Microwave Radiometry as a Non-Invasive Temperature Monitoring Modality During Superficial Hyperthermia

The application of microwaves in medicine has recently obtained renewed attention within the scientific community through emerging techniques in breast cancer detection (microwave tomography and ultra-wide-band radar imaging), high power ablation as well as microwave angioplasty and lipoplasty (Rosen et al., 2002). Nevertheless, more established techniques reported in the literature based on RF/microwaves are hyperthermia and medical radiometry. During the last decades, a number of research groups have studied various clinical applications based on radio-thermometry for detection of thermal anomalies in subcutaneous, or invasively more deeper, parts of the human body. The possibility of using microwave radiometry for non-invasive thermometry was originally suggested back in the early 1970’s (Edrich & Hardee, 1974; Enander & Larson, 1974) and the sensing principle was denoted microwave thermography. Prospected medical applications with highest potential include detection of breast cancer (often in conjunction with infrared thermometry) (Mouty et al., 2000), non-invasive temperature control of superficial (Ohba et al., 1995) and interstitial (Camart et al., 2000) hyperthermia, and control of brain temperatures of newborn infants during mild hypothermia (Hand et al., 2001; Maruyama et al., 2000). Additional applications, which have been investigated, comprise detection of inflammatory arthritis (MacDonald et al., 1994), extravasation rate of drugs (J. Schaeffer & Carr, 1986), changes of blood flow (Gabrielyan et al., 1992) or amount of lung water (Iskander et al., 1984) as well as post-mortem or cerebral temperature monitoring (Al-Alousi et al., 1994). Microwave radiometry in clinical medicine aims at deriving information on internal body temperature patterns by measurement of natural thermal black-body radiation from tissue in the lower part of the microwave region (<5 GHz). Knowledge on such thermal patterns can give valuable information in clinical disease detection and diagnosis as well as providing quantitative temperature feedback in monitoring of thermal therapeutic processes. The application of microwave radiometry raises several issues in determination of subcutaneous temperature heterogeneities. Most important is how to optimize design of the receiver hardware by identification of mutual compatible microwave devices with sufficiently low noise figures. The extremely low power levels (-174 dBm/Hz at 37oC body temperature) of electromagnetic thermal noise limit the applicability of microwave radiometers as the usable thermal signal competes both with external electromagnetic interference (EMI) and 2