|Figure 1. a) Volume-rendered image of a saccular aneurysm of the descending thoracic aorta (arrow). b) Plane of section through the aneurysm demonstrates mural thrombus within the aneurysm (arrowheads).|
There has been a substantial increase in the use of computed tomographic angiography (CTA) for the evaluation of the arterial system in recent years. This increase has been fueled by hardware advances, including spiral and multidetector CT (MDCT), which can be used for rapid imaging of large scan volumes using thin section collimation. The advent of MDCT allows for the reliable acquisition of volumetric datasets of unprecedented resolution. These advances have led to a rise in clinical indications for CTA. The purpose of this article is to discuss some of these indications for CTA of the systemic arterial system, as well as the implications of CTA for planning and follow-up evaluation of vascular interventional procedures.
|Figure 2. a) Volume-rendered image of a thoracic aortic dissection with entry tear along the inferior aspect of the aortic arch (arrow). Diminished flow is demonstrated within the false lumen (arrowheads). b) The dissection extends into the abdominal aorta where the true lumen supplies the right renal artery and the left renal artery has been stented (arrowhead). An infrarenal abdominal aortic stent-graft is also pictured (arrow).|
One of the most important applications of CTA is in the imaging of the thoracic aorta in the setting of trauma. Although aortography is considered the gold standard for aortic imaging, the volumetric acquisition of spiral CT offers several advantages, particularly for evaluation of overlapping structures in the chest. 1 Studies as early as 1995 demonstrate that the sensitivity of CTA exceeds that of conventional angiography for the detection of traumatic aortic injury. 2 Since that time, the indications for CTA of the thoracic aorta have expanded to include imaging of thoracic aortic aneurysm, thoracic aortic dissection, and intramural hematoma.
CTA evaluation of thoracic aortic aneurysm can be used to accurately define the size and extent of the aneurysm as well as the involvement of aortic branch vessels (Figure 1). 1 The use of curved planar reformations and volume-rendered images can facilitate this evaluation of branch vessel involvement. Double-oblique cross sections of the aortic lumen provide the most accurate and reliable measurement of aneurysm diameter, and three-dimensional reconstruction techniques for the determination of aneurysm volume promise to provide even more complete means of following aneurysm expansion over time. Additionally, 3D reformations can be used for the accurate determination of path lengths and to facilitate measurements of neck and landing zone dimensions for the purposes of device sizing. These techniques are helpful as an aid to surgical or endovascular repair planning and can be used in follow-up to determine the successfulness of an intervention.
Imaging of thoracic aortic dissection is another common indication for CTA of the chest. The near-universal availability of CT as well as the speed at which an examination can be performed on a hemodynamically unstable patient make CTA the first line diagnostic test at many institutions. Proper MDCT acquisition technique including adequate contrast enhancement of the aortic lumen, thin-section collimation with overlapping reconstruction interval, and appropriate use of three-dimensional rendering techniques facilitates evaluation of the dissection flap in relation to aortic branch vessels and the aortic valve. Retrospective cardiac gating can also be helpful for evaluation of aortic dissection, especially in distinguishing pulsation artifact in the ascending aorta from an intimal flap. The recent advances in MDCT have also led to an increased ability to locate the site of entry tear in thoracic aortic dissections, which can facilitate surgical and/or endovascular repair (Figure 2, page 27).
Intramural hematoma is a clinical entity distinct from aortic dissection. The imaging characteristics of intramural hematoma include a high-attenuation crescent within the wall of the aorta on non-contrast CT images with no perceptible intimal flap and no visualized blood flow demonstrated within the false lumen on contrast-enhanced CTA images. In the absence of associated luminal irregularity caused by aortic ulceration or dissection, intramural hematoma can be impossible to visualize by conventional angiography. MDCT is an established technique for the diagnosis of intramural hematoma, and recent articles have even identified imaging features that may predict progression to overt aortic dissection. 3
Until recently, the small diameter of coronary arteries and the inherent obstacle of cardiac motion and resulting artifacts have made reliable CT imaging of the coronary arteries problematic. The advent of MDCT has increased temporal and spatial resolution, allowing for a partial solution to these problems. The use of retrospective cardiac gating to reconstruct multiple sets of imaging data in different phases of the cardiac cycle provides the best opportunity for visualizing all segments of the coronary arteries. Increasing the diastolic proportion of the cardiac cycle using b-blockers to lower the heart rate below 75 beats per minute can also be helpful to minimize misregistration artifacts due to cardiac motion. Decreasing scan time and optimizing spatial resolution by using 16-detector-row MDCT and optimizing temporal resolution by using partial scan reconstruction techniques can further increase the likelihood of successful imaging of the coronary arteries. Although challenges still remain, particularly regarding the imaging of patients with high heart rates and cardiac arrhythmias, current techniques allow for imaging of calcified and soft atherosclerotic plaques in the majority of coronary arteries and for the accurate quantification of coronary artery stenosis in the majority of patients 4 (Figure 3, page 31).
