Veterinarians frequently ask "When should I use a CT and when should I use an MRI to make a diagnosis?" The short answer to this question is generally inane and goes something like "CT is better for bones and MRI is better for soft tissue." I always thought this was a nonsense answer because if the vet knew it was a bone or soft tissue problem they might not even have to order the test.
The long answer to the question usually involves a long list of use scenarios and people who own MRI's tell you to order and MRI and people who own CT's tell you to order a CT. Grrr..PAYOLA and self referral are masquerading as good medicine. Ghast.
Well, we don't own machines and we are not making recommendations about a specific case. What's not to trust?
You will find that our approach is a practical one. It is not the only one. There is no right or wrong in this discussion and I will likely give contradictory advice to what you may have heard in the past. I am genetically hard wired to be cheap. That is a fact. As such, it is our opinion that running the most expensive test (some would call it the "gold standard" is not always the right test. Spending more than the client needs to spend, even if they can afford it, is getting us (collectively) into trouble. We are seeing a syndrome that we call "I love my pet fatigue" where pet owners are getting tired of paying out the nose for expensive diagnostics. This fatigue inevitably leads to the resulting "I did that for my last dog" syndrome where owners "do everything: for their last pet and only have an urn full of ashes to show for it. A corollary to all of this syndrome only affects lawyers and is called "I cant get your dog back but I can get even." We are in the lawyers crosshairs. This guide should help prevent these horrible afflictions.
So, the following discussion is a practical guide to recommending axial imaging diagnostics that will help preserve your owners trust in you as well as the integrity of their bank account. Perhaps this approach is old fashioned. I am prepared to defend this list and I expect that the list will be updated based on your feedback but please dont tell me I am old fashioned. I might need a new hip but I will twitter you into submission in the blink of an eye. Try me.
With that - lets reframe our original question and ask "I have a Dog with XXXXXX should I do a CT or MRI?"
SEIZURES: This is the most common indication for axial imaging of the brain. The answer to this question is that it depends.
- If the DOG is <3 years old skip the CT and go straight to MRI. Young pets are less likely to have a brain tumor so it is not likely that you will have a lesion that you will identify on CT. You need to be worried about inflammatory and congenital disorders. Please spend the money and get the MRI.
- If the DOG is >5 years old and is a Boxer, Golden, or other breed prone to brain tumors: In these cases brain tumors are a primary consideration. CT will identify most of them so CT is a good screening test. If you dont have MRI, go for CT. If the CT is negative, get a CSF while you are there. If both of these tests are negative it is not likely you will find a lesion on MRI.
- If the DOG is >3-5 years old and is not a cancer factory breed, MRI is best but CT can be used. Just remember that if the CT is negative, you have not ruled out inflammatory or vascular disorders.
ACUTELY ATAXIA: Another very common indication for axial imaging
- If the dog is a Dachsund or other chondrodystrophic breed of dog and has a T3-L3 myelopathy: screening CT should be sufficient to rule out a herniated disk which is the most common disorder given this signalment.
- If the dog is a Dachsund or other chondrodystrophic breed of dog and has antyhing other than a T3-L3 myelopathy: you will need a CT myelogram or MRI. Please do not ever under any circumstances send us a screening cervical CT without a concurrent myelogram. Trust me on this one.
- If the dog is a large breed dog with a T3-L3 myelopathy: MRI first then think CT/myelogram. Vascular disorders are possible in this case. MRI is just better
* If the dog has any sort of chronic myelopathy: go for the MRI. You want to rule out primary spinal disorders. If MRI is not available CT/myelography is a second choice.
I have a dog that is SNEEZING or has a bloody nose: Go for the CT. You can get the same information as MRI and save the owners some money. If CT is not available MRI will work for nasal cavity disease but you could have taken that extra money and paid for a biopsy which you are gong to need after the study anyway.
I have a dog that has EOPHTHALMOS or FACIAL SWELLING: go for the CT. MRI will work too but, again, CT is sufficient.
I am worried about METASTATIC disease: CT is a slam dunk for evaluatiuon of the lungs. We routinely see lesions on CT that are not identified on survey radiogarphs of the thorax. Many institutions are now using CT to screen patients with osteosarcoma and especially with primary lung tumors if the pet owners will elect for lung lobectomy. Many times, metastatic disease can be seen at the time of surgery that are overlooked on survey radiographs of the thorax.
I have a dog with a HEAD TILT: If there is any indication that you have ear disease it is CT all the way. If you are worried about central vestibular disease, go with the MRI. If you really have no idea, go with the MRI. If you do a CT and it is negative you will be scratching your head wondering if you have idiopathic vestibular disease or if you missed a small, central, vestibular lesion. MRI will be sufficient for evaluation of the osseous bullae in this situation.
I am worries about LUMBOSACRAL DISEASE: This I really a tough one. For the most part, CT and MRI will give you similar information. I opt for CT in most cases just because it is less expensive. You just need to remember that the appearance of the CT or MRI does not always correlate with clinical signs so you NEED to correlate any findings in the MRI or CT report with the clinical picture.
I am worries about an ECTOPIC URETER: This is another slam dunk for CT. Stop with the IVU business. That is so 1980. Do a CT and get a diagnosis. Works every time.
I have a dog that is PAINFUL IN THE ELBOW: This one is tough and you have to promise me that you are positive that the pet is lame in the elbow. If so, do like referral hospitals do go straight to CT. Some recommend arthroscopic evaluation of the elbows if radiographs are normal. However, scoping is an invasive diagnostic procedure and exploring both elbows is a tough (and expensive) diagnostic for many owners to swallow. Therefore, many specialists now opt for non-invasive CT of the elbow.
You can click here for a PDF summary of this discussion. Print it out. File it next to the Purina poop scale handout that we were all given in vet school. "Yes maam your dog's poop is grade 2. Here look at the chart."