DOI: 10.1148/rg.242035104
RadioGraphics 2004;24:453-466
© RSNA, 2004
Multi–Detector Row CT Evaluation of Living Renal Donors Prior to Laparoscopic Nephrectomy1
Satomi Kawamoto, MD,
Robert A. Montgomery, MD, PhD,
Leo P. Lawler, MD,
Karen M. Horton, MD and
Elliot K. Fishman, MD
1 From the Russell H. Morgan Department of Radiology and Radiological Science (S.K., L.P.L., K.M.H., E.K.F.) and Department of Surgery (R.A.M.), Johns Hopkins Hospital, 601 N Caroline St, Room 3254, Baltimore, MD 21287-0801. Presented as an education exhibit at the 2002 RSNA scientific assembly. Received April 11, 2003; revision requested May 14 and received June 27; accepted June 27. All authors have no financial relationships to disclose. Address correspondence to E.K.F. (e-mail: efishman@jhmi.edu).
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Abstract
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Since its introduction in 1995, laparoscopic nephrectomy has become the preferred technique at many medical centers for the harvesting of kidneys from living donors for transplantation. Because the field of view at laparoscopic surgery is limited, preoperative radiologic evaluation of the donor’s anatomy—the renal veins and arteries, collecting system, and parenchyma—is critical. Spiral computed tomographic (CT) angiography is a fast, safe, minimally invasive, and generally accepted method for preoperative evaluation of the renal vessels. Multi–detector row CT scanners offer shorter image acquisition time, narrower collimation, better spatial resolution, and less tube heating than do single–detector row CT scanners. Multi-row scanners also provide more complete anatomic coverage, increased contrast enhancement of the arteries, and greater longitudinal spatial resolution—all of which are important both for accurate imaging of the renal vasculature and for three-dimensional postprocessing of image data. Dual-phase multi–detector row CT angiography combined with three-dimensional postprocessing enables minimally invasive and highly accurate depiction of the preoperative donor anatomy. To make the most effective use of this method, radiologists must be familiar with its technical aspects, advantages, and potential pitfalls. They also must be able to identify variations in vasculature and in renal and extrarenal anatomy that are important for laparoscopic donor nephrectomy.
© RSNA, 2004
Index Terms: Arteries, abnormalities, 96.131, 96.132, 96.134 • Computed tomography (CT), angiography, 81.12118, 96.12916, 96.12918 • Genitourinary system, calculi, 80.81 • Kidney, abnormalities, 81.141, 81.1421, 81.143, 81.3124, 81.3141, 81.32 • Kidney, transplantation • Veins, abnormalities, 96.131, 96.132, 96.134
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:
- Describe the multi–detector row CT angiographic technique used for preoperative evaluation of living renal donors.
- Identify anatomic variations of renal vessels and other renal conditions important for laparoscopic nephrectomy in living donors.
- Discuss the advantages and potential pitfalls of multi–detector row CT angiography for preoperative evaluation of renal donors.
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Introduction
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More than 50,000 patients awaited a kidney transplant in the United States in October 2001, but only approximately 9,000 cadaver kidney transplants were performed each year during the previous decade (1). Since the first reported use of laparoscopic nephrectomy in a living donor in 1995 (2), this method has lowered an important barrier to kidney donation and has become an important treatment option (3,4) by reducing postoperative pain and recovery time, as well as the length and cost of the hospital stay (5–7). Furthermore, long-term allograft function is comparable between kidneys obtained with laparoscopic nephrectomy and those obtained with the conventional open surgical procedure (8,9). Responses to a survey of the 31 largest U.S. renal transplantation centers, which perform 43% of all renal transplantation surgeries in the country, indicated that nearly all (97%) centers either offered the option of laparoscopic donor nephrectomy or planned to offer it soon (10).
Several technical challenges are associated with laparoscopic removal of a kidney (11). The surgical field of view is limited, and preoperative evaluation of the donor’s anatomy is critical. In addition to information about the renal arterial anatomy, complete information regarding the length and number of renal veins and the presence of venous anomalies is required. The left kidney is preferred for laparoscopic nephrectomy in living donors because it is technically easier to remove and has a longer renal vein than the right kidney (11,12). In addition, the right renal artery might be foreshortened during transperitoneal laparoscopic dissection, because the right renal vein is located behind the inferior vena cava. Even left kidneys with one or two accessory renal arteries or veins are considered by most surgeons to be acceptable for donor nephrectomy. Such vascular anomalies are not usually problematic, especially when they have been identified and localized with preoperative imaging (12).
