Automatic ultrasound determination of thermal coagulation front during laser tissue heating

Thermal therapies using laser, microwaves, radio frequency radiation, and high intensity focused ultrasound have shown great promise for minimally invasive treatment of benign and malignant lesions. To treat tissue effectively and safely, techniques that could monitor the advancement of coagulation front during treatment are highly desirable. This paper presents a noninvasive ultrasound technique for automatically determining the propagation of coagulation damage front during laser tissue heating. The basic assumption underlying this technique is that when coagulation is taking place in a tissue, owing to thermally induced structure changes in tissue, the waveform of echo signal scattered from that treated region should be changing accordingly. We first track echoes scattered from many small tissue regions during heating using a cross-correlation echo-tracking technique. We then use the waveform-change information to determine the position of coagulation front via an automatic calculation procedure. To test our technique, we carried out 35 experiments in which we irradiated fresh canine liver samples with a Nd:YAG laser (1064-nm wavelength) at various light fluence (62 to 167 W/cm²) and exposure time (20 to 350 s). A 13-mm diameter 10-MHz broadband single-element spherical focused ultrasound transducer was used to detect the thermal coagulation front. The root mean square difference between ultrasonically and visually determined coagulation depths was 0.77 mm. This good agreement between visually inspected and ultrasonically determined coagulation depths shows the potential of our technique for monitoring coagulative tissue damage during thermal therapy.