Use of Computerised Tomography for Diagnosis of Intervertebral Disc Disease - A Review of the Evidence
CT scan showing multiple mineralised discs which are still in position (yellow arrows). As well as mineralised herniated disc material in the thoracolumbar and lumbar region (red arrows).
Intervertebral disc disease (IVDD) is an inevitable part of aging in all species and with pets living to greater ages disc disease is a common finding in first opinion practice (Jeffery, et al., 2013). After stabilising the patient and performing a neurological exam to localise the lesion, imaging is often the next diagnostic step. In the majority of practices survey radiographs are the first line of imaging. Although these have a place in ruling out conditions causing gross bone destruction (e.g neoplasia, discospondylitis) they are very poor at characterising and localising IVDD as well as being subject to marked inter-observer variation (Lamb, et al., 2002). Advanced diagnostic imaging is always indicated where pain is persistent or symptoms worsen and is a prerequisite to any surgical intervention (Robertson & Thrall, 2011) (Jeffery, et al., 2013). The options for advanced imaging are myelography (either alone or in combination with CT), MRI or CT.
CT scan demonstrating a L7-S1 disc protrusion including intra-disc gas pockets (vacuum phenomenon) and partial mineralisation (yellow arrowheads). The spinal soft tissue series (right image) confirms the dorsal bulging of the L7-S1.
Myelography was for many years was the principal imaging modality used in diagnosis of IVDD. Several studies have found that when performed correctly, myelography had a similar overall sensitivity for disc disease compared to CT (Dennison SE, 2010) (Israel SK, 2009). However, in a more recent paper, when compared to a gold standard of computer tomographic myelography, CT was found to be more sensitive at detecting IVDD (Newcombe B, 2012). Myelography is invasive and not without risk; adverse reactions to contrast medium may result in seizures or exacerbation of the myelopathy and poor technique can result in spinal cord damage (Kishimoto M, 2004). Myelography also requires a high level of skill to perform safely and in a timely manner (Robertson & Thrall, 2011). As a result, cross sectional imaging has largely superseded myelography, fortunately both CT and MRI are becoming increasingly accessible to veterinary patients.
MRI is unrivalled in its ability to differentiate between soft tissues allowing the assessment of spinal cord and parenchyma for contusive injury and haemorrhage (Jeffery, et al., 2013). There is also evidence that in acute cases MRI may have some prognostic value (Ito D, 2005) (Boekhoff TM, 2012). In comparison to other imaging modalities MRI has been shown to be more effective than myelography alone at predicting site, severity and nature of spinal cord compression. One study by Cooper et al. (2014) found that MRI had a sensitivity of 98.5% compared to 88.6% using CT in dogs with surgically confirmed disc disease. However it must also be acknowledged that there is considerable variation in the quality of MRI in veterinary practice. For example, using low field MR it may be difficult to get diagnostic images in smaller patients (Robertson & Thrall, 2011).
CT scan demonstrating a large amount of mineralised material in the spinal canal.
Computerised Tomography has been found to be an accurate imaging modality for dogs with suspected disc herniation (Israel SK, 2009) (Dennison SE, 2010) (Hecht S, 2009) (Olby NJ, 2000). It has the same benefit as MRI in eliminating superimposition and is superior at identifying bone and mineralised disc material making it excellent at identifying chronic disc disease. It is also the imaging modality of choice for spinal fractures (Robertson & Thrall, 2011). CT may not be a good choice in cases which are per acute or with low volume disc extrusion where MRI and myelography have found to be more sensitive (Israel SK, 2009) (Jeffery, et al., 2013).
In conclusion, in cases where advanced imaging is required MRI and CT are preferred to myelography due to the risk and invasiveness involved in carrying out the procedure (with the possible exception of neurology referral centres). Between CT and MRI the latter is proven to be the more sensitive and may also be able to provide prognostic information. That said, in the study by Cooper et al (2014) CT accurately characterised IVDD in 88.6% of patients (compared to 98.5% for MRI) providing the information required to make a clinical decision for the majority of the patients. CT has the added advantage of being faster, more affordable and in many cases more accessible than MRI as well as allowing imaging in the presence of metal implants. With the recent advances in technology (e.g. thinner slices, shorter scan times and metal artefact reduction) modern scanners are producing far better images than their historical counterparts and there are still relatively few studies directly comparing modern CT scanners to MRI. Although not required for visualising mineralised discs, intravenous contrast is now being used more commonly in spinal CT and at the time of writing no studies were found on how this may influence the diagnosis of disc disease. The existing evidence suggests that CT is a valid alternative for the diagnosis of disc disease especially in cases where other factors may preclude the use of MRI.
CT scan showing herniated mineralised disc material at T13-L1 resulting in moderate to severe compression of the spinal cord (left image). Additional herniated mineralised disc material at L1-2 with mild compression of the spinal cord (middle image), as well as at L3-4 without compression of the spinal cord (right image).
Clare Sparks BVetMed(Hons), MRCVS
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