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Genitourinary Imaging |
1 From the Departments of Radiology (J.K.K., S.Y.P., H.j.K., K.S.C.) and Urology (C.S.K., H.J.A., T.Y.A.), Asan Medical Center, University of Ulsan, 388–1 Poongnap-dong, Songpa-gu, Seoul 138–736, Korea. Received September 2, 2002; revision requested October 31; final revision received March 21, 2003; accepted March 27. Address correspondence to K.S.C. (e-mail: kscho@amc.seoul.kr).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Four-channel multi–detector row CT scans were obtained in 77 patients. Vascular phase scans were used for CT angiography; excretory phase scans, for CT urography. At CT angiography, two independent observers evaluated the number of arteries and veins and the presence of early-branching arteries. CT urographic images were evaluated with regard to the opacification of the urinary tract and for abnormalities. Findings of CT angiography and urography were compared with surgical findings. Interobserver agreement between CT angiographic and surgical findings was quantified with weighted 
statistics. Sensitivity and specificity of CT angiography in identifying supernumerary vessels and early-branching arteries were also evaluated. To evaluate the radiation dose to patients, weighted CT dose index (DI) was assessed for each scan.
RESULTS: Agreement between CT angiographic and surgical findings was excellent for the number of renal arteries ( = 0.896) and veins ( = 0.843). Detection rate of CT angiography was 98% (89 of 91) for arteries and 98% (83 of 85) for veins. The respective sensitivity and specificity of CT angiography were 86% (12 of 14) and 100% (65 of 65) for supernumerary arteries, 100% (11 of 11) and 100% (66 of 66) for early-branching arteries, and 75% (six of eight) and 100% (69 of 69) for supernumerary veins. At CT urography, collecting systems and proximal ureters were well opacified in all patients; two patients had underrotated kidneys without obstruction. The weighted CT DI was 10.19 mGy for unenhanced and excretory phase scans and 12.88 mGy for the vascular phase scan.
CONCLUSION: Multi–detector row CT can help assess well the renal vasculature and the urinary tract of living renal donors.
© RSNA, 2003
Index terms: Computed tomography (CT), angiography, 81.12116 • Computed tomography (CT), multi–detector row, 81.12111, 81.12112, 81.12114, 81.12117 • Kidney, CT, 81.12111, 81.12112, 81.12114, 81.12117 • Kidney, transplantation, 81.455 • Urography, 81.1211
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Many authors have assessed the usefulness of single–detector row helical computed tomography (CT) in the preoperative evaluation of kidneys obtained from living donors and proved that CT angiography can replace conventional angiography (1–11). However, it is still necessary to improve the performance of CT, because detection of supernumerary arteries is yet incomplete; the sensitivity was 77% in a recent study with a large patient population (6). Furthermore, the use of CT urography has not been attempted in previous studies in which single–detector row helical CT was used because of limitations in obtaining a large data set with thin sections through the entire urinary tract in a single breath hold. Recent technical advances have introduced multi–detector row CT into clinical practice. This imaging modality allows thinner section imaging, faster scanning, and improved longitudinal spatial resolution than does single–detector row helical CT (12,13). These advantages can provide greater coverage during a single breath hold and reduce motion and partial volume artifacts. Therefore, the use of multi–detector row CT is anticipated to advance the quality of three-dimensional images generated from the CT data.
The purpose of this study was to evaluate in living renal donors the use of multi–detector row CT in the assessment of renal vasculature and the upper urinary tract.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Patients
Between July 2000 and July 2001, a total of 83 consecutive patients (39 men and 44 women; age range, 19–52 years; mean age ± SD, 38 years ± 9) were evaluated as potential living donors for renal transplantation. Among them, six patients were excluded because of urinary calculi (n = 3), a severe renal scar (n = 2), or a horseshoe kidney (n = 1), which were diagnosed at CT. Therefore, a total of 77 patients (age range, 19–52 years; mean age, 38 years ± 9), which included 36 men (age range, 25–52 years; mean age, 37 years ± 8) and 41 women (age range, 19–52 years; mean age, 39 years ± 12), were enrolled in a renal transplantation program; there was no difference in the age distribution between men and women.
