Review Dynamic Contrast Enhanced-MR CEST Urography: An Emerging Tool in the Diagnosis and Management of Upper Urinary Tract Obstruction Shaowei Bo 1, Farzad Sedaghat 1, KowsalyaDevi Pavuluri 1 , Steven P. Rowe 1,2, Andrew Cohen 2 , Max Kates 2 and Michael T. McMahon 1,3,* 1 The Russell H. Morgan Department of Radiology and Radiological Science, School of Medicine, The Johns Hopkins University, Baltimore, MD 21205, USA; [email protected] (S.B.); [email protected] (F.S.); [email protected] (K.P.); [email protected] (S.P.R.) 2 The James Buchanan Brady Urological Institute, Department of Urology, School of Medicine, Johns Hopkins University, Baltimore, MD 21205, USA; [email protected] (A.C.); [email protected] (M.K.) 3 F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD 21205, USA * Correspondence: [email protected]; Tel.: +1-443-923-9356 Abstract: Upper urinary tract obstructions (UTOs) are blockages that inhibit the flow of urine through its normal course, leading to impaired kidney function. Imaging plays a significant role in the initial diagnosis of UTO, with anatomic imaging (primarily ultrasound (US) and non-contrast computed tomography (CT)) serving as screening tools for the detection of the dilation of the urinary collecting systems (i.e., hydronephrosis). Whether hydronephrosis represents UTO or a non-obstructive process Citation: Bo, S.; Sedaghat, F.; is determined by functional imaging (typically nuclear medicine renal scintigraphy). If these exams Pavuluri, K.; Rowe, S.P.; Cohen, A.; reveal evidence of UTO but no discernable source, multiphase contrast enhanced CT urography Kates, M.; McMahon, M.T. Dynamic and/or dynamic contrast enhanced MR urography (DCE-MRU) may be performed to delineate Contrast Enhanced-MR CEST a cause. These are often performed in conjunction with direct ureteroscopic evaluation. While Urography: An Emerging Tool in the contrast-enhanced CT currently predominates, it can induce renal injury due to contrast induced Diagnosis and Management of Upper nephropathy (CIN), subject patients to ionizing radiation and is limited in quantifying renal function Urinary Tract Obstruction. (traditionally assessed by renal scintigraphy) and establishing the extent to which hydronephrosis is Tomography 2021, 7, 80–94. https:// due to functional obstruction. Traditional MRI is similarly limited in its ability to quantify function. doi.org/10.3390/tomography7010008 DCE-MRU presents concerns regarding nephrogenic systemic fibrosis (NSF), although decreased with newer gadolinium-based contrast agents, and regarding cumulative gadolinium deposition in Academic Editor: Brian D. Ross the basal ganglia. DCE-MR CEST urography is a promising alternative, employing new MRI contrast Received: 29 January 2021 agents and imaging schemes and allowing for concurrent assessment of renal anatomy and functional Accepted: 16 February 2021 parameters. In this review we highlight clinical challenges in the diagnosis and management of UTO, Published: 2 March 2021 identify key advances in imaging agents and techniques for DCE-MR CEST urography and provide perspective on how this technique may evolve in clinical importance. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: urinary tract obstructions; ultrasound; CT; contrast enhanced MRI; CEST published maps and institutional affil- iations. 1. Introduction Upper urinary tract obstructions (UTOs) are blockages in the flow of urine that Copyright: © 2021 by the authors. result in dilation of the renal collecting system (hydronephrosis), (Figure1A,B), impaired Licensee MDPI, Basel, Switzerland. glomerular/tubular function, renal injury, and, ultimately, irreversible loss of renal function. This article is an open access article The most common cause of acute UTO in adults is nephrolithiasis, with patients presenting distributed under the terms and with unilateral flank pain secondary to an obstructing stone. Additional causes of upper conditions of the Creative Commons UTOs vary according to age and sex, urothelial malignancy, benign strictures and congenital Attribution (CC BY) license (https:// ureteropelvic junction (UPJ) obstruction. While an elevation in creatine and decrease in creativecommons.org/licenses/by/ eGFR may be observed, fulminant renal failure is rare in the setting of unilateral UTO and 4.0/). Tomography 2021, 7, 80–94. https://doi.org/10.3390/tomography7010008 https://www.mdpi.com/journal/tomography Tomography 2021, 7, FOR PEER REVIEW 2 Tomography 2021, 7 81 congenital ureteropelvic junction (UPJ) obstruction. While an elevation in creatine and decrease in eGFR may be observed, fulminant renal failure is rare in the setting of unilat- eralthe absenceUTO and of the pre-existing absence of chronic pre-existing