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Department of Nuclear Medicine, Albert Einstein College of Medicine, Bronx, New York
| ABSTRACT |
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Key Words: transit time captopril renography renovascular hypertension renal artery stenosis
| INTRODUCTION |
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| MATERIALS AND METHODS |
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A second group, comprised of 84 subjects with more severe hypertension, had a high prevalence of renal artery stenosis (RAS) (group II). After exclusions, 60 of these subjects underwent a complete study, including baseline and captopril-stimulated renography, the in vitro captopril test, and arteriography.
Methods
Radionuclide renography was done supine, gamma camera underneath, with simultaneously administered 99mTcdiethylenediamine pentaacetic acid (DTPA) and 131I-orthoiodohippurate (OIH). After hydration, 185 MBq (5 mCi) 99mTc-DTPA and 5.5 MBq (150 µCi) 131I-OIH were given intravenously for the baseline study. One hour after the baseline injection, 25 mg captopril were given orally. Sixty minutes after administration of captopril, 370 MBq (10 mCi) 99mTc-DTPA and 11.1 MBq (300 µCi) 131I-OIH were injected. A medium-energy collimator on a model 409AT digital gamma camera (Elscint Ltd., Haifa, Israel) was used to acquire both the 140-keV 99mTc photopeak and the 364-keV 131I
peak. Dual isotope acquisition was performed at 3-s intervals for 8 min and at 30-s intervals for 22 min with a 64 x 64 pixel matrix. Kidneys and heart were included within the field of view.
Limits on injected activity of OIH necessarily restricted our analysis of MTT to data derived from 99mTc-DTPA injections. Within the 140-keV channel, downscatter from the 131I emissions constituted <3% of the total count rate.
MTTs of whole kidney and cortex were measured using a matrix method of deconvolution (911). The input function for deconvolution analysis was generated from the left ventricular region of interest (ROI), and the output function was the renogram curve derived from either a whole kidney or renal cortical ROI. Parenchymal MTT was calculated to reduce the effect of renal pelvic urinary retention.
Quantitative and Statistical Analysis
Abnormal values for parenchymal MTT were established using methods similar to those reported previously (2,8). Using group I subjects (i.e., those without RVH), mean values and the SDs of parenchymal MTT were determined for baseline and post-captopril studies as well as for the difference between post- and pre-captopril parenchymal MTT. Because an increase in parenchymal MTT after captopril is expected in studies positive for RVH, 1-tailed limits for change were used, which required an increase of 1.64 SDs to define a change with 95% confidence and an increase of 1.28 SDs for 90% confidence. Abnormal was defined to lie in excess of these upper limits.
The Wilcoxon signed rank test was used for comparison of pre- versus post-captopril paired data. Pooled data for group II versus group I subjects were compared using Mann-Whitney U testing. Contingency tables of true-positive, true-negative, false-positive, and false-negative results were evaluated by
2 analysis. Comparison between contingency tables was performed by calculating
2 heterogeneity.
The subject of this report is the quantitative statistical analysis of parenchymal MTT in group II subjects, using group I parenchymal MTT values to define the normal range. The statistical analysis of qualitative renography and other quantitative renographic parameters in group I and group II subjects has been reported (2).
| RESULTS |
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20% of group I subjects have abnormal, qualitatively observed, pelvic retention (PR) after captopril administration (12). In this subgroup of subjects (n = 17), the whole kidney MTT is prolonged (5.010 ± 0.66) in comparison with subjects (n = 55) without PR (4.177 ± 0.93) (P < 0.002). In contrast, no significant difference in cortical (parenchymal) MTT is observed after captopril in the 17 kidneys with PR (3.715 ± 0.50) compared with the 55 kidneys without PR (3.381 ± 0.68) (P > 0.05). As a result, all subsequent MTTs are understood to represent data from cortical ROIs. The parenchymal MTT among all baseline group I kidneys (n = 80 kidneys for 40 evaluable subjects) is 2.79 ± 0.70 and for all post-captopril group I kidneys is 3.44 ± 0.68 (P > 0.05) (Table 2).
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Among group II subjects with normal bilateral angiograms (n = 19), 3 were excluded because of blood background curves, and 1 was excluded for technical reasons.
