Postprocessing correction for distortions in T2* decay caused by quadratic cross‐slice B0 inhomogeneity

Relaxometric measurement of the effective transverse relaxation rate R  2* plays an important role in the quantitative evaluation of brain function, perfusion, and tissue iron content. However, accurate measurement of R  2* is prone to macroscopic background field inhomogeneity. In clinical applications and systems, postprocessing correction techniques are more flexible in implementation than unsupported protocol or hardware modifications. The current postprocessing correction approach assumes the cross‐slice background field inhomogeneity can be approximated by a linear gradient and corrects for a sinc modulation function. The importance of the high‐order terms in background field inhomogeneity has increased with the fast development of high‐ and ultrahigh‐field scanners in recent years. In this study, we derived an analytical expression of the free induction decay signal modulation in the presence of a quadratic cross‐slice background field inhomogeneity. The proposed quadratic correction method was applied to phantom and volunteer studies and demonstrated to be superior to the classic monoexponential model, monoexponential‐plus‐constant model, and the linear sinc correction method in recovering background field inhomogeneity‐induced. R  2* overestimations with visual inspection of R  2* parametric maps and a statistical model selection technique. We also tabulated 7‐T T  2* /R  2* measurements of several human brain structures and MnCl2 solutions with various concentrations for fellow researchers' reference. Magn Reson Med 63:1258–1268, 2010. © 2010 Wiley‐Liss, Inc.