between two contiguous sections should not be mistaken for an arterial filling defect. The choice of the scanning delay may also influence the quality of arterial enhancement at the top or bottom of the scanned volume. If the scanning delay is too short, there is insufficient time to allow adequate opacification of the pulmonary arteries on the first images. On the other hand, if the scanning delay is too long, there is not enough material left at the bottom of the selected subvolume, which may hamper detection of intraluminal thrombi. Physiologic data must also be integrated in the spiral volumetric CT evaluation of vascular enhancement. Several cases of unilateral increased pulmonary vascular resistance due to extensive airspace consolidation have led to a false-positive diagnosis of PE (2,4). The asymmetric pulmonary vascular resistance results in asymmetry of arterial perfusion due to slow flow through the pulmonary arteries ipsilateral to the lung consolidation, with normally circulating contralateral pulmonary arteries. Any cause of unilateral increased vascular resistance may lead to asymmetry in pulmonary arterial opacification. The various causes of nonopacification or faint unilateral opacification of a pulmonary artery that have been previously described on conventional angiograms and lung V-P scans (24,25) can be summarized as follows: (a) "occult" pulmonary artery; (b) unilateral pulmonary artery obstruction of extrinsic, mural, or endoluminal origin; (c) unilateral increase in pulmonary vascular resistance secondary to proximal or distal bronchial obstruction, bronchiolar obstruction, parenchymal destruction, extensive airspace consolidation, pleural restrictive or expansive process, or conditions with elevated venous pressure; and (d) congenital and acquired hemithoracic shunting of blood from the systemic to the pulmonary circulation.
The best way to demonstrate on CT scans the patency of a unilateral faintly opacified pulmonary artery is to perform a second spiral CT acquisition with a longer scanning delay to suppress flow phenomena at the level of this vessel. There are acquired disorders that are associated with anatomic or surgical shunting from the systemic to pulmonary circulation and that are thus responsible for antegrade or retrograde left-to-right shunts. This situation results in regional or unilateral changes in pulmonary blood flow that is interrupted at the level of the anatomic communication between the systemic and pulmonary circulations. The most common clinical situation is represented by patients with severe chronic inflammatory disease, particularly those with bronchiectasis, in whom prominent bronchopulmonary collateral circulation may develop and produce a proximal retrograde flow. In these patients, non-visualization of arteries or dilution defects may simulate emboli (Fig 3), as previously reported on conventional pulmonary angiograms (24). In patients with a substantial regional variation in pulmonary blood flow, a long start delay (eg, 20 seconds) and selection of a high concentration–high flow protocol can help suppress the focal hypoattenuation in the pulmonary arterial bed related to retrograde left-to-right shunting. However, the difficulties in obtaining optimal enhancement of the pulmonary arterial vasculature on CT scans are similar to those encountered on unilateral angiograms on which poor opacification or pseudoamputation are commonly seen. In most cases, hyperselective catheterization of the lobar or segmental arteries of this region is required to demonstrate their patency. A transient or permanent increase in pulmonary arterial pressure may be responsible for a right-to-left shunt through a patent foramen ovale, which results in a lower degree of pulmonary arterial enhancement on CT scans. This situation is suspected when a massive and early enhancement of the aorta is observed in conjunction with a faint pulmonary arterial opacification.
|
|
|
|
value was 0.85 with spiral CT angiography and 0.61 with V-P scanning. In the European multicentric trial (32), the agreement among CT scan readers was also better than that found among V-P scan readers (ie, 0.72 vs 0.39) (32). This study showed a considerable dependance of the interobserver agreement in CT angiography on the technical quality of the study, whereas interobserver agreement in V-P scintigraphy was neither dependent on study quality nor on the use of Biello or PIOPED criteria. These results explain why several authors suggest that spiral CT should be the initial imaging modality of choice, especially in the group of patients known to be associated with a high rate of indeterminate V-P scintigraphic studies (eg, all inpatients and patients with chronic obstructive pulmonary disease) (6,28,29,30,39). These conclusions are reinforced by the results of the cost-effectiveness analysis recently reported by Van Erkel et al (34). A limited number of patients could thus benefit from scintigraphy at first intention (ie, patients with low clinical suspicion of PE and no prior cardiopulmonary disease). An important question should then be considered: What are the remaining indications for pulmonary angiography? They should be reserved for patients in whom clinical suspicion for PE remains high despite normal results at spiral CT and duplex ultrasonography (US) of the legs. However, one should nuance the answer by considering the technical quality of the negative spiral CT angiogram. When the technical conditions of the latter examination limit the depiction of acute PE to central pulmonary arteries, pulmonary angiography should be reasonably indicated to search for thrombi at the level of the peripheral arterial bed. However, when a spiral CT angiogram is negative down to the subsegmental level, the clinical usefulness of pulmonary angiography is more debatable. Recent improvement in scanner technology is leading the radiologists to reevaluate the role of spiral CT in the detection of subsegmental PE; the frequency of subsegmental PE as reported in the literature varies between 2% and 33% (4,6,33,34,40,41). Over the past few years, several studies have stressed that subsegmental pulmonary arteries are not accurately evaluated with spiral CT. This inability was the logical consequence of the scanning parameters chosen in early reports (ie, 5-mm collimation and 5 mm/sec table feed [pitch = 1]) responsible for partial volume effects at the level of small branches. Since then, selection of a 3-mm collimation and a 5 mm/sec table feed (pitch = 1.7) has improved evaluation of peripheral pulmonary arteries at routine CT examinations. As previously noted, a 2-mm collimation and a 4 mm/sec table feed (pitch = 2) are now available in routine clinical practice. In a study group of 370 patients, the percentage of the analyzable subsegmental arterial bed was statistically significantly higher when scanning the patients with a 2-mm collimation and a pitch of 2 than when scanning the patients with a 3-mm collimation and a pitch of 1.7 (65% vs 43%, respectively). A question often raised over the past few years is, what is the clinical relevance of peripheral emboli in acute PE? Several authors consider that tiny clots are from calf veins and do not require anticoagulation (4,42–45). However, occlusion of a few subsegmental branches perfusing the most "normal" part of the lung parenchyma has been reported to lead to respiratory failure in patients with preexistent bronchopulmonary disease (6,33).
