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Genitourinary Imaging |
1 From the Departments of Radiology (G.M.I., V.S.L., G.A.K., M.T.L., J.C.W.) and Surgery (M.E., T.D.), NYU Medical Center, 560 First Ave, Suite HW 202, New York, NY 10016. Received October 12, 2001; revision requested December 5; revision received March 4, 2002; accepted April 16. Address correspondence to G.M.I. (e-mail: gary.israel@med.nyu.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Twenty-eight donor candidates who underwent subsequent laparoscopic nephrectomy were examined by using a torso phased-array coil at 1.5 T. Gadolinium-enhanced MR angiograms, MR venograms, and MR urograms were obtained in all patients by using an interpolated three-dimensional T1-weighted spoiled gradient-echo sequence (3.4–6.8/1.2–2.3 [repetition time msec/echo time msec], 25°–40° flip angle). Interpretation of the MR images was used to assess the arterial, venous, and ureteral anatomy, as well as parenchymal masses and scarring, and findings were compared with the surgical findings in all patients. Statistical evaluation was performed, with the surgical findings as the reference standard.
RESULTS: At MR imaging, 31 of 32 renal arteries and one of three early-branching arteries were identified correctly. The correct venous anatomy was identified in 23 of 28 patients, including a single left renal vein anterior to the aorta (n = 16), retroaortic left renal vein (n = 2), circumaortic left renal vein (n = 2), and single right renal vein (n = 3). A single collecting system in all harvested kidneys was identified correctly with MR urography. Overall, MR imaging correctly depicted vascular, ureteral, and parenchymal anatomy in 21 of 28 patients. Twenty-seven of 28 patients underwent successful laparoscopic donor nephrectomy on the basis of the MR findings. One procedure was converted to open nephrectomy on the basis of complex venous anatomy not prospectively identified on the MR images. The sensitivity and positive predictive value of MR imaging in correctly determining the combined vascular, ureteral, and parenchymal anatomy in the harvested kidney were 75% (21 of 28) and 95% (21 of 22), respectively.
CONCLUSION: Comprehensive gadolinium-enhanced MR imaging can depict the vascular anatomy, collecting system, and renal parenchyma preoperatively in patients who are candidates for laparoscopic living-donor nephrectomy.
© RSNA, 2002
Index terms: Kidney, MR, 81.12142 • Kidney, transplantation, 81.1269 • Renal arteries, MR, 961.12942
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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A comprehensive magnetic resonance (MR) examination can aid in the surgical planning of renal donation and helps avoid potential complications (8–10). Advantages include primary multiplanar capability, safer contrast agents, and superior soft-tissue contrast that is accentuated with the administration of a gadolinium chelate. The aim of this study, therefore, was to evaluate the accuracy of MR imaging in the preoperative evaluation of potential living reanl donors who are candidates for laparoscopic nephrectomy.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MR Imaging Technique
MR imaging was performed with a 1.5-T system (Vision or Symphony; Siemens Medical Systems, Erlangen, Germany) by using a torso phased-array coil. Informed consent was obtained from all patients for administration of both intravenous gadopentetate dimeglumine and intravenous furosemide. In all patients, MR imaging was requested by the referring physician as a routine examination in the evaluation of a potential renal donor.
All patients underwent transverse breath-hold T1-weighted imaging with a two-dimensional gradient-echo (GRE) sequence and coronal breath-hold T2-weighted imaging with a half-Fourier single-shot turbo spin-echo (HASTE; Siemens Medical Systems) sequence. Imaging parameters for the T1-weighted GRE sequence were as follows: 151–200/2–5.3 (repetition time msec/echo time msec); flip angle, 70°–90°; matrix, 80–118 x 256; section thickness, 5–8 mm; intersection gap, 0.6–2.0 mm; and field of view, 200–263 x 320–375. T2-weighted imaging was performed with the following parameters: 
/62–67; flip angle, 105°–180°; matrix, 180–256 x 256; section thickness, 5–8 mm; intersection gap, 0–1.2 mm; and field of view, 200–263 x 320–375.
