Table 1.

Effects of head motion on fMRI data grouped by a physical source or scanner subsystem. Severity of the effect is ranked from low to high, where low implies little or no effect on the fMRI data, and high implies considerable data corruption leading to an erroneous data analysis.

source		effect	consequence	severity
RF transmit	1.	motion of the object under investigation relative to the stationary, generally inhomogeneous transmit radiofrequency (TX-RF) fields	contrast modulation	low*
	2.	alteration of the TX-RF field(s) due to the coil loading changes and wave phenomena	contrast modulation	low*
	3.	motion of the object with respect to the signal preparation or slice/slab RF excitation pulses	contrast modulation	high
RF receive	4.	motion of the object relative to the stationary receiver coils and the corresponding radiofrequency fields (RX- RF), i.e. receiver coil sensitivities	intensity modulation	high
	5.	alteration of the spatial distribution of the RX-RF fields due to the coil loading changes, wave phenomena or motion of the coils in case of the flexible coils	intensity modulation	medium*
spatial encoding	6.	motion of the object, including internal structures relative to the image encoding coordinates defined by the gradient pulses, receiver frequency and phase offsets	partial-volume effect modulation	high
	7.	motion of the object during the acquisition of multi- slice packages	inconsistent 3D data	medium
	8.	local variations in spatial resolution corresponding to different positions due to the non-linearity of the spatial encoding fields	modulation of distortions or blurring	low†
Shimming & susceptibility	9.	motion of the object-generated magnetic field inhomogeneities relative to the stationary B0 inhomogeneities and the stationary shim coils.	B0 modulation	medium
	10.	alteration of the susceptibility-induced field distributions associated with a rotation of the object with respect to the B0 direction	B0 modulation	medium*
local B0 modulation	11.	alteration of the effective spatial encoding due to the changes in the local field inhomogeneities (motion to distortion interaction)	modulation of distortions or blurring	medium*
	12.	alterations due to local intra-voxel dephasing	intensity modulation	medium*

The consequence severity is based on the authors’ own experiences at 3T and may vary for different protocols and field strengths. In particular, severity tends to increase with higher main field strength for cells marked with asterisk (*), and also increases when a local head-only gradient coil is used for cells marked with dagger (†).