COMBINED ULTRASOUND AND FLUORESCENCE SPECTROSCOPY FOR PHYSICOCHEMICALIMAGING OF ATHEROSCLEROSIS

This paper describes a combined ultrasonic and spectroscopic system fo r remotely obtaining physico-chemical images of normal arterial tissue and atherosclerotic plaque. Despite variations in detector-tissue sep aration, R, fluorescence powers corresponding to pixels in the image a re converted to the same set of calibrated units using distance estima tions from A-mode ultrasound reflection times. An empirical model, val idated by Monte Carlo simulations of light propagation in tissue, is u sed to describe changes in fluorescence power as a function of R. Fluo rescence spectra of normal and atherosclerotic human aorta obtained wi th this system are presented as a function of R. To compensate for cha nges in fluorescence power with R, the empirical model was used in eac h case to calculate the fluorescence power at a constant reference val ue of R (R(ref) = 1.67 mm). Prior to compensation, tissue fluorescence power decreased more than a factor of two as R was increased from 2.5 to 5 mm. Following compensation, the fluorescence power varied less t han f 10% of the average compensated peak. The chemical composition of each sample was determined by fitting its fluorescence spectrum (in c alibrated units) to a model of tissue fluorescence incorporating struc tural protein and ceroid fluorescence, as well as structural protein a nd hemoglobin attenuation. Parameters of the fit were used to classify tissue type. Without compensation for distance variation, classificat ion of tissue type was frequently incorrect; however, with compensatio n, predictive value was high. A 1-D chemical image of a section of hum an aorta containing both normal and atherosclerotic regions obtained w ith this system is also presented. After compensation for detector-sam ple separation, tissue classifications along the cross-section closely resemble those obtained from histology. Regions of elevated ceroid co ncentration and intimal-thickening are clearly observable in the resul tant Chemical image. The potential value of this type of system in the diagnosis and treatment of coronary artery disease is discussed.