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1 Department of Diagnostic Radiology, Rambam Medical Center and Faculty of
Medicine, Technion-Israel Institute of Technology, Haifa 31096, Israel.
2 Department of Vascular Surgery, Rambam Medical Center and Faculty of Medicine,
Technion-Israel Institute of Technology, Haifa 31096, Israel.
3 Unit of Clinical Epidemiology, Rambam Medical Center and Faculty of Medicine,
Technion-Israel Institute of Technology, Haifa 31096, Israel.
Received June 6, 2002;
accepted after revision August 22, 2002.
Presented at the annual meeting of the Radiological Society of North
America, Chicago, November 2000.
Abstract
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SUBJECTS AND METHODS. Eighteen patients with peripheral vascular disease who were referred for elective digital subtraction angiography (DSA) also underwent CT angiography. We scanned patients from the level of the superior mesenteric artery to the pedal arteries in a single helical scan. CT angiograms were produced using maximum-intensity-projection reconstructions. Findings were graded according to six categories: 1, normal (0% stenosis); 2, mild (1-49% stenosis); 3, moderate (50-74% stenosis); 4, severe (75-99% stenosis); 5, occluded; and 6, nondiagnostic. CT angiography findings were compared with DSA findings for each arterial segment.
RESULTS. We found agreement for the degree of stenosis in 77.7% of the arteries and discrepancy for 22.3% of the arteries when all categories were considered. Grouping the six categories according to the threshold for treatment (categories 1 and 2 as one group and categories 3, 4, and 5 as the second group) resulted in an agreement of 91.95%. Compared with DSA, CT angiography yielded a sensitivity of 90.9% and a specificity of 92.4%.
CONCLUSION. Multidetector CT angiography is an accurate, noninvasive technique for the imaging of peripheral vascular disease.
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Conventional angiography is the gold standard for imaging of lower extremity occlusive disease. However, this method is invasive and expensive and has a definite, although low, morbidity [14, 15, 16]. One of the main limitations of CT angiography is the inability to cover the whole length of the vascular tree from the renal arteries to the toes [1, 3, 4, 5, 16]. The recent advance in helical CT technology—multidetector CT—enables scanning of long segments of the vascular tree [2, 12, 13], with a single contrast injection and thin sections, usually from 3 to 4 mm. Thus, multidetector CT has the potential to reveal peripheral vascular disease.
The purpose of this study was to evaluate the accuracy of CT angiography using a multidetector scanner for peripheral artery disease detection and grading relative to digital subtraction angiography (DSA). If CT angiography could provide all the information needed for planning revascularization procedures, then DSA could be replaced by this noninvasive method in the assessment of patients with peripheral vascular disease.
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The study included 15 men and three women with a mean age of 64.4 years (range, 50-79 years). According to the Society of Cardiovascular & Interventional Radiology categorization for chronic extremity ischemia [17], five patients were in category 2; nine, in category 3; three, in category 4 (rest pain); and one, in category 5 (tissue loss). Seven patients had previously undergone revascularization procedures.
Angiography was performed using the digital subtraction technique (MultiStar; Siemens Medical Systems, Forcheim, Germany). A 5-French pigtail or straight catheter was placed in the suprarenal aorta via the common femoral artery revealing the abdominal aorta and renal arteries. After the catheter was pulled down to the aortic bifurcation, the pelvis, thighs, knees, calves, and feet were studied by a bolus-chase technique.
CT angiography was performed with an MX8000 multidetector scanner (Philips, Cleveland, OH), which allows continuous helical acquisition of the entire vascular tree from the superior mesenteric artery to the pedal arteries in about 50 sec using a slice thickness of 3.2 mm. A low-dose automatic timing bolus protocol (Bolus ProUltra; Philips) was used to optimize the delay time from the start of injection to the beginning of the scan in each patient.
