by Merijeanne Hollingsworth
I’m not a veterinarian, but a lot of guidelines used with humans can be applied to dogs. When your dog is sick or injured, the vet may recommend further “images” (x-rays, CAT scans, etc.). Here’s a list of some of the tests, what they’re good for, and why your veterinarian may suggest one.
X-ray is basically a black and white photograph. Different tissues show up black, white, or shades of gray; in general the denser something is, the whiter it is. Bone is bright white, gas is black, and tissue (like heart or liver) is shades of gray. X-rays are excellent at showing problems with bones (such as breaks) and air/gas (like a collapsed lung, which should have air in it, or a bowel obstruction, which shows air inside the bowel). They’re not as good for showing “shades of gray”, like “is the liver large, or is that a tumor?” since it all looks rather alike (gray).
CT or CAT Scan (they’re the same thing) is actually a fancy x-ray, that shows things in 3-D. The name means “Computerized Axial Tomography”, which means that after a lot of x-rays are taken, a computer assembles them into a 3-dimentional picture. They’re good for pretty much everything x-rays are good for (just a lot more expensive than an x-ray) and have the added advantage of showing the “subtle shades of gray” a lot better, and seeing all sides of an organ. They are really good at showing the brain, because you can see “slices” of it (because of all those individual x-rays), not just a picture of the whole thing (envision how easy it would be to pick out the best cantaloupe if you could see the inside rather than just the outside). The CT machine is quite loud and obnoxious, so dogs (and kids) have to be sedated to get that many x-rays while being perfectly still.
MRI is Magnetic Resonance Imaging, formally called NMR (Nuclear Magnetic Resonance). The name was changed because people didn’t like the idea of having “nuclear stuff” near them, but everyone likes magnets, right? The machine uses a nuclear magnet to create a very strong magnetic field that makes all the hydrogen atoms in the body line up in the same direction. Radio waves are used to make the atoms produce signals that get translated into pictures. The MRI makes a lot of individual pictures, just like a CT, but the differences in “gray” areas become much more obvious. MRIs are great at distinguishing tumors and inflammation, because these usually have a high water content, and water has a high concentration of hydrogen atoms (water is H2O, or 2 hydrogen atoms and 1 oxygen atom). MRIs are great at looking at the brain and spine.
Ultrasound bounces sound waves off the organs, and creates a picture based on the acoustics of the different organs. Ultrasounds aren’t good at looking at bone or air-filled organs, because sound doesn’t transmit well through these (the sound just goes “thunk!”), but they’re great at looking at organs. They’re pretty good at telling the difference between an organ and a tumor, because a tumor “sounds different” due to it’s density. Ultrasounds also work in “real time”, like a movie camera, as opposed to an x-ray (which is a photograph). Ultrasounds are very good at looking at heart valves, because the vet can see the valves open and close.
There are things that imaging is not very good at, primarily neurological problems. The brain runs on electricity and chemistry, neither of which photograph at all. An analogy: if your car won’t start, you wouldn’t take a picture of the battery and dashboard to the mechanic, because it would be useless. It’s the same with seizures: a photograph of the brain doesn’t say anything about the electrical activity that’s going on (unless there’s a tumor or bleed in the brain, but that would have a lot more symptoms, showing pretty quickly, than just seizures). An EEG (electro encephalogram) is used to measure electrical activity in the brain, but it also picks up electrical activity in muscles. That’s why it’s not terribly useful in dogs (yet): the dog would have to be absolutely, perfectly still, and awake, for a long time to get the test.
So, let’s walk through a couple of scenarios.
1. You see your 2 year old English Shepherd get kicked by an unruly horse. He’s having trouble putting weight on one leg, and seems to be having trouble breathing. In addition to a good physical examination, what images would be logical for your vet to get first? Probably an x-ray. Why? It doesn’t involve anesthesia (which could make the dog’s breathing worse), shows broken bones and lung problems very well, and is relatively inexpensive. If the vet suspects internal organ damage they may also do an ultrasound (which also doesn’t involve anesthesia, and is reasonable in cost). The vet may order the more expensive tests that involve anesthesia based on their examination and expertise, but would probably start with the less expensive tests that don’t involve anesthesia.
2. Your 10 year old English Shepherd gradually becomes more listless, disinterested in eating and periodically vomits. What might be reasonable? In addition to a good examination and some blood/urine/stool tests, the vet would probably suspect something wrong with the digestive tract (bowel, liver, stomach or pancreas). What images could be helpful? A basic x-ray, of course, which could show backed-up air or stool, or a foreign body he’s eaten. An ultrasound could also be useful, as it can look at the digestive organs quite well, usually doesn’t involve anesthesia and is cost-effective. Depending on what the vet sees, they might progress to the more expensive tests that involve anesthesia (also, keep in mind that not every vet has access to a CT or MRI machine).
3. Your 6 month old English Shepherd develops “Grand Mal” (now called “Tonic-Clonic”) Seizures: loses consciousness and falls down with muscle spasms. What might the vet do? Besides a good examination (I know I keep repeating this part, but it’s critical to have a professional examine the dog) and lab work (looking for systemic anomalies), the vet might suspect congenital anomalies (such as hydrocephalus, or “water on the brain”, a brain tumor or bleeding in the brain from trauma or a bad blood vessel). What images might be useful? Not an x-ray, since the target organ is the brain, and the skull bone just shows up white. Not an ultrasound, for the same reason (bone just sounds “thunk!”) A CT or MRI would be far more useful, since either can show inside of the skull (thus the brain). Either scan is expensive and involves anesthesia. Why wouldn’t they get an EEG (which is one of the first examinations a human would be subjected to)? Because a seizure has to happen while the animal is hooked up to the EEG machine (with little electrical leads stuck all over its head). In a human, this can be accomplished by making a person stay awake overnight, hooking them up to the machine, and flashing a strobe light in their face. No English Shepherd ever born will happily participate with this test.
4. Your 5 year old English Shepherd develops “Limbic” or “Temporal Lobe” Seizures. She has staring spells where there’s “Just no dog there” and comes out of it being cranky and aggressive. What might the vet do? Again, an x-ray or ultrasound won’t be a help. If a good physical examination and lab tests for systemic problems doesn’t show anything, what might the vet order? They may order an MRI or CT scan to look for a brain tumor or bleed. They might also decide to order nothing and “wait and see” if medicine works. Why? Because MRIs and CTs are expensive and involve anesthesia, and the problem will either respond to medication or will become clear pretty quickly. It’s unlikely to be hydrocephalus “water on the brain” (because the dog would probably be dead already), a brain tumor in a middle-aged dog is very difficult to operate on successfully, and a bleed is unlikely (because it doesn’t “come and go” like Temporal Lobe Seizures). An EEG is impractical, because no self-respecting English Shepherd will allow herself to be subjected to the test. If medicine works, the dog will be (relatively) ok. If medicine doesn’t work, there isn’t really very much else to do.
Hopefully, you’ll find this useful the next time your vet suggests a test for your dog. Please keep in mind that it’s just an overview, and you and your veterinarian will decide together what tests, if any, are right for your dog.