ACCURATE DETERMINATION OF SPIN-DENSITY AND T-1 IN THE PRESENCE OF RF-FIELD INHOMOGENEITIES AND FLIP-ANGLE MISCALIBRATION

A method is presented for the accurate extraction of relative spin-den sity (rho(0)) and spin-lattice relaxation time (T-1) in the presence o f RF-field inhomogeneities and flip-angle miscalibration, The method r equires collecting images at several flip-angles with a three-dimensio nal, spoiled steady-state, gradient-echo imaging sequence, Results sho w that the predominant effect of an overestimated flip-angle is to shi ft the T-1 estimate to a higher value, whereas reductions in the norma lized RF-field from unity cause rho(0) and T-1 distributions to be ske wed toward lower values. Phantom and in vivo results demonstrate that the proposed method overcomes both of these systematic errors. The met hod 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 syste matic errors in the extracted rho(0) and T-1 values over a large numbe r of voxels, This made it possible to obtain a very precise estimate o f muscle T-1 at 1.5 T, yielding a 95% confidence interval of (1077.7 /- 3.5) ms.