For MR angiography, MR venography, and MR urography, a coronal breath-hold three-dimensional (3D) T1-weighted spoiled GRE sequence was performed before and after intravenous administration of 19 mL of gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ), which was followed by a 20-mL saline flush. Timing for MR angiography was based on a 1-mL test bolus of contrast material followed by a 20-mL saline flush that was injected at 2 mL/sec by means of a power injector (Spectris; Medrad, Pittsburgh, Pa) (11). Immediately after the test bolus, 10 mg of intravenous furosemide (American Pharmaceutical Partners, Los Angeles, Calif) was administered to augment diuresis. MR venography and MR urography were performed at approximately 1 minute and 5 minutes, respectively, after the arterial phase. Subtraction techniques were performed for all contrast material–enhanced 3D acquisitions. During the course of the study, the MR protocol evolved, and a second 3D fat-suppressed T1-weighted interpolated spoiled GRE sequence was subsequently performed in the transverse plane before and after administration of contrast material to better evaluate the renal parenchyma.
The coronal 3D sequence parameters were as follows: 3.4–6.8/1.2–2.3; flip angle, 25°–40°; matrix, 145–269 x 256–512; interpolated section thickness, 1.3–2.3 mm; and field of view, 250–350 x 325–500. Transverse 3D sequence parameters were 3.6–4.5/1.6–1.9; flip angle, 12°; matrix, 83–167 x 256; interpolated section thickness, 1.0–2.4 mm; and field of view, 203–350 x 350 x 400. All acquisition times were less than 30 seconds to facilitate breath holding at end expiration.
All images were evaluated by one of five MR radiologists (G.M.I., V.S.L., G.A.K., M.T.L., J.C.W.) by using commercially available workstations (Virtuoso, Siemens; and Vitrea, Vital Images, Minneapolis, Minn). The 3D acquisitions were evaluated by using several postprocessing techniques, including multiplanar reformatting, volume rendering, and/or maximum intensity projection. The postprocessing techniques were not compared with each other. All findings were reviewed with the transplantation team before donor nephrectomy. All imaging findings were compared with the surgical reports (n = 28) and surgical video recordings (n = 3) when available. For cases in which a discrepancy was noted between MR imaging and surgical findings, the MR images were reviewed again by a single radiologist (G.M.I.).
Images were interpreted for the following findings. For the arterial anatomy, the number of renal arteries supplying each kidney was determined. In addition, renal arteries were assessed for the presence of early renal artery branching (within 2 cm of the aorta) and stenosis. Evaluation of the venous anatomy entailed identifying the number of renal veins draining each kidney, as well as determining the relationship of the left renal vein to the aorta. Renal parenchyma was evaluated for mass lesions and scarring. The collecting system was evaluated for hydronephrosis and duplication.
MR examination requires approximately 20–30 minutes to perform and the images approximately 10–20 minutes to interpret, including postprocessing time. The left kidney is the preferred kidney to harvest because of the longer length of the left renal vein, which facilitates the venous anastomosis in the recipient. Therefore, MR imaging was considered to have influenced the side of harvest if the right kidney was harvested.
Data Analysis
The sensitivity and positive predictive value of MR imaging in determining the combined vascular, ureteral, and parenchymal anatomy of the harvested kidney were computed by using the surgical findings as the reference standard. A result was considered true-positive when, in the same patient, the predicted combined vascular, ureteral, and parenchymal anatomy was concordant with the surgical findings. A result was considered false-negative when a surgical finding related to the vascular, ureteral, or parenchymal anatomy was not identified prospectively on the MR images. If more than one false-negative result occurred in the same patient, this was considered a single false-negative result. A result was considered false-positive when a finding on the MR images was not confirmed at surgery.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In 23 patients, the left kidney was donated. In four of five patients in whom the right kidney was donated, MR imaging demonstrated anatomic variations related to the left kidney, influencing the surgeon to harvest the right kidney. These variations included a duplex left collecting system (n = 1) (Fig 1) and two left renal arteries (n = 3) (Fig 2). The fifth patient demonstrated mild stenosis of the proximal right renal artery. In this patient, it was decided to harvest the right kidney because it was thought that the right renal artery could be adequately anastomosed to the recipient, leaving the donor with a normal left renal artery and kidney.
