TIME-RESOLVED SPECTROSCOPY BY AMPLITUDE MODULATION INDEX

THE USE OF PHASE MODULATION IN RECORDING THE TIME CONSTANT FOR THE DECAY OF THE PHOTONS EXITING FROM HUMAN ORGANS SUCH AS THE BRAIN AND LIMBS HAS LED TO THE DEVELOPMENT OF SEVERALSUCCESSFUL DEVICES IN WHICH PRECISE CONTROL OF OSCILLATOR FREQUENCIES HAS RESULTED IN PHOTON MIGRATION MEASUREMENTS ACCURATE TO A FEW MILLIMETERS USING FREQUENCIES OF 220 MHZ. RECENT IMPROVEMENTS IN SENSORS EMPLOYING SILICON DIODE DETECTORS SUGGEST THAT FREQUENCIES APPROACHING 500 MHZ CAN BE USED. AT THIS FREQUENCY, THE AMPLITUDE MODULATION INDEX OF THE EMERGENT PHOTONS DEPENDS UPON THE PATH LENGTH AS, FOR EXAMPLE, IN THE ATTENUATION CHARACTERISTIC OF A PARALLEL RC NETWORK. WHEREAS THE PHASE CHARACTERISTIC OF SUCH AN RC NETWORK FOR TISSUE STUDIES HAS BEEN PREVIOUSLY EMPHASIZED, IN THIS PROJECT THE AMPLITUDE CHARACTERISTIC OF THE PHOTON MIGRATION WILL BE STUDIED. THIS RESEARCH WILL EVALUATE MEASUREMENT OF THE AMPLITUDE INDEX AT 440 MHZ WITH 140 MHZ AS A REFERENCE FREQUENCY. THEPROPOSED INSTRUMENT MEASURES THE RATIO (OR DIFFERENCE) OF THE AMPLITUDE MODULATION INDEX AT A LOW AND A HIGH FREQUENCYAND AT TWO WAVELENGTHS. THE ATTENUATION CHARACTERISTIC AT THE TWO FREQUENCIES AND TWO WAVELENGTHS BECOMES A MEASURE OFPATH LENGTH CHANGES. DEMONSTRATIONS IN A MODEL SYSTEM AND IN HUMAN TISSUES AT 97 AND 220 MHZ HAVE ENSURED THE FEASIBILITY OF THIS PROJECT.