Artefacts

The main sources of artefact in functional MRI are the same as for structural MRI (see Chapter2.3.7). Movement

Movement of the subject's head during fMRI data acquisition is inevitable, and attempts to eliminate it by fixing the head in the scanner may paradoxically exacerbate the problem. The best approach to minimizing movement is to ensure that the subjects are not unduly anxious about the scanning procedure, that they understand clearly what they are being asked to do, and that they are comfortable in the scanner before data acquisition begins. Experiments should be designed so that they do not require the subject to move extensively; small finger movements required for button pressing do not generally cause severe head movement. However, even very small movements of the head (less than 1 mm) can cause significant artefacts in fMRI data.

Involuntary or physiological movements are mostly due to the cardiorespiratory cycle causing pulsation of the cerebrospinal fluid and vascular spaces. Therefore these movements often occur at a higher frequency than the frequency of image volume acquisition, and are aliased into the signal as a low-frequency confound. The problem can be minimized by cardiac gating, which means timing the acquisition of images relative to the cardiac cycle measured by ECG.

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