MR Imaging of Optic Neuropathy with Extended Echo-Train Acquisition Fluid-Attenuated Inversion Recovery

Discussion

The benefits of FS techniques are well-known for orbital imaging and especially for imaging the optic nerve.^2,5,6 Despite their widespread use, FS techniques, such as STIR, SPIR, and T2 FSE FS, all have the production of high signal intensity from CSF in the perineural sheath. High signal intensity from CSF poses 2 major problems: First, optic nerve signal-intensity abnormalities are made less conspicuous by volume averaging with CSF. Second, the surrounding CSF creates limitations on T2-weighting because the signal intensity from the fluid rises at a faster rate than that of the optic nerve. Furthermore, optic nerve sheath dilation has been shown in the setting of acute optic neuritis, compounding the problem of volume averaging from adjacent CSF signal intensity.⁷ Jackson et al⁸ showed the added advantages of combining fluid and fat suppression in optic nerve imaging with a combined SPIR-FLAIR technique. The authors stressed the benefits of the SPIR-FLAIR sequence resulting from an increased TR and the consequent increased ratio of signal intensity between neuritic and non-neuritic nerve segments and clearer delineation of the optic nerve due to the absence of signal intensity from perineural CSF. However, the SPIR-FLAIR sequence is prone to artifacts from field inhomogeneity, flow, and blurring due to the use of long echo trains.⁸

XETA or Cube FLAIR is a new 3D FSE sequence that can be used to perform whole-brain FLAIR T2-weighted imaging at isotropic high spatial resolution. XETA is a technique that combines the FSE technique, with its desirable T2-weighted contrast and insensitivity to off-resonance artifacts, and 3D volumetric acquisition, with its inherent high signal intensity–to-noise ratio and the potential for very thin sections and ultimately isotropic resolution. XETA was developed with short echo spacing, optimized for very long echo-train readout, enabling fewer TRs to collect all the data. Second, it uses a technique for self-calibrated 2D-accelerated parallel imaging, thereby using fewer echoes to encode more resolution. Finally, by using a novel technique of refocusing flip angle modulation, XETA constrains relaxation during the long readout and voids the blurring that is commonly associated with long echo trains as noted in the SPIR-FLAIR imaging.

The optional chemically-selective FS uses an adiabatic spectral inversion pulse to null fat. This high-resolution volumetric imaging coupled with both fat and fluid suppression permits evaluation of cranial nerve anatomy and signal-intensity abnormality and is ideal for evaluation of optic nerve signal intensity. Very long echo trains and 2D-accelerated parallel imaging combine to vastly improve efficiency, making isotropic acquisition clinically realistic with XETA FLAIR. The variable flip refocusing pseudo-steady state sequence controls signal-intensity decay to produce sharp clear images reformatted in any plane.

We found that XETA is also particularly helpful to perceive signal-intensity abnormality in noninflammatory optic neuropathy and subacute-chronic optic neuropathy, where there may be no gadolinium enhancement. Youl et al³ studied the pathophysiology of acute optic neuritis and the timing of gadolinium leakage. Enhancement, signaling leakage of gadolinium across the blood-brain barrier, was a consistent finding in the acute lesion, but gadolinium leakage had ceased in 9/11 nerves imaged 4 weeks after the onset of acute symptoms. Our finding of no gadolinium enhancement in the 6 patients imaged after 4 weeks, are congruent with the study of Youl et al.³ The lack of gadolinium enhancement in subacute and chronic cases also emphasizes the importance of XETA, because 2/6 patients also had no signal-intensity abnormality and were false-negatives on our standard T2-weighted sequence. The signal-intensity abnormality was only perceived on XETA in these 2 cases.

Our study also suggests that XETA may also be particularly useful for intracranial segment disease. We had 2 patients with isolated intracranial segment disease, and neither patient had signal-intensity abnormality perceived on the standard coronal T2-weighted sequence. We hypothesize that the surrounding CSF signal intensity in this location decreases the conspicuity of T2 hyperintensity.

Another added advantage of XETA over our conventional coronal FSE T2 FS orbital sequence is that XETA includes whole-brain imaging that can be reconstructed in any plane. This is ideal for evaluating demyelinating plaques in MS and is arguably the most important component of imaging for optic neuritis. In the Optic Nerve Treatment Trial, the 5-year risk of developing MS was 16% in patients with normal brain MR imaging findings, 37% with 1–2 lesions, and 51% with ≥3 lesions. At 10 years, the only statistically significant difference was between no lesions (22% risk) and ≥1 lesions (56% risk).¹⁰ Morrissey et al¹¹ also showed that the risk of developing MS was much higher in patients with abnormal initial MR images (approaching 100% at long-term follow-up), and the risk of developing MS remained low in patients with normal MR imaging findings. Therefore, MR imaging during an acute episode of optic neuritis has the unique ability to help predict the likelihood of developing MS after optic neuritis. The use of XETA FLAIR may reduce imaging time because it could be used for evaluation of the optic nerve and whole-brain imaging. Any plane can be reconstructed from a single acquisition at high resolution, thus enabling further reduction in total imaging time if multiple planes of FLAIR imaging are required. XETA FLAIR imaging may also be useful for documenting chronic optic injury and could reduce the need for evaluation by a clinical specialist.

The primary limitations of our study include the small sample size and the lack of comparison with healthy controls. However, reviewers were blinded to the clinically abnormal side. Therefore, in the 13 patients with unilateral abnormality, a clinically normal contralateral optic nerve served as an internal control. Our study strongly suggests that XETA is superior to standard coronal FSE T2 FS for imaging the optic nerve. However, a larger study with healthy controls would be helpful to confirm these findings. Furthermore, evaluations compared with functional assessments such as VEPs could be very useful for assessing the utility of XETA imaging for identifying clinically silent lesions of the optic nerve. XETA FLAIR is a very useful sequence in a combined MR imaging brain and orbits protocol because it not only provides multiplanar imaging for detection of intracranial demyelinating lesions but also has greater sensitivity for optic nerve signal-intensity abnormality than the conventional T2-weighted orbital sequence. Therefore, this single sequence could potentially replace 2 planes of whole-brain imaging and the orbital T2-weighted sequence, streamlining imaging protocols.