Radiography/Radiology Activity for 2017 Activity No: A3(17) Topic MR Urography Article Pediatric MR Urography: indications, techniques and approach to review Approved for (3) Clinical Continuing Educational Units (CEU s)
PEDIATRIC IMAGING 1208 Note: This copy is for your personal non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, contact us at www.rsna.org/rsnarights. Pediatric MR Urography: Indications, Techniques, and Approach to Review 1 Elliot C. Dickerson, MD Jonathan R. Dillman, MD Ethan A. Smith, MD Michael A. DiPietro, MD Robert L. Lebowitz, MD Kassa Darge, MD, PhD Abbreviations: FSE = fast spin echo, MIP = maximum intensity projection, SSFSE = single shot fast spin echo, 3D = three-dimensional, 2D = two-dimensional RadioGraphics 2015; 35:1208 1230 Published online 10.1148/rg.2015140223 Content Codes: 1 From the Department of Radiology, Section of Pediatric Radiology, C. S. Mott Children s Hospital, University of Michigan Health System, 1540 E Hospital Dr, B1D502 UH, Ann Arbor, MI 48109-5030 (E.C.D., J.R.D., E.A.S., M.A.D.); Department of Radiology, Boston Children s Hospital, Harvard Medical School, Boston, Mass (R.L.L.); and Department of Radiology, Children s Hospital of Philadelphia, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, Pa (K.D.). Recipient of a Certificate of Merit award for an education exhibit at the 2013 RSNA Annual Meeting. Received May 15, 2014; revision requested July 29 and received August 29; accepted September 12. For this journal-based SA-CME activity, the authors, editor, and reviewers have disclosed no relevant relationships. Address correspondence to E.C.D. (e-mail: dickerse@med.umich.edu). Magnetic resonance (MR) urography is a valuable imaging modality for assessing disorders of the pediatric urinary tract. It allows comprehensive evaluation of the kidneys and urinary tract in children by providing both morphologic and functional information without exposing the child to ionizing radiation. Pediatric MR urography can be used to thoroughly evaluate renal and urinary tract abnormalities that are difficult to identify or fully characterize with other imaging techniques, and it has the potential to allow earlier diagnosis while decreasing the number of imaging studies performed. Common indications for pediatric MR urography include evaluation of complex renal and urinary tract anatomy, suspected urinary tract obstruction, operative planning, and postoperative assessment. MR hydrography (T2-weighted imaging of urine) excellently depicts dilated or obstructed urinary systems, whereas postcontrast imaging (gadolinium-enhanced T1-weighted imaging of the kidneys and urinary system) excellently depicts nondilated or nonobstructed urinary systems. Postcontrast MR urography also allows a functional evaluation of the kidneys and urinary tract that includes estimation of differential renal function. The authors review common indications for pediatric MR urography, detail MR urography techniques, compare the strengths and weaknesses of MR urography with those of alternative imaging strategies for children, and describe numerous common and uncommon abnormalities of the pediatric kidneys and urinary tract. RSNA, 2015 radiographics.rsna.org SA-CME LEARNING OBJECTIVES After completing this journal-based SA-CME activity, participants will be able to: List common indications and contraindications for pediatric MR urography and discuss its effectiveness relative to other modalities for diagnostic imaging of the pediatric urinary tract. Describe basic technical aspects of performing pediatric MR urography. Recognize common pathologic features in the pediatric urinary tract at MR urography. See www.rsna.org/education/search/rg. Introduction Magnetic resonance (MR) urography allows comprehensive evaluation of a child s kidneys and upper urinary tract in a single examination by providing both morphologic and functional information. This imaging modality has the potential to provide high-spatialresolution and high-contrast images of the kidneys and urinary tract in children of all ages, including neonates and adolescents, while avoiding ionizing radiation. State-of-the-art MR urography techniques performed with intravenous hydration, furosemide, and gadolinium-based contrast material also provide a urinary system stress test, which can reveal partial or intermittent upper urinary tract obstructions, and allow quantitative assessment of renal and upper urinary tract function (1,2). Information provided by so-called
RG Volume 35 Number 4 Dickerson et al 1209 TEACHING POINTS MR urography allows comprehensive evaluation of a child s kidneys and upper urinary tract in a single examination by providing both morphologic and functional information. With use of dynamic postcontrast imaging, functional MR urography can be performed to estimate differential (split) renal function and to generate numerous quantitative parameters related to the passage of contrast material through the kidneys and collecting systems. In girls experiencing both daytime and nighttime wetness despite successful toilet training, MR urography should be considered to rule out the presence of an ectopic ureter. Although many cases of congenital ureteropelvic junction narrowing are attributable to intrinsic abnormality of the ureteropelvic junction and manifest with antenatal hydronephrosis, some cases of narrowing (especially in older children) are attributable to extrinsic mass effect from a crossing vessel (eg, accessory renal artery). Obstructive congenital primary megaureter is attributable to disordered peristalsis of the distal ureter (smooth muscle in the distal ureter wall is abnormal and ureteral fibrosis may be present) and can cause variable degrees of upper urinary tract obstruction. functional MR urography includes differential renal function, based on both glomerular filtration of contrast material (Patlak method) and volume of enhancing renal parenchyma, and numerous parameters related to transit of contrast material through the urinary tract (eg, renal transit time) (3). Thus, the information provided with MR urography is similar to that acquired with a combination of ultrasonography (US), computed tomography (CT), excretory urography, and renal scintigraphy. The purpose of this review article is to present common indications for pediatric MR urography, compare the strengths and weaknesses of MR urography with those of alternative imaging strategies for children, describe state-of-the-art pediatric MR urography techniques, and provide an approach to pediatric MR urography evaluation. Numerous common and uncommon abnormalities of the renal and urinary systems will be described, and we will show that many classic pediatric uroradiology findings originally described at excretory urography and US have MR urography correlates. Common Pediatric MR Urography Indications Common indications for pediatric MR urography include evaluation of complex renal and urinary tract anatomy, suspected urinary tract obstruction, operative planning, and postoperative assessment. Pediatric MR urography is most helpful in children, for whom conventional imaging tests such as US and voiding cystourethrography provide insufficient information for medical and surgical management. For example, upper urinary tract anatomic structures can be difficult to define at US in the absence of dilatation or, conversely, when there is massive hydroureteronephrosis. Examples of complex urinary tract anatomic structures for which MR urography might be beneficial include certain duplex urinary systems and suspected ectopic ureters (4 7). MR urography can also be used to directly characterize sites of urinary tract narrowing and obstruction. Location of the narrowing (eg, ureteropelvic junction versus midureteral), degree of narrowing, and potential extrinsic cause (eg, crossing vessel) can be evaluated. MR urography with formal functional analysis also allows quantitative assessment of differential renal function based on both glomerular filtration of contrast material and volume of enhancing renal parenchyma in the context of urinary tract obstruction (1,8,9). The excellent depiction of the urinary tract and adjacent structures provided by MR urography can aid in surgical planning; this may be increasingly important as more pediatric urologic surgical interventions are being performed with laparoscopic or robotic techniques that afford limited visualization (8,10). In children who have previously undergone urinary tract surgery, MR urography can be used to assess surgical anastomoses (eg, in the context of ureteropelvic junction obstruction repair) and reimplanted ureters. MR Urography versus Alternative Urinary Tract Imaging Strategies for Children Ultrasonography Although US can depict the kidneys and urinary bladder without ionizing radiation, it has certain limitations. First, as children increase in size and as anatomic structures of interest (eg, ureter) become relatively smaller, the capabilities of US diminish. Thus, nondilated ureters or ectopic ureteric insertions can easily remain undetected at US. When marked hydroureteronephrosis is present, US may be incapable of fully showing urinary tract anatomy. Finally, US generally provides little or no information regarding renal function or whether dilated upper urinary tract structures are truly obstructed. Advantages of US over MR urography include shorter examination time, no need for sedation or anesthesia, lower cost, and increased availability. For many children, US can sufficiently define renal (eg, horseshoe kidney) and urinary tract abnormalities (eg, duplication of the upper urinary tract), detect and characterize hydroureteronephrosis, and provide accurate appraisal of the renal parenchyma (eg, focal scarring, diffuse thinning, or
