Renal Ultrasound in Transplanted Kidneys

Some points to consider:

• Corticomedullary differentiation may be exaggerated due to superficial location of transplanted kidney.
• Pelvicaliectasis: Usually due to overhydration, but reflux and obstruction may be seen as well.
• Main renal artery velocity: < 200 cm/s.
• Resistive indices (RI): Normal is less than 0.7. Between 0.7-0.8 is indeterminate. Above 0.8 is considered abnormal, but it has low sensitivity and specificity. Abnormally elevated resistive indices can be seen with any of the following:
  • Rejection
  • Acute tubular necrosis (more below)
  • Pyelonephritis
  • Renal vein thrombosis (more below)
  • Ureteral obstruction
  • Extrinsic compression
  • Drug toxicity
• Reversal of diastolic flow (seen in the image above) is nonspecific, but is a poor prognostic indicator. It can be seen in:
  • Acute tubular necrosis
  • Acute rejection
  • Renal vein thrombosis

Immediate (first week) complications:

• Acute tubular necrosis (ATN): Reversible ischemic damage. More likely in cadaveric transplant, with long ischemic times, and patient hypotension. Kidney may look normal or edematous. Elevated resistive indices may be seen in severe acute tubular necrosis, but can be normal.
• Accelerated acute rejection: Presents within a few days to a few weeks following transplantation. Nonspecific sonographic findings similar to those of acute tubular necrosis.
• Renal vein thrombosis: Occurs in about 1% of cases. Ultrasound shows an enlarged kidney with absent venous flow. There may be reversal of arterial flow in diastole. Look for correctable causes: Clot in the renal vein or compression of the renal vein by fluid collection.
• Renal artery thrombosis: No arterial flow on Doppler. Angiography can be used for confirmation

Early (1-4 weeks) complications:

• Acute rejection: Look for fluid around the kidney. The kidney may be edematous (loss of corticomedullary differentiation, obliteration of renal sinus echo complex). Also look for edema in the walls of the collecting system. There may be parenchymal infarctions. Resistive indices may be elevated, but has low sensitivity and specificity.
• Urine leak or fistula: Due to ischemia and necrosis of the distal ureter. Look for fluid collections. Lymphoceles (usually late complication) or mature hematomas can look the same. Ultrasound-guided aspiration can be used.
• Ureteral obstruction: Look for hydronephrosis, but beware that overhydration and reflux can fake you out.

Late (> 4 weeks) complications:

• Drug toxicity
• Renal artery stenosis: Up to 10% of cases. Normal peak systolic velocity in renal artery is < 200 cm/s. A ratio of greater than 3-3.5:1 between the iliac artery and the renal artery is also suggestive. RI can be decreased.
• Recurrence of native renal disease
• Lymphocele: Occurs in up to 15% of cases. Well-defined and may have fine septations.

Special thanks to Dr. Hansel Otero for the images.

References

• Friedewald SM, Molmenti EP, Friedewald JJ, Dejong MR, Hamper UM. Vascular and nonvascular complications of renal transplants: sonographic evaluation and correlation with other imaging modalities, surgery, and pathology. J Clin Ultrasound. 2005 Mar-Apr;33(3):127-39.
• Kaveggia LP, Perrella RR, Grant EG, Tessler FN, Rosenthal JT, Wilkinson A, Danovitch GM. Duplex Doppler sonography in renal allografts: the significance of reversed flow in diastole. AJR Am J Roentgenol. 1990 Aug;155(2):295-8.
• Zwirewich CV. Renal Transplant Imaging and Intervention: Practical Aspects.