Reduction of the grating lobes of annular arrays used in focused ultrasound surgery

An annular array with variable focus, used in focused ultrasound surgery, generates grating lobes responsible for undesirable lesions. It is known that the amplitude and the location of the lobes depend on both the geometry and the frequency of the transducer. A new procedure based on the use of a wide-band CW signal, i.e., a signal phase modulated by a pseudorandom code, is proposed to reduce the amplitude of these lobes. The theoretical study enables us to determine the location and the amplitude of these lobes and to simulate the effect of the transmitted signal bandwidth. In particular, a simple analytical relation gives the intensity ratio between the grating lobes and the main lobe. This equation shows that this ratio is inversely proportional to the number of rings and to the bandwidth of the transmitted signal. A system was developed and tested with two transducer arrays of 35- and 150-mm diameter, respectively. The simulations and experiments demonstrate the validity of the theoretical study and the efficacy of the proposed procedure. In conclusion, it is possible to reduce the grating lobes without geometric modification of the array by increasing the bandwidth of the transmitted signal.