Kidneys and Ureters
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10 Kidneys and Ureters Technique Computed Tomography Conventional Radiography The introduction of multislice helical CT has and Urography had a major impact on uroradiology. Multislice CT allows imaging during specific contrast Indications for intravenous (IV) pyelography opacification phases. Depending on the study (the term IV urography is used by some) have indication, the CT protocol used can be decreased considerably, but this examination modified accordingly. Thus suspected acute is still indicated if pyelocalyceal visualization urinary obstruction by a stone is studied is needed. Small transitional cell carcino- without contrast, in the workup of hypertension mas, pyeloureteritis cystica, medullary sponge and suspected renal artery stenosis emphasis is kidney, and papillary necrosis continue to be on arterial phase images, while some infections best identified with IV urography. A current are detected only during later phases; small major indication for IV urography is in investi- renal tumors are readily imaged in a breath- gating hematuria in adults, although here com- hold; potential living renal donor evaluation puted tomographic urography combined with is simplified and therapy aided by three- cystoscopy are making inroads, and magnetic dimensional (3D) imaging. Using a two-bolus resonance (MR) is not far behind. technique both a nephrogenic and excretory Intravenous urography is insensitive in phases can be obtained with one scan, although detecting small renal parenchymal tumors. such a protocol tends not to visualize all of the Thus about one third of tumors <3cm in diam- ureters completely. eter are not detected. Even with larger tumors Are screening serum creatinine levels neces- urography is generally insensitive in differenti- sary prior to contrast CT studies? Only 3% of ating benign from malignant tumors. This test, over 2000 consecutive outpatients undergoing however, does provide information on renal contrast-enhanced CT had elevated serum cre- function and gross anatomy, and it aids in eval- atinine levels, and risk factors were identified in uating disorders of the collective systems. It a majority of these (1); screening for risk factors tends to be somewhat inferior in infants com- appears adequate in this patient population. pared to older children. Renal opacification can be divided into Retrograde ureteropyelography has a role if three phases: first is an early vascular or bolus insufficient contrast is excreted during an IV phase, then a nephrogram phase (consisting urogram. For suspected obstruction, however, of a nephrogram), followed by a pyelogram currently noncontrast computed tomography or equilibrium (delayed) phase. A cortico- (CT) is considered to be superior. medullary phase, providing maximum differen- 571 572 ADVANCED IMAGING OF THE ABDOMEN tiation between renal cortex and medulla, and in planning a partial nephrectomy, such as occurs during the early phase. Whether a corti- orientation of blood vessels to a tumor or other comedullary phase or a later nephrogram phase structure. One should not rely only on 3D should be used for subtle tumor detection is images, however, because to an experienced eye debatable,although some evidence suggests that axial and coronal images provide more detailed the nephrogram phase is superior. Both neo- information, especially about small vessels that plasms and normal renal parenchyma enhance tend to be overlooked on 3D images. significantly more during the nephrogram In a patient with microscopic hematuria phase than during the corticomedullary phase. and a normal excretory urogram, should CT In general, more tumors <3cm in diameter are or ultrasonography (US) be performed next? detected on the nephrogram phase than on the Although this topic generated considerable corticomedullary phase. The onset of a nephro- controversy in the 1990s, currently many inves- gram phase varies among patients and tech- tigators believe that CT detects more tumors nique used; a faster injection rate results in an overall, especially smaller ones. In fact, a more earlier onset—roughly 100 seconds at 2mL/sec pertinent current question is whether CT or MR and 90 seconds at 3mL/sec. The cortico- is indicated as a primary imaging modality in medullary phase is more useful, however, for such a clinical setting. detection of such conditions as an aneurysm, arteriovenous malformation, or fistula, and in evaluating tumor vascularity. Also, the earlier Ultrasonography phase is more advantageous if optimal liver Renal cortex is isoechoic to liver, and the cen- and other abdominal structure visualization is trally located renal sinus is hyperechoic to sur- required. Renal tumors tend to be detected with rounding renal parenchyma. greater confidence