There are 2 techniques that ideally are used in conjuction with each other,however in circumstances where the renal artery is not seen in its entirety then the indirect approach can give an indication of vascular disease.
Assessing the renal artery from the aorta to the kidney and any accessory arteries for any stenosis.A >60% stenosis is reported when there is a >3.5:1 Renal to Aortic Ratio (RAR) or a >180 cm/sec velocity in the renal artery at any point from the origin to the kidney.
Assessing the arteries within the kidney parenchyma to assess any alteration in the waveforms.The RI should be low resistance.The Acceleration Time (AT) should be <70msec. The probe is slowly moved superior and inferior to search for additional renal arteries. Any vessels identified must be traced to the kidney to confirm their identity.
The kidneys will atrophy with chronic renal failure and the length should be >9cm. The RI wil be >0.8 for untreatable medical renal disease.
The renal arteries are clearly imaged in B Mode from an anterior,subcostal approach however as it is perpendicular to the ultrasound beam it is not suitable for Doppler assessment. Supernumerary (duplicate) arteries can be seen looking posterior to the IVC in B Mode and Colour in a sagittal plane.
By moving the probe to the left of midline and angling toward the patient’s right, an acceptable Doppler angle of 60 degrees is achieved. To avoid aliasing set the colour scale high enough so it is minimized. If the scale is too low then it is difficult to determine which veesel is the vein and which vessel is the artery.
Roll the patient into a decubitus position to void bowel gas and improve visibility of the renal artery,especially the mid to distal portion.
PRONE APPROACH The patient is lying prone or decubitus and the probe is moved from the spine laterally using the muscles as an acoustic window to find the kidney initially and then the renal hilum using Colour Doppler.