1210 July-August 2015 radiographics.rsna.org evidence of dysplasia, such as loss of corticomedullary differentiation, increased echogenicity, and cysts) (11). Voiding Cystourethrography Voiding cystourethrography is a fluoroscopic imaging examination that requires placement of a catheter in the urinary bladder and ionizing radiation. This imaging test, most often performed in awake children to evaluate for vesicoureteral reflux, provides detailed anatomic evaluation of the urinary bladder and urethra but allows upper urinary tract assessment only when vesicoureteral reflux is present. In many children, this imaging test provides either no or only limited information on upper urinary tract anatomic variations (eg, vesicoureteral reflux into only one of four upper urinary tract moieties with bilateral duplex kidneys). MR urography generally does not directly depict vesicoureteral reflux but may reveal indirect findings of the condition, such as upper urinary tract dilatation and parenchymal scarring. Renal Scintigraphy In children, renal scintigraphy can be used to show evidence of scarring or pyelonephritis, estimate differential renal function, and assess urinary tract obstruction, but it provides minimal anatomic information. Technetium 99m ( 99m Tc) dimercaptosuccinic acid is a cortical binding agent that allows relatively high spatial resolution (pinhole) imaging. It is most often used to evaluate for scarring or pyelonephritis (focal parenchymal photopenic defects), search for ectopic renal tissue, and determine differential renal function. It is often impossible to determine whether a given photopenic defect is attributable to acute pyelonephritis or scarring, and repeat imaging over several months may be required to distinguish the two (12). Both 99m Tc diethylenetriaminepentaacetic acid and 99m Tc m-mercaptoacetyltriglycine can be used to assess renal perfusion, estimate differential renal function, and evaluate for urinary tract obstruction on the basis of delayed passage of radiotracer from the kidneys into the more distal urinary system. Renal scintigraphy, unlike MR urography, exposes children to ionizing radiation. 99m Tc dimercaptosuccinic acid may result in an estimated effective dose of 1 msv (13). The limited anatomic information commonly leads to incorrect interpretation of scintigraphy findings (eg, not recognizing a duplex kidney or inaccurately describing the level of urinary tract obstruction). Limited visualization of anatomic structures also complicates functional assessment because of difficulties in defining the renal parenchyma or collecting system compared with background regions of interest. Scintigraphy may be similar or only slightly less costly than MR urography, and children usually remain awake when the procedure is performed. MR Urography Techniques Although variations in pediatric MR urography technique exist, the following discussion is generally reflective of commonly described practices (Table 1) (1,2,14,15). For most MR urography examinations, images are obtained using a combination of two basic approaches: MR hydrography and postcontrast MR urography. Typical MR urography examinations last 35 70 minutes. MR urography can be performed in both 1.5 T and 3.0 T scanners; there are advantages to both field strengths (16,17). MR Hydrography MR hydrography involves imaging of urine (or water) in the urinary tract by using a variety of T2-weighted pulse sequences (eg, single shot fast spin echo [SSFSE], two-dimensional [2D] FSE, and three-dimensional [3D] FSE). This approach exquisitely depicts dilated and/or obstructed upper urinary tract segments without requiring contrast material but performs less well in nondilated systems. SSFSE images provide an excellent overview of renal and urinary tract anatomic structures and guide anatomic coverage of subsequent pulse sequences (Fig 1a). These images, which can be rapidly acquired, generally have lower spatial resolution and signal-to-noise ratio than do images obtained with other T2-weighted pulse sequences. Two-dimensional and 3D FSE sequences, which are most often heavily T2 weighted (very long repetition and echo times), provide detailed assessment of the kidneys and urinary system. Two-dimensional T2-weighted FSE imaging can provide high-spatialresolution evaluation of the kidneys and ureters in either the axial or coronal plane and allows detailed assessment of renal parenchyma, aiding in evaluation of both dysplasia and scarring. Threedimensional T2-weighted FSE pulse sequences can be designed to depict both urinary tract and soft-tissue structures (including the kidneys) (Fig 1b) or to show only fluid in the urinary tract (similar to MR cholangiopancreatography). The 3D technique allows imaging with very thin sections with high in-plane spatial resolution with resultant isotropic (eg, 1 1 1 mm) or near-isotropic voxels. These images provide detailed assessment of anatomic structures and can be used to create high-quality multiplanar reformatted, maximum intensity projection (MIP), and volume-rendered images (Fig 1c). MR hydrography can be repeated in a time-resolved fashion to produce cine images to show ureter peristalsis (18).
RG Volume 35 Number 4 Dickerson et al 1211 Table 1: Example of a Pediatric MR Urography Protocol Pulse Sequence Planes Common Uses SSFSE with fat saturation T2-weighted FSE with fat saturation High-spatial-resolution 3D T2- weighted FSE with fat saturation Coronal, sagittal (axial optional) Axial (coronal optional) Coronal Provide general overview of renal and urinary tract anatomy; can be used to plan subsequent pulse sequences Anatomic assessment of renal parenchyma (including corticomedullary differentiation and presence of cysts), ureters, and bladder Anatomic assessment of dilated urinary tracts; can be tailored to also allow detailed evaluation of renal parenchyma 2D T1-weighted GRE Coronal (axial optional) Not commonly used for urinary tract assessment but may help characterize incidental findings, fluid collections, etc Dynamic postcontrast imaging for 10 15 min ( 50 3D T1- weighted GRE imaging volumes) High-spatial-resolution delayed postcontrast 3D T1-weighted GRE with fat saturation Optional: thick-slab T2-weighted SSFSE cine imaging Optional: delayed postcontrast 3D T1-weighted GRE with fat saturation 1 2 h after contrast material injection Note. GRE = gradient-recalled echo. Coronal-oblique (parallel to long axis of the abdominal aorta and kidneys) Axial, coronal, sagittal Coronal Axial, coronal, sagittal Assess renal perfusion, parenchymal enhancement, and excretion of contrast material; identify focal renal parenchymal perfusion defects due to scarring or pyelonephritis; used for quantitative assessment of renal function Anatomic assessment of kidneys and urinary tract, including ureteropelvic junction and distal ureter (including ureterovesical junction and ectopic ureter) Assess ureteric peristalsis Evaluation of suspected calyceal diverticulum; assessment of delayed pelvicalcyeal drainage Postcontrast MR Urography A second approach involves imaging the urinary tract after the administration of intravenous gadolinium-based contrast material (usually 0.1 mmol/kg) and allows for evaluation of both renal and urinary tract anatomic structures and function. Postcontrast MR urography excellently depicts nondilated, nonobstructed upper urinary tracts but may be limited in the context of severe urinary tract obstruction or poor renal function. When functional analysis is planned and the Patlak method (described under Differential Renal Function) will be used for determining differential renal function, the preferred gadolinium-based contrast agent is one that is filtered by the glomerulus, lacks significant tubular secretion or reabsorption, and has minimal plasma protein binding. Gadopentetate dimeglumine (Magnevist; Bayer HealthCare, Whippany, NJ) and gadoteridol (Prohance; Bracco Diagnostics, Monroe Township, NJ) are two particular contrast agents that meet these criteria. State-of-the-art published pediatric MR urography protocols frequently describe obtainment of both dynamic and delayed postcontrast MR urography images. Dynamic imaging allows assessment of renal perfusion (including visualizing renal arteries), timeliness and quality of parenchymal enhancement, and timeliness of contrast material excretion into the renal collecting systems and ureters (Fig 2a). These images are also used for formal functional analysis. Dynamic imaging is based on the acquisition of numerous (typically 50 or more) 3D gradient-recalled echo imaging volumes through the kidneys and urinary tract. These images are obtained serially (at least six imaging volumes per minute during the first 5 minutes of dynamic imaging) from the time contrast material is injected until approximately 10 15 minutes later. Each volume should have a temporal resolution of less than 10 seconds, contain an average of 20 30 images with a 2 4-mm section thickness (depending on patient size), and be oriented in the coronal-oblique plane (parallel to the long axis of the abdominal aorta). These images can be rapidly reviewed by placing derived coronal MIP images in a single series. Delayed postcontrast imaging provides highspatial-resolution depiction of the kidneys and urinary tract in the excretory phase. Imaging in
1212 July-August 2015 radiographics.rsna.org Figure 1. Bilateral hydroureteronephrosis in a 3-year-old girl. (a, b) Coronal SSFSE (a) and coronal 3D T2-weighted FSE (b) fatsaturated MR hydrograms reveal severely dilated ureters (arrows) and collecting systems ( * ) and bilateral renal parenchymal thinning; left kidney parenchymal heterogeneity and striations are better seen on b than on a. (c) Posterior-oblique volumerendered 3D T2-weighted FSE fat-saturated MR hydrogram shows similar findings and allows visualization of the upper urinary tracts and bladder on a single image. three planes (axial, sagittal, and coronal) is usually performed. Small field-of-view, high-spatialresolution imaging of the pelvis allows excellent depiction of the ureteric insertions, including ectopic ureters. Delayed postcontrast images can also be used to generate 2D reformations that allow optimal visualization of anatomic structures of interest (eg, the ureteropelvic junction) and 3D reconstructions, including MIP and volumerendered images, which provide an overview of renal and urinary tract anatomic structures on a single image (Fig 2b). Postcontrast MR urography is generally inappropriate for children with acute kidney injury or chronic kidney disease and an estimated glomerular filtration rate less than 30 ml/min/1.73 m 2 because of the potential risk for nephrogenic systemic fibrosis (19 22). Because MR urography is most often performed for children who have increased risk for impaired renal function, serum creatinine concentration should be measured and estimated glomerular filtration rate should be calculated with accepted formulas before contrast material administration (23). If injection of gadolinium-based contrast material is deemed to be necessary and the estimated glomerular filtration rate is approximately less than 40 ml/min/1.73 m 2, use of a macrocyclic (as opposed to linear) contrast agent is preferred at some institutions because there is likely either no or minuscule risk of nephrogenic systemic fibrosis when using these particular agents (22,24). Fortunately, pediatric
