Non-contact transmittance photoplethysmographic imaging (PPGI) for long-distance cardiovascular monitoring
This enables non-contact, long-distance heart rate monitoring, potentially expanding cardiovascular function monitoring beyond at-rest, short-term contact-based methods, though it appears incremental as it builds on existing PPGI concepts with a new mode.
The paper tackled the problem of non-contact cardiovascular monitoring by proposing a transmittance photoplethysmographic imaging (PPGI) system for long-distance use, achieving statistically significant increases in correlation to ground-truth PPG signals in both short- and long-distance scenarios (p-values as low as <0.0001).
Photoplethysmography (PPG) devices are widely used for monitoring cardiovascular function. However, these devices require skin contact, which restrict their use to at-rest short-term monitoring using single-point measurements. Photoplethysmographic imaging (PPGI) has been recently proposed as a non-contact monitoring alternative by measuring blood pulse signals across a spatial region of interest. Existing systems operate in reflectance mode, of which many are limited to short-distance monitoring and are prone to temporal changes in ambient illumination. This paper is the first study to investigate the feasibility of long-distance non-contact cardiovascular monitoring at the supermeter level using transmittance PPGI. For this purpose, a novel PPGI system was designed at the hardware and software level using ambient correction via temporally coded illumination (TCI) and signal processing for PPGI signal extraction. Experimental results show that the processing steps yield a substantially more pulsatile PPGI signal than the raw acquired signal, resulting in statistically significant increases in correlation to ground-truth PPG in both short- ($p \in [<0.0001, 0.040]$) and long-distance ($p \in [<0.0001, 0.056]$) monitoring. The results support the hypothesis that long-distance heart rate monitoring is feasible using transmittance PPGI, allowing for new possibilities of monitoring cardiovascular function in a non-contact manner.