Proton T₂ relaxation study of water, N‐acetylaspartate, and creatine in human brain using Hahn and Carr‐Purcell spin echoes at 4T and 7T

Abstract

spin-echo (SE) measurements were used to estimate the apparent transverse relaxation time constant (T 2 † ) of water and metabolites in human brain at 4T and 7T. A significant reduction in the T 2 † values of proton resonances (water, N-acetylaspartate, and creatine/phosphocreatine) was observed with increasing magnetic field strength and was attributed mainly to increased dynamic dephasing due to increased local susceptibility gradients. At high field, signal loss resulting from T 2 † decay can be substantially reduced using a Carr-Purcell-type SE sequence.Magn Theoretical and experimental studies have shown at least a linear increase in sensitivity with magnetic field strength (1,2). On the other hand, the transverse relaxation rate is known to increase with magnetic field strength (3,4), which can result in reduced sensitivity in spin-echo (SE) experiments. The apparent transverse relaxation time (T 2 † ) is related to the intrinsic transverse relaxation time (T 2 ) through the following equation:The first term on the right side of Eq.[1] is the inverse of the intrinsic T 2 and is governed by a number of possible mechanisms, including 1) homonuclear dipole-dipole interaction between protons, which is strongly dependent on rotational correlation time c ; 2) hyperfine (contact) interaction, namely, the change of transverse relaxation time due to interaction with a paramagnetic center; and 3) cross-relaxation, which can be significant in dipole-coupled systems. The second and third terms, T 2,Diffusion and T 2,Exchange , are the transverse relaxation times related to diffusion and exchange of spins between regions with different magnetic field strengths, respectively. These contributions describe the dynamic dephasing regime, whereby the net magnetization is reduced by diffusion and exchange between regions with different magnetic field strengths, which causes the phases of the different spin packets to average out. The opposite situation is defined as the static dephasing regime. NMR signal loss due to static dephasing can be refocused by SE sequences and is therefore not considered here.It is important to investigate: 1) how the increase of field strength causes T 2 † shortening, and 2) how the signal loss from T 2 † decay can be compensated for. Key experiments for answering these questions involve measuring T 2 † at different field strengths and attempting to estimate T 2 . The theory of NMR signal formation in the presence of local magnetic field inhomogeneity was first derived by Carr and Purcell (5), and later generalized by Torrey (6), who incorporated the diffusion effects into the Bloch equations to take into account the actual field distribution. The CarrPurcell (CP) method is the most valuable technique for determining transverse relaxation times. CP experiments are performed by applying a /2 pulse followed by a series of pulses spaced with time interval cp . The value of T 2 † determined with a CP technique can vary with cp because dynamic dephasing and homonuclear spin-spin coupling can cause signi...

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