Computed tomographic (CT) angiography is a fast and minimally invasive procedure that enables accurate visualization of arterial and venous anatomy for planning of laparoscopic nephrectomy. Several studies have demonstrated that spiral CT angiography may be used in place of excretory urography and renal angiography, which were traditionally used for the evaluation of potential renal donors (13–19). Accuracies reported for spiral CT angiography with a single–detector row scanner for the depiction of accessory arteries, prehilar branching, and renal venous anatomy, respectively, are 78%–98%, 89%–99%, and 90%–99% (13–18,20). Multi–detector row scanners offer shorter image acquisition times, reduced tube heating, narrower collimation, and improved spatial resolution, compared with single–detector row scanners. Multi–detector row scanners are particularly useful for angiographic applications because they provide more complete anatomic coverage, increased contrast enhancement of the arteries, and greater longitudinal spatial resolution (21,22), as well as more detailed and sensitive depiction of the renal vasculature (23).
We have examined more than 340 potential renal donors for laparoscopic nephrectomy with multi–detector row CT angiography since January 2000. In this article, we review the technical aspects of this method for evaluation of potential renal donors, and we discuss the advantages of multi–detector row scanners over single–detector row scanners, particularly in the evaluation of the renal vasculature. We describe anatomic variations in the renal arteries and veins, as well as other unexpected renal and extrarenal conditions that may affect the planning and success of laparoscopic donor nephrectomy. Clinical examples of variant anatomy are amply illustrated with radiologic images. We also discuss the potential pitfalls of multi–detector row CT angiography that are related to the imaging technique, difficulties of image interpretation, and anatomic variants in donors.
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CT Technique
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Optimal timing of image acquisition is a crucial technical aspect of multi–detector row CT angiography. With eight- and 16-section multi–detector row scanners, the timing of data acquisition is even more critical than with single–detector row scanners because the image acquisition time is substantially shorter. Dual-phase imaging for evaluation of the renal vessels and parenchyma is performed at most institutions. The optimal delay for each imaging phase after contrast material injection depends on the volume of contrast material administered, the rate of injection, and the subject’s cardiac output. Arterial phase images are usually acquired 15–30 seconds after the initiation of intravenous contrast material injection, but the timing of the second phase of imaging may vary. We perform venous phase imaging 55 seconds after intravenous contrast material injection. At some institutions, nephrographic phase imaging is performed 75–100 seconds after contrast material injection. For evaluation of the renal collecting system and ureters, a delayed topogram may be obtained 5 minutes after intravenous contrast material administration.
Some investigators recommend precontrast CT imaging for localization of the kidneys, exclusion of nephrolithiasis and urolithiasis, and characterization of any renal masses (17,23). However, potential renal donors are generally healthy young adults, and the radiation dose should be kept to a minimum. To minimize the dose of ionizing radiation, we do not usually perform CT imaging prior to contrast material administration. Since nephrolithiasis is an important finding in prospective renal donors, the omission of precontrast imaging is a potential limitation of our technique. However, in our experience, the presence of nephrolithiasis and urolithiasis can be detected on arterial phase images with careful scrolling through continuous images or with the use of maximum intensity projection (MIP) images (Fig 1), especially when calculi are more than several millimeters in diameter. In a recent multi–detector row CT study of 65 patients with urinary tract abnormalities, all five calculi present (two in the renal pelvis, one in the distal ureter, one in the bladder, and one in an ileal coduit) were seen both on the precontrast CT images and on the postcontrast CT images (24). The presence of renal neoplasms—another criterion for exclusion of a prospective donor from laparoscopic nephrectomy—is best evaluated with nephrographic phase imaging, approximately 90–100 seconds after intravenous contrast material administration. Our protocol is not optimal for the depiction of renal masses, because we perform the second (venous) phase of imaging 55 seconds after administering intravenous contrast material. The entire renal parenchyma should be carefully evaluated to rule out renal masses.
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Figure 1a. (a) Coronal volume-rendered image at the level of the abdominal aorta shows a normal left renal main artery (black arrows) and vein (white arrows) that have classic anatomic structure. (b) Coronal volume-rendered image slightly posterior to a shows an accessory artery (black arrow) and accessory vein (white arrows) connected to the lower pole of the left kidney and a small renal calculus in the upper pole (arrowhead). (c) Axial MIP image shows small bilateral renal calculi (arrowheads), as well as the classically structured main renal artery (black arrow) and vein (white arrow). (d) Axial volume-rendered image shows the polar artery (solid arrow) and the retroaortic polar vein (open arrows).
