# Practical Guide to Ultrasound Beam Forming: Beam Pattern and Image Reconstruction Analysis
## 3 Multi-Line Acquisition and Multi-Line Transmission Beam Forming
In particular, MLT beam forming is better than MLA when applied to tissue harmonic imaging, because the higher pressure amplitude that is generated--thanks to a focused beam in transmission--is fundamental to boost the generation of sufficiently strong harmonic components. When applied to harmonic imaging, MLT beam forming provides a further reduction of the side-lobe amplitudes and an increase in SNR [16]. To generate multiple beams in transmission, different approaches are possible. One approach is to simply distribute multiple focused beams in the volume of interest. This is achieved by a linear superposition of the signals that are used to generate each individual beam. As a side effect, this limits the maximum signal strength that is applicable to the formation of every beam, and thus lowers the maximum pressure that can be generated by a single focused beam [17]. The MLT approach can be used both with linear array and phased array beam forming, and with 1D and 2D arrays [17; 18; 19]. To minimize the possible inter-beam interference generated by neighboring transmitted beams, specific sets of apodization functions can be applied [20]. Additionally, another approach to reduce the interbeam interference is to separate the different beams in the frequency domain. With this approach, which is referred to as frequency division multiplexing, the available transducer bandwidth is divided into orthogonal sub-bands, each of which is allocated to a beam. Multiple beams, as many as the number of sub-bands, can thus be transmitted in parallel, and the generated echo signals can then be identified in the receive phase by means of band-pass filters [21]. The main disadvantage of this method is the loss in axial resolution due to the subdivision of the available band into smaller sub-bands. A smaller sub-band implies, in fact, a longer pulse. In general, when implementing MLT, it is beneficial to add small time delays between the signals that are used to generate multiple beams in transmission. This improves their capability to separate the different beams. As a side effect, it lengthens the transmit phase, and thus increases the depth for which the final imaging system will be blind [17; 18]. With MLT, the inter-beam interference level required by the specific application of interest limits the number of parallel beams. In general, a higher number of parallel beams results in a higher level of interference [18; 19; 20]. Figure 3 illustrates the effect of inter-beam interference on the final image. An ultrasound image of four wire targets obtained by linear array beam forming is shown (top left corner). The same targets are then imaged using MLT applied to linear array beam forming and performed by frequency division multiplexing, with three beams in transmission (top right corner). As can be seen, four MLT \"ghost\" wires appear before and in front of each \"actual\" wire. This type of artifact is also present when MLT  
is performed by spatially distributing the transmitted beams over the volume of interest. However, the location of the artifact is different. To illustrate this phenomenon, a single wire is imaged using MLT without frequency division multiplexing with six beams in transmission (bottom). In this case, the image was obtained by MLT applied to phased array beam forming. Ghost wires are visible on the sides of the actual wire. As for MLA, and also with MLT, the higher data acquisition rate achieved by generating multiple beams in transmission is not solely applicable to improve the frame rate. For example, when implementing MLT by means of orthogonal frequency division multiplexing, a multi-focusing imaging approach can be realized where the different sub-bands are used to generate beams with a focus at different depths. In particular, the lower the center frequency of the sub-band, the deeper the focus will be. Thanks to this approach, the penetration depth and signal-to-noise ratio (SNR) improves without affecting the frame rate [22].