In this chapter, we discuss coherent optical methods that hold much promise for applications in biomedicine, such as photon-correlation and diffusion wave spectroscopies; speckle interferometry; full-field speckle imaging; coherent topography and tomography; phase, confocal, and Doppler microscopy; as well as interferential measurements of retinal visual acuity and blood sedimentation.
8.1 Photon-correlation spectroscopy of transparent tissues and cell flows
The physical fundamentals of photon-correlation spectroscopy were discussed in Chapter 4. The description of the principles and characteristics of the main modifications of homodyne and heterodyne photon-correlation spectrometers, the laser Doppler anemometers (LDAs), differential LDA schemes, and laser Doppler microscopes (LDMs) can be found in Refs. 5, 6, 22, 76-79, 82, 343, 825-827, 829, 830, 833, 838, 842, 848, and 849. A review of medical applications, mainly limited to the investigation of eye tissues (crystalline lens, cataract diagnosis), hemodynamics in isolated vessels (vessels of eye fundus or any other vessels) with the use of fiber optic catheters, and blood microcirculation in tissues, is provided in Refs. 5, 6, 22, 67, 76–79, 82, 83, 343, 825–827, 829, 830, 833, 835, 838–842, 848–850, 853, 859, and 1164–1201. In this section, we will discuss the photon-correlation technique in application to early cataract diagnostics and to measurement of blood and lymph flow in microvessels.
8.1.2 Cataract diagnostics
The photon-correlation spectroscopy or quasi-elastic light scattering (QELS) technique was originally developed to study small colloidal particles in fluids. Three decades ago, Tanaka and Benedek proposed to use this technique to study the onset of cataract in the ocular lens; however, it did not find a wide-scale commercial acceptance in ophthalmology. Owing to innovations since then in the field of optoelectronics, QELS is now emerging as a potential ophthalmic tool, making the study of virtually every tissue and fluid comprising the eye possible.