|Figure 3. a) Volume-rendered and b) curved planar reformatted images of mixed calcified and soft plaque in the proximal (arrowhead) and mid (arrow) right coronary artery.|
The utility of CTA in the evaluation of abdominal aortic pathology is well established (Figure 4, page 31). Although routine postcontrast CT is usually adequate to establish the diagnosis of abdominal aortic aneurysm, CTA provides additional information regarding the size and extent of an aneurysm, as well as branch vessel involvement, which may be important for surgical planning. Historically, the preoperative work-up of aortic aneurysm repair has included conventional angiography; however, studies have shown that the evaluation of abdominal aortic aneurysm using CTA is less invasive and more accurate for the evaluation of aneurysm size than conventional angiography. 5 CTA also has the advantage of being able to image the outer wall of the aorta, rather than just the flow lumen. This allows for the visualization of mural thrombus and atheroma within the aneurysm and may have implications for the determination of suprarenal extension, an important part of treatment planning for aneurysm repair. 6
Another important application of CTA is in the planning and follow-up evaluation of endovascular stent-graft repair of abdominal aortic aneurysms. Accurate measurement of the arteries involved is an important factor in minimizing complications of endovascular aneurysm repair such as endoleak and branch occlusion. 7 Spiral CTA using thin-section collimation with overlapping reconstruction provides excellent volumetric spatial resolution. As with the thoracic aorta, double-oblique cross sections of the vessel lumen provide the most accurate measurement of aneurysm diameter and facilitate planning of endovascular stent-graft placement. CTA is also useful for ensuring successful stent-graft deployment. Biphasic MDCTA with delayed imaging through the aneurysm is currently the standard of care for postprocedure evaluation. This technique provides optimum visualization of stent-graft position and results in increased sensitivity for the detection of endoleaks. 8
Mesenteric and renal arteries
|Figure 4. a) Volume-rendered image of an abdominal aortic aneurysm (arrow), which extends down into the right common iliac artery (arrowhead). b) Lateral thin-section multi-planar reformatted image demonstrates posterior extravasation of contrast media (arrow) indicating active aneurysm leak.|
CTA has become an established technique for the noninvasive evaluation of renal and mesenteric arterial disease. Two common indications for renal artery CTA are in the work-up of renal artery stenosis and for preoperative evaluation of potential living renal donors. 9 Detection of renal artery stenosis due to atherosclerosis or fibromuscular dysplasia is well described, and 3D reformation of the renal arteries is an accepted approach to therapy planning (Figure 5). Volume-rendered or MIP images can accurately depict accessory renal arteries and allow measurement of stenoses even in relatively small branch vessels. Evaluation of these types of projections and the measurements they allow can provide guidance for percutaneous therapy, including catheter selection and planning for angioplasty and stenting. Renal artery CTA can also be helpful in the evaluation of other vascular disorders, such as renal artery aneurysm, dissection, thrombo-embolic disease, or vasculitis. 10
|Figure 5. a) Volume-rendered and b) thin-slab maximum intensity projection images demonstrate proximal stenosis of the main left renal artery (arrow) with a small accessory renal artery to the upper pole of the left kidney (arrowhead).|
Another important use of renal artery CTA is in the preoperative evaluation of potential living renal donors. CT has been shown to be able to depict important vascular anatomic variants, such as accessory renal arteries, prehilar renal artery branching, and retro-aortic and circum-aortic renal veins. 9 Additionally, CT can give information regarding renal parenchymal abnormalities, such as occult lesions or masses, collecting system obstruction or duplication, or evidence of prior renal scarring or reflux. Noncontrast CT imaging can also be helpful to evaluate for the presence of renal calculi, prior to organ harvest. These evaluations can be useful for planning of conventional or laparoscopic organ removal.
MDCT angiography has become an accepted tool for the evaluation of large and small branches of the mesenteric arteries. Applications include the evaluation of acute and chronic mesenteric ischemia as well as pre- and post-liver transplant evaluation. Recent studies have demonstrated the utility of CTA in the evaluation of acute mesenteric ischemia, detecting primary arterial disease and/or the resulting intestinal changes with high sensitivity and specificity. 11 CTA of the mesenteric arteries can be performed alone, but it is often combined with solid organ imaging of the liver or pancreas, and, in the clinical setting of acute abdominal pain, could suggest alternative diagnoses that may mimic mesenteric ischemia.
Mesenteric arterial MDCT has also become a valuable tool in the evaluation of liver transplantation, especially when other imaging modalities such as ultrasound and magnetic resonance imaging are suboptimal or contraindicated. Just as with renal transplant donor evaluation, the potential for preoperative detection of occult lesions as well as the postoperative assessment of vessel patency makes CT an important aid to clinical management.
