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This month we’re going to answer some basic ultrasound questions and go over imaging artifacts. The main benefits of learning how to perform bedside ultrasounds include efficiency and the ability to obtain repeat scans. Efficiency, for obvious reasons, is a critical consideration in the management of the unstable patient. Ultrasound offers real-time imaging within seconds of plugging in and turning on the machine. The average limited bedside scan only takes a few minutes and the results are immediately available to the provider. There is no waiting for a tech to come in or for a radiologist to receive, review, and finally communicate time-sensitive results. In cases such as a suspected ectopic pregnancy, pericardial tamponade, or a leaking abdominal aortic aneurysm, the results of an ultrasound can truly help expedite patient management options. Furthermore, in cases such as suspected intra-abdominal hemorrhage, where serial exams are warranted, bedside ultrasound can be used to obtain various data points during the patient’s course in the ED.

ED ultrasound should not replace a comprehensive study. It is an adjunct to expedite decision making in potentially unstable patients. In most institutions it is still recommended that your bedside interpretation be confirmed by a formal imaging study once the patient has been stabilized.

Now let’s talk about how it’s done. First off, your department needs an ED dedicated ultrasound machine. You need to know how to turn the machine on and how to select the proper probes. Make sure you know how to get started prior to a single coverage night shift when there is no one else to show you the ropes. Most ED sonography can be performed with one of two probes. The multi-use curvilinear probe is useful for scanning deep structures and internal organs. Although it has a larger footprint, it can be maneuvered in between ribs and other tight spaces when necessary. The linear probe is the other basic transducer you should learn how to use. It has a smaller footprint than the curvilinear probe, and provides better resolution at the expense of less penetration. Use the linear probe when you are scanning superficial structures <5-8cm deep such as blood vessels, musculoskeletal structures, and foreign bodies.

To ensure proper orientation of your images while you scan, there are a couple of markers and indicators you need to review before you begin. Remember that the indicator on the ultrasound screen corresponds to the indicator on your ultrasound probe. The screen indicator may be a dot, a symbol, or the logo of the machine manufacturer. Except for bedside echocardiography, the indicator on the screen should be positioned to the right of the ultrasound image while you scan. During the scan, remember to keep the marker on the ultrasound probe pointed towards the patient’s head or their right side at all times. This will ensure that the left side of the image on the screen corresponds to either the cephalad direction or towards the patient’s right side. By convention, during a bedside echo, the right-to-left orientation is reversed, and the indicator dot on the ultrasound screen should be positioned to the right of the image.

Air is the enemy of ultrasound. While you scan, note that ultrasound image quality can be adversely affected by obesity, COPD, a pneumothorax, subcutaneous emphysema, or bowel gas. These things are pretty much out of your control, but it is helpful to at least know the reason why you may be having a tough time getting a good view of the heart on a 350-pound patient with COPD. In most situations, scanning through a window or organ that contains more fluid will improve your image quality. Learn how to create your own acoustic windows for better imaging quality (To be covered in the next EPM edition)

Learn the language. When describing how dark or light a structure appears on ultrasound there are three common terms you should be familiar with. Hyperechoic structures reflect ultrasound waves well and will appear relatively “white” on your screen. Some examples include the diaphragm, the pericardium, and bones. In addition, structures behind very hyperechoic areas may be more difficult to see because the ultrasound waves do not penetrate well - more on this later. Hypoechoic structures reflect ultrasound waves partially and therefore appear gray on your screen. Some examples include the kidneys, the spleen and the uterus. Anechoic structures do not reflect ultrasound waves at all and will appear black on the screen. Some examples include unclotted blood (whether free in the abdomen or flowing in arteries and veins), bile, urine, ascites, and effusions. Anechoic, and to a lesser extent hypoechoic structures, allow good ultrasound wave transmission and therefore make good “windows” to improve visualization of deeper structures. This is why, for example, a full bladder helps to visualize the uterus on a transabdominal view.