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Figure 1b. (a) Coronal volume-rendered image at the level of the abdominal aorta shows a normal left renal main artery (black arrows) and vein (white arrows) that have classic anatomic structure. (b) Coronal volume-rendered image slightly posterior to a shows an accessory artery (black arrow) and accessory vein (white arrows) connected to the lower pole of the left kidney and a small renal calculus in the upper pole (arrowhead). (c) Axial MIP image shows small bilateral renal calculi (arrowheads), as well as the classically structured main renal artery (black arrow) and vein (white arrow). (d) Axial volume-rendered image shows the polar artery (solid arrow) and the retroaortic polar vein (open arrows).
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Figure 1c. (a) Coronal volume-rendered image at the level of the abdominal aorta shows a normal left renal main artery (black arrows) and vein (white arrows) that have classic anatomic structure. (b) Coronal volume-rendered image slightly posterior to a shows an accessory artery (black arrow) and accessory vein (white arrows) connected to the lower pole of the left kidney and a small renal calculus in the upper pole (arrowhead). (c) Axial MIP image shows small bilateral renal calculi (arrowheads), as well as the classically structured main renal artery (black arrow) and vein (white arrow). (d) Axial volume-rendered image shows the polar artery (solid arrow) and the retroaortic polar vein (open arrows).
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Figure 1d. (a) Coronal volume-rendered image at the level of the abdominal aorta shows a normal left renal main artery (black arrows) and vein (white arrows) that have classic anatomic structure. (b) Coronal volume-rendered image slightly posterior to a shows an accessory artery (black arrow) and accessory vein (white arrows) connected to the lower pole of the left kidney and a small renal calculus in the upper pole (arrowhead). (c) Axial MIP image shows small bilateral renal calculi (arrowheads), as well as the classically structured main renal artery (black arrow) and vein (white arrow). (d) Axial volume-rendered image shows the polar artery (solid arrow) and the retroaortic polar vein (open arrows).
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Our standard protocol for performing multi–detector row CT with a four-row scanner is outlined in the Table. The following parameters are used for both arterial and venous phase imaging: 120 kVp, 150–180 mAs, 4 x 1-mm detector collimation, 0.5-second rotation time, table feed of 6 mm per rotation, and pitch of 6. The duration of arterial phase imaging is approximately 20 seconds. All image data were reconstructed at 1-mm intervals by using the body soft-tissue algorithm. We routinely use water as a negative oral contrast agent. Positive oral contrast materials are not used because they obscure intravenous contrast enhancement on three-dimensional (3D) angiographic images.
For administration of intravenous contrast material, a 20-gauge peripheral line is inserted in an antecubital vein. After a scout CT image is obtained, arterial phase and venous phase volumetric image data sets are acquired at 25 seconds and 55 seconds, respectively, after the start of an intravenous injection of 120 mL of iohexol (Omnipaque 350; Amersham Health, Princeton, NJ) at an injection rate of 3 mL/sec. The imaging volume in arterial phase imaging extends from above the kidneys to just below the common iliac arteries (approximately 25 cm) and in venous phase imaging extends from above the kidneys to just below the bifurcation of the inferior vena cava (approximately 20 cm).
Renal veins enhance quickly and are usually visible on arterial phase images. We evaluate renal artery and venous anatomy primarily on arterial phase images; but if the renal veins are not enhanced on the arterial phase images, venous phase images may be used to further define the venous anatomy. The adrenal, gonadal, and lumbar veins enhance more slowly, and these veins also are evaluated on venous phase images. No timing of the intravenous contrast material bolus or computer-assisted triggering of bolus administration was used for arterial or venous phase imaging, because in previous experiments we had found that delays of 25 seconds for arterial imaging and of 55 seconds for venous imaging of the abdomen yield excellent results in most subjects.
A delayed topogram is routinely obtained with a low kilovoltage peak 5 minutes after intravenous contrast material administration to depict the collecting system and ureters. At some institutions, subjects are transferred to a standard radiography suite for conventional screen-film radiography of the abdomen and pelvis immediately after CT.
