Doppler US is a first-line noninvasive tool in the evaluation of suspected RAS and uses a combination of direct insonation of the anastomosis and main renal artery in addition to indirect intrarenal waveform morphology. Peak systolic velocity (PSV) in the renal artery is commonly used as the parameter to assess for the presence of RAS on US. Cutoff values of 200 to 300 cm/s have been proposed in various studies [18,19], but the lower limit suffers from low specificity, leading to unnecessary angiography procedures [20]. As in native kidneys, a tardus parvus waveform (small peak amplitude with delayed upstroke) can be seen within the transplanted kidney downstream to the stenosis. This morphologic waveform change is reflected quantitatively by the acceleration time (AT). De Morais et al [21] reported a sensitivity of 90% to 96.8% and a specificity of 87.5% to 70% using various PSV thresholds in the main renal artery and a sensitivity of 100% and specificity of 96.7% using an AT of 0.09 or less as normal. Another parameter that can be used is the renal artery to iliac artery ratio, which has been shown to The US appearance of RAT is striking, with complete absence of flow in the renal vessels on color flow and spectral analysis. Power Doppler imaging may be helpful because of its capability to detect low flow. However, it is important to remember that the absence of arterial flow within the kidney can also be seen in patients with hyperacute rejection and RVT [23]. Absence of renal venous flow and renal enlargement are classically seen in patients with RVT. Reversal of flow in the renal artery in diastole is often found in association with RVT [24]; however, this represents only approximately 10% of cases of reversed diastolic flow. Reversal of flow is seen more commonly in rejection or ATN and occasionally with nephrosclerosis [25].
Renal Transplant Dysfunction. Doppler US is a first-line noninvasive tool in the evaluation of suspected RAS and uses a combination of direct insonation of the anastomosis and main renal artery in addition to indirect intrarenal waveform morphology. Peak systolic velocity (PSV) in the renal artery is commonly used as the parameter to assess for the presence of RAS on US. Cutoff values of 200 to 300 cm/s have been proposed in various studies [18,19], but the lower limit suffers from low specificity, leading to unnecessary angiography procedures [20]. As in native kidneys, a tardus parvus waveform (small peak amplitude with delayed upstroke) can be seen within the transplanted kidney downstream to the stenosis. This morphologic waveform change is reflected quantitatively by the acceleration time (AT). De Morais et al [21] reported a sensitivity of 90% to 96.8% and a specificity of 87.5% to 70% using various PSV thresholds in the main renal artery and a sensitivity of 100% and specificity of 96.7% using an AT of 0.09 or less as normal. Another parameter that can be used is the renal artery to iliac artery ratio, which has been shown to The US appearance of RAT is striking, with complete absence of flow in the renal vessels on color flow and spectral analysis. Power Doppler imaging may be helpful because of its capability to detect low flow. However, it is important to remember that the absence of arterial flow within the kidney can also be seen in patients with hyperacute rejection and RVT [23]. Absence of renal venous flow and renal enlargement are classically seen in patients with RVT. Reversal of flow in the renal artery in diastole is often found in association with RVT [24]; however, this represents only approximately 10% of cases of reversed diastolic flow. Reversal of flow is seen more commonly in rejection or ATN and occasionally with nephrosclerosis [25].