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Background-The purpose of the present study was to define whether integrated backscatter (IB) combined with conventional intravascular ultrasound (IVUS) makes tissue characterization of coronary arterial plaques possible. Methods and Results-IB-IVUS was performed in coronary arteries (total 18 segments) of 9 patients at autopsy, and the findings were compared with the histology. RF signals, which were digitized at 2 GHz in 8-bit resolution, were obtained with an IVUS system with a 40-MHz catheter. IB values of the RF signal from the region of interest (ROI) (100-m depth, 1.4°per line) were calculated by use of a personal computer. IB values on the ROIs were divided into 5 categories, compared with each of the plaque histologies: category 1 (thrombus), Ϫ88 Ͻ IB Յ Ϫ80; category 2 (intimal hyperplasia or lipid core), Ϫ73 Ͻ IB Յ Ϫ63; category 3 (fibrous tissue), Ϫ63 Ͻ IB Յ Ϫ55; category 4 (mixed lesions), Ϫ55 Ͻ IB Յ Ϫ30; and category 5 (calcification), Ϫ30 Ͻ IB Յ Ϫ23. On the basis of these categories, we analyzed 5120 ROIs per segment in each ring-like arterial specimen. Color-coded maps of plaques were constructed by use of these IB data and conventional IVUS data, which reflected the plaque histology of autopsied coronary arteries well. Then, the same method was undertaken in 24 segments with plaque from 12 patients in vivo with angina pectoris. Comparisons between coronary angioscopy and IB-IVUS revealed that the surface color of plaques in angioscopy reflected the thickness of the fibrous cap rather than the size of the lipid core. Conclusions-IB-IVUS represents a new and useful tool for evaluating the tissue structure of human coronary arterial plaques.