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The position of the focus cursor can have profound effects on image quality (Figure 5.12). Most sonographers are familiar with gain, and its effect on making the image brighter or darker; and frequency, lower for better imaging of the larger patient and higher for more detail and imaging the smaller patient. However, the focus cursor is often forgotten. It is usually an arrow or an icon that moves along the centimeter scale. It can be increased from one to two or three or more on some machines, but generally it’s best to have only one, a single focus cursor, on your screen. Think of it as when having more focus cursors than just one, you are dividing up the energy in your ultrasound beam and the image will not be as high quality as a result.

The focus cursor should be directly across from the area or region of interest. A common mistake is moving the focus cursor to the “lung line” during Vet BLUE and then leaving the cursor in the very near‐field when you then image deeper thoracic, like heart, and abdominal structures. The deeper structures will often not look good no matter what you do to the gain and frequency until you move the focus cursor back to the center of the screen. Conversely, you may have the focus cursor in the center of the screen while imaging abdominal and thoracic structures that may be less than optimum for lung, in which the focus cursor placed in the near‐field across from the “lung line” could make a big difference in image quality (see Chapters 22 and 23). In some machines, the focus cursor actually moves on its own as depth settings are changed, and it may move to a less than optimal level within your image, so learn what your machine does to the focus cursor when you change the depth.

Figure 5.12. Focus cursor and depth. In (A) there are four focus cursors (upper right circle) and in (B) a single focus cursor (upper right circle). The best image is usually obtained by having a single focus cursor directly across from the area of interest so generally in the middle of the screen, unless imaging the lung in which a position directly across from the "lung line" is best. The number of focus cursors on this machine is also noted at the bottom of the screen (circled).

Source: Courtesy of Dr Gregory Lisciandro, Hill Country Veterinary Specialists and FASTVet.com, Spicewood, TX.