CT Protocol
All CT data were obtained with a four-channel multi–detector row CT scanner (LightSpeed QX/i; General Electric Medical System, Milwaukee, Wis). Unenhanced, vascular phase, and phase scans were obtained during a breath hold. X-ray tube voltage, tube current, and gantry rotation time for all scans were 120 kV, 200 mA, and 0.8 second per rotation, respectively.
Unenhanced scans were obtained from the 11th thoracic vertebral body to the ischial tuberosities, with a detector array of 2.5 x 4 mm, a beam pitch of 1.5 (equivalent to a section pitch of 6, high-speed mode), a table speed of 15 mm per rotation (18.75 mm/sec), and a reconstruction increment of 2.5 mm. The mean acquisition time was 17 seconds ± 2. Thereafter, iopamidol (Iopamiro 300; Bracco, Milan, Italy) was intravenously administered into the antecubital vein through an 18-gauge angiographic catheter by using a mechanical injector; 140 mL was administered at a rate of 4.0 mL/sec for patients with a body weight 60 kg or greater (n = 58) and 130 mL at a rate of 3.0 mL/sec for patients (n = 19) with a body weight of less than 60 kg.
The start time of the vascular phase scanning was determined by using automatic bolus tracking. Scanning was initiated with a start delay of 5 seconds after triggering at a threshold of 80 HU in the region of interest, which was placed in the abdominal aorta at the level of the renal arteries. With this technique, scanning began 24–31 seconds (mean, 27 seconds) after intravenous injection of contrast material. Scan coverage for the vascular phase, which was determined by referring to the prior unenhanced scan, was from the celiac axis to the common iliac arteries. The parameters for the vascular phase scan included a detector array of 1.25 x 4 mm, a beam pitch of 1.5, a table speed of 7.5 mm per rotation (9.38 mm/sec), and a reconstruction increment of 0.5 mm. The mean acquisition time was 18 seconds ± 1. Excretory phase scanning started 180 seconds after intravenous injection of contrast material and extended from the diaphragm to the ischial tuberosities. The parameters included a detector array of 2.5 x 4 mm, a beam pitch of 1.5, a table speed of 15 mm per rotation (18.75 mm/sec), and a reconstruction increment of 1.25 mm. The mean acquisition time was 20 seconds ± 2.
Image Processing and Analysis
Immediately after the CT data were obtained, a radiologist (J.K.K) briefly reviewed the vascular phase CT images with regard to whether renal vessels were adequately enhanced; adequate enhancement was considered when both renal arteries and veins were well opacified throughout their entire length and the renal arteries could be easily differentiated from the renal veins, as the arteries enhanced stronger than did the veins.
The CT data sets were transferred to a workstation (Advantage Windows 3.0; General Electric Medical System). The CT data were manipulated by an experienced technologist (H.j.K.) who had worked in the three-dimensional image laboratory of our institution for 3 years. If he had any questions or technical difficulties during image processing, he consulted one of several genitourinary radiologists (J.K.K., S.Y.P., K.S.C.). CT data of the vascular phase were used for CT angiography. First, the technologist segmented the kidney, renal arteries, renal veins, aorta, inferior vena cava, and perinephric fat from the raw CT images by manually drawing a three-dimensional region of interest, which was prescribed by using a maximum intensity projection (MIP). Thereafter, he removed all of the other structures while attempting to avoid deleting the regions of interest. In this model, additional detailed editing was performed to eliminate irrelevant structures, such as the mesenteric and lumbar arteries, and residual bony structures. From these edited CT data, subsequent three-dimensional images were reconstructed by using volume rendering and MIP, which were photographed at 15° increments of rotation along the z axis of the body at window and level settings for optimal vascular demonstration.
