The effect of varying echo spacing within a multiecho acquisition:: better characterization of long T2 components

A 48-echo pulse sequence with five different echo-spacing combinations was examined to determine how one can most effectively measure the T-2 relaxation characteristics of cerebral tissue containing a long T-2 component. For each scan, the first 32 echoes had an echo spacing of 10 ms, while the spacing for Echoes 33-48 (Delta TE2) was 10, 20, 30, 40 or 50 ms. In an in vivo study using 10 normal volunteers, it was found that the resolution of T-2 distribution peaks for both myelin water (similar to 20 ms) and intracellular/extracellular (IE) water (similar to 80 ms) improved as Delta TE2 increased. The geometric mean T-2 values of the main peak agreed within the error for all Delta TE2 values. A phantom study simulated T-2 relaxation distributions that are expected in the brains of patients with demyelinating diseases. For phantoms in which the T-2 values of the IE and lesion (200-500 ms) water compartments were separated by at least a factor of 3, each compartment in the distribution was better resolved when Delta TE2=40 or 50 ms. On the basis of these results, we recommend the use of extended Delta TE2 values for imaging patients with lesions, without the risk of losing valuable short T-2 information. (c) 2007 Elsevier Inc. All rights reserved.