Therefore, renograms were available for quantitative parenchymal MTT in 22 subjects with positive angiograms, of whom 11 had bilateral disease and 11 had unilateral disease, for a total of 33 kidneys with positive angiograms.
Similarly, the 11 normal kidneys from subjects with unilateral disease plus the 15 subjects with bilateral negative angiograms (consisting of 30 kidneys) resulted in a total of 41 kidneys with negative angiograms.
Quantitative Parenchymal MTT Evaluation
Neither baseline MTT nor post-captopril MTT or the change in MTT differed significantly (P > 0.05) between group I and group II subjects (Table 2).
In comparing baseline parenchymal MTT, division of group II subjects into those with positive or negative angiograms or with unilateral or bilateral disease (or both) did not disclose any significant subgroup distinctions. Statistically significant differences were not found for post-captopril parenchymal MTT or the change in parenchymal MTT among these subgroups (not shown).
Establishment of Criteria for Abnormal Parenchymal MTT
Upper thresholds for baseline parenchymal MTT and the change in parenchymal MTT were established from the group I data (Table 2). The 90% confidence limit for the upper normal range (3.69 min) was calculated as lying within 1.28 SDs of the group I mean. Similarly, 1.64 SDs above the group I mean resulted in an upper 95% confidence limit of 3.94 min. A more sensitive threshold of 3.00 min was chosen arbitrarily, at a confidence limit of <50%.
Thresholds were calculated similarly for the change in parenchymal MTT after captopril administration at 1.49, 1.73, and 0.85 min for the 90%, 95%, and <50% thresholds, respectively.
Identification of Renovascular Disease Using Abnormal Parenchymal MTT Versus Angiographic Standard
The quantitative criteria were applied to all interpretable group II renograms to define each as positive or negative for RAS. The accuracy, sensitivity, and specificity of an abnormal change in parenchymal MTT versus angiography are displayed in Table 3. The most sensitive threshold (0.85 min) resulted in an accuracy per kidney of 61% (45 true diagnoses of 74 angiograms), a sensitivity of 27% (9 true-positive studies of 45 positive angiograms), and a specificity of 88% (36 true-negative studies of 41 negative angiograms). The least sensitive (most specific) threshold (1.73 min) resulted in an accuracy of 55%, a sensitivity of 12%, and a specificity of 90%. No statistically significant difference was found in diagnostic performance among the thresholds.
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The diagnostic performance of an abnormal baseline parenchymal MTT versus angiography for the most sensitive threshold (3.00 min) resulted in a 54% accuracy (among kidneys), a sensitivity of 55%, and a specificity of 37%. The least sensitive (most specific) threshold (3.94 min) resulted in an accuracy of 58%, a sensitivity of 27%, and a specificity of 83%. None of the contingency tables of varying thresholds was statistically different from each other by evaluation of
2 heterogeneity (P > 0.05). The accuracies for these thresholds when analyzed by patient were 59% (22/37), 54% (20/37), and 54% (20/37), respectively.
Using the change in parenchymal MTT (
MTT), the total number of correct patient studies among those with calculable MTT (true-positive + true-negative) versus incorrect studies (false-positive + false-negative) shows a clear trend toward decreased accuracy with decreasing renal function (Fig. 1). For glomerular filtration rate (GFR) <50 mL/min, no correct studies were identified for 7 subjects, whereas 14 of 19 studies were correct for GFR >80 mL/min and 4 of 7 studies were correct for GFR in the intermediate range (5080 mL/min). A similar relationship was observed using baseline parenchymal MTT to evaluate for RAS (not shown).
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| DISCUSSION |
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There are theoretic advantages of parenchymal MTT compared with other renographically derived measures of renal parenchymal retention such as 20/Pk, 30/Pk, and Tpk. Technical problems resulting from poor intravenous injections may cause delays in the latter 3 measures that would falsely be considered delays in transit or parenchymal retention. Parenchymal MTT, because it calibrates an intravenous injection against the cardiac arterial input function, would be expected to reduce these kinds of false-positive studies. Alternatively, parenchymal delay may be detected by prolongation only of Tpk or only of 20/Pk (or 30/Pk) on renogram curves. Parenchymal MTT, which, more inclusively, mathematically integrates all kinds of parenchymal retention, might be expected to detect either of these kinds of parenchymal transit delays and, therefore, should demonstrate more true-positive studies than either Tpk or 20/Pk alone. Improvements in both true-positive rates and false-positive rates should be reflected by improved accuracy.
