Feasibility of Photoplethysmography Imaging of the Sole of the Foot

Abstract

In photoplethysmography imaging the blood volume pulse is extracted from subtle skin color variations recorded with a camera. In most works, the camera is oriented towards the face or the palm of the hand. Recently, it has been shown that applying contact force to the skin, e.g. through contact with a glass plate, increases signal strength. However, this approach is prohibitive for the face and there are applications where the hands are not accessible. In this paper, we address this issue by applying photoplethysmography imaging to the sole of the foot, which is placed on a glass plate and exposed to controlled illumination. We investigate (1) whether this setup acquires signals with an adequate signal-to-noise ratio and (2) whether the feet can be used as a replacement for the more commonly used hands. Additionally, we analyze (3) whether using a carefully selected pixel subset instead of all pixels improves signal-to-noise ratio. We report on experimental data from $\mathrm {N} = 21$ healthy subjects that we make publicly available 1. We applied photoplethysmography imaging to the sole of the foot and the palm of the hand and used a commercial finger photoplethysmography unit as ground truth. It is problematic that there is no established best practice for signal-to-noise estimation in PPGi research; hence we use five different metrics with three from literature. Additionally, we compare the agreement of these metrics using the experimental data. Our results show that (1) the proposed setup acquires signals with an adequate signal-to-noise ratio, (2) using all pixels from foot videos results in slightly inferior performance compared to the hand, but (3) using pixel subsets improves signal quality at the foot to the level of the hand. These results pave the way for the development of a photoplethysmography imaging setup applied at the sole of the foot for different medical applications.1All data used in this study is freely available online: http://www.github.com/nspi/rise/