The ACR recommends that percent image uniformity (PIU) is calculated as follows:
PIU = 100 x [1 - ((maximum signal - minimum signal) / (maximum signal + minimum signal)) ]
IPEM Report 80 suggests acquiring images of a flood field / spherical test object. Profiles are drawn through the centre of the test object in both the frequency and phase encoding directions. An average of several profile lines is taken in order to reduce the effect of noise. The modal value of a ROI positioned centrally in the test object is determined. The fractional uniformity is then calculated, defined as the fraction of the profile which lies within ±10% of the modal value. Typical values lie between 0.6 and 1.0.