Development of non-invasive temperature measurement system for continuous monitoring of deep brain temperatures

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It has been shown that cooling the brain of newborn baby after hypoxia-ischemia can reduce damage and improve functional outcome. Clinical trials are, however, currently hindered by the difficulty in measuring brain temperature non-invasively. Microwave radiometry has been proposed and developed as a non-invasive technique for brain temperature measurement at Shizuoka University. The radiometer system detects the very weak spontaneous electromagnetic radiation from inside the brain by a waveguide antenna placed on the head surface and amplifies them with five receivers operating at center frequencies of 1.2, 1.65, 2.3, 3.0 and 3.6 GHz. The system also includes two lock-in amplifiers, control circuit, reference noise source, and the temperature retrieval algorithm.Among the five receivers, 2.3, 3.0 and 3.6 GHz receivers have been added to the 1.2 and 1.65 GHz receivers that were previously made, and a control program for the whole system has been developed. In addition to those, several … More circuits such as buffer circuits, constant voltage/current circuits, supply circuits, etc. were manufactured. In order to operate stably not in a shielded room but in a normal room, several countermeasures for reducing background noises and thermal fluctuation have been made to the system. After such improvements, the system became stable in a normal room, and a calibration experiment could be done using a waveguide antenna. Temperature resolutions for the five receivers were 0.183, 0.273, 0.148, 0.108, 0.118 ℃. New radiometric weighting functions were also obtained from baby's MRI data. Confidence interval at the brain center was estimated by the temperature retrieval simulation method, which also developed for the evaluation of the system, using the obtained resolutions and a temperature shape function in a baby's head. The interval thus obtained was about 0.3 ℃. This result was satisfactory in comparison with the target value of 1 ℃. Manufacturing the temperature distribution phantom of a baby's head and the further countermeasures for stabilization of the system for temperature measurement experiment are now underway. Less