Follow-up of Acute PE and Management of Chronic Thromboembolic Disease
In addition to facilitating the diagnosis of acute PE, spiral CT angiography may help us understand changes within central pulmonary arteries after acute PE. In a study group of 19 patients with acute PE initially identified with spiral CT, resolving clots were found in 13 of 19 patients (68%) at 6-week follow-up (46). Residual abnormalities consisted of eccentric, wall-adherent filling defects or filling defects with central contrast material, consistent with recanalized clots. In a study group of 62 patients, follow-up spiral CT angiograms over a mean period of 11 months showed an incomplete resolution of acute PE in 39% of cases and newly developed chronic PE in 13% of cases (47) (Fig 4). The morphologic changes directly available on spiral CT angiograms are expected to improve our knowledge of the evolution of acute PE toward chronic thromboembolic disease. To date, to our knowledge only a few scintigraphic studies have addressed this issue, and the development of chronic changes has been reported with a frequency varying between 2% and 18% of cases (48–50). The most conspicuous abnormality of chronic PE is the presence of complete filling defects at the level of stenosed pulmonary arteries. Several additional features are also suggestive of chronic PE, namely, eccentric thrombi, evidence of recanalization, and arterial stenosis or web. The creation of reformatted images through the longitudinal axis of obliquely oriented vessels can overcome some of the difficulties encountered with transverse sections in the identification of focal arterial stenosis (51). In addition, reformatted images provide a delineation of mural thrombi on a single image, which is usually considered a useful complement to pulmonary angiography whenever a surgical treatment is planned.
|
|
CT can also depict changes at the level of the systemic arterial circulation, the collateral supply of the occluded pulmonary arterial bed. The most common findings on CT scans consist of dilated bronchial arteries, which can be detected as tortuous and/or dotted vascular sections in their mediastinal and/or hilar pathways (54,55). Enlargement of the internal mammary arteries is also easily detectable on CT scans, especially in cases of asymmetric or unilateral systemic supply. In addition, the possibility of identifying lung parenchymal abnormalities, such as areas of ground-glass attenuation and/or bronchial dilatation in cases of chronic PE, should be emphasized (56).
Pretherapeutic Management of Bronchial Carcinoma
Pulmonary Arterial Invasion from Bronchial Tumors
Precise staging is the key to rational management of bronchial tumors. Incremental CT has fallen short in predicting tumor extension in the mediastinum and the hilum, including vascular invasion. An improvement in CT-surgical correlations is expected with spiral CT with routine use of 2–5-mm thin sections (Fig 5). From clinical experience, we recommend performance of a contrast-enhanced spiral CT examination of the hila after analysis of the nonenhanced CT scans of the entire thorax. Careful interpretation of these sections helps determine the most appropriate injection protocol with special attention directed toward the target vasculature, the iodinated concentration, and the optimal start delay. The routine protocol for spiral CT angiography of the pulmonary circulation virtually guarantees an optimal opacification not only of the mediastinal and hilar pulmonary arteries but also of the venoatrial confluence and of the cardiac chambers. Strictly comparative performances of spiral CT acquisition are mandatory, before and after neoadjuvant chemotherapy, for a better estimation and, if needed, for a response quantification. This has to be taken into account as neoadjuvant chemotherapy has an initial response rate of approximately 50%, and subsequent resection rates in the responders may approach 80%.
|
A hilar lymph node dissection can be influenced by the spiral CT findings. If an upper lobectomy is contemplated, lymph nodes smaller than 10 mm in diameter, outside the lobectomy field, can be discovered by their compression or invasion of the descending pulmonary artery. They run the risk of being unseen on an incremental CT study because they hardly modify the external pulmonary artery morphology. For the same reason, mediastinal lymph node imaging can also be improved by means of spiral CT angiography of the mediastinal vessels (60). Radiographically occult squamous cell carcinoma involving main stem bronchi and lobar bronchi can benefit from photodynamic therapy. These lesions may be extremely small and are very often beyond the ability of CT detection. But once they are endoscopically detected and pathologically proved, the two major problems are to determine whether the lesion is limited to the bronchial wall and to eliminate an N2 disease. Another point of interest is that, if there is an incomplete response after photodynamic therapy, the patient can undergo surgical resection. Spiral CT with thin sections and contrast medium can depict the peribronchial tissues and be used in the extrabronchial follow-up of the treated disease. Khanavkar et al (61) pointed out that centrally located squamous cell carcinomas represented a predisposing factor to hemorrhage and tried to identify patients at high risk of developing fatal hemoptysis after endobronchial brachytherapy. They emphasized the following anatomic factors: involvement of major arteries, considerable bronchial wall destruction, and mediastinal invasion. Posttreatment necrotic cavitation of a tumor located a few millimeters from a major vessel can also be considered as a potentially lethal situation. The preventive effect of spiral CT angiography should be seriously considered in the follow-up of this treatment, as well as after vaso-occlusive therapy. Whereas bronchial carcinoids can be highly vascular lesions, they may only be depicted with a moderate homogeneous contrast enhancement on incremental CT scans (62). The transmural extension of this tumor can reach the peribronchial tissues and vessels, as well as the adjacent pulmonary parenchyma. If an endobronchial biopsy is anticipated, the highly vascular component should be demonstrated to prevent any further hemorrhagic complications. A sleeve resection can be performed depending on the peribronchial extent. Both findings can be searched for on pulmonary and systemic spiral CT angiograms.
Venoatrial Extension of Bronchial Carcinoma
In addition to invasion and obstruction of a pulmonary vein by a lung tumor, embolization into the left atrium may occur (Fig 6). The most common tumor able to invade the left atrium in this way is bronchogenic carcinoma. However, metastatic carcinoma and pulmonary sarcoma of the lung have also been reported with a similar extension. A preoperative diagnosis is imperative since the consequence of intraoperatively dislodging a macroscopic tumoral thrombus from the pulmonary vein can be multiple systemic tumor emboli and can be fatal in the short term (63). In patients with large central carcinomas in contact with one or several major pulmonary veins, a preoperative transthoracic or transesophageal echocardiogram and a spiral CT angiogram are recommended because a carcinomatous thrombus of a pulmonary vein may go unnoticed at surgery and become dislodged during surgical manipulations. The CT signs of venous invasion are those of a pulmonary veno-obstructive syndrome, including pulmonary edema and hemorrhage, pulmonary venous infarct, interlobular septal thickening, increased bronchial wall thickening, and pleural effusion (64). Spiral CT angiography can additionally show a massive enlargement in diameter of the invaded vein with an absence of opacification or a more or less notable filling defect of the adjacent part of the left atrium. The focal increase in pulmonary vascular resistance can also result in absent or delayed opacification of the pulmonary arteries in the area of venous obstruction, followed later by an unusual intense opacification of these pulmonary arteries due to pulmonaryarterial stasis. On incremental CT scans, a space-occupying lesion in the left atrium could be easily missed on an insufficiently contrast-enhanced CT scan (65).