RG Volume 35 Number 4 Dickerson et al 1213 Figure 2. Left congenital primary megaureter in a 1-year-old boy. Arterial (left), nephrographic (middle), and excretory (right) phase (a) and oblique delayed phase (b) 3D MIP dynamic postcontrast MR urograms show left megaureter, with abrupt ureteric narrowing near the ureterovesical junction (arrow in b). Symmetric enhancement during the nephrographic phase suggests no or minimal obstruction. Figure 3. MR imaging findings in a 10-day-old boy who had unclear urinary tract anatomic features and severe bilateral hydroureteronephrosis at US; gadolinium-based contrast material was not used because of acute renal failure. (a) Coronal 3D T2-weighted FSE MR image shows severe left hydroureteronephrosis ( * ). (b) Volumerendered 3D T2-weighted FSE MR image (posterior view) shows a duplicated left upper urinary tract with severe lower moiety ureteropelvic junction narrowing (black arrow) and severe narrowing of the distal ureters (white arrows). Cutaneous pyelostomy was performed to relieve the urinary tract obstruction and preserve renal function. nephrogenic systemic fibrosis is very rare, with only 23 documented cases as of September 2012 (19,21). MR urography can still provide important insights into complex renal and urinary tract anatomic structures in children with estimated glomerular filtration rates less than 30 ml/min/1.73 m 2 by using an MR hydrography technique without injecting gadolinium-based contrast material (Fig 3). Other Basic Principles Both MR hydrography and postcontrast MR urography are commonly performed with fat saturation to increase urinary tract conspicuity and improve the quality of 3D reconstructions. MIP and volume-rendered images can be created from both T2-weighted and contrast-enhanced volumetric pulse sequences that provide an excellent overview of urinary tract anatomic structures. Preprocedural intravenous hydration and furosemide treatment (0.5 1 mg/kg, up to 20 mg; administered before or during MR urography) improve urinary tract distention and visualization, decrease the transit time for excreted contrast material to reach the ureters and bladder, and dilute excreted contrast material to minimize artifacts (and associated signal loss) related to T2* effect (Table 2) (15,25 28). In addition, diagnosis of urinary systems with partial or intermittent obstruction is improved by these maneuvers, which stress the urinary system by maximizing the urine produced and conducted by the urinary system (29). A Foley catheter is also commonly placed in the urinary bladder to allow free drainage of urine during the imaging examination, thus minimizing backpressure on the upper urinary tracts (which could theoretically slow passage
1214 July-August 2015 radiographics.rsna.org Table 2: Pediatric MR Urography Challenges and Solutions Challenge Patient discomfort during examination due to bladder distention from procedural hydration and diuretic treatment Backpressure on upper urinary tracts due to distended urinary bladder T2* effect causing signal loss in the urinary tract on postcontrast T1-weighted images Instances of partial or intermittent obstruction Poor visualization of distal ureter, ureteric insertion, ectopic ureter Delayed passage of excreted contrast material from the dilated renal pelvis into the ureter in the context of nonobstructive pelvicaliectasis Foley catheter Foley catheter Possible Solution(s) Intravenous hydration, furosemide treatment Intravenous hydration, furosemide treatment Use small field of view, high-resolution 3D T2-weighted FSE and postcontrast 3D T1-weighted gradientrecalled echo pulse sequences at 3.0-T field strength Image patient in prone position Table 3: Maneuvers for Optimizing Image Quality and Patient Experience at Pediatric MR Urography Maneuver Preprocedural intravenous hydration Diuretic (furosemide) administration Placement of Foley catheter in urinary bladder Sedation or general anesthesia Child-life (child behavioral) specialist involvement Function Improves urinary tract distention, minimizes T2* effect of excreted contrast material at postcontrast T1-weighted imaging, and helps stress urinary tract to reveal mild or intermittent obstructions Improves urinary tract distention, minimizes T2* effect of excreted contrast material at postcontrast T1-weighted imaging, and helps stress urinary tract to reveal mild or intermittent obstructions Improves patient comfort during examination and decreases backpressure on urinary tracts Helps decrease motion artifacts in young or uncooperative children; can improve image quality by allowing imaging during breath holding Can educate children and parents about what to expect and the importance of the Foley catheter, remaining still, breath holding, and other maneuvers and potentially decrease use of sedation and general anesthesia of contrast material through the kidneys and collecting systems and distort functional assessment). The catheter improves patient comfort during the examination by preventing urinary bladder overdistention due to intravenous hydration and furosemide diuresis. The catheter can also be clamped for a portion of the examination to distend and improve evaluation of the bladder. Although MR urography can be performed in older children without sedation or general anesthesia, children younger than 6 10 years of age may require sedation or anesthesia, depending on the patient s maturity, developmental delay, and claustrophobia (Table 3). Before anesthesia is used, the benefits of the examination should be weighed against the small risk that it presents. On some occasions, neonates and young infants may be imaged with a feed and swaddle approach. If sleeping, the baby will usually remain still enough to allow high-quality images with free-breathing technique because the motion of the kidneys and urinary tract is negligible due to patient size. All T2-weighted pulse sequences (including SSFSE, 2D FSE, and 3D FSE) can be performed by using respiratory triggering or navigator gating, thus avoiding the need for patient cooperation for breath holds. Dynamic postcontrast images, which are used for functional analysis, are acquired over 10 15 minutes with the patient free breathing, although patient motion can substantially impact image quality and functional analysis. Delayed excretory phase imaging of the kidneys and urinary tract can be performed with the patient free breathing, although higher quality images generally can be obtained when breath holds are used. Child behavioral (often called child-life ) specialists can decrease sedation and general anesthesia use by educating children and parents about MR urography and implementing age-appropriate coping strategies.
RG Volume 35 Number 4 Dickerson et al 1215 Functional Analysis With use of dynamic postcontrast imaging, functional MR urography can be performed to estimate differential (split) renal function and to generate numerous quantitative parameters related to the passage of contrast material through the kidneys and collecting systems. A freely available software package for pediatric MR urography functional analysis is available at http://www.chop-fmru.com that allows determination of the quantitative parameters mentioned below. This particular software program automatically segments the renal parenchyma and abdominal aorta after regions of interest are placed around and in these structures (1). Although evidence is limited, information acquired from functional MR urography may be capable of predicting which pediatric patients will most benefit from surgical relief of urinary tract obstruction, particularly in the context of ureteropelvic junction obstruction (9). Differential Renal Function Determination of differential renal function is perhaps the most basic and important functional ability of MR urography. Differential renal function can be calculated by using two methods: (a) based on the amount (volume) of enhancing renal parenchyma bilaterally and (b) based on glomerular filtration of contrast material from the blood (Patlak-Rutland, or Patlak, method). The simplest method for estimating differential function involves segmenting enhancing renal parenchyma from adjacent structures, such as the renal collecting systems and vessels, and determining the amount of parenchyma that each kidney contributes to overall enhancing renal parenchymal volume. The primary disadvantage of this volume-based method is that it treats all areas of enhancing renal parenchyma (based on a software-determined signal intensity threshold) as having similar function. It fails to reflect potential differences in renal function among enhancing regions in a single kidney and between kidneys. An alternative method for estimating differential renal function is the Patlak method (30). A simplified explanation of the Patlak method starts with the assumption that a change in signal intensity at postcontrast T1-weighted imaging that results from the presence of gadolinium-based contrast material is directly (and linearly) related to the concentration of contrast material. Flip-angle optimization, hydration, and furosemide are means to help satisfy this assumption. The Patlak method (Fig 4), which involves dynamic postcontrast MR urography, uses a mathematical transformation of renal parenchymal and aortic signal intensity transit time data to generate a separate Patlak plot for each kidney. These Patlak plots provide