Three-dimensional images reconstructed by the technologist were independently reviewed by two radiologists (J.K.K., S.Y.P) at a workstation. Thereafter, both radiologists independently evaluated the renal vessels in oblique coronal and oblique transverse planes using thin-slab MIPs of the raw CT images, which had not been edited by the technologist. The slab thickness was arbitrarily applied from 2 to 15 mm in each case. If there was a discrepancy between the thin-slab MIP images made by the radiologist and the volume-rendered or MIP images made by the technologist, each radiologist made a final decision after reevaluating all of those images.
On the three-dimensional images, the two radiologists independently evaluated the number of arteries and veins and the presence of early-branching arteries, late venous confluence, arterial abnormalities such as stenosis or calcified plaque, and venous abnormalities such as a retroaortic or circumaortic vein. The different observations made by the two reviewers were then resolved by consensus.
When the kidney had two or more arteries or veins, the vessel with the greatest diameter was considered to be dominant and the others, supernumerary. An early-branching artery was determined to be present when any branch diverged within 1.5 cm from the aorta, and late venous confluence was indicated when veins joined within 1 cm from the inferior vena cava.
CT urographic images were reconstructed from the excretory phase CT data, without preliminary editing by the technologist, by using MIP (n = 31) or an average intensity projection (n = 46) in either coronal or oblique coronal plane. The slab thickness was arbitrarily determined to include both pelvocalyces and ureters. The two radiologists independently reviewed the CT urographic images with regard to whether the entire urinary tract was well opacified and whether there were structural abnormalities in the urinary tract. Different interpretations by the two reviewers were also resolved by consensus. When the entire urinary tract was not completely opacified, we performed an additional intravenous urography examination to confirm normal urine passage throughout the upper urinary tract.
In addition to CT angiography and urography, the first reviewer (J.K.K.) evaluated the presence of a renal parenchymal abnormality, such as a tumor or a scar, and an abnormality in the other abdominal organs.
Surgical Correlation
Seventy-seven of 154 kidneys were donated for transplantation, with urologists (T.Y.A., C.S.K., H.J.A.) using an open extraperitoneal approach; therefore, surgical confirmation of CT findings was obtained in these 77 kidneys. The donated kidneys were selected on the basis of CT findings; presence of a complex vascular anatomy was the principal consideration, and, if vascular anatomy was simple and the urinary tract was normal, the left kidney was preferred for donation because of its greater venous length. Surgeons recorded the number of arteries, early-branching arteries, and veins in each kidney by using the same criteria as those used in CT angiography, as well as any abnormality in the vessels and the urinary tract. Findings of CT angiography and CT urography were compared with surgical findings in the 77 donated kidneys.
Statistical Analysis
Interobserver agreement between the two reviewers and between CT angiographic and surgical findings was quantified by using weighted statistics; of <0.20 was considered poor; 0.21–0.40, fair; 0.41–0.60, moderate; 0.61–0.80, good; and 0.81–1.00, excellent. Interobserver agreement between the two reviewers was assessed from the data of independent reviews by the two reviewers, and agreement between CT angiographic and surgical findings was evaluated from the consensus data of the two reviewers and the surgical findings.
Considering surgical findings as the standard, we evaluated the sensitivity and specificity of CT angiography in identifying supernumerary vessels and early-branching arteries.
Weighted CT Dose Index
For the evaluation of the radiation dose to patients, we recorded the manufacturer-reported weighted CT dose index (DI) for each scan, which was demonstrated on the CT console. The weighted CT DI is defined (14) as DI consisting of two-thirds of the peripheral CT DI100 plus one-third the central CT DI100; CT DI100 is a DI based on CT DI measurements over a 100-mm volume (14), and CT DI is demonstrated with the methodology in compliance with federal regulation (15).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Renal Arteries
Both reviewers agreed on the number of arteries (n = 179) in 146 (95%) of 154 kidneys (Table 1); interobserver agreement was excellent ( = 0. 856; 95% CI: 0.808, 0.904). In the remaining eight kidneys with disagreement, the first reviewer observed 17 arteries and the second reviewer noted 15 arteries. The two reviewers reached a consensus that those kidneys had 17 arteries. As a result, CT angiography depicted a total of 196 arteries in the 154 kidneys; 37 (24%) kidneys had multiple arteries (Fig 1).
