Quantitative renal magnetic resonance imaging: magnetic resonance urography.

The goal of functional renal imaging is to identify and quantitate irreversible renal damage and nephron loss, as well as potentially reversible hemodynamic changes. MR urography has evolved into a comprehensive evaluation of the urinary tract that combines anatomical imaging with functional evaluation in a single test without ionizing radiation. Quantitative functional MR imaging is based on dynamic contrast-enhanced MR acquisitions that provide progressive, visible enhancement of the renal parenchyma and urinary tract. The signal changes related to perfusion, concentration and excretion of the contrast agent can be evaluated using both quantitative and qualitative measures. Functional evaluation with MR has continued to improve as a result of significant technical advances allowing for faster image acquisition as well as the development of new tracer kinetic models of renal function. The most common indications for MR urography in children are the evaluation of congenital anomalies of the kidney and urinary tract including hydronephrosis and renal malformations, and the identification of ectopic ureters in children with incontinence. In this paper, we review the underlying acquisition schemes and techniques used to generate quantitative functional parameters including the differential renal function (DRF), asymmetry index, mean transit time (MTT), signal intensity versus time curves as well as the calculation of individual kidney glomerular filtration rate (GFR). Visual inspection and semi-quantitative assessment using the renal transit time (RTT) and calyceal transit times (CTT) are fundamental to accurate diagnosis and are used as a basis for the interpretation of the quantitative data. The importance of visual assessment of the images cannot be overstated when analyzing the quantitative measures of renal function.

Accuracy of subjective vesicoureteral reflux timing assessment: supporting new voiding cystourethrogram guidelines.

BACKGROUND: Bladder volume at the onset of vesicoureteral reflux (VUR) is an important prognostic indicator of spontaneous resolution and the risk of pyelonephritis. OBJECTIVE: We aim to determine whether pediatric urologists and pediatric radiologists can accurately estimate the timing of reflux by examining voiding cystourethrogram (VCUG) images without prior knowledge of the instilled contrast volume. MATERIALS AND METHODS: Total bladder volume and the volume at the time of reflux were collected from VCUG reports to determine the volume at the onset of VUR. Thirty-nine patients were sorted into three groups: early-/mid-filling reflux, late-filling and voiding only. Thirty-nine images were shown to three pediatric urologists and two pediatric radiologists in a blinded fashion and they were then asked to estimate VUR timing based on the above categories. A weighted kappa statistic was calculated to assess rater agreement with the gold standard volume-based report of VUR timing. RESULTS: The mean patient age at VCUG was 3.1±2.9 months, the median VUR was grade 3, and 20 patients were female. Overall agreement among all five raters was moderate (k=0.43, 95% confidence interval [CI] 0.36-0.50). Individual agreement between rater and gold standard was slight to moderate with kappa values ranging from 0.13 to 0.43. CONCLUSION: Pediatric radiologists and urologists are unable to accurately and reliably characterize VUR timing on fluoroscopic VCUG. These findings support the recently published American Academy of Pediatrics protocol recommending the routine recording of bladder volume at the onset of VUR as a standard component of all VCUGs to assist in a more accurate assessment of the likelihood of resolution and risk of recurrent urinary tract infections.

Comparison of ultrasound and magnetic resonance urography for evaluation of contralateral kidney in patients with multicystic dysplastic kidney disease.

