what can mri do for you? yi wang noninvasive imaging cellular/macromolecular contents molecular...
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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
Isotropic Restricted
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 BDiamagneticChemical shift at nucleus
B
Unpaired e- Spin response (torque): Magnetic moment parallel to Bparamagnetic
An external magnetic field puts force/torque on electrons.Protons are too heavy to respond.
00 / BHM
M
Chemical shift – NMR spectroscopy
Iron paramagnetism – fMRI, QSMElectronic configuration
fMRI Iron 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 relaxationPure water:Zero contents1/T2 small, T2 long, 2 sec
Normal tissue: Lots of contents1/T2 large, T2 short, 75 msec
Edema/lesion:Median amount1/T2 median, T2 median, 200 msec