One hundred twenty to 130 mL of nonionic, lowosmolar contrast medium ([iomeprol] Iomeron 300; Bracco, Milan, Italy) was injected into a peripheral vein, usually at the antecubital fossa, with a power injector at a rate of 3.5 mL/sec. Patients were instructed to breathe quietly throughout the acquisition. The 3.2-mm-thick slices were reconstructed every 1.6 mm to achieve a 50% overlap and to maximize the longitudinal resolution. Images were sent to an online workstation (MxView; Philips), and CT angiograms were produced using maximum-intensity-projection (MIP) algorithm in the frontal, sagittal, and both oblique views.
We produced MIP images by removing the bony structures through a process called segmentation. After we completed this process, we proceeded with a similar process to electronically erase the arterial wall calcifications, which took another 20 min and was subject to operator judgments but enabled the production of images similar to DSA images. Because of the large number of axial slices, reconstruction was done in two parts: one from the superior mesenteric artery to the mid thighs and the second, lower part from the mid thighs to the toes. Suspected or abnormal findings were enlarged, viewed on the relevant axial slices using the related slice technique and reconstructed using MasterCut formation (Philips). This formation is a complex curved oblique reconstruction showing a plane that is perpendicular to a line drawn in the center of the artery on the MIP image. Segmentation of the tibialis anterior artery, on its distal course on the lateral surface of the tibia, was impossible with our current automatic and semiautomatic segmentation software (Fig. 1A, 1B, 1C, 1D). Segmentation could only be accomplished by manually drawing both arteries over at least 80 axial slices. This proved to be time-consuming and of questionable diagnostic value. We ended evaluating this particular segment through the axial slices alone. Other difficult areas for segmentation were the superior gluteal artery as it passes under the sciatic foramen and the branches of the profunda femoris as they pass around the medial surface of the femur. Reconstructions, including hard-copy production, took about 50 min.
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Two interventional radiologists, unaware of the DSA findings, reviewed the CT angiography studies jointly, usually viewing the MIP images but also the axial images and the MasterCut formation, as needed. Window leveling was used freely in each suspected lesion to help delineate the contrast-filled lumen from calcifications or stents. Findings were graded according to five categories: normal (0% reduction in diameter), mild (1-49% reduction in diameter), moderate (50-74% reduction in diameter), severe (75-99% reduction in diameter), and complete occlusion. These categories corresponded to those used in other imaging studies on peripheral occlusive vascular disease [1, 4, 5, 18]. In some cases, dense calcifications and small-diameter arteries prevented the determination of patency from MIP and axial CT scans. These examinations were classified as nondiagnostic.
The aorta, renal arteries, and the common iliac, external iliac, internal iliac, common femoral, superficial and deep femoral, popliteal, anterior tibial, posterior tibial, peroneal, and dorsalis pedis arteries were graded separately, and findings for each artery were tabulated separately. The statistical significance of the difference in sensitivity between the two imaging techniques was determined with the Wilcoxon-Mann-Whitney test. The accuracy of CT angiography was evaluated, with DSA as the gold standard.
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Our study of CT angiography and DSA compared 444 arteries according to categories shown in Table 1. Two hundred seventy-two arteries were classified as normal in the DSA study. Although 233 of them were also classified as normal in the CT angiography study, 17 were classified as mild; nine, as moderate; three, as severe; and three, as complete occlusion. The clinically important mismatch between DSA and CT angiography involved those arteries classified as complete occlusion or severe or moderate stenosis on CT angiography while classified as normal or mild on DSA. This discrepancy occurred in 22 arteries. Of these mismatches, five were in the renal arteries and nine in the infrapopliteal arteries. Sixteen arteries were classified as nondiagnostic on CT angiography, six of which were classified as nondiagnostic on DSA. Thirteen of these arteries were the dorsalis pedis arteries. Twenty-four arteries were classified as nondiagnostic in the DSA study, 11 of which were the dorsalis pedis arteries. There was complete agreement in 85.7% in the normal artery category and 87.0% in the complete occlusion category.
