laparoscopic approach to donor nephrectomy is possible or whether an open approach should be adopted [3]. In view of this, the presence of a tiny single calculus (<5 mm diameter) or a cyst of same size in one of the donor kidneys is not a con- traindication for its retrieval for a recipient, irre- spective of the complexities in its vascular variants. If both donor kidneys are normal, then the kidney with the less complex vascular anatomy is preferred, making a less invasive laparoscopic ap- proach for nephrectomy feasible. The left kidney is usually preferred for a recipient because it provides a longer segment of the renal vein, which joins the inferior vena cava (IVC), and thus provides more maneuverability to the surgeon to suture the donor vessel patch to the recipient’s iliac vein. In addition to defining the vascular anatomy and variants, imaging should clearly depict pathologic conditions like renal artery atherosclerosis, fibro- muscular dysplasia, aneurysm, and thrombosis. Ac- cessory renal arteries are seen in up to 30% of cases, and they usually originate from the aorta. Occa- sionally, these arteries may arise from the iliac ar- teries and rarely, from the mesenteric and lumbar arteries [4]. Delineation and clear outlining of small accessory arteries, which can be as small as 1 to 2mm in diameter, are important imaging pre- requisites from a surgical standpoint. Furthermore, a clear differentiation between two separate acces- sory arteries from prehilar branching (renal artery branching within 20 mm of renal artery origin) is extremely helpful and can sometimes help avoid torrential bleeding complications [5]. Similarly, multiple renal veins are seen in up to 30% of patients. An important presurgical imaging communication is confirmation of the presence or absence of venous variants such as the circumaortic renal vein (a single renal vein that is split or two renal veins encircling the aorta before joining the IVC), an isolated retroaortic left renal vein, and abnorma- lities such as venous thrombosis and varices [4]. Recently, the assessing of kidney volume before transplant has also gained importance, because transplant of the larger donor kidney has a more fa- vorable posttransplant outcome rate. Imaging in donors With the advent of MDCT and advances in the MR scanner, current donor evaluation protocols are im- proving rapidly. Both these imaging modalities have proven promising in detecting vascular and collecting system variants with an established in- crease in readers’ confidence [6]. With this develop- ment, the use of catheter angiography for mapping renal vasculature has virtually faded. Furthermore, the value of image postprocessing has added to in- creased acceptability of the CT and MR images to referring physicians because postprocessed images provide a close simulation to the operative findings during surgery [7]. The high–resolution, thin-slice acquisitions provided by the newer CT and MR im- aging scanners make it now possible to detect thin accessory renal arteries (Fig. 1) [8]. CT and MR ur- ography also provide a clear delineation of the pye- loureteral anatomy, with added benefits provided by three-dimensional (3D) postprocessing. Multidetector CT versus MR imaging for evaluation of renal donors The better spatial resolution, faster speed, and greater cost effectiveness of CT have led to a wide ac- ceptance of CT over MR imaging in most centers. Al- though CT and MR angiography have demonstrated substantial agreement in the preoperative evalua- tion of renal donors [9], more published research data on the integrity of CT technique, contrast vol- ume, and injection rates, and various revolutionary CT protocol techniques, have definitely tilted the balance toward MDCT, leading to its widespread ac- ceptance for imaging renal donors. The interob- server disagreement in the interpretation of CT and MR angiography is related to overreading and underreading of small vessels (1–2 mm in diame- ter) (Fig. 2) [10,11]. With the similarity of CT and MR imaging accuracies, the potential advantages and disadvantages associated with each modality have been widely discussed recently. MR angiography is a safe and noninvasive tech- nique for comprehensive evaluation of renal donors. It is radiation free and particularly advanta- geous in patients who are prone to allergic reaction from iodinated contrast media. The limitations of Fig. 1. A console-generated coronal maximum inten- sity projection in a 56-year-old female donor showing three arteries (thin arrows) supplying the right kid- ney, branching of the right main renal artery (thick arrow), and two renal veins (asterisks). Singh & Sahani 80 Cr oss-sectional I maging Evaluation of Renal Masses Srinivasa R. Prasad, MD * , Neal C. Dalrymple, MD, Venkateswar R. Surabhi, MD Most renal masses are neoplastic in nature. In- fectious, inflammatory, and nonneoplastic masses constitute a small subset of renal masses. Many re- nal neoplasms demonstrate characteristic cell of or- igin, histology, and clinicobiologic behavior. Renal neoplasms may be primary or metastatic in origin. Primary renal tumors in adults are classified, based on histogenesis and histopathology, into renal cell, metanephric, mesenchymal, mixed epithelial and mesenchymal, and neuroendocrine neoplasms [1]. They are further categorized, based on tumor biology and histopathology, into benign and malig- nant neoplasms. The imaging characteristics of renal masses are protean; accurate distinction of benign and malignant neoplasms may not be pos- sible because of overlap of imaging findings. A re- cent trend is toward percutaneous biopsy of renal masses in an attempt to characterize renal masses for the purpose of making treatment decisions. The number of biopsies in patients who have advanced or multicentric renal neoplasms has in- creased, when a benign renal tumor is suspected, or in the presence of a known nonrenal primary or systemic malignancy [2]. Based on imaging findings, renal masses may be broadly classified into predominant soft tissue, ad- ipose tissue, or cystic masses. Renal cell carcinoma (RCC) is by far the most common soft tissue mass in the kidney (Fig. 1). However, RCCs may demon- strate significant tumor heterogeneity and may appear entirely cystic or show a small proportion of macroscopic fat (Fig. 2) [3,4]. Other uncommon soft tissue renal masses in adults include onco- cytomas, metanephric adenomas, benign and malignant mesenchymal neoplasms, and neuroen- docrine neoplasms. Cystic renal lesions include kidney cysts (including hemorrhagic cysts), ab- scesses, and cystic neoplasms (multilocular cystic RADIOLOGIC CLINICS OF NORTH AMERICA Radiol Clin N Am 46 (2008) 95–111 Department of Radiology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229, USA * Corresponding author. E-mail address: prasads@uthscsa.edu (S.R. Prasad). - Cross-sectional imaging techniques Multidetector-row CT technique MR imaging technique MR imaging versus multidetector-row CT - Pattern-based approach to renal mass characterization: tumor morphology Renal mass with predominant soft tissue component Renal mass with predominant macroscopic fat Renal mass with predominant (or exclusive) cystic component - Pattern-based approach to renal mass characterization: tumor topography - Percutaneous biopsy of renal masses - Staging of renal cell carcinomas - Management of renal masses: knife, needle, or pills? - Follow-up imaging after surgery and ablative treatment - Summary - References 95 0033-8389/08/$ – see front matter ª 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.rcl.2008.01.008 radiologic.theclinics.com . prasads@uthscsa.edu (S.R. Prasad). - Cross-sectional imaging techniques Multidetector-row CT technique MR imaging technique MR imaging versus multidetector-row CT - Pattern-based approach to renal mass characterization:. knife, needle, or pills? - Follow-up imaging after surgery and ablative treatment - Summary - References 95 003 3-8 389/08/$ – see front matter ª 2008 Elsevier Inc. All rights reserved. doi:10.10 16/ j.rcl.2008.01.008 radiologic.theclinics.com . (or exclusive) cystic component - Pattern-based approach to renal mass characterization: tumor topography - Percutaneous biopsy of renal masses - Staging of renal cell carcinomas - Management of renal