A regular theme here at Neuroskeptic is the worrying issue of head movement during brain scans. We've seen that motion can alter measures of functional and structural connectivity, and that common approaches to dealing with this problem may be inadequate. Now a new study reveals that even measures of the gross structure of the brain can be biased by excessive motion: Head motion during MRI acquisition reduces gray matter volume and thickness estimates. Harvard neurologists Martin Reuter and colleagues took twelve healthy people and told them to move their heads at certain points during a series of structural MRI scans. Reuter et al. measured the degree of movement using navigator scans throughout the experiment, allowing them to correlate the amount of motion with the measured brain structure. They found a strong correlation between the amount of movement and measures of brain cortical thickness and grey matter density (morphometry). The more someone moved, the less grey matter they seemed to have, and the size of this effect was pretty large:

As the authors put it, this shows

Spurious, systematic effects of motion in morphometric estimates. Even small amounts of motion are sufficient to bias results enough to potentially overshadow real effects.

Reuter et al. found that the motion effect was seen across a range of leading structural MRI analysis packages, including VBM8, FreeSurfer, and FSL Siena. They say that:

It is important to stress that our results imply that the spurious effects do not reflect a processing failure of the morphometry tools. Rather, the images themselves contain consistent changes, such as motion-induced blurring, that appear similar to gray matter atrophy and cause systematic bias.

In most studies, neuroscientists will manually screen scans that seem to be affected by motion (e.g. that appear blurry) and exclude those data. However, Reuter et al. say that this is only partially effective:

Critically, the exclusion of scans that fail a thorough quality check is not sufficient to account for motion as a confounding variable, as significant measurement bias can still be detected after removing these scans. Even keeping only the highest quality scans... we still detect a similar trend, i.e. that motion causes spurious thinning.

They conclude with a warning that motion could, plausibly, lead to incorrect conclusions from many types of studies, especially those studying brain degeneration and aging, conditions in which grey matter volume really does decrease:

These findings imply that great care needs to be taken when studying movement disorders or any disease/condition that affects motion directly or indirectly... in longitudinal studies, motion levels likely increase concurrent with disease severity, inducing increased spurious atrophy rates... any sedative, tranquilizing or neuromuscular blocking drugs may provide the desired "effect" of reduced atrophy rates or even apparent neural augmentation, simply because they inhibit motion rather than providing a true anti-disease effect on brain structure.

What's the solution? They say that efforts to physically prevent head movement, such using a pillow to ensure that the subject's head is snugly fit into the scanner, are important. But these can never remove 100% of the motion. Therefore, they suggest that using navigator scans to directly measure motion (as they did) is vital. Retrospective image quality control and motion correction is insufficient.

Reuter M, Tisdall MD, Qureshi A, Buckner RL, van der Kouwe AJ, & Fischl B (2014). Head motion during MRI acquisition reduces gray matter volume and thickness estimates. NeuroImage, 107C, 107-115 PMID: 25498430