Despite these theoretic advantages of parenchymal MTT, we were unable to show a clinical advantage of parenchymal MTT compared with the more widely used renographic parameters. The range of diagnostic accuracy in the present study was 46%49% for patients and 55%61% for kidneys, using various thresholds of change in parenchymal MTT as the criteria for a positive study. Similarly, the range of accuracy was 54%59% for patients and 54%58% for kidneys, using baseline parenchymal MTT for the diagnostic criterion. These values were not statistically different from each other or from our previously reported values using the more widely used renographic parameters in the same subjects. Furthermore, the parenchymal MTT tended toward more false-positive studies than the other diagnostic criteria we reported previously (2). Our results agree with those of Russell et al. (4), which indicate no diagnostic advantage of parenchymal MTT for captopril renography compared with Tpk, 20/Pk, or 30/Pk.
Gruenwald et al. (13,14), using methods similar to ours to determine MTT (10,11), report a sensitivity of 78% using parenchymal MTT for the prediction of RVH from clinical follow-up data of angioplasty in 31 patients with documented RAS (14). Because all patients studied had RAS, the selection bias makes it impossible to determine test specificity. Similarly, Rutland and Stuart (15) also report only on patients with documented RAS. These studies do not address the clinical problem of attempting to diagnose RVH from among patients with a variety of clinical presentations.
It is possible that the high proportion of subjects with renal dysfunction may be partially responsible for our results. Figure 1 showed decreasing accuracy of the change of parenchymal MTT with declining GFR. Parenchymal
MTT correctly predicted RAS in 14 of 19 subjects with GFR >80 mL/min but in 0 of 7 subjects with GFR <50 mL/min and a total of 4 of 14 with GFR <80 mL/min. Previously, we reported a similar decline in diagnostic accuracy of 20/Pk and Tpk with decreasing GFR (2). An additional 18 group II subjects were excluded from analysis because of renal dysfunction severe enough to cause the renogram to resemble a blood-pool disappearance curve. Therefore, among 32 subjects with GFR <80 mL/min, only 4 had correctly interpretable studies.
The statistical uncertainties and mathematic artifacts that arise when transit time calculations are performed also may compound the poor results in our subjects. These kinds of errors can be magnified in subjects with renal dysfunction. The matrix method of parenchymal MTT calculation produces reproducible results in individuals with normal renal function (10). It is less clear that precision is as good in subjects with renal dysfunction (11,13), even with 185 and 370 MBq (5 and 10 mCi) of injected activities of DTPA. Furthermore, the mathematic requirements for accurate deconvolution of renograms with high background activity and incomplete transit, as found in many subjects with renal insufficiency, are poorly defined (10). Our method of deconvolution has been shown to improve on the noise propagation usually found in matrix methods (10,11,13). We were able to analyze the parenchymal MTT in subjects with renal dysfunction more reproducibly with our modification (10) of Diffey et al. (11) than with the unmodified technique.
Following Whitfield et al. (16), Russell et al. (17) found the dispersion of intrarenal transit times to be a better discriminant of RAS than the parenchymal MTT itself. We have not evaluated the dispersion of parenchymal MTT in our data, but it would be important to confirm the results of Russell et al. in subjects with renal dysfunction.
We were unable to perform transit time analysis with a tubular agent because of the necessary limitation on injected activity of simultaneously administered 131I-OIH (5). It is possible that results would differ if 99mTc-labeled mercaptoacetyltriglycine were used.
In our subjects, parenchymal MTT without captopril stimulation had no greater diagnostic usefulness than other reported quantitative or qualitative measures of parenchymal transit. The addition of captopril stimulation caused no improvement.
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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For correspondence or reprints contact: Eugene J. Fine, MD, Department of Nuclear Medicine, Jacobi Medical Center, BN13, Pelham Pkwy. and Eastchester Rd., Bronx, NY 10461.
| REFERENCES |
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This article has been cited by other articles:
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K. E. Britton, E. J. Fine, M. D. Blaufox, and Y. Li Parenchymal Mean Transit Time Analysis J. Nucl. Med., September 1, 2001; 42(9): 1439 - 1440. [Full Text] [PDF] |
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