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1 Department of Radiology, Georgetown University Hospital, 3800 Reservoir Rd.,
N.W., Washington, DC 20007.
2 Department of Surgery, Georgetown University Hospital, Washington, DC
20007.
Received July 2, 2001;
accepted after revision December 19, 2001.
Address correspondence to R. C. Jha.
Abstract
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SUBJECTS AND METHODS. Sixty-four consecutive patients underwent successful laparoscopic donor nephrectomy. Imaging of the kidneys was performed before surgery with MR imaging and breath-hold three-dimensional gadolinium-enhanced MR angiography. All studies were reviewed prospectively by one of two attending radiologists. Results were compared with findings at the time of laparoscopic nephrectomy.
RESULTS. Of the 64 patients, MR imaging and MR angiography identified 30 patients with normal arterial, venous, and ureteric anatomy, and concordance was found at surgery in 29 of these patients. Vascular anomalies were depicted on MR imaging in 34 patients, with complete concordance at surgery in 29 patients. The use of MR angiography for revealing arterial anomalies had a sensitivity of 89.4%, specificity of 94.1%, and accuracy of 90.6%. For venous anomalies, there was a sensitivity of 98.3%, specificity of 100%, and accuracy of 98.4%. No important utereric anomalies were identified at surgery or on MR imaging.
CONCLUSION. Renal MR imaging and gadolinium-enhanced MR angiography provide a safe, accurate, and minimally invasive means of comprehensive assessment of the potential living renal donor.
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Multiple imaging modalities have been used for the preoperative evaluation of renal donors, including sonography, CT, scintigraphy, IV urography, conventional angiography, and MR imaging [2]. MR imaging is an attractive choice because it is minimally invasive, and it can evaluate the renal parenchyma as well as the vascular anatomy without using iodinated contrast media or exposing the patient to ionizing radiation. MR angiography has been used as a minimally invasive means of preoperative radiologic evaluation of renal donors. Earlier reports involving phase-contrast MR angiographic evaluation of the kidneys questioned its accuracy and thus its effectiveness [3,4,5]. Comparison of CT angiography with MR angiography using these earlier techniques found CT to have higher accuracy [6, 7]. However, recent studies with gadolinium-enhanced MR angiography have established it as an accurate imaging modality for the visualization and evaluation of living renal donors for open nephrectomy [8,9,10]. We evaluated the effectiveness and use of gadolinium-enhanced MR angiography in a large group of renal donors undergoing laparoscopic donor nephrectomy and compared our prospective interpretation of MR images with the surgical findings.
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MR Technique
All patients underwent imaging with a 1.5-T MR system (Magnetom Vision;
Siemens Medical Systems, Erlangen, Germany) and a torso phased array coil. All
patients fasted for a minimum of 4 hr before the examination. A 20- or
22-gauge IV catheter was placed in an arm vein before the study. Imaging was
typically completed in 45 min.
Images of the renal parenchyma were obtained before contrast agent administration with breath-hold axial and coronal T1-weighted in-phase and opposed-phase gradient-echo MR imaging (TR/first-echo TE, second-echo TE, 136/2.2, 5.3; flip angle, 70°), breath-hold axial and coronal T1-weighted fat-saturated gradient-echo imaging (TR/TE, 140/4.1; flip angle, 80°), and breath-hold axial and coronal half-Fourier acquisition single-shot turbo spin-echo imaging (TR/effective TE, infinite/64). The imaging matrix using a rectangular field of view was 128-180 x 256, with the field of view from 300 to 350 cm according to body habitus.
Gadolinium-enhanced MR angiography was performed using a coronal breath-hold three-dimensional fat-saturated spoiled gradient-echo technique (5.0/2.0; flip angle, 20°) in 51 patients with the following sequence parameters: matrix, 128-140 x 256; field of view, 250-340 cm; and a slice thickness of 1.5-3 mm. In 13 patients, a breath-hold three-dimensional interpolated fat-saturated gradient-echo sequence (4.6/1.8; flip angle, 30°) was performed with the following sequence parameters: matrix, 135-180 x 256; field of view, 244-350 cm; and effective slice thickness with interpolation of 0.7-1.7 cm.
