Abdominal ultrasonography is extremely useful and versatile in investigating diseases in companion animals. We as veterinarians have the advantage over human medicine; our patients are small enough that we get excellent images. When you are scanning a 150 pound Rottweiler, you experience what human ultrasonographers have to deal with.
Since depth and abdominal fat are less of a limitation for veterinary ultrasound, the main physical barrier we have is gas. Ultrasound has a much lower speed of travel through air compared to tissue, causing more reflection at the air-tissue interface, and reverberation artifacts between the transducer and the air. This results in a great difference in acoustic impendance between air
and soft tissue. The bottom line: you can’t see much.
The gastrointestinal tract always contains some air, mostly in the stomach, less in the small intestine, and some in the colon. The colon also contains feces, which is less reflective but much more absorptive of sound. If the ultrasound beam is not reflected by gas, it’s absorbed by the stool. Our goal is often to investigate the gastrointestinal tract for foreign bodies, masses or inflammatory disease, and it’s difficult if you can’t see what you need to see. The GI tract can also get in the way of imaging other organs near it, such as the adrenal glands, which are near the colon. How do you make the most of your scan?
My strategy is to take a look at the GI tract first, and decide what parts can be evaluated and what parts are not visible. The body and pylorus of the stomach are often filled with gas in dorsal recumbency. Imaging the colon is often restricted to the near-field wall, before reflection or attenuation obscures the far wall. Ultrasound of the small intestine is usually rewarding if you wait for peristalsis to move the gas from the loop you are interested in.
In an animal with clinical signs that might involve the part of the GI tract you can’t see, you need to strategize. I had some fantastic mentors during my residency that were masters of making ultrasound work for you. You need to use the natural tendencies of fluid to fall to the dependent side, and gas to rise to the top.
Most people scan in dorsal recumbency, and a smaller number in lateral. The natural protocol is to place the transducer on the uppermost surface and look down. This is the best way to scan routinely, but since gas rises, there will be parts of the GI tract that have gas between the transducer and the deeper portions. To see those regions, you need to fill them with fluid and get the gas out of the way.
The best way to do that is to position the animal in lateral recumbency, and place the transducer between the animal and the table. You may need to use both hands because of the weight of the animal. Next, angle the transducer up, away from the table. The portions of the GI tract that contain gas and fluid will now have the fluid-filled portion nearest to the transducer, and gas will be out of the way.
The technique is most useful for the pylorus, which is often filled with gas and a suspicious site for gastric outflow obstruction. You can get a very nice image in a barrel-chested breed from the down side. Deep-chested dogs are difficult, because the pylorus and duodenum are under the ribs. The other sites where it can be helpful is in finding the right adrenal gland and right lobe of the pancreas. The ascending colon often blocks them from view in dorsal and lateral recumbency. The dogs find this “scanning from the down side” position more uncomfortable, and so will you. But it can be invaluable in finding out what you need to know.
Try it in easy cases first, like a nice Shih Tzu that has urinary calculi. You’ll be able to trace the duodenum and pyloric outflow tract when it’s filled with fluid. The information you gain will be worth the frustration of struggling against the weight of the dog, getting gel all over you, and reorienting your brain to the view of the anatomy. I encourage you to persevere in developing your “down side” skills for the cases where it really counts.
How to get gas out of your field of view:
- Position the dog in the lateral recumbency with the part of interest in the dependent position
- Place your hand(s) and transducer between the animal and the table
- Look “up”, or away from the table
- Find an anatomic landmark to orient yourself, and/or the organ of interest to investigate
Good point Allison! Positional ultrasonography is very useful – I used it yesterday to specifically look at the pyloro-duodenal junction in a dog suspected to have gastric outflow obstruction. I have two comments though: ultrasound does not travel faster in air than in tissue. It’s the other way around. The speed of propagation is actually slower in air (a less dense medium) than in soft tissue and than bone (faster speed because very dense)> Also, the acoustic impendance is not a characteristic of ultrasound but a characteristic of each medium.
Last, I think personally that it is better to scan animals in dorsal recumbency, because it allows me to get a much better mental representation of the anatomy and the laterality of lesions. Especially on organs that can move and twist along their long axis, such as the bladder, having the animal perfectly straight helps localizing lesions along the wall of the organ. I have seen my residents making mistakes in localizing lesions which I think are in part due to scanning the animal in lateral recumbency. The last two I can remember of were (1) a neoplasia in the bladder that was thought not to invade the trigone when in fact it did and (2) a cranial abdominal nodule for which a decision could not be made as to its origin: hepatic lymph nodes or pancreas… I think that in lateral recumbency, the non dependent organs tend to fall down and twist which changes the anatomy as we learn it from dissection, text books and diagrams. The relationship between organs is changed in such a way that fized landmarks are more difficult to use. So, as much as I like positional ultrasound for specific applications, I think that in most cases scanning in dorsal recumbency makes my life easier.
Thanks for your input, Wil. I agree that the geometry of the abdominal organs is more consistent in the dorsal position. On a questionable lesion it can be very helpful to look from different positions.