Resistive Index (RI):
Peak Systolic Velocity (PSV) – End disastolic velocity (EDV)
Peak Systolic Velocity (PSV)
Normal Values: 0.53. Increases in RI may indicate intrinsic Kidney disease.
Acceleration Time: Time from the start of flow to the peak flow.
Normal Values: <100 ms. Increases may suggest renal artery stenosis.
Size: Normal size 9-12 cm. A kidney size less than 8 cm or > 1.5 cm difference between the two kidneys is abnormal.
Normal renal arteries have a peak systolic velocity (PSV) of about 100 cm/sec with a LOW RESISTIVE PROFILE with forward diastolic flow of about 30 cm/sec.
Renal Artery Ratio (RAR):
Peak Systolic Velocity renal artery
Peak Systolic Velocity Aorta
0-59% RAR < 3.5 without significant turbulent flow
60-99% PSV > 200 cm/sec; RAR > 3.5 with significant turbulent flow.
>80% EDV > 150 cm/sec.
Renal artery occlusion should only be documented with visulation of the renal artery and no color flow or Doppler waveforms. Kidney size will usually be under 8 cm and parynchymal waveforms and color signal will be markedly reduced.
AAA: Aortic velocities should be documented prior to the aneurysm, at the level of the renal arteries and distal to the anuerysm if possible. RAR may or may not be accurate in the presence of AAA and this should be noted.
Low Aortic Velocity (<40 cm/sec): Low aortic velocities may falsely elevate RAR. If RAR > 3 with this situation, then PSV should be > 200 cm/sec in order to report 60-99% stenosis.
High Aortic Velocity(>100 cm/sec) : RAR may also not be accurate with high aortic velocities as it may falsely lower RAR.
Secondary Criteria to establish significant stenosis if an RAR can not be calculated inlude:
Distal Velocity shift
Significant turbulence by color doppler
An Acceleration time > 100m/s
Renal Artery Ultrasound
1) Acute Renal Failure
2) New onset Hypertension
3) Refractory Hypertension
4) Abdominal Bruit
5) Flash Pulmonary Edema
6) Abdominal Bruit
7) Small Kidneys
8) Post intervention surveillance
The renal arteries may be technically difficult to image. Large body habitus, excessive bowel gas, and the depth or course of the arteries may all play role in contribute to this difficulty. Patients should be kept NPO before the procedure.
1) Survey the Aorta in transverse orientation.
2) Sample Celiac, SMA, IMA
3) Image the aorta in long axis. Document spectral waveform at 60 degrees in the center of the vessel between SMA and renal arteries. This velocity will be used to calculate the Renal/Aortic Ratio (RAR).
Identify the renal arteries in the transverse plane.
Scanning the Renal Arteries:
Stenotic lesions will lead to velocity increases and post stenotic turbulence. Once identified, the artery should be visualized through it’s entire course. The sample volume should be kept as small as possible keeping the angle as close to 60 degrees. The greatest peak velocity should be documented to facilitate the calculation of RAR. The Renal wave form is biphasic and LOW RESISTANCE with continuous diastolic flow. A high resistive signal may indicate renal stenosis or parenchymal disease.
Scanning the Kidneys:
Normal renal size is 9-12 cm, the left renal artery is generally slightly larger. Parenchymal flow should be documented to the upper, middle and lower kidney to rule out AV fistulas, infarcts or other abnormalities. The resistive index and acceleration time should be documented. Without significant pathology, the pressure waveform should resemble the renal artery ie..LOW RESISTIVE. A high resistive profile may indicate renal parenchymal disease or lobar artery occlusion.