Multislice CT accurately evaluates kidney donors

The prevalence of end-stage renal disease (ESRD) has increased over the last decade. Combined with a limited supply of cadaver kidneys, the increase has almost quadrupled the waiting list for kidney transplantation.¹ This mismatch between the demand and supply of kidney grafts has led to a tremendous rise in the number of renal transplant surgeries with kidneys procured from living donors.²

Use of the laparoscopic approach to harvest kidneys from living donors is also growing, as this approach lowers morbidity and shortens hospital stays compared with open nephrectomy, leading to a high degree of patient satisfaction. Laparoscopic surgery is technically challenging, however, due to limited surgical exposure and poor operative visibility.³ Detailed evaluation of the kidney donor with imaging is essential prior to such surgery to decrease intraoperative morbidity and complications.

Since the introduction of the spiral scanner, CT angiography has been extensively used in the body as a noninvasive alternative to catheter angiography. Results from single-slice spiral CT scanners for the assessment of living renal donors have been encouraging.⁴ Multislice CT offers several advantages over single-slice scanners and is therefore well suited for this indication.

Depending on the number of channels (four, eight, 16, and so on), MSCT scanners are four to 25 times faster than conventional single-slice spiral scanners. The shortened scan duration effectively reduces motion artifacts and allows scanning in the optimal arterial and venous phases separately. In addition, MSCT allows longer z-axis coverage, as tube heating is no longer an issue. Finally, MSCT offers better z-axis resolution, producing near-isotropic imaging data, which helps to create excellent-quality multiplanar reformations and 3D rendering.^5,6

CT TECHNIQUE

MSCT evaluation of kidney donors is technically challenging. Optimal timing of image acquisition after contrast administration is crucial because the scan times are very short (@7 seconds on 16-slice CT scanners). It is highly advisable to individualize the scan delay for arterial phase instead of using an empiric scan delay. This can be performed by injecting a test bolus to estimate the time of arrival of contrast agent in the arteries. Most current MSCT scanners offer automated bolus-tracking capability that enables initiation of scanning automatically in the artery of interest at the desired threshold of arterial enhancement.

At our institution, we use an automated triggering technique, placing the region of interest in the aorta and defining the threshold of aortic enhancement at 125 to 150 HU. We routinely use about 120 to 130 cc of iodinated nonionic contrast medium (300 mgI/mL), injected at a rate of 4 mL/sec through a peripherally placed 18 to 20-gauge IV cannula (see table).

Some investigators have proposed that optimal angiographic enhancement can also be achieved with the use of a low volume (80 mL) of high iodine concentration contrast media (400 mgI/mL). A saline flush after the administration of contrast medium has also been found useful. The field of scanning should include the origin of the celiac artery and the bifurcation of the iliac arteries, as the accessory renal arteries can originate from celiac, superior mesenteric, and common iliac arteries.

POSTPROCESSING

Each data set (arterial phase, venous phase, and excretory phase) is retroreconstructed with a 50% overlap for image postprocessing. A dedicated 3D workstation and trained technologists are imperative to achieve proper image reconstruction in various orthogonal planes and to create multiplanar reformations and 3D rendering.

^3,5,6

We routinely create coronal reformations, multiplanar subvolume maximum intensity projections (MIPs), and volume-rendered images.

Axial source images are the basis for making diagnosis, and these are reviewed on a workstation. As the number of images can be 300 to 600 per patient, viewing films is impractical. A dedicated workstation allows rapid scrolling through the images and provides tools for generating various measurements of donor anatomy. Noncontrast images are useful for detecting renal or urinary tract stones and for characterizing incidentally detected renal or adrenal tumors. Arterial phase images are evaluated for presence, location, and number of renal arteries and their branching pattern.

Multiple renal arteries may be seen in 25% to 30% of donors (Figure 1). These accessory arteries may arise from the aorta or common iliac arteries and, rarely, from celiac and superior mesenteric arteries. They are smaller than the main artery. An early branching (within 1.5 cm from the origin) of the main renal artery is important to recognize,7 because most surgeons require a length of renal artery before branching of at least 1.5 cm for proper anastomosis in the recipient. Bilateral occurrence of multiple renal arteries has been reported in up to 15% of individuals.^8-10

Identification of renal artery stenosis or fibromuscular dysplasia in the donor renal arteries is equally important. And the presence of a renal tumor or a congenital anomaly such as crossed fused ectopia, horseshoe kidney, or medullary sponge kidney is an absolute contraindication for donor nephrectomy.8-11 The presence of simple cysts, however, is not considered a contraindication. The presence of kidney stone(s) is a relative contraindication, and guidelines may vary among institutions and transplant surgeons.

