renal artery spectral analysis- origin,proximal,mid and distal artery .
if a stenosis is suspected then a velocity with an accurate angle (<60degrees) in the renal artery is compared with the aorta to give a ratio (>3.5:1 is a >60% stenosis which is haemodynamically significant). if an accurate angle of less than 60degrees cannot be obtained then a ratio should be done with no angle in both the renal artery and aorta to minimise the error caused by the cos q approaching 0.
ultrasound image- stenosis measurement :
this is a spectral trace done in the stenotic site with an angle correction for an accurate velocity measurement.
less than 180cm/sec suggests less than 60%.
this example is 173cm/s (top left)
ultrasound image- renal artery : aortic ratio (rar)
this is the same trace but the angle correction is taken off and the measurement is compared to the aortic measurement, also with no angle and a ratio is determined. the renal aortic ratio (rar).
the velocity without angle correction is 156cm/s (top left)
ultrasound image- the length of the stenosis is measured and its distance from the renal artery origin.
the blood travels down the aorta
into the renal artery
some arteriosclerotic plaque proximally causing a stenosis and high velocity flow with a renal to aortic ratio (rar) >3.5:1 and velocity >180 cm/sec.therefore it is >60% stenosis
there is post stenotic turbulent,aliasing flow
there is loss of the esp and and a slow rise (increased at)
the interlobar (segmental)
interlobular (arcuate) assessment will reflect the earlier stenosis with abnormal at>0.07sec
-spectral assessment within the kidney.
the resistive index (ri)is easily performed by placing a caliper on the early systolic peak (esp)and the other caliper on the lowest diastole.the ri is a ratio of peak systolic and end diastolic velocity.
the acceleration time (at) is done by placing a caliper on the level at which the gradient begins to rise and finished at the first peak ie the early systolic peak (esp).this should be less than 70ms
this is done by assessing the parenchymal haemodynamic changes in the waveform. initially there is an esp but with a stenosis this will be lost and a tardus parvus waveform will be the result. with chronic renal failure the waveform becomes high resistance (ri>0.80) which unfortunately cannot be repaired. this is also associated with high creatinine levels.