Brain CT in dogs when MRI is not available

Where brain CT fits

MRI is the gold-standard imaging method for intracranial tumours, yet CT remains commonly used for diagnosis and treatment planning (Stadler et al., Frontiers in Veterinary Science, 2017). The useful question is not whether CT equals MRI. It is which clinical questions CT can answer well, and which results should still lead to MRI, CSF analysis or referral.

What CT detects reliably

The most direct comparison (Gielen et al., Journal of the American Veterinary Medical Association, 2013) scanned 51 dogs and 7 cats with suspected intracranial lesions using both low-field MRI and single-slice CT, read by radiologists blinded to the case:

Question	Agreement between CT and MRI
Is there a lesion, and is it solitary or multiple?	Substantial
Is there a mass effect?	Almost perfect
Does the lesion enhance with contrast?	Almost perfect
Lesion visibility in most brain regions	Substantial to almost perfect
Enhancement pattern and lesion margins	Lower
Lesion dimensions	Poor

Mass effect and contrast enhancement were considered the principal imaging signs, and CT matched MRI closely for both. The authors called for more work on the relative sensitivity of the two methods.

Brain tumours: detection is not diagnosis

In 15 dogs with histologically confirmed glioma, contrast-enhanced measurements of the tumour on CT were equivalent to those on MRI. Neither method predicted the tumour well:

• accuracy for grade was 46.7% on CT and 53.3% on MRI
• accuracy for type was 53.3% on CT and 60% on MRI

Histopathology remained necessary for an accurate diagnosis.

Meningioma and radiotherapy planning

For radiotherapy planning, CT alone tends to underestimate tumour extent. In 22 dogs with an imaging diagnosis of meningioma (Berlato et al., Veterinary Radiology & Ultrasound, 2018), the tumour volume drawn on contrast MRI was larger than on contrast CT in all but two dogs. On average, 26% of the combined tumour volume was seen only on MRI and 6% only on CT. The authors describe the two methods as complementary.

A second study of 13 dogs, contoured by 18 radiation oncologists (Walther et al., Veterinary Radiology & Ultrasound, 2023), found that plans based on registered CT and MRI covered the true tumour volume better than plans based on CT alone. They were also more likely to meet the planning criteria.

Stroke: haemorrhage versus ischaemia

In eight dogs with brain infarction diagnosed on CT (Paul et al., Australian Veterinary Journal, 2010), three had non-haemorrhagic cerebellar infarcts and five had telencephalic infarcts, most of which appeared haemorrhagic. All dogs were 8 years or older. The authors found CT particularly sensitive for haemorrhagic infarction, but noted that it under-represents ischaemic and lacunar infarcts compared with MRI.

Inflammatory brain disease: the main gap

In 168 dogs with presumed meningoencephalomyelitis (Bach et al., Frontiers in Veterinary Science, 2023), 130 had MRI and 38 had CT. In dogs with inflammatory CSF, brain abnormalities were seen in 74% on MRI but in only 39% on CT. MRI was more sensitive than CT, but not 100% sensitive. The authors recommend CSF analysis even when MRI or CT shows no abnormality.

For a young or middle-aged dog with multifocal signs, this is the area where a normal brain CT is least reassuring.

Head trauma

CT is a valuable tool to detect structural damage after head trauma (Wyatt et al., Veterinary Radiology & Ultrasound, 2021). In 40 dogs that had CT within 72 hours of traumatic brain injury, 75% survived to discharge and 17.5% had post-traumatic seizures. No CT feature was associated with survival or seizures, including midline shift, intracranial haemorrhage, herniation, skull fractures or a lesion grading scale.

Brain CT protocol at a glance

Contrast enhancement is central to brain CT. In the CT and MRI comparison, mass effect and contrast enhancement were the principal imaging signs, and the glioma and meningioma studies above measured tumours on contrast-enhanced CT. The comparison used single-slice CT, so it does not describe the performance of modern multislice scanners.

What the radiologist looks at

The study is read against the neurological examination and history you send.

• Mass effect and enhancement. These are the signs where CT agrees most closely with MRI. Enhancement pattern, margins and dimensions agreed less well.
• A mass is not a diagnosis. CT and MRI both predicted glioma grade and type poorly.
• Haemorrhage or ischaemia. Haemorrhagic infarcts show well on CT, but ischaemic and lacunar infarcts are under-represented.
• After trauma. CT describes the structural damage, but no CT feature predicted survival or seizures.

What your report will include

Every Veheri case is read by a radiologist or neurologist, and when you order you can choose Diplomate-only reporting or the full specialist panel. Neurology reporting covers CT and MRI. You can chat directly with the specialist assigned to your case, and a simplified pet owner report helps you explain the findings to the owner.

When you take the report forward:

• A normal CT with a brain localisation does not exclude a small ischaemic infarct or inflammatory disease, so MRI or CSF analysis is the next step.
• Suspected inflammatory disease, or inflammatory CSF, calls for CSF analysis even when imaging shows no abnormality.
• A mass that will be irradiated is worth discussing for MRI, even if CT has already shown it, because CT alone underestimates tumour extent and combined CT and MRI plans cover the tumour better.
• After head trauma, use the CT findings to describe the injury, not on their own to predict the outcome.
• Talking to the owner: a CT can show a mass but not its type or grade, so histopathology is still needed for an accurate diagnosis.

What to include when you request a brain CT report

• signalment: age, breed and sex
• the neurological examination and the suspected lesion localisation
• onset and course: acute, progressive or episodic
• seizure history, trauma history and time since the event
• CSF results, if already available
• whether the result will guide surgery, radiotherapy or referral for MRI

How Veheri can help

Veheri works with veterinary radiologists and neurologists and reports brain CT and MRI studies remotely for clinics. When MRI is available, see brain MRI for dogs with seizures, and for trauma patients, the role of emergency CT.