|
|
|
In the particular group of patients who have undergone lung transplantation, spiral CT can be helpful for the evaluation of anastomosis-related stenoses of the arteries and veins. The frequency of pulmonary artery stenosis has been recently estimated by Griffith et al (69) who observed obstructions in five of 60 single-lung transplant recipients and in none of 74 double-lung transplant recipients. In one case, obstruction was associated with an intraluminal thrombus. The association of systemic hypoxia, normal chest radiographic findings, and perfusion scan abnormalities represents an indication for spiral CT angiography whenever the patient is able to support breath holding for a few seconds. It is not easy to make the differential diagnosis between a true stenosis and a distorted or kinked anastomosis resulting from excessive lengths of donor and recipient artery cuff. The latter can spontaneously improve over a follow-up period, in contrast to a true stenosis, which leads to a surgical revision or to endovascular procedures. Owing to pulmonary-to-bronchial artery retrograde flow, pulmonary artery stenosis can compromise the bronchial anastomosis. In the absence of contraindications to contrast medium administration, a spiral CT study initially indicated for the follow-up of airway anastomosis can also allow evaluation of arterial and venous anastomoses. When the transplant recipient has to be treated with surgery, percutaneous angioplasty, and/or endoprosthesis (70), the precise shape, site, and length of the stenosis can be better appreciated with two-dimensional and 3D reformations because they permit unlimited viewing angles (Fig 8). Perfusion scanning and 3D spiral CT angiography can also avert the use of more invasive and repeated angiographic follow-up.
|
|
|
Imaging studies can contribute to the care of patients with pulmonary hypertension by helping to (a) detect its presence, (b) indicate possible causes, (c) grade its severity, and (d) evaluate the functional state of the right ventricle. Routine chest radiography, CT, MR imaging, pulmonary function studies, and echocardiography have all been studied with variable success in predicting the presence or severity of pulmonary hypertension in patients with underlying cardiopulmonary disease. More than 10 years ago, a main pulmonary artery diameter of 29 mm or greater on a CT scan was reported to have a sensitivity of 69% and a specificity of 100% for predicting pulmonary hypertension in patients with cardiac or intrinsic pulmonary vascular disease (72). Because CT of the chest is often used in the diagnosis and treatment of patients with parenchymal lung disease, Tan et al (73) undertook a retrospective study aimed at determining the diagnostic utility of CT-determined diameter of the main pulmonary artery for predicting pulmonary hypertension in this patient population. The diameter was measured at the widest portion of the main pulmonary artery within 3 cm of the bifurcation; in addition, the right pulmonary arterial diameter and left pulmonary arterial diameter were measured at the widest portions after bifurcation of the main pulmonary artery. The authors concluded that a main pulmonary artery diameter of 29 mm or greater enables identification of patients with even a mild degree of pulmonary hypertension and therefore should enable identification of those patients who may have the greatest pulmonary vascular limitation to exercise.
In addition to the enlargement of proximal pulmonary arterial branches, CT may help identify morphologic changes suggestive of pre- or postcapillary pulmonary hypertension. In favor of precapillary pulmonary hypertension, one may note small peripheral pulmonary arteries and veins with the concurrent presence of normal lung parenchyma and left atrium. Presence of postcapillary pulmonary hypertension is usually suggested by the identification of CT features of interstitial and/or alveolar edema, including septal lines and abnormal lung attenuation varying from ground-glass opacities to consolidation. These parenchymal abnormalities are observed with the concurrent presence of enlarged pulmonary veins, sometimes associated with pleural effusion.
During the past few years, several studies have shown that a mosaic pattern of lung attenuation can be seen in patients with pulmonary hypertension of various causes (74). In an attempt to determine whether infiltrative, airway, or vascular lung disease is the cause of mosaic attenuation on thin-section CT scans of the lung, one may pay attention to the size of the pulmonary vascular sections within the areas of ground-glass attenuation and search for air trapping. The areas of increased lung attenuation can be attributable to blood flow redistribution when the size and number of vessels in the areas of increased attenuation are increased compared with those in the areas with normal or decreased attenuation in the absence of air trapping (75–77). As recently demonstrated by Worthy et al (78), it should be emphasized that, whereas the diagnoses of infiltrative lung disease and airway disease are frequently correct and confident, vascular disease is more difficult to diagnose on thin-section CT scans as the cause of areas of ground-glass attenuation.
Primary Pulmonary Artery Sarcomas
Primary pulmonary artery sarcomas are infrequent tumors that are very often confused with acute and chronic thromboembolic disease. Apart from the particular endovascular development of the tumor, which can be seen as a floating mass resembling a endoluminal clot within the pulmonary artery trunk and/or its proximal branches, it should be emphasized that this tumor is often associated with thrombus formation. The absence of predisposing factors for PE, the absence of "thrombus" dissolution despite anticoagulation, and atypical distribution of the filling defects as seen at two-dimensional or Doppler US, pulmonary angiography, MR imaging or CT, and V-P scanning should alert the radiologist (79). Peripheral pulmonary infiltrates can correspond to pulmonary infarcts due to thrombi or embolization of tumor material from the original site.
The diagnosis can be strongly suspected when chest radiography, CT, or MR imaging shows a lobulated and heterogeneous hilar mass that originates from the pulmonary artery trunk or from the main right or left pulmonary arteries and expands the artery (80). Such polypoid intraluminal masses can be partly hemorrhagic, necrotic, and hypervascularized with enhancement on spiral CT scans or after administration of gadopentetate dimeglumine on T1-weighted MR images. This finding helps eliminate a thrombus and indicates a neoplastic process (81,82–84). The pretherapeutic evaluation of this tumor can be performed with spiral CT, first during the passage of the contrast medium bolus, but also a few minutes later to search for contrast medium uptake by the tumor (Fig 9). In a minority of patients, the tumor arises from the right or left main pulmonary artery with or without contralateral spreading. Direct transmural growth in the hilum, the mediastinum, the lung parenchyma, and the pericardium can occur or it can be simultaneously multicentric. Such lesions can mimic a tumor from another origin, particularly a bronchial carcinoma invading the pulmonary artery. Extravascular infiltration and nodal invasion are also possible (85), and it should be noted that osteo-, chondro-, and carcinosarcomas may exhibit osteoid and bony components (79).
