All of the posts of the series Learning Ultrasound are excerpts from the book Point-Of-Care Ultrasound for Emergency Physicians — “The EDE Book”. I purchased the e book two years ago. The e book is only available on Apple devices. The e book is only about $15 and it is simply the best there is on learning to perform point of care ultrasound (POCUS). You need to buy it now.
This is the second in a series of posts to help all primary care clinicians quickly develop skill in primary care ultrasound [also called Point of Care Ultrasound (POCUS)].
The most important skill in primary care ultrasound is the ability to obtain high quality diagnostic images. As soon as you can do that you can easily transmit your studies over the internet to get immediate expert interpretation if you need it.
The very best resource for learning how to get reliably ultrasound scan images is the book, Point-Of-Care Ultrasound for Emergency Physicians — “The EDE Book”; “The clearest and most concise approach to emergency ultrasound.”
I have posted these Learning Ultrasound series, temporarily, for a friend who does not currently have access to an Apple device.
All the illustrations below are from the above book. Text that is also in quotes is also from the book [indented and in italics].
In this post we’ll go over the next steps in learning to scan. And first we’re going to talk about moving the probe around on the patient.
Sweeping the probe means moving the beam without moving the position of the probe. You are tilting the angle of the handle but not moving the probe on the skin (also called tilting or fanning the probe).
Rotating the probe means keeping the central axis of the probe (and the beam) unchanged as you rotate the probe through arc of a circle. An example which we will get to later is when you rotate the probe approximately 90 degrees to go from (in cardiac imaging) the parasternal long axis view to the parasternal short axis view.
Heeling the probe or heel-toeing the probe is rocking the probe back and forth along its length while keeping it at the same location on the skin.
Now we are going to go over the different probes you will be using in your various scans. But the workhorse is the curved array probe that you will be using for your abdominal scans. It has a frequency range of 2 – 5 MHz which allows for good depth penetration.
And here’s what the screen looks like from a curved array probe:
The phased array probe is a low frequency probe that is shaped for thoracic echocardiographic scanning. It can, however, also be used in the abdomen for examination of the gallbladder and the Inferior Vena Cava.
And this is what the screen looks like when you are using the phased array probe:
Here is the difference between the curved array probe and the phased array probe:
Next is the linear array probe that is used for scanning superficial structures like the carotid artery, for example (although the carotid scan is not typically a primary care ultrasound scan). It is used for the venous exam in the lower extremities.
Here is the screen for the linear array probe:
Here are two different linear array probes:
The endocavity probe is a mid frequency microconvex curved array probe set on a long handle (Figure 17). It has a greater amount of curvature than a curved array probe, with a resulting screen that fans out widely (Figure 18). You may be familiar with its use in transvaginal scanning in early pregnancy. Although less than ideal, the endocavitary probe can be an adequate substitute for a linear probe for some near-surface indications such as Ocular EDE [Emergency Department Echo – the term the authors use] and central line guidance.
What follows is text from the book on Knobology
Ultrasound machines offer different presents that optimize parameters for a particular type of scan. For example, you could select an abdominal preset that would adjust gain, starting depth, TGC, THI, and probe frequency to settings that usually produce the best image of abdominal structures.
You may already have noticed that ultrasound probes have a range of frequencies at which they can be set. For instance, a curved array probe may have a range of 2.5 – 5.0 MHz. When the machine is first turned on, the probe usually defaults to the mid-range (e.g. 3.5 MHz). However, one can toggle to a lower frequency (e.g. 2.5 MHz to allow for better penetration, or switch to a higher frequency (e.g. 5.0 MHz) to allow for better resolution. In some cases, a switch in frequency will noticeably improve your image quality. This is particularly true with the linear probe. When placing a peripheral intravenous line in the arm under EDE guidance, the highest possible frequency is preferred. However, when using that same probe to look for a DVT in an obese patient, you will choose a lower frequency.
