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Multiplanar and Three-dimensional Multi–Detector Row CT of Thoracic Vessels and Airways in the Pediatric Population¹

¹ From the Edward Mallinckrodt Institute of Radiology, Washington University School of Medicine, 510 S Kingshighway Blvd, St Louis, MO 63110. Received August 8, 2002; revision requested October 8; revision received November 20; accepted January 2, 2003; updated September 15. Address correspondence to the author (e-mail: siegelm@mir.wustl.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION PATIENT PREPARATION CONSIDERATIONS BEFORE SCANNING POSTPROCESSING APPLICATIONS CLINICAL VASCULAR APPLICATIONS CENTRAL AIRWAYS APPLICATIONS CONCLUSION REFERENCES

 
Multi–detector row computed tomography (CT) has changed the approach to imaging of thoracic anatomy and disease in the pediatric population. At the author’s institution, multi–detector row CT with multiplanar and three-dimensional reconstruction has become an important examination in the evaluation of systemic and pulmonary vasculature and the tracheobronchial tree. In some clinical situations, multi–detector row CT with reformatted images is obviating conventional angiography, which is associated with higher radiation doses and longer sedation times. Although multi–detector row CT with multiplanar and three-dimensional reconstruction is expanding the applications of CT of the thorax, its role as a diagnostic tool still needs to be better defined. The purposes of this article are to describe how to perform multi–detector row CT with multiplanar and three-dimensional reconstruction in young patients, to discuss various reconstruction techniques available, and to discuss applications in the evaluation of vascular and airways diseases.

© RSNA, 2003

Index terms: Aorta, CT, 56.12111, 56.12112, 56.12117 • Bronchi, CT, 671.12111, 671.12117 • Computed tomography (CT), in infants and children • Computed tomography (CT), multi–detector row, _**.12117² • Radiology and radiologists, How I Do It • Trachea, CT, 671.12111, 671.12117

	INTRODUCTION

TOP ABSTRACT INTRODUCTION PATIENT PREPARATION CONSIDERATIONS BEFORE SCANNING POSTPROCESSING APPLICATIONS CLINICAL VASCULAR APPLICATIONS CENTRAL AIRWAYS APPLICATIONS CONCLUSION REFERENCES

 
The introduction of multi–detector row computed tomography (CT) has transformed the approach to imaging the thoracic vessels and airways in infants and children. The advantages of multi–detector row CT, compared with single–detector row CT, include improved temporal and spatial resolution, greater anatomic coverage, more consistent contrast material enhancement, and higher quality reconstructions. These benefits have dramatically expanded the applications of CT in the evaluation of vascular and airways diseases. In vascular imaging in adults, multi–detector row CT provides image quality that is equal or superior to that of conventional angiography. It can aid in the diagnosis of pulmonary embolus, arteriovenous malformation, aneurysm, and dissection, and, in some clinical scenarios, it often obviates conventional angiography (1,2). Similarly, CT of the airways can improve confidence in the diagnosis of airway stenoses and provide a more accurate assessment of the extent of narrowing (1–3).

Although there is information about the techniques and applications of multi–detector row CT in adults, similar information is sparse with regard to the pediatric population. The goal of this article is to describe an approach to performing multiplanar and three-dimensional (3D) CT of the thoracic vessels and airways in infants, children, and adolescents. The technical factors in performing the examination and the best methods of data reconstruction are described. Finally, the usefulness of multi–detector row CT in the evaluation of vascular and airways conditions is described.

	PATIENT PREPARATION

TOP ABSTRACT INTRODUCTION PATIENT PREPARATION CONSIDERATIONS BEFORE SCANNING POSTPROCESSING APPLICATIONS CLINICAL VASCULAR APPLICATIONS CENTRAL AIRWAYS APPLICATIONS CONCLUSION REFERENCES

 
As the speed of CT scanning has increased, the need for sedation in infants and children younger than 6 years of age has decreased. The sedation rate for single–detector row CT is at least 30% (4,5). Early experience has suggested that the sedation rate for young children undergoing multi–detector row CT is less than 5% (6). These data were based on small groups of patients, so further experience will be required to determine the precise sedation rate in a young patient population. Although improvements have been made, sedation has not been eliminated and knowledge of safe and effective use of sedation remains of paramount importance (7,8).