|
|
|
|
Rounded Atelectasis
Any type of pleural inflammatory reaction can lead to round atelectasis. Asbestosis is the principal cause today and this diagnosis should be suspected when a peripheral nodule or mass is associated with signs of pleuritis and/or pleuroparenchymal changes related to asbestos. Usually, round atelectasis presents as a posterobasal, subpleural, round or lentiform lesion with some of the following features (93–95): (a) peripheral location, with the mass being incompletely surrounded by the lung; (b) increased attenuation value of the mass at its periphery; (c) pleural thickening in the vicinity of the mass; (d) curving of vessels and bronchi toward the mass; and (e) the presence of an air bronchogram within the mass. Correct diagnosis is very important, especially in asbestos-exposed patients owing to the increased incidence of mesothelioma and bronchogenic carcinoma in this population. Two main mechanisms are recognized to underlie round atelectasis: (a) pleural effusion or diffuse pleural thickening and (b) thickening of the visceral pleura with progressive wrinkling and folding of the subpleural lung (96). The lesion can be stable over time; however, it may progressively enlarge and then be confused with a malignant tumor (97).
The use of spiral CT angiography can be of particular interest, as a single data set can provide densitometric and morphologic clues to this diagnosis. On contrast-enhanced CT scans, an intense and homogeneous enhancement is usually observed at the level of a round atelectasis (98). This densitometric feature can help differentiate the atelectatic trapped lung from bronchial carcinoma or fibrosis in which enhancement is usually poor. Care should be taken to pay more attention to the degree of parenchymal enhancement rather than to the presence of the CT angiogram sign, that is, the ability to see normal pulmonary vasculature within parenchymal consolidations. As recently reported by Shah and Friedman (99), the CT angiogram sign is a common finding in lobar consolidations evaluated with contrast-enhanced CT whatever their cause. On thick horizontal sections or on thin interspaced sections, the internal architecture of this atelectatic pseudotumor is not easy to analyze. With spiral CT, thin overlapped sections can show a large curved tongue of the neighboring pleural cavity with fluid invaginating into the underlying lung parenchyma, in close correlation with the anatomic findings (100). Moreover, additional reconstructions may help identify a few additional criteria. The pulmonary vessels trapped in the atelectatic lung can be correctly opacified and enhanced by means of the MIP principle, unless the parenchyma is too highly vascularized. The distortion and regular bending of these vessels can provide another argument against suspected carcinoma.
By using MPR images from 3D SSD images, it is possible to acquire images in multiple planes parallel to the converging vessels and bronchi. According to Mc Hugh and Blaquiere (101), when it is possible to identify the previously cited criteria with conventional and spiral CT examinations, further diagnostic evaluation is unnecessary. If most of the criteria are absent, cautious follow-up or percutaneous needle biopsy is recommended.
Spiral CT Evaluation of Peripheral Vessels
STS MIP Appearance of Normal Peripheral Pulmonary Vasculature
Besides dichotomous branching, pulmonary vessels may display a disproportionate mode of division. Dichotomous branching is the division of a branch into two equal parts, both arising at an acute angle and having half the diameter of the original branch. The disproportionate mode occurs in small branches arising at nearly a right angle from the native vessel, the branches being of smaller diameter than in the case of the usual dichotomous mode of bifurcation (Fig 11). This asymmetric division mode, which was previously only seen with injections of autopsy specimens but not on angiograms, is now easily recognized on STS MIP images. This new morphologic aspect of the pulmonary circulation is expected to be further included in the description of disorders of the peripheral pulmonary circulation.
|
|
Clinical Applications of STS MIP
From the middle 1980s to the middle 1990s, thin-section CT was widely performed to evaluate respiratory symptoms or pulmonary function abnormalities suggestive of diffuse infiltrative lung disease. This technique has been shown to provide improved clarity of parenchymal abnormalities, thus enabling a better and more confident characterization of disease processes in even severely involved areas. However, a few limitations of this technique have been reported with regard to the detection of micronodular infiltration and in the assessment of arteriolocentric lesions. On thin sections, peripheral pulmonary arteries and veins are multifragmented and numerous vessels are not depicted in the nonacquired intervals of the lung. Conversely, on thick sections calculated with a high-spatial- frequency reconstruction kernel, the distal vessels are correctly depicted but tiny micronodules are masked by the high-amplitude volume averaging effect. An excellent compromise is the STS MIP technique because (a) it depicts submillimetric vessels over a longer length than on individual thin sections, (b) the contrast resolution is reinforced because of the MIP algorithm, (c) the background means is maintained at a minimal level, and (d) there is no background enhancement in the absence of injected contrast medium. The clinical value of this technique was evaluated in a study group of 81 patients with suspicion of mild micronodular patterns (102). In this study, the sensitivity of STS MIP was statistically significantly higher than that of conventional CT in the detection of lung micronodules. In addition, it helped recognize the peribronchovascular distribution of lung lesions, a feature indicative of infiltration of the peripheral compartment of lung interstitium. Similarly, the arteriolocentric distribution of micrometastases can be precisely demonstrated (103).
Another clinical application of STS MIP is the recognition of pulmonary arteriovenous microfistulas (Fig 12). In congenital heart disease, a Glenn anastomosis, unifying the superior vena to the right descending pulmonary artery, has a reported incidence of pulmonary arteriovenous malformations (AVMs) of up to 25%. Other palliative surgical treatments, consisting of the diversion of the normal hepatic venous flow away from the pulmonary circulation, can also be responsible for the development of tiny pulmonary AVMs analogous to those described in chronic hepatopathies. Surgical channeling of hepatic venous blood flow in the pulmonary circulation is able to successfully remove pulmonary AVMs (104), but whether this resolution is transient or permanent is unknown. According to Srivastava et al (105) who conducted a pathologic study of two patients with pulmonary AVMs after cavopulmonary anastomosis, the distal pulmonary arteries at the level of the terminal and respiratory bronchioles are dilated and clusters of thin-walled vessels of indeterminate origin are also seen in the pleura. In close correlation to blood gases and angiographic findings, these tiny pulmonary AVMs can be identified on STS MIP images without using contrast medium (103). In chronic liver disease, Schraufnagel and Kay (106) described pulmonary artery–to–pulmonary vein anastomoses through pleural channels and arteriovenous channels within the lung parenchyma at intralobular, interlobular, and subpleural sites, ranging in size from 0.2 to 1.0 mm in diameter. These arteriovenous anastomoses, morphologically similar to those found in Rendu-Osler-Weber disease, can also be identified on STS MIP images without using contrast medium.
|
Congenital Anomalies of the Pulmonary Artery
Left Pulmonary Artery Sling
Whereas most frequently diagnosed in symptomatic children, left pulmonary artery sling can be found in asymptomatic adults in whom it can mimic a mediastinal adenopathy (107). Until now, the need for surgical repair has necessitated sophisticated diagnostic evaluation that required bronchoscopy and tracheobronchography for identification of tracheal compression and associated abnormalities, barium swallow examination for depiction of an anterior esophageal indentation due to the intertracheoesophageal course of the aberrant left pulmonary artery, and pulmonary angiography for the definitive preoperative diagnosis. As recently emphasized, spiral CT angiography has the advantage of avoiding the above-mentioned invasive examinations because a precise diagnosis of both vascular and bronchial anomalies can be obtained with a single data set (108–110).