PURPOSE: The contralateral kidney is abnormal in up to 25% of patients with multicystic dysplastic kidney. Traditionally, anatomical and functional evaluation of the contralateral kidney has been performed with ultrasound and dimercapto-succinic acid renal scintigraphy, as indicated. Recently magnetic resonance urography has been used to evaluate renal anatomy and function in other urological abnormalities. We compared the results of magnetic resonance urography and ultrasound for evaluating the contralateral kidney in patients with multicystic dysplastic kidney and we describe the range of findings detected. MATERIALS AND METHODS: Patients with multicystic dysplastic kidney who underwent magnetic resonance urography were identified. Anatomical findings on magnetic resonance urography were analyzed and compared to those on renal ultrasound. Additional functional information derived from magnetic resonance urography was also recorded. RESULTS: We retrospectively identified 58 patients with a unilateral multicystic dysplastic kidney who had undergone magnetic resonance urography, of whom 54 also underwent ultrasound. Of the patients 19 (32.8%) had a contralateral abnormality. A discrepancy between magnetic resonance urography and ultrasound was seen in 9 patients (16.7%). Of these patients only 1 had a completely normal contralateral kidney by ultrasound on retrospective review. The incidence and range of parenchymal abnormalities was wider than previously reported. CONCLUSIONS: Contralateral abnormalities in children with multicystic dysplastic kidney are common and more definitively evaluated with magnetic resonance urography vs ultrasound. Renal ultrasound remains the most appropriate modality for the initial evaluation of children with multicystic dysplastic kidney, and magnetic resonance urography is recommended when a functional study is required either to confirm the diagnosis of multicystic dysplastic kidney or to evaluate suspected abnormalities of the contralateral kidney.

Pediatric magnetic resonance urography.

Magnetic resonance urography (MRU) is a powerful clinical tool that fuses anatomic information with functional data in a single test without the use of ionizing radiation. This article provides an overview of the technical aspects, as well as common clinical applications with an emphasis on the evaluation of hydronephrosis. A fluid challenge is an essential part of our MRU protocol and enables the definition of compensated or decompensated kidneys within the spectrum of hydronephrosis. This classification may have prognostic implications when surgery is being considered. In addition, underlying uropathy can be identified on the anatomical scans and renal scarring can be seen on both the anatomical and dynamic scans. MRU can identify and categorize dysmorphic kidneys in vivo and may provide insight into congenital abnormalities seen in conjunction with vesicoureteric reflux. MRU is still in its infancy and as the technique develops and becomes widely available, it seems likely that it will supplant renal scintigraphy in the evaluation of renal tract disorders in children.

Pediatric uroradiology: state of the art.

Recent years have witnessed major strides in imaging modalities, including US and contrast-enhanced voiding urosonography, fluoroscopy, CT and MRI, and these have significantly impacted paediatric uroradiology. The trend is towards reduced or no radiation exposure and combined morphological and functional imaging. This review presents the currently available and recommended modalities in modern paediatric uroradiology practice.

Bilateral congenital midureteric strictures associated with multicystic dysplastic kidney and hydronephrosis: evaluation with MR urography.

We report a case of bilateral congenital midureteric strictures diagnosed using MR urography. The severity of obstruction differed in the two ureters, resulting in a multicystic dysplastic kidney (MCDK) with an atretic ureter on one side and hydronephrosis that worsened over time due to progressive stenosis on the other. Although midureteric strictures are usually misdiagnosed as ureteropelvic junction (UPJ) or ureterovesical junction (UVJ) obstruction on conventional imaging, MR urography was able to clearly demonstrate both the anatomical and functional abnormalities. Additionally, because of the excellent anatomical resolution, similarities in the underlying pathological lesions could be contrasted with the severity of the pathophysiological impact upon each kidney.

Radiologic features of implants after endoscopic treatment of vesicoureteral reflux in children.

OBJECTIVE: Implants after endoscopic treatment of vesicoureteral reflux (VUR) in children will be more frequently detected on imaging studies and may lead to misinterpretation and unnecessary intervention. This article reviews the radiologic appearance of implants. CONCLUSION: Radiologic findings of implants depend on the imaging technique, bulking agent, and time after injection. A history of VUR or an antireflux procedure and the absence of hydronephrosis in cases of suspected urolithiasis are important clues to suggest implants.

Individual kidney blood flow measured with contrast-enhanced first-pass perfusion MR imaging.

