Molecular magnetism This category of MRI contrast mechanisms is useful to assess molecular properties and their changes in metabolism (oxygen metabolism, iron metabolism): Electron cloud response – chemical shift at molecule nuclei, diamagnetic field outside molecule Unpaired electrons – strong paramagnetic field outside molecule
Tissue magnetism Orbital response (Lorentz force): Magnetic moment opposing B Diamagnetic Chemical shift at nucleus B Unpaired e - Spin response (torque): Magnetic moment parallel to B paramagnetic An external magnetic field puts force/torque on electrons. Protons are too heavy to respond. M
Chemical shift – NMR spectroscopy
Iron paramagnetism – fMRI, QSM Electronic configuration fMRIIron metabolism
Relaxation (T1, T2) This category of contrast mechanism is useful to examine macromolecular/cellular contents in water. T2 is very sensitive to cellular content change, a must in all MRI protocols in clinical practice. T1 is sensitive to tissue global or “lattice” environment, used with contrast agents.
cellular contents: T2 relaxation Pure water: Zero contents 1/T2 small, T2 long, 2 sec Normal tissue: Lots of contents 1/T2 large, T2 short, 75 msec Edema/lesion: Median amount 1/T2 median, T2 median, 200 msec