Whatever the patient's age and condition, the left pulmonary artery sling should be regarded as a multiplanar anomaly because it requires multiple planes of reformations for an optimal pretherapeutic evaluation. Transverse CT scans are particularly suited for depicting both the abnormal origin and the abnormal course of the left pulmonary artery. Owing to its orientation, the abnormal vessel can be completely hidden behind the proximal part of the right pulmonary artery on a frontal pulmonary angiogram, which explains some angiographic misinterpretations (108). Moreover, performance of spiral CT can avert endoscopy and tracheobronchography, which cannot be considered as safe procedures in patients with compromised ventilation. An overall understanding of the tracheobronchial tree can be obtained from the spiral CT data set either using 3D SSD and a double thresholding segmentation or bronchographic-like reconstructions generated with the volume-rendering technique (111). As recently emphasized by Lacrosse et al (112), MPR images or preferably 3D images can demonstrate the tracheal stenosis and the associated anomalies of the bronchial bifurcation.
Occult Pulmonary Artery
From a semantic viewpoint, it is preferable to use the term "occult pulmonary artery" rather than proximal "absence" of a pulmonary artery branch, because in the majority of cases, the so-called absent pulmonary artery does exit in the hilum and has to be identified for therapeutic purposes. When the occult pulmonary artery is not associated with congenital cardiac disease, the presence of a left occult pulmonary artery associated with a right aortic arch can be diagnosed late in life (113). In the majority of patients, spiral CT angiography should replace conventional or digital angiography as it enables a more complete evaluation of the malformations (114). First, it allows a noninvasive recognition of the pulmonary artery anomaly and helps detect its hilar postobstructive portion. Second, it permits estimation of the importance and variable origins of the systemic collateral supply, which develops from birth to adulthood and which may be responsible for massive hemoptysis.
Pulmonary AVMs
Conventional CT has already proved useful and highly sensitive for the detection of pulmonary AVMs (115), but it suffers from false-negative results due to variations in respiratory depth, partial volume averaging effects, and motion artifacts. It is now accepted that spiral CT of the entire thorax represents the method of choice for routine detection of pulmonary AVMs (116). In addition, this technique appears to be able to overcome the limitations of the most frequently recommended screening procedures. Contrast-enhanced two-dimensional echocardiography has proved to be an excellent technique for the identification of a right-to-left shunt (117). However, definite conclusions regarding the sensitivity of contrast-enhanced two-dimensional echocardiography cannot be drawn from previous investigations because pulmonary angiograms, until now the reference standard technique for the detection of pulmonary AVMs, are not always obtained in patients with hereditary hemorrhagic telangiectasia and negative results of echocardiography. In addition, this technique cannot determine the number of pulmonary AVMs, their location in the lung, and their size. It should also be pointed out that this technique is not devoid of complications in patients with pulmonary AVMs. A transitory cerebral accident may occur after an intravenous peripheral injection of agitated saline solution or of a mixture of saline solution and a small amount of air used as a microbubble contrast agent during echocardiography (98). Several articles have recently appeared concerning estimation of the anatomic intrapulmonary shunt by measurements of arterial oxygen saturation, or SaO2, and measurements of arterial partial pressure of oxygen, or PaO2, and hemoglobin after breathing 100% oxygen for 15 minutes, and intravenous injection of technetium 99m–labeled albumin macroaggregates or microspheres (118,119). Despite being able to detect and quantify a right-to-left shunt, these techniques suffer from the same drawbacks as those cited for echocardiography. Consequently, they can be routinely used in families with hereditary hemorrhagic telangiectasia for the detection of pulmonary AVMs (120), but not for the detection and the pretherapeutic evaluation of these malformations.
Apart from the detection of pulmonary AVMs, spiral CT is useful in the pretherapeutic evaluation of the angioarchitecture of a pulmonary AVM, namely, in the assessment of the number of vessels connected to the aneurysmal sac arteries. With regard to the embolization procedure, the number and orientation of the feeding arteries are the most important anatomic information that determines the technical difficulty and thus the duration of the embolization procedure (Fig 13). Contradictory results on the respective proportions of simple and complex pulmonary AVMs have been reported in the literature. According to White et al (121), 80% of the malformations have a simple angioarchitecture (ie, a single artery being connected to the aneurysmal sac), whereas this percentage decreased to 20% in the Haitjema et al series (122). After the nonenhanced spiral CT evaluation of the thorax aimed at determining the number of malformations, a selective acquisition of the pulmonary AVM to be treated is performed. This second acquisition is usually performed without administration of contrast medium, with selection of a volume of interest that includes the aneurysmal sac and the vascular pedicles. The main technical aspects of this focal spiral CT acquisition can be summarized as follows. The section collimation depends on the location of the malformation. Pulmonary AVMs with vertically oriented pedicles can require a z-axis coverage of up to 10 cm. These acquisitions can be obtained with a 3–5-mm collimation and a pitch of 2. When the vascular pedicles are horizontally oriented, 4–5 cm of the thorax is surveyed with a 2–3-mm collimation and a pitch of 2. Transverse CT scans of the volume of interest are systematically reconstructed, which can be completed by means of 3D reconstructions of the malformations to obtain an overall view of the malformation. For 3D SSD, the threshold values range from -500 to -700 HU to depict pulmonary AVMs from nonenhanced data sets. Whenever transverse CT scans suggest additional tiny arterial branches to be participating in the pulmonary AVM, an additional 3D display is systematically obtained with a lower thresholding segmentation (-850 HU). This lower threshold may allow identification of additional small feeding arteries but markedly increases the profusion of vessels in the volume, which may prevent optimal viewing of the malformation. This is the reason why such a reconstruction is not obtained at first intention. As previously pointed out, an MIP image can also be generated from the same data acquisition. Whereas the depth impression of a 3D SSD enables a clear understanding of the malformation with each viewing angle, analysis of MIP images may be confusing since crossing and looping vessels are not correctly identified, unless the cine loop technique is used.
|
Congenital Anomalies of the Pulmonary Veins
Spiral CT is now the method of choice in the assessment of congenital anomalies of the pulmonary venous return. By means of a single data set, this technique enables a noninvasive assessment not only of pulmonary venous anomalies but also of the associated tracheobronchial and pulmonary arterial malformations. The congenital anomalies of the pulmonary venous return can be conveniently classified into one of the following categories: (a) anomalous pulmonary venous drainage with or without abnormal course in the lung, (b) anomalous pulmonary venous route without abnormal connection, or (c) abnormal venous diameters, including varices, stenoses, and atresia.
