Effect of the spin-locking B₁ radiofrequency field strength on the signal enhancement with Magic Sandwich Echo...

Contrast methods based on dipolar coupling are of great interest for imaging tissues containing large macromolecules, such as myelin. Most of these conventional methods deal with various "relaxation" phenomena influenced by dipolar coupling such as inhomogeneous magnetization transfer. In this work we propose to investigate the benefit of using another method, called magic sandwich echo (MSE), which allows direct modulation of the dipolar coupling (H_d) as described by the work of Matsui and the Redfield theory. To verify the potential of this method in biological tissue, we first proposed an experimental model for dipolar coupling modulation in an ex vivo tendon (as a highly anisotropic tissue) and used it to prove H_d modulation by varying the amplitude of the spin-lock radiofrequency pulse of this sequence. We then proposed a potential in vivo usable metric, directly related to the residual amount of H_d, which we called MaSteR for Magic sandwich echo to Stimulated echo ratio, as it is based on the ratio of the signal acquired with the MSE sequence and a stimulated echo sequence. First, we show that the higher H_d, the more effective the spin-lock radiofrequency amplitude. We measured with MaSteR that the change in radiofrequency amplitude allowed us to distinguish between different H_d intensities, with a greater MaSteR when H_d is higher.