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Three-dimensional Image Processing and Evaluation
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The CT image data sets are transferred for subsequent processing and review to a freestanding workstation (either Onyx Infinite Reality or O2; Silicon Graphics, Mountain View, Calif) equipped with postprocessing software (3D Virtuoso; Siemens Medical Solutions, Malvern, Pa).
Reviewers can edit the data sets to create optimal 3D angiographic images at the workstation at real-time frame rates of 10–30 frames per second. We use source images as well as 3D images for analysis. For 3D CT angiography, volume rendering is most commonly used, but MIP also is used as an adjunct technique, especially for the depiction of small vessels. Alternative postprocessing techniques for depiction of complex vascular anatomy include multiplanar reformation and stereoscopic display. Three-dimensional volume-rendered images depict the vascular anatomy in a format that is familiar to most surgeons. Although some authors have reported that axial CT images are best for depiction of accessory arteries and early branching, multiplanar reformations and 3D reconstructions can provide supplemental information (25), especially about vessels that are located in oblique planes (23) and regarding prehilar branching of the renal artery (13).
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Advantages of Multi–Detector Row CT
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Volume Coverage in a Single Breath Hold
Multi–detector row CT offers shorter image acquisition time than single–detector row CT, as well as the ability to image large volumes without a decrease in the signal-to-noise ratio. Because accessory renal arteries may arise from the aorta or common iliac arteries (26), the volume of coverage is important for preoperative assessment of prospective living kidney donors. At our institution, the usual scan length at arterial phase imaging to depict the main and potential accessory renal arteries is approximately 25 cm (range, 20–30 cm, depending on the donor’s body habitus). With a single–detector row scanner, 3-mm collimation, rotation time of 1 second, a table increment of 6 mm per rotation, and a pitch of 2, a 25-cm volume can be imaged in approximately 42 seconds. With a four-section multi–detector row scanner and 4 x 1-mm collimation, rotation time of 0.5 second, a table increment of 6 mm per rotation, and table speed of 12 mm per second, the same volume can be imaged in approximately 21 seconds. With a 16-section multi–detector row scanner and 16 x 0.75-mm collimation, rotation time of 0.5 second, a table increment of 12 mm per rotation, and table speed of 24 mm per second, the same volume can be imaged in approximately 10 seconds. Z-axis resolution is better with faster scanners, even with shorter acquisition times. Also, shorter acquisition times allow for better separation of arterial and venous phases if necessary, as long as imaging is performed after contrast material reaches the renal artery and before it empties into the renal vein.
Better Z-Axis Resolution
Because of the size and course of the renal arteries, the z-axis resolution is critical for adequate depiction of renal vascular abnormalities and for image postprocessing (27). The combination of thin-section collimation and large-volume coverage likely enhances the identification of accessory vessels, which are occasionally missed with single–detector row scanners.
With single–detector row CT scanners, the need to optimize z-axis resolution resulted in the limitation of anatomic coverage. Multi–detector row CT scanners avoid this dilemma (27). With the use of four-section multi–detector row CT scanners, 1-mm section thickness is routinely possible even for imaging of large volumes, because there is less of a compromise between section thickness, acquisition time, and volume of coverage. Isotropic data sets (1-mm spatial resolution in all planes) are ideal for 3D imaging and CT angiography because of superior z-axis resolution. With single–detector row CT, data sets are anisotropic, and images are less accurate for evaluation of small vessels. In addition, with multi–detector row CT scanners, unlike single–detector row scanners, the section thickness can be chosen retrospectively, and thin and thick sections can be retrospectively reconstructed for review or archiving.
Decreased Tube Heating
Because of tube heating limitations, single–detector row CT scanners may not allow the acquisition of thin sections covering a large area, which is necessary to obtain useful 3D image data sets. Tube heating is a potential limitation in the single–detector row CT evaluation of an obese potential renal donor. Tube heating limitations may prevent an increase in amperage to the levels needed to image the appropriate volume in a subject with a large body habitus and during a single breath hold (17). With multi–detector row CT scanners, tube heating is not typically a problem in such situations.
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Anatomic Variations and Pathologic Conditions
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Preoperative CT evaluation of prospective renal donors may depict the presence or absence of renal arterial and venous variants, abnormalities of the renal parenchyma and collecting system, renal calculi, and other renal and extrarenal abnormalities. The information obtained with multi–detector row CT is used to select the donor and the kidney for nephrectomy and to help plan the surgical procedure and prevent complications.