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= 0.885; 95% CI: 0.829, 0.941). In the remaining four kidneys with disagreement, consensus was reached that two kidneys had early bifurcation. Therefore, CT angiography demonstrated that 20 (13%) of the 154 kidneys had early bifurcation. On the basis of CT angiographic findings, 53 left and 24 right kidneys were chosen for donor nephrectomy. In those kidneys, surgeons identified a total of 91 arteries; 65 kidneys had single arteries, 11 kidneys had two arteries in each kidney, and one kidney had four arteries. Therefore, 12 kidneys had 14 supernumerary arteries. Surgeons also found 11 early-branching arteries. Both reviewers agreed that there was no arterial abnormality such as stricture or calcified plaque.
In the 77 donated kidneys, the first reviewer observed 11 supernumerary arteries and 11 early-branching arteries, whereas the second reviewer noted 12 supernumerary arteries and 10 early-branching arteries. The two reviewers reached the consensus that there were 12 supernumerary and 11 early-branching arteries.
On the basis of the consensus of the two reviewers regarding the number of renal arteries, CT angiographic findings agreed with surgical findings in 75 (97%) of the 77 kidneys (Table 2, Fig 2). Each of the remaining two kidneys with disagreement between CT angiographic and surgical findings had a supernumerary artery. Therefore, the detection rate of CT angiography was 98% (89 of 91 arteries), and the agreement between CT angiographic and surgical findings was excellent ( = 0.896; 95% CI: 0.753, 1.038) (Table 2). The two supernumerary arteries, which were 1.5 and 2.0 mm in diameter, were noted only at surgery but not at prospective CT angiography. Both were identified at retrospectively reconstructed CT angiography (Fig 3). Therefore, the retrospective detection rate of CT angiography for arteries was 100%. Both of the missed arteries were tied off because they were too small to be anastomosed to the recipient arterial system. The respective sensitivity and specificity of CT angiography in identifying supernumerary arteries were 79% (11 of 14) and 100% (65 of 65) for the first reviewer, 79% (11 of 14) and 98% (64 of 65) for the second reviewer, and 86% (12 of 14) and 100% (65 of 65) for the consensus between the two reviewers.
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= 1). The respective sensitivity and specificity of CT angiography in identifying early-branching arteries were 100% (11 of 11) and 100% (66 of 66) for the first reviewer, 91% (10 of 11) and 100% (66 of 66) for the second reviewer, and 100% (11 of 11) and 100% (66 of 66) for the consensus between the two reviewers.
Renal Veins
The two reviewers agreed on the number of veins and the presence or absence of late confluence or venous abnormalities in 150 (97%) of 154 kidneys; interobserver agreement was excellent ( = 0. 861; 95% CI: 0.793, 0.929) (Fig 2). The two reviewers disagreed on whether each kidney had two veins or one vein of late confluence in the remaining four (3%) kidneys, all of which were right kidneys. The two reviewers reached the consensus that two of those kidneys had two veins each, and the other kidneys had a single vein with late confluence. Therefore, CT angiography demonstrated a total of 175 veins, including 95 (54%) veins in the right kidney and 80 (46%) veins in the left kidney. Sixteen (21%) of 77 right kidneys and three (4%) of 77 left kidneys had multiple veins. Twelve (16%) patients had late venous confluence (eight patients in the right and four patients in the left kidney). Three patients (4%) had circumaortic left renal veins, and two (3%) patients had retroaortic left renal veins.
