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Effect of variable echo spacing in multi-echo sequences for resolving long T2 componenets in multiple sclerosis Skinner, Matthew Garrett

Abstract

Five 48-echo CPMG pulse sequences were examined to determine their effectiveness in measuring the T2 relaxation characteristics of cerebral tissue containing multiple sclerosis (MS) lesions. To this end, two sets of experiments were conducted in which the first 32 echoes had an echo spacing of 10 ms while the secondary echo spacing (SES), the final 16 echoes, was increased from 10 to 50 ms. In the first study 10 normal volunteers were imaged and the geometric mean T2 (GM T2) , the peak widths and the myelin water fractions of the resultant T2 distributions were measured and compared to the results of a previous study. The second study simulated the T2 relaxation distributions that are expected in the brains of MS patients using a series of NiCI2 water phantoms. The GM T2, peak widths and myelin water fractions were measured in these phantoms. The volunteer study showed that the T2 distributions in several gray matter structures contained a single main peak (70 - 95 ms) with a low, or zero, myelin water fraction. The resolution of both myelin (10 - 50 ms) and intra/extracellular (70 - 95 ms) water peaks decreased while the accuracy of the GM T2 of the main peak improved as SES increased. Sampling of the decay curves at longer times improved the separation of the short and intermediate T2 compartments. In the phantom study it was demonstrated that there was a clear clinical benefit to the use of a longer SES. When imaging lesions for which the T2 of the intra/extracellular 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 SES was increased. When the lesion T2 was 179 ms, the intermediate and long T2 peaks could not be resolved. The fraction that each component contributed to the mixtures also determined the resolution of their peaks. As the fractional contributions of each component increased so too did its resolution. Increased resolution of one component often resulted in better estimation of the other components. This study demonstrated that for specific clinical situations, it is advantageous to use extended secondary echo spacings for imaging the lesions of MS patients, without the risk of losing valuable short T2 information.

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