The use of broadband signals to reduce grating lobe effects in HIFU tissue ablation

In an array transducer, simple ultrasound theory states that the location of grating lobes depends on both transducer geometry and frequency. One can reduce these sidelobes by using wideband signals as either frequency modulated chirp or pseudo random coded signals. Such signals were evaluated for tissue ablation with HIFU. A system was built and tested. A simple equation for calculating the ratio of intensity between the first on-axis grating lobe and the main-lobe was evaluated. The equation shows that this ratio is inversely proportional to the bandwidth B of the transmitted signal and to the number of rings in the array. Computer simulations and experimental results clearly show the validity of this equation. Reduction of grating lobe levels without increase of annular array ring number was demonstrated. The greater is the bandwidth of the transmitted signal, the better is the reduction