What Can MRI Do for You? Yi Wang Noninvasive imaging cellular/macromolecular contents molecular magnetism transport processes

Transport processes This category of MRI contrast mechanisms is useful for assessing live tissue functions: Flow Perfusion, convection, permeability Diffusion (tensor)

Diffusion – incoherent motion IsotropicRestricted Diffusion increase in tumor (bright) Diffusion decrease in acute ischemia (dark) Fiber tractography

Perfusion & flow – coherent motion Coherent Ischemic stroke Flow (vessel blocked in left) Reduced perfusion (yellow)

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