HIFU beam visualization using beam profile extraction from tissue speckle pattern

HIFU Beam Imaging (HBI) is used to visualize High Intensity Focused Ultrasound (HIFU) beam intensity profiles. Our group previously proposed a novel HBI method to minimize the risk of bio-effect during the beam visualization process. In this method, a HIFU transducer irradiates a microsecond pulsed HIFU beam and an imaging probe detects the scattered HIFU beam signal. After scanning the scattered beam, an intensity profile of HIFU beam is visualized. Echo intensity of HBI at the sampling point reflects both, beam intensity and backward-scattering coefficient. In this study, an extraction method of HIFU beam profile from the background tissue pattern is proposed and its performance is estimated in experiments. The main concept of this method is that a HIFU beam is electronically scanned to discriminate a moving HIFU beam pattern from a fixed inhomogeneous scattering patterns in the focus area. In the experiment, the HIFU frequency was set to 2 MHz with a pulse duration of 2 cycles and a HIFU acoustic intensity of 2 kW/cm². We used agar phantom mixed with powdery graphite as a tissue sample. In the case of a non-extracted beam profile, the prominent artifact of echo was observed at the boundary between high and low density scattering areas of the powdery graphite. On the other hand, in the case of the extracted beam profile, several artifacts in the profile were successfully suppressed and the beam profile could easily be recognized. This result clearly demonstrates that the proposed method can be used as a high accuracy HBI method.