Non-invasive measurement of brain temperature with microwave radiometry: demonstration in a head phantom and clinical case

Abstract

This study characterizes the sensitivity and accuracy of a non-invasive microwave radiometric thermometer intended for monitoring body core temperature directly in brain to assist rapid recovery from hypothermia such as occurs during surgical procedures. To study this approach, a human head model was constructed with separate brain and scalp regions consisting of tissue equivalent liquids circulating at independent temperatures on either side of intact skull. This test setup provided differential surface/deep tissue temperatures for quantifying sensitivity to change in brain temperature independent of scalp and surrounding environment. A single band radiometer was calibrated and tested in a multilayer model of the human head with differential scalp and brain temperature. Following calibration of a 500MHz bandwidth microwave radiometer in the head model, feasibility of clinical monitoring was assessed in a pediatric patient during a 2-hour surgery. The results of phantom testing showed that calculated radiometric equivalent brain temperature agreed within 0.4°C of measured temperature when the brain phantom was lowered 10°C and returned to original temperature (37°C), while scalp was maintained constant over a 4.6-hour experiment. The intended clinical use of this system was demonstrated by monitoring brain temperature during surgery of a pediatric patient. Over the 2-hour surgery, the radiometrically measured brain temperature tracked within 1-2°C of rectal and nasopharynx temperatures, except during rapid cooldown and heatup periods when brain temperature deviated 2-4°C from slower responding core temperature surrogates. In summary, the radiometer demonstrated long term stability, accuracy and sensitivity sufficient for clinical monitoring of deep brain temperature during surgery.

Keywords: brain temperature; core temperature; microwave radiometry; non-invasive thermometry.

Figure 1

CT scan of a 66-year-old female head. The collective thickness of skull and scalp is 10.9 mm in the forehead region and the distance from skin to ventricle deep in brain is 45.5 mm.

Figure 2

Human head model with variable temperature brain tissue-equivalent liquid circulating through a balloon that fills the inside of the skull, and temperature controlled scalp tissue-equivalent liquid circualting through an adjustable thickness “scalp” under the skull. The black cable connects the external radiometer readout to the front stage radiometer and antenna sensor which is coupled to the scalp through a thin mylar window and facing upwards into the skull.

Figure 3

2.5 cm diameter tapered log spiral microstrip antenna encapsulated together with the radiometer electronics printed circuit on the back surface of antenna inside a cylindrical copper tube with 1 mm wall thickness.

Figure 4

HFSS-simulated SAR pattern of a 2.5 cm diameter log spiral microwave antenna on the human head model. Simulations performed at the center band frequency of 1.35 GHz demonstrate that the radiometer is sensitive to scalp temperature (maximum SAR and received power in red) but receives a significant portion of the energy at 1.35 GHz from brain tissues inside the skull.

Figure 5

Equivalent brain tissue temperature (black) calculated from total power receiveed by a non-invasive radiometric sensor closely mirrors the actual temperature of the cirulating variable temperature brain phantom (red) as measured through constant temperature scalp tissue (blue), with no significant error from drift in radiometer calibration over 4.5 hours of monitoring.

Figure 6

Absolute temperature difference between the equivalent brain temperature calculated from radiometer received power readings and the fiberoptic probe measured temperature of circulating brain phantom.

Figure 7

Equivalent brain temperature (black) from 400-point median calculations of raw radiometric readings during surgery of pediatric patient, correlated with manually recorded rectal (red) and nasopharyngeal (blue) core temperature measurements and skin surface just under the radiometric sensor (green dashed line). Note the large (up to 4°C) difference in core temperature readings during the time of rapid temperature change in the patient and closer agreement of core measurements after a period of steady state (1-2°C).