A noninvasive cross-correlation ultrasound technique for detecting spatial profile of laser-induced coagulation damage-an in vitro study

A cross-correlation A-mode ultrasound technique is proposed for noninvasively detecting the spatial profile of coagulation damage in tissue being irradiated by laser. The basic assumption underlying this technique is that when coagulation is taking place in a tissue region, owing to thermally-induced structure changes in tissue, the waveform of echo signal scattered from that region should be changing accordingly. The authors´ technique consists of four steps: (1) repeatedly sending the same acoustic signal to the tissue being heated; (2) tracking echo signals scattered from many small tissue regions using a cross-correlation echo-tracking technique; (3) quantifying waveform change of echo signal scattered from each region by means of cross-correlation coefficient between the currently acquired signal and a reference signal; (4) using the spatial profile of the degree of the waveform change to represent the tissue coagulation status at different depths. The authors carried out 23 heating experiments on fresh canine liver samples using a Nd:YAG laser (1064 nm wavelength) at various light intensities (62 to 105 W/cm ²) and exposure times (20 to 350 s). A 13-mm-diameter 10-MHz broadband single-element spherical focused ultrasound transducer was used. The spatial profiles of the degree of coagulation damage in the heated tissues, as determined by our technique, qualitatively agreed with the grossly inspected results. They also appeared to be consistent with the experimental and theoretical findings in the literature on laser-tissue interaction. Moreover, we developed an automatic procedure to compute the coagulation depth using the spatial profiles of the waveform change. The authors used the result as an indirect but quantitative means for evaluating the technique. Good overall agreement with a root mean square (rms) difference of only 0.81 mm was obtained between the computed and visually inspected final coagulation depths for the 23 experiments.