Renal Arterial Anatomy
Between 70% and 75% of people may be expected to have one renal artery on each side (ie, one renal artery per kidney), with the remainder being expected to have two or more renal arteries on each side (26,28). Renal arteries typically arise at the level of the upper margin of the second lumbar vertebral body, 1 cm below the origin of the superior mesenteric artery (29). Each renal artery supplies smaller, inferior adrenal arteries, which may be single or multiple.
In a study of 400 cadaver donors with 800 kidneys, Pollak et al (28) found that 23% had double renal arteries (Fig 2), 4% had triple arteries(Fig 3), and 1% had quadruple arteries. Multiple renal arteries occur on the left side in 26%–32% of people and on the right side in 23%–29% (28,30). Bilateral multiple renal arteries occur in 15% (28). Higher or lower origins are not uncommon among accessory arteries. So-called polar arteries are connected to the superior or inferior renal pole (Figs 1, 4). Accessory arteries are considered to be persistent embryonic lateral splanchnic arteries (26). First and second accessory arteries are often small in caliber, ranging from 3 to 4 mm in diameter (30). Rarely, accessory renal arteries arise from the celiac or superior mesenteric arteries, near the aortic bifurcation, or from the common iliac arteries (26).
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Figure 2a. (a) Coronal volume-rendered image shows two left renal arteries (white arrows) and a single right renal artery, as well as a retroaortic left renal vein (black arrow). (b) Oblique coronal volume-rendered image shows the left renal vein behind the aorta (arrow).
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Figure 2b. (a) Coronal volume-rendered image shows two left renal arteries (white arrows) and a single right renal artery, as well as a retroaortic left renal vein (black arrow). (b) Oblique coronal volume-rendered image shows the left renal vein behind the aorta (arrow).
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Prehilar branching is a variant in which the renal artery branches before it reaches the renal hilum. It is important to detect any prehilar branching that occurs within 2 cm of the origin of the renal artery from the aorta, because most surgeons require at least a 2-cm length of renal artery before hilar branching, to ensure adequate control and anastomosis (Figs 5, 6).
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Figure 5. Coronal MIP image shows accessory renal arteries that arise from the aorta near the bifurcation (white arrows) and that supply the lower pole of each kidney, as well as early branching of two left renal arteries (black arrows).
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Figure 6. Oblique coronal volume-rendered image shows a circumaortic left renal vein (white arrow) and early branching of the left renal artery approximately 1.5 cm from its origin (black arrow).
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Venous Anatomy
From the surgeon’s perspective, knowledge of the venous anatomy prior to laparoscopic surgery is important to avoid vascular injuries and bleeding. The appropriate venous structures, including renal veins, adrenal veins, gonadal veins, and lumbar veins, must be identified and localized.
Renal veins have a more uniform anatomic pattern than do renal arteries, and 92% of people have one renal vein on each side (28). The left renal vein is approximately 7.5 cm long, and the right renal vein is approximately 2.5 cm long (26). Duplicate renal veins are more common on the right side. Two to four entirely separate right renal veins are found in 15% of people, whereas multiple left renal veins with separate renal hilar origin and separate caval entry are rare (1%) (31). The most common left renal venous variant, a circumaortic renal vein, is seen in 5%–7% of individuals (Fig 6). A retroaortic renal vein is seen in 2%–3% (Figs 2, 7). The retroaortic component may receive flow from lumbar veins and may run caudad and enter the inferior vena cava in the lower lumbar region (Fig 8), or it may subdivide before entering the inferior vena cava.
The left adrenal vein and gonadal vein empty into the left renal vein in nearly all individuals. However, on the right side, the gonadal vein empties into the right renal vein only in 7%, and the accessory branch of the adrenal vein drains into the right renal vein in 31% (31). The left adrenal vein joins the left renal vein from above, lateral to the abdominal aorta. The reported average distance between the left lateral wall of the aorta and the junction of the left adrenal vein in 61 living renal donor subjects evaluated with CT angiography was 5.2 mm (Figs 9, 10) (32). The left gonadal vein joins the left renal vein from below, lateral to the left adrenal vein. Multiple (usually two) left gonadal veins occur in about 15% of people (31). Three-dimensional CT angiography with volume rendering is highly accurate for the detection of adrenal veins, and the left adrenal vein was consistently localized with single–detector row as well as multi–detector row scanners in 92.5% of preoperative living renal donors (Figs 9, 10) (32).
