Real-time monitoring of the transients of HIFU-induced lesions

RF data from standard B-mode and pulse inversion (PI) imaging of HIFU lesion formation of freshly excised tissue was collected before, during, and after lesion formation experiments in ex vivo tissue. Exposures at intensity levels of 1100 W/cm ² to 2500 W/cm ² for durations of 2, 3, and 5 seconds in a single shot were used. Also continuous raster scan of longer duration (10-20 seconds) to form slice or volumetric lesions were monitored. Monitoring was done with a diagnostic scanner and RF data was acquired at 1 frame/second for 60 seconds starting 5 seconds before each shot. Lesion maps from grayscale B-mode and PI images were obtained using level-set methods for each frame and compared with the actual lesion found by inspection. Lesion maps from PI imaging were consistently smaller in size and more in line with the actual lesion size. Transient analysis of harmonic content of lesion echoes show sustained harmonic activity for 10-15 seconds after the therapy pulse is turned off (in ex vivo liver tissue). A gradual drop in this activity follows with steady state reached within 50-60 seconds. It was also shown that the use of short microsecond pulses from the therapy transducer to expose the lesion location during real-time imaging significantly increased the scattering from lesion location.