Non-invasive measurement of the human core temperature

The direct measurement of the temperature of the human body core, while being of great significance for the evaluation of patient health, is inconvenient. This realization has motivated attempts to measure the core temperature indirectly, for the most part at locations which do not require invasion of the body. In the investigation described here, a new approach to noninvasive measurement of the core temperature is set forth. The site of the proposed measurement is on the forehead where the anatomy is relatively uniform amongst individuals and also well established to be modeled accurately. The approach described here consists of simple hardware and a logic-based and customized control algorithm. The connection between the core temperature, the sensor temperature, and the temperature of a sensor-system heater was determined by numerical simulations of the heat transfer in the tissue bed fronted by the forehead skin and backed by the frontal bone. For heat transfer through each tissue layer in the tissue bed, the bioheat equation was employed, while for a resistance element in the sensing system, the Fourier heat conduction equation was used. These were solved in the transient mode. The controller read the temperatures of the core, the sensor, and the ambient, and the algorithm instructed the sensor-system heater to obtain a specific temperature value. The iterative use of the control algorithm resulted in near-equality of the sensor and core temperatures. An optimization routine performed operations needed to find the best performing sensor system. After optimization, that system is capable of yielding core temperature results that are approximately in error by 0.2 °C. A model problem was formulated to assess the capabilities of the sensing system to follow a time-varying core temperature. It was found that for core temperatures that varied by 0.05 °C per minute, the sensor followed the core within 0.2 °C or better.