estimated differential renal function based on the glomerular filtration rate. Functional analysis software can be used to generate renal parameteric maps based on Patlak differential renal function that allow regional assessment of renal function (including upper versus lower moieties in the context of a duplex kidney). Such information can assist in presurgical planning, for example, when deciding whether to perform upper pole nephrectomy or ureteric reimplantation in the context of a duplex kidney with an obstructed upper moiety. Moredetailed descriptions of the Patlak method for determining differential renal function are available elsewhere (1,2,31). Calyceal Transit Time The time required for contrast material to pass from the renal cortex into calyces is referred to as the calyceal transit time (1,9). In general, calyceal transit time should be bilaterally symmetric. Asymmetry between kidneys (or between moieties in a duplex kidney) may be related to nephron injury. Delayed calyceal transit time can be seen in the context of acute obstruction, whereas rapid calyceal transit time may be observed in long-standing partial or intermittent urinary tract obstructions or after surgical relief of obstruction (9). Rapid calyceal transit time has been hypothesized to be attributable to tubular damage and reduced tubular resorption of fluid (1,3,9). Renal Transit Time Renal transit time is the time required for contrast material to pass from the renal cortex into the proximal ureter (at the level of the lower pole of the ipsilateral kidney) (1). Jones et al (32) observed that renal transit time, when used in conjunction with morphologic MR urography, generally allows normal kidneys to be differentiated from obstructed kidneys. They found good agreement between MR urography and renal scintigraphy and concluded that a renal transit time greater than 490 seconds suggests obstruction. Renal transit time can be falsely positive for obstruction in the context of either a capacious but nonobstructed renal collecting system or a nondependent anteriorly positioned ureteropelvic junction, because of the higher specific gravity of gadolinium-based contrast material and its tendency to layer dependently. In such cases, prone imaging and careful assessment of anatomic imaging may confirm the absence of significant urinary tract narrowing. Signal Intensity Transit Time Curves Similar to renal scintigraphy using 99m Tc diethylenetriaminepentaacetic acid or 99m Tc m-mercaptoacetyltriglycine, postcontrast MR urography
1216 July-August 2015 radiographics.rsna.org Figure 4. Summary of functional analysis of MR urography (MRU) for a patient with left urinary system obstruction. Illustration shows regions of interest overlying the kidneys after segmentation. Time-versus signal intensity curves from the kidneys and abdominal aorta are obtained from dynamic postcontrast MR urograms; the left renal curve shows both delayed and decreased parenchymal peak enhancement and delayed loss of signal intensity due to obstruction ( hyperintense nephrogram ). After Patlak transformation is performed, the slopes from the resulting plots are used to estimate differential renal function based on the glomerular filtration rate (GFR) of gadolinium (Gd) based contrast material. Finally, the results are summarized in a table. with dynamic imaging allows the generation of signal intensity time curves. Renal parenchymal signal intensity time curves provide information pertaining to renal perfusion, uptake of contrast material by the kidneys, and contrast material excretion, allowing subjective assessment of renal function (1). Asymmetry between curves for the right and left kidney (or curves for the upper and lower renal moiety) should be investigated. Systematic Approach to Pediatric MR Urography Review In our experience, a systemic approach to pediatric MR urogram review can aid in detecting all pertinent renal and urinary tract findings and in generating accurate conclusions. The abnormalities and findings that can be observed at pediatric MR urography are detailed in this section. Kidneys Initial assessment of the kidneys should begin by confirming their presence and locations. When a kidney is not seen in its respective renal fossa, a careful search should be undertaken to detect possible renal ectopia or a small dysplastic kidney (located either within the renal fossa or elsewhere). The possibility of renal fusion (eg, either horseshoe kidney or cross-fused renal ectopia) should also be considered (Fig 5). These anomalies generally can be detected on both T2-weighted and postcontrast images; however, thin-section 3D T2-weighted FSE and delayed
RG Volume 35 Number 4 Dickerson et al 1217 Figure 5. Renal fusion anomalies. (a) Axial 2D T2-weighted FSE fat-saturated MR urogram in a 12-year-old girl shows fusion of the kidneys in the midline ( * ), with abnormally rotated renal moieties, findings typical of a horseshoe kidney. (b) Coronal postcontrast delayed phase 3D T1-weighted spoiled gradient-recalled echo MR urogram in a 3-year-old girl shows both kidneys located to the right of the midline and fused and an empty left renal fossa, findings typical of cross-fused renal ectopia. Figure 6. Imaging findings in an 8-year-old girl with a history of recurrent pyelonephritis. (a) Axial 2D T2-weighted FSE fat-saturated MR urogram shows multiple areas of focal parenchymal thinning (arrows) due to scarring; linear areas of underlying signal alteration may be attributable to superimposed infection. (b) Coronal 3D T2-weighted FSE fatsaturated MR urogram excellently depicts the areas of focal parenchymal loss (arrows). postcontrast images may be particularly useful for identifying small, ectopic, often dysplastic kidneys. Parenchymal Loss. Renal injury (eg, due to urinary tract obstruction, vesicoureteral reflux, and/or infection) can cause permanent loss of nephrons that may appear as focal parenchymal scarring or generalized loss of renal parenchyma at MR urography (Figs 6 and 7). Focal scarring appears as discrete parenchymal defects at T2-weighted and postcontrast MR urography and is easily distinguished from infection (unlike at 99m Tc dimercaptosuccinic acid renal scintigraphy). Focal scarring is commonly multifocal, often located directly over medullary pyramids; can be associated with loss of corticomedullary differentiation; and may be seen more frequently at the poles. Subtle cortical scarring is often most evident on axial 2D and coronal 3D T2-weighted FSE images. Diffuse renal parenchymal thinning can occur in the context of long-standing impaired renal drainage or hypoperfusion due to renal artery narrowing (33). MR urography allows renal parenchymal thickness, including both cortical and medullary thicknesses, to be compared bilaterally. In some instances, parenchymal thinning may be clearly attributable to loss of medullary height (eg, due to hypoplasia in the context of primary megacalyces or long-standing renal obstruction) as opposed to cortical thinning. An important research
1218 July-August 2015 radiographics.rsna.org Figure 7. Imaging findings in a 5-year-old boy with right vesicoureteral reflux. (a) Axial 2D T2-weighted fat-saturated MR urogram shows a small right kidney with areas of asymmetric parenchymal thinning due to scarring and mild compensatory left kidney hypertrophy. (b) Excretory phase MIP dynamic postcontrast MR urogram shows nonobstructive right hydroureteronephrosis ( * ) and extensive parenchymal scarring (bracket). Figure 8. Coronal postcontrast 3D T1-weighted spoiled gradientrecalled fat-saturated MR urogram in a 17-year-old girl shows multiple areas of bilateral renal parenchymal linear and wedge-shaped signal abnormality without parenchymal loss (arrows), findings indicating acute multifocal pyelonephritis. frontier lies in identifying the significance of different patterns of renal parenchymal thinning with respect to surgical (eg, pyeloplasty) and long-term outcomes (9,34). Parenchymal Infection. MR urography is both sensitive and specific for the diagnosis of pyelonephritis (35). Areas of parenchymal infection are most often peripherally located in the kidney and appear as striated, wedge-shaped, or masslike areas of altered signal intensity on T2-weighted and postcontrast MR urograms (Fig 8). Pyelonephritis may be multifocal and bilateral, and adjacent perinephric inflammatory changes can occur in some children. However, unlike with focal renal scarring, there should be no associated parenchymal loss in the context of acute infection at MR urography. A primary advantage of MR urography over 99m Tc dimercaptosuccinic acid renal scintigraphy is its ability to accurately allow discernment between acute pyelonephritis and irreversible focal scarring in a single examination (without ionizing radiation) (33,35). Dysplasia and Cysts. Renal cysts are commonly seen in children with renal and urologic abnormalities. A common cause of multiple renal cysts is developmental dysplasia of the kidney. Renal dysplasia encompasses a spectrum of abnormalities that may be attributable to impaired development of the metanephros (which leads to nephrons) or mesonephric duct (which leads to the ureters) or a failure of these two structures to meet and induce normal renal and urinary system development (36,37). The most severe form of renal dysplasia is multicystic dysplastic kidney. The exact cause of multicystic dysplastic kidney remains unknown, although the result is a nonfunctional kidney that contains numerous variably sized noncommunicating cysts with severely dysplastic inter-
