(Radiology. 2000;217:813-818.)
© RSNA, 2000
Captopril MR Renography in a Swine Model: Toward a Comprehensive Evaluation of Renal Arterial Stenosis1
Pottumarthi V. Prasad, PhD,
James Goldfarb, MS,
Chandru Sundaram, MD,
Agus Priatna, PhD,
Wei Li, MD and
Robert R. Edelman, MD
1 From the Departments of Radiology (P.V.P., J.G., A.P., W.L., R.R.E.) and Surgery, (C.S.), Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Mass. From the 1998 RSNA scientific assembly. Received September 13, 1999; revision requested October 26; final revision received March 3, 2000; accepted March 13. P.V.P. supported in part by a grant-in-aid from the American Heart Association and National Institutes of Health grant R01 DK53221. R.R.E. supported in part by National Institutes of Health grant R01 DK48769-01A1. Address correspondence to P.V.P., Department of Radiology, MRI, Evanston Northwestern Healthcare, 2650 Ridge Ave, Evanston, IL 60201 (e-mail: pprasad@enh.org).
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ABSTRACT
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PURPOSE: To test the feasibility of captopril magnetic resonance (MR) renography and to validate the technique in an animal model of renal arterial stenosis.
MATERIALS AND METHODS: Seven pigs with induced renal arterial stenosis were studied. MR renography was performed with a T1-weighted approach by using three-dimensional fast imaging with steady-state precession, or FISP, sequences after administration of a bolus of 0.1 mmol of gadopentetate dimeglumine per kilogram of body weight. Captopril was administered to improve the specificity.
RESULTS: The results demonstrate that differences in renographic curves and indices are observed only if an anatomically substantial stenosis, typically a diameter reduction of more than 70%, is present and captopril is administered.
CONCLUSION: In this preliminary experience in an animal model, captopril MR renography provided data consistent with expectations based on conventional renographic results.
Index terms: Animals • Captopril, 961.12944 • Hypertension, renovascular, 961.723 • Magnetic resonance (MR), experimental studies, 961.12944 • Magnetic resonance (MR), phase-contrast imaging, 961.12944 • Magnetic resonance (MR), vascular studies, 961.12944 • Renal arteries, flow dynamics, 961.91 • Renal arteries, MR, 961.721 • Renal arteries, stenosis or obstruction, 961.721
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INTRODUCTION
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Hypertension is a significant health problem that affects up to 60 million people in the United States (1). In an estimated 5% of cases of hypertension, the cause is stenosis of a renal artery (2). Because a stenosis can be corrected through balloon angioplasty, stent placement, or surgery, it is important to be able to identify patients eligible for this treatment. However, no cost-effective screening method with low risk currently exists.
The currently accepted reference standard for detection of renal arterial stenosis is x-ray angiography, which is invasive and involves injection of iodinated contrast material and exposure to ionizing radiation. Moreover, results of this test do not indicate the hemodynamic significance of the stenosis. Conventional techniques to assess hemodynamic significance include intravenous pyelography and peripheral renin. Newer diagnostic examinations for renal arterial stenosis include intravenous digital subtraction angiography for anatomy, duplex Doppler ultrasonography for blood flow, captopril renography for renal function, intraarterial and computed tomographic (CT) angiography for anatomy, and magnetic resonance (MR) imaging for anatomy along with potential blood flow and renal function. The best imaging method for this important diagnosis remains unknown to date.
For arteriography in the abdomen and thorax, dynamic contrast material–enhanced MR angiography is proving to be a useful clinical test for the diagnosis of vascular disorders (3). Although the technique has been shown to be robust and provides three-dimensional angiograms, its accuracy in the estimation of the level of stenosis is not yet established. Phase-contrast MR angiography has been suggested for estimating qualitative hemodynamic significance (4). Alternatively, flow profiles derived from quantitative phase-contrast MR images are being used to grade the hemodynamic significance of renal arterial stenosis (5). It was previously hypothesized that renal perfusion imaging could provide the hemodynamic significance of renal arterial stenosis, and it demonstrated some preliminary evidence (6). However, because of variable capacity of the kidneys to accommodate the presence of a stenosis, conflicting outcomes were obtained with two different perfusion imaging techniques (6,7).
