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Effects of selective pulses on the measurement of T 2 and apparent diffusion in multiecho MRI
Author(s) -
Majumdar S.,
Gore J. C.
Publication year - 1987
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.1910040204
Subject(s) - nutation , spins , transverse plane , diffusion , nuclear magnetic resonance , physics , slab , plane (geometry) , computational physics , optics , condensed matter physics , mathematics , geometry , structural engineering , astronomy , geophysics , engineering , thermodynamics
The effects of using spatially selective and nonuniform radiofrequency pulses on the apparent T 2 of a train of multiple echoes have been calculated by computer simulation. It is demonstrated that the use of selective pulses produces errors in estimates of T 2 in imaging experiments because regions of the slice are effectively off resonance and therefore receive spin rotations less than in an ideal case. Components of the transverse magnetization that are incompletely rotated by 180° refocusing pulses produce longitudinal components that are later returned to the transverse plane and add to subsequent echoes. These so‐called ghost components influence the precise value of T 2 estimated, but even when completely removed the resultant estimate is too short. The apparent T 2 is influenced by the magnitude of the nutation angle inhomogeneity across the slice and paradoxically is not always more accurate with larger numbers of echoes. The reduction of apparent T 2 caused by increasing the slice‐selection gradient across a thin slab of material replicates the effects previously ascribed to diffusion and affords an alternative explanation for these without recourse to motion of the Spins. © 1987 Academic Press, Inc.