Which of the following is/are true regarding the assessment of renal artery obstruction with duplex scanning?

a. There is no flow reversal in early diastole in the renal artery

b. Renal artery to aortic peak systolic velocity ratios that exceed 3.5 indicates the presence of a 60% diameter stenosis

The kidneys represent a low resistance vascular bed, therefore flow in renal arteries is positive throughout the cardiac cycle and there is no flow reversal in early diastole as is seen during the assessment of peripheral arteries. A ratio of renal artery to aortic peak systolic flow that exceeds 3.5 indicates the presence of a 60% diameter stenosis. Sensitivities greater than 80% and specificities greater than 90% have been reported using these criteria to predict significant stenoses of main renal arteries. Unfortunately duplex scanning often fails to detect accessory renal arteries or segmental branch disease. It is quite reasonable as a screening technique for patients with suspected renal artery hypertension and as an accurate method for monitoring renal artery reconstruction.