Multiple spin echoes in heterogeneous systems: physical origins of the observed dips.

Dipolar interactions in liquids have recently offered a new challenge to investigate porous media by exploiting intermolecular quantum coherences, which are obtained through a simple two-pulse sequence (90 degrees -tau-120 degrees ). This sequence, in the presence of an external gradient (G), refocuses a train of echoes at multiple integer values of time tau. The first and second echo amplitudes are acquired for heterogeneous systems such as porous media at different time values (tau). In our first experiments on bovine bone samples we have observed unpredicted dips on the second echo time behavior. We argue that a strict relation occurs between the average pore dimensions and the dips time position through the correlation distance d=pi/(gamma G tau) (defined as half a cycle of the magnetization helix, which originates in the presence of an external gradient). Although the experimental results have revealed an exceptional connection between the porous structure and the correlation distance, no physical explanation was so far provided. In this paper we propose a possible physical cause of the observed phenomenon. In addition we report an accurate analysis of new experiments performed on glass beads phantoms, which confirms our conclusions.