Tissue Harmonic Imaging
Ultrasound waves transmitted into the body generate echoes that return to the probe and are interpreted as an image on the screen. Tissues actually generate more than one echo for each ultrasound wave that contacts them. Harmonic waves are double the frequency of the regular echo. Because of how these harmonic waves are generated, they sometimes have fewer artifacts than the original echo, particularly edge artifact. Tissue densities with reflective qualities can generate a lot of edge artifact. Therefore bone, some fat layers, and the smooth echogenic membranes of cystic structures may be better visualized if you only looked at the harmonic frequency echo they reflect.
Turning on tissue harmonic imaging (THI) filters out the normal frequency echo and only reads the double frequency harmonic (Figure 20). When imaging cystic structures on objects deep within an obese patient, turn the THI on and off to see if the artifact is reduced. THI can be particularly useful for cardiac scanning and it often improves the image in Gallbladder and DVT EDE. You can easily see if THI will improve your image. A click of a button will turn it off and on. Use it frequently.
Dynamic range controls how large an assortment of ultrasound echoes is shown on the screen. Echoes return from tissues from weakly echogenic to strongly echogenic (Figure 21). If the dynamic range is high, then all of these echoes are shown on the screen. This provides a lot of detail so by default the dynamic range is usually set high.
Sometimes you may wish to image a very reflective Area of Interest. All the tissue around this area generates weaker echoes, and these weaker echoes interfere with the visualization of the primary object. Decreasing the dynamic range will filter out these weaker echoes, leaving only the more intense reflection of the object of interest. The image will have less detail and look more pixelated the object will stand out more (Figure 22). This is sometimes useful when useful when imaging nerves or blood vessels that are surrounded by layers of soft tissue.
Time Gain Compensation (TGC)
When an ultrasound beam penetrates the body it gets weaker as it goes through successive layers of the body. So deep reflected ultrasound waves are weaker (meaning the deep picture is darker) than the more superficial ultrasound waves (which means that they are brighter). The time gain compensation (TGC) sliders allow you to increase or decrease the brightness of the picture at each level of the scan.
Focus allows you to determine the level of the scan where lateral resolution is highest. The effect of changing the level of focus isn’t that great so don’t be surprised when you don’t see much change.
Color Doppler Mode
This mode generates color for moving objects (blood in blood vessels) and superimposes the color on top of the B-mode image.
As a default on most systems, color Doppler applies red color to objects moving toward the probe, and blue color to motion moving away from the probe (Figure 26). This does not mean red-colored flow is arterial and blue-colored flow is venous. These colors simply indicate the direction of flow. By convention, the color bar on the side of the screen will have the color of movement towards the probe at the top of the bar and color movements away from the probe at the bottom (Figure 27).
To get the color gain correct, you want to turn the doppler gain control up until you see spontaneous color, speckling, and then turn down the color gain until it just goes away.
Doppler is useful for detecting the flow of fluid in vascular structures (e.g., for Vascular Access and DVT EDE). Doppler delivers greater amounts of energy to tissue than B-mode; this produces a small amount of heat. It is therefore important to avoid Doppler in potentially vulnerable tissue such as in the small fetus or the retina.
M-mode or Motion-mode measures motion of tissue seen by one crystal in the probe as represented by a vertical line on the screen. The graph generated shows motion of tissue seen by one crystal in the probe as represented by a vertical line on the screen. The graph generated shows motion of tissue at all the depths intersected by that line, measured over time (Figure 34 Above).
M-mode is used by emergency physicians to assess respiratory movements in lung tissue.
In the previous post we discussed three types of artifacts: enhancement, shadowing, and refraction (edge) artifact.
Now we’re going to consider four other types of artifact: reverberation, comet tails, mirror, and side lobe artifact.
Highly reflective interfaces can cause ultrasound waves to reflect multiple times back to the probe which the ultrasound machine interprets as sperate objects. This creates an artifact that looks like duplicates of the reflective surface spaced equally apart (Figure 35 Above). This is often referred to as “ring-down” artifact.
Comet Tail Artifact
We’ll go over this artifact when we go over respiratory ultrasounds.
Point-Of-Care Ultrasound for Emergency Physicians — “The EDE Book”; “The clearest and most concise approach to emergency ultrasound.”
There is also an e-book of the above available but unfortunately it is only available on Apple devices. It is not available for Android or Windows users. The e-book is only about $15 and I strongly recommend you buy it if you have an Apple Device.