RG Volume 35 Number 4 Dickerson et al 1219 Figure 9. Imaging findings in a 9-year-old girl with left multicystic dysplastic kidney. (a) Coronal 3D T2-weighted FSE fat-saturated MR urogram shows multiple noncommunicating cysts in the left renal fossa with no normal intervening renal parenchyma (black arrows). The right upper urinary tract is duplicated and extensive right kidney parenchymal scarring is seen. An apparent simple cyst (white arrow) arising from the lower right kidney pole is actually an obstructed lower moiety collecting system with severely thinned overlying renal parenchyma. (b) Axial postcontrast excretory phase 3D T1-weighted spoiled gradient-recalled fat-saturated MR urogram shows minimal enhancement of the left multicystic dysplastic kidney (black arrows), with no excretion of contrast material into associated cysts, and extensive right kidney scarring (white arrows). This case highlights the frequent association between multicystic dysplastic kidney and contralateral renal abnormalities, including duplication and ureteropelvic junction obstruction. vening renal tissue (37). Multicystic dysplastic kidneys tend to decrease in size over time, and small or ectopic multicystic dysplastic kidneys may remain unrecognized with multiple other modalities (38). At T2-weighted MR urography, multicystic dysplastic kidneys appear as a cluster of variably sized hyperintense cysts (Fig 9). At postcontrast imaging, dysplastic renal parenchyma may mildly enhance, although no excreted contrast material is seen in a renal collecting system or ureter. Approximately onethird of cases of multicystic dysplastic kidneys are associated with contralateral urinary tract abnormalities, including ureteropelvic junction obstruction and midureteric strictures (39,40). Less severe forms of renal dysplasia also occur. Severely dysplastic kidneys that have some minimal residual function and that drain into an associated ectopic ureter are a source of persistent wetness in girls. These kidneys are generally very small, contain numerous tiny cysts, and may be ectopic (38). Such kidneys are best appreciated at T2-weighted imaging and sometimes may be located by identifying an associated dilated, urine-filled, obstructed ureter. Postcontrast imaging may be less useful because of minimal renal function and delayed excretion of contrast material into the ureter. MR urography has been shown to be useful for identifying severely dysplastic yet functional kidneys that are impossible to locate with US, scintigraphy, or other imaging methods (5). Mild and moderate forms of renal dysplasia are presumably attributable to in utero renal obstruction and are commonly seen in neonates with posterior urethral valves and prune belly syndrome. In mild cases of renal dysplasia, the kidneys most often maintain their normal morphologic features (although they may be smaller than expected for age), demonstrate variable degrees of parenchymal and corticomedullary differentiation loss, and contain parenchymal cysts. These cysts, which are best appreciated on axial 2D T2-weighted and coronal 3D T2-weighted FSE images, vary in number and size depending on the severity of dysplasia and are commonly subcapsular. MR urography commonly shows evidence of renal dysplasia affecting the obstructed upper moieties of duplex kidneys (Fig 10). MR urography can also be used to characterize other apparent cystic abnormalities of the kidney, including strictured infundibula (hydrocalyx) (Fig 11), vascular abnormalities (eg, traumatic pseudoaneurysms), and large upper pole simple cysts (which may be mistaken for a duplex kidney with upper moiety obstruction). Upper Urinary Tracts (Renal Collecting Systems and Ureters) Duplex Kidney. Duplex kidney, or duplication of the upper urinary tract, is the most common congenital abnormality of the urinary tract and may be incomplete or complete. Both incomplete and complete duplications are most often
1220 July-August 2015 radiographics.rsna.org Figure 10. Coronal SSFSE fat-saturated MR hydrogram in a 3-week-old boy with right ureterovesical junction obstruction and left duplex kidney, obtained to define urinary tract anatomic features (because US findings were unclear), shows right hydroureteronephrosis to the level of the distal ureter, dilatation and tortuosity of both left upper urinary tracts, and termination of the left upper moiety in a large ureterocele ( * ). The presence of many cysts (arrow) indicates severely dysplastic left upper moiety renal parenchyma. Figure 11. Imaging findings in a 13-year-old boy with recurrent right flank pain after a remote history of right high-grade renal injury; US (not shown) showed a right renal cystic structure of uncertain significance. Coronal postcontrast delayed phase 3D T1-weighted spoiled gradientrecalled fat-saturated MIP MR urogram confirms filling of the cystic structure with contrast material ( * ), a finding consistent with hydrocalyx due to posttraumatic infundibular stenosis in the right upper pole. asymptomatic, although complete duplication is more likely to be symptomatic. In the context of complete duplication, the affected kidney is typically divided into two moieties (upper and lower) that are anatomically and functionally grouped to drain into a single collecting system and ureter; each ureter has a separate ureteric insertion (41) (Fig 12). Triplication and even quadruplication of the upper urinary tract can also occur (42). Ureters of duplicated upper urinary tracts can be very tortuous and intimately related; thus, establishing the relationship with US is difficult (Fig 10). Upper moiety ureteric insertions are typically abnormally inferior and medial in location (the so-called Weigert-Meyer rule) (Fig 13). In girls, ectopic upper moiety ureters that insert into the vagina or into the urethra below the level of the sphincter mechanism can manifest as persistent wetness. In boys, ectopic upper moiety ureters may insert into the posterior urethra above the level of the sphincter mechanism or into the genital tract (eg, seminal vesicles, vas deferens, or epididymis); such ectopic insertions can manifest with recurrent urinary tract infections or pelvic pain. It is not uncommon for a wide variety of Figure 12. Coronal postcontrast 3D T1- weighted spoiled gradient-recalled fat-saturated MIP MR urogram in a 6-year-old girl with right duplex kidney shows two right-sided collecting systems and ureters, right lower moiety mild pelvicaliectasis (*) ( drooping lily appearance) due to vesicoureteral reflux (as opposed to ureteropelvic junction obstruction), and extensive scarring in the right kidney lower moiety parenchyma (bracket) and left kidney lower pole (arrow).
RG Volume 35 Number 4 Dickerson et al 1221 Figure 13. Imaging findings corresponding to the Weigert-Meyer rule in a 4-month-old boy with a duplex kidney. (a) Coronal 3D T2-weighted FSE MIP MR urogram shows marked left upper moiety hydroureteronephrosis ( * ); the left upper moiety ureteric insertion is ectopic (arrow), inserting into the posterior urethra below the level of the urinary bladder. (b) Coronal postcontrast excretory phase 3D T1-weighted spoiled gradientrecalled fat-saturated MIP MR urogram shows normal excretion of contrast material by the left kidney lower moiety at 20 minutes after contrast material injection and only a very small amount of contrast material in the left upper moiety collecting system (arrow), findings confirming a left duplex kidney. Figure 14. Three-dimensional T2-weighted FSE MIP MR urogram in a 1-year-old girl with right ureterovesical junction obstruction shows marked right hydroureteronephrosis ( * ), with ureterectasis to the level of the right distal ureter, and Y-type incomplete duplication of the left upper urinary tract (arrow), with two separate collecting systems and proximal ureters. imaging examinations to have been performed before the diagnosis of an ectopic ureter (43). MR urography can also be used to successfully characterize duplex kidneys with ureterocele disproportion (ie, large ureterocele with associated tiny dysplastic upper moiety collecting system), a finding that is sometimes difficult to recognize at US (44). Although many cases of duplex kidney can be accurately characterized at US, voiding cystourethrography, and renal scintigraphy, MR urography may be necessary to completely understand certain complex upper urinary tract duplication anomalies (4). In particular, MR urography can be used to (a) confirm the presence of a duplex kidney, (b) establish whether the duplication anomaly is incomplete (Fig 14) or complete, (c) determine exact ureteric courses and insertions (Figs 13 and 15), (d) evaluate lower moiety renal function and hydronephrosis (determine whether lower moiety hydronephrosis is attributable to ureteropelvic junction obstruction or vesicoureteral reflux) (Fig 16) (41,45), (e) evaluate upper moiety renal function and hydronephrosis (determine whether upper moiety hydronephrosis is attributable to ectopic ureteric insertion or an obstructing ureterocele) (41,45), and (f) identify evidence of renal parenchymal focal scarring, diffuse thinning, and/or dysplasia.
1222 July-August 2015 radiographics.rsna.org Figure 15. Imaging findings in a 7-year-old girl with daytime and nighttime wetness despite successful toilet training. (a) Coronal postcontrast delayed phase 3D T1-weighted spoiled gradient-recalled fat-saturated MIP MR urogram reveals duplication of the left kidney with upper moiety obstruction ( * ). The upper moiety renal collecting system is composed of a single dilated calyx, and no contrast material is seen in its mid or distal ureter 20 minutes after administration. (b) Axial 2D T2-weighted FSE fat-saturated MR urogram shows a small round structure (arrow) in the anterior wall of the vagina, consistent with an ectopic left upper moiety ureter; fluid (possibly urine) is present in the vagina. (c) Axial delayed phase 3D T1-weighted 3D spoiled gradient-recalled fat-saturated MR urogram reveals the ectopic ureter as a hypointense lesion (arrow) located posterior to the urethra (a catheter is present in the urethra) just above the vaginal introitus. Both T2-weighted and postcontrast MR urography are useful for identifying and characterizing duplex kidneys. MR urography allows direct visualization of both renal moieties and associated upper urinary tracts; this is unlike voiding cystourethrography, which commonly relies on indirect evidence of duplication, such as the drooping lily sign (46). Three-dimensional T2- weighted FSE images are very useful for assessing dilated and/or obstructed moieties and may help locate ectopic ureteric orifices. Postcontrast MR urography is required to assess upper and lower moiety function and evaluate for lower moiety ureteropelvic junction obstruction. In our experience, MR urography can play a critical role in surgical planning in the context of complex urinary tract duplications (eg, guiding the decision whether to perform total nephrectomy, partial nephrectomy, or a diversion procedure [cutaneous ureterostomy or pyelostomy]). Ureteric Course and Insertion. As mentioned above, MR urography is an excellent modality for detecting ureteric ectopia. Ectopic ureters are most often associated with the upper moieties of duplex kidneys and certain dysplastic (and often ectopic) kidneys. In girls experiencing both daytime and nighttime wetness despite successful toilet training, MR urography should be considered to rule out the presence of an ectopic ureter (Fig 15). MR urography allows direct visualization of the ectopic ureter, often without intravenous contrast material, and can guide surgical management (either nephrectomy or ureteric reimplantation) (47). Narrowing and Obstruction. MR urography allows direct visualization of sites of upper urinary tract narrowing and can be used to assess the effect of these sites on renal function and the passage of contrast material through the
