Numerical study on sector-vortex phased irradiation method using annular array transducer in High-Intensity Focused Ultrasound treatment.

Sector-vortex phased irradiation from annular array transducer was numerically studied with breast model constructed from MRI data of real patient. Phase compensation (PC) based on time reversal pre-computation was applied in order to handle phase delay caused by heterogeneity of breast tissues, and results showed great effectiveness on single-focus case, insignificant effectiveness on multi-focus cases with 4 and 8 phase-sectors, but ineffectiveness on multi-focus case with 12 phase-sectors, where enormous undesired outer ablation occurred. For single-focus case, phase compensation not only produced real focus very close to targeted site (0.1 mm deviation), but also decreased thermal peak ratio (outer/focal) largely by 30%. However, phase compensation did not increase total ablated size. For multi-focus cases with 4 and 8 phase-sectors, deformed focal shapes by tissue heterogeneity were restored by phase compensation, but the 4-phase-sector case had higher thermal peak ratio and smaller ablation than 8-phase-sector case for strong cancelling effect between phase-sector borders. Ineffectiveness of phase compensation on multi-focus case with 12 phase-sectors had three considerable reasons. 1st, inequality of piezo-element number between sectors; 2nd, heterogeneous attenuation of breast model; 3rd, insufficient number of piezo-elements per sector; where the 2nd reason originated from breast model, and other two reasons were related to array transducer. This research gave several preliminary indications. 1st, ineffectiveness of phase compensation occurs on case with large phase-sector number when using annular array transducer; 2nd, with same input energy and same irradiation time, sector-vortex phased irradiation creates smaller focal ablation, but withstands longer than single-focus irradiation free of outer ablation; 3rd, phase-difference π between neighboring phase-sectors is disadvantageous because of energy loss; 4th, phase compensation is effective on single-focus for improving pinpoint ablation but not for increasing total ablated size.