renal arteries normal

there are 2 techniques, direct method and indirect method, used to investigate renal arterial supply.
both 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 alone can give an indication of vascular disease.

direct method

this is directly examining the main extra-renal arteries.

involves:

anterior approach to examine the aorta and proximal renal arteries.
oblique/decubitus approach to examine the remainder of the renal arteries.
coronal approach may need to be employed if large patient habitus or obscurring bowel gas.

anterior approach.the patient is lying supine and the probe is moved inferiorly and superiorly to identify the renal arteries and any supernumerary arteries.look in b-mode and colour doppler to help identify.

a spectral analysis is made of the aorta at the level of the renal arteries. the velocity is taken with an angle for an accurate measurement.if an accurate angle (<60degrees) cannot be obtained then another measurement is taken with no angle so it can be compared to the renal artery at a stenosis site to do a renal artery:aorta ratio (rar ratio). this should be less than 3.5:1.

the aorta is examined for any abdominal aortic aneurysm or stenosis.

oblique approach
by moving the probe intercostally to the right of midline and angling toward the patient’s left, 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 vessel is the vein and which vessel is the artery.

oblique approach.

angling 45 degrees to right renal artery or rolling the patient in a semi left decubitus position to avoid the bowel gas and improve the doppler angle.

oblique approach

anterior v’s oblique (ultrasound image)

oblique approach.

angling 45 degrees to right renal artery.in most individuals, the renal arteries arise from the abdominal aorta immediately distal to the origin of the sma.by moving the probe to the right of midline and angling toward the patient’s left, an acceptable doppler angle of 45-60 degrees is achieved

anterior approach. (ultrasound image)

the renal arteries are clearly imaged in b mode from an anterior approach however as it is perpendicular to the ultrasound beam it is not suitable for doppler assessment.

in most individuals, the renal arteries arise from the abdominal aorta immediately distal to the origin of the superior mesenteric artery (sma). the right renal artery passes underneath the inferior vena cava (ivc) and posterior to the right renal vein

anterior approach: ultrasound image of the left renal artery.

the left renal vein passes between the aorta and sma.

the left renal artery is located posterior to the renal vein.

normal waveform

a normal waveform obtained from the main renal artery demonstrates a rapid upstroke in systole and a low resistance waveform with continuous forward flow throughout the cardiac cycle.

the normal peak systolic velocity of the main renal artery is less than 180 cm/sec.

the resistive index is less than 0.70 .

doppler ultrasound sampling throughout the entire artery from the origin to the hilum.

coronal appoach

roll the patient into a decubitus position to avoid bowel gas and improve visibility of the renal artery,especially the mid to distal portion.

coronal approach.

angling 90 degrees to left renal artery.the patient is rolled decubitus and a coronal view through the left loin and liver.an intercostal view through the ribs on a deep inspiration is ideal.

coronal approach.

this ultrasound view provides an excellent angle for imaging the origins of both the right and left renal arteries using color/power doppler. using the coronal approach to visualise the renal artery from the aorta to the kidney on a tortuous vessel may result in the mid section not being seen.

supernumerary arteries (also called accessory arteries)

this coronal plane ultrasound image gives a better appreciation of the supernumerary arteries. another useful view to demonstrate supernumerary (duplicate) renal arteries is a coronal image of the aorta.

there are 2 renal arteries posterior to the ivc in this ultrasound image.

in this ultrasound image, there are 3 renal arteries posterior to the ivc

indirect method

this is examining the intra-renal arteries to indirectly infer the status of the main extra-renal artery(s).

an indirect assessment requires a good colour ultrasound image to determine the position of the interlobar and interlobular arteries, which in turn will determine the best angle to get an accurate measurement of the ri and at.

this is a normal spectral trace of an interlobar (segmental) artery

ris and ats (resistive index and acceleration time)

resistive index

the resistive index (ri)is easily performed by placing a caliper on the peak systolic velocity and the other caliper on the lowest diastole.the ri is a ratio of peak systolic and end diastolic velocity.

acceleration time

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

scan protocol

role of ultrasound

identify a haemodynamically significant stenosis of a renal artery that may be the cause of hypertension.
rarely, as a cause for renal failure.

limitations

this examination requires the patient to be cooperative and hold respiration in inspiration and expiration depending where the sonographer can best see the artery. if the patient cannot hold their breath then adequately obtaining an accurate doppler signal will be difficult or impossible.

patient preparation

fast for 6 hours. no food. drink 2 litres of water over the two hours prior to the appointment.the bladder can be emptied as needed. book the appointment in the morning preferably to reduce bowel gas.

equipment setup

transducer:
highest frequency curved linear array probe possible. start with 7mhz and work down to 2 or 3mhz for larger patients.colour and spectral doppler capabilities . assess the depth of penetration required and adapt.

a high sweep speed will improve accuracy of the measurement taken on the spectral trace.

common pathology

renal artery stenosis
renal artery occlusion
aneurysm

indications

renovascular hypertension,usually it is uncontrolled, (resistant hypertension).
may be caused by renal artery stenosis,parenchymal disease,renal neoplasms,renal vein thrombosis,or an adrenal mass.

it may be caused by atherosclerosis in the renal artery or less commonly fibromuscular dysplasia(fmd) particularly in a young woman.

scanning technique

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.

direct
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.

indirect
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.

anterior approach
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.
oblique approach
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.
coronal approach
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.

criteria

a >60% stenosis is suspected if:

direct method

peak systolic velocity is > 180cm/s

and/or

renal artery aortic ratio (rar) >3.5:1.

if an accurate angle cannot be obtained in the renal artery then a ratio is done with no angle on either the renal artery or the aorta.

indirect method

interlobar/segmental artery spectral analysis.

acceleration time (at) >70msec

and

resistive index (ri) >0.7.

ri >0.8 suggests chronic renal disease.

basic hardcopy imaging

direct method

peak systolic velocity in the aorta -taken above the level of the renal arteries origin.taken with and without an angle for a ratio with the renal artery.
renal artery assessment- initially with colour doppler.
renal artery spectral analysis- origin,proximal,mid and distal artery .if a stenosis is suspected then a velocity with an angle to compare with the aorta to give a ratio (>3.5:1 is a >60% stenosis which is haemodynamically significant). if an accurate angle cannot be obtained in the renal artery then a ratio is done with no angle on either the renal artery or the aorta.
interlobar/segmental artery spectral analysis- acceleration time (at) and resistive index (ri).(at <70msec and ri >0.8 for chronic renal disease)

indirect method

aorta b mode -longitudinal and transverse to assess for an abdominal aortic aneurysm.
kidney lengths-cortex assessment for reduced size.
perfusion kidney- colour doppler used to assess the perfusion to the edge of the renal cortex.
resistive index- spectral doppler of kidney parenchyma at the interlobar (segmental) arteries and interlobular (arcuate)arteries.
acceleration time