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