Use of MR Urography in Pediatric Patients
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Purpose of Review
In this article, we describe the basics of how magnetic resonance urography (MRU) is performed in the pediatric population as well as the common indications and relative performance compared to standard imaging modalities.
Although MRU is still largely performed in major academic or specialty imaging centers, more and more applications in the pediatric setting have been described in the literature.
MRU is a comprehensive imaging modality for evaluating multiple pediatric urologic conditions combining excellent anatomic detail with functional information previously only available via renal scintigraphy. While generally still reserved for problem solving, MRU should be considered for some conditions as an early imaging technique.
KeywordsImaging Children Kidneys Urinary tract Hydronephrosis Renal transplant
There are many clinical indications to image the urinary tract in the pediatric population. Urinary tract dilatation (UTD), detected pre- or post-natally, is one of the most common reasons to image the urinary tract. Magnetic resonance urography (MRU) is increasingly being used for comprehensive anatomic and functional evaluation of the urinary tract in children. MRU has been in clinical development in children since the early 2000s and has been subsequently refined and improved over time. It is now routinely used in clinical care in many institutions over the last 5 to 10 years [1, 2]. The information that can be provided with MRU is similar to that acquired with a combination of ultrasound, computed tomography (CT), excreted urography, and renal scintigraphy and it does so with no exposure to ionizing radiation.
Common Imaging Modalities for Pediatric Urologic Conditions
Ultrasonography (US) is the most commonly employed imaging modality to evaluate the kidneys and bladder pre- and post-natally. US has the advantages of being performed without sedation or ionizing radiation and is non-invasive. US generally provides sufficient anatomical detail of renal anatomy and any parenchymal changes (diffuse thinning, altered echogenicity, cysts, etc.) and is the primary imaging modality used to identify and grade hydronephrosis. However, ultrasound is limited for visualization of the ureters, especially when non-dilated, and is particularly limited at the levels of the mid-ureter and ureterovesical junction. On the other hand, when there is marked ureterectasis, it also can be difficult to fully characterize urinary tract anatomy by US due to anatomic distortion and the relatively limited field of view. Furthermore, US provides no information about renal function; although, speculatively US performed with an intravascular contrast material (i.e., microbubble contrast) may provide some information regarding differential perfusion in the future avoiding both nuclear medicine and MRI-based contrast agents. US technique can be affected by many patient-specific parameters such as bowel gas, body habitus (e.g., scoliosis and obesity), and patient cooperation.
Voiding cystourethrography (VCUG) is another commonly employed imaging modality for urologic conditions in the pediatric population and is most often used for diagnosing vesicoureteral reflux and assessing the morphology of the bladder and urethra. VCUG requires placement of a urethral catheter and uses intermittent, low-dose fluoroscopy to image contrast material instilled into the urinary tract. In the absence of vesicoureteral reflux, no information is gained regarding the upper tract collecting system, and VCUG does not provide information about the renal parenchyma.
Scintigraphic studies can provide a range of information about the urinary tract depending on the radiopharmaceutical employed. Diuretic renal scintigraphy using mercaptoacetyltriglycine (MAG3) provides functional (i.e., differential renal function based on plasma flow) and drainage information. Renal cortical scintigraphy using dimercaptosuccinic acid (DMSA) provides information about the renal parenchyma (i.e., differential renal function based on cortical binding and detection of focal scarring), while diethylenetriaminepentaacetic acid (DTPA) provides information about renal functional (i.e., differential renal function based of glomerular filtration) and drainage. The anatomic detail provided by scintigraphy is inherently limited, but the functional information provided remains the imaging reference standard. Scintigraphic studies necessarily expose patients to ionizing radiation but only rarely require sedation.
