Synchro-Thermography: Monitoring ~10 mK Facial Temperature Changes with Heartbeat Referencing for Physiological Sensing

TL;DR

This work tackles the difficulty of non-invasively tracking minute facial temperature changes linked to cardiac activity by introducing Synchro-Thermography, which references the heartbeat to stabilize thermographic measurements. The method resamples thermal videos to the mean RR interval, aligns motion, applies a sliding-window synchronous additive average, and analyzes the time series with Fourier transform to detect heartbeat-periodic fluctuations in facial skin temperature. Across eight young adults, the approach detects ~10 mK temperature changes on facial regions and yields at least a two-fold improvement in temperature resolution compared to raw thermography, with successful applicability in both frontal and profile views and without external stimuli. The results suggest Synchro-Thermography as a viable tool for non-contact physiological sensing of blood flow changes, with potential applications in injury/disease monitoring and stress assessment, while highlighting the need for broader validation and deeper understanding of the underlying vascular mechanisms.

Abstract

Infrared thermography has gained interest as a tool for non-contact measurement of blood circulation and skin blood flow due to cardiac activity. Partiularly, blood vessels on the surface, such as on the back of the hand, are suited for visualization. However, standardized methodologies have not yet been established for areas such as the face and neck, where many blood vessels are lie deeper beneath the surface, and external stimulation for measurement could be harmful. Here we propose Synchro-Thermography for stable monitoring of facial temperature changes associated with heart rate variability. We conducted experiments with eight subjects and measured minute temperature changes with an amplitude of about \SI{10}{mK} on the forehead and chin. The proposed method improves the temperature resolution by a factor of 2 or more, and can stably measure skin temperature changes caused by blood flow. This skin temperature change could be applied to physiological sensing such as blood flow changes due to injury or disease, or as an indicator of stress.

Figures (7)

• Figure 1: Overview of our proposed Synchro-Thermography using the heartbeats as a reference timing signal
• Figure 2: The experimental environment
• Figure 3: The biosignal amplifier
• Figure 5: Raw data in left column and results of the proposed method in right column: Female frontal face. \ref{['fig:rawFemale']}, \ref{['fig:synchroFemale']} The thermal video frame. \ref{['fig:rawECGfemale']} Heartbeats of raw data. \ref{['fig:synchroECGfemale']} Heartbeats resampled to the mean RR interval. \ref{['fig:rawAfemale']}, \ref{['fig:synchroAfemale']} Temperature at the right cheek A (269, 248). \ref{['fig:rawBfemale']}, \ref{['fig:synchroBfemale']} Temperature at the right cheek B (261, 382)
• Figure 6: Raw data in left column and results of the proposed method in right column: Left profile of female subject 5. \ref{['fig:raw5']}, \ref{['fig:synchro5']} The thermal video frame. \ref{['fig:rawECG5']} Heartbeats of raw data. \ref{['fig:synchroECG5']} Heartbeats resampled to the mean RR interval. \ref{['fig:rawA5']}, \ref{['fig:synchroA5']} Temperature at the inner corner of the left eye A (308, 224). \ref{['fig:rawB5']}, \ref{['fig:synchroB5']} Temperature at the nose B (293, 217).