UNLABELLED: The study design was HIPAA-compliant and approved by the Institutional Review Board, with all participants providing signed informed consent prior to the study. The purpose of this study was to prospectively evaluate the feasibility of determining renal blood flow (RBF) by using a technique based on intravenous administration of gadolinium chelate and evaluation of first-pass gadolinium chelate perfusion by using highly accelerated three-dimensional (3D) gradient-echo magnetic resonance (MR) imaging of the kidney in freely breathing subjects. Flow is determined with Kety-Schmidt formalism by modeling the uptake of gadolinium chelate in the kidney prior to its leaving through the venous system. Validation of the gadolinium chelate perfusion technique is based on comparison of values determined for participants with phase-contrast gradient-echo imaging. The model fit to the measured data is excellent over the first 7-8 seconds of gadolinium chelate uptake and diverges after its appearance in the renal vein. The perfusion data analysis technique showed less than 10% interobserver variation. The average difference between phase-contrast and gadolinium chelate perfusion measurements was 0.08 mL/sec (95% confidence interval: -3.73, 3.58) for left and right kidneys. This study demonstrates feasibility of the gadolinium chelate perfusion method for RBF measurement and discusses potential applications. SUPPLEMENTAL MATERIAL: http://radiology.rsnajnls.org/cgi/content/full/246/1/241/DC1.

Magnetic resonance urography in children.

Magnetic resonance (MR) urography is a powerful tool that fuses anatomic information with functional data in a single test without the use of ionizing radiation. This article provides an overview of the technical aspects of MR urography and common clinical applications, such as the evaluation of hydronephrosis, reflux nephropathy, and renal dysplasia.

MR urography: technique and results for the evaluation of urinary obstruction in the pediatric population.

MR urography has the potential to revolutionize imaging of the urinary tract in both adults and children, because of its ability to provide an unprecedented level of anatomic information and quantitative functional evaluation of each kidney. MR urography can now provide useful assessment of obstructive uropathy and may provide predictive information about which children will benefit from surgery. It has the potential to identify parameters that indicate a significant obstruction as opposed to self-limited hydronephrosis. Further technical developments in the field will produce greater insights into the pathophysiology of not only urologic disorders but also disorders of the kidney itself.

Magnetic resonance urography for diagnosis of pediatric ureteral stricture.

PURPOSE: Ureteral stricture is a rare cause of hydronephrosis in children and is often misdiagnosed on ultrasound (US) and diuretic renal scintigraphy (DRS), requiring intraoperative diagnosis. We evaluated ureteral strictures diagnosed by magnetic resonance urography (MRU) at our institution. MATERIALS AND METHODS: Children with ureteral stricture who underwent MRU were identified. Patient demographics, prior imaging, MRU findings, and management were assessed. The efficacy of MRU in diagnosis of stricture was compared with US and DRS. Patients with ureteropelvic or ureterovesical junction obstruction were excluded. RESULTS: Twenty-eight ureteral strictures diagnosed by MRU between 2003 and 2013 were identified; 22% of strictures were diagnosed by DRS ± US. The mean age at MRU diagnosis was 2.4 years (range 4 weeks-15 years). Hydronephrosis was the most common presentation, accounting for 20 (71%) cases. Other etiologies included pain (3), incontinence (2), and urinary tract infection, cystic kidney, and absent kidney, present in one case each. A mean of 2.7 imaging studies was obtained prior to MRU diagnosis. Twenty-one (75%) ureteral strictures required surgical intervention, with the approach dependent upon location. CONCLUSIONS: MRU provides excellent anatomic and functional detail of the collecting system, leading to accurate diagnosis and management of ureteral stricture in children.

Spinal cord infarction following minor trauma in children: fibrocartilaginous embolism as a putative cause.

Spinal cord infarctions following seemingly innocuous trauma in children are rare, devastating events. In the majority of these cases, the pathophysiology is enigmatic. The authors present 3 cases of pediatric spinal cord infarction that followed minor trauma. An analysis of the clinical, radiographic, and laboratory features of these cases suggests that thromboembolism of the nucleus pulposus into the spinal cord microcirculation is the likely mechanism. A review of the human and veterinary literature supports this notion. To the authors' knowledge, this is the largest pediatric series of myelopathy due to thromboembolism of the nucleus pulposus reported to date, and it is the first report of this condition occurring in an infant.

Magnetic resonance imaging-guided laparoscopic-assisted anorectoplasty for imperforate anus.

