Transverse relaxation in fixed tissue: Influence of temperature and resolution on image contrast in magnetic resonance microscopy.

To describe transverse relaxation of water in fixed tissue, we propose a model of transverse relaxation accelerated by diffusion and exchange (TRADE) that assumes exchange between free (visible) and bound (invisible) water, which relax by the dipole-dipole interaction, chemical exchange, and translation in the field gradient. Depending on the prevailing mechanism, transverse relaxation time (T 2 ) of water in fixed tissue could increase (when dipole-dipole interaction prevails) or decrease with temperature (when diffusion in the field gradient prevails). Chemical exchange can make T 2 even temperature independent. Also, variation of resolution from 100 to 15 μm/pxl (or less) affects effective transverse relaxation. T 2 steadily decreases with increased resolution ( T 2 ∝ ∆ x 2 , ∆ x is the read direction resolution). TRADE can describe all of these observations (semi)quantitatively. The model has been experimentally verified on water phantoms and on formalin-fixed zebrafish, mouse brain, and rabbit larynx tissues. TRADE could help predict optimal scanning parameters for high-resolution MRM from much faster measurements at lower resolution.