The study population consisted of 127 subjects who were evaluated for kidney donation at Emory University Hospital between February 1998 and March 2001. Review of patient records was approved by the institutional review board. Preoperative imaging studies included ^99mTc MAG3 renography as a functional study; most subjects also underwent an anatomic study, either percutaneous contrast renal angiography, MR angiography, or CT angiography. Ten patients were excluded because the technologist entered a whole number—that is, 10.0 mCi (370.0 MBq)—as the dose injected; the dose has to be assayed in a dose calibrator and it is highly unlikely that exactly 10.0 mCi would be in the injection syringe. An incorrect dose entry invalidates the clearance measurement. Five more patients were excluded from analysis because data sets were missing; another four were excluded because of ^99mTc MAG3 dose infiltration exceeding 1%, four were excluded because the camera was started late, one was excluded because of an unsuspected renal mass, and one was excluded because of unexpected bilateral renal artery stenosis.

Of the remaining 106 potential renal donors, 54 had normal results on MR angiography, 32 had normal results on percutaneous angiography, and five had normal results on 3D CT. Anatomic data were not obtained or not available in the remaining 15 subjects. A creatinine clearance was obtained in 99 of the 106 subjects (mean = 133 ± 38 mL/min/1.73 m²). All but four subjects (two men and two women) had a normal 24-hour urinary creatinine clearance (normal range for our laboratory is 90-139 mL/min/1.73 m² for men and 80-125 mL/min/1.73 m² for women). The four subjects with a reduced creatinine clearance had serum creatinine values ranging from 0.9 to 1.1 mg/dL, which all fall within our normal laboratory range of 0.6-1.4 mg/dL; in addition, three had normal results on MR angiography and the fourth had normal results on 3D CT. Because of the normal serum creatinine values, absence of history of renal disease, and normal findings on anatomic studies, these patients were included in the data analysis. A creatinine clearance was not obtained in seven subjects; five of the same seven subjects had a normal serum creatinine level (mean, 1.04 mg/dL; range, 0.6-1.2 mg/dL), and in the remaining two subjects the serum creatinine level was not measured or not available.

These remaining 106 subjects constituted the study group. Forty-four men and 62 women were evaluated. The mean age of the subject population was 39.9 ± 10.8 (SD) years (mean age for men, 41.0 ± 11.9; for women, 39.1 ± 9.9 years). The mean body surface area (BSA) for men was 2.04 m² and for women was 1.80 m².

Each study was performed with 7-11 mCi (259-407 MBq) of ^99mTc MAG3 (Mertiatide, Mallinckrodt Medical). Radiochemical purity was 95.0% ± 2.7% (Sep-Pak Cartridge, Millipore).

The subjects were hydrated with approximately 500 mL of water 30 minutes before the study. Images were acquired in a 128 × 128 matrix with a 15-inch (38-cm) field-of-view gamma camera fitted with a low-energy all-purpose collimator. Each subject was imaged supine with the kidneys and bladder in the field of view. After the IV injection of ^99mTc MAG3, serial 2-seconds-per-frame digital images were obtained for the first 48 seconds followed by 15-seconds-per-frame images (n = 16) and 30-seconds-per-frame images (n = 40), for a total study duration of 24 minutes 48 seconds. Time 0 was defined as the 16-second interval required for the dose to reach the kidney [21].

At the end of the acquisition, one additional postvoid 2-minute image was obtained of the kidneys with the patient in the supine position, and 1-minute anterior prevoid and postvoid bladder images were also obtained to determine residual urine volume [20] and postvoid (30 minutes) over maximum (postvoid/max) count ratios. The data were processed using the QuantEM 2.0 software (developed at Emory University and licensed by Emory University to GE Healthcare), which was developed specifically for ^99mTc MAG3 renography [21, 22]. Processing details are summarized in the following text. The display of a representative study is shown in Figure 1.

All subjects received a dose of 7-11 mCi (259-407 MBq) of ^99mTc MAG3. Dead-time losses may be significant when counting larger doses, depending on the camera used. To avoid dead-time losses and determine the counts injected, a small syringe of approximately 1 mCi (37 MBq) was counted by placing it in a syringe holder 30 cm above the face of the camera. The small syringe and the syringe containing the dose to be injected were counted in a dose calibrator to yield the injected dose-to-small-syringe count ratio. The software multiplied the counts in the small syringe by the injected dose-to-small-syringe count ratio to obtain the counts in the injection syringe; the decay-corrected counts in the postinjection syringe were then subtracted from the counts in the injection syringe to yield the counts actually injected.

Infiltration was calculated by first drawing an ROI over the injection site at the conclusion of the study. Counts in the injection site ROI were corrected for decay and divided by dose injected to obtain a conservative estimate of the infiltrated dose. Four subjects were excluded from the study because of infiltration that exceeded 1% of the injected dose.

Whole-kidney ROIs were automatically assigned over each kidney using the 2- to 3-minute postinjection image and were modified by the operator as necessary. An automated cortical ROI was assigned using an algorithm to identify the area of the renal pelvis and calyces and then subtracting that area from the whole-kidney ROI to generate the cortical ROI. A 2-pixel-wide C-shaped perirenal background ROI was generated 1 pixel outside of the whole-kidney ROI. To calculate relative uptake, the counts per pixel in the C-shaped perirenal background ROI were normalized to the number of pixels in the whole-kidney ROI and subtracted from counts in the whole-kidney ROI to determine the background-corrected counts.

