Point-of-Care Ultrasound Techniques for the Small Animal Practitioner. Группа авторов

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.

       Not Paying Attention to the Centimeter Scale

Every machine has a centimeter (cm) scale (Figure 5.13). Be familiar with where the maximum depth is on the screen, and how the marks are set up – as one hash mark every centimeter or are the hash marks every 5 mm or even every 2.5 mm? This will help you to locate the area of interest; for example, starting the FAST DH (subxiphoid) view at a depth of 5 cm even in medium‐sized dogs is too superficial (shallow or magnified) and won't properly image the diaphragm and gallbladder. The wise sonographer knows to start with a depth between 9 and 13 cm, get a feel for the four DH targets – diaphragm, gallbladder, cardiac bump, and caudal vena cava – and then magnify (decrease depth) once the targeted structures are identified from afar (see Chapters 6 and 7). On the other hand, lung is imaged at more shallow depths between 4 and 8 cm for smaller and larger patients, respectively.

Figure 5.13. Depth setting and the centimeter scale. The image is the same as Figure 5.12B with the focus cursor near center. The image has the centimeter scale to the right of the screen and then added numbers by the author for the depth of 12 cm. See how the hash marks correlate with 1 cm increments. Every ultrasound screen has a centimeter scale. Individual hash marks are often 1 cm (10 mm) on abdominal presets but can be every 0.5 cm (5 mm) or less. Find your cm scale and be aware of its depth scale. The screen itself has the depth (cm) and frequency (5 MHz), circled upper left and also at the screen's bottom along with number of focus cursors (1), plus other parameters noted (including the preset). Take the time to look over all the information provided on your machine's screen.

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

       Not Using the Centimeter Scale

What looks really large on ultrasound may in fact be quite small. This is especially true with pleural effusion that looks like a lot on ultrasound but the maximum dimension is only 2 cm (or less than an inch), and merely a fissure line on the thoracic radiograph (TXR) or is missed altogether by TXR. As for masses, the seemingly large splenic mass that deforms the spleen, a serious finding, is really only 3 × 3 × 3 cm (or approximately 1¼ × 1¼ × 1¼ inches), easily missed by palpation during your physical exam and on abdominal radiography; and the lung lesion that appears so obvious is in fact a mere few millimeters (<1/3 inch) being missed or occult by thoracic radiography (and possibly CT) (Figure 5.14). Don't get me wrong, these findings may have a significant clinical bearing on your case, but a small pocket of pleural effusion in a stable case requires a calm and collected mind for decision making, and not overresponding.

Pearls: Trouble Shooting a Poor Image

      See the trouble‐shooting algorithm (Figure 5.15).

       Check your frequency. In small dogs and cats, you can get away with higher frequencies such as 7–9 MHz; however, in large dogs (and other patients), the image will not be optimized until you lower the frequency to the 4–6 MHz range. You may have to change your preset, for example from small abdomen to medium or large abdomen, to change your frequency on some machines. Get used to looking at the frequency setting when starting to image the patient (see Figure 5.13).

       Check your gain. Generally, the time gain compensation (TGC) should be a gentle curve from the near‐field to the far‐field (Figures 5.16 and 5.17). Then, only the overall gain is used for the rest of the Global FAST ultrasound examination. However, using the TGC sliders is another strategy, depending on what level of the image needs to be brighter or darker. A common mistake when moving from AFAST and TFAST to Vet BLUE is having too much gain, in other words too bright in the near‐field for lung because you needed more gain for soft tissue than lung surface imaging. Thus, the “lung line” is now too bright, lacking the contrast you need for Vet BLUE. Conversely, moving from Vet BLUE to AFAST or TFAST, you are now too dark (undergained) because the “lung line” imaging did not need as much gain as the soft tissue of abdominal and cardiac structures.

       Check your depth. Depending on the area of interest and the size of your patient, paying attention to the depth will help you to get where you want to go more quickly (see Figure 5.13). Generally, it’s best to start with too much depth to gain an overview of what you are imaging, and to survey for recognizable landmarks prior to magnifying (decreasing the gain). And time is valuable on many fronts, including the patience of your patient and its tolerance of your physical and chemical restraint.Figure 5.14. Splenic mass and estimating size from the centimeter scale. These images were captured during a POCUS spleen following AFAST. In (A) is the spleen (*) and arrows showing its borders. The spleen is sonographically recognized by its hyperechoic (bright white) capsule, and its splenic vessels splitting the capsule (not shown). In (B) the mass detected is eyeballed at approximately 4 × 4 cm using the centimeter scale. The mass appears large when in fact it is quite small. The finding is important and concerning because the mass deforms the capsule of the spleen.Source: Courtesy of Dr Gregory Lisciandro, Hill Country Veterinary Specialists and FASTVet.com, Spicewood, TX.Figure 5.15. An algorithm for trouble shooting image acquisition with some clinical pearls and descriptors.Source: Courtesy of Dr Gregory Lisciandro, Hill Country Veterinary Specialists and FASTVet.com, Spicewood, TX.Figure 5.16. Time gain compensation (sliders) and the overall gain (wheel). Shown in (A) is an ultrasound machine with its keyboard and screen. The time gain compensation (TGC) is in the upper right on this keyboard. These TGC sliders can be intimidating but gain adjusts your gray scale when in B‐mode. The sliders represent the different levels from the near‐field to the far‐field. In general, they should look like the circled upper two images in (B) and not like the lower two images (marked with an "X"). In (C) is the overall gain (circled wheel with arrow pointing to it) that may be turned clockwise and counterclockwise that alters the overall picture. If your sliders are adjusted as in (B) then you should mostly only be using the overall gain thereafter. If the image still doesn't look good then go through the trouble‐shooting algorithm (see Figure 5.15).Source: Courtesy of Dr Gregory Lisciandro, Hill Country Veterinary Specialists and FASTVet.com, Spicewood, TX.

       Check the location and numbers of your focus cursor(s). Generally, a single focus cursor is best that is centered on the screen or directly across your area of interest.

       Check your probe–skin contact (coupling). Is the probe head directly on skin? Do you have ample acoustic coupling medium?

       Check your probe hand placement and beam direction. Make