Compact photoacoustic imaging system using LED light source for monitoring skin condition

Non-invasively monitoring skin condition that can be performed near the patient is helpful for diagnosis of skin diseases. Photoacoustic and ultrasound imaging is one of the biomedical imaging technique. Both of these can be detected with an ultrasound transducer and can be acquired complementarily. Photoacoustic provides functional information while ultrasound provides structural information. However, since a solid-state laser has been used as a light source of a photoacoustic imaging system, it was difficult to carry and expensive. In this study, we propose a compact imaging system combining ultrasound and photoacoustic imaging using a LED light source for monitoring skin condition. The system performance was tested in vivo by imaging a human heel.

I. Introduction

Non-invasively examine internal structures and function of biomedical tissue is useful for diagnosis of various diseases. Ultrasound imaging is widely used for examining internal structures of biomedical tissue and it can be performed near the patient. Although it can observe deep part of the body, the resolution of this system (about 1 mm-l cm) is not sufficient for a small blood vessel. On the other hand, optical techniques such as OCT (Optical Coherence Tomography) [1] delivers high resolution (about 1-10μm) and high contrast. But, due to the scattering characteristics of tissue, it is only possible to image with a depth of about 1 mm in the human skin. In recent years, photoacoustic imaging has been studied widely in the field of biomedical imaging [2]–[5]. In photoacoustic imaging, a short pulse light is exposed into the body. This light is absorbed by tissue, where adiabatic expansion occurs and acoustic wave are emitted. This wave can be detected with an ultrasound transducer array at the tissue surface. The time from the pulse light irradiation to the detection of the detected signal corresponds to the distance from the light absorber to the detector. And, the intensity of the signal is proportional to the absorbed energy density. Since ultrasonic scattering characteristics in biological soft tissues are weak, photoacoustic combines high penetration depth (about 5cm) and high resolution. Photoacoustic imaging can detect the distribution of blood. Therefore photoacoustic can be used to identify the functional activities of tissues by visualizing the presence of small blood vessels, the content of hemoglobin and its degree of oxygenation. Moreover, photoacoustic uses an ultrasound transducer, it is possible to easily obtain the morphological image and the functional image of the same cross section without changing the positional relationship of the detector with respect to the object. However, since a solid- state laser has been used as a light source of a conventional photoacoustic imaging system, the system is large, expensive and difficult to carry. For these reason, it has not been used clinically.