Figure 9 MIP image from a CTA in a patient with a 70% ostial stenosis of the left renal artery (long arrow) and a 50% nonostial stenosis of the right renal artery (short arrow). Note the associated small infrarenal aortic aneurysm. Abbreviations: CTA, computed tomography angiography; MIP, maximum intensity projection. Source: Courtesy of Joe Brookes, The Middlesex Hospital, London.
level. As such, the vessel lumen and calcified plaques remain separate. This rendering method can produce an angioscopic view from within the artery, allowing the arterial ostium to be viewed from the aortic lumen.
The accuracy of CTA varies with the display technique utilized. Early work by Galanski et al. showed that review of the axial images with additional MPRs was superior to MIP or SSD (72). CTA was sensitive for detecting accessory renal arteries on axial images. MIP images were superior for the diagnosis of ostial stenosis. Rubin et al. found that MIP images were 92% sensitive and 83% specific for the detection of stenoses greater than or equal to 70%. SSD was only 59% sensitive and 82% specific. The accuracy for grading the stenosis was 80% for MIP but only 55% for SSD (74). A prospective comparison (75) of multislice CTA and contrast-enhanced magnetic resonance angiography (MRA) [with digital subtraction angiography (DSA) as the gold standard] showed no statistical difference between the two for evaluating aorto-iliac and renal arteries (sensitivity 92%, specificity 99%), but the authors commented that the CT images took significantly more time to read than MRA studies.
Nevertheless, many studies have shown CTA to be a highly effective method for the noninvasive diagnosis of RAS, with sensitivities of 88% to 100% and specificities of 87% to 99% (Table 8) (68,76-82) and in close agreement with DSA for grading of stenosis (80). A normal CTA virtually rules out significant RAS with a negative predictive value of 95% (83). A meta-analysis (51) found that CTA along with contrast MRA performed significantly better than ultrasound and captopril scintigraphy.
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