Assessing signal enhancement in distant dipolar field-based sequences

Wilson Barros Jr., Daniel F. Gochberg and John C. Gore

Abstract
The possibility of improving the signal-to-noise efficiency of NMR signal refocused by long-range dipolar interactions has been discussed recently [Branca et al., JMR 187 (2007) 38-43]. For systems where T1<< T2, by including an extra radio-frequency pulse in a standard CRAZED sequence, it is possible to increase the available signal by exploiting its sensitivity to T1 relaxation. Here, we use analytical calculations to investigate the source of this improved signal and determine the maximum enhancement provided by the method.

Apparent Longitudinal Relaxation in Solutions with Intermolecular Dipolar Interactions and Slow Chemical Exchange

Shengchun Zhang, Xiaoqin Zhu, Zhong Chen, Shuhui Cai and Jianhui Zhong

Abstract
A modified CRAZED sequence with selective inversion before excitation was designed to investigate the longitudinal relaxation under the effects of slow chemical exchange and distant dipolar interactions in highly polarized spin systems. Analytical expressions for the apparent longitudinal relaxation time of such systems were derived from a combination of the dipolar field theory and product operator formalism. The result shows that the signal intensity follows a multi-exponential function of the inversion-recovery time. Experimental results support the theoretical predictions.

Finite Element Formulation of the Bloch Equations with Dipolar Field Effects

Louis-S. Bouchard
(Submitted on 24 Jun 2007)
arXiv.org

A Galerkin finite element (FEM) formulation for the Bloch equations with dipolar field is presented which makes possible the derivation of weak solutions to the Bloch equations. The FEM formulation has the advantage that the equations of motion are local in real space, eliminating the global truncation errors associated with calculations of the dipolar field in Fourier space. The dipolar field and other geometric parameters are calculated only once, before the simulation, and used as an initial condition rather than re-calculated at every time step of some numerical integration.