How to Differentiate Foreign Body Shadowing vs Gas in Veterinary Ultrasound
Oct 02, 2024
Written by Dr Camilla Edwards, DVM, CertAVP, MRCVS
Introduction
When performing a veterinary ultrasound, one of the challenges professionals often face is distinguishing between foreign body shadowing and gas. Accurately identifying these two artefacts is crucial for providing a correct diagnosis and guiding treatment decisions. This skill is especially important for veterinarians, sonographers, and students working in the veterinary field, as even small diagnostic errors can lead to missed or misinterpreted findings
In this post, I’ll explain the significance of understanding foreign body shadowing and gas in ultrasound imaging, outline key methods for differentiating between the two, and offer practical tips for improving diagnostic accuracy. Whether you’re new to veterinary ultrasound or looking to refine your skills, this guide will help you enhance your imaging interpretation.
What is Foreign Body Shadowing vs Gas, and Why Does it Matter?
This is a common concern that many veterinarians face, and understandably so. We all dream of having a highly sensitive, specific test that can definitively tell us if there’s a gastrointestinal foreign body present or not. It’s a difficult situation—do we proceed with exploratory surgery, or do we hold back? Both decisions carry weight, and the uncertainty can be stressful.
Fortunately, what we do know is that some studies suggest ultrasound is more sensitive than X-rays in detecting foreign bodies. However, distinguishing between foreign body shadowing and gas can be tricky. Knowing how to interpret these artefacts correctly is crucial for making confident, informed decisions that can improve the outcome for our patients.
Identifying Gas in Ultrasound Scans
In veterinary ultrasound, identifying gas can be challenging, but there are key characteristics that help distinguish it from other artefacts. Gas, due to its unique interaction with sound waves, often causes acoustic shadowing that may be confused with foreign bodies.
Key Signs of Gas in Ultrasound:
- Comet tails and A-lines: Small bubbles of gas often cause hyperechoic lines that stretch to the bottom of the screen; these are known as b-lines in the lungs, but in the gastrointestinal tract they will often be called comet tails and can coalesce. When faeces are mixed in with the gas, this often creates a “dirty shadow” of more hypoechoic shadowing beyond the surface.
- Acoustic Shadow: In contrast, foreign bodies often form acoustic shadows. This is where the surface of the foreign body causes complete reflection of the ultrasound waves that reach it due to the large difference in acoustic impedance. This means you are left with a hyperechoic surface and anechoic (complete blackout) beyond that surface all the way to the bottom of the image, as no sound waves can reach or return from that area.
- Movement: Gas tends to be more mobile than foreign bodies. If you manipulate the animal's position during scanning, gas bubbles may shift easily within the intestines or stomach, helping you differentiate it from a more static foreign body. Moving the patient from left lateral to right lateral recumbency can help judge how an object is moving in the gastrointestinal tract also.
Key Differences Between Foreign Body Shadowing and Gas
Foreign body shadowing differs from gas in several important ways. Understanding these differences is essential for an accurate diagnosis.
Foreign Body Characteristics:
- Hyperechoic Line with Shadowing: Foreign bodies often appear with a very distinct hyperechoic (bright) line on the ultrasound, followed by acoustic shadowing beneath them. This occurs because foreign bodies have a different acoustic impedance than body tissues, causing them to reflect sound waves more effectively.
“Foreign bodies tend to be quite hyperechoic… They will reflect at their surface, reflecting sound waves quite thoroughly, resulting in a hyperechoic line with shadowing beneath them.”
- Shape and Movement: Foreign bodies, particularly synthetic objects, often have an abnormal shape, such as being unusually straight or perfectly round, which is uncommon for organic tissue. Additionally foreign bodies may move freely in fluid and will either sink or float, whereas gas always rises. Corn on the cob for example, can shift within fluid depending on the animal’s position.
“Foreign bodies like corn on the cob for example, might look similar to tissue but are quite different. We get a high hyperechoic line with shadowing, and the object can move depending on the fluid present.”
- Fluid Presence: When scanning animals with suspected foreign bodies, especially those that have been vomiting, the presence of fluid in the stomach can highlight the foreign object. Fluid may accumulate around the foreign body, making it more visible on ultrasound.
“Often, with foreign bodies in the stomach, we see a lot of fluid, which helps highlight the object. If the animal is vomiting, it’s important to scan the stomach thoroughly, as fluid may shift depending on the position.”
Click here to watch a video showing you an example of the key differences between foreign body shadowing and gas in veterinary ultrasound.
Practical Tips for Accurate Diagnosis
- Thorough Scanning: To avoid missing a foreign body, ensure you scan the entire gastrointestinal tract, in the stomach, include areas like the fundus under the ribs and the pyloroduodenal junction and then each part of the small and large intestine.
“Make sure to scan all the way up towards the fundus under the ribs and fan down towards the pylorus.”
- Observe for Movement: As you scan, pause and carefully observe how potential foreign bodies or gas move within the stomach or intestines. Is it floating, sinking or rising?
“With objects like the corn on the cob, it moved around the stomach depending on the animal's position, which can be a big clue in identifying it.”
- Look for Clues in Shape: Foreign bodies often appear abnormally straight or unnaturally round. If you notice something with a very defined shape, it is more likely a synthetic object rather than gas or faeces.
“We wouldn’t normally expect to see perfectly straight or perfectly round objects in the body—this is a strong clue of a foreign body.”
Improve Your Ultrasound Skills—Stay Informed!
Differentiating between foreign body shadowing and gas in veterinary ultrasound is a crucial skill that can greatly enhance diagnostic accuracy and improve patient outcomes. By recognising the distinct features of each, from hyperechoic lines to acoustic shadowing and object behaviour in fluid, you can confidently identify foreign bodies and make informed clinical decisions.
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