An ultrasonic annular array based on quadrature sampling

An ultrasound pulse echo system is described which uses an annular array to achieve high resolution and beam geometry control over an extended depth. The continuous wave point spread functions of arbitrary individual elements were derived by expanding the Fresnel integral about an average radial coordinate. The extra term that is retained in the expansion gives the delays required for focusing. The general equations are solved for the case of an annular array, and the results are used in a computer model to predict the point spread function for any annular array geometry given the phasing between elements and their relative intensities. A system which uses the direct RF quadrature sampling technique to provide dynamic focusing for an annular array is described. Results are presented to demonstrate the utility of the sampling process. Several theoretical point spread functions are presented for comparison with the measured results. Doppler beam plots and spectral recording demonstrating the ability of the sampled delay lines to obtain physiological Doppler information are included.