OPTICAL INVESTIGATIONS OF PHYSIOLOGY - A STUDY OF INTRINSIC AND EXTRINSIC BIOMEDICAL CONTRAST

The utility and performance of optical studies of tissue depends upon the contrast and the changes of contrast in health and disease and in functional activity. The contrast is determined both by the optical pr operties of extrinsic and intrinsic chromophores and scatterers but es pecially upon the changes evoked by physiological activity and patholo gical states. Here, we have focused upon absorption changes of the int rinsic probe: blood absorbance changes due to cortical hypoxia and to haematomas, giving, for particular conditions, absorbance changes of 0 .15 and over 0.4 Delta OD respectively. Functional activity may give c hanges of blood volume of over 0.05 Delta OD with some variability due to individual responses that is best expressed as histogram displays of the distribution of response among a significant population. Respon ses have been observed in prefrontal parietal and occipital functions (242 tests). Extrinsic probes afford signals dependent upon the dose t olerance of the subject and can readily equal or exceed the blood volu me and oxygenation signals, and currently afford vascular volume and f low indications. However, contrast agents for the functional activity of cellular function are ultimately to be expected. Finally, lightscat tering changes afford osmolyte-related responses and are here shown to indicate a large signal attributed to cortical depolarization and KC release in hypoxia/ischaemia. Thus, the optical method affords imaging of manifold contrasts that greatly enhance its specificity and sensit ivity for diagnostic procedures.