Coherent detection imaging for medical laser tomography

Abstract

Optical spectroscopy techniques are providing essential ways to characterize physical and chemical properties in living tissues and cells and then to monitor their functional changes occurring inside them. Thus they offer exciting possibilities for developing spectroscopic computed tomography in the optical region, especially in the red and near infrared regions, to acquire functional/physiological information through their unique and valuable reconstructed images. Nevertheless, its development and application in living tissues and systems are complicated and prevented practically by the diffuse nature of the image quality due to strong multiple scattering of light. In the present paper, we will review and discuss our recent studies on Coherent Detection Imaging (CDI) method which could provide at present one of the most reliable and feasible schemes for achieving the optical/spectroscopic computed tomography using various kinds of lasers for biomedical applications. This method is basically realized with the optical heterodyne detection technique, possessing both the properties of a highly directional antenna and an ultra—sensitive receiver, and the image reconstruction based on the projection slice theorem from sets of line integrals of laser absorption along a large number of rays crossing the object with the parallel beam geometry. It is also postulated that the three fundamental conditions should be satisfied in principle in order to establish the optical absorption computed tomography on the basis of the conventional projection slice theorem. We have demonstrated experimentally the ful— fillment of these fundamental conditions using several biological tissues as well as scattering and diffusive samples by virtue of the optical heterodyne detection technique. Hence the applicability of the CDI method was confirmed in highly scattering and turbid media such as living tissues and bodies in which an object is completely ob— scured from normal visual observation and from conventional direct detection tech— niques. In consequence, we have achieved experimentally the two—dimensional direct (projection) imaging with some test objects immersed in biological tissues and the reconstructive tomographic imaging with various in vitro and in vivo objects such as chicken leg, chicken egg and human tumor specimens together with some plants and new—born mouse head. The collimated optical beam from a cw laser operated usually less than 10 mW of the output power, selected suitably from a set of Ar, He—Ne, Kr, Ti:A1203 and Nd:YAG lasers in our CDI system, enabled us to display these images with the spatial resolution as good as approximately 200 tim to 700 tun in this experi— ment. Several typical results of these measurements are also shown and discussed in this paper.