Dr. Moore is Chief Resident in Diagnostic Radiology in the Department of Radiology and Radiological Sciences; and Dr. Fishman is Professor of Radiology and Oncology and Director of Diagnostic Radiology and Body CT, Johns Hopkins Hospital, Baltimore, MD.

The development of the 4-slice multidetector computed tomography (MDCT) scanner, combined with state-of-the-art three-dimensional (3D) reconstruction software, has optimized the resolution of today's computed tomographic angiography (CTA) to the extent that clinicians are increasingly relying on this noninvasive means of evaluating vascular disease rather than conventional arterial angiography. CT angiography truly applies to whole-body vascular imaging, ranging from the detection of cerebral aneurysms and coronary artery disease to the evaluation of peripheral vascular disease in the lower extremities. For the evaluation of abdominal pathology, CTA has become a routine diagnostic tool as well as an integral step in preoperative planning.

Technical advances

The increased scanning speeds provided by MDCT have made detailed imaging of the vasculature possible. With the old single-detector CT scanners with rotation speeds of 1 second, only 30 rotations could be made in a 30-second breath hold. Therefore, scanning parameters were chosen to either cover a small region with high resolution or a larger volume using thick slices. ¹ Today's MDCT scanners have four rows of detectors simultaneously acquiring data combined with sub-
second rotation (0.5 to 0.8 sec) of the gantry, increasing scanning speed by a factor of eight. ¹ As a result, a greater portion of the z-axis can be covered (150 cm) in a shorter amount of time, and ultra-thin slices can be obtained (1 mm). ¹ These thin sections are then reconstructed at small intervals (1 mm), producing high-resolution 3D images. Since the image is acquired as a volume, it follows that a very efficient method for viewing such large amounts of data is via a 3D display. This is accomplished using a computer workstation and graphics software. The radiologist processes the image by selecting the optimal 3D rendering technique, windowing parameters, and viewing cut-plane. By rotating the data set in any plane on the workstation, the entire length of the vessel may be displayed and evaluated on one image (Figure 1).

3D rendering techniques

The three principle techniques for 3D processing of CT datasets include surface rendering, maximum intensity projection (MIP), and volume rendering. Surface rendering was one of the first methods developed, and it relies on the comparison of a voxel intensity to some defined threshold value, which the computer uses to define an edge contour. ² The MIP algorithm selects out the maximum value in a voxel along a line from the viewer's eye through the image and displays only that value in the corresponding pixel. ² The limitations of both MIP and surface rendering lie in the forfeiting of most of the available data in order to increase image-processing speed. ² Volume rendering, on the other hand, sums the contributions of each voxel along a line from the viewer's eye through the dataset, such that all of the information is included in the final image. ² Therefore, volume rendering is more accurate and allows the visualization of multiple tissue types simultaneously. For these reasons, volume rendering has become the preferred method for 3D postprocessing. With MDCT, MIP can play a complementary role and is used in conjunction with volume rendering in our practice.

CTA of the kidney

CT angiography has come to the forefront in the preoperative evaluation of candidates for renal transplants and partial nephrectomies. It is also a reliable noninvasive exam for the detection of renal artery stenosis. A triphasic CT vascular map is now indicated for noninvasive evaluation of both the donor organ and selection of the organ to be transplanted, identifying any aberrant vessels or anatomic variations ³ (Figure 2). In addition to evaluating the vasculature, CTA has the advantage of simultaneously excluding the presence of renal cell carcinoma or renal artery stenosis in renal donors, both of which may render the patient inoperable. CT angiography has also played a key role in the presurgical evaluation of patients for laparoscopic partial nephrectomies and in the postoperative evaluation of complications including urinoma, pseudoaneurysm, and perinephric hemorrhage. ⁴ Halpern et al ⁵ prospectively studied 56 patients who underwent both CTA and Doppler ultrasound of the renal arteries for the evaluation of renal artery stenosis, and compared the results with conventional angiography. The results indicated that CTA was more accurate than Doppler ultrasound for the detection of renal artery stenosis. Kaatee et al ⁶ prospectively studied 71 patients who had been examined with CTA and digital subtraction angiography (DSA) for renal artery renal artery stenosis. They found that the sensitivity and specificity of CTA for renal artery stenosis >50% was 96%, which is similar to that of DSA.

CTA of the pancreas

The accurate identification of vascular invasion is crucial in the staging and evaluation for resectability in patients with pancreatic cancer. In evaluating patients who are potential candidates for a Whipple procedure, the superior mesenteric artery (SMA) and the superior mesenteric vein (SMV) must be clearly defined in relation to the pancreatic mass, and a decision must be made as to whether the mass invades or just abuts the vasculature (Figure 3). It has been shown in the literature that while evaluation of vascular invasion with axial images alone has a poor correlation with surgical findings, 3D CTA detection has a near one-to-one correlation. ⁷ This is in part due to the multiplanar capabilities of MDCT scanning, but is also a result of enhanced resolution provided by thinner slices.

