MRI of the kidney and urinary tract has evolved with advancements in MR technology combined with further radiologic experience. A combination of simple pulse sequences and the use of gadolinium (Gd) allow a comprehensive evaluation of the genitourinary system and facilitate the detection and accurate characterization of renal and urinary tract masses. In this review an MRI technique used to evaluate the kidneys and urinary tract is presented with an emphasis on the characterization of renal masses. RECENT TECHNOLOGICAL ADVANCES have expanded the role of MRI in evaluating the kidney and the urinary tract. The high spatial resolution and intrinsic tissue contrast of MRI have resulted in superb detection and characterization of renal and urinary tract anatomy and pathology. Recently, new imaging methods (such as parallel imaging) have resulted in sequences with higher spatial resolution that are performed during shorter breath-holds. Gadolinium (Gd) can be administered to patients without concern for contrastinduced nephrotoxicity, and is well tolerated by patients with a history of iodinated-contrast allergy. The superior safety profile of Gd combined with the lack of ionizing radiation makes MRI an ideal modality for evaluating the genitourinary system, especially in patients who have renal disease or have undergone nephrectomy or partial nephrectomy. In this review the MRI findings of the kidneys and urinary system are discussed with an emphasis on differentiating renal masses and evaluating the collecting system.
TECHNIQUEA comprehensive examination of the kidneys and collecting system includes an evaluation of the renal vasculature, parenchyma, and collecting system. Unenhanced imaging includes axial breath-hold T1-weighted gradient-echo (GRE) (performed in and out of phase), coronal or axial breath-hold T2-weighted half-Fourier single-shot fast spin-echo (FSE), and breath-hold frequency-selective fat-suppressed 3D T1-weighted spoiled GRE (Table 1) sequences. The 3D fat-suppressed T1-weighted sequence is also performed at multiple time points after intravenous Gd administration, and may be used to produce MR angiographic, venous, parenchymal, and excretory phase images. If it is necessary to determine the renal arterial anatomy (e.g., in patients about to undergo partial nephrectomy), this sequence should be performed in the oblique-coronal plane during an arterial phase. Using the proper scan delay and acquiring the low-spatial-frequency (high contrast) lines of k-space during peak arterial enhancement are critical for minimizing venous contamination and preventing scanning before sufficient contrast reaches the aorta. The arterial phase may be optimized with the use of fluoroscopic triggering (1), an automated bolus detection technique (2), a timing run (3), or a "best-guess" method. With improved gradient strength, pulse sequences, and parallel imaging methods, a "time-resolved" approach can also be used, which will obviate the need for a timing strategy (4). Scanning with rapid temporal resolution (approxi...