# Practical Guide to Ultrasound Beam Forming: Beam Pattern and Image Reconstruction Analysis
## 3 Multi-Line Acquisition and Multi-Line Transmission Beam Forming  
As briefly introduced above, the beam does not need to be the same in the transmit and receive phase. This is certainly the case with multi-line acquisition (MLA) beam forming. The basic idea behind this approach is to transmit a wide beam, so that a large area is covered, and then make use in receive of multiple, narrower beams, in order to form several A-scans along different directions for each transmission event. In this way, multiple lines are formed in parallel, thus increasing the frame rate and improving the temporal resolution. The receive phase is in fact defined by how the different signals received by all of the array elements are combined to form a line of the image. Therefore, it is possible to apply different phase sets and apodization masks to the signals received after a single transmission event, thus allowing the formation of multiple lines in parallel. In fact, these techniques are also referred to as parallel receive beam forming. In Equation (10), we can see that a wider beam can be achieved by using, without focusing, a small sub-aperture at the center of the array during transmission [9; 12; 13; 14]. Since not all of the elements are utilized, and as a result the active aperture is reduced in transmit, the maximum pressure generated is consequently lower compared to the case where all of the elements are used. Furthermore, the spatial resolution (although not in the axial direction) is not as good, as focusing is applied only in the receive phase. Not only can MLA be applied to achieve a gain in the frame rate, it can also be applied to improve the SNR and contrast by simply averaging consecutive images obtained at a higher temporal resolution than with standard beam forming (i.e., techniques where only one line is generated per each transmission event). Figure 2: This figure shows the different spatial distribution of pressure amplitudes when unfocused, focused, and steered beams are generated. Moreover, the typical field of view (FOV) that is achievable with linear and phased array beam forming is also shown. These beam profiles were generated using the software package k-Wave [11]. Moreover, MLA techniques can also be used to image a larger FOV. In this case, the gain in acquisition rate is used to widen the area covered by the imaging system. To summarize with a simple example, in case 4, image lines could be formed in parallel, which means that: (a) the temporal resolution could be improved by factor of four, or (b) four consecutive images could be averaged to improve the SNR, or (c) a FOV that is four times larger could be in principle imaged, or (d) a combination of these gains could be achieved by spending the higher data acquisition rate in the most desirable way (e.g., averaging only two consecutive images and thus improving the SNR while still improving also the frame rate by factor of two). A similar concept could be also applied at inverted phases: instead of having parallel lines being formed in the receive phase, they could be generated during transmission. This approach is referred to as multi-line transmission (MLT) or parallel transmit beam forming. Even in the case where the very same phase sets and apodization functions are used, and are simply swapped between the transmit phase and the receive phase, advantages can already be obtained. This is the case for tissue harmonic imaging applications. Unlike standard (fundamental) ultrasound imaging, this modality makes use of the harmonic components that are generated during ultrasound propagation, and not the pressure fields directly emitted by the array, to form an image. The harmonic components represent a part of the pressure wave fields, which is located around multiples of the transmitted center frequency. For a pulse-echo imaging system, an improved spatial resolution, a reduction of reverberation, grating, and side-lobe artifacts [15] are among the advantages of utilizing tissue harmonic imaging.