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Figure 3 Spectral Doppler trace in a patient with right RAS. The Doppler gate has been placed at the site of stenosis. Measured velocities in excess of 5 m/sec indicate a high-grade stenosis in this case. Abbreviation: RAS, renal artery stenosis.

Table 4 Criteria for Diagnosis of Stenosis on Ultrasound

Peak systolic velocity at least 100-200 cm/sec Ratio of renal artery to aortic peak systolic velocity > 3.5 Turbulent flow distal to the stenosis on color Doppler No Doppler signal in the artery indicates occlusion may be seen on ultrasound, their detection rate is universally very low in all series and much below the performance of other techniques (33,38).

Intrarenal Waveform Analysis

Due to the high rate of nondiagnostic scans with measurement of the renal artery velocity, considerable interest has been focused on the intrarenal arteries. These are much easier to scan, with waveform analysis being Doppler angle-independent. The results are thus far less operator dependent. The theory is that stenosis of the main renal artery will dampen the waveform of the downstream arteries, resulting in the so-called parvus-tardus effect ("parvus" meaning "small," and "tardus" meaning "slow" or "late"), with decreased peak velocity and a decreased slope of the systolic upstroke (Fig. 4) (39). Various parameters have been suggested to objectively measure this: prolonged systolic acceleration time (>0.07 seconds), decreased acceleration index (>4) (40), and resistive index (peak systolic velocity minus maximum end-diastolic velocity/peak systolic velocity). There are many reports of excellent results in the literature (41-43). Ripoles quoted a sensitivity of 89% and a specificity of 99% for the detection of RAS greater than 75%. However, others have shown that the tardus effect relates to vessel compliance (38), which changes with age and blood pressure, and has a poor correlation with stenosis (44). Most convincing is a study in which endovas-cular Doppler flow wires were placed in the patient's main and intrarenal arteries. Dop-pler traces obtained in this manner avoid the technical pitfalls of transabdominal ultrasound. The investigators found good correlation between main arterial peak systolic velocity and stenosis but no association with intrarenal parameters (45). Captopril can improve intrarenal Doppler ultrasound but not sufficiently (46).

Ultrasound Contrast Media

Because one of the main drawbacks of measuring renal artery peak systolic velocity is the inability to adequately image the artery in many subjects, efforts have been

Table 5 Performance of Doppler Ultrasound for the Detection of RAS Based on the Measurement of Main Renal Artery Velocity

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