Imaging Artifacts: An ED Ultrasound Overview

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

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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.

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. alt

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.

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.

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