on delayed images than on Similar to other structures, use of an intra- early-phase images. One solution is to obtain vascular US contrast agent (such as Levovist; images during both phases (corticomedullary Schering AG, Berlin, Germany) enhances vascu- and nephrogram), but whether the extra com- lar signals and makes vascularity more evident. plexity and cost justify such an approach for a Diagnostic accuracy is improved, especially for limited gain is not clear; a decision based on hyperechoic tumors and complex cysts. individual indications appears reasonable. Doppler US provides data for the intrarenal Renal CT performed shortly after excretory arterial blood flow resistive index (RI) and pul- urography—called CT urography—is a varia- satility index (PI). These indices increase with tion of delayed-phase CT combining high age, acute obstructive uropathy, use of certain spacial resolution of conventional filming with drugs, and in some nephropathies. high contrast resolution of CT. Even pyelove- Endoluminal US using a high-frequency nous backflow can be identified on CT urogra- transducer housed in a catheter and advanced phy (2). Only about half of renal parenchymal endoscopically into a ureter is moving from tumors identified on CT are detected on the pre- research into clinical practice. Potentially, the vious excretory urogram, but such a combina- information obtained helps guide biopsy and tion study tends to increase the clinicians’ laser therapy and defines vessels adjacent to a confidence in some findings. A variant of this ureter. technique is to obtain delayed postcontrast coronal images or a delayed CT scout image. Validity of these various combined procedures Magnetic Resonance Imaging in evaluating hematuria is yet to be established in larger studies. Currently for most suspected renal conditions Furosemide-enhanced CT urography, ob- CT is performed rather than magnetic reso- tained 10 minutes after contrast agent injection, nance imaging (MRI), but MRI is used in a outlines pelvicaliceal structures and identifies setting of contrast allergy or renal failure, and calculi inside opacified urine and differentiates for studying some complex masses. It is also them from phleboliths (3). useful in evaluating venous thrombosis in a Three-dimensional CT imaging techniques setting of renal carcinoma. It provides both are useful both in evaluating suspected tumors spatial resolution and information on renal 573 KIDNEYS AND URETERS function and potentially is more useful than good correlation exists between MR renography either CT or nuclear medicine. Magnetic reso- and radionuclide renography results (4). nance applications range from multiphase 3D A general disadvantage of MR in children, magnetic resonance angiography (MRA) to especially younger ones, is the need for seda- evaluate renal artery stenosis and renal perfu- tion. A relative disadvantage in certain renal sion abnormalities, MR nephrography, and MR applications is its poor sensitivity in detecting urography of the renal collecting systems (espe- calcifications. cially useful in posttransplant complications). Surprisingly, diagnostic accuracy in patients Potentially, a single MR study evaluates reno- with a clinical suspicion for renal tumor was vascular disease, assesses renal function, detects comparable when studies were performed using renal tumors,and identifies urinary tract abnor- either a low field [0.2 tesla (T)] or a high field malities, all without radiation exposure. More (1.5T) magnet, although the signal-to-noise and often, however, the specific MR sequences best contrast-to-noise ratios were significantly worse suited for each application are selected. at low field strength (5). Gadolinium diethylenetriamine pentaacetic Conventional acid (Gd-DTPA) is an ionic agent. A nonionic version is also available in some countries. No As with CT, several distinct phases—cortical, significant differences exist in either signal medullary, and pyelocaliceal—are evident after intensity or function between these two agents. IV paramagnetic gadolinium contrast injection. Some investigators add an intermediate cortico- Magnetic Resonance Urography medullary junction phase. Magnetic reso- nance signal intensity normally decreases in the Magnetic resonance urography both without pyelocaliceal phase due to contrast agent con- and with contrast agents is evolving into viable centration. When searching for small renal alternative studies in select patients. No con- tumors, use of a body phased-array coil in com- sensus is yet apparent on which specific MR bination with fast low-angle shot (FLASH) and sequences constitute MR urography. A more fat suppression pre- and postcontrast thin- basic question concerns the role of MR urogra- section MR allow imaging in single breath- phy: Does it have any advantages