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Technical Developments |
1 From the Department of Radiology, Northwestern University Medical School, 676 N St Clair St, Suite 800, Chicago, IL 60611 (F.S.P., R.M.M., V.B., J.C.C., V.K., J.P.F.); and University of Arizona Health Sciences Center, Tucson (E.A.K.). From the 2001 RSNA scientific assembly. Received May 24, 2001; revision requested June 25; revision received August 20; accepted September 28. Address correspondence to F.S.P. (e-mail: s-pereles@northwestern.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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© RSNA, 2002
Index terms: Aorta, dissection, 941.74, 943.74 • Aneurysm, aortic, 941.73, 943.73 • Magnetic resonance (MR), cine study, 941.12949, 943.12949 • Magnetic resonance (MR), vascular studies, 941.12942, 943.12942
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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Contrast-enhanced MR angiography has largely replaced nonenhanced MR angiographic techniques. Typical MR imaging protocols in use today for the aorta vary among institutions but generally involve a combination of multiple nonenhanced gradient- and spin-echo techniques and contrast-enhanced 3D gradient-echo imaging. Total examination time for confident diagnostic MR evaluation of the aorta ranges between 10 and 45 minutes and is therefore suitable only for medically stable patients. With the combination of steady-state gradient-echo techniques and advanced gradient hardware, the stage has been set for the introduction of a family of high-speed pulse sequences with very favorable vascular imaging properties (2–4).
True fast imaging with steady-state precession (FISP) (TrueFISP; Siemens Medical Systems, Iselin, NJ) is a coherent steady-state technique that uses a fully balanced gradient waveform to recycle transverse magnetization. Contrast is determined on the basis of the ratio of T2 to T1 rather than on the basis of inflow effects, as in spoiled gradient-echo methods. This difference eliminates sensitivity to saturation effects from absent or slow flow.
Relative to spoiled gradient-echo methods, true FISP is extremely rapid, which allows performance of a cine MR sequence in as little as 5–6 seconds. Gated single-phase true FISP images can be acquired in a single heartbeat. This allows a rapid survey of the thorax in multiple planes in less than 1 minute.
To our knowledge, a study of nonenhanced true FISP MR imaging of the thoracic aorta for diagnosis and evaluation of suspected or known aortic dissection or aneurysm has not been published. We hypothesized that a short imaging protocol that combined single-shot true FISP and cine true FISP sequences would be adequate for confident initial evaluation of these abnormalities. The basis for this hypothesis was our experience with these sequences as part of our routine clinical evaluation of patients. The purpose of our study was to retrospectively evaluate our experience with single-shot true FISP and cine true FISP MR imaging of the thoracic aorta for the diagnosis of aortic dissection or aneurysm.
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Materials and Methods |
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TOPABSTRACTINTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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Imaging
All patients underwent MR imaging of the chest with a 1.5-T whole-body system (Magnetom Sonata; Siemens Medical Systems) equipped with high-performance gradients (slew rate, 200 mT/m/msec; amplitude, 40 mT/m). Our comprehensive routine imaging protocol included (a) transverse and coronal single-shot true FISP imaging of the chest followed by (b) four breath-hold cine true FISP acquisitions, (c) transverse nonenhanced and contrast-enhanced gradient-echo T1-weighted fat-saturated two-dimensional acquisitions, and (d) sagittal oblique and coronal contrast-enhanced time-resolved MR angiography of the chest followed by (e) contrast-enhanced high-spatial-resolution 3D MR angiography. The single-shot true FISP images were cardiac gated to acquire images in diastole. A stack of two-dimensional images can be acquired in approximately 30–40 seconds depending on patient size and heart rate. The cine angiographic true FISP movies consisted of three transverse sequences performed through the aorta at the arch, middle ascending aorta, and aortic root and one left anterior oblique sagittal sequence of the aorta. Each cine sequence takes approximately 6 seconds. Therefore, the actual sequence times combined for the transverse and coronal single-shot stacks and the four cine sequences was less than 2 minutes. The entire time for scout localization imaging, set up of sequences, and acquisition of images was less than 4 minutes. The following parameters were used for both single-shot and cine imaging: 3.2/1.6 (repetition time msec/echo time msec); flip angle, 50°–70°; readout bandwidth, 975 Hz/pixel; rectangular field of view, 320–380 mm; matrix, 140–256 x 256; section thickness, 6 mm; single-shot section interspace, 4 mm.
As part of our routine clinical protocol, all patients also underwent high-spatial-resolution contrast-enhanced 3D MR angiography performed immediately after true FISP imaging. This 3D MR angiographic examination served as proof of diagnosis. The parameters of the high-spatial-resolution 3D MR angiographic sequence were as follows: coronal plane; 3.2/1.4; 25°–30° flip angle; resolution, 200 x 512; rectangular field of view, 320–380 mm; slab thickness, 80–120 mm; 80 partitions (interpolated); effective thickness, 1.0–1.5 mm; imaging time, 15–21 seconds.
