Fetal Magnetic Resonance Imaging of Congenital Renal Anomalies: A Pictorial Review

Congenital renal anomalies comprise a vast spectrum of pathologies and are often associated with oligohydramnios, which can make prenatal ultrasonography (US) evaluation difficult. However, fetal magnetic resonance imaging (MRI) is less affected by the diminished amniotic fluid and can provide superior image quality. Understanding of fetal renal anomalies and associated malformations is essential for patient counseling and appropriate pregnancy, delivery, and neonatal management. MRI is a well-established and effective diagnostic tool complementing US due to its superior tissue contrast, large field of view, enhanced anatomic evaluation, and more detailed rendering of renal malformations when US findings are equivocal or inconclusive, especially when oligohydramnios is present. Correspondence Author to: Su-Chiu Chen Department of Radiology, Mackay Memorial Hospital, Taipei, Taiwan No. 92, Sec. 2, Chung-Shan N. Road, Taipei 104, Taiwan J Radiol sci 2014; 39: 77-84 fetal MRI of congenital renal anomalies 78 J Radiol Sci September 2014 Vol.39 No.3 bright but is isointense to maternal fat, and represents fetal urine. Occasionally, a long-T2 rim surrounds the kidney, suggesting perirenal fat (Fig. 1). On thick-slab T2-weighted images, a topographic view of the normal bulk fluids within the spinal canal, lungs, stomach, intestines, and urinary tract, including bilateral renal pelves and urinary bladder, is visible (Fig. 2). Types of feTal Renal anoMalIes Renal agenesis Renal agenesis, congenital absence of one or both kidneys, possibly results from a failure of the ureteral bud to induce development of the metanephrogenic blastema. Bilateral renal agenesis is a rare and devastating anomaly, occurring in only one or two per 10,000 births; the condition is usually associated with severe oligohydramnios and is one causative agent of Potter sequence [8]. Unilateral renal agenesis is much more common and usually found incidentally along with compensatory hypertrophy of the contralateral kidney, or may be associated with ipsilateral urogenital anomalies or adrenal agenesis [8, 9]. On MRI, anhydramnios or oligohydramnios, bowel loops in place of normal kidney in the expected renal fossa, and a small or signal void of the urinary bladder [5] suggest bilateral renal agenesis (Fig. 3). In a fetus with unilateral agenesis, absence of the bright urine signal from the involved renal pelvis, contralateral renal hypertrophy, normal visualization of the urinary bladder and a normal amniotic fluid volume are observed (Fig. 4) [3, 5, 10, 11]. Renal hypoplasia Renal hypoplasia refers to a small kidney containing essentially normal residual parenchyma with intact, but fewer nephrons, whereas a dysplastic kidney contains disorganized and maldifferentiated tissue [12]. In practice, the diagnosis of congenital renal hypoplasia is favored under the following conditions: the renal size is decreased by two standard deviations from the mean size according to age and the contralateral kidney shows compensatory hypertrophy; renal scarring is excluded using postnatal Doppler US as an initial renal screen or 99mTc–dimercaptosuccinic acid radionuclide scan [12]. Renal hypoplasia is not generally associated with urinary tract malformations but may be a component of a larger genetic syndrome such as renal coloboma and branchio-oto-renal syndromes, though it may be an isolated finding [13]. On MRI, the abnormal small kidney maintains a normal renal pelvis (Fig. 5). congenital cystic renal diseases Congenital cystic renal disease can develop in the fetus but more typically develops after birth. The condition has two MRI patterns, frank cysts and long-T2 renal parenchyma, which can be associated with each other. The cysts can be miniscule or large, and can develop anywhere in one or both kidneys. Long T2 of the renal parenchyma suggests microscopic cysts and/or dilated tubules. The Potter