Captopril renography in nuclear medicine is considered the best predictor of blood pressure changes after surgical correction of renal arterial stenosis (8). This technique is based on the pharmacologic inhibition of the renin-angiotensin response, which plays a major role in the manifestation of renovascular hypertension. The glomerular filtration rate in a kidney with partial obstruction is reduced by means of administering angiotensin-converting enzyme inhibitor, despite the preservation of renal plasma flow; meanwhile, the glomerular filtration rate in the contralateral kidney is maintained. A potential response to the administration of captopril enables prediction of the outcome of revascularization. Because captopril renography still lacks the ability to depict the anatomic stenosis, conventional, CT, or MR angiography is nevertheless performed. If captopril renography were feasible by using MR imaging (ie, captopril MR renography), it could, in combination with MR angiography, provide a more complete evaluation of renal arterial stenosis.
MR renography in principle was known to be feasible even when gadopentetate dimeglumine was first introduced as an MR imaging contrast material (9). This idea was based on the simple fact that gadopentetate dimeglumine is similar to technetium 99m diethylenetriaminepentaacetic acid, the tracer commonly used for nuclear medicine renal imaging. Many investigators (10–14) pursued MR renography. However, owing to the limitations of the MR acquisition parameters, T1 and T2 effects could not be separated and made interpretation qualitative. With recent advances in MR imaging gradient hardware, short echo times on the order of 1 msec are possible with gradient-echo sequences, which makes them predominantly T1 weighted and forms the basis for contrast-enhanced MR angiography. We have previously demonstrated the same sequences could be useful for imaging the ureteral system (15), without any evidence for T2* effects. Herein we will show that these sequences could be used to perform MR renography, which provides data that can be interpreted in a similar manner to the way those obtained by means of the nuclear medicine counterpart are interpreted.
The purpose of this study was to evaluate the feasibility of performing captopril MR renography in concert with angiotensin-converting enzyme inhibition to assess the hemodynamic significance of renal arterial stenosis in a controlled animal model.
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MATERIALS AND METHODS
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Animal Model
Ameroid constrictors (6) (Research Instruments, Corvallis, Ore) were placed surgically around the left renal arteries of seven female Yorkshire pigs (50–70 lb; Team Associates, Canterbury, Conn) that were then studied during the next 6 weeks. The ameroid plastic is hydrophylic and swells when exposed to moisture. When the outward swelling is restricted, the inner diameter progressively reduces over about 4–6 weeks. For anatomic stenosis characterization, selective arteriograms were obtained prior to surgery and every MR imaging study. The diameter reductions were measured after digitizing the angiograms and measuring the full width at half maximum of the signal intensity profiles drawn across the vessels in the region of interest (P.V.P.). The contralateral renal artery was used as a control. For absolute diameter measurement calibration, we measured the diameter of the catheter used to administer contrast material. All the animal studies were performed with prior approval from the institutional animal care and use committee.
MR Imaging Methods
All studies were performed with a 1.5-T whole-body imager (Vision; Siemens Medical Systems, Erlangen, Germany) by using a standard four-element phased-array body coil. MR renography was performed with a T1-weighted approach by using a three-dimensional fast imaging with steady-state precession sequence (3.5/1.4 [repetition time msec/echo time msec]; flip angle, 20°) to image 10 partitions within a single breath hold of about 5 seconds or less. Other relevant parameters include a bandwidth of 890 Hz/pixel, one signal acquired, section thickness of 6–8 mm, field of view of 300 mm, and matrix size of 128 x 256. No fat saturation or suppression was used. A 0.1-mmol bolus of gadopentetate dimeglumine (Magnevist; Berlex, Wayne, NJ) was administered per kilogram of body weight.