A 2–3-mL timing bolus with 15-mL saline flush at 2–3 mL/sec was used to determine the time of bolus arrival in the aorta. For subtraction imaging, acquisitions were performed before and after infusion of 30–40 mL of gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ) at a rate of 2–3 mL/sec with a 15-mL saline flush at the same rate.
In addition, all abnormal cases had further imaging studies: CT aortography (n = 14), Doppler ultrasonography (US) (n = 4), and DSA (n = 3). Five patients underwent surgical repair. Several patients had more than one type of imaging study or surgery. In 16 patients, these additional correlative studies were performed within approximately 1 month of the MR examination (mean, 9.6 days ± 10.7; range, 0–35 days). Four patients had been followed up yearly for at least 3 years before their MR examinations for stable abnormalities (three aneurysms and two type B dissections; one of these aneurysms also had a dissection, hence, five abnormalities in four patients).
Study Design
Two MR fellowship–trained radiologists who interpret clinical vascular MR studies on a daily basis (F.S.P., R.M.M.) independently performed a retrospective review of images to determine if true FISP MR sequences alone were adequate to confirm or rule out aortic dissection and aneurysm. Radiologists were blinded to the patients’ identities, clinical histories, and MR sequence parameters. For this type of identity-free retrospective review of standard clinical studies, our institutional review board does not require its approval or patient informed consent.
Examinations were viewed with PACS workstations (Pathspeed 8.0; GE Medical Systems, Milwaukee, Wis) with capability to display cine movie loops. Viewing parameters (ie, brightness, contrast, magnification, movie loop speed and direction) were adjusted at the radiologists’ discretion. Each radiologist made a diagnosis based solely on the true FISP portion of the MR examinations and rated the confidence of diagnosis on a five-point scale: 5, extremely confident; 4, confident; 3, marginally confident; 2, suspected but uncertain; 1, nondiagnostic study, no confidence. In addition to the diagnosis of aortic dissection or aneurysm, radiologists were also asked to note any other pertinent aortic abnormalities and rate their confidence of these findings on the basis of this scale.
Proof of Diagnosis
In all abnormal cases, proof of diagnosis was based on the MR angiographic findings. There was further corroboration with any other imaging studies that were available and with surgical findings. In the normal cases, MR angiographic findings alone served as proof.
Statistical Analysis
The conclusions derived with the true FISP images were compared with the final conclusions based on the proof of diagnosis. Sensitivity (number of true-positive findings divided by the number of actually positive cases) and specificity (number of true-negative findings divided by the number of actually negative cases) were calculated. The decision confidence ratings were analyzed by using multireader-multicase receiver operating characteristic techniques (5). A 
analysis was performed to determine rating agreement between the two observers.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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value was 1.0.
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All seven aneurysms were proved with high-spatial-resolution MR angiography. Five had CT angiographic findings to support the MR findings. Four underwent surgical repair, which confirmed imaging findings. Two had supporting correlation with echocardiographic US. The total does not equal seven because three patients had more than one additional study and two patients underwent surgical repair on the basis of the MR findings alone. Three aortic aneurysms involved aortic root dilatation and were confined to the ascending aorta only. Three aneurysms involved only the descending aorta, without ascending aortic involvement. The six fusiform aneurysms ranged in diameter from 4.6 to 7.5 cm. The one saccular aneurysm was a 2.7-cm eccentric lateral outpouching from the normal-caliber 3-cm aortic arch, 2 cm distal to the origin of the left subclavian artery.
The two cases of severe atherosclerotic disease with ulcerations were proved with high-spatial-resolution MR angiography and CT angiography.
The one case of intramural hematoma was known because of its iatrogenic occurrence during catheter injection at cardiac angiography. This hematoma was followed up to resolution over a 6-week period by means of serial 2-week MR imaging examinations and transesophageal US examination at 6 weeks.
All nine normal cases were proved with high-spatial-resolution MR angiography. Two patients with history of allergy to iodinated contrast material also underwent nonenhanced chest CT examinations, which were normal.
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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Many institutions capable of rapid MR examinations are using MR imaging as the modality of choice for thoracic aortic evaluation. One of the larger early aortic MR studies included 110 patients and found spin-echo imaging was more accurate than transthoracic or transesophageal echocardiography and even contrast-enhanced CT for diagnosis of aortic dissection (6). MR imaging, US, and CT technologies have all advanced substantially since this early study. However, with the exception of contrast-enhanced 3D MR angiography, relatively little is reported on the accuracy of newer fast MR imaging techniques for evaluating the aorta. Urata et al (7) recently reported a promising nonenhanced fresh-blood imaging technique that relies on 3D half-Fourier fast spin-echo imaging for detection and characterization of aortic disease. Many institutions still use the relatively slow spin-echo techniques in combination with gradient-echo techniques and contrast-enhanced 3D gradient-echo MR angiography. Although some progressive groups may use cine and single-shot techniques as routine sequences in aortic MR imaging, many radiologists are not using these newer techniques for aortic evaluation. When single-shot and cine techniques are in use, they are typically performed in concert with other sequences, including contrast-enhanced 3D MR angiography.
