Annular array beamforming for 2D arrays with reduced system channels

Annular arrays can operate with a small number of channels because the shape of the physical elements matches the acoustic phase fronts. The symmetry of the annular array provides axisymmetric focusing without the need for additional beamforming channels. An annular array structure can be constructed by connecting elements of a 2D array and therefore a similar reduction in the number of delay channels required can be achieved [1,2]. We present simulation results for annular array apertures with a diameter of 20-26 mm and design frequency in the 5-8 MHz range. The simulations are used to determine the required number of annular rings (around 20), the effect of the size of the underlying 2D array elements (should be <200 um), and the optimal ring widths for a particular design with 20 rings (325 um). Simulations of a 20 ring design with a 26 mm aperture show that the -6dB beamwidths are less than 0.5 mm over a depth of field from I to 5 cm with a single transmit focus. The azimuthal and elevation beam widths are, as expected, equivalent. Experimental measurements of the field at the focal point show good agreement with the model.