Partial Anomalous Pulmonary Venous Drainage without Abnormal Course in the Lung
Isolated anomalous drainage of the left superior pulmonary vein into the left brachiocephalic vein is reported to be one of the most common types of abnormal venous drainage. It runs the risk of being confounded with two other anomalies characterized by the presence of a vertical vein on the left border of the mediastinum (ie, a left superior vena cava ending in the coronary sinus, and a left superior vena cava draining into the left atrium). These differential diagnoses can be easily ruled out with spiral CT and specific injection techniques. In the case of a left superior vena cava draining into either the coronary sinus or the left atrium, the systemic venous flow is directed superoinferiorly, in the usual direction. Opacification of this abnormal vein is obtained with a very low concentration of iodinated contrast material, injected at a slow flow rate into the left arm positioned alongside the thorax to avoid the risk of obstruction at the thoracic inlet. An 8%–12% iodine concentration and a flow rate of 3 mL/sec are usually optimal technical parameters because the contrast medium is directed primarily toward the left superior vena cava owing to the absence or the small size of the left brachiocephalic vein. The start delay between injection and acquisition is short, usually 5 seconds.
In the case of partial anomalous venous return of the left upper lobe in a vertical vein, the blood flow is directed inferosuperiorly, partially draining the left upper lobe venous blood into the origin of the left brachiocephalic vein (Fig 14). Owing to the reversed flow, the vertical vein cannot be opacified by using the aforementioned technique because the small and slow bolus would be diluted in the left brachiocephalic vein. Consequently, the optimal injection technique employs a high iodine concentration (eg, 300 mg/mL, a flow rate of 5 mL/sec, and a minimum start delay of 12–15 seconds. As previously suggested, the most appropriate injection protocol can only be selected once the morphologic criteria are identified on nonenhanced scans. In the case of partial anomalous venous return in the vertical vein, its connection with the origin of the vertical vein is easily identified. Consequently, the rest of the normally positioned left superior pulmonary vein anterior to the left upper lobe bronchus is depicted as a small structure or is absent. The left brachiocephalic vein, which drains the left subclavian vein, the left internal jugular vein, and the vertical vein, is normal or slightly dilated. The coronary sinus is normal.
|
|
|
When isolated, the degree of the left-to-right shunt is equal to or less than 25% of the cardiac output and is compatible with a normal life. However, this acceptable hemodynamic condition can decompensate after contralateral surgery and can become a major left-to-right shunt, almost 50% of the cardiac output, after a right pneumonectomy (125). Consequently, when routinely using preoperative CT of the thorax, it is important to bear in mind the surgical importance of an unsuspected partial anomalous venous drainage of the left superior pulmonary vein into the left brachiocephalic vein whenever the patient is a candidate for right lung surgery. Depending on the importance of the lung volume to be resected, the anomalous vein may be preventively reimplanted into the left atrial appendage.
The diagnosis of partial anomalous pulmonary venous return of the right upper lobe into the right superior vena cava or the azygos arch can be made on incremental CT scans (126,127). Cross-sectional images, without contrast material enhancement, show right dilated pulmonary veins in close contact with the superior vena cava and/or the terminal portion of the azygos arch. As previously pointed out with regard to the left superior pulmonary vein, the expected normal part in the hilum and near the left atrium is not found or is too small. The diagnosis is easily confirmed with spiral CT angiography, which can use 3D reconstructions with the aim of clarifying this anomaly (128). Optimally, however, contrast material should be injected into the inferior vena cava to avoid simultaneous opacification of the pulmonary venous return and the superior vena cava from an injection into the arm.
Abnormal Venous Drainage with Abnormal Route in the Lung
In the majority of patients, a partial or total pulmonary anomalous venous return of the right lung, usually into the inferior vena cava or the right atrium, is easily diagnosed on a chest radiograph provided that the right lung is not too hypoplastic. The abnormal vein can be an isolated finding or may be associated with other anomalies termed the "scimitar syndrome" or "scimitar spectrum." The abnormal right pulmonary vein drains the right lung or only one portion of it into the inferior vena cava just above or below the diaphragm, looking like a Turkish sword. It receives the abnormal systemic arterial supply of the right lung, which triggers an additional more or less severe left-to-right shunt. It can be stenosed at its implantation into the inferior vena cava. The scimitar syndrome can be considered as a multiplanar anomaly, particularly suited for a spiral CT angiographic evaluation. It requires 3D reformations with or without contrast medium to provide an overall view of the abnormal vein (Fig 15), while thin-section CT scans can depict septal lines when the abnormal pulmonary venous return is obstructed. In the context of a preoperative evaluation, depiction of the pseudo-fissural line corresponding to the close contact between the right lung herniated behind the heart (horseshoe lung) and the left lung also requires thin sections. Three-dimensional reformations of the tracheobronchial tree are useful for an easier description of the abnormal right bronchial tree and 3D reformations of the abnormal systemic artery or arteries require good opacification. These vessels can be identified by their spontaneous high contrast if the right lower lobe has a normal attenuation. But their subdiaphragmatic origin and course are only clearly seen after injection of contrast medium. When considering the above-mentioned advantages of spiral CT in the recognition of the various anomalies observed in patients with scimitar syndrome (Table 2), one should strongly disagree with the conclusions of Schramel et al (129). In 1995, these authors stated that "angiography of the aorta, pulmonary arteries and veins, bronchoscopy, bronchography, spirometry, lung scintigraphy or bronchospirometry, and measurement of left/right shunt are essential in the management of patients with scimitar syndrome" (129). When surgery is needed, noninvasive follow-up after reimplantation of the abnormal vein in the left atrium can also require spiral CT angiography, especially when postoperative obstruction to the reimplanted abnormal pulmonary venous return in itself represents a further indication for pneumonectomy.
|
|
|
|
|
|
|
|
|
|
|
|
|
CONCLUSION
Owing to the multiple possibilities inherent to this technique, spiral CT angiography has the potential for substantial cost savings without reduction in the quality of patient care or diagnostic accuracy. In addition, the concept of using spiral CT as a physiologic tool has just appeared. Advanced computer technology and image-processing techniques will allow automated volume segmentation as well as anatomy and physiology integration with present and future rendering techniques. Undoubtedly, these new technical possibilities are able to offer a large field of investigation. It is also very likely that simulation, treatment planning, and guidance of therapeutic or diagnostic interventions using 3D and multimodality imaging technologies will be routinely applied in the near future.
