Removing facial features from structural MRI images biases visual quality assessment
Fig 1
An example of T1w image before and after defacing.
Defacing is typically implemented by zeroing the voxels around the face (left-hand side, panel “T1-weighted”). The “Background noise” panel shows two visualizations extracted from the MRIQC visual report, in which a window is applied to select the lowest intensities and then the latter are inverted to enhance patterns in the background. The red arrows indicate aliases along the anterior–posterior axis—which had the lowest bandwidth in the example—produced by eye motion. These aliases are straightforward to notice in front of the ocular globes in the “Background noise” panel of the “nondefaced” image because of the absence of other signal sources. This aliasing also spreads in the opposite direction, overlapping brain tissue. However, this overlap is often very hard to notice against the signals of interest within the brain. The corresponding “defaced” version of the “Background noise” panel shows how defacing eliminates valuable information for quality assessment.