Reliability of CT scans in the diagnosis of intervertebral disc extrusion in dogs

Imagine this clinical scenario: you are presented with a Dachshund with acute onset paraplegia that you suspect has an intervertebral disc extrusion between the T3 and L3 spinal cord segments. You plan to perform a hemilaminectomy procedure and want to evaluate the evidence behind whether computed tomography (CT) is a reliable modality in comparison to magnetic resonance imaging (MRI) for the diagnosis and surgical planning of thoracolumbar disc extrusions in dogs.

The evidence

Four papers were critically appraised: two were prospective studies and two were retrospective studies.

Prospective studies

The study by Cooper et al. (2014) directly compared the sensitivity of MRI and CT in the diagnosis of thoracolumbar intervertebral disc disease in 44 dogs. MRI was deemed to have a sensitivity of 98.5 percent for detecting intervertebral disc herniation, compared to a sensitivity of 88.6 percent with CT examination. MRI was also more accurate (93.2 percent) when compared with CT (84.1 percent) for locating intervertebral disc herniation, as well as in differentiating disc protrusion versus extrusion (MRI at 94.4 percent versus CT at 85.7 percent) and lesion lateralisation (MRI at 95 percent versus CT at 69.9 percent). A limitation of the study is that there was no evaluation of CT-positive MRI-negative disc herniations and therefore no ability to fully compare MRI to CT for lesion characterisation.

The “golf tee sign” and subarachnoid filling deficits were observed in three of the eight cases using CT myelography, suggesting that it may be more sensitive for detecting intradural disc herniations

Tamura et al. (2015) studied eight dogs, three of which had a CT myelogram and five of which had an MRI to identify the presence of intradural herniation (as confirmed at surgery). MRI misdiagnosed all cases in the study, failing to identify any intradural herniation and, instead, characterising the cases as standard intervertebral disc herniation. The “golf tee sign” and subarachnoid filling deficits were observed in three of the eight cases using CT myelography, suggesting that it may be more sensitive for detecting intradural disc herniations. Limitations include the fact that MRI and CT examinations were not performed in the same animal, so findings could not be directly compared, and that the MRI which was used was a low-field scanner, so findings may differ if a high-field scanner was used.

Retrospective studies

A third study (Emery et al., 2018) examined 555 dogs that underwent CT imaging, with the outcome studied being the need for additional imaging for the diagnosis of a thoracolumbar myelopathy. Dachshunds were the least likely breed to need additional imaging following non-contrast CT, with only 12/335 (3.6 percent) requiring further imaging compared to 30/220 (13.6 percent) of the other dog breeds. The presence of a normal CT or multiple intervertebral disc herniations identified on non-contrast CT (and therefore difficulty distinguishing acute from chronic lesions) were both cited as common reasons for requiring additional imaging techniques. A limitation is that the accuracy of lesion localisation and lateralisation was not assessed in this study and therefore cannot be evaluated.

The presence of a normal CT or multiple intervertebral disc herniations identified on non-contrast CT […] were both cited as common reasons for requiring additional imaging techniques

In the final study (Noyes et al., 2017), 40 dogs underwent both CT and MRI and the influence on preoperative planning considerations for hemilaminectomies based on CT versus MRI was studied. There was a large range of inter-observer agreement for both imaging modalities when assessing the site, side and size of laminectomy approach. All observers planned a larger laminectomy defect based on MRI compared to CT imaging. All lesions were identified in cases when using MRI; however, lesions were not identified in four of these cases when assessing CT images alone. A limitation of the study is that no statistical analysis was performed to directly compare the reliability of CT and MRI in detecting lesion localisation.

Conclusion

Overall, MRI was found to have an increased sensitivity for the detection of intervertebral disc disease in dogs compared to non-contrast CT. However, low-field MRI misdiagnosed all cases of thoracolumbar intradural disc herniation when compared with CT myelography in dogs. Non-contrast CT appears to be a sufficient imaging modality for intervertebral disc disease and surgical planning in the majority of Dachshunds.

The reason behind the diagnostics performed and the likelihood of other differential diagnoses should be taken into account before deciding to use one imaging modality over the other

The strength of evidence for each paper is considered moderate, although no systematic reviews or meta-analyses have been performed for this topic. The reason behind the diagnostics performed (eg surgical planning) and the likelihood of other differential diagnoses (eg spinal neoplasms) should be taken into account before deciding to use one imaging modality over the other. Cost, procedure time and need for a general anaesthetic are also factors that should be considered when deciding between MRI and CT examinations.