Diagnostic Quality of 3D T2-SPACE Compared with T2-FSE in the Evaluation of Cervical Spine MRI Anatomy

Discussion

In this study, we compared the visibility scores of 12 anatomic structures of the cervical spine and CSF pulsation artifact on T2-FSE and T2-SPACE MR imaging sequences. Five anatomic structures, namely the intervertebral disc, neural foramina, ligamentum flavum, ventral rootlets, and dorsal rootlets, were better seen on T2-SPACE compared with T2-FSE for both reviewers, and CSF pulsation artifact was less on T2-SPACE. The remaining structures showed statistical equivalency in visualization or discordance in visualization between both reviewers.

This is the second study to focus on visualization of normal anatomic structures of the C-spine at 1.5T MR imaging strength by using T2-SPACE. Compared with the first such study by Meindl et al,⁸ which included only healthy volunteers, ours included older patients with clinical indications warranting C-spine MR imaging and is arguably more clinically relevant. Similar to Meindl et al,⁸ we also found that intraspinal structures (rootlets) and neural foramina were better visualized with T2-SPACE, while also evaluating additional anatomic structures not evaluated in that study, namely the ligamentum flavum and longitudinal ligaments.

Comparable with Meindl et al,⁸ our study also found less CSF pulsation artifact (better CSF visibility) on T2-SPACE in comparison with T2-FSE. Conventional T2-FSE images are often wrought with pulsation artifact,¹¹ which greatly diminishes their utility in visualizing thecal sac contents, especially small structures such as the rootlets. Many patients undergo invasive CT myelography¹² or contrast-enhanced MR myelography¹³ to interrogate these structures. The ability of T2-SPACE to better identify the rootlets could be helpful in clinical diagnosis and surgical planning because no intrathecal contrast is needed and there is no radiation exposure. This “myelographic” application of T2-SPACE should be explored via prospective comparative trials with CT and MR myelography.

In contrast to studies of degenerative disease by using T2-SPACE^{2⇓⇓⇓⇓–7} and the MR imaging anatomy study by Meindl et al,⁸ our interobserver agreement for T2-SPACE ranged from slight to fair. It is unclear if this reflects any underlying difference in experience with this sequence between the 2 reviewers. Reviewer 1 (attending musculoskeletal radiologist) had no experience with this newer sequence, whereas reviewer 2 (attending neuroradiologist) had approximately 1 year of experience. It is also unclear if this difference may reflect an underlying learning curve associated with the T2-SPACE sequence or may be related to the different approaches to spine MR imaging interpretation between these 2 subspecialties. These questions could be assessed in future studies by testing multiple reader interobserver agreement after various amounts of experience with the sequence. In addition, future studies may benefit from training observers before the onset of the study to establish baselines for the measurements and thereby improve interobserver agreement.

Interestingly, we also found slight to fair interobserver agreement for the T2-FSE sequence. Although this finding has not been reported thus far for C-spine MRIs, a 2005 study by van Rijn et al¹⁴ found greater than 50% interobserver disagreement when they evaluated lumbar spine MRIs for disc herniation. That study included only neuroradiology-trained reviewers and examined degenerative lumbar spine disease (defined as osteophyte at endplates, disc herniation, central canal stenosis, and lateral recess stenosis). Interobserver studies of C-spine MR imaging have reported a wide range of agreement for detection of degenerative disease on T2-FSE or T2-SPACE.^{15⇓⇓–18} None, however, have reported interobserver agreement for T2-FSE visualization of C-spine anatomic structures. Our interobserver agreement results may represent the first description of visualization and detection differences of C-spine MR imaging anatomy on T2-FSE between musculoskeletal radiologists and neuroradiologists.

Our study limitations are as follows:

1. We used a retrospective study design, which raises the concern for selection bias and the presence of unknown confounders, both of which can be better addressed by using a prospective, randomized design;

2. Although we used a paired study design, we assessed visualization with only 2 reviewers. Future studies could use a multireader design, which would allow better evaluation of interobserver agreement and allow a lower sample size to find statistical significance¹⁹; and,

3. For T2-SPACE evaluation, we did not compare visualization scores between inexperienced versus experienced readers.

We feel the biggest advantage of T2-SPACE is the ability to acquire isotropic imaging data at millimeter or sub-millimeter section thickness with a single sagittal acquisition followed by multiplanar reformats. This is in contradistinction to conventional T2-FSE imaging, where the increased section thickness, and the variable addition of skip sections in some clinical settings, leads to an averaging of the area imaged rather than depicting the true anatomy. In addition, the artifacts that can cause diagnostic problems on T2-FSE tend to stem from CSF pulsation, which is minimized on T2-SPACE.

Potential roles for the T2-SPACE sequence may include, but are not limited to: 1) supplanting contrast-enhanced CT or MR myelography; 2) replacing conventional T2-FSE sequences in the imaging of degenerative C-spine disease, similar to a recent study examining lumbar spine MR imaging²⁰; 3) assessing traumatic C-spine ligamentous injury and nerve root avulsions; and, 4) generating oblique sagittal MPR images for the evaluation of C-spine neural foramina without the time cost associated with T2-FSE oblique imaging.²¹ However, robust comparative effectiveness studies are needed to further characterize the benefits and limits of this sequence's uses.