Parallel transmit beamforming using orthogonal frequency division multiplexing for harmonic imaging

Nowadays real-time volumetric ultrasound imaging systems are used for medical diagnosis. To achieve the required data acquisition rate, the system relies on parallel beamforming, i.e. a single wide-angled beam is used for transmission, and several narrow parallel beams are used for reception. When applied to harmonic imaging, the demand for high-amplitude pressure wave fields, necessary to generate the harmonic components, clashes with the idea of using a wide-angled beam in transmission since this results in a large spatial decay of the acoustic pressure. To enhance the harmonics amplitude it is preferable to do the reverse; transmit several narrow parallel beams and use a wide-angled beam in reception. Here it is investigated whether this concept can be used for harmonic imaging. The method proposed in this paper relies on orthogonal frequency division multiplexing (OFDM), which is used to create distinctive parallel beams in transmission. To test the proposed method, a numerical study has been made, where the transmit, receive and combined beam profiles have been simulated up to the second harmonic component. Compared to standard parallel beamforming, application of the proposed technique results in a reduction of side-lobes amplitude and a gain of 23 dB for the main beam. Experimental verification in water has also been performed. The measurements confirm the feasibility of the proposed technique for a transducer with a realistic bandwidth.