In the 77 donated kidneys, surgeons identified a total of 85 veins; 69 (90%) kidneys had a single vein, eight (10%) kidneys had two veins in each kidney, and two (3%) kidneys had veins with late confluence. At CT angiography, the two reviewers agreed on the number of veins in all (100%) of the 77 kidneys; they found six kidneys with two veins and two kidneys with one vein at late confluence. Therefore, CT angiography depicted 83 (98%) of 85 renal veins that were identified during surgery, and the agreement between CT angiographic and surgical findings was excellent ( = 0.843; P < .001; 95% CI: 0.735, 0.952). The sensitivity and specificity of CT angiography in depicting supernumerary veins were 75% (six of eight) and 100% (69 of 69), respectively. Both of the two veins missed at CT angiography were tied because they were as small as 2 mm in diameter, and both of them were identified in the retrospective evaluation.
Urinary Tract Findings
Interpretation of CT urographic findings was concordant between the two reviewers in all patients. At CT urography, both pelvocalyces and proximal ureters that would be removed for transplantation were well opacified in all patients. The distal or the middle part of ureters were not opacified in 10 (6%) patients, and additionally performed intravenous urography revealed normal contrast material passage throughout the urinary tract in all of those patients. Two patients had incomplete duplication in the unilateral urinary tract, in which the upper and lower ureters joined at the proximal ureter. These kidneys were not donated because of increased surgical difficulty. Another two patients had mild under-rotation of the left kidney. In these kidneys, the renal pelvis was slightly dilated, but CT urography demonstrated normal contrast material passage throughout the entire urinary tract (Fig 4). Therefore, these kidneys were donated for transplantation. In 77 donated kidneys, surgical and CT urographic findings were concordant.
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Radiation Dose
The weighted CT DI was 10.19 mGy for each of the unenhanced and excretory phase scans and 12.88 mGy for the vascular phase scan. Therefore, the weighted CT DI for the entire CT examination was 33.26 mGy for each patient.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Adequate vascular enhancement, which is one of the key factors for high-quality angiograms, is greatly affected by scanning delay. Scanning delay for adequate vascular enhancement can vary widely according to various patient factors. Rubin (16) and Sandstede et al (17) commented that with age there is a substantial increase in the peak transit time of contrast medium from the antecubital vein to the abdominal aorta. Kaatee et al (18) showed that CT angiography of the renal arteries can be performed best with a scanning delay based on the transit time of a test bolus injection. In our study, according to our preliminary experience, we used a scanning delay of 5 seconds after triggering at a threshold of 80 HU in the region of interest located in the abdominal aorta at the renal hilar level. By using this method, the scanning delay varied widely, from 24 to 31 seconds (mean, 27 seconds), as was already demonstrated in previous reports (16,17), and vascular enhancement was adequate in all patients.
In this study, we used a thin–slab MIP technique for vascular assessment, as well as volume rendering and MIP. This technique needs minimal computational complexity, because the volume is restricted to a thin slab that is only several voxel widths in depth (19), and segmentation for organ of interest is not necessary. Therefore, with this technique less time is needed and information about adjacent structures can be provided. On the basis of these advantages, a thin-slab MIP technique seems to be able to play a supplementary role in depicting small vessels, the presence of which was suspicious but not clearly depicted with volume rendering or MIP.
The performance of CT angiography in our study seems to be superior to that in previous studies. According to our data, CT angiographic findings agreed with surgical findings regarding the number of arteries in 97% of donated kidneys, in comparison with previously reported values of 90%–95% (4–10). Interobserver agreement for the interpretation of CT angiographic findings between the two reviewers is higher in our study ( = 0.856) than it was in a comparative study ( = 0.73) (8). Both sensitivity and specificity for detecting supernumerary arteries are also higher in our study (86% and 100%, respectively) than they were in a study with a large population (77% and 89%, respectively) (6). We believe that multi–detector row CT, automatic bolus tracking, and advanced three-dimensional reconstruction techniques contributed to this improvement.
