Patient dose reduction during voiding cystourethrography

Prior to beginning VCUG, the radiologist should obtain a thorough clinical history and confirm that the referring physician’s reason for performing the examination is a correct clinical indication such as a febrile urinary tract infection or prenatal hydronephrosis [1]. Of note, there are instances when the diagnosis of vesicoureteral reflux is best made by radionuclide cystography (RNC) rather than VCUG [1]. The clinical indications for RNC include family screening, follow-up of known vesicoureteral reflux, or follow-up after surgery (for example ureteral reimplantation surgery) [1]. VCUG delivers a much higher effective radiation dose to a child than RNC (unpublished data; [7‐9]). RNC provides continuous monitoring of bladder filling and voiding [8]. However, RNC provides less anatomic conspicuity than VCUG, such that RNC precludes the ability to evaluate the urethra and bladder (for example RNC does not allow the determination of the presence of a para-ureteral diverticulum, which could change patient management) [1, 8, 10]. Therefore, it is important to make certain that the child actually needs an imaging examination based on the clinical history, and if so, then to choose between VCUG and RNC based on the appropriate clinical indication.

Methods to reduce radiation exposure while performing VCUG in a child include important technique choices by the radiologist as he/she operates the fluoroscope. For example, a radiologist might choose to perform multiple cycles of bladder filling and emptying (i.e., cyclic VCUG) to increase the diagnostic yield of detecting vesicoureteral reflux [11, 12]. However, the radiologist must remain aware that each cycle incrementally increases the radiation exposure of a child, so cyclic VCUG should only be performed after the pretest probability of vesicoureteral reflux has been considered and in selected patient populations [11, 12].

Similarly, the radiologist can control the overall amount of real-time fluoroscopy and number of actual recorded images that comprise the examination and hence the resultant total radiation exposure [10, 13, 14]. Investigators have tailored VCUG protocols to exclude preliminary images, utilize the fluoroscopy capture mode and optimize digital fluoroscopy video techniques [10, 13‐17] all in an attempt to reduce radiation exposures.

The radiologist can also reduce radiation exposure by choosing to use pulsed fluoroscopy rather than the conventional, continuous fluoroscopy [18]. Unlike continuous fluoroscopy, pulsed fluoroscopy has multiple available pulse rates (number of radiation beam pulses per second) and pulse widths (duration of each pulse) [19, 20]. If the radiologist performing VCUG selects a lower pulse rate, then the radiation exposure to the patient is reduced. If a narrower pulse width is used for small children who sometimes move during imaging, then image quality is improved. Pulsed fluoroscopy has recently been shown to decrease radiation exposure in an adult study and in a pediatric phantom study [18, 19, 21, 22].

Last, other VCUG radiation exposure reduction techniques include increasing the source-to-skin distance (i.e., maximize the distance between the child and the X-ray source), and removing the antiscatter grid between the patient (source of scattered radiation) and the detector [23].

The manufacturer and medical physicist can work together to optimize a fluoroscope’s design to achieve ALARA levels [24, 25]. A survey of pediatric hospitals in 2000 showed that those with a pulsed fluoroscopy unit had the lowest radiation exposures, especially those fluoroscopes that were optimized for pediatric imaging [24]. Our group recently compared radiation exposures between pulsed and continuous fluoroscopy in a pediatric-size porcine model of vesicoureteral reflux during the performance of VCUG with a pulsed fluoroscope that was modified by the manufacturer for children [25]. These design features included spectral beam filtration (adding filters in the path of the X-ray beam), proper selection of the tube current and high voltage by the automatic brightness control system as a function of the child’s size, reductions in the selectable pulse rates to 7.5, 3.75 and 1.88 pulses/s, and reductions in the available selectable pulse widths to 5 ms and 10 ms [25]. These design modifications enhanced our ability to achieve statistically significant reduction in total radiation exposures while performing VCUG and maintaining diagnostic image quality.

The animal model investigation of VCUG with pulsed fluoroscopy that was performed by our group was as follows. We compared an optimized, grid-controlled variable-rate pulsed fluoroscopy (GCPFL) unit to a continuous fluoroscopy unit in a pediatric porcine model of vesicoureteral reflux [25]. The pigs underwent VCUG at three different abdominal girth ranges that corresponded to the girth of a newborn to 6-month-old human infant (group 1), a young child approximately 2 to 3 years of age (group 2), and an older child approximately 10 years of age (group 3). An electronic online patient exposure monitoring device was used to accurately calculate the exposure at the entrance skin plane of the animal [26, 27]. Table 1 and Fig. 1 summarize our results for total radiation exposure, fluoroscopic radiation exposure, recorded VCUG image exposure and fluoroscopy time for all three groups [25]. The optimized GCPFL unit delivered radiation exposures to a pig that ranged from 4.6 to 7.5 times lower than with a continuous fluoroscopy unit. The fluoroscopy times for VCUG between the fluoroscopy units were not significantly different. Figures 2 and 3 are examples of actual recorded VCUG images demonstrating that anatomic conspicuity and diagnostic image quality were maintained. Tables 2 and 3 underscore the anatomic conspicuity and diagnostic image quality scores for the 48 VCUGs assigned by two experienced pediatric radiologists [25].

Subsequently, our group translated these findings to our routine clinical pediatric uroradiology practice. The optimized GCPFL unit used in the animal model investigation was compared with the same conventional, continuous fluoroscopy unit in a human study. We found that the GCPFL unit could significantly reduce the radiation exposures of children with common clinical indications for VCUG. The statistically significant radiation exposure reductions with the GCPFL unit were achieved during both fluoroscopy and the actual recording of VCUG images (unpublished data). Moreover, at these lower radiation exposures, the GCPFL unit still provided diagnostic quality images. Hence, an optimized GCPFL unit has the potential to provide an optimal balance between reducing radiation exposure of a child and maintaining diagnostic image quality (unpublished data).