When it comes to imaging options for patients at your veterinary practice, veterinary computed tomography (CT) and magnetic resonance imaging (MRI) are each viable options. Indeed, you should be recommending both of them. There are circumstances where the former is clearly preferable, while other times the latter is the obvious choice which is why it’s essential to understand each of them, so you can be certain to offer the proper option.
Here’s a breakdown of the different use cases, benefits, advantages and disadvantages of veterinary CT vs. MRI for imaging your patients.
What You’re Imaging Determines Whether You Recommend Veterinary CT or Veterinary MRI
These two imaging solutions are optimized for different parts of the anatomy. Veterinary CT is best for imaging bone and mineralized tissue. Therefore, it’s very effective for detecting skull abnormalities, spinal fractures, mineralized disc extrusion herniations and joint disease.
CT is also able to rapidly image a large area of the body while the patient is just sedated. Although MRI is superior for soft tissue imaging, it’s difficult to image the entire thorax and abdomen on MRI with the same rapidity as with a CT scan. For these tissues and areas, CT should be performed first.
MRI is usually the imaging modality which is preferable for soft-tissue imaging and is excellent for depicting central nervous system (CNS) lesions. Unless the CNS disease is bone-related, CT likely won’t pick it up clearly, which can cause delays. For spinal cord imaging, veterinary CT often requires an iodine-based contrast dye to highlight the spinal cord, which can carry risk and add time to the process. In summary, veterinary MRI is the best option for soft tissue, muscles, the spinal cord, brain, ears and nose.
MRI Provides Higher Quality Images to Support Definitive Diagnoses
Both veterinary CT and MRI can be used for diagnosis, but each has specific advantages for certain use cases. Soft tissue and muscle imaging on MRI, for example, are so well defined that a problem’s severity can be observed, which is rarely the case when using CT. For example, it enables you to identify the margins of a lesion prior to surgery. You can determine the size of an individual lesion in the brain and spinal cord, even those as small as 1-2mm.
With veterinary CT, you might see a reasonably sized mass, but MRI allows you to identify smaller lesions, such as hemorrhages. You can even identify their stage. The detail that MRI provides regarding nervous system anatomy allows a precise determination of the structures affected (such as white matter, grey matter, cranial nerve nuclei, etc) which is essential to more accurately considering the responsible disease.
Beyond showing the extent of the primary disease, MRIs can reveal the impact it’s had on the tissue surrounding it. For example, you might see a mass or lesion in the brain and diagnose it as a tumor, and you might also observe edema as a reaction to it. Seeing the secondary consequences of the disease provides direction for a clear treatment plan and your prescription can address all the complications the patient is experiencing. Whereas CT too often lacks this level of detail. Instead, everything merges into a single abnormality.
Types of tumors can be defined more clearly with MRI, especially using contrast. This helps you arrive at a better diagnosis so you can create a more targeted treatment plan.
Some Patients Aren’t Good Candidates for MRIs
Having explained the benefits of MRIs, it’s time to address occasions they are not a viable option. For example, if a patient has any metal in their body (say, from surgical implants), it can distort the MR imaging. The metal may be far enough away from the scan for it not to be a factor: a hip implant won’t impact a brain scan, for example. It would, however, distort a pelvic scan. Even a pet’s microchip can interfere with image interpretation of surrounding tissues, so it is vital to check with the MRI referral centre when considering the animal’s individual circumstances. A distorted image is of limited value in diagnosing patients. It should be noted that pets with pacemakers can’t be MR imaged at all, since the magnet could interfere with the device.
There are also other potential drawbacks to MRIs, which will be explained below.
Additional Factors to Consider
Unlike CTs, MRIs require anesthesia to keep the patient still during the study, which could be deemed a health risk for some pets. Accordingly, older pets or ones with existing health complications may require additional testing to see if MRI is a good fit for them. The tests to ensure that anesthesia is safe for the patient can add time, drive up the cost and generally frustrate clients.
Another advantage CT has over MRI is speed. MRI takes 40-50 minutes, while CT can produce images in just 10 minutes, and a CT can image several regions such as thorax and abdomen during this time. This makes CTs far more manageable for traumatized patients.
It’s also usually easier to get a CT. There can be a waiting list for MRIs since the length of the procedure means fewer can be administered each day. For emergency cases, in particular, veterinary CT is generally more available than an MRI.
The fact is that veterinary CT and MRI should be seen as complementary. If you have had a patient for many years, it’s very possible you’ll eventually wind up recommending both of these imaging modalities, taking full advantage of what each performs best.
Authored by Dr Simon Platt, Small Animal Medical Director with Hallmarq Veterinary Imaging
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