Diagnostic Ultrasound - Abdomen and Pelvis
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Diagnostic Ultrasound - Abdomen and Pelvis

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PART II SECTION 6 Kidney Transplant Introduction and Overview Approach to Sonography of Renal Allografts 552 Allograft Hydronephrosis 558 Perigraft Fluid Collections 562 Transplant Renal Artery Stenosis 566 Transplant Renal Artery Thrombosis 570 Transplant Renal Vein Thrombosis 572 Renal Transplant Arteriovenous (AV) Fistula 574 Renal Transplant Pseudoaneurysm 576 Renal Transplant Rejection 578 Delayed Renal Graft Function 580

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 552

PART II<br />

SECTION 6<br />

Kidney Transplant<br />

Introduction <strong>and</strong> Overview<br />

Approach to Sonography of Renal Allografts 552<br />

<br />

Allograft Hydronephrosis 558<br />

Perigraft Fluid Collections 562<br />

Transplant Renal Artery Stenosis 566<br />

Transplant Renal Artery Thrombosis 570<br />

Transplant Renal Vein Thrombosis 572<br />

Renal Transplant Arteriovenous (AV) Fistula 574<br />

Renal Transplant Pseudoaneurysm 576<br />

Renal Transplant Rejection 578<br />

Delayed Renal Graft Function 580

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