Heartbeat Detection using Infrared Thermometry in the Ear

Abstract

Heart rate is a key factor in cardiovascular system monitoring and sports science. Some recent commercial applications use sensors in the ear but are faced with motion artifacts which corrupts the signal. In this report, a comprehensive review of heart rate detection methodologies was completed in order to find possible solutions with better performance. Infrared thermography is a non-contact technique and was selected because it may minimize motion effects with better user comfort and lower power consumption. Thermopiles were chosen as the sensors used in the system based on their responsivity, relevant wavelength detection and size. The anatomical distribution of the arteries in the external ear was studied in order to select the best position of the sensor. The cavum conchae and anti-tragus were studied considering the irrigation of the posterior auricular and superficial temporal arteries. The signal analysis was performed using a continuous wavelet transform which extract frequency features of the bioheat transfer waveforms.
A digital sensor was evaluated first in the neck, nose, wrist and ear but did not render relevant results mainly due to quantization errors. Subsequently, analog thermopiles were studied with ultra-low noise amplifiers. First, the Noise Equivalent Temperature Difference from a ST150 thermopile was calculated using a Peltier cooler and a Pt100 reference sensor. This sensor was then used to measure the infrared signals of the wrist, neck and ear in order to find pertinent heart rate signal. Next, a smaller sensor was used to try compare the locations of the anti-tragus and the cavum conchae in the ear and under the effect of a fan. The fan did not had any effect on the measurement and the smaller sensor had significantly less signal than the previous sensor. The localization of the arteries was not possible as a result of the reduction on the detection area of the skin. Finally, the effect of the detection area of the skin was tested under 2 conditions using the ST150 thermopile. The results show that a separation of the sensor from the skin increased the detection area and the quality of the heart rate signal measured. The proposed system successfully uses infrared differential thermometry to detect the heart rate in the auricle.