Orally administered chloral hydrate and intravenously administered pentobarbital sodium are the two most widely used sedative agents for diagnostic imaging (9). In our practice, we use chloral hydrate, (50–100 mg per kilogram body weight, maximum dose of 2,000 mg) administered orally for children younger than 18 months and pentobarbital sodium (6 mg/kg, maximum dose of 200 mg) administered intravenously for children older than 18 months. In general, children older than 5 years will cooperate after verbal reassurance and explanation of the procedure.

	CONSIDERATIONS BEFORE SCANNING

TOP ABSTRACT INTRODUCTION PATIENT PREPARATION CONSIDERATIONS BEFORE SCANNING POSTPROCESSING APPLICATIONS CLINICAL VASCULAR APPLICATIONS CENTRAL AIRWAYS APPLICATIONS CONCLUSION REFERENCES

 
Before the initiation of scanning, decisions must be made about the protocol for intravenous contrast material administration and the CT technical parameters.

Intravenous Contrast Material
The issues in the administration of intravenous contrast material for CT angiography in infants and children include the type and volume of contrast material, method of administration (manual vs power injection), and the delay between the start of injection and the initiation of scanning.

Intravenous contrast material is not administered routinely for CT evaluation of the airways. However, it is indicated in patients suspected of having paratracheal abnormalities such as vascular ring, anomalous origin of the pulmonary artery, and mediastinal masses.

If possible, an intravenous catheter should be in place when the child arrives in the CT suite. This reduces patient agitation that otherwise would be associated with a venipuncture performed just prior to the administration of contrast material. In our practice, nonionic contrast material is administered at a dose of 2.0 mL/kg (maximum dose, 125 mL). The use of a nonionic agent minimizes gastrointestinal side effects (nausea and vomiting), discomfort at the site of injection, patient motion during intravenous contrast material administration, and complications arising from contrast material extravasation (10,11). With multi–detector row CT, successful contrast enhancement for CT angiography can be achieved with total volumes as small as 4–5 mL (Fig 1).

View larger version (133K): [in this window] [in a new window] [Download PPT slide]   Figure 1. Interrupted aortic arch in a 2-kg neonate. Volume-rendered sagittal reconstruction from CT data (collimation, 1.0 mm; table feed, 8 mm per rotation) shows a markedly hypoplastic transverse aortic arch (long upper arrow) and a large patent ductus arteriosus (PDA; long lower arrow) that supplies the distal aorta. Scanning began 12 seconds after the start of the contrast material injection. Total volume of contrast material was 4 mL. I = brachiocephalic trunk (innominate artery), short arrow = left common carotid artery.

The accepted use of power injection through a 24-gauge peripheral catheter is not as well established in infants and children. However, there is some experience that suggests that this approach is safe, provided there is proper intravascular positioning of the access, verified by observing unimpeded return of blood and unimpeded delivery of a saline flush (14). Injection through a central venous access also is controversial in the pediatric population, although at our institution we will use central access if peripheral access is not available. Contrast material is administered at a rate of 1.0 mL/sec.

Although a power injector is preferred for the intravenous delivery of contrast material, manual injection is used when intravenous access is via a catheter placed in the dorsum of the hand or wrist. The complication rates from manual and power injections are similar (<0.4%), provided the catheter is properly positioned and functions well (14).

Scanning Delay Times
The delay time is the time between the start of the contrast material injection and the start of the scan data acquisition. In our experience, we use either an empiric delay, based on the weight of the patient and the clinical indication for the examination, or an automated tracking system. An empiric delay of 12–15 seconds after the start of the intravenous contrast material injection usually produces excellent vascular enhancement in patients who weigh less than 10 kg (Fig 1). A delay of 20–25 seconds is used in larger patients. Of note, the delay for angiography is shorter than that for routine chest CT (ie, tumor staging, evaluation of a mediastinal mass or congenital lung anomaly). In the latter scenario, the delay after the start of contrast material administration is usually set at 30 seconds.

