Modeling the detectability of vesicoureteral reflux using microwave radiometry

Abstract

We present the modeling efforts on antenna design, frequency selection and receiver sensitivity estimation to detect vesicoureteral reflux (VUR) using microwave (MW) radiometry as the warm urine from the bladder maintained at fever range temperature using a MW hyperthermia device reflows into the kidneys. Radiometer center frequency (f c ), frequency band (Δf), and aperture radius (r a ) of the physical antenna for kidney temperature monitoring are determined using a simplified universal antenna model with circular aperture. Anatomical information extracted from computed tomography (CT) images of children age 4-6 years is used to construct a layered 3D tissue model. Radiometric antenna efficiency is evaluated in terms of the ratio between the power collected from the target at depth and the total power received by the antenna (η). Power ratio of the theoretical antenna is used to design a microstrip log spiral antenna with directional radiation pattern over f c ± Δf/2. Power received by the log spiral from the deep target is enhanced using a thin low-loss dielectric matching layer. A cylindrical metal cup is proposed to shield the antenna from electromagnetic interference (EMI). Transient thermal simulations are carried out to determine the minimum detectable change in antenna brightness temperature (δT B ) for 15-25 mL urine refluxes at 40-42°C located 35 mm from the skin surface. Theoretical antenna simulations indicate maximum η over 1.1-1.6 GHz for r a = 30-40 mm. Simulations of the 35 mm radius tapered log spiral yielded higher power ratio over f c ± Δf/2 for the 35-40 mm deep targets in the presence of an optimal matching layer. Radiometric temperature calculations indicate δT B ≥ 0.1 K for the 15 mL urine at 40°C and 35 mm depth. Higher η and δT B were observed for the antenna and matching layer inside the metal cup. Reflection measurements of the log spiral in saline phantom are in agreement with the simulation data. Numerical study suggests a radiometer with f c =1.35 GHz, Δf = 500 MHz and detector sensitivity better than 0.1 K would be the appropriate tool to noninvasively detect VUR using the log spiral antenna.

Utility of microwave radiometry for diagnostic and therapeutic applications of non-invasive temperature monitoring

“…Optimization parameters included the number and winding of spiral turns, and thickness and dielectric properties of the substrate and matching layer. A 7cm tapered log spiral was determined most appropriate for recording temperature rise in kidney located 3-5cm below the skin [13,14], and a 2.6cm diameter tapered log spiral was found to maximize coupling through the human skull into brain. [15] Fig .…”

“…[10,11,13] This application requires radiometric monitoring of temperature rise in kidney while controlling MW heating of bladder urine with radiometry. Fig.…”

Current management of vesicoureteral reflux in pediatric patients: a review

“…9 Noninvasive, nonionizing imaging facilitated by transabdominal manipulation of bladder temperature is still in the early stages of development, but holds hope for future clinical application in the coming years. 62 We can also look forward to the much-anticipated findings from the Randomized Intervention for Children with Vesicoureteral Reflux study, a multicenter, double-blind, randomized, placebo-controlled trial with over 15 participating institutions and more than 600 enrolled pediatric patients. The study is designed to assess UTI recurrence, renal scarring, antimicrobial resistance, patient quality of life, medication compliance, resource utilization, and VUR status in children randomized to antibiotic prophylaxis versus placebo.…”

Radiometric temperature reading of a hot ellipsoidal object inside the oral cavity by a shielded microwave antenna put flush to the cheek

“…In the proposed VUR detection system described by Snow and Taylor (2010), the urine temperature inside the bladder is raised to [40][41][42][43][44][45] o C applying an external 915 MHz microwave hyperthermia device (Stauffer et al 2010). When heating the bladder to such high temperatures, the skin temperature has to be controlled with the use of a water circulated bolus to prevent blisters from the high-energetic nearfields.…”

“…The method has also been evaluated through numerical calculations (Arunachalam et al 2010) and phantom experiments (Arunachalam et al 2011). Common to all this studies is the use of a well established heating device, the so called dual concentric conductor (DCC) antenna, originally developed to treat large area chest wall disease by superficial hyperthermia (Stauffer et al 2010). The novel VUR detecting principle, based on radiometry and applying temperature as a contrast agent, comprises a two steps sequential procedure: i) Heating of the bladder using microwaves where the heating process is quality assured with microwave radiometry, ii) detection of heated urine reflux from the bladder back into the kidney by radiometric temperature probing of the kidney.…”