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Figure 9. Coronal volume-rendered image shows typical locations of the left adrenal vein and left gonadal vein: The left adrenal vein (black arrow) enters the left renal vein from above and lateral to the abdominal aorta. The left gonadal vein (white arrow) enters the left renal vein from below and lateral to the left adrenal vein.
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Figure 10. Coronal volume-rendered image shows an atypically located left adrenal vein (arrow) that drains medially to the superior portion of the left renal vein where it overlaps the aorta.
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There is great variation in the number and location of the lumbar veins. Retroperitoneal veins (lumbar, ascending lumbar, and hemiazygos veins) communicate with the left renal vein in 75% of people (Fig 11) and with the right renal vein in 3% (31). Communications occur directly through the lumbar veins or indirectly through the gonadal veins. Baniel et al (33) reported that a lumbar vein entered the left renal vein in 43% of 102 consecutive patients who underwent retroperitoneal lymph node dissection.
Other Pathologic Conditions
Unexpected renal and extrarenal conditions detected at preoperative CT examination may affect the planning and performance of donor nephrectomy. Renal arterial stenosis due to fibromuscular dysplasia or atherosclerosis (Fig 12) may be found and may influence the plan for donor surgery. Other renal arterial diseases, such as aneurysm and arteriovenous malformation (Fig 13), also may be found preoperatively. Multi–detector row CT angiography can provide excellent preoperative information about such arterial lesions.
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Figure 12. Volume-rendered image shows mild stenosis (arrow) of the right main renal artery just past its origin, with minimal poststenotic dilatation. Note the accessory renal artery that arises near the main artery and feeds the upper pole of the right kidney.
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Figure 13. Axial MIP image shows two nodular hypervascular lesions (arrow) in the right kidney that exhibit a level of contrast enhancement similar to that of the renal vasculature, a finding compatible with renal artery aneurysm or small arteriovenous malformation.
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The presence of retroperitoneal varices is one of the possible contraindications for donor nephrectomy (Fig 14). Other unexpected vascular abnormalities that may be found at preoperative evaluation with multi–detector row CT include aneurysms of the abdominal aorta and splenic artery (Fig 15).
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Figure 15. Coronal MIP image shows unusual accessory arteries that arise from the superior mesenteric artery and aorta and extend in a serpentine pattern along both sides of the aorta (arrows). At surgery, the corkscrew-like arteries were seen to course through the mesocolon and along the gonadal vein. Although great caution was used during surgery, these arteries were extremely fragile and avulsed from the aorta during elevation. Although good hemostasis was achieved by the end of the procedure, the subject developed intraperitoneal hemorrhage a few hours after nephrectomy and underwent emergent exploratory laparotomy to achieve hemostasis.
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A renal donor must have two kidneys of normal size and position. Although criteria for the exclusion of laparoscopic donor nephrectomy have not been clearly defined, the presence of a renal anomaly such as unilateral agenesis, renal ectopia, or horseshoe kidney (Fig 16) is a contraindication for the procedure. Other exclusionary criteria are the presence of renal arterial disease, renal neoplasm, hydronephrosis, cortical atrophy, sponge kidney (Fig 17), renal papillary necrosis, or retroperitoneal varices (23). Renal parenchymal disease such as polycystic disease can be well evaluated with multi–detector row CT. Tumors such as renal cell carcinoma (Fig 18) or angiomyolipoma (Fig 19) also may be found incidentally during preoperative evaluation with multi–detector row CT. Small, isolated renal cysts are not usually considered contraindications for donor nephrectomy (23).
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Figure 17. Topogram from delayed CT shows medullary blush (arrows), a finding compatible with sponge kidney. The diagnosis was confirmed with intravenous pyelography. Medullary blush is more evident in the right kidney than in the left kidney because there is no superimposed bowel air.
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Nephrolithiasis may be a contraindication for living donor nephrectomy because of the risk that recurrent stones, obstruction, and infection may injure the remaining kidney. This condition also places the recipient at risk (34). With the use of nonenhanced CT, however, small renal calculi are found much more frequently than with previously used modalities, and urolithiasis is detected in many asymptomatic subjects. The definition of nephrolithiasis as an exclusionary criterion varies according to the medical center. The presence of a single calculus larger than 5 mm or of multiple calculi is the definition used in one center (17), whereas the presence of a single stone larger than 8 mm or of more than three stones located unilaterally and including at least one stone larger than 3 mm is the definition used in another center (23).
