Biomedical Signals and Sensors II

After the interface between physiologic mechanisms and the resultant biosignals has been examined (Volume I), the subsequent interface between acoustic biosignals and the associated sensing technology is discussed here. A large variety of acoustic biosignals—permanent biosignals—originates in the inner human body, including heart sounds, lung sounds, and snoring sounds. These biosignals arise in the course of the body’s vital functions and convey physiological data to an observer, disclosing cardiorespiratory pathologies and the state of health. The genesis of acoustic biosignals is considered from a strategic point of view. In particular, the introduced common frame of hybrid biosignals comprises both the biosignal formation path from the biosignal source at the physiological level to biosignal propagation in the body, and the biosignal sensing path from the biosignal transmission in the sensor applied on the body up to its conversion to an electric signal. Namely, vibrating structures in the body yield acoustic sounds which are subject to damping while propagating through the thoracic tissues towards the skin. Arrived at the skin, different body sounds interfere with each other and induce mechanical skin vibration which, in turn, is perceived by a body sound sensor and then converted into the electric signal. It is highly instructive from an engineering and clinical point of view how sounds originate and interact with biological tissues. Discussed phenomena teach a lot about the physics of sound (as engineering sciences), and, on the other hand, biology and physiology (as live sciences). Basic and application-related issues are covered in depth. In fact, these issues should remain strong because these stand the test of time and mine knowledge of great value. Obviously, the highly interdisciplinary nature of acoustic biosignals and biomedical sensors is a challenge. However, it is a rewarding challenge after it has been coped with in a strategic way, as offered here. The chapter is intended to have the presence to answer intriguing “Aha!” questions.

After the interface between physiologic mechanisms and the resultant biosignals has been examined (Volume I), the subsequent interface between optic biosignals and the associated sensing technology is discussed here. In the genesis of optic biosignals—induced biosignals—an artificial light is coupled into biological tissue. The resulting transmitted light intensity is strongly governed by the light absorption and scattering in tissue. The light absorption, for instance, is modulated by blood oxygenation and local pulsatile blood volume. Consequently, the transmitted light intensity reflects multiple physiologic parameters—which are vital for the assessment of cardiorespiratory pathologies and the state of health—and comprises the optic biosignal. The genesis of optic biosignals is considered from a strategic point of view. In particular, the introduced common frame of hybrid biosignals comprises both the biosignal formation path from the biosignal source at the physiological level to biosignal propagation in the body, and the biosignal sensing path from the biosignal transmission in the sensor applied on the body up to its conversion to an electric signal. Namely, the optical sensor is comprised of a light source on the skin to generate the incident light and couple it into tissue, and a distant light sink to detect the resulting transmitted light. The transilluminated region can be approximated as an arrangement of tissue layers and blood vessels. If an arterial vessel is considered with a blood pressure pulse propagating along the vessel, then there is a local pulsatile change in the arterial radius. Provided that blood in vessels absorbs the light to a larger extent compared to the tissues surrounding these vessels, it is clear that the transmitted light intensity temporarily decreases for increasing arterial radius in the transilluminated region. Thus, the propagating light is modulated by diverse physiological phenomena. A certain portion of light leaves the body and is detected by the light sink applied on the skin. The sink converts the transmitted light intensity into the electric signal. It is highly instructive from an engineering and clinical point of view how light interacts with biological tissues. Discussed phenomena teach a lot about the physics of light (as engineering sciences), and, on the other hand, biology and physiology (as live sciences). Basic and application-related issues are covered in depth. In fact, these issues should remain strong because these stand the test of time and mine knowledge of great value. Obviously, the highly interdisciplinary nature of optic biosignals and biomedical sensors is a challenge. However, it is a rewarding challenge after it has been coped with in a strategic way, as offered here. The chapter is intended to have the presence to answer intriguing “Aha!” questions.