Today, EPI (Echo Planner Sequence) sequence is widely used, due to fast acquisition rate, which allows the activation response to short stimuli to be detected. EPI also has the benefit of reduced artefact from subject motion. To obtain the maximum signal change for a region with a particular value of T2*, the optimal value of echo time can be shown to be equal to the T2* value of that tissue. The contrast to noise ratio of the BOLD signal also depends on voxel size and slice thickness. Smaller voxels have less proton signal due to the reduced number of spins, however larger voxels may reduce the contrast to noise ratio by partial volume effects. This occurs if the signal changes on activation come from only a small region within the voxel, and so makes less of an impact on the total signal change in that voxel.