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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.
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