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Arterial Findings
Overall, the correct arterial supply was depicted with MR imaging in 25 (89%) of 28 patients. Of the 32 renal arteries found during laparoscopic donor nephrectomy, 31 (97%) were prospectively identified at MR imaging, including a single renal artery (n = 24) and two renal arteries (n = 3). For the three patients in whom two renal arteries were identified preoperatively, the decision to harvest that kidney (left kidney [n = 3]) was made because the contralateral kidney also demonstrated accessory arteries (n = 2) or the harvesting surgeon thought the contralateral (right) renal vein was too short for adequate anastomosis in the recipient (n = 1). In a single patient in whom a single renal artery was prospectively identified, a second undetected renal artery was subsequently identified at surgery and ligated without consequence. In retrospective review, this artery was seen on the MR images and measured 2 mm in diameter.
A single patient with an early arterial branching pattern was prospectively identified with MR imaging (Fig 3). However, two additional patients were found to have early arterial branching at surgery. In retrospect, this could be demonstrated with MR angiography only in the patient whose procedure was converted to open nephrectomy. Difficulty in identifying the branch artery prospectively can be attributed to its small size (2 mm) and the inability to visualize it in the renal hilum.
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In another patient, a circumaortic left renal vein was prospectively identified on the MR images (Fig 4). At surgery, the retroaortic component of this was not identified. This represents the only false-positive finding in this series.
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Parenchymal Findings
In one patient, a 2-cm cyst was demonstrated in the upper pole of the kidney on the MR images and was confirmed surgically. Evaluation of the remaining 27 patients demonstrated no parenchymal abnormality on MR images or at nephrectomy.
Collecting System
In all patients, the MR images demonstrated the harvested kidney to have a single collecting system and ureter without hydronephrosis, findings that were all confirmed surgically. In one patient, a duplicate collecting system and ureter drained the left kidney (Fig 1), and this finding contributed to the decision to harvest the right kidney.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Donor nephrectomy has been shown to be safe for the donor and subsequent transplantation effective for the recipient. Laparoscopic techniques have been developed that reduce the morbidity of donor nephrectomy (1,2). With the modified visualization and surgical access of laparoscopic methods, however, evaluation of potential donors suitable for donor nephrectomy becomes critical for safe and successful laparoscopic nephrectomy.
Gadolinium-enhanced MR angiography provides accurate information regarding renal arterial anatomy, including evaluation for accessory arteries, early-branching patterns, and stenoses (14,15). In the present study of 28 patients, one accessory artery and two early-branching arteries were not prospectively identified. Our results compare favorably with those of Bakker et al (14), who identified 21 of 22 accessory renal arteries, and with those of Korst et al (15), who identified 13 of 17 accessory renal arteries by using digital subtraction angiography as the reference standard. However, studies comparing conventional angiography with surgical findings have shown that up to 8% of accessory renal arteries may not be identified at angiography, and therefore the true number of accessory renal arteries may be underestimated (6). Similar results have been reported in studies comparing gadolinium-enhanced MR angiographic findings with surgical findings at donor nephrectomy. Rankin et al (8) missed two accessory arteries in 18 patients, and Halpern et al (10) missed one accessory renal artery and one early-branching renal artery in 18 patients. Finally, Low et al (9) compared gadolinium-enhanced MR angiographic with conventional angiographic findings and/or surgical findings in 22 potential renal donors and missed an early-branching renal artery in one patient.
The results of our study and previous other studies indicate that gadolinium-enhanced MR angiography can accurately depict the arterial anatomy of potential living, related renal donors. However, small (<2 mm) accessory arteries and branches may not be seen with the limited spatial resolution of current MR sequences. In some cases, these vessels supply an insignificant portion of the renal parenchyma and can be ligated without detriment to the recipient. However, small accessory lower polar arteries may also provide blood supply to the ureter, and ligation of these can lead to ureteral ischemia. MR imaging strategies, including elliptical centric phase encoding and parallel imaging techniques such as simultaneous acquisition of spatial harmonics, or SMASH, and sensitivity encoding, or SENSE, may further improve spatial and contrast resolution of breath-hold MR angiography (16,17).
