Time at which the maximum amplitude of a gradient echo (GRE) and/or RF echo is observed in NMR and MR imaging , measured from the center of the initial excitation pulse . The echo time determines the degree of T2 - or T2* - contrast weighting of an image acquired with a spin echo (SE) or gradient echo (GRE) sequence, respectively: the longer the echo time the stronger the corresponding contrast contribution. In standard spin- and gradient-echo imaging, the echo time relevant for a MR image is well defined, as all signal echoes are acquired at identical echo time. In contrast, in the RARE pulse sequence , echo-planar imaging pusle sequences or GRASE pulse sequence imaging, the echo time cannot be exactly defined. An effective echo time is specified to indicate the degree of T2 or T2* weighting.