CT can be useful for some pediatric urologic conditions but is typically only used as a first-line imaging modality for renal masses and urinary tract calculi in the pediatric population. In part, this is due to the fact that CT necessitates exposure to ionizing radiation. CT urography (CTU) is relatively commonly employed in the adult population but is infrequently used in pediatrics because it generally requires multiple image acquisitions (non-contrast, parenchymal or nephrographic phase, ureteral or excretory phase). The number of image acquisitions can be decreased using dual-energy CT which provides a virtual non-contrast imaging series or by performing “split-bolus” CTU, where two separate administrations of intravenous contrast material allow nephrographic and excretory phase information to be obtained from the same image acquisition . CT can provide a qualitative assessment of renal function if multiple phases are acquired, but this is typically impractical and comes at a cost of radiation dose.
Basics of MRU Technique
Pediatric MRU can be performed at 1.5 or 3 Tesla (T) in children of any age. 3 T magnets generally offer superior spatial resolution, which is helpful particularly in younger children, with improved visualization of small urinary tract structures. However, 1.5 T magnets generally allow for more homogeneous fat saturation and are less susceptible to artifacts, such as dielectric effect, T2* effects of excreted gadolinium, and any artifacts from surgical material. Imaging is performed with multi-element phased-array surface coils.
MR urography can refer to anatomic imaging of the kidneys and collecting system but more commonly refers to anatomic imaging in combination with functional imaging, the latter of which requires administration of intravenous gadolinium-based contrast material. Anatomic imaging of the abdomen and pelvis is performed, including sequences that focus on the renal parenchyma (T1- and T2-weighted sequences) and sequences focused on the urinary tracts. Sequences targeted at the urinary tract include high-resolution 2D and 3D T2-weighted images, which when obtained in a 3D fashion allow multiplanar reformatting and can be used to make a variety of reconstructions (e.g., volume-rendered and maximum intensity projection images).
Functional MR urography allows the determination of differential renal function and allows assessment of renal excretion into the collecting systems. Functional imaging is obtained dynamically over a 10- to 15-min period of time following administration of intravenous gadolinium-based contrast material. By imaging multiple times over 15 min, renal parenchymal contrast uptake and excretion are visualized and later quantified with post-processing techniques. This allows the measurement of differential renal function (based on renal volumes or glomerular filtration) and time vs. signal intensity washout/excretion curves. Detailed reviews of these calculations have been described [4•]. The provided data is comparable to that obtained by scintigraphic studies; however, scintigraphy remains an accurate and reliable modality for cases that do not require the additional anatomic information provided by MRU. Newer MRI techniques likely will be forthcoming to more specifically non-invasively evaluate the renal parenchyma for findings of inflammation and fibrosis [5, 6, 7, 8].
While MRU has the advantages of being a radiation-free imaging modality and providing the greatest anatomic detail of any modality for imaging the urinary tract, it does have some limitations that need to be considered. First, MRU exams require the patient to lie still in the bore of the magnet for up to 60 to 90 min. Some children can achieve this without difficulty, particularly if distraction techniques (video goggles, etc.) are employed, but others will require sedation/anesthesia or anxiolysis to complete their exam. Second, administration of intravenous gadolinium-based contrast material used to be considered entirely benign but is being increasingly scrutinized due to evidence of retention of gadolinium in the body .
How the MRU Works in Practice
Generally children younger than 8–10 years of age or those with developmental delay will require some form of anesthesia or sedation to prevent motion artifacts, which is a relative disadvantage compared to other imaging techniques. The age at which children require sedation also will depend on prior experience and tolerance of bladder catheterization, which is a necessary part of the exam.
