Magnetic Resonance Imaging Overview

Magnetic Resonance Imaging Mike Hoaglin ECE381 Professor Max Epstein Northwestern University June 2, 2004 History of MRI  Magnetic resonance (1946)  Stanford & Harvard researchers   Nuclear Magnetic Resonance (NMR) Clinical application (early 1980s)  Known as MRI  Soft tissue resolution MRI Physics     Principle of spin flipping Nuclear spin Hydrogen probe Application of external field Spin flipping MRI Physics     Principle of spin flipping Nuclear spin Hydrogen probe Application of external field Magnetic Moment MRI Physics     Principle of spin flipping Nuclear spin Hydrogen probe Application of external field Why hydrogen ions? MRI Physics     Principle of spin flipping Nuclear spin Hydrogen probe Application of external field Precession about Bo Larmor freq. RF Magnetic dipole moment Relaxation time      Take away RF signal T1=time for magnetic moment to realign T2=time for Mx-y to disperse (weaker) Analagous to RC circuit Interpreted by FT  Compare T1/T2 to knowns Slices   3D capability Take slices by:    varying RF frequency/duration Varying axes gradient fields Varying phase Time course of scan Basic equipment requirements   $2 million Superconducting solenoid    Surrounds patient comfortably Shielding Niobium-titanium alloys @ 4K High SNR High uniformity High temporal stability ESSENTIAL FOR GOOD IMAGE Contrasting agents   Provide extra contrast in areas with low res Not visualized directly   Enhance signal by increasing relaxation times Complex metal-chelate: Gd(EDTA)  Paramagnetic (unpaired electrons)  Short half life (~90min)  Safely cleared by kidney in 24 hours Contrasting agent  Gadolinum-chelate over time  Increases M Cardiac MRI  Most challenging MRI modality   Can’t stop beating heart Can hold breath Data acquisition with each heartbeat (diastole)   Manage movement by cardiac triggering  EKG leads (cine) Piece together into fMRI Cardiac triggered multiphase imaging Cardiac wall movement  Important in ventricular health  Can intervene early, preventing muscle damage Saturation bands  Provide a dynamic topography  Cardiac tagging  Cardiac mapping   Stress map Areas of ventricular stress Ventricular thinning Risk for MI Future of MRI  Continued refinementssaved lives  Study of ventricular function Better real-time imaging techniques  Improvement of computer systems   Better contrasting agents  Longer half-life & safe  Coronary angiographycompletely noninvasive bloodflow evaluation