IV gadodiamide (Omniscan; Nycomed, Princeton, NJ) was administered at a dose of 0.1 mmol/kg of body weight through a peripheral arm vein at a rate of 2 mL/sec followed by 20 mL of normal saline flush. A test bolus was used in three patients with 1 mL of gadolinium followed by 20 mL of normal saline to assess circulation time. However, a test bolus was not routinely used because of concern for venous contamination obscuring arterial vascular anatomy. After bolus injection, patients underwent imaging without a scan delay. A total of three acquisitions were obtained in each patient, with the first acquisition approximately 20 sec after contrast agent administration. Delayed imaging of the renal parenchyma was performed with breath-hold axial and coronal T1-weighted fat-saturated gradient-echo imaging (140/4.1; flip angle, 80°).
MR Analysis
The radiologic findings for each renal donor were correlated with the
surgical findings to determine whether there were discrepancies. The
preoperative MR imaging was interpreted by an experienced cross-sectional
imager. Interpretation was based on film hard-copy images from all the
available pulse sequences described. Maximum-intensity-projection imaging and
multiplanar reconstruction were performed on an independent workstation. In
each patient, findings from a comprehensive assessment of the vascular anatomy
of each kidney included the number and size of renal arteries and the presence
of early extrahilar branching, defined as branching occurring within the
proximal 2.0 cm from the renal artery ostium (Figs.
1 and
2). Renal vein anatomy was
evaluated to detect the presence of accessory renal veins, retroaortic or
circumaortic variants, and lumbar vein drainage into the renal veins
(Fig. 3). The renal parenchyma
was evaluated to assess focal lesions and to determine the number of ureters
present.
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A transplantation surgeon performed laparoscopic nephrectomy after MR imaging evaluation. The left kidney was removed in 62 patients, and the right kidney was removed in two. The MR imaging findings were compared with the surgical findings, which were based on reviews of the operative reports and interviews of the surgeons. The cases of all patients with discrepant findings at surgery were reviewed independently by two examiners unaware of the surgical findings.
Statistical Analysis
We calculated the sensitivity, specificity, and accuracy of MR angiography
for the determination of vascular anomalies. The results found at surgery were
used as the gold standard in our analysis. Patients were classified on the
basis of absolute agreement between their respective MR angiographic findings
and those findings reported at surgery. Our statistical results were obtained
on the basis of these criteria. For further analysis, a paired Student's
t test was used to compare data obtained using an interpolated MR
angiography technique versus a noninterpolated MR angiography technique.
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MR angiography prospectively described normal arterial, venous, and ureteric anatomy in 30 patients, with concordance in 29 at laparoscopic nephrectomy. The left kidney was chosen as the harvest site in all these patients, as is preferred, because it has a longer vascular pedicle. In one patient, the initial MR angiography reported a single left renal artery, but at surgery two renal arteries, one with extrahilar branching, were seen. This patient's case was reviewed by the two MR imaging interpreters. Both reviewers described one renal artery and noted the extrahilar branching, but a tiny accessory superior left renal artery was again missed (Fig. 4A,4B). However, both reviewers commented on the poor image quality, with a suboptimal slice profile of 2.9-mm thickness and a 340-cm field of view.
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Vascular anomalies were described on MR angiography as involving the arterial or venous system in 34 patients, with complete concordance seen with the surgical findings in 29 patients and discrepancy in five.
Arterial Anomalies
Twenty-nine patients had multiple renal arteries or early extrahilar
branching of the renal arteries identified on MR angiography.
Isolated right renal artery anomalies were seen on MR angiography in eight patients. Of these, five patients had two right renal arteries identified on MR angiography, and three had early extrahilar branching. Only one of these patients had the right kidney harvested because of the complex left renal vein anatomy and the finding of an accessory right renal artery that was confirmed in this patient. The remaining seven patients had the left kidney removed, and the findings of a single left renal artery and vein were confirmed at surgery.
Bilateral artery anomalies were seen on MR angiography in eight patients, with supernumerary arteries seen in seven and early extrahilar branching in one. All of these patients had laparoscopic left nephrectomy, with confirmation of the findings seen on MR angiography in seven of eight and a discrepant result in one. In this patient, bilateral dual renal arteries were described on initial MR angiography, but they were not seen at left nephrectomy. Rather, a single left renal artery and a small circumaortic left renal vein were seen. Both interpreters saw these structures retrospectively. The initial MR angiography was misinterpreted as showing small accessory branches supplying each kidney, when in fact there was early venous opacification of the retroaortic component of the left renal vein (Fig. 5A,5B,5C).
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Isolated left renal artery anomalies were seen on MR angiography in 12 patients, with accessory renal arteries seen in eight, early extrahilar branching in three, and both of these findings in one patient. Of these 12 patients, surgery confirmed the MR findings in eight, and discrepant findings were identified in four patients.
