In the case of diagnostic imaging it increases the

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In the case of diagnostic imaging, it increases the brightness but does not increase the number of pixels per image. A knob or up/down button on the control panel allows the operator to adjust gain. The gain function has no effect on the acoustic power. Time gain compensation (TGC) controls on an ultrasound machine allow the operator to adjust the gain at varying depths ( Figure 1.19 ). Echoes returning from deeper structures are more attenuated simply because they have to travel through more tissue. Without TGC, the far field (bottom of the screen, deeper tissue) would always appear darker than the near field (top of the screen, tissue Figure 1.19 Time gain compensation (TGC) may be controlled with sliders ( left ) or knobs ( right ), and machines vary in the number of regions which may be controlled individually. Figure 1.18 Gain. Increasing gain from the left to right panels. Fundamentals 13 Fundamentals
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closest to probe). TGC can boost the gain on the echoes returning from the far fields. Some machines have one button that allows you to adjust the near field relative to the far field. Other machines have multiple slider levers that allow gain control throughout the entire scanning depth. Figure 1.20 demon- strates the impact of near- and far-field gain on image quality. Scanning modes There are a variety of imaging modalities used in diagnostic ultrasound. A , or amplitude , mode is an imaging modality largely of historical interest, although it is used in ophthalmologic applications today ( Figure 1.21 ). It uses an oscilloscope display for returning amplitude information on the vertical axis and the reflector distance information on the horizontal axis. There is no picture; distance and amplitude are represented by a graph. In Figure 1.21 the vertical axis A represents the amplitude of the signal returned to the transducer, and the depth D is calculated based on the round-trip time of the ultrasound beam signal. B , or brightness , mode is the modality which has been described up to this point; it is what we use for diagnostic imaging. B-mode scanning con- verts these amplitude waveforms into an image by using the gray-scale converter discussed previously. Most scanners now display images with up to 256 shades of gray, allowing for visualization of subtle differences within tissues/structures. As mentioned, the gray-scale assignment of each pixel is based on the signal amplitude or strength of the returning wave from a given point. M , or motion , mode plots a waveform that depicts the motion of the tissue/ structure of interest relative to the transducer’s image plane (line through the Figure 1.21 A-mode. Figure 1.20 Near-field gain too high ( left ), far-field gain too high ( right ), and a well-gained image in the center. 14 Fundamentals Fundamentals
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structure) on the vertical axis, and time on the horizontal axis ( Figure 1.22 ).
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