Accuracy and Advantages of Computed Tomography for Suspected Intervertebral Disc Herniation
Suspected intervertebral disc herniation is a common indication for spinal imaging in dogs. Survey radio graphs alone are not an accurate imaging modality for the diagnosis of intervertebral disc herniation, so advanced spinal imaging is a prerequisite to surgical intervention. This allows for accurate assessment of location, laterality, and degree of spinal cord com pression to guide surgical correction.
Computer tomography (CT) is an accurate imaging modality for dogswith suspected disc herniation. In general, CT can accurately detectdisc herniations in about 80-85% of cases. CT is even more sensitive when there is herniation of mineralized disc material and hemorrhage, as is the case with the vast majority of chondrodystrophic breeds.
Such cases make up a significant portion of the population of dogs that present for suspected inter vertebral disc herniation. When a disc herniation is suspected, but a CT is inconclusive, CT myelography can be performed to further improve sensitivity in identifying extradural compression.
There are many benefits of CT when compared to magnetic resonance imaging (MRI). Total anesthesia time, including patient positioning and image acquisi tion, is about 10-20 minutes for CT compared to 60-90 minutes for MRI. In addition, image acquisition is completed in a minute or less with multislice CT scanners and therefore a patient can be sedated rather then placed under general anesthesia. This is Particularly beneficial with patients at a greater anesthetic risk or in cases where the patient will be woken up after the imaging study is completed. Low field MRI is still used commonly in veterinary medi cine mainly due to cost, space requirements and simplicity of maintenance. Low field MRI has signifi cant limitations for spineimaging, particularly in smaller patients where image resolution in thetrans verse plane is often inadequate for appropriate interpretation and to guide subsequent clinical deci sions. In larger patients, multiple sequences must be run to obtain coverage of larger areas of the vertebral column, dramatically increasing the length of the scan. CT may be preferred to MRI for assessment of vertebral fractures. This is because CT allows for extremely thin slices (1mm) in a relatively short time period, with subsequent three dimensional recon struction. This facilitates visualization of complex fractures for surgical planning.
While CT is adequate for the majority of cases with suspected intervertebral disc herniation, there is a subset of patients where MRI may be indicated or provide additional pertinent information. A small percentage of cases, particularly non-chondrodystro phic breeds, will have non-diagnostic or inconclusive CT or CT myelography imaging studies. MRI may be necessary in these cases to confirm the diagnosis and define the lesion for surgical planning. Cases that present paraplegic with loss of pain sensation may also benefit from an MRI, which can provide further prognostic information based on the degree of spinal cord edema.