Ultrasound, like other imaging modalities, may at times be affected by certain types of imaging artifacts. These artifacts can impair accurate interpretation of an image when their presence is either not noticed or not understood.

What 4 types of imaging artifact can you identify in these four images?
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Image 2
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Image 3
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Image 4
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See the next pages for the answers. 
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Image 1 shows a gallbladder with many very small stones or stonelets. Shadowing and posterior enhancement are also visible behind the small stones and the stone-free area of the gallbladder respectively. It is important to be aware that a single or a few very small stones may not cast a visible acoustic shadow, therefore making ultrasound less sensitive for tiny gallstones.
 
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Image 2 shows an ultrasound of the eye of a patient with a retinal detachment. The vitreous is black. The retina is white and is partially detached. Note the intense posterior enhancement artifact below the globe in the far field.
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Image 3 shows the paired femoral artery and vein. Notice the large echogenic clot within the femoral artery. Below the artery in the far field there are two areas of linear shadowing know as edge artifact, which occur in these locations because the sound waves are more attenuated after passing through the arterial wall.
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Image 4 shows the gallbladder with a gallstone. Behind the gallstone a dark anechoic stripe is seen. This is an ultrasound shadow, which is one type of common artifact. The presence of the shadow helps to identify the gallstone. It is important to realize that very small stones may not shadow. Directly behind the rest of the gallbladder the liver appears more echogenic, or lighter gray, than other parts of the liver. This is due to posterior enhancement, which is an artifact of improved sound wave transmission through the anechoic gallbladder.

Continue next for Pearls and Pitfalls of ultrasound description and artifacts.
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Pearls & Pitfalls: Ultrasound Description and Artifacts
 
Echogenic: the propensity to reflect ultrasound waves. The more echogenic a structure, the whiter or lighter grey it appears.

Hyperechoic: a relatively “white” appearance. Examples include the diaphragm, the pericardium and bones.

Hypoechoic: a gray appearance. Examples include the kidneys, the spleen and the uterus.

Anechoic: a black appearance. Examples include unclotted blood (whether free in the abdomen or flowing in arteries & veins), bile, urine, ascites and effusions.

Imaging Artifact: an echo or line that does not correspond to a true anatomical feature (i.e. acoustic shadows, edge shadows, acoustic enhancement). Knowledge of imaging artifacts helps avoid misinterpretation of images.

Posterior Enhancement Artifact: seen when tissues appear more echogenic and bright because they are being visualized through a fluid-filled structure like the uterus sitting behind the urinary bladder.

Shadowing Artifact: an anechoic area behind calcified structures such as bones and stones. It is difficult to obtain information about objects farfield or within the shadow artifact.

Edge Artifact: a shadow far-field from the edge of hyperechoic structures
Reverberation Artifact: bright arcs seen on the screen from the sound waves bouncing back and forth between two highly reflective objects. Scan at a different angle, adjust the time gain compensation (TGC), or change the depth gain to reduce the amount of reverberation artifact present.

Practice Makes Perfect: With bedside ultrasound there is no substitute for experience. The more ultrasounds you do, the better you will be able to differentiate abnormal from normal, even when you may not be sure exactly what the abnormality is. An image library of normal and abnormal ultrasounds helps immensely and EPMonthly.com can take you there. Just go to EPMonthly.com and click the ultrasound library link within the “Real-Time-Readings” department.

Brady Pregerson manages a free online EM Ultrasound Image Library and is the editor of the Emergency Medicine Pocketbook series. For more information visit www.ERPocketBooks.com

Teresa Wu is the Director of Simulation Education and Training for Graduate Medical Education, and Ultrasound Faculty at Orlando Regional Medical Center in Orlando, FL.

 
 

Comments   

# Visit...James 2013-01-30 08:52
Hi Teresa... I am taking notes of these overview. Bookmarked and will be shared to my friends who also need to write up for our defense... Thanks
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