The clinical utility of fMRI for identifying covert awareness in the vegetative state: a comparison of sensitivity between 3T and 1.5T

Abstract

In the last few years, mental imagery fMRI paradigms have been used successfully to identify covert command-following and awareness in some patients who are thought to be entirely vegetative. However, to date there is only evidence supporting their use at magnetic fields of 3T, which limits their applicability in clinical settings where lower field strengths are typically used. Here, we test the 'gold standard' fMRI paradigm for detecting residual awareness in non-responsive patients by comparing its sensitivity at 1.5T and 3T in the same group of healthy volunteers. We were able to successfully detect brain activity showing command-following in most participants at both 3T and 1.5T, with similar reliability. These results demonstrate that fMRI assessment of covert awareness is clinically viable and therefore justify a broader use of these methods in standard assessments in severely brain injured patients.

Figure 1. Example of individual results for the motor imagery task versus rest.

For each subject, the ROI showing highest consistency across scanning sessions is displayed. All participants, except C03, C08 and C10, showed activation of SMA in at least one scanning session. Participants C03 and C10 activated other anatomically appropriate areas (dorsal premotor cortex and inferior parietal lobule respectively). Participant C08 failed to show any significant activation for any scanning session. Participant C01 failed to show any significant activation in the scanning session at 1.5T while participant C12 failed to do so at 3T. Results are thresholded at a FWE-corrected p<0.05.

Figure 2. ROI group results showing activation of SMA and dorsal premotor cortex for the motor imagery task (i.e. imagine playing tennis) compared to rest at 3T and 1.5T (including 1 and 2 runs).

For display, results are thresholded at an uncorrected p<0.001 and rendered on a canonical single subject T1 MRI image.

Figure 3. Example of individual results for the motor imagery task versus rest.

For each subject, the ROI showing highest consistency across scanning sessions is displayed: occipito-parietal junction (C01, C07 C14), parahippocampal cortex (C02, C06, C13), dorsal premotor cortex (C03, C04, C08 at 3T and 1 run at 1.5T), retrosplenial cortex (C05, C11), and precuneus (C08 at 1.5T, C09, C10, C12). Participants C03 and C12 failed to show significant activation in the scanning session at 1.5T when only 1 run was analyzed but succeeded when both runs were analyzed. Results are thresholded at a FWE-corrected p<0.05.

Figure 4. ROI group results showing activation of the parahippocampal cortex and the occipito-parietal junction for the spatial navigation task (i.e. imagine walking around your house) compared to rest at 3T and 1.5T (including 1 and 2 runs).

For display, results are thresholded at an uncorrected p<0.001 and rendered on a canonical single subject T1 MRI image.