Abstract
Imaging photoplethysmography (iPPG) is an emerging technology used to assess microcirculation and cardiovascular signs by collecting backscattered light from illuminated tissue using optical imaging sensors. An engineering approach is used to evaluate whether a silicone cast of a human palm might be effectively utilized to predict the results of image registration schemes for motion compensation prior to their application on live human tissue. This allows us to establish a performance baseline for each of the algorithms and to isolate performance and noise fluctuations due to the induced motion from the temporally changing physiological signs. A multi-stage evaluation model is developed to qualitatively assess the influence of the region of interest (ROI), system resolution and distance, reference frame selection, and signal normalization on extracted iPPG waveforms from live tissue. We conclude that the application of image registration is able to deliver up to 75% signal-to-noise (SNR) improvement (4.75 to 8.34) over an uncompensated iPPG signal by employing an intensity-based algorithm with a moving reference frame.
Highlights
The ability to monitor subcutaneous blood flow in living tissues has been an area of intensive biomedical research for decades [1,2,3]
Imaging photoplethysmography has since been used to demonstrate the feasibility of remote blood perfusion imaging as an inexpensive alternative method where tissue surface is illuminated by ambient [5] or artificial light [6,7,8], and modulated backscattered light is captured by an image sensor, typically a digital camera
The noise floor of an Imaging photoplethysmography (iPPG) signal extracted from a static region of interest (ROI) may contain at least 0.16% of its DC value and reach around a quarter of the available signal headroom, masking some of the desired cardiac-related fluctuations
Summary
The ability to monitor subcutaneous blood flow in living tissues has been an area of intensive biomedical research for decades [1,2,3]. Until recently laser Doppler and laser speckle imaging were the only viable non-contact techniques for mapping capillary blood flow [4]. Imaging photoplethysmography (iPPG) has since been used to demonstrate the feasibility of remote blood perfusion imaging as an inexpensive alternative method where tissue surface is illuminated by ambient [5] or artificial light [6,7,8], and modulated backscattered light is captured by an image sensor, typically a digital camera. The extracted iPPG waveforms have been found to be sensitive to optical distortions, in particular body motion, which is able to instantly change the amount of backscattered and stray light hitting the camera sensor [9,10]. Focusing on the same skin area during the whole cardiac cycle is paramount in obtaining a high-resolution perfusion image of a rich capillary bed in body areas such as the face, palms, Sensors 2018, 18, 4340; doi:10.3390/s18124340 www.mdpi.com/journal/sensors
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