RG Volume 35 Number 4 Dickerson et al 1223 Figure 16. Imaging findings in an 8-year-old girl with bilateral hydronephrosis seen at US. (a) Coronal 3D T2-weighted FSE MIP MR urogram shows bilateral duplex kidneys with lower moiety pelvicaliectasis, bilateral lower moiety renal parenchymal thinning, normal upper moiety parenchyma, hyperintensity in the right lower moiety parenchyma that indicates edema and acute obstruction (arrows), and a dysmorphic left lower moiety collecting system lacking well-defined calyces. (b) Coronal postcontrast delayed phase T1-weighted 3D spoiled gradient-recalled fat-saturated MIP MR urogram shows normal excretion of contrast material by the upper moieties and delayed renal transit times in both lower moieties, findings that indicate bilateral lower moiety ureteropelvic junction obstructions. urinary system. In children, congenital narrowing of the upper urinary tract usually occurs at one of three locations: the ureteropelvic junction, midureter, or ureterovesical junction. Although many of these obstructive lesions can be characterized adequately with US and renal scintigraphy, some require additional radiologic evaluation. Ureteropelvic Junction Obstruction The most common site of upper urinary tract narrowing is at the ureteropelvic junction (the junction of the renal pelvis and proximal ureter). Although many cases of congenital ureteropelvic junction narrowing are attributable to intrinsic abnormality of the ureteropelvic junction and manifest with antenatal hydronephrosis (Fig 17), some cases of narrowing (especially in older children) are attributable to extrinsic mass effect from a crossing vessel (eg, accessory renal artery) (48,49) (Fig 18). MR urography can be used to evaluate suspected ureteropelvic junction obstructions that have atypical features at conventional imaging. MR urography, in particular 3D T2-weighted FSE imaging, allows direct visualization of ureteropelvic junction anatomic structures and assessment of degree of luminal narrowing, presence of ureteropelvic junction kinking or tortuosity, and location of ureteral insertion on the renal pelvis (eg, abnormally high insertion). Dynamic postcontrast imaging allows assessment of differential renal function and passage of contrast material through the kidney and collecting system into the ureter. Crossing vessels, which may be the only source of ureteropelvic junction narrowing in older children, can be directly seen on postcontrast dynamic images, whereas they may appear as flow voids on T2-weighted images. The site of ureteropelvic junction narrowing in the context of a crossing vessel is often the very proximal ureter, as previously described with excretory urography (Fig 18) (49). As the use of laparoscopic and robotic pyeloplasty procedures which have limited intraoperative visualization increases in children, the identification of crossing vessels will become increasingly important. MR urography can also be used to depict the postoperative ureteropelvic junction and confirm patency in the context of recurrent symptoms (Fig 19) (9). Midureteral Obstruction Congenital midureteral obstruction (also called congenital midureteral stricture or ureteral valve)
1224 July-August 2015 radiographics.rsna.org Figure 17. Imaging findings in a 7-month-old girl with right hydronephrosis at US. (a) Axial 2D T2-weighted FSE fatsaturated MR urogram shows severe right pelvicaliectasis ( * ) due to intrinsic ureteropelvic junction obstruction, very thin right kidney parenchyma, and a normal left kidney. (b) Coronal postcontrast delayed phase T1-weighted 3D spoiled gradient-recalled fat-saturated MR urogram shows markedly delayed calyceal transit time on the right, with no substantial excreted contrast material in the right renal collecting system, and visible enhancing right kidney parenchyma (arrows). Because the right kidney contributed approximately 23% to renal function at MR urography functional analysis (based on amount of enhancing renal parenchyma), pyeloplasty to relieve obstruction was performed instead of nephrectomy. Figure 18. Imaging findings in a 13-year-old boy with intermittent left flank pain. (a) Coronal 3D T2-weighted FSE fat-saturated MIP MR urogram shows severe left-sided pelvicaliectasis and abnormal narrowing of the left ureteropelvic junction; the caliber of the left ureter is normal. (b) Sagittal SSFSE MR urogram also shows left-sided pelvicaliectasis ( * ) and a short segment of urine-filled proximal ureter (arrow), a finding commonly observed in ureteropelvic junction obstruction due to a crossing vessel. (c) Coronal postcontrast arterial phase T1-weighted 3D spoiled gradient-recalled fat-saturated MIP MR urogram shows bilateral accessory renal arteries, with the lower left accessory renal artery (arrows) proven to be the cause of obstruction at surgery, and left kidney hypoenhancement due to hypoperfusion. is a rare cause of upper urinary tract obstruction theorized to be attributable to failure of ureter recanalization, prenatal vascular insult, or persistent ureteral fold (50). Although this entity can be mistaken for ureteropelvic junction or distal ureteric obstruction, correct diagnosis is imperative for surgical treatment (most often ureteroureterostomy). MR urography can be used to distinguish midureteral obstruction from more proximal and distal causes through direct visualization of the area of narrowing. At T2-weighted imaging, congenital midureteral obstructions demonstrate pelvicaliectasis and proximal ureterectasis, whereas the distal ureter is normal in caliber and may be difficult to visualize (Fig 20). Postcontrast images may show evidence of renal obstruction. Both T2- weighted and postcontrast images may show a
RG Volume 35 Number 4 Dickerson et al 1225 Figure 19. Coronal postcontrast delayed phase T1-weighted 3D spoiled gradient-recalled fat-saturated MIP MR urogram in a 16-year-old girl with solitary left kidney status after pyeloplasty (and recurrent left flank pain) shows a surgically reconstructed left ureteropelvic junction that is patent and without narrowing (arrow), normal left kidney parenchyma, and congenitally absent right kidney. Figure 20. Coronal 3D T2-weighted FSE subvolume MIP MR urogram in a 2-month-old girl with left hydronephrosis at US shows dilatation of the left renal collecting system and proximal ureter, beaking of the left mid ureter (arrow), and decompressed left distal ureter, findings consistent with midureteric stricture or valve. Multiple noncommunicating cysts in the right renal fossa are consistent with multicystic dysplastic kidney. transverse filling defect or abrupt ureteral narrowing with beaking at the obstruction. The contralateral kidney should also be closely evaluated for abnormalities, including multicystic dysplastic kidney (51). Ureterovesical Junction Obstruction, Including Obstructive Congenital Primary Megaureter Distal ureteric obstruction may occur at or proximal to the ureterovesical junction. Obstructive congenital primary megaureter is attributable to disordered peristalsis of the distal ureter (smooth muscle in the distal ureter wall is abnormal and ureteral fibrosis may be present) and can cause variable degrees of upper urinary tract obstruction (36,52) (Fig 21). Because obstructive congenital primary megaureter is often related to a functional rather than a mechanical cause of obstruction, spontaneous resolution can occur in some children and it is often managed with observation (47). Most primary megaureters are left-sided, and about two-thirds are in boys (53). MR urography demonstrates a narrowed aperistaltic segment of distal ureter of variable length (Fig 22). MR urography also allows detailed evaluation of the pelvicalyceal system, which can be dilated because of the ureteric obstruction or a concomitant abnormality, such as ureteropelvic junction obstruction or congenital megacalyces (both of which have been associated with congenital primary megaureter) (54,55). Renal parenchymal thinning is commonly observed, and focal scarring may be seen in children with pyelonephritis due to urinary stasis. Congenital Megacalyces. Congenital megacalyces refers to nonobstructive caliectasis associated with congenital medullary hypoplasia characterized by normal cortical thickness, substantial loss of medullary height, and papillary flattening (56). Although this condition is rare, it is important to differentiate from other causes of caliectasis that may lead to deterioration in renal function and require surgical intervention, such as ureteropelvic junction obstruction or vesicoureteral reflux. MR urography findings are similar to those observed at excretory urography: calyceal dilatation (caliectasis) that may be unilateral or bilateral and either diffuse or segmental. The number of calyces is often increased, and they have a polygonal or faceted shape. Postcontrast dynamic imaging should not show significant renal obstruction, although there may be delayed passage of contrast material because of the capacious calyces. Coexistence with congenital primary megaureter has also been described (54).
