An anti-focus is defined as the point where the pressure amplitude is zero. As an active protection means in focused ultrasound, the anti-focus was researched through simulations. A 513 element, 1 MHz, 15 cm focal depth, spherical shape transducer with a central hold was used for the simulation. An anti-focus was implemented with two different approaches and the simulation results were compared. First, the interference among foci was compared to the summation of the individual focus fields. Second, the interference among anti-foci was compared to the multiplication of the individual anti-focus fields. The minimum distance between a focus and an anti-focus was determined by the comparison between intended control values and the simulation results. The field simulation results indicate that the minimum distance to control the field is approximately 3 mm in the lateral direction and 15 mm in the axial direction. The results also demonstrate that a specific zone can be effectively protected with the addition of anti-foci and that the field can be effectively implemented by switching type amplifier systems. Results demonstrate the potential for anti-foci to significantly reduce the collateral damage in focused ultrasound through improved field control.
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