Feasibility of in vivo intravascular ultrasound tissue characterization inthe detection of early vascular transplant rejection

Background-Unprocessed ultrasound radiofrequency (RF) signal analysis has b een shown to distinguish different tissue structures more reliably than gra y-scale interpretation of conventional ultrasound images. Methods and Results-The objective of this study was to test the feasibility of in vivo intravascular ultrasound (IVUS) RF signal analysis in an animal model of allograft rejection. Six cynomolgus monkeys underwent transplanta tion of 3-cm aortic allograft segments distal to the renal arteries from im munologically mismatched donors. IVUS imaging with a 30-MHz system was perf ormed 84 to 105 days after the operation. RF signals were acquired from cro ss sections of the recipient and the allograft aortas in real time with a d igitizer at 500 MHz with 8-bit resolution. Sixty-five cross sections and 68 regions of interest (31 in host aorta and 37 in allograft) were analyzed i n the adventitial layer with a total number of 8568 vectors processed. For each region of interest, a weighted-average attenuation was calculated on t he basis of the attenuation and length for each individual vector. Histolog ical examination was performed at every cross section imaged by IVUS, When the gray-scale images of conventional IVUS scored by an independent observe r were compared, no distinction between adventitia of the native aorta and allograft was possible. Analysis of the average RF backscatter power also s howed no significant difference (70.32+/-3.55 versus 70.72+/-3.38 dB). Howe ver, the average attenuation of allografts was significantly lower than tha t of the host aortas (2.64+/-1.38 versus 4.02+/-1.16 dB/mm, P<0.001). Histo logy demonstrated a marked adventitial inflammatory response in all allogra fts, with no inflammation observed in the host aortas. Conclusions-In vivo IVUS tissue characterization can be performed during ro utine imaging. In this model of transplant vasculopathy, RF attenuation mea surements were more sensitive than visual or quantitative gray-scale analys is.