A synthetic-focusing strategy for real-time high-frequency annular-array imaging

A single-pass approach of collecting all transmit-to-receive (T/R) echo data from an annular array was utilized and the data were synthetically focused (SF) to create an arbitrary number of focal zones. A spatial-impulse response model was implemented to simulate the acoustic field of a 5-ring, 38 MHz annular array with 12 mm geometric focus and 6 mm aperture. Acoustic field simulations included lateral beamwidth (LBW) and axial depth-of-field (DOF) for focusing over the 8 to 20 mm range. Full SF of all 25 T/R pairs was compared to fixed focusing and to cases with receive channels eliminated. Due to the reciprocal nature of the acoustic path, it was possible to recover some of the lost aperture by doubling the contribution of the reciprocal T/R pairs. The technique was applied to mouse embryo T/R data and as receive channels were dropped, SNR decreased and resolution degraded. Simulations and experiments showed that when using an SF beamforming approach based on individual T/R pairs, the outer receive channels of an annular array could be eliminated without drastically affecting LBW, DOF, or SNR. This permitted faster frame rates by reducing the amount of data that had to be beamformed.