Selected Clinical Applications of Functional PPGI Perfusion Mapping in Dermatology

Abstract

Assessment of dermal blood perfusion before and after standardized clinical skin tests is mostly provided by clinical inspection. This chapter aims to supplement this qualitative assessment with the Photoplethysmography Imaging (PPGI) method. Our experimental PPGI system contains a light source consisting of multiple LEDs that illuminate a larger part of the skin surface with selective light intensity and wavelength. The light intensity scattered back from the skin is detected by a highly sensitive CCD camera (UltraPix FE250) acting like a PPG detector array to detect weak light modulation caused by the arterial pulsation. Our setup utilizes a silicon frame-transfer CCD chip with a pixel resolution of 512x512, cooled down to -40 °C to reduce the readout noise. Depending on the camera lens, the observed skin region can range from a few square centimeters to whole extremities, observed from a distance of 50 cm. After the recording of a short video sequence (e.g., 120 s) of the skin surface, the operator can choose different regions of interest (ROIs acting like the “virtual sensors”). Then the time-dependent backscattered light intensity of the ROIs is calculated. The resulting signals visualize the arterial blood volume changes in the respective skin regions. Fourier and Wavelet Transforms, artifact reduction algorithms, and algorithms for point-to-point multidimensional perfusion calculation and visualization (functional perfusion mapping) provide a sound basis for quantified clinically relevant skin perfusion studies. Our first clinical trials validate the diagnostic potential of the proposed method in dermatology (especially at skin allergy testing) and dermato-angiology.