MRI and CT in Dogs and Cats: What Each Scan Shows and When
Reviewed by the LabReadAI medical team
When a vet says a pet needs a scan, the part owners rarely understand is why one case goes to MRI and another to CT — and why both need anaesthesia. Here is what each scan shows in dogs and cats, how the two differ and how to read the report you take home.
How MRI Differs From CT in Dogs and Cats
The difference is physical, and everything else follows from it.
| MRI | CT | |
|---|---|---|
| Sees best | Soft tissue: brain, spinal cord, discs, ligaments, muscle | Bone, lungs, gas, bleeding, metal |
| Radiation | None (magnetic field) | Yes (X-rays) |
| Time | 30–60 minutes or more | Seconds to a couple of minutes |
| Anaesthesia | Almost always | Almost always, but shorter |
| Metal implants | Pins and plates are a problem | Not a barrier, but cause artefacts |
The short version: MRI answers what is wrong with the nervous system and soft tissue, CT answers what is wrong with bone or lungs and how urgent it is. Neither replaces ultrasound, which is explained in ultrasound for dogs and cats.
What an MRI Shows in a Dog or Cat
MRI is the main tool when a nervous system problem is suspected:
- Weak or dragging hind legs. Usually a disc extrusion — near-routine in dachshunds, French bulldogs and Pekingese.
- Seizures, odd behaviour, head tilt, sudden blindness. Here the brain is imaged for inflammation, a mass or a vascular event.
- Neck or back pain with no obvious cause.
- Lameness that plain films did not explain — ligaments, menisci, soft tissue inside the joint; the canine version of this story is in dog limping.
An MRI shows a picture, not a diagnosis: inflammation and tumour can look alike, and the vet reads the images alongside the exam, blood work and sometimes a spinal fluid tap.
What a CT Scan Shows: Bone, Lungs and Speed
CT is an X-ray reassembled by a computer into a three-dimensional picture. It is strongest where density and time matter:
- Complex fractures, pelvic and skull trauma — CT finds lines plain films miss; the basics of plain films are in dog X-ray.
- Lungs. Small nodules are invisible on plain radiographs; when metastases are being looked for, CT finds far more of them, which is why it is done before cancer surgery.
- Nose and sinuses — chronic discharge, nosebleeds, turbinate destruction.
- Ears, teeth and jaws — structures that overlap on a plain film.
- Vessels and thromboembolism — with contrast; in cats this is a common scenario, and the first step is usually a cat X-ray.
CT takes minutes. For a sick animal that is decisive: shorter anaesthesia, lower risk.
Anaesthesia for a Pet Scan: the Real Deciding Factor
A person lies still on request. An animal does not, and any movement turns a scan into a set of useless blurred slices. So imaging is done under general anaesthesia, and in practice the decision to scan or not comes down to that.
What an owner should know, without naming any drug:
- Anaesthetic risk in a healthy dog or cat is low but not zero, and it rises with age, weight, breed (flat-faced dogs) and the severity of the underlying illness.
- Before anaesthesia the clinic examines the pet and usually runs blood work; what the numbers mean is covered by pet results interpretation and the article on the dog blood test.
- Fasting before anaesthesia is required; water is usually allowed. The clinic sets the exact timing.
- Cats and small dogs lose heat quickly on the table, and the clinic monitors that.
If the patient is unstable, the vet often picks the faster scan. That is not cost-cutting — it is less time under anaesthesia.
Preparing a Dog or Cat for a Scan
- Ask how long the fasting period should be and whether water is allowed.
- Bring every previous image and report: the comparison with an older study is sometimes worth more than the new one.
- Remove metal collars and clothing with hardware before an MRI.
- Tell the vet about implants, pins, plates and the microchip — metal distorts MRI images and can heat up.
- After anaesthesia a pet is wobbly and sleepy for several hours and needs a warm, quiet place.
- If the scan was ordered because of lameness, plain joint films usually come first — see dog leg and joint X-ray.
How to Read an MRI or CT Report
Reports describe location, size, the nature of a change and how it takes up contrast. Common phrases:
| Phrase | What it usually means |
|---|---|
| Disc extrusion or protrusion | Disc material has moved into the spinal canal |
| Spinal cord compression | The cord is squeezed — degree and length matter |
| Contrast-enhancing mass | Active tissue: tumour or inflammation, told apart by further tests |
| T2 hyperintense signal | Oedema or inflammation, non-specific |
| Pulmonary nodules | Nodules — number and nature are clarified |
| Bone lysis | Bone destruction, needs an explanation |
Clinics hand out a disc or DICOM files. If they will not open, see how to open an MRI or CT disc and how to open a DICOM (.dcm) file.
When Pets Do Not Need a Scan
Advanced imaging is rarely first. Coughing, breathlessness and a heart murmur start with an echo — see dog heart ultrasound and ECG. Vomiting, diarrhoea and weight loss start with an abdominal scan, described in dog abdominal ultrasound. A CT or MRI is ordered once a simpler test has narrowed the question or failed to answer it.
What to Do With the Images and the Report
The report is written for a clinician, and most of its wording is gone by the time you get home. Upload the files or a photo of the report to pet imaging interpretation: the service explains the phrasing for the species and builds the questions worth asking at the next visit. If there is no scan yet, only a worrying picture at home, you can describe the symptoms and see which test makes sense first.
This article is informational. Diagnosis and treatment of your pet are the job of a veterinarian.
For informational purposes only
This article is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. Please consult a healthcare professional for medical guidance.