1226 July-August 2015 radiographics.rsna.org Figure 21. Imaging findings in a 5-month-old boy with bilateral obstructing congenital primary megaureters and normal voiding cystourethrography findings. (a) Coronal 3D T2-weighted FSE MIP MR urogram shows bilateral hydroureteronephrosis (left greater than right). (b) Coronal delayed phase T1-weighted 3D spoiled gradient-recalled fat-saturated MIP MR urogram obtained 20 minutes after contrast material injection shows asymmetric excretion of contrast material, with holdup of contrast material at the level of the right distal ureter (arrow); contrast material in only the renal collecting system on the left ( * ); and diffuse bilateral renal parenchymal thinning (left greater than right). Figure 22. Imaging findings in a 6-year-old boy with long-standing increasing left hydroureteronephrosis. (a) Coronal-oblique 3D T2-weighted FSE MIP MR urogram shows dilatation of the mid left ureter and kinking of the distal ureter (arrow); the most distal portion of the ureter is normal in caliber. (b) Coronal-oblique postcontrast delayed phase T1-weighted 3D spoiled gradient-recalled fat-saturated MIP MR urogram shows findings similar to those in a, including a long segment of normal-caliber distal ureter (arrow). Intraoperative examination of the distal left ureter revealed an aperistaltic segment, consistent with congenital primary megaureter. Fibroepithelial Polyps. Fibroepithelial polyps are benign projections of fibrovascular connective tissue that can develop from the collecting system or ureter and cause urinary tract obstruction. The polyps appear as focal elongated filling defects in the urinary tract that enhance at postcontrast imaging (Fig 23). They may be idiopathic or develop at a prior urinary tract surgical site and can be treated with surgery or endoscopic laser ablation (57,58). Calculi. Although US and CT are most often used to evaluate known or suspected urolithiasis, MR urography can also be used to detect some urinary tract calculi. The calculi usually appear as hypointense filling defects in the urinary system
RG Volume 35 Number 4 Dickerson et al 1227 Figure 23. Sagittal SSFSE (a) and axial 2D T2-weighted FSE fat-saturated (b) MR urograms in a 3-year-old girl with right renal obstruction and collecting system dilatation show low-signal-intensity filling defects (arrow) adherent to the ureter wall at the site of obstruction, which were determined to be fibroepithelial polyps. Figure 24. Sagittal prone postcontrast delayed phase T1-weighted 3D spoiled gradient-recalled fat-saturated MR urogram in a 7-year-old boy with left hydroureteronephrosis shows a hypointense filling defect (arrow) in the distal left ureter, a finding consistent with a calculus. More proximal dilatation of the upper urinary tract and pelvicalyceal system fluid-fluid levels suggest obstruction and stasis of contrast material. Urinary Bladder US is most often used to assess the urinary bladder in children and can be used to evaluate volume, wall thickening, masses, and calculi. MR urography assessment of the urinary bladder is generally limited, in part because of the decompressed state related to Foley catheter placement. Nonetheless, the bladder should be carefully inspected for ureteroceles, diverticula, and masses. Ureteroceles appear as variably sized thin-walled cystic structures in the urinary bladder lumen at MR urography and most often are associated with the upper moiety ureter of a duplex kidney (Figs 10 and 25). Rarely, they can be associated with nonduplicated upper urinary tracts. Ureteroceles causing urinary tract obstruction require incision or resection to improve urinary drainage and minimize risk for infection (59 62). on both T2-weighted and postcontrast images (Fig 24). In our experience, calculi seen at MR urography are generally large (>5 mm) and may be associated with urinary stasis and hydronephrosis. Postcontrast imaging can also show whether an associated obstruction is partial or complete. Renal Vasculature The renal vasculature can be assessed with both T2-weighted and postcontrast MR urography. Renal arteries appear as hypointense flow voids on 2D T2-weighted FSE images and are best seen during the arterial phase at postcontrast imaging. Multiple renal arteries are commonly observed, especially in the context of horseshoe kidney or cross-fused renal ectopia. As mentioned above, crossing vessels (either a crossing artery or vein) are a cause of ureteropelvic junction obstruction that is amenable to surgical correction (the ureteropelvic junction is transected, moved anterior to the crossing vessel, and anastomosed) (9,48,63). Most renal vein anomalies (eg, retrocaval left renal vein or
1228 July-August 2015 radiographics.rsna.org Figure 25. Coronal 3D T2-weighted FSE fat-saturated MR urogram in a 6-month-old girl with left duplex kidney shows upper moiety obstruction due to an associated ureterocele (*), severe left kidney upper moiety parenchymal thinning, and normal caliber in the left lower moiety collecting system and ureter (not shown). Figure 26. (a) Coronal postcontrast early venous phase T1-weighted 3D spoiled gradient-recalled fat-saturated MIP MR urogram in a 16-year-old girl with chronic flank pain and hematuria shows retrograde filling of the left gonadal vein (arrow) and pelvic varices (arrowhead), findings attributable to abnormal narrowing of the left renal vein between the abdominal aorta and superior mesenteric artery (so-called nutcracker syndrome ). (b) Axial-oblique gadofosveset trisodium enhanced subvolume MR venogram in a different 16-year-old girl shows the configuration of high-grade narrowing (arrow) of the left renal vein as it passes between the aorta and superior mesenteric artery ( * ) and multiple vessels collateral to the left renal vein. drainage to the common iliac vein) are of little clinical significance. However, MR urography can show evidence of impingement of the left renal vein as it passes between the abdominal aorta and superior mesenteric artery in the context of hematuria and flank pain ( nutcracker syndrome ) with or without left gonadal vein dilatation and pelvic varices (Fig 26). Conclusion Pediatric MR urography is an extremely useful radiologic tool that allows comprehensive assessment of the pediatric kidneys and upper urinary tracts, providing information that would otherwise require multiple other imaging tests. This imaging technique has the potential to allow earlier diagnosis of a wide variety of renal and urinary tract abnormalities, while decreasing the number of imaging studies performed for a child. Pediatric MR urography can be used to successfully evaluate complex renal and urinary tract anatomy and a variety of causes of urinary tract obstruction and can provide critical information for operative planning. In our experience, findings from pediatric MR urography using a state-of-the-art protocol can directly affect medical and surgical decision making for children with urologic abnormalities, and use of the modality may lead to improved patient outcomes. References 1. Khrichenko D, Darge K. Functional analysis in MR urography made simple. Pediatr Radiol 2010;40(2): 182 199.
RG Volume 35 Number 4 Dickerson et al 1229 2. Jones RA, Schmotzer B, Little SB, Grattan-Smith JD. MRU post-processing. Pediatr Radiol 2008;38(suppl 1): S18 S27. 3. Grattan-Smith JD, Little SB, Jones RA. MR urography evaluation of obstructive uropathy. Pediatr Radiol 2008;38(suppl 1):S49 S69. 4. Adeb M, Darge K, Dillman JR, Carr M, Epelman M. Magnetic resonance urography in evaluation of duplicated renal collecting systems. Magn Reson Imaging Clin N Am 2013;21(4):717 730. 5. McMann LP, Kirsch AJ, Scherz HC, et al. Magnetic resonance urography in the evaluation of prenatally diagnosed hydronephrosis and renal dysgenesis. J Urol 2006;176(4 Pt 2):1786 1792. 6. Gylys-Morin VM, Minevich E, Tackett LD, Reichard E, Wacksman J, Sheldon CA. Magnetic resonance imaging of the dysplastic renal moiety and ectopic ureter. J Urol 2000;164(6):2034 2039. 7. Riccabona M, Riccabona M, Koen M, et al. Magnetic resonance urography: a new gold standard for the evaluation of solitary kidneys and renal buds? 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A single-system ectopic ureter draining an ectopic dysplastic kidney: delayed diagnosis in the young female with continuous urinary incontinence. Br J Urol 1998;81(3):474 478. 39. Atiyeh B, Husmann D, Baum M. Contralateral renal abnormalities in multicystic-dysplastic kidney disease. J Pediatr 1992;121(1):65 67. 40. Schreuder MF, Westland R, van Wijk JA. Unilateral multicystic dysplastic kidney: a meta-analysis of observational studies on the incidence, associated urinary tract malformations and the contralateral kidney. Nephrol Dial Transplant 2009;24(6):1810 1818. 41. Bisset GS 3rd, Strife JL. The duplex collecting system in girls with urinary tract infection: prevalence and significance. AJR Am J Roentgenol 1987;148(3):497 500. 42. Finkel LI, Watts FB Jr, Corbett DP. Ureteral triplication with a ureterocele. Pediatr Radiol 1983;13(6):346 348. 43. Gharagozloo AM, Lebowitz RL. Detection of a poorly functioning malpositioned kidney with single ectopic ureter in girls with urinary dribbling: imaging evaluation in five patients. AJR Am J Roentgenol 1995;164(4):957 961. 44. Share JC, Lebowitz RL. Ectopic ureterocele without ureteral and calyceal dilatation (ureterocele disproportion): findings on urography and sonography. AJR Am J Roentgenol 1989;152(3):567 571. 45. Fernbach SK, Zawin JK, Lebowitz RL. Complete duplication of the ureter with ureteropelvic junction obstruction of the lower pole of the kidney: imaging findings. AJR Am J Roentgenol 1995;164(3):701 704. 46. Callahan MJ. The drooping lily sign. Radiology 2001;219(1):226 228. 47. McLellan DL, Retik AB, Bauer SB, et al. Rate and predictors of spontaneous resolution of prenatally diagnosed primary
1230 July-August 2015 radiographics.rsna.org nonrefluxing megaureter. J Urol 2002;168(5):2177 2180; discussion 2180. 48. Park JM, Bloom DA. The pathophysiology of UPJ obstruction: current concepts. Urol Clin North Am 1998;25(2):161 169. 49. Hoffer FA, Lebowitz RL. Intermittent hydronephrosis: a unique feature of ureteropelvic junction obstruction caused by a crossing renal vessel. Radiology 1985;156(3):655 658. 50. Docimo SG, Lebowitz RL, Retik AB, Colodny AH, Bauer SB, Mandell J. Congenital midureteral obstruction. Urol Radiol 1989;11(3):156 160. 51. Cauchi JA, Chandran H. Congenital ureteric strictures: an uncommon cause of antenatally detected hydronephrosis. Pediatr Surg Int 2005;21(7):566 568. 52. Kang HJ, Lee HY, Jin MH, Jeong HJ, Han SW. Decreased interstitial cells of Cajal-like cells, possible cause of congenital refluxing megaureters: histopathologic differences in refluxing and obstructive megaureters. Urology 2009;74(2):318 323. 53. Meyer JS, Lebowitz RL. Primary megaureter in infants and children: a review. Urol Radiol 1992;14(4):296 305. 54. Vargas B, Lebowitz RL. The coexistence of congenital megacalyces and primary megaureter. AJR Am J Roentgenol 1986;147(2):313 316. 55. McGrath MA, Estroff J, Lebowitz RL. The coexistence of obstruction at the ureteropelvic and ureterovesical junctions. AJR Am J Roentgenol 1987;149(2):403 406. 56. Kozakewich HP, Lebowitz RL. Congenital megacalyces. Pediatr Radiol 1974;2(4):251 257. 57. Williams TR, Wagner BJ, Corse WR, Vestevich JC. Fibroepithelial polyps of the urinary tract. Abdom Imaging 2002;27(2):217 221. 58. Kojima Y, Lambert SM, Steixner BL, Laryngakis N, Casale P. Multiple metachronous fibroepithelial polyps in children. J Urol 2011;185(3):1053 1057. 59. Shankar KR, Vishwanath N, Rickwood AM. Outcome of patients with prenatally detected duplex system ureterocele: natural history of those managed expectantly. J Urol 2001;165(4):1226 1228. 60. Valla JS, Breaud J, Carfagna L, Tursini S, Steyaert H. Treatment of ureterocele on duplex ureter: upper pole nephrectomy by retroperitoneoscopy in children based on a series of 24 cases. Eur Urol 2003;43(4):426 429. 61. Husmann DA, Ewalt DH, Glenski WJ, Bernier PA. Ureterocele associated with ureteral duplication and a nonfunctioning upper pole segment: management by partial nephroureterectomy alone. J Urol 1995;154(2 Pt 2):723 726. 62. Hagg MJ, Mourachov PV, Snyder HM, et al. The modern endoscopic approach to ureterocele. J Urol 2000;163 (3):940 943. 63. Silverman SG, Leyendecker JR, Amis ES Jr. What is the current role of CT urography and MR urography in the evaluation of the urinary tract? Radiology 2009;250(2):309 323. This journal-based SA-CME activity has been approved for AMA PRA Category 1 Credit TM. See www.rsna.org/education/search/rg.