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The study revealed an agreement between the degree of stenosis in 77.7% and disagreement in 22.3% of the arteries when all the categories were considered (Table 2). Excluding the nondiagnostic category and grouping the remaining five categories according to the threshold for treatment (categories 1 and 2 as one group and categories 3, 4, and 5 as the second group) resulted in agreement of 91.95% (Table 3). Compared with DSA, CT angiography yielded a sensitivity of 90.9% and a specificity of 92.4%, with a kappa value of 0.812 (p = 0.001) for lesions that needed treatment. The overall accuracy of CT angiography was 91.5%—that is, according to the threshold for treatment; 377 of 410 arteries were judged similarly on CT angiography and DSA.
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Figure 3 shows the agreement between DSA and CT angiography in specific arteries according to the threshold for treatment; all the arteries except the dorsalis pedis, tibialis posterior, profunda femoris, and renal arteries are greater than 90% agreement. Calcified plaques were the main reason for misinterpretations on CT angiography.
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Lawrence et al. [4] compared CT angiography with DSA of the legs with good correlation, but these researchers did not show the inflow to the legs and found the overlapping of contrast-filled veins a major problem. In 1996, Raptopoulos et al. [3] described a CT angiography technique using two sequential helical scans to image the aortoiliac segment only. Two studies have described the use of CT angiography in trauma to the extremities with good diagnostic results [15, 21].
Although all these studies dealt with a limited segment only, our study examined the entire vascular tree, including the inflow and runoff vessels down to the feet. CT angiography of the whole peripheral vascular tree is a new technique made possible only recently by the new generation of CT scanner, the multidetector scanner [2, 13].
In 2001, the initial experience of Rubin et al. [13] with multidetector CT angiography of the lower extremities was published. The authors concluded that arteries of the lower extremities can be reliably depicted with this technique, although its diagnostic accuracy and effectiveness have yet to be determined. However, if we consider CT angiography as an alternative to angiography, we must generate CT angiograms in a presentation format similar to that of conventional angiography with which referring surgeons are accustomed [10, 21]. It is not possible for clinicians to view more then 700 axial slices containing much irrelevant, nonvascular information. In two studies, one by Rubin et al. [2] and the other by Rubin [22], this data "explosion" was considered to be the greatest challenge of the new scanning technology. Many display formats, including multiplaner reformation, MIP, shaded-surface display, and, most recently, volume rendering have been developed [19]. The MIP algorithm, one of the most commonly used formats [11], has the capability to reveal the entire vascular tree in one image [20] but, at present, has limitations. Bones and dense calcifications obscure the true lumen of the arteries and must be erased from the projections produced [3, 5, 19]. In the presence of extensive calcified plaques, especially in the distal small tibial arteries, it is difficult to produce MIP images with good diagnostic value. Continuous calcification of the wall of an artery may cause a false diagnosis of patency, whereas the process of erasing these calcifications may result in a false diagnosis of high-grade stenosis or occlusion. When dense calcifications are present, the end product is of no, or questionable, diagnostic value.
In large iliac arteries, this problem was satisfactorily solved by the related slice and MasterCut techniques. Using the MasterCut formation is not always possible in the distal tibial arteries that are sometimes visualized suboptimally, even in the axial slices. We agree with Kramer et al. [20] that transverse slices, as opposed to MIP findings alone, should be viewed in cases of stent placement and dense calcifications to delineate the lumen from the stent or from calcified plaques. In our series, we did not encounter the problem of veins filling or the inadequacy of spatial resolution of the MIP images for the proximal tibialis anterior as noted by other researchers [1, 4], probably because of the much faster scanning and thinner slices (3.2 mm compared with 5 mm) of our protocol, made possible by the multidetector CT scanner we used.
