Research On Key Techniques Of The Noncontact Detection Of Physiological Signals

Imaging photoplethysmography (IPPG) is noncontact physiological signal detectiontechnology based on the traditional photoplethysmography (PPG), which could achievesome specific cases of clinical and daily detection such as physiological signaldetection with open wounds and motion state. IPPG technology with its non-contactmeasurement, low cost and easy operation, has become one research hotspot in the field ofthe instrument and biomedical engineering. At present, although some importantphysiological parameters such as heart rate, respiration rate, etc, have been detected throughIPPG technology, the stability and comprehensiveness of the detection is a key problemneeding to be resolved.Based on the comprehensive analysis of the IPPG optical and physiology principle, thepaper discusses the technical features of IPPG, analyzes and summarizes the key problems,and proposes the corresponding solutions. It carries out the concrete research content andthe results as follows:(1) The paper analyzes and summarizes the selection of sensitive areas, differentimaging equipment performance requirements to different physiological parameters andvideo image signal processing technique.(2) The existing IPPG system is not suitable for the extraction of oxygen saturationbecause it is sensitivity to ambient light. Based on the existing system, IPPG systemplatform suitable for the measurement of blood oxygen saturation is designed in this paper.On the hardware design, two low illumination cameras each mounted with a narrow bandpass filter are chosen as video acquisition unit. In consideration of the spectral responsecurve of imaging equipment, visible wavelength of the filter is660nm and520nmrespectively. In terms of software design, algorithm suitable for oxygen saturation andsome other physiological parameters detection, including average gray value of sensitivearea, wavelet transform (WT), eigenvalue extraction of pulse wave, template matching,blind source analysis for noise attenuation, is discussed. (3) Experiments used IPPG system have been carried out to verify the feasibility andperformance in detecting blood oxygen saturation. With subjects holding their breath,empirical constant of blood oxygen saturation is calibrated. Then comparison tests areperformed between the standardized IPPG system and traditional PPG system with subjectsin the state of spontaneous breathing. The results show that the IPPG system designed inthe paper can detect blood oxygen saturation precisely under ambient light and theexperimental results are in high consistency within4%error. Moreover, programs writtenwith C++, matlab and mixed programming achieve quasi real-time measurement for bloodoxygen saturation.(4) The paper discusses the feasibility of using color imaging device to achieve bloodoxygen saturation basing on RGB three-channel signal collection characteristics. ColorCCD is applied as the imaging device. Red and blue channels of the color CCD replace thetraditional red and infrared channels to obtain two PPG signals to detect blood oxygensaturation. The experimental results show that the measurement system based on colorcamera is restricted by the wide bandwidth of red and blue channel. Although measurementresults can reflect the change of blood oxygen saturation, the parameter can’t be measuredaccurately. Thus the system used color camera is not suitable for routine and clinical care.(5) IPPG system with high speed camera is used to detect pulse rate variability (PRV).The results obtained through time domain analysis, frequency domain analysis andtime-frequency joint analysis methods are compared with that detected by traditional PPGand ECG system. The comparison results with high consistency show the feasibility ofPRV detection with IPPG technology. Furthermore, pulse transit time and blood pressurewould be measured through IPPG. The results also provide theoretical and technicalsupport to detect physiological parameters simultaneously through IPPG system.(6) In order to improve the practical application ability of IPPG system, blind sourceseparation technology is applied in eliminating motion artifacts. According to thecharacteristics of the RGB signal extracted from color imaging device, the sensitive areatracking matching method combined with independent component analysis method is usedto remove motion artifacts. Besides, for black and white imaging device, pulse wave signalis extracted from monochromatic signal through single channel independent component analysis (ICA) method. The heart rate extracted from the method above is consistent withthat from traditional PPG system, which proves the strong practical application ability ofthe IPPG system. However, blind source separation method cannot be applied in theextraction of oxygen saturation and PRV.