Dynamic-transmit focusing using time-dependent focal zone and center frequency.

A method for achieving dynamic-transmit focus is presented. Within an initial high bandwidth pulse, successively higher frequencies are focused to successively closer focal zones along a transmitted beam line. During the receive operation, a time varying bandpass filter initially passes the higher frequencies that are focused closer in and successively passes lower center frequencies as time evolves and the focal zone moves out. The dynamically focused receive pulses are digitally sampled and processed by a matched digital filter to minimize phase anomalies. In this way, an improved resolution image is obtained with no loss of frame rate. The method is evaluated using comprehensive simulations that account for realistic levels of phase aberration and tissue attenuation. The method is relatively robust with respect to these perturbations. When the appropriate conditions apply, the new method can achieve an improvement in mean lateral resolution similar to that found in a multiple transmit zone implementation but without the frame rate penalty. A discussion of implementation considerations and limitations is presented.