Measurement of Local Chest Wall Displacement by a Custom Self-Mixing Laser Interferometer

Several studies demonstrated that the human chest wall is made of at least two but perhaps more independent mechanical components, and that their relative contribution to the total chest wall mechanics is significantly affected by several respiratory disorders, supporting the possible clinical usefulness of this information. Unfortunately, the determination of local chest wall mechanics requires the accurate assessment of local chest wall displacement, which, up to now, can only be obtained by complex, expensive, and bulky equipment. In this paper, we present a new approach based on a custom-made low-cost self-mixing laser interferometer able to directly measure vibrations on human skin with very high accuracy and temporal resolution. A novel signal processing algorithm has also been developed to estimate the motion of the subject's skin also in the presence of signal fading. The system has been validated both in vitro and in vivo on seven healthy volunteers. The results showed good accuracy and very low signal noise compared with available techniques. We conclude that this approach, considering its advantages, might allow the development of new diagnostic tools for the clinical evaluation of respiratory diseases.