Echo-spacing optimization for the simultaneous measurement of reversible (R2') and irreversible (R2) transverse relaxation rates

Accurate measurement of reversible (R2') and irreversible (R2) transverse relaxation rates plays a key role in various magnetic resonance imaging research and applications. Although optimization of echo spacing for a multiecho pulse sequence measuring a single exponential decay has been investigated, optimization in sequences such as Gradient-Echo Sampling of Free Induction Decay and Echo (GESFIDE), in which two echo trains are simultaneously measured to obtain both R2 and R2', has not been reported. In this work, optimum echo spacings for the GESFIDE sequence are determined to improve the accuracy of measured relaxation parameters. Various relaxation rates and the number of acquired echoes are considered, as well as whether the receiver bandwidth is kept fixed or is varied with echo spacing. In the case of constant receiver bandwidth, results show that the echo train length approximately equal to T2* should be used for each echo train in GESFIDE to minimize uncertainty in R2 or R2'. If the receiver bandwidth is allowed to change with echo spacing in order to maximize the image signal-to-noise ratio, the optimum echo train length will vary, generally increasing with the number of echoes.