IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, . 60, . 11, NOVEMBER 2013 2310 0885–3010/$25.00 © 2013 IEEE Implementation of Parallel Transmit Beamforming Using Orthogonal Frequency Division Multiplexing—Achievable Resolution and Interbeam Interference Libertario Demi, Jacopo Viti, Lieneke Kusters, Francesco Guidi, Piero Tortoli, and Massimo Mischi Abstract—The speed of sound in the human body limits the achievable data acquisition rate of pulsed ultrasound scanners. To overcome this limitation, parallel beamforming techniques are used in ultrasound 2-D and 3-D imaging systems. Different parallel beamforming approaches have been proposed. They may be grouped into two major categories: parallel beamform- ing in reception and parallel beamforming in transmission. The first category is not optimal for harmonic imaging; the second category may be more easily applied to harmonic imaging. However, inter-beam interference represents an issue. To over- come these shortcomings and exploit the benefit of combining harmonic imaging and high data acquisition rate, a new ap- proach has been recently presented which relies on orthogonal frequency division multiplexing (OFDM) to perform parallel beamforming in transmission. In this paper, parallel transmit beamforming using OFDM is implemented for the first time on an ultrasound scanner. An advanced open platform for ultra- sound research is used to investigate the axial resolution and interbeam interference achievable with parallel transmit beam- forming using OFDM. Both fundamental and second-harmonic imaging modalities have been considered. Results show that, for fundamental imaging, axial resolution in the order of 2 mm can be achieved in combination with interbeam interference in the order of -30 dB. For second-harmonic imaging, axial reso- lution in the order of 1 mm can be achieved in combination with interbeam interference in the order of -35 dB. I. I A wide range of parallel beamforming techniques cur- rently exist which have been developed to generate ultrasound images at a high data acquisition rate. A high data acquisition rate can be used to increase the frame rate, to increase the field of view, to produce independent images that later can be averaged to reduce noise, or to reduce the scanning time. Three-dimensional ultrasound imaging is an application which requires the high data acquisition rate enabled by parallel beamforming tech- niques [1], [2]. Different parallel beamforming approaches are proposed and reported in the literature. They may be grouped into two major categories: parallel beamforming in reception and parallel beamforming in transmission. The techniques which belong to the first category [3]– [7] employ a wide beam or a plane wave in transmission to insonify a large volume and, when receiving, multiple parallel lines are acquired by means of narrow receiver beams. As a consequence, they are not optimal to perform harmonic imaging. In fact, the utilization of a wide beam or plane wave in transmission results in the generation of lower absolute pressure values, when compared with focused ultrasound beams, and conflicts with the demand for high-amplitude pressure wave fields, which are neces- sary to generate the harmonic components. On the other hand, the techniques which belong to the second category are more suitable for harmonic imaging, because narrow parallel beams may be used in transmis- sion and focusing may be employed to increase the am- plitude of the generated pressure wave fields. Hence, the benefit of combining harmonic imaging and a high data acquisition rate may be exploited for imaging. Harmon- ic imaging is becoming the standard in pulse–echo ap- plications because it both improves the image resolution and reduces the effects of clutter and side and grating lobes [8]–[12] with respect to fundamental imaging. Nev- ertheless, in case parallel beamforming is performed in transmission, interbeam interference represents an issue. A possible solution to this problem may come from dif- ferent combinations of transmit and receive apodizations [13] or beam transformation techniques [14]. Recently, an alternative solution has been presented. This novel ap- proach relies on orthogonal frequency division multiplex- ing (OFDM) [15], [16] to perform parallel beamforming in transmission. Frequency division multiplexing is not new to ultrasound applications [17]. With this technique, mul- tiple beams may be generated by allocating to each beam a portion of the available bandwidth in transmission. Fur- thermore, each beam may be independently steered in a specific direction. The pressure wave fields generated in this fashion propagate in each specific steered direction and next, appropriate coherent demodulation and applica- tion of low-pass filters in reception is used to discriminate between echoes coming from the different directions of ob- servation. Other than OFDM, alternative methods exist which can be used as simultaneous transmit methods, i.e., methods taking advantage of coded excitation and chirp signals [18]–[21]. Manuscript received May 13, 2013; accepted August 7, 2013. This paper has been partially supported by the Italian Ministry of Education, University, and Research (PRIN 2010-2011). L. Demi, L. Kusters, and M. Mischi are with the Laboratory of Bio- medical Diagnostics, Eindhoven University of Technology, Eindhoven, the Netherlands (e-mail: l.demi@tue.nl). J. Viti, F. Guidi, and P. Tortoli are with the Laboratory of Microelec- tronics Systems Design, Università degli Studi di Firenze, Florence, Italy. DOI http://dx.doi.org/10.1109/TUFFC.2013.2828