Computer Simulations of 3D MPRAGE in Human Brain with Inclusion of Inadvertent Magnetization Transfer Effects

Introduction: In the human brain, both white matter and gray matter contain significant amounts of bound water. Although the bound water is not directly seen in conventional MRI experiments, due to its very short T2, the effects of RF pulses on the bound water can influence the free water signal through proton exchange. This effect is known as magnetization transfer (MT) [1], and when MT effects are unintentionally produced, the result has become known as inadvertent MT. Our simulations show that, for conventional 3D MPRAGE experiments at 4.0 T, the inadvertent MT effects are significant, but not substantial. At 4.0 T and higher strength fields, the B1 inhomogeneity over the human head becomes quite significant [2], producing deleterious effects, including regionally varying contrast and loss of signal to noise (S/N). Although B1 shimming has become a popular approach for reducing the B1 inhomogeneity, B1 shimming requires both the MRI instrument and the excitation coil be equipped with multiple RF channels. Another approach, suitable for 3D MPRAGE experiments, is the use of RF pulses which produce uniform tips even in the presence of non-uniform B1 fields (B1 insensitive pulses) [3,4]. We have recently designed our own B1 insensitive pulses [ISMRM 2010, submitted]. However, as the B1 insensitive pulses are invariably much longer than the rectangular pulses they replace in the conventional 3D MPRAGE experiment, the inadvertent MT effects can be quite substantial. Thus, our aim was to develop a simulation program for 3D MPRAGE experiments that incorporated MT effects into the simulation.