The automated tracking system has the advantage of allowing customization of contrast enhancement for each patient, taking into account factors such as cardiac output and circulation time. This technique uses continuous monitoring of the attenuation value within a large target vessel (eg, aorta or pulmonary artery) by use of a series of low-dose transverse images. A cursor is placed within a region of interest in the target vessel. Once the designated threshold has been achieved (generally 100 HU), low-dose scanning terminates and the diagnostic examination begins at the desired region (15). A default delay can be programmed in case the desired threshold is not achieved. This method is best used with a power injector. Although a hand injector can be used, it is more difficult to reach a high enough level of contrast material to trigger initiation of the acquisition.

Technical Parameters
Once the patient is sedated and an intravenous catheter is in place, several technical parameters need to be selected before the initiation of scanning. These include collimator thickness, table speed, tube current or milliamperage, kilovoltage, and anatomic coverage.

The collimator thickness determines the effective section thickness (ie, minimum section thickness) that can be acquired after scanning is completed (17,18). If thin sections are desired, this must be planned before the image acquisition process. For example, if the collimation is set at 5 mm, the minimum section thickness is 5 mm. If the collimation is set at 1 mm, the minimum section thickness is 1 mm. Although 1-mm-thick sections can improve resolution, they are not routinely used in children because a decrease in section thickness requires an increase in tube current to maintain the same signal-to-noise ratio. With this scenario, the radiation dose increases.

The table speed should be as fast as possible. A faster speed increases temporal resolution and decreases radiation dose.

The imaging techniques described below reflect experience with Plus 4 Volume Zoom and Sensation 16 scanners (Siemens, Iselin, NJ) with a 0.5-second rotation time (Tables 1, 2). The techniques for evaluation of vascular lesions and airways are similar. With a four-row detector, most CT examinations are performed with 2.5-mm collimation and a table speed of 15–20 mm per rotation. Because the major thoracic vessels and airways are long, of large caliber, and usually course perpendicular to the transverse plane, thinner (1-mm) sections (which impart more radiation dose) are not necessary. Thinner sections, however, have proved to be useful in the evaluation of small structures, such as a patent ductus arteriosus and subsegmental pulmonary arteries or bronchi, that course parallel to the transverse plane. With a 16-row detector, the collimation is 1.5 mm with a table speed of 36 mm per rotation. The 2.5-mm-thick sections are reconstructed at 3-mm thickness, and the 1.5-mm-thick sections are reconstructed at 2-mm thickness.

Radiation Exposure: Milliamperage and Kilovoltage
The issue of radiation exposure is extremely important in children. Because children are more radiosensitive than adults to the same organ dose and because they have a longer life span, a potential for the development of radiation-induced malignancies exists. In general, multi–detector row CT should be performed with techniques that provide acceptable image quality and the lowest possible radiation exposure. These techniques include the use of low milliamperage and kilovoltage settings, appropriate section thickness, and a faster table speed (19–25). Additionally, multiphasic studies should be performed only when necessary, rather than be used as a routine protocol.

The lowest possible milliamperage and kilovoltage should be used for CT in children. The tube currents recommended for thoracic CT examinations in children are shown in Table 3. In patients with smaller body habitus (50 kg), the studies can be performed at 80 kV. The 80-kV approach lowers the dose to the patient (compared with standard 120-kV protocols) by approximately 30%, with better contrast visualization (unpublished data, 2003). A higher kilovoltage will be needed in patients with larger body habitus (>50 kg).

In several clinical scenarios, a greater degree of coverage is warranted. For example, in the setting of pulmonary sequestration, inclusion of the upper abdomen is necessary because the anomalous feeding artery may arise from the proximal abdominal aorta, particularly in cases of extralobar sequestration. Similarly, in the setting of aortic dissection, imaging through the abdominal aorta is needed because intimal flaps may extend into the aortic branches. Finally, some disease entities may require inclusion of the cervical portions of the carotid and vertebral arteries. This is particularly relevant when assessing large-vessel arteritides, such as Takayasu or Kawasaki disease, which can involve the brachiocephalic branches.

