Radionuclides in nephrourology, Part 2: pitfalls and diagnostic applications

Abstract

Radionuclide renal scintigraphy provides important functional data to assist in the diagnosis and management of patients with a variety of suspected genitourinary tract problems, but the procedures are underutilized. Maximizing the utility of the available studies (as well as the perception of utility by referring physicians) requires a clear understanding of the clinical question, attention to quality control, acquisition of the essential elements necessary to produce an informed interpretation, and production of a report that presents a coherent impression based on data contained in the report and that specifically addresses the clinical question. To help achieve these goals, part 1 of this review addressed the available radiopharmaceuticals, quality control, and quantitative indices, including the measurement of absolute and relative renal function. Part 2 assumes familiarity with part 1 and focuses on the common clinical indications of suspected obstruction and renovascular hypertension; part 2 also summarizes the status of radionuclide renal imaging in the evaluation of the transplanted kidney and the detection of infection, discusses potential pitfalls, and concludes with suggestions for future research. The series of SAM questions accompanying parts 1 and 2 has been designed to reinforce and extend points made in the review. Although the primary focus is the adult patient, aspects of the review also apply to the pediatric population.

Keywords: MAG3; diuresis renography; diuretic renography; renal obstruction; renovascular hypertension.

FIGURE 1

A 77-y-old man with a complicated history had bilateral obstruction resulting in placement of bilateral nephrostomy tubes. Four years later, he was voiding normally as well as draining from nephrostomy tubes. ^99mTc-MAG3 scan was obtained to evaluate drainage. His serum creatinine was 2.0 mg/dL and had been stable for several years. (A) Baseline whole-kidney renogram (sequential 2-min ^99mTc-MAG3 images obtained for 24 min) shows prolonged time to peak activity bilaterally. Relative function was 53% on left and 47% on right. Camera-based ^99mTc-MAG3 clearance was 113 mL/min/1.73 m², compared with reference range of 238–433 mL/min/1.73 m². (B) Sequential 2-min images obtained after 40 mg of furosemide show pelvic retention. Time–activity curve shows minimal excretion; T½ measured 98 min for left kidney and 54 min for right kidney. (C) Even though appearance of images and curves was consistent with obstruction, patient was clearly not obstructed since nephrostomy tubes had not been clamped. By increasing image intensity on computer, percutaneous nephrostomy tubes and ureters could be visualized although percutaneous nephrostomy tubes had not been apparent on standard image display. (D) Patient returned for repeated study, and nephrostomy tubes were clamped. Sequential 2-min ^99mTc-MAG3 images were again obtained for 24 min; last image in sequence is postvoid image. Whole-kidney renogram (lower left panel) shows prolonged time to peak activity bilaterally. Relative function was 54% on left and 46% on right. Camera-based ^99mTc-MAG3 clearance was 84 mL/min/1.73 m². (E) Patient received 80 mg of furosemide because of inadequate response to 40 mg of furosemide on initial study. Sequential 2-min images obtained for 20 min after furosemide administration and time-activity curves demonstrate prompt washout of tracer from both kidneys; much of the activity in left kidney had drained on postvoid image (last image in sequence); T½ was 12 min for left kidney and 13 min for right kidney. Obstruction could be excluded, and nephrostomy tubes were pulled. Patient’s renal function has remained stable. (Reprinted with permission of (23))

FIGURE 2

A 57-y-old man with prostate adenocarcinoma had undergone radical cystoprostatectomy and urinary diversion with construction of neobladder from piece of large intestine. Abdominal CT performed 2 y after resection showed mild left-sided hydronephrosis. (A) Reno-gram after administration of 395.9 MBq (10.1 mCi) of ^99mTc-MAG3 showed persistent activity within left urinary collecting system; 40 mg of furosemide (Lasix; Sanofi-Aventis) were subsequently administered followed by 20-min acquisition, which showed minimal clearance from collecting system and abnormal T½ of 19 min for left kidney; however, poor drainage may have resulted from relatively high pressure within reservoir and ureteral reflux. (B) Baseline renogram was repeated with 355.6 MBq (9.61 mCi) of ^99mTc-MAG3 and again shows accumulation of activity within left collecting system; right kidney is not obstructed. To minimize possibility of reflux, Foley catheter was placed into ileostomy. Postfurosemide renogram showed complete emptying of left collecting system with half-life of 9.9 min, excluding obstruction. In adults with urinary diversion, incidence of reflux is relatively high. Given ease of placing indwelling catheter into ileostomy, protocol for diuretic renogram should include catheterization to maximize diagnostic accuracy. (Reprinted with permission of (25))

FIGURE 3

A 52-y-old hypertensive patient with normal level of serum creatinine underwent CT for possible incarcerated abdominal hernia. CT scan revealed heavily calcified right renal artery. Subsequent CT angiogram confirmed RAS, and ACE inhibition renal scan was requested to determine its functional significance. (A) Baseline scan was obtained after injection of 52 MBq (1.4 mCi) of ^99mTc-MAG3. Upper panel shows sequential 2-min ^99mTc-MAG3 images. Lower left panel shows whole-kidney renogram curves (blue, left kidney; green, right kidney); lower right panel shows cortical renogram curves. Relative uptake was 51 % (left) and 49% (right), with ^99mTc-MAG3 clearance of 295 mL/min/1.73 m². Images appear normal, with time to maximum counts (Tmax) and 20-min/maximum count ratios in reference range for both kidneys although there was asymmetry, with Tmax for right kidney (4.8 min) more delayed than Tmax for left kidney (1.8 min); in addition, 20-min/maximum ratio for right kidney, 0.30, was higher than that of left (0.19). (B) Patient received intravenous injection of 2.5 mg of enalaprilat followed 20 min later by second ^99mTc-MAG3 injection of 333 MBq (9.0 mCi). Left kidney was normal. Relative function was essentially unchanged, 49% on left and 51% on right, but sequential 2-min images showed marked parenchymal retention of tracer in right kidney, with correspondingly abnormal whole-kidney and cortical renogram curves. Marked change in right kidney from baseline to ACE study indicates high probability for renovascular hypertension. (Reprinted with permission of (81).)

FIGURE 4

Middle-aged man with refractory hypertension was referred for ACE inhibition renography. (A) Sequential 2-min images and whole-kidney renogram curves were obtained after baseline injection of approximately 37 MBq (1.0 mCi) of ^99mTc-MAG3. (B) Patient received 50 mg of captopril, and 370 MBq (10 mCi) of ^99mTc-MAG3 were injected 1 h later. Two-minute images demonstrated parenchymal retention, with bilateral rising whole-kidney renogram curves. Usually, renovascular hypertension produces asymmetric abnormalities. Further review showed that patient’s precaptopril blood pressure had been 165/71 but fell to 102/41 during the study, even though patient remained asymptomatic. (C) Several days later, study was repeated with 50 mg of captopril and intravenous hydration to maintain blood pressure. (D) Two-minute sequential images and renogram curves were normal. Bilateral symmetric abnormalities after ACE inhibition are a nonspecific finding and are often due to volume depletion and hypotension. (Reprinted with permission of (82).

FIGURE 5

Common renogram patterns used for visual interpretation of ACE inhibition renography: type 0, normal; type 1, time to peak (Tmax) of >5 min and 20-min/maximum count ratio of >0.3 for background-subtracted ¹³¹l-orthoiodohippurate and ^99mTc-MAG3 curves; type 2, more exaggerated delays in time to peak and in parenchymal washout; type 3, progressive parenchymal accumulation (no washout detected); type 4, renal failure pattern but with measurable renal uptake; type 5, renal failure pattern representing blood background activity only.