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What Can MRI Do for You?
Noninvasive imaging
• cellular/macromolecular contents
• molecular magnetism
• transport processes
Yi Wang
Transport processes
This category of MRI contrast mechanisms is
useful for assessing live tissue functions:
• Flow
• Perfusion, convection, permeability
• Diffusion (tensor)
Diffusion – incoherent motion
Diffusion increase in tumor (bright)
Diffusion decrease in acute ischemia (dark)
Fiber tractography
Perfusion & flow – coherent motion
Reduced perfusion (yellow)
Flow (vessel blocked in left)
Ischemic stroke
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
• Unpaired electrons – strong paramagnetic
field outside molecule
Tissue magnetism
M   H   B0 /  0
An external magnetic field puts force/torque on electrons.
Protons are too heavy to respond.
Orbital response (Lorentz force): Unpaired e- Spin response (torque):
Magnetic moment parallel to B
Magnetic moment opposing B
Chemical shift at nucleus
Chemical shift – NMR spectroscopy
Iron paramagnetism – fMRI, QSM
Electronic configuration
Iron metabolism
Relaxation (T1, T2)
This category of contrast mechanism is useful to
examine macromolecular/cellular contents in
• 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
Median amount
1/T2 median,
T2 median, 200 msec

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