Acoustic Field of Phased-Array Ultrasound Transducer with the Focus/Foci Shifting

Abstract

Background

High-intensity focused ultrasound (HIFU) is becoming popular in the treatment of solid tumors because of its non-invasiveness with few complications. The acoustic field is of importance in evaluating the safe focus shifting distance and determining the treatment plan.

Methods

The propagation of finite-amplitude acoustic wave from a 331-element HIFU phased-array with focus steering along and transverse to the transducer axis and 4-foci shifting on the focal plane was simulated using the angular spectrum approach (ASA) with a marching second-order operator-splitting scheme. In addition, the acoustic field produced by a truncated asymmetric transesophageal HIFU annular array was also simulated, and the effects of driving frequency and the number of concentric rings were investigated.

Results

Because of the nonlinear effects, the peak negative pressure is lower than that of peak positive pressure at the main lobe but has a larger beam size. However, the peak positive and negative pressures at the grating lobe are quite similar. The effects of the focus shifting distances on the main and grating lobe (both acoustic pressure and − 6 dB beam size) were evaluated. With the focus shifting axially away from the transducer surface, the main lobe has decreased acoustic pressure by ~ 1.9 fold and increased beam size by ~ 4.5 fold while the grating lobe has the increased acoustic pressure by ~ 1.8 fold. The focus shifting laterally leads to the reduced pressure at the main lobe by ~ 1.4 fold but the slight decrease at the grating lobe by ~ 1.1 fold. In comparison, the shifting of multi-foci has similar influences on the main lobe but increases the pressure at the grating lobe. Driving frequency of annular array is found to have greater influences on the peak pressure and beam size.

Conclusion

Our algorithm can simulate the acoustic field of phased-array in arbitrary shape and optimize the transducer design, and the focus shifting distance and strategy should be selected appropriately for the safe HIFU exposure.

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