QUESTIONNAIRE A3(17) Pediatric MR Urography: indications, techniques and approach to review INSTRUCTIONS Read through the article and answer the multiple choice questions provided below Some questions may have more than one answer; in which case you must please mark all the correct answers Question 1: Benefits of magnetic resonance (MR) urography as a tool to assess disorders of the pediatric urinary tract, include which of the following? A: It provides both functional and morphologic information B: There is no need for sedation in young children C: Evaluation takes place in a single examination D: Lower cost than ultrasonography (US) E: There is no exposure to ionizing radiation Question 2: Which one of the items below refers to the Patlak method? A: Volume of enhancing renal parenchyma B: Glomerular filtration of contrast material Question 3: The information provided by MR urography can be compared to that gained by a combination of...,..., and : A: Ultrasonography (US), computed tomography (CT), voiding cystourethrography and renal scintigraphy B: Ultrasonography (US), computed tomography (CT), excretory urography and renal scintigraphy Question 4: Common indications for pediatric MR urography include which of the following? A: Evidence of pyelonephritis B: Evaluation of complex renal and urinary tract anatomy C: Suspected urinary tract obstruction D: Operative planning E: Post-operative assessment Question 5: A little girl is experiencing day- and nighttime wetness despite successful toilet training. MR urography should be considered to rule out: A: Cancer B: Urinary tract infection C: The presence of an ectopic ureter Question 6: MR urography can be used to evaluate which of the following in relation to sites of urinary tract narrowing and obstruction? A: Location of the narrowing B: Degree of narrowing C: Potential extrinsic cause D: Intrinsic abnormality E: All of the above Question 7: Identify the weaknesses of US when compared to MR urography: A: US capabilities diminish as children increase in size and as anatomic structures of interest become relatively smaller B: Longer examination time C: Exposure to ionized radiation D: Little or no information is provided regarding renal function Question 8: Voiding cystourethrography is mostly performed whilst children are sedated. Is this statement TRUE or FALSE? A: TRUE B: FALSE Question 9: In children,... can be used to show evidence of scarring or pyelonephritis but it provides minimal anatomic information: A: Computed tomography B: Ultrasonography C: MR hydrography D: Renal scintigraphy Question 10: Which of the following are TRUE with regard to renal scintigraphy? A: It provides inadequate anatomic information B: There is no need for repeat imaging over several months C: It exposes children to ionizing radiation D: It is much more costly than MR urography E: Limited visualization of anatomic structures complicates functional assessment Question 11: MR urography examinations typically last: A: 15 35 minutes B: 35 70 minutes C: 40 85 minutes D: 60 90 minutes Question 12: Which type of T2-weighted pulse sequence provides an excellent overview of renal and urinary tract anatomic structures, and guides anatomic coverage of subsequent pulse sequences? A: SSFSE B: 2D FSE C: 3D FSE
Question 13: MR hydrography can be repeated in a timeresolved fashion to produce cine images to show: A: Ureter peristalsis B: Vesicoureteral reflux C: Pyelonephritis Question 14: Which pulse sequence targets the coronal-oblique plane? A: SSFSE with fat saturation B: T2-weighted FSE with fat saturation C: 2D T1-weighted GRE D: High-spatial-resolution 3D T2-weighted FSE with fat saturation E: Dynamic post contrast imaging for 10-15 min Question 15:... allows for evaluation of both renal and urinary tract anatomic structures, and excellently depicts... and... upper urinary tracts: A: Post-contrast MR urography, dilated and nonobstructed B: MR hydrography, non-dilated and non-obstructed C: Post-contrast MR urography, non-dilated and nonobstructed D: Pre-contrast MR urography, non-dilated and nonobstructed Question 16: With reference to MR urography, each dynamic imaging volume should meet the following criteria: A: Temporal resolution of less than 10 seconds B: Contain an average of 20-30 images with a 2-4 mm section thickness C: Be oriented in the coronal-oblique plane D: A and B Question 17: Which of the following are FALSE with regard to delayed post-contrast imaging? A: Provides low-spatial-resolution depiction of the kidneys and urinary tract in the excretory phase B: Can be used to generate 2D reformations C: Cannot generate 3D reconstructions that include MIP and volume-rendered images to give an overview of renal and urinary tract anatomic structures on a single image D: Imaging is usually done in axial and coronal planes and never in the sagittal plane Question 19: Because MR urography is most often performed for children who have an increased risk for impaired renal function, which of the following should be done? A: Serum creatinine concentration should be measured B: Estimated glomerular filtration rate should be calculated with accepted formulas before contrast material administration Question 20: Pre-procedural intravenous hydration and furosemide treatment administered before or during MR urography is done to achieve which of the following? A: Improve urinary tract distention and visualization B: To provide an excellent overview of urinary tract anatomic structures C: Decrease the transit time for excreted contrast material to reach the ureters and bladder D: Dilute excreted contrast material to minimize artifacts related to T2 effect E: To allow free drainage of urine during the imaging examination Question 21: Which of the following can be used to minimize patient discomfort during examination due to bladder distention from procedural hydration and diuretic treatment? A: Furosemide treatment B: Foley catheter C: Image patient in a prone position Question 22:... can help to decrease motion artifacts in younger or uncooperative children? A: Feed and swaddle approach B: Breath holds C: Sedation or general anesthesia Question 23: Child-life or child-behavioural specialists can decrease sedation and general anesthesia use by doing which of the following? A: Educating children and parents about MR urography B: Implementing age-appropriate coping strategies Question 18: The potential risk for nephrogenic systemic fibrosis is associated with which technique and is therefore generally inappropriate for children with acute kidney injury or chronic kidney disease? A: Voiding cystourethrography B: Post-contrast MR urography C: Post-contrast MR hydrography
Question 24: Which quantitative functionality is possibly viewed as the most basic and important functional ability of MR urography? A: Determination of calyceal transit time B: Determination of differential renal function C: Determination of renal transit time D: Determination of signal intensity transit time curves Question 25: Which type of calyceal transit time may be observed in intermittent urinary tract obstructions? A: Delayed B: Rapid C: Intermediate Question 26: Jones et al concluded that a renal transit time suggests obstructions: A: Greater than 590 seconds B: Less than 490 seconds C: Greater than 490 seconds D: Between 360 and 590 seconds Question 27: Renal parenchymal signal intensity-time curves provide information in relation to: A: Renal perfusion B: Uptake of contrast material by the kidneys C: Contrast material excretion D: A and C Question 28: can cause permanent loss of nephrons that may appear as focal parenchymal scarring and is easily distinguished from infection at MR urography: A: Pyelonephritis B: Dysplasia and cysts C: Renal injury Question 29: Which of the following is the most common congenital abnormality of the urinary tract and may be complete or incomplete? A: A multi-cystic dysplastic kidney B: Renal fusion C: Parenchymal scarring D: A duplex kidney or duplication of the upper urinary tract Question 30: The most common site of upper urinary tract narrowing is at the: A: Midureter B: Ureteropelvic junction C: Ureterovesical junction Question 31: is theorized to be attributable to failure of ureter recanalisation, pre-natal vascular insult or persistent ureteral fold: A: Congenital midureteral stricture B: Ureteropelvic junction obstruction C: Distal ureteric obstruction Question 32: Most primary megaureters are : A: Right-sided B: More prevalent in girls than in boys C: Left-sided D: Most prevalent in boys Question 33: Which of the following appears as a variably sized, thin-walled cystic structure in the urinary bladder lumen? A: Diverticula B: Ureteroceles C: Masses D: Calculi End
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