Renal venous anatomy is less variable than the arterial anatomy; however, accurate characterization of venous anomalies is valuable to the laparoscopic surgeon. Common left renal venous anomalies include circumaortic and single retroaortic renal veins, which occur in 5%–7% and 2%–3% of the population, respectively (18). Multiplicity of the right renal vein occurs in approximately 30% of individuals (18). We correctly identified the renal venous anatomy of the harvested kidney in 23 of 28 patients. Reports of MR evaluation of renal venous anatomy are limited. In the series of Halpern et al (10), disagreement concerning venous anatomy was related to very small retroaortic venous structures. In our study, we used 19 mL (plus 1 mL for the timing run) of a gadolinium-based contrast agent, and an increased dose may help identify small venous structures. Furthermore, a different approach, such as timing of the venous phase instead of a fixed venous imaging delay, may be helpful.
MR urography may be performed with T2-weighted turbo spin-echo sequences or with a 3D T1-weighted GRE sequence after the administration of a gadolinium-based contrast agent. Low et al (9) correctly characterized the collecting system in 20 of 22 patients by using a T2-weighted turbo spin-echo sequence with the application of compression paddles and following a 1-L saline bolus and 10 mg of furosemide. In our study, in which a 3D T1-weighted GRE sequence was used, we correctly identified in all harvested kidneys a single collecting system and ureter without evidence of hydronephrosis. The results of a study that compared both methods support the use of 3D T1-weighted GRE imaging over T2-weighted turbo spin-echo imaging (8).
We found a single false-positive result in our series; the retroaortic component of a prospectively identified circumaortic left renal vein was not identified at surgery. We hypothesize that at surgery, a retroaortic renal vein could be mistaken for a draining retroperitoneal vein.
One major limitation of MR imaging in evaluating potential kidney donors is the inability to depict calcifications. To exclude urolithiasis, a second examination method, such as US, could be performed. At autopsy, the prevalence of urolithiasis is only approximately 1% of patients, and it is uncertain if donor candidates need to be screened routinely (19). The transplantation surgeons at our institution consider US to be sufficient for excluding renal calculi. All patients underwent US examination before the MR examination and were shown to be free of renal calculi.
Our study has recognized limitations. First, the number of patients is relatively small, and therefore all anatomic variants may not be represented. Second, comparison with surgical findings was only available for the transplanted kidney. Our results may be biased toward nonvariant anatomy, as the surgeon typically chooses the kidney with the less complex vascular anatomy. Third, correlation with conventional angiography was not performed, and therefore the prevalence of intrinsic renal artery disease may be underestimated. In addition, in the present study, we did not directly compare the MR findings with those of more conventional imaging methods in the evaluation of potential donors. Therefore, the accuracy of MR imaging, as compared with that of conventional methods, as well as the cost-effectiveness of MR imaging, cannot be evaluated. Finally, the surgeon was aware of the MR findings before donor nephrectomy, and potential bias may be introduced in the use of surgical findings to confirm the MR imaging results.
In conclusion, laparoscopic donor nephrectomy may replace open nephrectomy as the surgical approach of choice, provided donors can be evaluated safely and comprehensively for anatomic variants before surgery. Without exposure to ionizing radiation and nephrotoxic contrast agents, MR imaging can provide a minimally invasive and accurate tool for the preoperative evaluation of potential renal donors. MR imaging provides important vascular, ureteral, and parenchymal information, thereby reducing the risk to the donor and improving the chances of a successful outcome for the recipient. In the future, improved MR imaging methods should be explored to overcome limitations in visualization of small arteries and veins.
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, G.M.I., V.S.L.; study concepts, T.D., G.M.I., V.S.L., M.E.; study design, T.D., G.M.I., V.S.L.; literature research, G.M.I.; clinical studies, G.M.I., V.S.L., M.T.L., G.A.K., J.C.W.; data acquisition, J.C.W., M.E., G.M.I., V.S.L., M.T.L., G.A.K.; data analysis/interpretation, G.A.K., J.C.W., G.M.I., V.S.L.; statistical analysis, G.M.I.; manuscript preparation and definition of intellectual content, G.M.I., V.S.L.; manuscript editing, G.M.I., V.S.L., M.T.L., G.A.K., J.C.W., M.E.; manuscript revision/review and final version approval, all authors.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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