CT of the canine thoracolumbar spine for suspected disc extrusion

Why CT works well in chondrodystrophic dogs

In chondrodystrophic dogs, extruded disc material is often mineralised, and plain CT enables the diagnosis in most cases (Robertson and Thrall, Veterinary Radiology & Ultrasound, 2011). In 46 dogs with acute myelopathy (Dennison et al., Veterinary Radiology & Ultrasound, 2010), 21 of 23 Hansen type I disc extrusions were mineralised, but none of the Hansen type II protrusions were.

CT is also more widely available than MRI, less costly and much faster, and it can be performed under sedation, as a 2020 review of imaging in disc disease (da Costa et al., Frontiers in Veterinary Science) notes. In 19 chondrodystrophic dogs with acute thoracolumbar extrusion (Hecht et al., Veterinary Radiology & Ultrasound, 2009), the level of extrusion agreed with surgery in 100% of dogs on conventional CT, 94.7% on helical CT and 94.7% on myelography. Median examination times were 4 minutes for helical CT, 8 for conventional CT and 32 for myelography.

One outcome study supports this. In 560 non-ambulatory chondrodystrophic dogs with acute thoracolumbar extrusion (Low and Markovic, Veterinary Surgery, 2026), management guided by conditional CT-myelography (42 dogs) was non-inferior to MRI (518 dogs) for recovery of ambulation. Rates of postoperative deterioration, progressive myelomalacia and major complications did not differ, and costs were lower.

How CT compares with MRI

MRI remains the more sensitive test.

• Detection. In 44 dogs with surgically confirmed disc herniation (Cooper et al., Veterinary Radiology & Ultrasound, 2014), non-contrast CT had a sensitivity of 88.6% and MRI 98.5%. MRI was more accurate for the site of compression and for separating extrusion from protrusion.
• Who CT misses. In that study, CT was less accurate for localisation in peracute cases and in chondrodystrophic, female, older and smaller dogs (under 7 kg). In 182 dogs (Israel et al., Veterinary Radiology & Ultrasound, 2009), myelography was more sensitive than CT in dogs under 5 kg, while CT was more sensitive in chronically affected dogs.
• Surgical plan. When four observers planned hemilaminectomies in 40 chondrodystrophic dogs (Noyes et al., Veterinary Surgery, 2017), plans based on CT and MRI of the same dog were similar in 43.5% to 66.6% of dogs. The side was the most consistent element; observers tended to plan larger defects on MRI.

When CT is not enough

Non-contrast CT shows mineralised disc material and loss of epidural fat, but not the spinal cord itself, and a disc extrusion can go undetected on CT. The evidence supports further imaging, with CT-myelography or MRI, in these situations:

Situation	Evidence
No lesion on CT, or a lesion that does not match the neurolocalisation	Further imaging may be required; CT-myelography was often necessary when CT showed no lesion
Non-chondrodystrophic dog	Additional imaging was needed in 17.4% of non-chondrodystrophic breeds versus 3.6% of Dachshunds in a study of 555 dogs; CT-myelography was often necessary in these dogs
Plegic dog	Concurrent extradural compression and spinal cord swelling can require CT-myelography for a correct diagnosis
Very small dog	CT was less accurate under 7 kg and less sensitive than myelography under 5 kg
Suspected non-compressive or hydrated extrusion	MRI is the modality of choice for acute non-compressive and hydrated nucleus pulposus extrusion

Thoracolumbar spine CT protocol at a glance

Because extruded disc material in chondrodystrophic dogs is often mineralised, plain CT enables the diagnosis in most cases. Multiplanar reconstructions add confidence: in 111 dogs they increased mean certainty scores for a correct diagnosis by 3–11% compared with 2D CT images. When more is needed, CT-myelography was the most sensitive technique for compression in 46 dogs with acute myelopathy, with the highest agreement between observers.

What the radiologist looks at

The 2020 review summarises a series of 23 surgically confirmed thoracolumbar extrusions that fell into three CT patterns:

Pattern	CT appearance
Acutely extruded, mineralised nucleus pulposus	Heterogeneous, hyperattenuating extradural mass (mean 219 HU) with severe compression
Acute extrusion with haemorrhage	Less compression, extending over several vertebrae, only slightly distinguishable from the cord (mean 59 HU)
Chronic mineralised nucleus pulposus	Very hyperattenuating, more homogeneous material (mean 745 HU)

• The haemorrhagic pattern deserves particular care, because its attenuation is close to that of the cord.
• Does the lesion match? The radiologist checks the lesion against the neuroanatomical localisation you send, because a mismatch is a reason for further imaging.
• Which side, and how far? The side was the most consistent element between CT- and MRI-based plans, but overall plans from CT and MRI of the same dog were similar in only 43.5% to 66.6% of dogs.

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 negative or mismatched CT is a reason for CT-myelography or MRI, not a normal result, because a disc extrusion can go undetected on CT.
• Prognosis needs the cord. For acute non-compressive and hydrated nucleus pulposus extrusion, MRI findings have been linked to the likelihood of recovery. Where that would change the decision, MRI is worth discussing even when CT has already shown the likely cause.
• Surgical planning: observers tended to plan larger defects on MRI than on CT, so discuss the extent of the approach with the surgeon.
• Talking to the owner: in non-ambulatory chondrodystrophic dogs, management guided by conditional CT-myelography was non-inferior to MRI for recovery of ambulation, and costs were lower.

What to include when you request a spinal CT report

• signalment: age, breed and sex
• body weight, since CT was less accurate in small dogs
• onset and duration of signs, since peracute and chronic cases behave differently on CT
• neurological grade, including ambulation and nociception
• neuroanatomical localisation, so the radiologist can check the lesion matches
• previous spinal imaging or surgery
• whether CT-myelography or MRI is available if CT is inconclusive

How Veheri can help

Veheri works with veterinary radiologists and neurologists and reports CT and MRI studies remotely for clinics. For the MRI side of disc disease, see MRI of the thoracolumbar spine in dogs, and for scanning, see mastering veterinary spinal CT.