Duplicate collecting systems are found in approximately 1% of the general population (Fig 20). Other abnormal findings may include hydronephrosis, ureteropelvic junction obstruction, or uroepithelial tumor. These anomalies also may affect the surgical planning or procedure.
Extrarenal abnormalities such as benign and malignant lesions in other abdominal organs occasionally may be found incidentally during preoperative renal donor evaluation. Such findings may affect the selection and planning of laparoscopic donor nephrectomy.
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Potential Pitfalls
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Technical Failure
The timing of image acquisition relative to intravenous administration of the contrast material bolus is crucial for adequate assessment of renal vasculature. With multi–detector row CT scanners that have a large number of detectors, such as 16-section scanners, acquisition time is short, yet imaging must be performed during adequate contrast enhancement. Some authorities recommend that the imaging delay be adjusted for the individual subject by using a test bolus injection to determine the contrast medium transit time or computer-assisted software to automate or trigger bolus administration (27). We use fixed delays of 25 seconds for arterial imaging in the abdomen and 55 seconds for venous imaging. Most potential renal donors are relatively young and healthy, and a fixed delay time has yielded excellent image quality for evaluation of arteries and veins with the four-section multi–detector row CT scanner and protocol described in this article. Macari et al (35) evaluated 70 patients who had infrarenal abdominal aortic aneurysms by using a four-section multi–detector row CT scanner and a fixed 25-second delay after intravenous administration of a 150-mL bolus of contrast material at a rate of 4 mL/sec. Adequate enhancement of the aorta and of iliofemoral runoff was achieved in all subjects. However, further evaluation is needed with faster (eight- or 16-section) multi–detector row CT scanners.
Since an accessory renal artery can arise from anywhere in the abdominal aorta or common iliac arteries, the volume scanned at preoperative evaluation is important. The scan length, to adequately depict the main and potential accessory renal arteries, is usually 20–30 cm.
Any motion of subjects will result in degradation of images, especially of 3D images. However, with multi–detector row CT scanners, imaging time is significantly shorter than with single-row scanners, and most subjects can hold their breath for the duration of the sequence so as not to cause severe artifacts from respiration or other movement.
Interpretive Errors
Interpretive errors primarily result from insufficiently attentive review of the axial source images (15). Rydberg et al (23) reported that there is a learning curve for radiologists and that errors of perception are common during radiologic evaluation of the venous system, especially early in the radiologist’s experience. Small arterial branches, such as inferior adrenal arteries or capsular branches of the renal artery, may be mistaken for prehilar branching of the renal artery or for polar arteries (Fig 21). In our experience, a prominent inferior left adrenal artery has been incorrectly considered a prehilar branch of the renal artery. However, careful evaluation of the entire course of the artery by scrolling through continuous images helped us to differentiate the artery supplying the adrenal gland from a prehilar branch of the renal artery.
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Figure 21. Oblique coronal volume-rendered image shows a small arterial branch (arrows) that arises from the superior aspect of the left renal artery and extends to the left adrenal gland.
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In previous studies performed with single-row spiral CT scanners, even when thin-section collimation was used, small accessory renal arteries with diameters of less than 2 mm were missed at initial image interpretation and seen only in retrospect (18,36). On some occasions, such arteries have not been identifiable even in retrospect.
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Conclusions
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Dual-phase multi–detector row CT angiography combined with 3D postprocessing is a minimally invasive and highly accurate method for preoperative evaluation of renal donors. It provides comprehensive depiction of the renal arterial and venous anatomy, which is critical for the planning and performance of laparoscopic living donor nephrectomy. The capabilities of multi–detector row scanners, including shorter image acquisition time, narrower collimation, and improved spatial resolution, make multi–detector row CT advantageous for evaluation of renal donors because of greater anatomic coverage, increased contrast enhancement of the arteries, and greater longitudinal spatial resolution, which are important both for image acquisition and for 3D postprocessing.
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Footnotes
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Abbreviations: MIP = maximum intensity projection,
3D = three-dimensional
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References
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S. S. Raman, S. Pojchamarnwiputh, K. Muangsomboon, P. G. Schulam, H. A. Gritsch, and D. S. K. Lu
Surgically Relevant Normal and Variant Renal Parenchymal and Vascular Anatomy in Preoperative 16-MDCT Evaluation of Potential Laparoscopic Renal Donors
Am. J. Roentgenol.,
January 1, 2007;
188(1):
105 - 114.
[Abstract]
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Abstract
LEARNING OBJECTIVES
