Early postoperative ultrasound of kidney transplants: evaluation of contrast medium dynamics using time-intensity curves.

OBJECTIVE: To evaluate kidney recipients in the early posttransplant phase by semiquantitative analysis of the arterial arrival of ultrasound (US) contrast medium using time-intensity curves. PATIENTS AND METHODS: Twenty-two kidney recipients underwent US examination after intravenous bolus administration of 2.4 ml of US contrast medium (SonoVue, Bracco Altana) 5 to 7 days after transplantation. The examinations were performed with the Aplio US system (Toshiba) and a 3.5-MHz wideband transducer using contrast harmonic imaging at a low mechanical index of 0.1. Arterial arrival was documented digitally over 60 sec (image repetition rate: 10 images per sec) for subsequent evaluation of contrast medium kinetics in the main renal artery, interlobar artery, subcapsular area, and renal vein using the system's integrated time-intensity curve (TIC) software. The increase, decrease, and percentage enhancement factor were calculated from the curves. Four patients were excluded from analysis because of perirenal hematoma (n = 3) or a polar perfusion loss demonstrated by power Doppler (n = 1). RESULTS: Twelve of the remaining 18 patients assigned to the nonrejection group showed an uneventful clinical course. These had uniform TICs with an early and steep increase of similar magnitude in the main renal artery (11.7 +/- 4.5 intensity units/sec), interlobar artery (8.7 +/- 4.6 intensity units/sec), and subcapsular area (8.3 +/- 3.7 intensity units/sec) followed by a washout and subsequent plateau phase. Six patients showed histologically proven acute rejection on day 5 or 6 after transplantation (rejection group). This group had a delayed (time to peak in the subcapsular area: 32.9 +/- 8.3 sec in the rejection group versus 20.9 +/- 4.7 sec in the nonrejection group, p < 0.05) and smaller subcapsular percentage increase (41.2 +/- 21.9 % versus 114.4 +/- 59.8 %, p < 0.05). In the rejection group the subcapsular area (3.8 +/- 2.3 intensity units/sec) showed a less pronounced increase than the main renal artery (7.9 +/- 5.9 intensity units/sec) and interlobar artery (8.7 +/- 3.8 intensity units/sec). The RI in the rejection group was in the normal range at the time of contrast-enhanced US (day 5: 0.78 +/- 0.06) and increased to abnormal levels in the further course (day 7: 0.94 +/- 0.09). CONCLUSIONS: Quantitative determination of arterial arrival of an US contrast medium in the early phase after kidney transplantation is possible. This new US procedure might identify acute rejection earlier than conventional techniques.