PURPOSE: Magnetic resonance imaging (MRI) has been well described as a modality for evaluation of a failed anorectal pull-through procedure for imperforate anus. To the authors' knowledge, intraoperative MRI has not been previously used to guide a laparoscopic-assisted anorectoplasty (LAARP). We propose that such a procedure would assure anatomically correct placement of the pulled-through rectum. METHODS: Three male patients with imperforate anus and a prostatic urethral fistula underwent an MRI-guided LAARP in an operative MRI suite. The patients' ages ranged from 5 to 6 months at the time of their pull-through procedure. Preoperative MRIs with mineral oil within the distal colostomy were performed on all patients to document the anatomy of the rectourethral fistula and its relationship to the parasagittal and vertical muscle complex. The perineum was pierced with an MRI compatible needle at the central portion of the parasagittal muscle complex as determined by a direct muscle stimulator. Further incremental advancement of the needle within the muscle complex was guided by serial MRIs in axial, coronal, and sagittal planes until the levator floor was penetrated, and the peritoneal cavity was entered. LAARP was then completed. RESULTS: Completion MRI demonstrated placement of the pulled-through segment in a central location through the length of the muscle complex. Serial MRIs performed intraoperatively during advancement of the localization needle demonstrated a curved path of the vertical fibers. Attempts to nonincrementally advance the needle in a straight plane resulted in a breach of the vertical muscle complex or eccentric placement of the needle. CONCLUSIONS: Magnetic resonance imaging-guided LAARP results in anatomically correct placement of the rectum within the vertical muscle complex. Straight needle advancement techniques in LAARP could result in a deviation of the pulled-through rectum from the central muscular path. Further follow-up will be required to demonstrate functional advantage.

Appearance of Deflux implants with magnetic resonance imaging after endoscopic treatment of vesicoureteral reflux in children.

OBJECTIVE: With the increasing popularity of endoscopic treatment for vesicoureteral reflux (VUR) in children, dextranomer/hyaluronic acid copolymer (Deflux) implants are more frequently detected by magnetic resonance imaging (MRI). Such findings on MRI may be misinterpreted and lead to unnecessary intervention. The objective of this study was to characterize the appearance of Deflux implants on MRI. MATERIALS AND METHODS: Between July 2001 and November 2007, hospital charts of patients with a history of Deflux injection for VUR were evaluated to identify those who had subsequent MRI studies. The indications were determined, and the appearance of Deflux implants analyzed and compared to findings on ultrasound, voiding cystourethrography and kidney ureter bladder radiography. RESULTS: Of 893 patients who underwent endoscopic treatment for VUR, subsequently 16 patients (1.8%) had MRI scans. Twenty-seven Deflux implants were identified on MRI as bright structures on T2-weighted sequences only. Neither T1-weighted images nor excretory MR urography visualized the Deflux implants, which did not enhance with gadolinium. Deflux was not detected by kidney ureter bladder radiography; however, voiding cystourethrography and more reliably ultrasound could identify implants. CONCLUSIONS: Deflux implants appear on MRI as bright structures on T2-weighted sequences. History of VUR/VUR treatment and the presence of a single urinary system should provide reassurance and prevent inappropriate intervention for misdiagnosed ureteroceles.

Evaluation of UPJ obstruction before and after pyeloplasty using MR urography.

This article builds on the previous article in this symposium and shows how MR urography contributes to the postoperative evaluation of children with UPJ obstruction. By analyzing the postoperative results, we derived new insights into their preoperative evaluation. With MR urography we combine simultaneous physiological and anatomic evaluation that enables us to identify changes in renal pathophysiology that occur in association with impaired drainage and obstruction. We studied 35 children before and after pyeloplasty. The pyeloplasty was considered successful in 30 and unsuccessful in 5. Both anatomic and functional criteria were used. The anatomic parameters included the degree of hydronephrosis, the appearance of the renal parenchyma, the quality of the nephrogram and the presence of crossing vessels. The functional criteria included the renal transit time, the calyceal transit time, the volumetric differential function, the Patlak differential function, the difference between the volumetric and Patlak differential function and the Patlak number per milliliter of renal tissue. No single parameter was sufficient to fully characterize UPJ obstruction, but by synthesizing all the information we were able to subdivide UPJ obstruction into compensated, decompensated and uropathic kidneys. Decompensated systems had the most significant improvement following successful pyeloplasty. Compensated systems showed little improvement in renal function, and uropathic kidneys were associated with a poor prognosis. It is clear that not all UPJ obstructions are the same, and it seems logical that treatment should be individually tailored rather than using a standard approach for all cases. Because MR urography can identify pathophysiological differences in children with UPJ obstruction that are occult to renal scintigraphy, it has an important potential role in identifying those who will benefit most from pyeloplasty and those who are probably best observed.

