An Optimization Method for Quantifying Magnetization Vectors in Magnetic Resonance Measurements
Keywords:
Optimization, Magnetic Resonance (MR), Nuclei Magnetization Vector, Quantitative Analysis, MR Data Acquisition, Longitudinal Relaxation Time, Flip Angle.Abstract
There are increasing interests in quantitative magnetic resonance (MR) measurement techniques for a variety of experimental physics and clinical applications. Recently methods for quantification of local nuclei magnetization magnitude, M0, and its longitudinal relaxation time (T1) have been proposed. An efficient method for measuring T1 values is based on acquiring two spoiled gradient-recalled echo (SPGR) data sets in steady states with different radiofrequency (RF) excitation flip angles, which has also been extended, with additional acquisitions, to obtain M0 values representing the object’s proton density. Several factors, including inaccurate flip angles due to inhomogeneity of the exciting RF magnetic fields and low signal-to-noise ratios (SNR) may negatively affect the accuracy of this method and produce systematic errors in the T1 and M0 estimations. In this paper, a modified dual-acquisition method based on an optimization of the SPGR MR sequence formulism is presented for reliable M0 and its actual flip angle (i.e., magnetization vector, M) measurements with a high spatial resolution and a relatively short experimental time. The optimal nominal flip angles for M0 measurements and the optimal repetition times for estimation of the RF excitation angles are found analytically and numerically using the SPGR MR sequence. The proposed method can measure the magnetization vectors with an isotropic spatial resolution of 1×1×1 mm3 of a large 3D object in less than 5 minutes.
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