# Practical Guide to Ultrasound Beam Forming: Beam Pattern and Image Reconstruction Analysis
## 4 Plane and Diverging Wave Beam Forming  
With these techniques, the emphasis is certainly more on improving the achievable frame rate. Plane wave imaging originates from studies aimed at imaging the transient propagation of shear  
Figure 3: This figure illustrates the effect of inter-beam interference on the image. A standard linear array beam forming image of four wires is shown on the top left corner. The same wires are then imaged with multi-line transmission (MLT) performed by frequency division multiplexing, and applied to linear array beam forming. The corresponding image is shown in the top right corner, and “ghost” wires are clearly visible before and after each wire. At the bottom, an image of a single wire obtained with MLT performed without frequency division multiplexing, and applied to phased array beam forming, is shown. Also in this case, ghost wires are visible, but at a different position relative to the actual wire. mechanical waves in real time [24; 25; 26]. For this type of application, a frame rate in the order of thousands of frames per second is needed. The basic concept is that if one can reduce the number of transmission events that are needed to form an image to the bare minimum, this implies maximizing the frame rate. The absolute minimum is of course one transmission event per image. In this case, the frame rate is, in essence, only limited by the speed of the ultrasound wave in the imaging medium, the depths that need to be visualized, and the processing time that is necessary to form the image. Thus, the idea is to approximate in transmission the generation of a plane wave, achieving, in this way, a very wide and homogeneous beam (as wide as the array aperture). This can be achieved by simply exciting all of the transducer elements with the same phase for each transmission event. Then, in the receive phase, the signals acquired by all of the elements are processed with different phase sets and amplitudes, and multiple lines are generated in parallel. In particular, all of the lines that form an image are generated with the echo signals, which are received after a single transmission. In simple words, this approach can be seen as a radical MLA approach where only one large beam is used in transmission. As in the case of MLA, this technique suffers from lower pressure amplitudes being generated compared to focused beams, thus affecting the SNR and penetration depths. Moreover, the straightforward approach of plane wave imaging suffers from low image quality in terms of spatial resolution and contrast [27]. In fact, all of the imaging features are sacrificed to maximize the temporal resolution. A way to balance the performance of plane wave imaging among the different imaging features is to compromise, or in other words, to apply image compounding. Compounding essentially means averaging. However, it is not a simple averaging of consecutive frames. With plane wave coherent compounding, steering is applied, and thus the \"plane wave\" is no longer propagating only straight in front of the transducer array, it is also propagating under a given angle [27]. An image is then formed for varying transmission angles, and in the end, averaging is performed over the images obtained with all of the different angles. In this way, the gain in frame rate is reduced by a factor that is equal to the number of angles. On the other hand, the other imaging features (spatial resolution, SNR, penetration depth, contrast) are improved. However, in order to obtain a performance that is comparable to standard beam forming, the amount of compounded angles is very high (in the order of 70), and the extreme gain in frame rate that is achievable with plane wave imaging is substantially lost [27; 28]. The number of angles, as well as the maximum steering angle, can be adjusted to tune plane wave imaging for a specific application.