Accurate determination of spin‐density and T1 in the presence of RF‐field inhomogeneities and flip‐angle miscalibration

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Abstract
A method is presented for the accurate extraction of relative spin‐density (ρo) and spin‐lattice relaxation time (T1) in the presence of RF‐field inhomogeneities and flip‐angle miscalibration. The method requires collecting images at several flip‐angles with a three‐dimensional, spoiled steady‐state, gradient‐echo imaging sequence. Results show that the predominant effect of an overestimated flip‐angle is to shift the T1 estimate to a higher value, whereas reductions in the normalized RF‐field from unity cause ρo and T1 distributions to be skewed toward lower values. Phantom and in vivo results demonstrate that the proposed method overcomes both of these systematic errors. The method was shown to be valid for up to a 50% reduction in RF sensitivity. A self‐consistency argument was used to validate the absence of systematic errors in the extracted ρo and T1 values over a large number of voxels. This made it possible to obtain a very precise estimate of muscle T1 at 1.5 T, yielding a 95% confidence interval of (1077.7 ± 3.5) ms.