kidney ch diseaseronic kidney (CKD). disease These entities(CKD). oftenThese haveentities an ofteninsidious have presentation an insidious and presentation may go undetected, and may leadinggo undetected, to irreversible leading kidney to irreversible damage. Thus, kid- neythe earlydamage. identification Thus, the of early functional identification renal impairment of functional is vital renal in promptingimpairment intervention is vital in promptingand the preservation intervention of renaland the function. preservation of renal function. Figure 1. Coronal contrastcontrast enhancedenhanced CTCT ( A(A)) and and retrograde retrograde pyelogram pyelogram (B ()B depicting) depicting a hydronephrotica hydronephrotic right right kidney. kidney. Coronal Cor- onalCT after CT after nephroureteral nephroureteral stent stent (C) placement (C) placement demonstrating demonstrating resolution resolution of hydronephrosis. of hydronephrosis. Note Note the presencethe presence of an of IVC an IVC filter (arrow). filter (arrow). The treatment of upper UTOs depends on th thee cause of the obstruction and degree of renalrenal impairment.impairment. ForFor example,example, in in the the setting setting of nephrolithiasis,of nephrolithiasis, conservative conservative management manage- mentis often is theoften choice the forchoice small for stones, small asstones, these spontaneouslyas these spontaneously pass and rarelypass and cause rarely permanent cause permanentfunctional impairment.functional impairment. Conversely, Conversely, lithotripsy and lithotripsy stenting and are thestenting standard are treatmentthe standard for treatmentlarger obstructing for larger calculi. obstructing In other calculi. settings In other where settings hydronephrosis where hydronephrosis is present without is present signif- withouticant functional significant impairment, functional treatment impairment, centers treatment around centers addressing around the underlyingaddressing etiologythe un- derlyingfor UTO. etiology UTO related for UTO. to malignancy UTO related is typically to malignancy managed is typically surgically, managed irrespective surgically, of the irrespectivedegree of functional of the degree impairment. of functional However, impa inirment. some cases, However, the quantification in some cases, may the still quanti- play ficationa role, particularly may still play in determininga role, particularly the utility in determining of intervening the to utility preserve of intervening a hydronephrotic to pre- servekidney. a hydronephrotic When functional kidney. impairment/acute When functional kidney impairment/acute injury (AKI) is kidney secondary injury to (AKI) partial is secondaryor complete to ureteralpartial or obstruction, complete ureteral percutaneous obstruction, nephrostomy percutaneous (Figure nephrostomy1C) and retrograde (Figure 1C)ureteric and stentingretrograde are well-establishedureteric stenting choices are we forll-established minimizing choices renal parenchymal for minimizing injury renal and parenchymalpreserving renal injury function. and preserving While these renal treatments function. are While generally these safe treatments and well-tolerated, are generally they safeare not and without well-tolerated, risk and therethey are is inherent not without morbidity risk and related there to is the inherent presence morbidity of urinary related stents, towhich the presence can become of urinary a nidus stents, of infection. which Furthermore,can become a thenidus presence of infection. of hydronephrosis, Furthermore, andthe presenceeven the documentationof hydronephrosis, of a functionaland even the UTO, do doescumentation not suggest of a whetherfunctional damage UTO, fromdoes thatnot suggestUTO is reversible.whether damage Interventions from that are, UTO therefore, is reve undertakenrsible. Interventions to alleviate are, obstructions therefore, without under- takenknowing to alleviate if the intervention obstructions will without actually knowing improve if the kidney intervention function. will Finally, actually monitoring improve kidneyUTO after function. surgical Finally, intervention monitoring is a diagnostic UTO after conundrum. surgical intervention Interventions, is a such diagnostic as ureteric co- surgeries or even ureteric stents, are often associated with persistent hydronephrosis. In the case of stents, such persistent hydronephrosis may or may not suggest incomplete treatment and persistent obstruction. For UPJ obstruction, evidence of obstruction often Tomography 2021, 7, FOR PEER REVIEW 3 nundrum. Interventions, such as ureteric surgeries or even ureteric stents, are often asso- ciated with persistent hydronephrosis. In the case of stents, such persistent