CTA of the mesentery

Timed administration of the intravenous contrast bolus for both arterial and venous phases provides a dynamic evaluation of the mesenteric vasculature that may aid in the diagnosis of bowel ischemia and inflammatory bowel disease. The CT evaluation of patients with suspected bowel ischemia has previously relied on the detection of pneumatosis, pneumoperitoneum, and bowel wall thickening. However, bowel wall edema is a relatively nonspecific inflammatory finding and benign causes of pneumatosis have been reported due to steroid use, pulmonary disease, and collagen vascular disease. ⁸ Therefore, a more specific dynamic evaluation of the mesenteric vasculature is desired for the detection of low-flow states or embolic vessel occlusion, which may precipitate ischemia to the bowel. CT angiography of affected bowel loops may demonstrate absence of enhancement, delay in enhancement, or persistent enhancement when compared with unaffected loops ⁸ (Figure 4). Visualization of the entire data set in one image improves the detection of collateral vessels between the celiac, SMA, and inferior mesenteric artery (IMA), which may suggest chronic ischemia in selected segments of bowel. ⁸ Improved resolution with MDCT allows the detailed visualization of small branching vessels that may contain tiny emboli. ⁸ In addition, 3D postprocessing using cut-planes removes superimposed vessels, isolating the vessel of interest. ⁸ Hypervascularity and engorgement of the bowel wall has also been observed in patients presenting with an acute flare of inflammatory bowel disease ⁸ (Figure 5).

CTA of the abdominal aorta

CT angiography plays an important role in the preoperative and postoperative evaluation of abdominal aortic graft placement, aneurysms, and dissections. The multiplanar capabilities of 3D CTA are extremely useful in obtaining accurate measurements of the extent and maximum diameter
of tortuous aortic aneurysms with extensive calcified mural plaque. ⁹ Postoperatively, CTA is a low-cost and noninvasive means of surveillance of aortic stents (Figure 6). Complications after graft placement including supragraft aneurysms, distal anastomotic aneurysms, graft infections, perigraft fluid collections, and graft thrombus are also optimally detected. ¹⁰ Imaging the entire aorta in one plane is ideal for the evaluation of aortic dissections. CT angiography is used to depict the relative patency of the true and false lumen and the origin of the renal arteries can be clearly defined (Figure 7).

CTA of the liver

CT angiography evaluation of the liver is very useful for preoperative vascular mapping in the evaluation of candidates for liver transplant as well as detecting vascular complications of cirrhosis and early identification of hepatocellular carcinoma. The "typical" hepatic arterial anatomy occurs in only 55% of the population, and at least 10 variations exist. ¹¹ In liver transplant patients, CTA identifies aberrant anatomy pre-operatively, and simultaneously evaluates liver volume ¹² (Figure 8). Another benefit of CTA is the full visualization of the extent of varices and collateral vascularization in cirrhotic patients on a single image (Figures 9 and 10). CT angiography is also particularly useful for the detection of small hepatomas in patients with hepatitis or cirrhosis. When contrast was administered directly via canalization of the hepatic artery, Kanematsu et al ¹³ found that the conspicuity of tumors and tumor-feeding arteries was significantly better on MIP CTA than on conventional angiograms.

Intra-arterial chemotherapy or che-moembolization for recurrent hepatocellular carcinoma requires superselective catheterization of small branches of the hepatic artery under fluoroscopic guidance. Often, the identification of the main arterial feeding vessels supplying the tumor mass is made in real time during the interventional angiogram by analysis of the vascular blush. However, Sze et al ¹⁴ found that 3D CTA with selective hepatic arterial contrast administration identified additional hepatic tumors undetected by conventional angiography in 30% of patients. Also, the vascular map provided by 3D CTA altered the decision as to which arterial branches were optimal for catheterization. Thus, it was concluded that the use of 3D CTA prior to selective chemoembolization frequently alters treatment plans. ¹⁴

Conclusion

The increase in imaging speed and resolution provided by MDCT, combined with improved 3D processing software, has brought CTA to the forefront as an important diagnostic tool for the evaluation of abdominal pathology. Three-dimensional CTA ideally depicts the vasculature because the entire length of the vessel may be visualized on a single image and vascular patency is accurately defined even in small branching vessels. CT angiography is increasingly being implemented in the pre-operative evaluation of patients undergoing hepatic chemoembolization procedures and transplants of both the liver and kidney. It is also an excellent noninvasive means of surveillance in patients with cirrhosis for the evaluation of hepatoma and for stent placement in patients with aortic grafts. The newest MDCT scanners will be able to acquire 16 slices simultaneously, further increasing scan speed and the resolution capabilities for optimal 3D imaging of the abdominal viscera and its vascular supply.