To ensure coverage of the clinical area of interest, CT angiography begins with an unenhanced frontal scout image of the chest and upper abdomen. Precontrast transverse scans are not obtained routinely for CT angiography in children, in order to minimize patient radiation exposure. The exceptions are the evaluation of endoluminal stents, usually for repair of coarctation, and dissections. In the former scenario, calcification around the graft, which can mimic an endoleak on contrast-enhanced scans, will be seen best on unenhanced scans. In the setting of dissection, unenhanced sections are useful for localizing high-attenuating hematoma in the false lumen.

Field of View
The field of view selected should closely approximate the cross-sectional size of the part being studied. Spatial resolution is improved by using a smaller field of view because the pixel size decreases as the field of view decreases. An extremely large field of view results in a waste of matrix space, a loss of resolution, and poor quality images, and it may yield erroneous attenuation values because of partial volume averaging.

Breath Holding
CT examinations are performed with breath holding at suspended inspiration in cooperative patients, usually children older than 5–6 years of age. Scans are obtained during quiet respiration in children who are unable to cooperate with breath-holding instructions and in patients who are sedated.

Reconstruction Algorithms
A standard reconstruction algorithm usually suffices for routine studies and CT angiograms. A high-resolution algorithm is used for 3D reconstructions of the airways.

	POSTPROCESSING APPLICATIONS

TOP ABSTRACT INTRODUCTION PATIENT PREPARATION CONSIDERATIONS BEFORE SCANNING POSTPROCESSING APPLICATIONS CLINICAL VASCULAR APPLICATIONS CENTRAL AIRWAYS APPLICATIONS CONCLUSION REFERENCES

 
A number of image reconstruction options are available for postprocessing of volumetric data sets. We currently use three reconstruction displays to highlight areas of interest—multiplanar reformations, variable-thickness displays, and 3D volume renderings. The 3D volume renderings use all the information initially acquired in the raw data set for image reconstruction, whereas other methods (eg, shaded-surface display or maximum or minimum intensity projections) use only a small amount of the data (16).

Multiplanar Reconstructions
Multiplanar reformations are 1-voxel-thick, two-dimensional tomographic sections that can be displayed in coronal, sagittal, or parasagittal planes or in a curved plane, such as along the axis of the mediastinal vessels or airways (Fig 2). The advantages of this technique are that it is fast, can be easily performed at the CT scanner, and uses all of the attenuation values in the data set, presenting them in off-axis views. Multiplanar reformatting is used to assess the extent of disease processes in the craniocaudal direction (Fig 2). The major disadvantage of this technique is that it provides only a two-dimensional display of data; information regarding the spatial relationships of anatomic structures is absent.

View larger version (113K): [in this window] [in a new window] [Download PPT slide]   Figure 2. Multiplanar coronal reconstruction from CT data (collimation, 2.5 mm; table feed, 15 mm per rotation) of the trachea (T) and right and left main bronchi in a 15-year-old boy with cough. Coronal reformation obtained at end inspiration shows normal caliber of the trachea. Image was reconstructed by using 3-mm section thickness and 1-mm table intervals.

Variable-Thickness Displays
Peripheral vessels and airways are often better seen as an assimilation of sections in a volume slab rather than in individual sections of equivalent thickness. With this technique, CT images are acquired at their routine section thickness and then combined in multiples, or "slabs," to create a thicker image. The volume slab technique can be used with either maximum intensity projections (MIPs), which display data on the basis of the maximum attenuation value, or minimum intensity projections (MINIPs), which display data on the basis of the minimum attenuation value. MIPs have been applied to the examination of pulmonary vessels, while MINIPs can be used to enhance evaluation of the airways.

3D Volume Rendering
Volume rendering has largely replaced other 3D reformatting techniques in evaluations of airway and vascular pathologic processes. Every voxel is assigned a proportional value that is based on the full range of tissue attenuation values in the original data set (Fig 3). The use of a transfer function allows mapping of a CT number to brightness, color, and opacity. This allows the visualization of different tissue types with varying color and opacity (transparency) levels and provides valuable information regarding the spatial relationship of anatomic structures. It also permits the data to be displayed from an external or internal perspective (ie, "virtual bronchoscopy") (1–3,26,27). The volume-rendering technique is particularly useful for displaying structures that course parallel or oblique to the transverse plane.