Group 1.—In four animals, MR imaging was performed 3 and 6 weeks after the placement of the constrictors around the left renal artery. We administered 50 mg of captopril (Capoten; Bristol-Meyers, New York, NY) intravenously 1 hour before MR imaging. MR renography was performed before contrast enhancement; 30 seconds after contrast enhancement, which represented the perfusion phase; and every minute starting 1–20 minutes after contrast enhancement. One animal developed complete obstruction by week 3, so no MR renography was performed.
Group 2.—In the other three animals, MR imaging was performed twice during week 6—once with and the other time without administration of captopril. The perfusion phase was monitored by means of a two-dimensional fast low-angle shot, or FLASH, sequence (3/0.9; flip angle, 25°) that was used to obtain a single 10-mm coronal section every 0.5 second. Other relevant parameters include a bandwidth of 976 Hz/pixel, field of view of 350 mm, matrix size of 128 x 128, and one signal acquired. MR renography data was performed starting 2 minutes after contrast enhancement at a rate of one image every minute for 20 minutes.
The time versus signal intensity curves for MR renography performed in each animal were obtained by one author (A.P.) using region-of-interest analysis (AVS; Advanced Visual Systems, Waltham, Mass). A single region of interest covering the entire renal parenchyma was defined by segmentation performed to separate renal parenchyma from the collecting system on the basis of signal intensity differences. Parenchymal signal intensity was measured as the mean signal intensity: summation of signal intensity of nonzero pixels divided by the number of nonzero pixels within a region defined by the operator to include each entire kidney. Data from all the sections containing the kidneys were summed. No specific procedures to include or exclude intrarenal fat were used.
To facilitate semiquantitative or quantitative comparisons of data obtained in different animals, we defined the renographic index as the ratio of signal intensity in the renal parenchyma at 20 minutes to the peak signal intensity and expressed as a percentage. For perfusion images in the second set of animals, the mean time versus signal intensity curves were based on regions of interest containing at least 25 pixels drawn on the renal parenchyma (P.V.P.) by using the tools available on the MR imager.
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RESULTS
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Tables 1 (group 1) and 2 (group 2) summarize the data obtained in all seven animals. Figures 1 and 2 show data obtained in pig 3, representative of group 1. Angiography showed the differences in the level of stenosis between weeks 3 and 6 (Table 1). MR renography performed after captopril administration clearly showed differences between weeks 3 and 6. During week 3, there was minimal asymmetry between the left and right kidneys. However, in the same animal, during week 6 there was a substantial difference between the kidneys. The kidney supplied by the stenosed vessel (left) illustrated little washout of contrast material. In Figures 1 and 2, renographic indices are shown along with the time versus signal intensity plots; when there was continuous accumulation of contrast material, the peak was taken to be the 20-minute time point, so the renographic index value was 1.00.
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Figure 1a. Group 1. Pig 3. Week 3.
(a) Anteroposterior angiogram (far left image) obtained 3 weeks after surgical placement of the constrictor shows the stenosed vessel. Note the moderate degree of stenosis (arrow). Coronal MR renograms (three right images) obtained with a T1-weighted approach by using three-dimensional fast imaging with steady-state precession (3.5/1.4; flip angle, 20°) to image 10 partitions within a single breath hold of about 5 seconds or less show one representative section at three representative time points after captopril administration. Prior to administration of gadopentetate dimeglumine (left middle image), the kidneys have little signal intensity. The right middle image obtained 30 seconds after gadopentetate dimeglumine administration shows perfusion or an uptake phase. The single-section images (three right images) show small but perceptible left-to-right differences in signal intensity. However, when combined signal intensities from all sections were included, as in b, the uptake and washout curves from both kidneys were comparable. At 16 minutes after gadopentetate dimeglumine administration (far right image), both kidneys show a decrease in signal intensity, which signifies symmetric washout or excretion. (b) Signal intensity (arbitrary units) versus time curves illustrate symmetric uptake and washout phases. Note the renographic indices (RI) included in the plot. This figure illustrates that even when captopril was administered, no substantial left-to-right differences in washout of contrast material were apparent when the level of stenosis was not hemodynamically significant (ie, no stimulation of renin-angiotensin response). RAS = kidney with renal arterial stenosis.