Footnotes
Abbreviations: AVM = arteriovenous malformation MIP = maximum intensity projection MPR = multiplanar reformation PE = pulmonary embolism SSD = shaded surface display STS = sliding thin slab V-P = ventilation-perfusion 3D = three-dimensional
References
This article has been cited by other articles:
|
|
 |
|
  K. Fukuhara, A. Akashi, S. Nakane, and E. Tomita Preoperative assessment of the pulmonary artery by three-dimensional computed tomography before video-assisted thoracic surgery lobectomy Eur. J. Cardiothorac. Surg., October 1, 2008; 34(4): 875 - 877. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. M. Terra, A. Fernandez, R. H. Bammann, J. J.M. Junqueira, and V. L. Capelozzi Pulmonary Artery Sarcoma Mimicking a Pulmonary Artery Aneurysm Ann. Thorac. Surg., October 1, 2008; 86(4): 1354 - 1355. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 E. Y. Lee, P. M. Boiselle, and R. H. Cleveland Multidetector CT Evaluation of Congenital Lung Anomalies Radiology, June 1, 2008; 247(3): 632 - 648. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. Murata, T. Sakai, S. Goto, and K. Sumikawa Three-Dimensional Computed Tomography for Difficult Thoracic Epidural Needle Placement Anesth. Analg., February 1, 2008; 106(2): 654 - 658. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Scarsbrook, K. Bradley, F. Gleeson, A. M. Groves, S. J. Yates, T. Win, I. Kayani, J. Bomanji, and P. J. Ell Perfusion Scintigraphy Still has Important Role in Evaluation of Majority of Pregnant Patients with Suspicion of Pulmonary Embolism Radiology, August 1, 2007; 244(2): 623 - 625. [Full Text] [PDF]
|
|
|
|
 |
|
 D. A. Froehling, P. R. Daniels, S. J. Swensen, J. A. Heit, J. N. Mandrekar, J. H. Ryu, and P. L. Elkin Evaluation of a Quantitative D-Dimer Latex Immunoassay for Acute Pulmonary Embolism Diagnosed by Computed Tomographic Angiography Mayo Clin. Proc., May 1, 2007; 82(5): 556 - 560. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. H. Lee, J. M. Goo, H. J. Lee, K. G. Kim, J.-G. Im, and K. T. Bae Determination of Optimal Timing Window for Pulmonary Artery MDCT Angiography Am. J. Roentgenol., February 1, 2007; 188(2): 313 - 317. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. T. Nguyen, C. I. S. Silva, J. M. Seely, S. Chong, K. S. Lee, and N. L. Muller Pulmonary Artery Aneurysms and Pseudoaneurysms in Adults: Findings at CT and Radiography Am. J. Roentgenol., February 1, 2007; 188(2): W126 - W134. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Allen, T. Demetriades, and D. J. Brenner Radiation risk overestimated. Radiology, August 1, 2006; 240(2): 613 - 614. [Full Text] [PDF]
|
|
|
|
|
|
B. Ghaye, A. Nchimi, C. T. Noukoua, and R. F. Dondelinger Does Multi-Detector Row CT Pulmonary Angiography Reduce the Incremental Value of Indirect CT Venography Compared with Single-Detector Row CT Pulmonary Angiography? Radiology, July 1, 2006; 240(1): 256 - 262. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. M. Hurwitz, T. Yoshizumi, R. E. Reiman, P. C. Goodman, E. K. Paulson, D. P. Frush, G. Toncheva, G. Nguyen, and L. Barnes Radiation Dose to the Fetus from Body MDCT During Early Gestation. Am. J. Roentgenol., March 1, 2006; 186(3): 871 - 876. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 E. Castaner, X. Gallardo, J. Rimola, Y. Pallardo, J. M. Mata, J. Perendreu, C. Martin, and D. Gil Congenital and acquired pulmonary artery anomalies in the adult: radiologic overview. RadioGraphics, March 1, 2006; 26(2): 349 - 371. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. Uchida, M. Tabata, K. Kiura, Y. Tanimoto, A. Kanehiro, M. Aoe, N. Ohohara, H. Ueoka, and M. Tanimoto Successful Treatment of Pulmonary Artery Sarcoma by a Two-drug Combination Chemotherapy Consisting of Ifosfamide and Epirubicin Jpn. J. Clin. Oncol., July 1, 2005; 35(7): 417 - 419. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 L. K. Moores, W. L. Jackson Jr., A. F. Shorr, and J. L. Jackson Meta-Analysis: Outcomes in Patients with Suspected Pulmonary Embolism Managed with Computed Tomographic Pulmonary Angiography Ann Intern Med, December 7, 2004; 141(11): 866 - 874. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y. Ohno, T. Higashino, D. Takenaka, K. Sugimoto, T. Yoshikawa, H. Kawai, M. Fujii, H. Hatabu, and K. Sugimura MR Angiography with Sensitivity Encoding (SENSE) for Suspected Pulmonary Embolism: Comparison with MDCT and Ventilation-Perfusion Scintigraphy Am. J. Roentgenol., July 1, 2004; 183(1): 91 - 98. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
U. J. Schoepf, C. R. Becker, B. M. Ohnesorge, and E. K. Yucel CT of Coronary Artery Disease Radiology, July 1, 2004; 232(1): 18 - 37. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. C. Demos, H. V. Posniak, K. L. Pierce, M. C. Olson, and M. Muscato Venous Anomalies of the Thorax Am. J. Roentgenol., May 1, 2004; 182(5): 1139 - 1150. [Full Text] [PDF]
|
|
|
|
|
|
D. A. Froehling, P. L. Elkin, S. J. Swensen, J. A. Heit, V. S. Pankratz, and J. H. Ryu Sensitivity and Specificity of the Semiquantitative Latex Agglutination D-Dimer Assay for the Diagnosis of Acute Pulmonary Embolism as Defined by Computed Tomographic Angiography Mayo Clin. Proc., February 1, 2004; 79(2): 164 - 168. [Abstract] [PDF]
|
|
|
|
|
|
M. S. Ginsberg, V. King, and D. M. Panicek Comparison of Interpretations of CT Angiograms in the Evaluation of Suspected Pulmonary Embolism by On-Call Radiology Fellows and Subsequently by Radiology Faculty Am. J. Roentgenol., January 1, 2004; 182(1): 61 - 66. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