|Figure 6. a) Volume-rendered image of a lower extremity CTA runoff study demonstrating focal traumatic occlusion of the dorsalis pedis artery (arrow). b) Postoperative follow-up CTA demonstrates a patent distal anterior tibial artery bypass graft to the dorsalis pedis artery (arrow) with prominent venous shunting (arrowheads).|
As the number of detector rows used for MDCT has increased from four-row to eight- and 16-row scanners, faster scanning has allowed for significant improvements in longitudinal anatomic coverage. Advances in scan efficiency have also improved fine spatial resolution, allowing for visualization of smaller and smaller branch vessels. This increase in image detail has allowed for new applications of CTA that were previously unavailable. In no region of the body have these advances been more apparent than in the extremities, where CTA evaluation of arterial runoff has become an accepted alternative to conventional angiography. Several recent papers have validated the use of MDCT in the setting of peripheral vascular disease, demonstrating reliability in the detection of atherosclerotic occlusive disease similar to that of conventional DSA. 12, 13 CTA also can be used to characterize inflow and runoff vessels as a guide to therapy planning and has been shown to be useful in follow-up evaluation of peripheral vascular intervention. 14
Future applications of MDCT angiography of the extremities include evaluation of the arteries of the upper extremity. Unpublished data from our institution indicate that CTA of the upper extremity is suitable for the evaluation of arterial stenoses as well as for the evaluation of AV dialysis fistula patency. Additionally, MDCT can be used in the setting of blunt and penetrating extremity trauma to assess for vascular injury. Prior published reports have described the use of spiral single-detector CTA to evaluate for proximal arterial injury in the setting of lower extremity trauma. 15,16 Unpublished data from our institution show that MDCT can be used to image the vessels of the distal extremities as well to evaluate for traumatic injury (Figure 6).
CTA provides a noninvasive means to evaluate the arterial system throughout the body and has become an increasingly important diagnostic tool. As CT technology continues to evolve, the applications for CTA will continue to expand. An increasing number of detector rows, faster gantry rotation speeds, and improved methods of postprocessing promise to improve temporal and volumetric spatial resolution even further. Future applications including functional studies of the vascular system, quantification of myocardial perfusion abnormalities, and postdeployment evaluation of new endovascular therapy devices promise to continue to increase the utilization of this important imaging modality.
Eric E. Williamson, MD, is a fellow, Department of Radiology, Stanford University School of Medicine, Stanford, Calif.
Geoffrey D. Rubin, MD, is chief of cardiovascular imaging and associate professor of radiology, Department of Radiology, Stanford University School of Medicine, Stanford, Calif.
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- Gavant ML, Menke PG, Fabian T, Flick PA, Graney MJ, Gold RE. Blunt traumatic aortic rupture: detection with helical CT of the chest. Radiology. 1995;197:125-33.
- Choi SH, Choi SJ, Kim JH, et al. Useful CT findings for predicting the progression of aortic intramural hematoma to overt aortic dissection. J Comput Assist Tomogr. 2001;25:295-9.
- Pannu HK, Flohr TG, Corl FM, Fishman EK. Current concepts in multi-detector row CT evaluation of the coronary arteries: principles, techniques, and anatomy. Radiographics. 2003;23(Spec No.): S111-25.
- Pavone P, Di Cesare E, Di Renzi P, et al. Abdominal aortic aneurysm evaluation: comparison of US, CT, MRI, and angiography. Magn Reson Imaging. 1990;8:199-204.
- Rubin GD. MDCT imaging of the aorta and peripheral vessels. Eur J Radiol. 2003; 45(Suppl 1): S42-9.
- Tillich M, Hill BB, Paik DS, et al. Prediction of aortoiliac stent-graft length: comparison of measurement methods. Radiology. 2001;220:475-83.
- Golzarian J, Dussaussois L, Abada H, et al. Helical CT of aorta after endoluminal stent-graft therapy: value of biphasic acquisition. AJR Am J Roentgenol. 1998;171:329-31.
- Chow LC, Rubin GD. CT angiography of the arterial system. Radiol Clin North Am. 2002;40:729-49.
- Fleischmann D. Multiple detector-row CT angiography of the renal and mesenteric vessels. Eur J Radiol. 2003;45(Suppl 1): S79-87.
- Kirkpatrick ID, Kroeker MA, Greenberg HM. Biphasic CT with mesenteric CT angiography in the evaluation of acute mesenteric ischemia: initial experience. Radiology. 2003;229:91-8.
- Ofer A, Nitecki SS, Linn S, et al. Multidetector CT angiography of peripheral vascular disease: a prospective comparison with intraarterial digital subtraction angiography. AJR Am J Roentgenol. 2003;180:719-24.
- Rubin GD, Schmidt AJ, Logan LJ, Sofilos MC. Multi-detector row CT angiography of lower extremity arterial inflow and runoff: initial experience. Radiology. 2001;221:146-58.
- Kramer SC, Gorich J, Aschoff AJ, et al. Diagnostic value of spiral-CT angiography in comparison with digital subtraction angiography before and after peripheral vascular intervention. Angiology. 1998; 49:599-606.
- Soto JA, Munera F, Cardoso N, Guarin O, Medina S. Diagnostic performance of helical CT angiography in trauma to large arteries of the extremities. J Comput Assist Tomogr. 1999;23:188-96.
- Soto JA, Munera F, Morales C, et al. Focal arterial injuries of the proximal extremities: helical CT arteriography as the initial method of diagnosis. Radiology. 2001;218:188-94.