Prospective and retrospective MR angiography in the first of these four patients revealed a dominant left renal artery entering the renal hilum and a tiny capsular branch originating from the left common iliac artery supplying the lower left renal pole. This vessel was not confirmed at surgery.
In another patient, a 3-mm accessory left renal artery was prospectively described as originating from the aorta and extending into the renal hilum. This finding was not confirmed at surgery. On retrospective analysis, both reviewers described a small arterial branch arising from the aorta, but they could not confidently show its relationship to the renal parenchyma (Fig. 6A,6B).
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In the third patient, early extrahilar branching of the left renal artery was seen on initial MR angiography with a very short common origin, measuring approximately 5 mm from the renal ostium. Both examiners at retrospective analysis held to this interpretation. At surgery, two separate ostia were identified from the aorta.
Prospective MR angiography in the fourth patient revealed the findings of a malrotated kidney with a horizontal axis. Two left renal arteries were seen: one supplied the upper pole and a second artery was thought to arise from the inferior mesenteric artery and supply the lower pole (Fig. 7A,7B,7C,7D). Other imaging findings were suggestive of stenosis of the upper left renal artery. On angiography, the upper renal artery was seen with no definite stenosis, but the lower renal artery was not identified. At surgery, two left renal arteries were seen, but the lower renal artery originated from the aorta itself, rather than from the inferior mesenteric artery. Because of the position and length of the surgical staple placed at laparoscopic surgery, focal stenosis of the upper left renal artery could not be confirmed. At review, both examiners agreed that the origin of the lower accessory artery could not definitively be described because the volume of acquisition did not completely include the origin of the inferior mesenteric artery.
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Venous Anomalies
Venous anomalies involving the left-sided venous structures were identified
on preoperative MR imaging in seven patients: Three patients had a
circumaortic left renal vein, one patient had a retroaortic left renal vein,
one patient had three left renal veins merging anteriorly to the aorta, one
patient had a duplicated inferior vena cava, and one patient had a prominent
lumbar vein entering the left renal vein. In six patients, the findings were
confirmed at the time of laparoscopic resection of the left kidney. In the
patient with a trifurcation of the left renal veins, the right kidney was
chosen as the site for harvesting because of the complex venous anatomy.
Ureteric Anomalies and Parenchymal Abnormalities
No significant anomalies were detected at the time of laparoscopy, although
two patients were reported to have bifid renal pelves on MR imaging. This
finding was not noted at the time of surgery.
Statistical Analysis
If complete concordance between surgical findings and MR imaging findings
is used in analysis, the statistical results are as follows, with 95%
confidence levels given in parentheses: for depiction of all arterial
anomalies, MR angiography has a sensitivity of 89.4% (78.7%, 97.9%) and
specificity of 94.1% (76.5%, 100%), with an accuracy of 90.6% (81.3%, 96.9%).
For all venous anomalies, MR angiography has a sensitivity of 98.3% (93.2%,
100%) and specificity of 100% (80.0%, 100%) for all, with an accuracy of 98.4%
(93.8%, 100%). In comparing the interpolated versus noninterpolated MR
angiography techniques, we found no significant difference between the two
groups of patients in the detection of vascular anomalies.
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Surgical correlation between the use of laparoscopic donor nephrectomy and preoperative evaluation with CT angiography has been documented by Del Pizzo et al. [11]. In their study, CT arteriography was 96% accurate in identifying the arterial anatomy before surgery. In addition, the venous anatomy was identified accurately in 97% of the patients. They suggest that CT angiography is accurate enough to be used as a single modality for the evaluation of renal donors. Others have also recommended the use of CT angiography as an effective and accurate modality to be used independently for the evaluation of potential donors [12].
Recent studies have found gadolinium-enhanced MR angiography to have high rates of accuracy and to be comparable to conventional angiography and CT angiography in the evaluation of living renal donors for open nephrectomy. In a recent study, Halpern et al. [13] compared the effectiveness of CT angiography and MR angiography in the preoperative evaluation of living renal donors. In their study, of the 18 patients who had surgical correlation available, MR imaging missed only one of the 23 arteries transplanted. Additionally, two small arteries were suggested on CT but not on MR imaging, and these vessels were not found during nephrectomy. Buzzas et al. [9] determined gadolinium-enhanced MR angiography to be 100% accurate in evaluating the renal vascular anatomy in 15 patients for open nephrectomy. In a study by Low et al. [8], 22 patients were evaluated with gadolinium-enhanced MR angiography, and the results were compared with those found during conventional angiography and open nephrectomy. They found 100% accuracy in identifying the renal arteries in all patients. Bakker et al. [14] have also described the advantage of gadolinium-enhanced MR angiography in a study of 24 living renal donors, with 100% accuracy in the detection of main and accessory renal arteries.
