Improvement in the contrast of CEST MRI via intermolecular double quantum coherences.

The tremendous potential of chemical exchange saturation transfer (CEST) agents as an emerging class of magnetic resonance imaging contrast media has been demonstrated in recent years. In a CEST experiment, a high CEST contrast is always welcome. However, when the exchange rate is low, which may happen in exchangeable solute protons of low concentration, it is usually hard to obtain an excellent CEST efficiency. Recently, we noted that the intermolecular multiple quantum coherence signal is more sensitive to the changes of the magnetization magnitude than a conventional single quantum coherence signal. Consequently, it may be easier when used in obtaining a CEST contrast. In this note, a modified COSY (two-dimensional correlated spectroscopy) revamped with an asymmetric Z-gradient echo detection (CRAZED) sequence combined with an off-resonance saturation pulse followed by a standard spin-echo imaging sequence was designed to obtain a better CEST contrast image based on the intermolecular double quantum coherence method. An analytical expression was derived from a dipolar field theory. Experiments were performed on an agar-glucose phantom, and the results demonstrate the feasibility of our method.