We routinely use nonenhanced single-shot true FISP in transverse and coronal planes as an overview of the chest and aorta. These series of images each take approximately 30–40 seconds to acquire. The transverse coverage includes the entire thoracic aorta to the level of the diaphragm. The coronal single-shot images and the sagittal oblique cine sequence cover the entire thoracic aorta and often include a portion of the upper abdominal aorta to the level of the renal arteries. Evaluation of the abdominal portion of the aorta was not included in this study. A more focused initial evaluation of the chest for aortic dissection is then performed rapidly with several segmented cine true FISP movie acquisitions (approximately 6 seconds per cine sequence). We used cine true FISP imaging rather than cine fast low-angle shot, or FLASH, imaging because of the inherently higher signal-to-noise ratio and faster acquisition speed of true FISP (3). These cine sequences are performed transversely at the aortic root, middle ascending aorta, and aortic arch. The fourth sequence is a sagittal oblique view. These cine sequences effectively screen the thoracic aorta for dissection and help clarify any artifacts that may be seen on single-shot images, as true FISP is susceptible to magnetic field inhomogeneities and pulsatile flow. For example, in patients with surgical clips or sternotomy wires, local field inhomogeneities will occur that can create local susceptibility artifact. In addition, with true FISP, there is a predictable phase cancellation dark stripe artifact at fat and water interfaces that is analogous to that seen in opposed-phase gradient-echo imaging for characterization of adrenal adenomas. Despite predictable susceptibility and phase cancellation artifacts seen with true FISP imaging, these seldom represent a relevant limitation in clinical practice. Even in patients with history of prior aortic graft repair or prosthetic valves, true FISP yields clinically diagnostic images in most cases (4).
A limitation of these true FISP sequences, including cine imaging, is that they provide limited dynamic flow information, such as demonstrating differential flow in true and false lumina, which time-resolved techniques can reveal (8,9). Despite limited flow information, true FISP sequences are very useful for rapidly determining presence or absence of disease and consequently deciding if additional imaging of the aorta is necessary. For example, on detection of a dissection, aortic MR angiography can be used to provide additional anatomic and dynamic information related to amount of flow in true and false lumina and branch vessels. If aneurysm is encountered, one could choose to obtain high-spatial-resolution 3D MR angiographic images to assess aneurysm diameter and map origins of aortic branch arteries. In these scenarios, nonenhanced imaging is effectively performed in less than 4 minutes. Supplemental contrast-enhanced MR angiography can be performed if clinically desired with the caveat that this incurs additional imaging time.
Shortcomings of this study are that it is retrospective and that the study group is not large. The study included only one case of intramural hematoma, which is not a large enough sample to make meaningful inferences regarding true FISP imaging characteristics of this entity. Furthermore, our case of intramural hematoma was atypical in that it was iatrogenic due to intramural catheter injection of contrast material. Some would argue that this should be called a dissection based on its mechanism; however, no flap was visible at transesophageal US, CT, or MR imaging. The only demonstrable abnormality was transient (approximately 10 days) focal asymmetric aortic wall thickening. Typical intramural hematomas are thought to occur as focal perforations through the intima into the media that wall off or as spontaneous hemorrhage into the media. They are usually detectable with nonenhanced MR imaging as increased signal intensity on T1-weighted images owing to the presence of extracellular methemoglobin. Our study group also lacked cases of actual aortic rupture, but given the image quality with these sequences we believe that such abnormalities would also be easily detected.
In conclusion, findings in this preliminary study demonstrate that nonenhanced true FISP sequences alone are promising for the initial, rapid, accurate diagnosis of aortic dissection and aneurysm in less than 4 minutes.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, F.S.P., J.P.F.; study concepts and design, all authors; literature research, F.S.P., V.B., J.C.C., V.K., E.A.K., J.P.F.; clinical studies, F.S.P., V.B., R.M.M., J.C.C., V.K., J.P.F.; data acquisition, F.S.P., V.B., R.M.M., J.C.C., V.K., J.P.F.; data analysis/interpretation, all authors; statistical analysis, E.A.K., V.B., F.S.P.; manuscript preparation and definition of intellectual content, all authors; manuscript editing, F.S.P., V.B., E.A.K., J.P.F.; manuscript revision/review and final version approval, all authors.
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