Laparoscopic donor nephrectomy is gaining favor as a minimally invasive alternative to open techniques of renal donor harvesting. Both arterial and venous vessels must be identified and, in particular, possible branching and accessory vasculature must be known before laparoscopic nephrectomy. This information is important because of the technical challenges associated with the procedure and the limited surgical field of view. Therefore, an accurate and effective preoperative evaluation is necessary, and anything short of this can lead to surgical complications and injury to the patient or the organ being harvested.
Our results show the capability of gadolinium-enhanced MR angiography to delineate the arterial and venous anatomy with a high level of accuracy in patients who are candidates for laparoscopic nephrectomy. Furthermore, patients tolerate the procedure well, and it is convenient to schedule them for this minimally invasive outpatient test rather than conventional angiography and excretory urography. Our surgeons have found the anatomic detail provided by MR imaging to be satisfactory for selection of the site for harvest. The MR angiographic images are presented in a format similar to conventional angiography, with multiplanar reconstruction images also available to show complex anatomy.
Our study has some limitations. First, our accuracy rates reflect only the findings from the single harvested kidney. We could not analyze the contralateral nonharvested kidney. Second, we did not attempt to quantify the exact caliber and length of vessels or the ureteric anatomy. Furthermore, the imaging technique was not consistent because the technologists and the junior physicians who often were responsible for monitoring the study had various levels of experience. When we reviewed the cases with discrepant findings, we were able to see that a learning curve exists in the interpretation of renal MR angiography and that most of the misinterpretations occurred during the early part of the study. Furthermore, although we did not retrospectively analyze the quality of all the MR angiographic images, review of the discrepant cases revealed the importance of optimizing image acquisition parameters of slice thickness and field of view and the problems that may be encountered with venous contamination. Some centers advocate the use of a timed bolus for all MR angiographic examinations, but it is our experience that venous contamination and the inconsistent quality of subtraction images make it preferable to use a fixed delay in these generally young and healthy living donors.
We considered any discrepancy between MR imaging and surgery as a false-positive or false-negative finding in calculating the accuracy rates of MR imaging. It is clear, however, that from a clinical point of view, some differences do not alter surgical management. The detection of an early extrahilar renal artery branch on MR angiography that is noted as two separate renal artery ostia at laparoscopic nephrectomy does not alter the surgical approach: both are treated as two separate renal arteries, as was done in one of our patients. Information about a suspected early extrahilar branch can prove valuable to the transplantation surgeon in surgical planning—for selection of the harvest site and for determination of the number of anastomoses that will be required.
In one patient, MR angiography was able to show a second renal artery supplying the lower pole in a malrotated kidney, a finding that was missed on conventional angiography. The origin of this vessel was from the lower abdominal aorta rather than from the inferior mesenteric artery, which was excluded from the lower aspect of the volume of the acquisition. Regardless, the findings on MR angiography were important in surgical planning, especially because this accessory vessel was seen neither prospectively nor retrospectively on conventional angiography. In two other patients, vessels were described on MR angiography as being 2- to 3-mm accessory arteries, but they were not seen at surgery. In a third patient, a 3-mm accessory artery was missed on MR angiography but seen at laparoscopy. This vessel was anastomosed with the dominant artery using a GORE-TEX patch (W. L. Gore, Flagstaff, AZ), and the reconstructed vessel was used in the anastomosis with the iliac artery. Review of these images revealed the tiny vessels, but visualization of their exact relationship to the renal parenchyma was difficult despite multiplanar reformatting. These cases show the limitations of spatial resolution and the importance of optimizing imaging parameters.
The case of the final patient with discrepant findings illustrates the learning curve involved in interpretation of renal MR angiography: Although the prospective evaluation reported bilateral duplicated renal arteries, retrospective analysis of this case by both examiners more than 2 years after the initial reading was correctly interpreted as venous contamination of a circumaortic renal vein that created an image that simulated supernumerary renal arteries.
In conclusion, renal MR imaging with gadolinium-enhanced MR angiography is an accurate and minimally invasive technique for obtaining a complete preoperative evaluation of renal donors. It has multiple advantages over other means of evaluation and, more important, it provides the necessary detail to allow harvest-site selection for laparoscopic donor nephrectomy. Continued experience with interpretation and identification of vascular anatomy and the optimizing of imaging techniques should continue to improve accuracy rates in preoperative assessment.
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