Gray-scale ultrasound has little to contribute in the diagnosis of RAS except for the measurement of renal length and the exclusion of important morphological abnormalities.
Since the development of color Doppler encoding, ultrasound has played an important role in imaging of the vascular system, including the renal arteries. Ultrasound has many advantages as a screening method. It is inexpensive, readily available, particularly noninvasive, and well tolerated by patients. Vascular sonography generally centers around the measurement of angle-corrected absolute arterial velocity, but historically, in the kidneys, direct velocity measurement has been difficult. Sonographic strategies for the detection of RAS broadly divide into the measurement of velocity and velocity change in the main artery and sampling of intrarenal arterial waveforms.
An ultrasound examination of the entire renal artery is made, scanning the patient with the probe in an anterior or anterolateral position. Both color and power Doppler may be used to follow the course of the artery. The stenosis itself may be visualized on color Doppler imaging as a change in color due to turbulence (Fig. 2). Objective evidence is obtained by measurement of velocities on spectral Doppler— narrowing beyond the critical threshold (>70% reduction) results in a gradually increasing arterial velocity (the Bernoulli effect); the velocity gradient can help to grade the stenosis. Significant arterial stenosis produces an increase in the peak systolic velocity at or just distal to it (Fig. 3). Criteria for diagnosing a significant stenosis are presented in Table 4. The exception to this is in the near occlusive state when flow slows to a trickle. The artery is often tortuous, and scanning may be hampered by body habitus or interposed bowel gas, and in routine practice, complete and accurate interrogation is impossible in 10% to 50% (29).
Consequently, early reports on the use of this technique were somewhat less than promising, with quoted sensitivities as low as 0% (30). However technology and operator experience have moved on and current results are much better. Miralles et al. (31) reported that peak systolic velocity was the best ultrasound parameter for separating a less than 60% stenosis from a greater than 60% one. With a threshold of 198 cm/sec and a renal-to-aortic velocity ratio of 3.3, they obtained sensitivities of 87% and specificities of 91%. Other workers have reported sensitivities of 79% to 98% and specificities of 77% to 98% depending on the thresholds used (31-37). The results of these studies are shown in Table 5. Although accessory renal arteries
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