In our study, CT urography was used for evaluating the urinary tract of potential kidney donors. Recently, CT urography has become more feasible since multi–detector row CT was introduced into clinical practice (20–22). We generated CT urographic images from the excretory phase CT data. In previous studies, the excretory phase CT data have been used only for the evaluation of a renal parenchymal abnormality (4,8,10), and urinary tract was assessed with a CT scanogram or a screen-film radiograph.
There may be some doubts regarding the application of CT urography for the preoperative evaluation of living donor kidneys, because the necessary observations are simple for these kidneys, that is, focus on the detection of unsuspected urinary obstruction or ureteral duplication. Moreover, previous reports have already demonstrated that a CT scanogram or screen-film radiograph is sufficient for urinary tract evaluation. However, a CT scanogram may be limited by the relatively poor spatial resolution (<1 line pair per millimeter) (23), and screen-film radiograph may cause complexity of examinations, because patients should be immediately transferred to a urography suite within a few minutes of the initiation of the main bolus of contrast material at CT. With CT urography in our study, the urinary tract, as well as the renal parenchyma, can be demonstrated with high resolution, and all components of the donor kidney, including vessels, the parenchyma, and the urinary tract, can be evaluated thoroughly in a single session.
Another major criticism may arise against increased radiation dose, in particular that caused by an additional third CT acquisition for CT urography. To reduce radiation dose, we used a tube current of 200 mA, which is slightly lower than that used in previous studies (210–300 mA), a short gantry rotation time (0.8 second per rotation), and a high-speed mode (section pitch of 6, equivalent to beam pitch of 1.5). In our study, the weighted CT DI for the entire CT examination was 33.26 mGy for each patient, which satisfies a proposed guideline of 35 mGy for general abdominal CT (24). It may be possible to reduce more of the radiation dose by changing a detector array and the tube current value. Furthermore, overlapping reconstruction can substitute an excessively thin-section scanning, which potentially increases radiation dose. However, it should not be overlooked that excessive reduction of radiation dose as a result of lowering the tube current or changing detector array may deteriorate the image quality. Therefore, the CT protocol should be balanced between the radiation dose and the image quality. We hope that a future study will be performed to determine a reasonable CT protocol.
A potential limitation of this study is that the surgeons preferentially removed the kidney that had less complicated vascular anatomy. Therefore, the kidneys with more complex vessels had no pathologic proof, and the sensitivity and specificity values were given only for the less-complicated kidneys.
In our study, we used a four-channel multi–detector row CT scanner. Therefore, all parameters for CT acquisition and intravenous contrast material injection were determined to a four-channel system and should be changed in a further advanced system. Recently, a 16-channel system has been developed, which can surprisingly increase the scanning speed without requiring additional output from the x-ray tube. With this system, abdomen and pelvis could be scanned in 7 or 8 seconds, with a 2.5-mm nominal section thickness and a table speed of 120 mm/sec (detector array of 1.5 x 16 mm, pitch of 2.0, gantry rotation time of 0.4 second).
In summary, with use of a four-channel multi–detector row CT scanner, CT angiography can allow vascular assessment and dedicated CT urography can be performed. Given the merits of this advanced technology, we believe that multi–detector row CT may improve the performance of CT in the comprehensive evaluation of living renal donors.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, K.S.C.; study concepts and design, K.S.C., J.K.K.; literature research, S.Y.P., H.j.K.; clinical studies, C.S.K., H.J.A., T.Y.A.; data acquisition, J.K.K., S.Y.P.; data analysis/interpretation, J.K.K., S.Y.P., K.S.C.; statistical analysis, J.K.K.; manuscript preparation and editing, J.K.K.; manuscript definition of intellectual content, revision/review, and final version approval, J.K.K., K.S.C.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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