External bladder warming of urine to moderate hyperthermic temperatures (40–45°C) using MW hyperthermia and passive radiometric temperature monitoring of the kidneys to detect warm reflux was recently proposed for safe and noninvasive VUR detection [6
]. The proposed radiometry diagnostic system is noninvasive and uses non-ionizing microwave radiation for gentle warming of the urine. In [7
], we presented the modeling efforts for radiometer frequency selection and design of the microstrip log spiral antenna with matching layer and shield cup to acquire thermal noise radiation from the warm urine at depth. Here we evaluated the performance of the 1.375 GHz radiometer and EM shielded microstrip log spiral antenna to detect warm reflux in temperature controlled tissue mimicking phantoms.
Low viscous liquid phantoms made of ethylene glycol proposed for the bulk EM property of head and body tissues [24
] and Diacetin are well suited to measure antenna receive patterns and maintain homogeneous temperature load for radiometry experiments. Phantom formulations presented here differ in their composition and closely match the individual EM property of muscle and kidney tissues. Dielectric measurements of tissue equivalent phantoms in indicated good correspondence with literature data [20
] and the human urine sample. The non-toxic liquid and solid fat phantoms were stable with long shelf life, if appropriate storing using plastic wrap (Saran®, SC Johnson, Racine, WI USA) was used. Deterioration in the dielectric property of liquid phantoms due to dehydration was easily compensated with rehydration.
Antenna return loss from the tissue load in indicated better than −10 dB for the 1 mm dielectric disks. However, |S11
less than −10 dB with relatively flat response was observed over the radiometer frequency band for the 0.25 mm Mylar film alone. It can be concluded that the Mylar window with permittivity close to the antenna substrate (ε′=3.66) improved antenna input matching and power reception. The use of a low loss dielectric matching layer to enhance the signal to noise ratio of the radiometric signal was also studied in [22
]. The signal to ambient EMI ratio was further enhanced using a metal cup surrounding the antenna to minimize EMI (TEMI
) picked up by the antenna from the environment. Radiometer power spectral measurements of clearly demonstrated the potential EMI inside the radiometer frequency band and the shielding provided by the metal cup in continuous contact with the phantom surface and antenna ground plane. This result is of fundamental importance to translate in the clinic this EMI ultrasensitive technology.
It should be noted that the normalized volume average power density is the antenna radiometric weighting function (W
) in Equation 2
. of the normalized antenna power spectral density distributions of showed directional receive pattern with about 5% of the power density from the deep target location (30–40 mm from tissue surface). indicated comparable depth profiles in the power density distributions of the spiral antenna measured in the presence and absence of the metal cup. Radiometer spectral data of and power density distributions of together demonstrated that the metal cup surrounding the antenna provided effective EM shielding without deteriorating antenna power reception. EM return loss from the tissue phantom inside the scan tank was better than −10 dB for all scan configurations ().
The severity of VUR is graded I–V by the International Reflux Grading system based on the radiographic images. Grades III and above exhibit significant dilation of the ureter, renal pelvis and calyces and the degree of dilation increases for higher grade VUR. Radiometer measurements of clearly indicated the ability to detect temperature change of a large reflux volume (30 mL) inside the kidney, which corresponds to higher reflux grades that lasts for few minutes. Stable temperatures recorded in on the antenna surface and inside phantoms confirm that the radiometer response was due to localized temperature change at depth and not due to heat diffusion inside the layered phantom. Stable radiometer measurements also indicate good thermal stability of the total power radiometer. System response in indicates linear relation between the radiometer signal change (ΔP) and temperature differential (ΔT) of the 30 mL urine at depth.
The warm urine refluxing from the bladder comes in contact with the highly perfused kidney which receives arterial blood supply at 36.5–37°C. This could potentially accelerate heat loss into the kidney parenchyma which might degrade the ability to detect transient temperature change with MW radiometry. Thus, experiments were conducted to assess the transient response of the radiometry system to sudden temperature changes at depth. The system response of measured for warm urine injections followed by complete drainage clearly demonstrated the ability to detect VUR using radiometry however. The lack of change in radiometer power for the reflux event R0 in indicated that the signal detected during warm reflux is associated with change in temperature and not volume. also indicated smaller signal changes for lower reflux volumes (R2 vs. R7) and lower urine temperatures (R1 vs. R2). This is because the brightness temperature (TB, i
) given in Eqn (2)
is related to phantom temperature and the ratio of power received from the refluxed urine volume at depth to the power received from the total sensing volume as discussed in . Furthermore, the power gathered by the antenna reduces with increase in target depth (d1
) and, decrease in target volume (V1
) and urine temperature (T0
). Radiometer response of clearly demonstrated that warm reflux on the order of 20–30 mL can be detected using MW radiometry (R1–R5) and the visibility of smaller and deeper refluxes could be challenging (R6, R7). Efforts to improve system sensitivity to detect lower grade reflux are underway. System design is also being improved to address the influence of internal organ motion, reliable antenna positioning for kidney temperature monitoring and, miniaturization of the antenna and receiver for clinical implementation. Following successful system characterization in tissue phantoms, efforts are being focused on conducting animal experiments to evaluate the clinical potential of the proposed technique for non-invasive detection of VUR.