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Figure 1b. Group 1. Pig 3. Week 3.
(a) Anteroposterior angiogram (far left image) obtained 3 weeks after surgical placement of the constrictor shows the stenosed vessel. Note the moderate degree of stenosis (arrow). Coronal MR renograms (three right images) obtained with a T1-weighted approach by using three-dimensional fast imaging with steady-state precession (3.5/1.4; flip angle, 20°) to image 10 partitions within a single breath hold of about 5 seconds or less show one representative section at three representative time points after captopril administration. Prior to administration of gadopentetate dimeglumine (left middle image), the kidneys have little signal intensity. The right middle image obtained 30 seconds after gadopentetate dimeglumine administration shows perfusion or an uptake phase. The single-section images (three right images) show small but perceptible left-to-right differences in signal intensity. However, when combined signal intensities from all sections were included, as in b, the uptake and washout curves from both kidneys were comparable. At 16 minutes after gadopentetate dimeglumine administration (far right image), both kidneys show a decrease in signal intensity, which signifies symmetric washout or excretion. (b) Signal intensity (arbitrary units) versus time curves illustrate symmetric uptake and washout phases. Note the renographic indices (RI) included in the plot. This figure illustrates that even when captopril was administered, no substantial left-to-right differences in washout of contrast material were apparent when the level of stenosis was not hemodynamically significant (ie, no stimulation of renin-angiotensin response). RAS = kidney with renal arterial stenosis.
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Figure 2a. Group 1. Pig 3. Week 6.
(a) Anteroposterior angiogram (far left image) of the stenosed vessel (solid arrow) obtained 6 weeks after surgical placement of the constrictor in the same animal as in Note the increased severity in degree of stenosis as evidenced by poststenotic dilatation (dotted arrow). Coronal MR renograms (three right images) obtained with a T1-weighted approach by using three-dimensional fast imaging with steady-state precession (3.5/1.4; flip angle, 20°) to image 10 partitions within a single breath hold of about 5 seconds or less show one representative section at three representative time points after captopril administration. Prior to administration of gadopentetate dimeglumine (left middle image), the kidneys have little signal intensity. The right middle image obtained 30 seconds after gadopentetate dimeglumine administration shows perfusion or an uptake phase. Note the similar uptake in both kidneys. At 16 minutes after gadopentetate dimeglumine administration (far right image), note the substantial accumulation of contrast material in the kidney supplied by the stenosed vessel. (b) Signal intensity (arbitrary units) versus time curves illustrate asymmetry in the washout phase. Note the renographic indices (RI) included in the plot. This figure clearly demonstrates that in the presence of hemodynamically significant stenosis and when captopril is administered, substantially reduced washout of contrast material from the kidney supplied by the stenosed vessel is observed. RAS = kidney with renal arterial stenosis.
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Figure 2b. Group 1. Pig 3. Week 6.
(a) Anteroposterior angiogram (far left image) of the stenosed vessel (solid arrow) obtained 6 weeks after surgical placement of the constrictor in the same animal as in Note the increased severity in degree of stenosis as evidenced by poststenotic dilatation (dotted arrow). Coronal MR renograms (three right images) obtained with a T1-weighted approach by using three-dimensional fast imaging with steady-state precession (3.5/1.4; flip angle, 20°) to image 10 partitions within a single breath hold of about 5 seconds or less show one representative section at three representative time points after captopril administration. Prior to administration of gadopentetate dimeglumine (left middle image), the kidneys have little signal intensity. The right middle image obtained 30 seconds after gadopentetate dimeglumine administration shows perfusion or an uptake phase. Note the similar uptake in both kidneys. At 16 minutes after gadopentetate dimeglumine administration (far right image), note the substantial accumulation of contrast material in the kidney supplied by the stenosed vessel. (b) Signal intensity (arbitrary units) versus time curves illustrate asymmetry in the washout phase. Note the renographic indices (RI) included in the plot. This figure clearly demonstrates that in the presence of hemodynamically significant stenosis and when captopril is administered, substantially reduced washout of contrast material from the kidney supplied by the stenosed vessel is observed. RAS = kidney with renal arterial stenosis.