Our pediatric MRU protocol
Patient arrives 60–90 prior to exam time
NPO 4 h if non-sedate (otherwise set by anesthesia)
Place bladder catheter and clamp
Place IV catheter (two IV catheters if sedation is needed)
10 mL/kg IV saline over 15 min for sedated patients or 30 min for non-sedated
T2-weighted single-shot fast spin-echo without/with fat suppression
Unclamp bladder catheter
0.5 to 1 mg/kg (max dose = 40 mg)
T2-weighted fast spin-echo with fat suppression
High spatial resolution 3D T2-weighted fast spin-echo without and with fat suppression
3D T1-weighted gradient recalled echo with fat suppression
0.2 mL/kg at 0.2 mL/s
3D T1-weighted gradient recalled echo with fat suppression
Coronal, 15 min dynamic post-contrast
3D T1-weighted gradient recalled echo with fat suppression
Sagittal, coronal, axial
Remove IV and bladder catheter
Common Indications for MRU
Common indications for pediatric MRU include evaluation of complex renal and upper urinary tract anatomy, suspected urinary tract obstruction, operative planning, post-operative complications, and functional assessment. Generally, patients have already undergone conventional imaging tests such as US, VCUG, and/or renal scintigraphy and yet the clinician still needs additional information for management. As such, MRU is typically employed as a problem solving or surgical planning modality. MRU can delineate anatomy in the presence or absence of collecting system dilation, which can be a limiting factor in US evaluation. Additionally, MRU can visualize the entire course of the ureter and identify ectopic insertions as well as sites and potential causes of narrowing or obstruction, including identification of crossing vessels as a cause of ureteropelvic junction obstruction. In planning for surgery or evaluating post-surgical changes, MRU can provide detailed anatomic assessment for the surgeon with the ability to make 3D reconstructions of the entire renal and upper urinary tract. MRU functional assessment can provide quantitative data of differential renal function.
Common Clinical Applications
Collecting System Abnormalities
One of the most common abnormalities detected by prenatal ultrasound is UTD, occurring in 1–2% of all pregnancies [10, 11, 12]. Most often, this finding is transient and resolves early in life. However, multiple clinically relevant abnormalities initially present in this fashion and require further evaluation or intervention to prevent complications such as urinary tract infection (UTI), urinary stone formation, and renal dysfunction/injury [10, 11, 13, 14, 15]. Of note, it has recently been recognized that children with congenital obstructive nephropathy will go on to develop end-stage renal disease in adulthood at a higher rate than previously expected, with need for renal replacement therapy not manifesting until the fourth decade of life [13, 15]. Following transient/physiologic UTD, the most common etiologies of UTD include ureteropelvic junction (UPJ) obstruction, vesicoureteral reflux, ureterovesical junction obstruction (megaureter), multicystic dysplastic kidney disease (MCDK), and posterior urethral valves [10, 14].
MRU can be used in the assessment and characterization of the majority of causes of UTD and obstruction with the exception of vesicoureteral reflux and probably posterior urethral valves, which are better assessed with VCUG as described above. The added value of MRU to the traditional imaging modalities of US, VCUG, and renal scintigraphy in the setting of dilated collecting systems largely relates to accurate anatomic descriptions, parenchymal evaluation, and functional assessment. Many of the common causes of collecting system dilation have one or more congenital anatomical abnormalities that can go undetected on traditional imaging modalities. Additionally, MRI is superior to US and renal scintigraphy for evaluating the renal parenchyma for inflammation, scarring, and cortical thinning, which are frequently associated with the various causes of urinary tract dilation. Further, the functional information provided by MRU can be helpful for surgical planning (e.g., guiding the decision for heminephrectomy vs. ureteral reimplantation) and prognosticating.
Ureteropelvic Junction Obstruction
Ureterovesical Junction Obstruction and Congenital Primary Megaureter
Renal Ectopia/Fusion Anomalies
MRU provides probably the most complete assessment of the urinary tract in children, allowing detailed evaluation of the renal parenchyma, collecting systems and ureters, and the bladder and providing both static and dynamic functional information. As such, MRU has the potential to be contributory to the evaluation of a wide variety of pediatric urologic abnormalities. Currently, MRU is typically reserved for problem solving after traditional imaging modalities of US, VCUG, and/or renal scintigraphy have not provided all the necessary information for clinical decision making. In the future, the use of MRU may increase in the evaluation of children with genitourinary anomalies, particularly in complex patients in which urologists have specific questions to be answered.
Compliance with Ethical Standards
Conflict of Interest
Cara E. Morin, Morgan P. McBee, Andrew T. Trout, Pramod P. Reddy, and Jonathan R. Dillman each declare no potential conflicts of interest.
Human and Animal Rights and Informed Consent
This article does not contain any studies with human or animal subjects performed by any of the authors.
Papers of particular interest, published recently, have been highlighted as: • Of importance •• Of major importance
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