An ultrasonic diagnostic imaging system has a two dimensional array arranged in multiple patches of multiple transducer elements. Each patch of transducer elements is coupled to a group of microbeamformer delay lines having outputs which can be coupled to a selected channel of a system beamformer, which beamforms the partially summed beams of each patch into a final beamformed signal. The ability to direct a delayed signal from a transducer element to a selected beamformer channel enables the patch boundaries to be changed as the aperture is translated across the array.