EDUCATIONAL REVIEW The old becomes new: advances in imaging techniques to assess nephron mass in children Marissa J. DeFreitas 1 & Chryso P. Katsoufis 1 & Juan C. Infante 2 & Michael L. Granda 3 & Carolyn L. Abitbol 1 & Alessia Fornoni 4 Received: 21 October 2019 /Revised: 9 December 2019 /Accepted: 8 January 2020 # IPNA 2020 Abstract Renal imaging is widely used in the assessment of surrogate markers of nephron mass correlated to renal function. Autopsy studies have tested the validity of various imaging modalities in accurately estimating “true” nephron mass. However, in vivo assessment of nephron mass has been largely limited to kidney volume determination by ultrasonography (US) in pediatric populations. Practical limitations and risks create challenges in incorporating more precise 3D volumetric imaging, like magnetic resonance imaging (MRI), and computed tomography (CT) technologies, compared to US for routine kidney volume assessment in children. Additionally, accounting for structural anomalies such as hydronephrosis when estimating renal parenchymal area in congenital anomalies of the kidney and urinary tract (CAKUT) is important, as it correlates with chronic kidney disease (CKD) progression. 3D imaging using CT and MRI has been shown to be superior to US, which has traditionally relied on 2D measurements to estimate kidney volume using the ellipsoid calculation. Recent innovations using 3D and contrast-enhanced US (CEUS) provide improved accuracy with low risk. Indexing kidney volume to body surface area in children is an important standard that may allow early detection of CKD progression in high-risk populations. This review highlights current understand- ing of various imaging modalities in assessing nephron mass, discusses applications and limitations, and describes recent advances in the field of imaging and kidney disease. Although renal imaging has been a long-standing, essential tool in assessing kidney disease, innovation and new applications of established technologies provide important tools in the study and manage- ment of kidney disease in children. Keywords Nephron mass . Kidney volume . Renal imaging . Children . Chronic kidney disease . Ultrasound Introduction The importance of non-invasive and low radiation imaging in diagnosing and predicting the prognosis of kidney disease in adults and children has long been recognized, with efforts to establish normal references from the early 1960s [1–4]. It was not until the mid-1980s that renal ultrasonography offered a non-invasive assessment of renal size with standardization in children [5–8]. This expanded to the routine diagnosis of con- genital anomalies of the kidneys and urinary tract (CAKUT) by fetal ultrasound in the 1990s [9–11]. Methodologies have included 2-dimensional (2D) and expanded to 3-dimensional (3D) imaging to more precisely define the potentially func- tional renal parenchyma [12, 13]. The natural extension was to try to use non-invasive imaging to estimate functioning neph- ron mass and, by inference, the prognosis of renal disease, whether congenital or acquired [14–16]. Obviously, function- al studies had to coincide with the estimate of nephron mass, posing tactical challenges related to concurrent renal function measurements, which are difficult in children as well as adults. * Marissa J. DeFreitas mdefreitas@med.miami.edu 1 Division of Pediatric Nephrology, University of Miami Miller School of Medicine, P.O. Box 016960 (M714), Miami, FL 33130, USA 2 Section of Pediatric Radiology, Department of Diagnostic Radiology, University of Miami Miller School of Medicine, Miami, FL, USA 3 Division of General Internal Medicine, Department of Medicine, University of Miami Miller School of Medicine, Miami, FL, USA 4 Katz Family Division of Nephrology and Hypertension, Department of Medicine, University of Miami Miller School of Medicine, Miami, FL, USA Pediatric Nephrology https://doi.org/10.1007/s00467-020-04477-8