View larger version (115K): [in this window] [in a new window] [Download PPT slide]   Figure 3a. Reconstructions based on CT data (collimation, 2.5 mm; table feed, 15 mm per rotation) show normal trachea. (a) Coronal 3D volume-rendered image with lower opacity values shows normal-caliber trachea and right and left main bronchi. Lower opacity values render a more transparent image. (b) A 3D image from virtual bronchoscopy at level of main bronchi shows patency of right main bronchus. Orifice of the left main bronchus was best seen at a different level. This volume-rendered image with higher opacity levels and more color enhances the visibility of the airway surface.

View larger version (108K): [in this window] [in a new window] [Download PPT slide]   Figure 3b. Reconstructions based on CT data (collimation, 2.5 mm; table feed, 15 mm per rotation) show normal trachea. (a) Coronal 3D volume-rendered image with lower opacity values shows normal-caliber trachea and right and left main bronchi. Lower opacity values render a more transparent image. (b) A 3D image from virtual bronchoscopy at level of main bronchi shows patency of right main bronchus. Orifice of the left main bronchus was best seen at a different level. This volume-rendered image with higher opacity levels and more color enhances the visibility of the airway surface.

	CLINICAL VASCULAR APPLICATIONS

TOP ABSTRACT INTRODUCTION PATIENT PREPARATION CONSIDERATIONS BEFORE SCANNING POSTPROCESSING APPLICATIONS CLINICAL VASCULAR APPLICATIONS CENTRAL AIRWAYS APPLICATIONS CONCLUSION REFERENCES

 
The rapid imaging afforded by multi–detector row CT is the basis for the growth of CT angiography. This technique is challenging the role of conventional angiography and, in some instances, has even replaced it (1,2). Multi–detector row CT has advantages over conventional angiography. The volumetric acquisition allows superb image resolution and clear delineation of the aorta, superior vena cava, and pulmonary arteries and veins and their branches. The major vessels and their branches commonly overlap on conventional arteriograms, which obscures their depiction. Other advantages of CT over angiography include shorter acquisition times; superior 3D renderings; and greater range of coverage, which increases detection of vascular and nonvascular lesions. Compared with the radiation dose for angiography, the radiation dose for CT angiography is at least two to three times lower. In our experience, these advantages have resulted in CT obviating conventional arteriography in the assessment of some vascular abnormalities, particularly those related to the great vessels.

CT also has gained increasing acceptance as an alternative to magnetic resonance (MR) imaging in the diagnosis of vascular anomalies. CT angiography has the advantage over MR angiography of shorter acquisition times, which means a reduced need for sedation and the ability to scan extremely ill patients who cannot tolerate the long imaging times for MR examinations. There is the risk of radiation exposure in CT angiography, but in the critically ill patient the risk of prolonged sedation may be greater than that of radiation.

Transverse images are often sufficient for diagnosis. However, the use of multiplanar and 3D reconstructions can help in the detection of short focal stenoses and coarctations (28). In addition, these images mimic the perspective seen by the surgeon, who is oriented to a traditional coronal or sagittal visualization of the aorta; thus, the reconstructions can help in surgical planning.

Aortic Arch Anomalies
Both vascular ring and coarctation can be exquisitely demonstrated with CT angiography (Fig 4). In the setting of vascular ring, multiplanar and 3D reconstructions can improve understanding of the origin of the great vessels and the relationship of the vessels to the adjacent airway, compared with the understanding achievable with transverse images alone. This can help provide a more accurate assessment of airway compromise (29,30). In the setting of coarctation, the location and extent of the coarctation, the relationship of the coarctation to the great vessels, and the extent of collateral vessel formation become better defined with imaging planes customized to the course of the aorta (Fig 5). Volume-rendered images also may prove valuable in postoperative evaluation of the aorta because the positions of endovascular stents and their relationships to the origin of the great vessels can be clearly demonstrated (Fig 6).

