Accuracy of the Compressed Sensing Accelerated 3D-FLAIR Sequence for the Detection of MS Plaques at 3T

Discussion

Reducing the MR imaging scan time may improve patient comfort, cost-effectiveness, motion-related artifacts, and thus reproducibility. This improvement may be particularly useful for 3D FSE sequences, such as FLAIR, that usually require long acquisition times. In this study, we found that by adding CS to standard PI acceleration strategies, we can reduce the scan time of 3D-FLAIR by 27% (1 minute 25 seconds) while preserving the accuracy of the detection of MS plaques. This result appears clinically relevant, considering the widespread use of 3D-FLAIR in MR imaging protocols dedicated to MS.⁴ Additionally, the agreement of results produced automatically by postprocessing further strengthens our conclusion, providing an unbiased analysis and taking into account all the visible MS lesions, including those involving the deep white matter.

To our knowledge, the combination of PI and CS to achieve FLAIR imaging has never been evaluated in patients with MS. The use of CS in neuroimaging has been evaluated in healthy volunteers with diffusion imaging¹⁴ and in patients with intracranial aneurysms using TOF-MRA.¹⁵ Our results are also fully concordant with recent studies evaluating CS in cardiac, abdominal, or knee imaging.^{16⇓⇓–19} Indeed, these studies have shown comparable image quality and lesion conspicuity between accelerated and nonaccelerated sequences. Admittedly, higher CS factors and scan time reductions can be achieved in MR sequences producing high contrast images with high data sparsity such as TOF-MRA, but such analysis is beyond the scope of the present study.

We observed image blurring in some of the CS FLAIR acquisitions. Such findings appear inherent to the CS technique that uses a variable random undersampling of the k-space, more pronounced at the periphery, thus leading to blurred images. While the quality of CSF suppression was identical in CS FLAIR and conventional FLAIR, a slight difference in deep brain nuclei and gray-white matter junction delineation was observed due to minor motion-related blurring that did not affect the diagnostic performance. We used a simplified 3-point scale to evaluate image quality. A more detailed scale may have exposed these minor differences, even though they have no impact on diagnostic performance. We performed accelerated FLAIR with a CS factor of 1.3 after testing a wide range of acceleration factors (detailed in the preliminary study) because this value appeared to us as a good compromise among blurring, reduced scan time, and accuracy.

In this study, we aimed at reducing the scan time of the 3D-FLAIR sequence with CS and evaluated its diagnostic performance. However, CS could also be used to further improve image quality in the same acquisition time of the conventional sequence by, for example, increasing the spatial resolution and/or contrast. Such an approach could be particularly useful to reduce partial volume effects and improve delineation of the cortical ribbon.

Of note, the diagnostic performance of both conventional and CS FLAIR appeared similar regardless of the topography of the inflammatory lesions, including those involving the optic nerves. Such findings may be particularly relevant in patients suspected of having MS and for whom spatial dissemination is customarily evaluated on the basis of FLAIR images.¹

This study has some limitations. First, we did not include healthy controls since SNR values were also measured within the normal-appearing white matter of patients. Second, the number of MS lesions detected tended to differ between the 2 blinded readers (particularly for periventricular lesions). Such differences may be related to the small size of some lesions. However, both readers found no significant difference between conventional and CS FLAIR, whatever the lesion topography. We did not evaluate the impact of the acceleration factor value on diagnostic performance because we systematically applied a previously determined (by a preliminary study) CS factor of 1.3. Such a study design would have led to an important increase in protocol duration, which was not acceptable in a clinical setting. Further studies may investigate the optimal combination of acceleration factors of PI and CS according to the artifacts observed. Because we only included follow-up MRIs of patients with relapsing-remitting MS, we did not evaluate the performance of CS FLAIR in other types of MS, such as clinically isolated syndrome. However, it is reasonable to expect that the detectability of MS plaques using FLAIR may not differ according to MS types.