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On the other hand, Figures 3 and 4 illustrate the differences between MR renography performed without captopril and MR renography performed with captopril during week 6 in pig 5, when the stenosis typically was more than 70% diameter reduction (Table 2). Perfusion images show no left-to-right differences whether or not captopril was administered. However, the MR renograms clearly illustrate enhanced left-to-right differences when captopril was administered.
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Figure 3a. Group 2. Pig 5.
Images and data obtained without the use of captopril. (a) Coronal first-pass perfusion MR images obtained 6 weeks after surgical placement of the constrictor show 16 representative time points. The perfusion phase was monitored with a two-dimensional fast low-angle shot sequence (3/0.9; flip angle, 25°) to acquire a single 10-mm section every 0.5 second. Note the symmetric uptake of the contrast material. (b) Coronal MR renograms of one representative section of 10 at nine representative time points. The images were obtained with three-dimensional fast imaging with steady- state precession (3.5/1.4; flip angle, 20°). Note the symmetric washout of the contrast material in both kidneys. (c) Signal intensity (arbitrary units) versus time curves illustrate symmetric uptake and washout phases. Note the apparent inconsistency in the signal intensities in the uptake and washout phases owing to use of different sequences to image each phase. Note the renographic indices (RI) included in the plot. RAS = kidney with renal arterial stenosis. (Reprinted, with permission, from reference 16.)
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Figure 3b. Group 2. Pig 5.
Images and data obtained without the use of captopril. (a) Coronal first-pass perfusion MR images obtained 6 weeks after surgical placement of the constrictor show 16 representative time points. The perfusion phase was monitored with a two-dimensional fast low-angle shot sequence (3/0.9; flip angle, 25°) to acquire a single 10-mm section every 0.5 second. Note the symmetric uptake of the contrast material. (b) Coronal MR renograms of one representative section of 10 at nine representative time points. The images were obtained with three-dimensional fast imaging with steady- state precession (3.5/1.4; flip angle, 20°). Note the symmetric washout of the contrast material in both kidneys. (c) Signal intensity (arbitrary units) versus time curves illustrate symmetric uptake and washout phases. Note the apparent inconsistency in the signal intensities in the uptake and washout phases owing to use of different sequences to image each phase. Note the renographic indices (RI) included in the plot. RAS = kidney with renal arterial stenosis. (Reprinted, with permission, from reference 16.)
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Figure 3c. Group 2. Pig 5.
Images and data obtained without the use of captopril. (a) Coronal first-pass perfusion MR images obtained 6 weeks after surgical placement of the constrictor show 16 representative time points. The perfusion phase was monitored with a two-dimensional fast low-angle shot sequence (3/0.9; flip angle, 25°) to acquire a single 10-mm section every 0.5 second. Note the symmetric uptake of the contrast material. (b) Coronal MR renograms of one representative section of 10 at nine representative time points. The images were obtained with three-dimensional fast imaging with steady- state precession (3.5/1.4; flip angle, 20°). Note the symmetric washout of the contrast material in both kidneys. (c) Signal intensity (arbitrary units) versus time curves illustrate symmetric uptake and washout phases. Note the apparent inconsistency in the signal intensities in the uptake and washout phases owing to use of different sequences to image each phase. Note the renographic indices (RI) included in the plot. RAS = kidney with renal arterial stenosis. (Reprinted, with permission, from reference 16.)
