After interface layer was simulated by the magnetic nano-particles in the egg white phantom, high intensity focused ultrasound (HIFU) at the same dosage was introduced to radiate the phantom in different depths to blow the acoustic interface layer to mimic "point" exposure. The results showed that the volumes of biological focal region (BFR) were enlarged when the acoustic focal region (AFR) is close with interface layer. This meant that the magnetic nano-particles enhanced the therapeutic efficiency of HIFU. When the distance of the AFR from the interface layer was 10 mm, the size and shape of the BFR were similar with those of the control group, but a larger lesion at the interface, which was harmful for treatment, was observed. When the distance of the AFR to the interface layer increased to 30 mm, the size and shape of the BFR were also similar to those of the control group. When the thickness of the interface layer diminished, the utility of enhancement decreased. Continuous increase of the safe area for treatment and decrease of the utility of enhancement were observed along with the abatement of the thickness of the interface
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