Background: Hyperthermia, i.e., raising tissue temperature to 40-45°C for 60 min, has been demonstrated to increase the effectiveness of radiation and chemotherapy for cancer. Although multi-element conformal heat applicators are under development to provide more adjustable heating of contoured anatomy, to date the most often used applicator to heat superficial disease is the simple microwave waveguide. With only a single power input, the operator must be resourceful to adjust heat treatment to accommodate variable size and shape tumors spreading across contoured anatomy. Methods: We used multiphysics simulation software that couples electromagnetic, thermal and fluid dynamics physics to simulate heating patterns in superficial tumors from commercially available microwave waveguide applicators. Temperature distributions were calculated inside homogenous muscle and layered skin-fat-muscle-tumor-bone tissue loads for a typical range of applicator coupling configurations and size of waterbolus. Variable thickness waterbolus was simulated as necessary to accommodate contoured anatomy. Physical models of several treatment configurations were constructed for comparison of simulation results with experimental specific absorption rate (SAR) measurements in homogenous muscle phantom. Results: Accuracy of the simulation model was confirmed with experimental SAR measurements of three unique applicator setups. Simulations demonstrated the ability to generate a wide range of power deposition patterns with commercially available waveguide antennas by controllably varying size and thickness of the waterbolus layer. Conclusion: Heating characteristics of 915 MHz waveguide antennas can be varied over a wide range by controlled adjustment of microwave power, coupling configuration, and waterbolus lateral size and thickness. The uniformity of thermal dose delivered to superficial tumors can be improved by cyclic switching of waterbolus thickness during treatment to proactively shift heat peaks and nulls around under the aperture, thereby reducing patient pain while increasing minimum thermal dose by end of treatment.
Background: Brown adipose tissue (BAT) plays an important role in whole body metabolism and could potentially
mediate weight gain and insulin sensitivity. Although some imaging techniques allow BAT detection, there are currently
no viable methods for continuous acquisition of BAT energy expenditure. We present a non-invasive technique for long
term monitoring of BAT metabolism using microwave radiometry.
Methods: A multilayer 3D computational model was created in HFSSTM with 1.5 mm skin, 3-10 mm subcutaneous fat,
200 mm muscle and a BAT region (2-6 cm3) located between fat and muscle. Based on this model, a log-spiral antenna
was designed and optimized to maximize reception of thermal emissions from the target (BAT). The power absorption
patterns calculated in HFSSTM were combined with simulated thermal distributions computed in COMSOL® to predict
radiometric signal measured from an ultra-low-noise microwave radiometer. The power received by the antenna was
characterized as a function of different levels of BAT metabolism under cold and noradrenergic stimulation.
Results: The optimized frequency band was 1.5-2.2 GHz, with averaged antenna efficiency of 19%. The simulated
power received by the radiometric antenna increased 2-9 mdBm (noradrenergic stimulus) and 4-15 mdBm (cold
stimulus) corresponding to increased 15-fold BAT metabolism.
Conclusions: Results demonstrated the ability to detect thermal radiation from small volumes (2-6 cm3) of BAT located up to 12 mm deep and to monitor small changes (0.5 °C) in BAT metabolism. As such, the developed miniature
radiometric antenna sensor appears suitable for non-invasive long term monitoring of BAT metabolism.
Background: There are numerous clinical applications for non-invasive monitoring of deep tissue temperature. We
present the design and experimental performance of a miniature radiometric thermometry system for measuring volume
average temperature of tissue regions located up to 5cm deep in the body.
Methods: We constructed a miniature sensor consisting of EMI-shielded log spiral microstrip antenna with high gain onaxis
and integrated high-sensitivity 1.35GHz total power radiometer with 500 MHz bandwidth. We tested performance
of the radiometry system in both simulated and experimental multilayer phantom models of several intended clinical
measurement sites: i) brown adipose tissue (BAT) depots within 2cm of the skin surface, ii) 3-5cm deep kidney, and iii)
human brain underlying intact scalp and skull. The physical models included layers of circulating tissue-mimicking
liquids controlled at different temperatures to characterize our ability to quantify small changes in target temperature at
depth under normothermic surface tissues.
Results: We report SAR patterns that characterize the sense region of a 2.6cm diameter receive antenna, and radiometric
power measurements as a function of deep tissue temperature that quantify radiometer sensitivity. The data demonstrate:
i) our ability to accurately track temperature rise in realistic tissue targets such as urine refluxed from prewarmed bladder
into kidney, and 10°C drop in brain temperature underlying normothermic scalp and skull, and ii) long term accuracy
and stability of +0.4°C over 4.5 hours as needed for monitoring core body temperature over extended surgery or
monitoring effects of brown fat metabolism over an extended sleep/wake cycle.
Conclusions: A non-invasive sensor consisting of 2.6cm diameter receive antenna and integral 1.35GHz total power
radiometer has demonstrated sufficient sensitivity to track clinically significant changes in temperature of deep tissue
targets underlying normothermic surface tissues for clinical applications like the detection of vesicoureteral reflux, and
long term monitoring of brown fat metabolism or brain core temperature during extended surgery.