Upper Pole

SV Angle -80* Dep 1.7 cm Size 2.0 mm Freq 2.5 MHz WF Low

Dop 73% Map 2 PRF3731 Hz

7 120

"r GO

Figure 4 Intrarenal waveform analysis in a transplant kidney. The parvus-tardus effect is demonstrated with decreased peak velocity and a decreased slope of the systolic upstroke.

made to increase its visibility with the use of ultrasound contrast. These are micro-bubbles of air or other gas surrounded by a protective shell, allowing them to circulate around the vascular system several times. They provide a high-impedance interface, enhancing reflection of the ultrasound beam (47) and improving visibility (Fig. 5A,B). A study on a first-generation contrast (Levovist®, Schering-Plough, Berlin, Germany) showed that visualization of the renal artery could be significantly improved with contrast (75% of arteries demonstrated without enhancement compared to 90% with enhancement) (34). Other authors have reported accompanying improvements in sensitivity and specificity for the diagnosis of RAS with the use of ultrasound contrast and second harmonic imaging (48,49). A report that 97% of normal arteries could be visualized with newer contrast agents would suggest that a normal scan could effectively exclude stenosis (50). However, current agents still have a limited life, and long-lasting agents are keenly awaited, as are further refinements in ultrasound technology.

In conclusion, measurements of peak systolic velocity in the main renal artery and renal-to-aortic velocity ratios are the most accurate ultrasound methods for recognition of RAS. Waveform analysis is less accurate, but in the significant proportion of patients in whom the renal artery cannot be fully visualized, ultrasound contrast may help. Even then, sonographic evaluation has been lately overshadowed by the technical leaps in computed tomography (CT) and magnetic resonance imaging (MRI), and its routine use is limited to a few specialized centers.

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