Optimal echo spacing for multi-echo imaging measurements of Bi-exponential T2 relaxation

Calculations, analytical solutions, and simulations were used to investigate the trade-off of echo spacing and receiver bandwidth for the characterization of bi-exponential transverse relaxation using a multi-echo imaging pulse sequence. The Cramer-Rao lower bound of the standard deviation of the four parameters of a two-pool model was computed for a wide range of component T2 values and echo spacing. The results demonstrate that optimal echo spacing (TEopt) is not generally the minimal available given other pulse sequence constraints. The TEopt increases with increasing value of the short T2 time constant and decreases as the ratio of the long and short time constant decreases. A simple model of TEopt as a function of the two T2 time constants and four empirically derived scalars is presented.