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.

Assessing the feasibility of time-resolved fNIRS to detect brain activity during motor imagery

“…However, the fMRI timecourse for this subject was similar to the other subjects and did not indicate inverse oxygenation, and hence this result warrants further investigation. As for the 3 subjects with no detectable fMRI or fNIRS activity, this is not uncommon and similar results have been reported previously in the literature 5 .…”

“…The motor imagery experimental protocol used in this study was previously described elsewhere 5 , which consisted of five cycles of 30 seconds alternating blocks of rest and tennis imagery, with a total experiment time of five and half minutes. The two cue words used in this study were "rest" and "tennis".…”

“…Using motor imagery (i.e., imagining playing tennis), Owen et al first introduced the concept of command following by regulating brain activity using fMRI 4 . Motor imagery such as imagining playing tennis activates brain regions such as the SMA and the PMC 1,4,5 . The results of that study showed that a patient who was clinically diagnosed as being in a vegetative state was able to follow commands by regulating her brain activity.…”

On the difficulty to communicate with fMRI-based protocols used to identify covert awareness

“…Our results confirm those of Fernandez-Espejo in which the protocol has been realized at 1.5T and 3T providing similar patterns of activations at the group level but with a non-systematic activation of ROIs at the individual level (Fernandez-Espejo et al, 2014). Before being offered as standard clinical care in a unit, fMRI paradigms used to study volitional brain activity in DOC patients first have to be validated on cohorts of healthy volunteers.…”

“…However, although other studies have successfully reproduced similar activations at the group level using this paradigm of detecting cerebral activity modulation on aware subjects (Aslaksen et al, 2012, Liang et al, 2014, the robustness of this paradigm (on aware subjects) has yet to be confirmed at the individual level. Indeed, if the healthy controls recruited in the seminal works of Boly (Boly et al, 2007), Monti (Monti et al, 2010) and Owen (Owen et al, 2006) activated the SMA during motor imagery and the PPA during spatial imagery, a recent article from the same team (Fernandez-Espejo et al, 2014) could not replicate such a result. In that study, at best 85.7% of healthy subjects activated the SMA during motor imagery, and at best 57.1% activated the PPA during spatial imagery.…”