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Figure 4a. Group 2. Pig 5.
Images and data obtained with the use of captopril. (a) Coronal first-pass perfusion MR images obtained 6 weeks after surgical placement of the constrictor in the same animal as in Figure 3 show 16 representative time points. The perfusion phase was monitored with a two-dimensional fast low-angle shot sequence (3/0.9; flip angle, 25°) to acquire a single 10-mm section every 0.5 second. Note the symmetric uptake of the contrast material. (b) Coronal MR renograms of one representative section at nine representative time points. The images were obtained with three-dimensional fast imaging with steady-state precession (3.5/1.4; flip angle, 20°). Note the retention of contrast material in the affected kidney. (c) Signal intensity (arbitrary units) versus time curves illustrate symmetric uptake but asymmetric washout phases. Note the apparent inconsistency in the signal intensities in the uptake and washout phases owing to the use of different sequences to image each phase. Note the renographic indices (RI) included in the plot. RAS = kidney with renal arterial stenosis. (Reprinted, with permission from reference 16).
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Figure 4b. Group 2. Pig 5.
Images and data obtained with the use of captopril. (a) Coronal first-pass perfusion MR images obtained 6 weeks after surgical placement of the constrictor in the same animal as in Figure 3 show 16 representative time points. The perfusion phase was monitored with a two-dimensional fast low-angle shot sequence (3/0.9; flip angle, 25°) to acquire a single 10-mm section every 0.5 second. Note the symmetric uptake of the contrast material. (b) Coronal MR renograms of one representative section at nine representative time points. The images were obtained with three-dimensional fast imaging with steady-state precession (3.5/1.4; flip angle, 20°). Note the retention of contrast material in the affected kidney. (c) Signal intensity (arbitrary units) versus time curves illustrate symmetric uptake but asymmetric washout phases. Note the apparent inconsistency in the signal intensities in the uptake and washout phases owing to the use of different sequences to image each phase. Note the renographic indices (RI) included in the plot. RAS = kidney with renal arterial stenosis. (Reprinted, with permission from reference 16).
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Figure 4c. Group 2. Pig 5.
Images and data obtained with the use of captopril. (a) Coronal first-pass perfusion MR images obtained 6 weeks after surgical placement of the constrictor in the same animal as in Figure 3 show 16 representative time points. The perfusion phase was monitored with a two-dimensional fast low-angle shot sequence (3/0.9; flip angle, 25°) to acquire a single 10-mm section every 0.5 second. Note the symmetric uptake of the contrast material. (b) Coronal MR renograms of one representative section at nine representative time points. The images were obtained with three-dimensional fast imaging with steady-state precession (3.5/1.4; flip angle, 20°). Note the retention of contrast material in the affected kidney. (c) Signal intensity (arbitrary units) versus time curves illustrate symmetric uptake but asymmetric washout phases. Note the apparent inconsistency in the signal intensities in the uptake and washout phases owing to the use of different sequences to image each phase. Note the renographic indices (RI) included in the plot. RAS = kidney with renal arterial stenosis. (Reprinted, with permission from reference 16).
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As noted in the Tables, there were technical difficulties in two individual experiments. In animal 4 during week 6, captopril was not administered. However, the data were consistent with those observed with the second set of animals. Animal 6, during imaging without captopril, was not physiologically stable, for reasons not fully investigated.
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DISCUSSION
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Our preliminary data support the feasibility of performing captopril MR renography for the evaluation of renal arterial stenosis. The time versus signal intensity curves obtained in the present study are similar to those obtained with nuclear medicine techniques. This is different from results with previously proposed techniques that involved T2* weighting that resulted in a decrease in signal intensity as the gadopentetate dimeglumine was concentrated in the tubules (9–13, 17,18). Whereas those techniques were useful in evaluating renal function in a qualitative fashion, they were not amenable to any semiquantitative or quantitative interpretation similar to those possible with the nuclear scans.
