33rd Annual Scientific Meeting proceedings

Magnetic resonance imaging (MRI) and computed tomography (CT) have become more widely available and are commonly used for the diagnosis of acute spinal disease. Both imaging modalities are associated with specific advantages and limitations.

Magnetic resonance imaging (MRI)
MRI has become more accessible in recent years and should be considered the diagnostic modality of choice for most spinal and brain disorders. It allows clear visualisation of the neuroparenchyma, associated soft tissue structures, and anatomical components of the intervertebral disc. Intervertebral disc degeneration, spinal cord compression, and intraparenchymal signal intensity changes are readily visualised on MRI. Although the surrounding bony structures (i.e. vertebrae) can be identified, detailed assessment of the vertebrae is limited with MRI. MRI is non-invasive, does not expose the patient to radiation, and allows direct multiplanar imaging. MRI is however an expensive diagnostic technique, requires general anaesthesia, and studies can take a substantial amount of time before completed. Interpretation of MR images requires perfect technique (including straight positioning), training, expertise and careful correlation of imaging abnormalities with results of the neurological examination. The sensitive nature of MRI is associated with a risk of over-diagnosing clinically irrelevant imaging findings. Another disadvantage is that MRI is predominantly used for neurological disorders. A large neurology caseload is therefore required to justify the investment in MRI. Most patients with metal implants cannot undergo MR imaging, limiting the use of this diagnostic technique for post-operative imaging. Although titanium implants are considered MRI-safe, this type of implants can still be associated with substantial susceptibility artefacts.

Computed tomography (CT)
CT is more widely available and less expensive than MRI. In contrast to MRI, CT is particularly useful for assessment of bony abnormalities, such as the skull and vertebral column. Evaluation of the neuroparenchyma is limited compared to MRI. Spinal cord compression and intraparenchymal signal intensity changes cannot be observed on CT. Visualisation of spinal cord compression requires injection of intrathecal contrast, creating a CT-myelogram. CT is associated with substantial radiation exposure and multiplanar imaging is allowed by reconstruction algorithms. CT can however be performed under sedation, imperfect positioning can be compensation for by reconstruction algorithms, and large anatomical areas can be scanned in seconds to minutes. Although metal implants are associated with streaking artefacts, CT can be used for post-operative imaging. CT can reliably be used for assessment of the head, thorax, abdomen, and appendicular skeleton. This allows CT to be used for more widespread indications compared to MRI.

Which technique for the patient with spinal trauma?
It is important to realise that diagnostic imaging cannot replace clinical judgement. It should be questioned if further diagnostics will result in different clinical decision making and which diagnostic modality should be considered most appropriate for your patient. To answer the latter question, it will be important to have recognised the most important differential diagnosis in your individual patient. In general, MRI is superior for conditions predominantly involving soft-tissue structures, while CT is superior for conditions predominantly involving bony structures. Several conditions are however associated with a combination of soft-tissue and bony abnormalities; MRI and CT can be considered complementary for such disorders.

Although both CT and MRI have been used for detection of spinal fracture and luxation, CT should be considered superior for this purpose. CT allows detection of subtle fracture and luxations, detection of optimal implant trajectories, and post-operative evaluation of implant placement. CT can be performed under sedation, is forgiving for imperfect positioning, and can be completed in several minutes. These are important advantages in patients that have experienced external trauma, which might be cardiovascular unstable and for which manipulation should be kept to a minimum due to possible vertebral instability. CT also allows evaluation of multiple body systems during one diagnostic examination, thereby facilitating detection of co-morbidities.

Although MRI is probably more sensitive, most animals with Hansen type I intervertebral disc disease (also referred to as intervertebral disc extrusion) can be diagnosed with CT. The calcified nature of extruded material allows detection of this condition in most cases. CT can therefore be considered in animals with a high clinical suspicion of Hansen type I intervertebral disc disease: young chondrodystrophic dog with an acute onset of progressive, often symmetrical and painful myelopathy. MRI however allows assessment of the spinal cord parenchyma, which might be important in animals with suspected myelomalacia. Hansen Type II intervertebral disc disease cannot be diagnosed by native CT and requires injection of intrathecal contrast medium.

Ischaemic myelopathy and non-compressive nucleus pulposus extrusion are two common causes of peracute paralysis. Both conditions can occur spontaneously during intense exercise (which can be confused for trauma by the owner). Acute non-compressive nucleus pulposus extrusion can also occur after external trauma and can occur in animals with concurrent spinal fracture and luxation. MRI is the diagnostic modality of choice for both conditions and specific MRI abnormalities have been reported for ischaemic myelopathy and acute non-compressive nucleus pulposus extrusion respectively.