MR urography evaluation of obstructive uropathy.

Hydronephrosis and the evaluation of obstructive uropathy are the most common indications for MR urography in our practice. Typically our patients fall into one of two groups: infants with antenatal hydronephrosis and older children who present with abdominal pain, hematuria or urinary tract infection. Obstruction in children is usually chronic and partial. Intermittent episodes of increased pressure occur when the urine production exceeds the capacity for drainage. MR urography uses a fluid and diuretic challenge to assess the hydronephrotic kidney. High-quality anatomic images provide a morphologic assessment of the hydronephrotic system. Although it is relatively straightforward to determine if a system is not obstructed on the basis of the renal transit time (RTT), no single parameter is adequate to fully characterize obstruction. By evaluating the changes in signal intensity in the renal parenchyma following contrast administration, the hydronephrotic systems are classified as compensated or decompensated. Delayed RTT and the presence of urine-contrast levels indicate stasis. Calyceal transit time and the difference between the volumetric and the Patlak differential renal function (vDRF-pDRF) are measures of the physiologic changes within the kidney. Additionally, MR urography provides prognostic information by assessing the quality of the renal parenchyma and identifying uropathy preoperatively. MR urography combines both anatomic and functional information in a single test and is capable of providing a comprehensive evaluation of obstructive uropathy that could ultimately help select those patients most likely to benefit from surgical intervention.

Evaluation of reflux nephropathy, pyelonephritis and renal dysplasia.

MR urography has the potential to significantly improve our understanding of the relationship between reflux nephropathy, pyelonephritis, vesicoureteric reflux and renal dysplasia. MR urography utilizes multiple parameters to assess both renal anatomy and function and provides a more complete characterization of acquired and congenital disease. Pyelonephritis and renal scarring can be distinguished by assessing the parenchymal contours and signal intensity. Characteristic imaging features of renal dysplasia include small size, subcortical cysts, disorganized architecture, decreased and patchy contrast enhancement as well as a dysmorphic pelvicalyceal system. Because of its ability to subdivide and categorize this heterogeneous group of disorders, it seems inevitable that MR urography will replace DMSA renal scintigraphy as the gold standard for assessment of pyelonephritis and renal scarring. MR urography will contribute to our understanding of renal dysplasia and its relationship to reflux nephropathy.

MR urography in children: how we do it.

In this article we introduce the topic of MR urography in children, focusing on the details required to obtain consistently high-quality scans. Much of the information presented is based on our experience during the last 7 years. We have performed almost 1,000 MR urograms in children, and the technique has evolved considerably during this time. We have learned through trial and error and have improved our protocols to the point that our approach is now standardized and reliably generates high-quality studies. From this standardized protocol, further refinements in technique can be readily implemented. It is important to remember that this clinical application is in its infancy and will improve significantly with further technical development. This paper provides an overview of the practical issues associated with obtaining high-quality scans as well as an introduction into the interpretation of MR urograms.

MRU post-processing.

Dynamic magnetic resonance urography (MRU) scans acquired in conjunction with an injection of a contrast agent can be used to estimate a number of parameters that reflect renal function. This article discusses the methodologies and assumptions used in the estimation of these parameters, with special attention to the problem of deriving the concentration of the contrast agent from the change in the MR signal. The estimates of split renal function derived from MRU are in good agreement with those obtained using nuclear medicine studies. The time-intensity curves show subtle differences from those measured using nuclear medicine but still allow the transit of the contrast agent through the kidney to be assessed. Quantitative estimates of renal function (GFR) can be derived from MRU but have yet to be validated in a pediatric population.