Prior to renal ultrasound exam, fasting for 8 hours is recommended to reduce bowel gas. During the exam, having the patient hold breath will push the kidneys inferiorly in the abdomen. This will also help with motion artifact.
In abdominal aorta, the blood flow at the level of celiac and SMA is low resistance. Below the level of renal arteries, the aorta assumes high resistance flow pattern. To calculate renal-to-aortic velocity ratio (RAR), aortic velocity should be taken at the level of SMA origin.
Renal Artery Velocity
- Normal main renal artery PSV < 180 cm/s
- Normal PSV for interlobar arteries: 30-40 cm/s
- Arcuate arteries: 20-30 cm/s
- Renal/Aortic ratio (RAR) < 3.5. If perirenal aorta PSV outside of 40- 100 cm/s (normal range), RAR is not reliable.
- Renal PSV > 180 cm/s and RAR > 3.5, it is > 60% stenosis
- PSV > 180, RAR < 3.5, < 60% stenosis
- PSV < 180, RAR < 3.5, normal
- For transplant kidney, renal-iliac ratio (RIR) of 2.5 -3.0 or greater is consistent with stenosis of allograft renal artery
- For renal stent or bypass PSV of 280 and RAR of 4.5 are used for thresholds
Renal Hilum Velocities
Renal hilum velocities can be easily obtained by scanning through the kidney at the patient’s flanks, and can yield clues to the renal artery. It can be used as adjunct but not suitable for primary screening for renovascular disease. Below are some normal values:
- For acceleration time (AT), more than 100 msec is abnormal
- For acceleration index (AI), or slope of the acceleration, less than 300 cm/s^2 is abnormal
Parenchymal Assessment
End diastolic ratio:
EDR = Parenchymal EDV/ Parenchymal PSV
Normal cortical EDR vales are 0.30 – 0.47
Resistive index:
RI = 1- EDR
High RI, or low EDR, signify worse kidney disease. RI > 0.8 identifies patients unlikely to respond to renal revascularization.
Size
- Normal 10-12 cm long
- 4.5 – 6 cm wide
- Cortical thickness 1 – 1.5 cm
- 1 cm decrease in length is considered renal atrophy