Optimum acquisition times of two spin echoes for MR image synthesis

Spin‐echo images can be synthesized at arbitrary values of echo time T_Es if two images are acquired at the same repetition time and two different echo times T_EI and T_E2. Depending on the value of T_Es, the noise in the synthetic images can either be greater or less than the acquisition noise. This note shows that if the time between the acquired echoes T_E2‐T_E1 is equal to T₂, the noise level in the synthetic images is no larger than the acquisition noise for T_Es ⩾ T_E1. This is the lowest possible noise bound for two‐echo acquisition. Also, the noise bound for images synthesized with O ⩽ T_Es ⩽ T_E1 is minimized by making T_E1 as short as possible. © 1986 Academic Press, Inc.