The background-corrected counts were then corrected for renal depth using published equations [23, 24] with weight in kilograms and height in centimeters:andand were subsequently corrected for attenuation [21, 22]. To generate the background-subtracted renogram curve, an automated C-shaped perirenal background ROI was assigned so that the background ROI would not overlap the ureter and would lead to an inappropriately high background correction when there was marked retention of activity in a ureter or enlarged renal pelvis. The counts per pixel in the C-shaped perirenal background ROI were normalized to the number of pixels in the kidney ROI and subtracted from counts in the whole-kidney and cortical ROIs to determine the background-corrected counts used to generate the renogram curve.

The clearance of ^99mTc MAG3 was measured in all subjects using a camera-based method without blood or urine sampling as previously described [21]. Briefly, the counts in the kidneys from 1 to 2.5 minutes after injection were corrected for background, renal depth, and attenuation and then were divided by the dose injected to calculate the percentage of injected dose in the kidneys at the 1- to 2.5-minute interval. The value was adjusted for BSA, and a regression equation was used to convert the BSA-adjusted percentage of dose in the kidney at 1-2.5 minutes to a MAG3 clearance [21].

The following parameters were generated from the ^99mTc MAG3 renograms: time to maximum counts (Tmax), time to half-peak counts (T½), the ratio of renal counts at 19-20 minutes to the maximum counts (20 min/max), and the ratio of counts at 19-20 minutes to the 2- to 3-minute counts (20 min/2-3 min). These parameters were generated for both whole-kidney and cortical or parenchymal ROIs; the terms “cortical” and “parenchymal” are used interchangeably and refer to an ROI over the renal cortex or parenchyma that excludes any activity in the collecting system (calyces or pelvis). Relative renal uptake was determined using whole-kidney ROIs and the 1- to 2.5-minute interval as described previously. The whole-kidney ROI was repositioned over the postvoid image of the kidney, and whole-kidney postvoid/prevoid and postvoid/maximum count ratios (postvoid/max) were generated. Finally, the camera-based MAG3 clearance (mL/min/1.73 m²) was calculated for each subject.

Postvoid urine determinations were available in 63 subjects; in the remaining subjects, the data were either not obtained or not recorded.

The mean, SD, range, and percentiles were used to describe the normal values. Two-way analysis of variance was used to determine whether a difference existed between sex and age groups (≤ 40 years vs > 40 years). No change was made. Adults 40 years old were included in the younger age group for purposes of analysis; hence, ≤ 40 years. A simple linear regression analysis was used to determine the association between MAG3 clearance and age. Statistical tests were performed at the 5% level of significance.

The MAG3 and creatinine clearances were normalized to 1.73 m². The mean BSA-corrected MAG3 and creatinine clearances were significantly higher in men than in women (p < 0.001); however, no decrease was seen in the MAG3 clearance with age for either men or women (Table 1). For adult men, the lower range of a normal MAG3 clearance defined by the fifth percentile was 238 mL/min/1.73 m² versus 226 mL/min/1.73 m² for women.

The relative uptake was 51% ± 4.0% for the left kidney and 49% ± 4.0% for the right kidney (Table 2). No significant difference was seen in relative uptake between men and women or between the two age groups.

Time to maximum counts (Tmax)—The Tmax for both kidneys was significantly greater in women than in men (p < 0.05) using whole-kidney ROIs, but no significant difference was seen between the sexes when a cortical ROI was used to calculate the Tmax; moreover, the Tmax for both the right and left kidneys using cortical ROIs was significantly less than the Tmax obtained with whole-kidney ROIs (p < 0.005) (Tables 3 and 4).

Time to half-maximum counts (T½)—The T½ was calculated from the time of the maximum counts to the time when the renogram curve decreased to half of the maximum counts. The T½ for the right kidney using whole-kidney ROIs was significantly higher in women than in men—8.29 minutes versus 5.64 minutes, respectively—but this difference was minimized when cortical ROIs were used (Tables 3 and 4). The mean values for cortical T½ were significantly less than the mean whole-kidney T½ values for both men and women (p < 0.01).

Twenty-minute/maximum count ratio (20 min/max)—The mean right cortical 20-min/max count ratio was 0.19 ± 0.07 for the right kidney and 0.19 ± 0.04 for the left kidney. Both were significantly less than the whole-kidney values (p < 0.001) (Tables 3 and 4). No significant difference was seen between men and women, and no significant change with age.

Twenty-minute/2-3 minute count ratio (20 min/2-3 min)—The mean right cortical 20 min/2-3 min ratio was 0.16 ± 0.07 for the right kidney and 0.15 ± 0.04 for the left kidney (Table 4). No significant difference was seen between sexes and no significant change with age.

Postvoid/maximum renal count ratio— The postvoid/max renal count ratio was determined using whole-kidney ROIs (Table 5). The mean left postvoid/max count ratio was 0.09 ± 0.03. No significant difference was seen between the right and left kidneys, nor was there a significant difference between men and women. The postvoid/max ratio increased slightly with age for the left kidney (p < 0.05), but the mean values for both age groups and both sexes were all < 0.1.

Postvoid/prevoid renal count ratio—The postvoid/prevoid renal count ratio was also determined using whole-kidney ROIs (Table 5). The mean left postvoid/prevoid count ratio was 0.59 ± 0.15, and the mean right postvoid/prevoid count ratio was 0.52 ± 0.19. No significant difference was seen between men and women. The postvoid/prevoid ratio increased slightly with age for the left kidney (p < 0.05).

No significant difference was seen in the voided volume between men and women. When the data were analyzed by age groups, no significant difference was seen in voided volume between men and women younger than or equal to 40 years old, but a significant difference was seen in voided volume between younger (≤ 40 years) and older (> 40 years) men (p < 0.01) (Table 6). No significant difference was seen in the residual urine volumes of men and women (Table 6). For women and men ≤ 40 years, the 95th percentile for residual bladder volume did not exceed 42 mL. Older men had a significantly higher residual volume than younger men (Table 6), possibly due to prostatic hypertrophy.