View larger version (144K): [in this window] [in a new window] [Download PPT slide]   Figure 4a. Double aortic arch. (a) Transverse contrast-enhanced CT image produced with variable-thickness display (ie, average of three images together to create thicker image) and maximum intensity projection shows double aortic arch surrounding the trachea (T), without airway compression. The two arches meet posteriorly. (b) Coronal volume-rendered CT image shows the relationship of the great vessels to both right arch (R) and left arch (L). A = ascending aorta, P = main pulmonary artery.

View larger version (137K): [in this window] [in a new window] [Download PPT slide]   Figure 4b. Double aortic arch. (a) Transverse contrast-enhanced CT image produced with variable-thickness display (ie, average of three images together to create thicker image) and maximum intensity projection shows double aortic arch surrounding the trachea (T), without airway compression. The two arches meet posteriorly. (b) Coronal volume-rendered CT image shows the relationship of the great vessels to both right arch (R) and left arch (L). A = ascending aorta, P = main pulmonary artery.

View larger version (133K): [in this window] [in a new window] [Download PPT slide]   Figure 5a. (a) Transverse CT and (b) multiplanar reconstruction images of coarctation in a newborn boy. Short-segment coarctation (arrow) is demonstrated on b, but the area of narrowing was not identified on a. (c) Coarctation in an adolescent patient. Descending aorta is narrowed (black arrowhead) just beyond the left subclavian artery (S), which is mildly dilated. Also note large internal mammary artery collateral vessel (white arrowheads). Posterior collateral intercostal arteries (arrow) are also seen.

View larger version (113K): [in this window] [in a new window] [Download PPT slide]   Figure 5b. (a) Transverse CT and (b) multiplanar reconstruction images of coarctation in a newborn boy. Short-segment coarctation (arrow) is demonstrated on b, but the area of narrowing was not identified on a. (c) Coarctation in an adolescent patient. Descending aorta is narrowed (black arrowhead) just beyond the left subclavian artery (S), which is mildly dilated. Also note large internal mammary artery collateral vessel (white arrowheads). Posterior collateral intercostal arteries (arrow) are also seen.

View larger version (150K): [in this window] [in a new window] [Download PPT slide]   Figure 5c. (a) Transverse CT and (b) multiplanar reconstruction images of coarctation in a newborn boy. Short-segment coarctation (arrow) is demonstrated on b, but the area of narrowing was not identified on a. (c) Coarctation in an adolescent patient. Descending aorta is narrowed (black arrowhead) just beyond the left subclavian artery (S), which is mildly dilated. Also note large internal mammary artery collateral vessel (white arrowheads). Posterior collateral intercostal arteries (arrow) are also seen.

View larger version (82K): [in this window] [in a new window] [Download PPT slide]   Figure 6. Postoperative coarctation in a 10-year-old boy. Sagittal volume-rendered CT reconstruction, viewed from behind, demonstrates an endovascular aortic stent (arrows). The left subclavian artery (S) is narrowed at its origin. C = left common carotid artery.

View larger version (86K): [in this window] [in a new window] [Download PPT slide]   Figure 7a. Annuloaortic ectasia secondary to Marfan disease. (a) Transverse contrast-enhanced CT scan shows a dilated ascending aorta (A). (b) Multiplanar coronal CT reconstruction demonstrates craniocaudal extent of dilatation. P = normal-caliber main pulmonary artery. (Case courtesy of Jack Sty, MD, Milwaukee, Wis.)

View larger version (83K): [in this window] [in a new window] [Download PPT slide]   Figure 7b. Annuloaortic ectasia secondary to Marfan disease. (a) Transverse contrast-enhanced CT scan shows a dilated ascending aorta (A). (b) Multiplanar coronal CT reconstruction demonstrates craniocaudal extent of dilatation. P = normal-caliber main pulmonary artery. (Case courtesy of Jack Sty, MD, Milwaukee, Wis.)