In the present study, the time versus signal intensity curves for MR renography included pixels covering the entire renal parenchyma. These should, in principle, be amenable to deriving relative functional parameters such as split-renal-function (relative uptake in each kidney expressed as a percentage of total uptake). However, since only a single section was obtained for perfusion phase imaging, the signal intensity curves were derived from selected regions of interest defined over the renal parenchyma. Future implementations could involve a single acquisition sequence to image both perfusion and renographic phases. This may compromise the time resolution for the perfusion phase but will allow for deriving consistent time versus signal intensity data to cover both uptake and washout phases.
The changes observed with different anatomic degrees of stenosis and between data obtained with and data obtained without captopril are consistent with the expected behavior on the basis of captopril scintigraphy. During week 3, when the anatomic degree of stenosis was moderate, the gadopentetate dimeglumine washout curves appeared symmetric, even when captopril was administered. Whereas when the same animals were studied during week 6, there was substantial asymmetry consistent with hemodynamically significant stenosis (ie, activating the renin-angiotensin response). Similarly, during week 6, the asymmetry enhanced substantially when captopril was administered, which signified again the dependence on the renin-angiotensin response.
The absence of any substantial asymmetry during the perfusion phase, even when changes were observed during the renographic phase, is interesting and is consistent with our previous finding with quantitative perfusion imaging in the same model (7). On the basis of CT measurements, it was shown that there was no correlation between degree of stenosis and renal perfusion (19). This in turn is consistent with earlier reports of xenon 133 washout (20) and radionuclide studies (21) performed in human subjects.
The exact cause for the absence of a substantial correlation between regional blood flow and anatomic degree of stenosis is not yet clear. It is thought to be related to the variable capacity of the kidneys to regulate blood flow in the presence of a stenosis. Because of this fact, most of the human data in the literature usually differ from data acquired in acute animal models of renal arterial stenosis (22). The agreement of our results with those in humans suggests that the graded (chronic) stenosis model used in this study may be a better physiologic analogue of human renal arterial stenosis.
Further studies are warranted to correlate the MR renography with conventional renography and/or other clinical parameters in a large number of human subjects. Also, future implementations may use sequences that can provide direct estimates of T1 (or 1/T1) (23,24) rather than just T1-weighted signal intensity. These may avoid any potential nonlinearities associated with higher concentration of the contrast material.
In conclusion, although the MR counterpart shares the positive attributes of conventional renography, the technique is also limited by the same problems associated with conventional renography. The ability to predict the effect of revascularization remains controversial (25). Standardized testing procedures and diagnostic criteria currently are lacking, which limits the widespread acceptance of renography.
Practical application: We believe that captopril MR renography in combination with contrast-enhanced MR angiography could become a single comprehensive examination in patients suspected of having renovascular hypertension. This parallels our previous experience with combining functional MR urography and MR angiography in the evaluation of ureteropelvic junction obstruction (26). The substantially higher spatial resolution compared with that of nuclear scans and differentiation of renal parenchyma from the collecting system allows for better evaluation of the uptake and washout dynamics. Also, since tomographic sections can be obtained with MR imaging, regional changes can be depicted much better.
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ACKNOWLEDGMENTS
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The authors thank Stephen Gladstone, MS, and Vojkan Suslic, DVM, for their help in implementing the animal model and Jean Yang, BS, for technical assistance.
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FOOTNOTES
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Author contributions: Guarantor of integrity of entire study, P.V.P.; study concepts, P.V.P., R.R.E.; study design, P.V.P.; definition of intellectual content, P.V.P.; literature research, P.V.P.; experimental studies, C.S., P.V.P.; data acquisition, J.G., A.P., W.L.; data analysis, J.G., A.P., P.V.P.; manuscript preparation, P.V.P.; manuscript editing, P.V.P.; manuscript review, P.V.P., R.R.E.
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ABSTRACT
INTRODUCTION
