Perinatal urology has changed significantly since the advances of prenatal ultrasound. Prenatal ultrasound is a noninvasive tool that can determine gestational age, fetal number, viability, placental location, and some fetal anomalies. Although prenatal ultrasound screening may detect congenital abnormalities, the benefit of this knowledge in measured postnatal outcomes is unproven. The a priori deduction that early diagnosis can prevent future complications is controversial, as in many cases, early diagnosis may lead to unnecessary interventions. Additionally, it can lead to significant parental anxiety.

Although the use of prenatal ultrasound in diagnosing genitourinary abnormalities has not been proven to change postnatal outcome relative to renal function, more women are undergoing routine prenatal counseling and screening ultrasound [1].

In addition to anatomic detail, it is important to determine the degree of obstruction and the relative function of the kidneys. The purpose of a diuretic renogram is to distinguish between nonobstructive hydronephrosis and mechanically obstructed hydronephrosis (Figs. 10.2 and 10.3). Historically, the intravenous urogram was the study of choice, often requiring delayed images with dilute contrast in an attempt to identify the level of abnormality. The IVU has been replaced primarily by nuclear medicine as the functional modality of choice. Technetium-99m is the ideal agent because of availability, short half-life, low-energy photopeak (140 keV), and ability to bind to agents such as MAG3 (mertiatide). Appropriate gamma camera and collimator are necessary. Because radioactive isotope is excreted into the bladder, care should be taken that the bladder is emptied by continuous bladder drainage or as soon as possible after the exam, especially in females where the ovaries are adjacent. This is aided by appropriate hydration. Because time is required to acquire images, swaddling of infants or sedation if the child cannot cooperate may be necessary.

Magnetic resonance urography (MRU) is a more recently developed modality for evaluation of the genitourinary tract, particularly with obstruction. Magnetic resonance imaging was first introduced in 1986 [2]. MRU later came along and has been used to not only assess differential function and drainage as with a diuretic renogram but also to provide anatomic detail of the kidneys and collecting system. MRU also avoids exposure to radiation and iodine contrast. Volumetric differential renal function (DRF) is calculated by comparing the volume of enhancing renal parenchyma of each kidney prior to contrast excretion in the kidneys. Gadolinium-diethylenetriaminepentaacetate dimeglumine (Gd-DTPA) is the contrast used. Sequential imaging allows for evaluation of contrast parenchymal concentration, excretion, and drainage [3]. The anatomic detail of MRU is considered its main advantage. With a variety of technical sequences, exquisite anatomic detail is possible. With administration of MR contrast agents, such as gadolinium, functional information is possible as well. With the addition of diuretics, dynamic information is obtained as with renal scintigraphy but with the additional benefit of superior contrast, spatial, and temporal resolution. The major disadvantages are cost, access, and time required for imaging acquisition, necessitating sedation in younger or certain disabled patients.

Computed tomography is a modality that provides excellent anatomic detail. With intravenous contrast, it also performs the additional role of a functional modality. Non-contrast CT has become the modality of choice for the detection of calculi in adults. Contrast CT can demonstrate a delayed nephrogram, as was seen with intravenous urography, or, with delayed images, can illustrate the detail of the etiology of the obstruction such as mass, stricture, or congenital abnormality. Radiation exposure to children from CT scans can be quite high, and evaluation by alternative imaging procedures when possible is the recommended approach to evaluation.

The voiding cystourethrogram remains the best imaging modality to diagnose bladder outlet obstruction because it defines the anatomy of the bladder, bladder neck, and urethra. Imaging of the urethra is best obtained during the voiding phase of the VCUG.

It can, however, result in significant radiation exposure. Advances in equipment development now allow varying degrees of pulsed fluoroscopy in which the X-ray beam is functioning only intermittently rather than continuously. In addition, “fluoro store” is a function where the image on the monitor screen can be stored without requiring additional spot film exposures.