Pulmonary Sequestration
CT angiography has become the primary screening tool for definitive establishment of the diagnosis of pulmonary sequestration in infants and children (34). Precise timing of contrast material administration allows optimal evaluation of the thoracic aorta, and the rapid acquisition of images allows evaluation of the abdomen with the same contrast material bolus. Transverse data sets reconstructed with thin collimation can be used to diagnose the anomalous vessel, which is recognized by its opacification immediately after peak aortic enhancement (Fig 8). The ability to produce multiplanar and 3D volume-rendered images can help follow the route of those feeding arteries that course obliquely through the imaging planes.

View larger version (135K): [in this window] [in a new window] [Download PPT slide]   Figure 8a. Extralobar pulmonary sequestration. (a) Transverse contrast-enhanced CT scan demonstrates feeding artery (arrowhead) arising from the proximal abdominal aorta (A). (b) Coronal volume-rendered CT image shows entirety of feeding artery (arrow) as it courses from aorta (A) to left lower lobe sequestration (S).

View larger version (110K): [in this window] [in a new window] [Download PPT slide]   Figure 8b. Extralobar pulmonary sequestration. (a) Transverse contrast-enhanced CT scan demonstrates feeding artery (arrowhead) arising from the proximal abdominal aorta (A). (b) Coronal volume-rendered CT image shows entirety of feeding artery (arrow) as it courses from aorta (A) to left lower lobe sequestration (S).

View larger version (112K): [in this window] [in a new window] [Download PPT slide]   Figure 9. Right hemitruncus. Coronal 3D volume-rendered CT image demonstrates right pulmonary artery (P) arising from proximal ascending aorta (A).

View larger version (130K): [in this window] [in a new window] [Download PPT slide]   Figure 10. Tetralogy of Fallot with failed right Blalock-Taussig shunt. Coronal volume-rendered CT image demonstrates multiple transpleural collateral vessels arising from a dilated tortuous right mammary artery (arrows).

Pulmonary Arteries and Veins
The complex curving anatomy of the pulmonary artery and vein is also well suited to CT angiography (35–37). Three-dimensional volume-rendered images are valuable in studies of malformations such as pulmonary arteriovenous malformations and anomalous venous return. Three-dimensional imaging has been shown to be accurate in defining the angioarchitecture of pulmonary arteriovenous malformations and can improve the depiction of these small-caliber structures (38). On transverse images, it may be difficult to discern with thin sections whether a small structure is a vessel and whether it is an artery or vein. Small vessels may be better appreciated on multiplanar and 3D renderings, where they can be shown to connect to nearby vascular structures. Multiplanar and 3D volume rendering may also prove to be valuable in evaluation of anomalous pulmonary venous return, because the course of these obliquely oriented vessels can be better shown than on transverse images (Fig 11).

View larger version (86K): [in this window] [in a new window] [Download PPT slide]   Figure 11a. Scimitar syndrome with partial anomalous venous return. Transverse CT scans obtained at the level (a) of the ventricles and (b) of the liver show anomalous pulmonary vein (arrow) entering the inferior vena cava (C) just below the junction with the right atrium. Note also slightly smaller right hemithorax and ipsilateral mediastinal shift. Right ventricle (V) is dilated due to left-to-right shunt. (c) Left oblique volume-rendered 3D CT image shows entire course of the anomalous vessel (arrow). Most of the right lung in this patient drained into the anomalous vein. Only a small part of the upper lobe drained normally. C = inferior vena cava.

View larger version (102K): [in this window] [in a new window] [Download PPT slide]   Figure 11b. Scimitar syndrome with partial anomalous venous return. Transverse CT scans obtained at the level (a) of the ventricles and (b) of the liver show anomalous pulmonary vein (arrow) entering the inferior vena cava (C) just below the junction with the right atrium. Note also slightly smaller right hemithorax and ipsilateral mediastinal shift. Right ventricle (V) is dilated due to left-to-right shunt. (c) Left oblique volume-rendered 3D CT image shows entire course of the anomalous vessel (arrow). Most of the right lung in this patient drained into the anomalous vein. Only a small part of the upper lobe drained normally. C = inferior vena cava.

