Radiation force imaging for detection of irreversible changescaused by high intensity focused ultrasound therapy

Abstract

Therapy monitoring based on echo-time-shift imaging during high intensity focused ultrasound (HIFU) treatment was described. The echo shift estimated by radio frequency (RF) correlation between adjacent frames is potentially useful for mapping coagulation and tissue temperature. B-mode images are also useful for real-time monitoring, but cannot show the denatured region formed below the boiling point. Echo-shift images are, however, can. They are affected by temperature-dependant changes in the speed of sound, by thermal expansion of tissues, and by tissue expansion caused by irreversibly denatured protein and the radiation force generated by HIFU. To separate the effect of radiation force from other thermal changes, we used a split-focus technique with which the peak of ultrasonic intensity can be shifted from the peak tissue temperature. Tissue expansion was mapped with a split HIFU beam with large separation in an in vitro experiment. With a narrow HIFU beam, in contrast, the effect of radiation force exceeded that of expansion and focal tissue displacement away from the transducer was observed. Since the time course of tissue expansion did not follow that of the temperature change, it was suggested that echo-shift imaging could detect region in which coagulation occurred below the boiling temperature, which regions are could not be detected by B-mode imaging. Because tissue expansion could be also detected in an in vivo experiment, the detection of tissue expansion is useful for monitoring coagulation during thermal therapy. I. INTRODUCTION High intensity focused ultrasound (HIFU) therapy requires image guidance for targeting and monitoring the tissue to be treated. Tissue temperature changes occurring during thermal therapy can be mapped by detecting the shifts in echo location due to the temperature dependence of the speed of sound (1). These echo shifts are also affected by thermal expansion of tissues, by irreversible changes in tissue properties and by the radiation force generated by HIFU (3), and this echo-shift- based mapping is potentially useful for monitoring focused ultrasound treatment. One of the most important purposes of monitoring is the visualization of the profile of the HIFU- treated region. This profile cannot be determined from only the temperature because tissue coagulation depends not only on the temperature but also on the HIFU exposure time, tissue motions during the exposure make it is difficult to know the exposure time for each local part of the target region. Locating the boundary between treated and untreated regions help us decide whether the therapy can be ended or another exposure will be required. The location of boundary is easier determined by detecting coagulation detection than by mapping tissue temperature, because the profile of the temperature distribution is continuous and the boundary is not clear. In this study we estimated each component of the echo shift caused by several different tissue changes. To separate the effect of radiation force from other thermal or irreversible changes, we used a split-focus technique (4), with which the peak ultrasonic intensity could be shifted from the peak tissue temperature.