On the Feasibility of Pulse Wave Velocity Imaging for Remote Assessment of Physiological Functions

Remote Photoplethysmography with a High-Speed Camera Reveals Temporal and Amplitude Differences between Glabrous and Non-Glabrous Skin

Photoplethysmography (PPG) is a noninvasive optical technology with applications including vital sign extraction and patient monitoring. However, its current use is primarily limited to heart rate and oxygenation monitoring. This study aims to demonstrate the utility of PPG for physiological investigations. In particular, we sought to demonstrate the utility of simultaneous data acquisition from several regions of tissue using remote/contactless PPG (rPPG). Specifically, using a high-speed scientific-grade camera, we collected rPPG from the hands (palmar/dorsal) of 22 healthy volunteers. Data collected through the red and green channels of the RGB CMOS sensor were analyzed. We found a statistically significant difference in the amplitude of the glabrous skin signal over the non-glabrous skin signal (1.41 ± 0.85 in the red channel and 2.27 ± 0.88 in the green channel). In addition, we found a statistically significant lead of the red channel over the green channel, which is consistent between glabrous (17.13 ± 10.69 ms) and non-glabrous (19.31 ± 12.66 ms) skin. We also found a statistically significant lead time (32.69 ± 55.26 ms in the red channel and 40.56 ± 26.97 ms in the green channel) of the glabrous PPG signal over the non-glabrous, which cannot be explained by bilateral variability. These results demonstrate the utility of rPPG imaging as a tool for fundamental physiological studies and can be used to inform the development of PPG-based devices.

Remote photoplethysmography with consumer smartphone reveals temporal differences between glabrous and nonglabrous skin: Pilot in vivo study

Photoplethysmography (PPG) is a noninvasive optical technology, with applications including vital sign extraction and patient monitoring. The PPG acquisition skin type may be of importance. Skin is either nonglabrous (~90%) or glabrous (~10%). Clinical PPG collection is typically from glabrous (fingerpad), while proliferating wearables collecting PPG, which may perform critical functions like arrythmia detection, often acquire from atypical sites. Glabrous skin has significant differences from nonglabrous, including microcirculation, yet comparisons between their PPG signals have not been well reported. Using a smartphone-based remote/contactless PPG, a pilot dataset was collected from the hands (palmar/dorsal) of five healthy volunteers. The data shows statistically significant lead time (52 ± 36 ms) of glabrous over nonglabrous. Further, a trend of glabrous amplitude increase over nonglabrous (31%) was found. Although our study has a small number of participants, these results further the characterization of PPG skin differences, and can be used to inform development of PPG-based devices.

Feasibility Study of Remote Contactless Perfusion Imaging with Consumer-Grade Mobile Camera

A non-invasive, contactless, inexpensive and easy-to-operate perfusion imaging method using a consumer-grade mobile camera (iPhone 8) developed in our group can visualise blood flow in tissue.

METHODS

Ischemia was induced in one hand using a blood pressure cuff inflated over the systolic blood pressure to stop the blood flow. Using an iPhone, data was collected from 5 subjects, beginning with no occlusion (a baseline), followed by one hand occluded, and then release of the occlusion to restore blood circulation. This protocol was repeated for each hand for a total of 10 videos. The data were analysed to extract the oscillating and quasi-constant components of the photoplethysmogram signal representing blood flow. In addition, we introduced a scoring parameter to reflect perfusion (i.e., perfusion score).

RESULTS

The proposed perfusion score was used to create a pseudo colour map of perfusion across the protocol, demonstrating the ability to detect ischemia caused by occlusion. The difference in perfusion score was statistically significant between ischemia and baseline/recovery.

CONCLUSIONS

Pilot results on healthy volunteers demonstrate the feasibility of perfusion imaging using a consumer-grade camera. A further developed method can be used to assess the viability of transplanted tissue.