View larger version (145K): [in this window] [in a new window] [Download PPT slide]   Figure 11c. Scimitar syndrome with partial anomalous venous return. Transverse CT scans obtained at the level (a) of the ventricles and (b) of the liver show anomalous pulmonary vein (arrow) entering the inferior vena cava (C) just below the junction with the right atrium. Note also slightly smaller right hemithorax and ipsilateral mediastinal shift. Right ventricle (V) is dilated due to left-to-right shunt. (c) Left oblique volume-rendered 3D CT image shows entire course of the anomalous vessel (arrow). Most of the right lung in this patient drained into the anomalous vein. Only a small part of the upper lobe drained normally. C = inferior vena cava.

	CENTRAL AIRWAYS APPLICATIONS

TOP ABSTRACT INTRODUCTION PATIENT PREPARATION CONSIDERATIONS BEFORE SCANNING POSTPROCESSING APPLICATIONS CLINICAL VASCULAR APPLICATIONS CENTRAL AIRWAYS APPLICATIONS CONCLUSION REFERENCES

View larger version (81K): [in this window] [in a new window] [Download PPT slide]   Figure 12. Tracheal stricture caused by prior trauma in a 12-year-old boy. Coronal volume-rendered CT images obtained at inspiration (A) and expiration (B) show smoothly marginated asymmetric narrowing above the bifurcation. Arrows = longitudinal extent of narrowing. Lack of change in airway caliber during the respiratory cycle indicates stenosis or fixed narrowing rather than tracheomalacia.

View larger version (62K): [in this window] [in a new window] [Download PPT slide]   Figure 13a. Tracheomalacia. (a) Sagittal reconstruction of CT data obtained at full expiration shows approximately 50% tracheal narrowing. (b) Coronal 3D volume-rendered reconstruction of CT data with lower opacity (transparency) values obtained during full inspiration shows normal-caliber trachea.

View larger version (71K): [in this window] [in a new window] [Download PPT slide]   Figure 13b. Tracheomalacia. (a) Sagittal reconstruction of CT data obtained at full expiration shows approximately 50% tracheal narrowing. (b) Coronal 3D volume-rendered reconstruction of CT data with lower opacity (transparency) values obtained during full inspiration shows normal-caliber trachea.

Virtual bronchoscopic images almost never result in an altered diagnosis. However, in selected cases, such as a high-grade stenosis or large intraluminal tumor, these images can allow evaluation of the airways beyond the site of stenosis or neoplasm, which otherwise can be difficult to visualize at conventional bronchoscopy. In addition, pulmonologists often prefer these images since they mimic the perspective as seen through an endoscope.

The capability of multi–detector row CT for dynamic inspiratory and expiratory imaging helps in the identification of strictures, areas of air trapping, and tracheobronchomalacia (Fig 13) (39).

	CONCLUSION

TOP ABSTRACT INTRODUCTION PATIENT PREPARATION CONSIDERATIONS BEFORE SCANNING POSTPROCESSING APPLICATIONS CLINICAL VASCULAR APPLICATIONS CENTRAL AIRWAYS APPLICATIONS CONCLUSION REFERENCES

 
Multi–detector row CT with multiplanar and 3D imaging has expanded the role of thoracic CT in the pediatric population. Multiplanar and 3D reconstructions can improve communication of anatomic detail to clinicians and, in selected instances, provide additional information about the nature and extent of disease. However, the exact role of CT with reconstructions in noninvasive imaging of pediatric vessels and airways still needs to be clarified. Which children should be evaluated in this fashion? What is the benefit of this advanced technology versus the risk from radiation exposure? Further research is needed to answer these questions and improve our understanding of how we should use CT and reconstruction techniques in the diagnosis and management of thoracic diseases in the pediatric population.

	FOOTNOTES

 
²_**. Multiple body systems

Abbreviation: 3D = three-dimensional

	REFERENCES

TOP ABSTRACT INTRODUCTION PATIENT PREPARATION CONSIDERATIONS BEFORE SCANNING POSTPROCESSING APPLICATIONS CLINICAL VASCULAR APPLICATIONS CENTRAL AIRWAYS APPLICATIONS CONCLUSION REFERENCES

 

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