12.3 Liver & Renal Transplant Sonography

Renal Transplant: Anatomy and Protocol

A transplanted kidney is placed extraperitoneally in the iliac fossa — usually the right — with the renal artery and vein anastomosed end-to-side to the external iliac vessels (living-donor grafts may use the internal iliac artery end-to-end) and the ureter implanted into the bladder. Because the graft lies only a few centimeters deep, it can be scanned with a mid- to high-frequency curved or linear transducer, and its orientation may be reversed from a native kidney, so orient yourself by the hilum before labeling poles.

A complete protocol combines gray scale and Doppler:

• Gray scale: measure graft length (often slightly larger than a native kidney), assess cortical echogenicity and corticomedullary differentiation, and survey for hydronephrosis and perinephric collections
• Color Doppler: confirm global perfusion to upper pole, mid, and lower pole; a focal perfusion defect suggests infarct or a branch vessel problem
• Spectral Doppler: sample the main renal artery and vein at the anastomosis plus arcuate/interlobar arteries in at least three regions, recording the resistive index (RI) = (peak systolic velocity − end-diastolic velocity) / peak systolic velocity

Keep the Doppler angle at or below 60 degrees, use a small sample volume within the vessel, and average at least three representative RI measurements.

Interpreting the Resistive Index and Vascular Complications

A normal graft shows low-resistance arterial flow with continuous forward diastolic flow; the RI is normally below about 0.7–0.8. The key exam concept is that an elevated RI is sensitive but nonspecific — it rises in acute rejection, acute tubular necrosis (ATN), renal vein thrombosis, extrinsic compression, and obstruction. Clinical timing helps narrow the differential: ATN typically appears in the first days after transplantation (especially in cadaveric grafts) and resolves, whereas acute rejection usually emerges after the first week. Renal vein thrombosis produces the most extreme pattern — a plateau-like waveform with reversal of diastolic flow (RI effectively above 1.0) and absent venous flow — and is a surgical emergency. Renal artery stenosis, usually at or near the anastomosis and often presenting months later with hypertension, shows peak systolic velocity above roughly 200–250 cm/s with a renal-artery-to-iliac ratio greater than about 2–3.5, post-stenotic turbulence, and a downstream tardus-parvus waveform — a delayed systolic upstroke (prolonged acceleration time, often >100 ms) with diminished amplitude. Biopsy-related injuries create arteriovenous fistulas (high-velocity, low-resistance flow with arterialized venous pulsatility and perivascular tissue vibration) and pseudoaneurysms (a cystic structure with swirling yin-yang color flow and a to-and-fro neck waveform).

Perinephric fluid collections are dated by their typical appearance:

Liver Transplant: Anastomoses and Vascular Surveillance

A whole-graft liver transplant has four anastomoses to evaluate: the hepatic artery, the portal vein, the hepatic venous/IVC outflow (end-to-end IVC or a piggyback side-to-side configuration), and the bile duct (choledochocholedochostomy, or a Roux-en-Y hepaticojejunostomy). Sonography is the first-line tool both in the immediate post-operative period — where scans may be performed daily to confirm vascular patency — and in long-term surveillance.

The hepatic artery is the most critical vessel to document because the biliary tree receives its entire blood supply from the hepatic artery. A normal hepatic artery waveform shows a rapid systolic upstroke with continuous diastolic flow, an RI of roughly 0.55–0.8, and an acceleration time under about 80 ms. A tardus-parvus pattern — RI below 0.5 and a systolic acceleration time longer than 80 ms — indicates proximal hepatic artery stenosis at or upstream of the anastomosis. Both criteria must be met, and the further the acceleration time runs beyond 80 ms, the more confident the call. Absent arterial flow despite optimized technique suggests hepatic artery thrombosis, the most common major vascular complication of liver transplantation; because the bile ducts depend on arterial perfusion, thrombosis leads to biliary ischemia, strictures, bilomas, and hepatic abscesses, and often mandates retransplantation. Portal venous gas or new perihepatic fluid in this setting is ominous.

The portal vein should show hepatopetal flow; mild focal narrowing and a velocity step-up at the anastomosis are expected, but a focal velocity roughly three times or more the preanastomotic segment with post-stenotic turbulence indicates hemodynamically significant portal vein stenosis. The hepatic veins and IVC normally retain some cardiac pulsatility; conversion to a flat monophasic hepatic vein waveform raises concern for outflow (hepatic vein or IVC) stenosis, especially with new ascites. Biliary complications — anastomotic stricture, bile leak with biloma, cholangitis, and stone or sludge formation — are common and are screened by looking for ductal dilatation and perihepatic collections. Early post-operative scans also document baseline findings (small perihepatic hematoma or seroma is expected) so that interval change on surveillance studies can be judged against them.

For the renal graft, no fasting or special preparation is needed: the patient lies supine with the transducer placed directly over the iliac fossa, and an overlying surgical dressing or tender incision may require a light touch and copious gel. Because the superficial graft sits near the skin line, higher frequencies (5–9 MHz) are often possible, but the deep vascular anastomoses at the iliac vessels still need a lower-frequency curved probe, a low PRF, and careful angle correction at or below 60 degrees for velocity measurements.