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Fig. 7A —Sodium (23Na) MRI. Sodium (23Na) has
spin quantum number (I) of 3/2 and four possible spin orientations (-3/2,
-1/2, +1/2, +3/2). Three single-quantum (SQ) transitions are possible: an
"inner" or -1/2 
+1/2 transition, and two
"outer" or -3/2 -1/2 and + 1/2 +3/2 transitions.
When 23Na cation is transiently bound to macromolecules, electric
field gradients created allow outer transitions to relax more quickly than
inner transition. In these circumstances, double-quantum (DQ) or
triple-quantum (TQ) transitions become possible. Multiple-quantum transitions
tend to occur in intracellular space due to high concentration of
macromolecules in this compartment. Normal extracellular space is aqueous and
predominantly shows SQ transitions. In disease, accumulation of collagen, as
in hepatic fibrosis, enables multiple-quantum transitions to occur in
extracellular space as well.