Knowledge of donor venous anatomy is crucial prior to undertaking a laparoscopic surgery to avoid inadvertent injury to the accessory renal veins and their tributaries. The left kidney is usually preferred for the open approach as well as for the laparoscopic approach because of the longer left renal vein. Multiple renal veins are encountered less frequently than multiple arteries. When present, accessory renal veins usually occur on the right side (Figure 2).

Other critical donor venous anomalies that may preclude donor nephrectomy on that side include a retro-aortic left renal vein, circumaortic left renal vein, and renal arteriovenous malformation/fistulae.⁹ Identification of anomalous venous drainage of adrenal, lumbar, and gonadal veins is important to minimize vascular injury during laparoscopic surgery.

The excretory phase images are assessed for the presence of urinary tract duplication, hydronephrosis, or any ureteric pathology. Though simple duplication of a ureter (Figure 3) is not an absolute contraindication for donor nephrectomy, it affects the surgical technique for ureteric implantation.

In addition to the evaluation of the kidney and urinary tract, the other abdominal organs should be evaluated for the presence of incidental pathologies that may preclude a donor nephrectomy or may affect selection of surgical procedure.

ACCURACY OF MSCT IN RENAL ARTERY EVALUATION

The accuracy of MSCT for the detection of accessory renal arteries has been shown to range from 89% to 97%.

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A small accessory artery missed on MSCT is usually due to interpretation errors and rarely from nonvisualization of the artery. Careful review of axial and reformatted images can minimize these errors.

Three-D imaging with MSCT has come to the forefront in the evaluation and selection of open versus laparoscopic approaches to harvesting the donor kidney. The exquisite detail provided by MSCT prepares a transplant surgeon for what to expect during the surgery and thereby minimizes unexpected intraoperative complications that may jeopardize the outcome of a renal graft. Although it is critical that the radiologist be familiar with the transplant surgeon's requirements, the resolution provided by thin-section MSCT and the quality of 3D images have partly eliminated the need for a reader with extensive experience to interpret these dedicated exams.^5,6,9

MSCT is highly accurate in evaluating renal donors. A proper CT technique and careful review of CT data are important to provide the highest levels of accuracy in evaluating renal donors by MSCT.

DR. SAHANI is a clinical instructor of radiology at Harvard Medical School and an assistant in radiology at Massachusetts General Hospital in Boston. DR. KALVA is a clinical fellow in the department of vascular imaging and intervention at MGH.

References

1. The OPTN national patient list for organ transplantation: Annual report of the U.S. Scientific Registry for Transplant Recipients and the Organ Procurement and Transplantation Network: Transplant Data 2002. U.S. Department of Health and Human Services, Health Resources and Services Administration, Office of Special Programs, Division of Transplantation, Rockville, MD; UNOS, Richmond, VA. www.unos.org.

2. Evans R. Need, demand and supply in kidney transplantation: a review of the data, an examination of the issues and projections through the year 2000. Semin Nephrol 1992;12:234-255.

3. Kawamoto S, Montgomery RA, Lawler LP, et al. Multidetector CT angiography for preoperative evaluation of living laparoscopic kidney donors. AJR 2003;180:1633-1638.

4. Kim TS, Chung JW, Park JH, et al. Renal artery evaluation: comparison of spiral CT angiography to intra-arterial DSA. J Vasc Interv Radiol 1998;9:553-559.

5. Saini S. Advances in Multi-Detector CT. JCAT 2004;28 Suppl:S1.

6. Saini S, Dsouza RV. Optimizing technique for multi-slice CT. Eur Radiol 2003;13 Suppl 5:M21-24.

7. Kim JK, Park SY, Kim HJ, et al. Living donor kidneys: usefulness of multi-detector row CT for comprehensive evaluation. Radiology 2003;229:869-876.

8. Kawamoto S, Montgomery RA, Lawler LP, et al. Multi-detector row CT evaluation of living renal donors prior to laparoscopic nephrectomy. Radiographics 2004;24:453-466.

9. Fleischmann D. MDCT of renal and mesenteric vessels. Eur Radiol 2003;13 Suppl 5:M94-101.

10. Janoff DM, Davol P, Hazzard J, Lemmers MJ, et al. Computerized tomography with 3-dimensional reconstruction for the evaluation of renal size and arterial anatomy in the living kidney donor. J Urol 2004;171:27-30.