In addition to the frequency, the focus position and focus number also have a major impact on the image. Frequency is also affected by presets. Smaller parts, pediatric and smaller abdominal and cardiac presets generally increase the frequency automatically and adult and larger abdominal and cardiac presets decrease the frequency automatically.
Focal Position and Number
The ultrasound beam has a focus position where the beams narrow to give a more detailed image at a certain depth. The beams do not converge, as we may think of light focusing on the retina, because they will again diverge beyond the focal position. The physics of this can be found in additional references (Nyland et al. 2002). Both the focus position and number of focal points can be set and adjusted by the sonographer. However, the processor can only handle a certain amount of information and by asking it to do more, it will reduce other items, normally the frame rate or how many times/second the image is refreshed. High frame rates make for a smooth image but take a lot of processing power. Low frame rates give a choppy image. Ask the processor to do more and it will respond by giving you a lower frame rate, a choppier image.
Pearl: For the POCUS and FAST scans, generally keep the focal point number at 1, and set the focal point's position at, or just deep to, the area of interest.
Presets, Abdominal, Cardiac, Small Parts, etc.
Even with just these four settings, that's still a lot of knobs to be adjusting in the emergent situation. Modern ultrasound machines have a collection of imaging settings which the user may select, based upon the area of interest, such as cardiac vs abdomen vs small parts and others, and patient size, adult versus pediatric. Trying to image an abdominal structure if the machine is on the small parts preset will lead to frustration until the sonographer realizes that the preset must be changed first to be able to adjust and manipulate the Big 4 knobs.
Alternate Imaging Tools
Up to now, we have been talking about B‐mode, or standard two‐dimensional, ultrasonography. A‐mode has no practical bearing on the emergency scans outlined in this book and therefore will not be discussed. However, M‐mode and color flow Doppler imaging are used in many of the POCUS and FAST protocols and during ultrasound‐guided procedures.
M‐Mode
The “M” in M‐mode stands for “motion.” This mode has also been called the “ice pick” mode because it reflects a small column of ultrasound waves but follows it over time. Cardiac ultrasound is where M‐mode is best known. It can be a little challenging to understand what is being displayed on the screen but using the B‐mode view to show just where that “ice pick” is cutting through is helpful. M‐mode is used not only for certain cardiac studies but also in certain lung and pleural space studies, vascular measurements for volume assessment, and fetal imaging. Importantly, M‐mode speed may be adjusted to slow, medium, and fast, and the M‐mode speed setting can significantly impact what you are attempting to interrogate, depending on the area of interest.
Color Flow Doppler
Color flow Doppler is used in combination with B‐mode ultrasonography. It allows you to see blood flow within a vessel and helps to determine the direction of that flow. Doppler is best when the flow is parallel with the sound beam. Color signatures are usually set up so that flow toward the probe is red – remember by “you are getting warmer” – and flow away from the probe is blue – remember by “you are getting colder” or that “blue” and “away” have the same number of letters – although this can be set on most machines to user preference. Color flow Doppler has its limitations, with low velocities and is also affected by patient movement, a phenomenon called “jumping” (see Chapter 25).
To maximize accurate image acquisition and proper interpretation, the sonographer should be familiar with settings that affect color flow Doppler (Pozniak et al. 1992). Briefly, the Big 3 are gain, pulse repetition frequency, and wall filter setting, and their effects are as follows.
Gain: the overall sensitivity to flow signals.
Pulse repetition frequency (also known as PRF, scale, or velocity scale): user‐defined setting. Use a lower PRF with low velocities; use a higher PRF with higher velocities to correct aliasing. If too high a PRF is selected, low velocities may not be recorded, and a vessel may appear as if thrombosed (no flow); conversely, too low a PRF will lead to overlap of the signal on the image.
Wall filter: removes unwanted low‐velocity Doppler signals; however, if set too high, it will remove important low‐velocity signals.
For more information, the reader is referred to more detailed texts (Nyland et al. 2002; Evans et al. 1989) and the machine's technical support team.
An alternate form of color flow Doppler, called power Doppler imaging (PDI), can be employed. Similar to color flow, this shows flow of fluid but at much lower velocities. The trade‐off is a lack of directionality. Blood flowing at 0.5 cm/sec away from the probe will have the same color signature as blood flowing at 0.5 cm/sec toward the probe. Consider using PDI for small vessels in POCUS studies such as the optic artery and vein
Pearl: Color signatures are usually set up so that flow toward the probe is red and flow away from the probe is blue – remember “red is getting warmer and toward you” and “blue is away from you and getting colder” or that “away” and “blue” have the same number of letters.
Pearl: Color flow Doppler is optimized by understanding its settings and their effect on the image: gain, pulse repetition frequency, and wall filter setting.
On the Horizon
Single Crystal Probes
Single crystal probes emit a large bandwidth of sound beams instead of just one, thereby combining the benefits of high‐frequency resolution and low‐frequency penetration. The learning curve for imaging is generally quite different from that of traditional multicrystal ultrasound probes.
Smartphone Applications
At the time of this writing, there are several smartphone‐powered ultrasound devices approved by the Federal Drug Administration. Technology is advancing quickly and ultrasound machines may now be carried around in the healthcare provider's pocket. One must wonder what the future holds for the ease and availability of ultrasound imaging.
Documentation of POCUS and FAST Ultrasound Examination Findings
Save and label the images. A medical record is not complete with just a written description of an image, whether that is a radiograph, an ultrasound image, computed tomography (CT), or magnetic resonance imaging (MRI). The image must be there to back up the interpretation. Furthermore, the other imaging modalities contain information to indicate exactly where an image was obtained. For example, the radiograph has anatomical landmarks and for both CT and MRI, there is a pilot image that records where all the remaining images are obtained. For ultrasound images, there may not be any definitive markers.
An ultrasound
