Reliability of CT myelography versus MRI in the assessment of spinal epidural disease

Abstract

The spine is the third most common site of metastatic disease after the lungs and liver [1–2]. Several imaging modalities exist for the evaluation of epidural disease, namely computed tomography (CT) and magnetic resonance imaging (MRI). CT myelography (CTM) involves the injection of contrast material into the cerebrospinal fluid prior to imaging and requires ionizing radiation. Though previously widely used, CTM has steadily fallen out of favor to MRI at many institutions and in some cases, is used alone in radiation therapy planning. MRI is notable for its non-invasive nature, excellent visualization of soft tissues, and lack of exposure to ionizing radiation. CTM, however, can provide improved visualization of bony structures and is more resistant than MRI to geometric distortion and patient motion due to the rapid acquisition of images [3]. Additionally, CTM can be used in patients with contraindications to MRI and can also provide a direct correlation of electron density to doses used for radiation therapy planning [4]. With recent advances in radiation therapy, including intensity-modulated and image-guided radiation therapy, which aim to improve tumor targeting thereby sparing normal tissue, precise tumor localization for treatment planning often requires the use of CT integrated with MRI [5–7]. A previous study comparing CTM and MRI in the evaluation of degenerative disease in the cervical spine found that MRI was superior in its reliability for assessing the degree of nerve root compression while CTM provided greater reliability in the assessment of bony lesions and foraminal stenosis [8]. Other studies focusing on the lumbar spine also found that CTM provided greater reliability and reproducibility in the assessment of the extent of lumbar stenosis than did MRI [9] and that MRI underestimated the degree of spinal root compression in the lumbar spine [10]. While both imaging modalities have complementary advantages, studies to date have not investigated a composite evaluation of spinal epidural disease. Furthermore, given the limitations of MRI, it has not been studied whether there is merit in continuing to use CTM to complement MRI for radiation therapy planning. This study is the first to investigate whether axial and sagittal projections of the spinal epidural space using both T1 and T2-weighted MRI compared to CTM produce compatible levels of inter- and intra-rater reliability in measuring the extent of epidural disease by neuroradiologists trained in both imaging modalities. The results of this study will be useful in determining whether CTM should continue to be used in conjunction with MRI when evaluating spinal metastases and planning radiation therapy.