Experimental evaluation of targeting accuracy of an ultrasound-guided phased-array high-intensity focused ultrasound system

Bowl-shaped phased array therapeutic probes are becoming a standard module in magnetic resonance imaging-guided high-intensity focused ultrasound (MRgHIFU) systems because single beam focus can be electronically steered and multiple foci can be simultaneously synthesized. B-mode imaging provides only two dimensional image, therefore phased array HIFU probes are rarely used in ultrasound imaging-guided HIFU (USgHIFU) systems. An 144-element phased-array USgHIFU system was developed, and a B-mode probe was mounted in a rotating platform and harnessed to track the focus steered in three-dimensional volume. In this study, the targeting accuracy of this system was estimated experimentally using beef-embedded phantoms with four reference markers. The plane of treatment crossing the reference markers was reconstructed by acquired image sequence, and targeting area was predetermined as circular treated zone in the center of a square with corners in the middle of the markers. After sonication the contour of lesion was compared with the outline of planned targeting area. For three treated zones with a diameter of 5.8 mm, 9.7 mm, and 13.5 mm, the ratio of the thermal lesion area in the relevant treated zone to the planned targeting area was 89%, 92%, and 96%, respectively. The displacements between centers of the lesions and of the planned treated zones were no more than 1 mm. Moreover, the maximal crossing of borders outside the targeting area was lower than 2 mm. The targeting accuracy of this system was found to be around 1 mm being comparable to other image-guided HIFU systems. Therefore, the rotation-based 3D image reconstruction is effective for the targeting, treatment planning, and monitoring by using our system. Besides, such arrangement of treated zones is simple but useful in treatment planning.