Comparison of our results with those of other studies is difficult because of the different methods in grading and the different segments of arteries evaluated. In a study of CT angiography of the aortoiliac segment, Rieker et al. [5] found a sensitivity of 93% for high-grade stenosis (defined as 75% stenosis) of the common and external iliac arteries using the cine and the axial modes for diagnosis and a sensitivity of 53% when using MIP images only. Raptopoulos et al. [3] found a sensitivity of 93% for the detection of aneurysms and significant stenosis (defined as 85% stenosis) in their study of CT angiography of the aortoiliac segment only. Rieker et al. [1] used CT angiography of the legs from the groin to the lower calf and calculated the accuracy of CT angiography in the detection of significant stenosis (defined as 75% stenosis) for each arterial segment, finding a sensitivity of 67% for the tibial and peroneal arteries.
The study by Lawrence et al. [4] most closely resembles ours in design and the definition of significant stenosis. Lawrence et al. found a sensitivity of 92.2% for the detection of 50% stenosis of the arteries of the legs from the groin to the mid calf. In our study, we found a sensitivity of nearly 91% for the detection of significant stenosis along the entire vascular tree including the inflow and the very distal crural arteries. Although some of the sensitivities of these previously mentioned studies [3, 4, 5] are higher than ours, the definition of a significant stenosis as a 50% reduction in diameter (compared with 75% or higher) and the display of the entire vascular tree result in a more difficult correlation.
CT angiography includes the following advantages. This method provides all the cross-sectional tissue and organ information (vascular and nonvascular) shown on CT [6]. For instance, in one of our patients, a 52-year-old man with severe right intermittent claudication, CT angiography showed a small aneurysm with a large thrombus in the distal aorta (Fig. 1C). This information, crucial for planning treatment, was not seen on DSA. Second, CT angiography allows one to view a three-dimensional model from any angle to best see a stenosis, specific vessel, or graft anastomosis. Third, no arterial puncture is necessary with CT angiography [1]; thus, patients on anticoagulation treatment or with blood coagulopathy or thrombocytosis need no preparation and have no added morbidity while undergoing the vascular evaluation. Fourth, CT angiography takes less time than DSA. Fifth, CT angiography may be useful in patients with limited or no peripheral access who are not candidates for MR angiography.
Presently, the major limitation of CT angiography is the limited value of the current software for MIP image production, especially in the presence of dense calcifications and in the distal anterior tibial and peroneal arteries. The presence of calcification is likely in a high portion of older patients with multiple and multisegmental diseases. The inability to quickly evaluate the distal peroneal, tibialis anterior, and dorsalis pedis arteries is a drawback when a distal bypass is needed. MR angiography has been shown to be a good imaging modality to show these regions and has the inherent advantage of using nonionizing radiation or nonnephrotoxic contrast media. However, MR angiography has other limitations including machine time, retrograde flow artifacts, indwelling stent artifacts, and availability. MR imaging does not provide a reliable depiction of vascular calcifications and is still a time-consuming procedure [13, 23]. Contrast-enhanced moving-table MR angiography is a new technique that may overcome some of these obstacles. It provides rapid, high-contrast arterial imaging with reduced flow artifacts. Three other groups of researchers [16, 24, 25] have found that moving-table MR angiography may be an effective alternative to catheter angiography in patients with peripheral arterial occlusive disease.
In 1996, Rieker et al. [1] concluded that the results of CT angiography were encouraging and that CT angiography protocols covering the pelvic to the pedal vessels in a single scan still had to be established. In 2001, Rubin et al. [13] concluded that the arteries of the lower extremities could be reliably depicted with a multidetector scanner, but the scanner's diagnostic accuracy still had to be determined. Our study shows that, excluding the most distal crural arteries, the multidetector scanner has broadened CT angiography applications to include peripheral artery evaluation with acceptable accuracy. Newer, faster scanners with the ability to scan the entire vascular tree with thinner, submillimeter slices, and improved software for image reconstruction, may overcome these remaining limitations.
Acknowledgments
We thank Alex Rosenberger for his help in preparing this manuscript, Shmuel
Weitzman and the CT technicians for their excellent work in CT angiography,
Szabo Ronit and the angiography technicians for their excellent work in the
DSA studies, and Boris Futerman for statistical analysis.
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