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Thank you for our first ask an expert question! It's a good one.
For renal arteries (and any artery really for that matter) you ideally want to maintain a consistent Doppler angle as close to 60degrees as possible. It's my understanding that the flow velocities published in the literature are based on Doppler angles corrected to 60degrees. For grading proximal renal artery stenosis I prefer using renal-aortic-ratio (RAR) which is renal artery stenosis velocity divided by aortic velocity at renal level.
It can be hard with abdominal vascular scans to maintain an angle of 60 because you can't steer the beam so you need to heel and toe the probe to the best of your ability to create this angle. In the event you simply can't achieve an angle of 60, and don't I necessarily advocate for this, then angles of 45-60 have been shown to produce results with acceptable degrees of error. It goes without saying though that if you interrogate a renal artery stenosis with an angle of say, 48degrees then you must use the same angle in the aorta to produce a reliable RAR. General rule of thumb is that an RAR of >3.5 equates to a >60% stenosis.
For intra-renal arteries we don't correct Doppler angle (angle of 0degrees) at the lab I work at because we only measure resistive indices at the upper, mid and lower poles.
I'm sure the other vascular SIG members will be able to provide supplementary information - hope this helps!
Matt