Diagnostic Ultrasound - Abdomen and Pelvis

Approach to Sonography of Renal Allografts
Diagnoses: Kidney Transplant
Imaging Anatomy
Renal transplants are placed in the pelvis where they are
readily accessible for ultrasound imaging. The transplant renal
artery is typically anastomosed end to side to the external iliac
artery and the renal vein to the external iliac vein. The
transplant ureter anastomosis is at the dome of the bladder.
The ureter is not usually visible unless it is dilated or contains a
stent. Stents may be placed if the surgeon is concerned about
the integrity of the ureteral anastomosis.
The first renal transplant or solitary renal transplants are
preferentially placed in the right pelvis, where the iliac vessels
are more superficial, in an extraperitoneal location. Second
transplants or those accompanied by a pancreas transplant
are typically placed in the left pelvis. Patients may have dual
simultaneous bilateral transplants or sequential transplants, in
both iliac fossae. Pediatric donor kidneys are harvested en
bloc with the intervening aorta and vena cava, leaving the
renal vascular pedicle intact. They appear as two small, closely
opposed kidneys in one iliac fossa with intervening aorta and
vena cava anastomosed to iliac artery and vein. These kidneys
are intraperitoneal. When patients have had multiple failed
renal transplants, the newest transplant may be placed
intraperitoneally with anastomoses to aorta and vena cava.
The renal transplant appears like a normal kidney but is
oriented obliquely. The position of the hilum is variable; it can
be facing medially or laterally, depending on size match
between the donor and recipient. Renal pyramids are well
seen and are hypoechoic to renal cortex and sinus fat. Color
perfusion should be seen up to the arcuate arteries. Doppler
shows a low-resistance arterial waveform with continuous
flow throughout the cardiac cycle. The resistive index is
normally < 0.7 and tends to be higher in the main renal artery
than in the intrarenal arteries. The renal vein and intrarenal
venous waveforms are monophasic or slightly undulating with
the respiratory and cardiac cycle.
Gas may be seen in the renal pelvis or calyces secondary to
reflux from the bladder in the setting of bladder
catheterization.
Anatomy-Based Imaging Issues
At baseline, it is crucial to determine if there are any variations
to the standard surgery. These include accessory renal arteries
separately anastomosed to the iliac artery or arterial
reconstruction onto a patch of aortic graft or interposed vein
graft. Accessory veins are much less common.
Mild dilatation of the collecting system is not uncommon early
after transplantation and is more commonly seen in the renal
pelvis. Calyceal dilatation is a more concerning sign. Dilated
calyces should be distinguished from cortical cysts or sinus
cysts, by demonstrating communication with the renal pelvis.
Hypoechoic pyramids should not be mistaken for calyces.
Color Doppler should be used to rule out a vascular lesion
mimicking a cyst. If the bladder is distended and there is
dilatation of renal pelvis or ureter, the patient should be
rescanned after emptying the bladder.
Pathologic Issues
Pathologic evaluation of renal transplant biopsy is the gold
standard for the diagnosis of nonsurgical noninfectious causes
of renal transplant dysfunction. Evaluation includes histology,
immunofluorescence, immunohistochemistry, and electron
microscopy.
In the first few days after transplantation, acute tubular
necrosis is the most common cause of delayed function.
Hyperacute rejection is not common with current screening.
After the first three-five days, acute cell mediated rejection
and acute antibody mediated rejection occur. Pathological
evaluation is key to making an accurate diagnosis of rejection
so that the antirejection therapy can be tailored to the
patient. The Banff classification for rejection is in wide use.
Toxic effects from calcineurin inhibitors (used to counter
rejection) may be manifest as tubulopathy or thrombotic
microangiopathy on biopsy.
In the first year after transplantation, recurrent glomerular
disease (such as focal segmental glomerulosclerosis) can
significantly impact graft survival and can be diagnosed by
biopsy. Polyomavirus nephropathy is an important cause of
chronic graft failure, secondary to viruses such BK and JC
viruses. Later allograft dysfunction can result from a
combination of all the above with the addition of
hypertension, pyelonephritis, and post-transplant
lymphoproliferative disorder.
Pathology-Based Imaging Issues
Imaging is not sensitive or specific for differentiating
parenchymal causes of dysfunction; biopsy is required.
Imaging Protocols
Curved transducers are ideal for evaluation, using as high a
frequency as allowed by body habitus and depth of the
kidney, typically between 2-5 MHz but up to 8 MHz if feasible.
A linear transducer may be used for focused assessment of a
lesion. Use of compound and harmonic imaging can improve
image quality and decrease artifacts.
Patients are typically imaged supine but the decubitus
position is very helpful to mitigate the presence of intervening
gas or pannus. Additionally, in the immediate postoperative
period, wound staples and dressings may prevent optimal
access. Grayscale evaluation of the renal transplant should
include assessment of renal size, cortical echogenicity,
presence of hydronephrosis or hydroureter, and perinephric
collections. The bladder should also be evaluated for degree
of distension, wall thickness, and intraluminal content such as
stent, clot, or debris.
Color Doppler is essential for identification of vessels and to
determine patency and luminal narrowing. The iliac artery and
vein are assessed in color Doppler and Doppler waveforms are
obtained. More detailed devaluation of the iliac artery
waveforms may be obtained if there is a suspicion for iliac
artery dissection or atherosclerosis.
The main renal artery or arteries are evaluated with color and
spectral Doppler for patency and the presence of stenosis.
Stenosis most commonly occurs at the renal artery to iliac
artery anastomosis and causes aliasing in color Doppler with
elevation of peak systolic velocity on spectral Doppler. This is
compared to the peak systolic velocity in the iliac artery.
Downstream effects can be observed, notably the tardus
parvus waveform in the intrarenal arteries, which is quantified
using the acceleration index and the resistive index.
Starting at the renal hilum and progressing toward the cortex
the intrarenal arteries are the segmental, interlobar, arcuate,
and interlobular arteries. Doppler waveforms are obtained of
these arteries with measurements of the resistive index. Such
measurements may be obtained in the upper, mid, and lower
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