This post is in response to the post “It’s Artifactual”
To summarize, this was an eleven-year-old boy who was brought into the ED for left foot pain with a concern for a foreign body in his foot. An ultrasound was performed and the following images were obtained:
Thank you to everyone who took the quiz!
For question #1, 52% of you guessed correctly that the foreign body visualized was wood, and not glass (29%) or plastic (19%).
For question #2, 57% of you guessed correctly that posterior enhancement is an artifact not commonly seen with foreign bodies, compared to ring down (14%), shadowing (14%) and reverberation (14%).
Not all foreign bodies are radiopaque on radiographs, so ultrasound can be used to identify radiolucent foreign bodies, such as wood or plastic.
Foreign bodies are hyperechoic on ultrasound compared to the surrounding soft tissue. If a foreign body has been in the soft tissue for an extended period of time, there may be some surrounding inflammatory edema visualized as a hypoechoic collection around the foreign body seen on ultrasound. However, they may not always be visualized sonographically, so the presence of artifacts may provide clues to the location of the foreign body.
Attenuation and Acoustic Impedance
Before discussing ultrasound artifacts, it is important to understand the basic physics of ultrasound energy through tissue. Attenuation is the loss of energy, which weakens sounds waves, as it travels through tissue. The amount of energy that is either reflected or absorbed by a structure is determined by its acoustic impedance, or the density and stiffness of the structure. The greater the stiffness, the faster the wave will travel and the faster it will weaken as it travels through tissue. Structures with a high acoustic impedance, such as bone or gas, will absorb a large portion of the energy, leaving less energy to visualize deeper structures. Structures with a low acoustic impedence, such as blood or urine, will absorb less energy, leaving more energy to visualize deeper structures.
Shadowing is a type of attenuation artifact. Materials such as wood or plastic are highly reflective (high attenuation) surfaces sonographically and tend to produce shadowing because the ultrasound waves are either partially or fully reflected off the surface, leaving little energy to penetrate into and visualize deeper structures.
Reverberation is another type of propagation artifact. It occurs when sound encounters two highly reflective layers (metal, plastic) and is bounced back and forth between the two layers before traveling back to the ultrasound probe. The probe will detect a prolonged traveling time and assume a longer traveling distance and display additional ‘reverberated’ images in a deeper tissue layer which appear as multiple hyperechoic parallel lines deep to the foreign body.
Ringdown is a type of reverberation. It is caused by sound waves encountering the front and back of a strong reflector, such as an air bubble or thin piece of metal, are is visualized sonographically as a very narrow band of evenly spaced hyperechoic parallel lines deep to the foreign body.
Posterior enhancement is another type of attenuation artifact, and essentially the opposite of shadowing. Fluid-filled structures, such as the bladder or gallbladder, have low attenuation, which allows a greater amount of energy to penetrate into deeper structures. This increase in energy causes the posterior structures to seem more hyperechoic than the surrounding structures that are not posterior to the fluid-filled structure.
How to obtain a better image
Since foreign bodies are usually superficial, sonographic visualization may be challenging since sound is not transmitted or reflected well in the area immediately inferior to the probe. Using a stand-off pad, water-filled glove or water bath can elevate the transducer several millimeters above the structure of interest, allowing for better sound transmission and visualization of superficial soft tissue structures.
Hand without water bath. Hand with water bath.
Learn more about how to perform a soft tissue ultrasound.
Think you know all there is to know about soft tissue ultrasound? Test yourself!
If you want to see YOUR image included in the next Image of the Month, please email interesting stills and/or clips in addition to a small blurb on the patient to Lorraine Ng at PEMFellowscom@gmail.com.
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