In dual line ultrasound beamforming, an ideal transmit beam should have two peak locations, each following one of the receive beams. Dual beam steering by apodization creates such transmit beam; however, the resulting beam is optimized for only a single depth and significant line warping can occur away from that depth. The transmit beam shape in ultrasound multiline acquisition however, is improved by numerically optimizing the transmit beam, i.e., the aperture function, such that no line warping occurs over the entire depth of field. The aperture function is numerically optimized by numerically optimizing a Cost function which is dependent on the aperture function. The numerically optimized aperture function is then used in a multiline acquisition mode of an ultrasound imaging system.