I. Hasegawa, P. M. Boiselle, and H. Hatabu Bronchial Artery Dilatation on MDCT Scans of Patients with Acute Pulmonary Embolism: Comparison with Chronic or Recurrent Pulmonary Embolism Am. J. Roentgenol., January 1, 2004; 182(1): 67 - 72. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
I. Hasegawa, P. M. Boiselle, V. Raptopoulos, and H. Hatabu Tracheomalacia Incidentally Detected on CT Pulmonary Angiography of Patients with Suspected Pulmonary Embolism Am. J. Roentgenol., December 1, 2003; 181(6): 1505 - 1509. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B. Trotman-Dickenson Radiology in the Intensive Care Unit (Part 2) J Intensive Care Med, September 1, 2003; 18(5): 239 - 252. [Abstract] [PDF]
|
|
|
|
|
|
E. E. Chiang, P. M. Boiselle, V. Raptopoulos, K. F. Reynolds, M. P. Rosen, and M. Simon Detection of Pulmonary Embolism: Comparison of Paddlewheel and Coronal CT Reformations--Initial Experience Radiology, August 1, 2003; 228(2): 577 - 582. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. P. Kanne, M. B. Gotway, N. Thoongsuwan, and E. J. Stern Six Cases of Acute Central Pulmonary Embolism Revealed on Unenhanced Multidetector CT of the Chest Am. J. Roentgenol., June 1, 2003; 180(6): 1661 - 1664. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. B. Henk, S. Grampp, K. F. Linnau, M. M. Thurnher, C. Czerny, C. J. Herold, and G. H. Mostbeck Suspected Pulmonary Embolism: Enhancement of Pulmonary Arteries at Deep-Inspiration CT Angiography—Influence of Patent Foramen Ovale and Atrial-Septal Defect Radiology, March 1, 2003; 226(3): 749 - 755. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S.-i. Watanabe, K. Arai, T. Watanabe, W. Koda, and H. Urayama Use of three-dimensional computed tomographic angiography of pulmonary vessels for lung resections Ann. Thorac. Surg., February 1, 2003; 75(2): 388 - 392. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. Arakawa, Y. Kurihara, K. Sasaka, Y. Nakajima, and W. R. Webb Air Trapping on CT of Patients with Pulmonary Embolism Am. J. Roentgenol., May 1, 2002; 178(5): 1201 - 1207. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 U J Schoepf, C R Becker, R D Bruening, B M Ohnesorge, A Huber, L-G Haw, H Hildebrandt, and M F Reiser Multislice CT angiography Imaging, December 15, 2001; 13(5): 357 - 365. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. Reissig, J.-P. Heyne, and C. Kroegel Sonography of Lung and Pleura in Pulmonary Embolism : Sonomorphologic Characterization and Comparison With Spiral CT Scanning Chest, December 1, 2001; 120(6): 1977 - 1983. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. P. Lawler and E. K. Fishman Multi-Detector Row CT of Thoracic Disease with Emphasis on 3D Volume Rendering and CT Angiography RadioGraphics, September 1, 2001; 21(5): 1257 - 1273. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G.R. Ferretti, I. Bricault, and M. Coulomb Virtual tools for imaging of the thorax Eur. Respir. J., August 1, 2001; 18(2): 381 - 392. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. L. Nickoloff and P. O. Alderson Radiation Exposures to Patients from CT: Reality, Public Perception, and Policy Am. J. Roentgenol., August 1, 2001; 177(2): 285 - 287. [Full Text] [PDF]
|
|
|
|
 |
|
 M. RIEDEL Emergency diagnosis of pulmonary embolism Heart, June 1, 2001; 85(6): 607 - 609. [Full Text]
|
|
|
|
 |
|
 J. A. Kline, E. G. Israel, E. A. Michelson, B. J. O'Neil, M. C. Plewa, and D. C. Portelli Diagnostic Accuracy of a Bedside D-dimer Assay and Alveolar Dead-Space Measurement for Rapid Exclusion of Pulmonary Embolism: A Multicenter Study JAMA, February 14, 2001; 285(6): 761 - 768. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. G. Crystal Research Opportunities and Advances in Lung Disease JAMA, February 7, 2001; 285(5): 612 - 618. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Riedel VENOUS THROMBOEMBOLIC DISEASE: Acute pulmonary embolism 1: pathophysiology, clinical presentation, and diagnosis Heart, February 1, 2001; 85(2): 229 - 240. [Full Text]
|
|
|
|
|
|
R. C. Gilkeson, J. R. Haaga, and L. M. Ciancibello Anomalous Unilateral Single Pulmonary Vein: Multidetector CT Findings Am. J. Roentgenol., November 1, 2000; 175(5): 1464 - 1465. [Full Text] [PDF]
|
|
|
|
|
|
T. P. Smith Pulmonary Embolism: What's Wrong with This Diagnosis? Am. J. Roentgenol., June 1, 2000; 174(6): 1489 - 1497. [Full Text]
|
|
|
|
|
|
V. Raptopoulos and P. M. Boiselle Multi-Detector Row Spiral CT Pulmonary Angiography: Comparison with Single-Detector Row Spiral CT Radiology, December 1, 2001; 221(3): 606 - 613. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
U. J. Schoepf, N. Holzknecht, T. K. Helmberger, A. Crispin, C. Hong, C. R. Becker, and M. F. Reiser Subsegmental Pulmonary Emboli: Improved Detection with Thin-Collimation Multi-Detector Row Spiral CT Radiology, February 1, 2002; 222(2): 483 - 490. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| RADIOLOGY | RADIOGRAPHICS | RSNA JOURNALS ONLINE |
).
) due to the presence of multicentric malignant histiocytofibroma (surgically confirmed).
in d). (e) Three-dimensional SSD (lateral view, threshold of +150 HU) generated from the same data set provides an overall view of the sub- and retro-hilar route of the abnormal vein (•) and of its normal drainage into the left atrium (
