Principles of MRI

Principles of MRI

Principles of MRI • Some terms: • Nuclear Magnetic Resonance (NMR) • quantum property of protons • energy absorbed when precession frequency matches radio frequency • Magnetic Resonance Imaging (MRI) • uses spatial differences in resonance frequencies to form an image • basis of anatomical MRI • functional Magnetic Resonance Imaging (fMRI) • exploits magnetic properties of hemaglobin to create images changes in cortical blood flow

Principles of NMR • Protons are like little magnets • they orient in magnetic fields like compass needles • what way do they normally point?

Principles of NMR • Protons are like little magnets • they orient in magnetic fields like compass needles • what way do they normally point? • normally aligned with Earth’s magnetic field

Principles of NMR • Protons are like little magnets • they orient in magnetic fields like compass needles • what way do they normally point? • normally aligned with Earth’s magnetic field • NMR uses a big magnet to align all the protons in a sample (e.g. brain tissue)

Principles of NMR • Protons are like little magnets • Radio Frequency pulse will knock protons at an angle relative to the magnetic field

Principles of NMR • Protons are like little magnets • Radio Frequency pulse will knock protons at an angle relative to the magnetic field • once out of alignment, the protons begin to precess

Principles of NMR • Protons are like little magnets • Radio Frequency pulse will knock protons at an angle relative to the magnetic field • once out of alignment, the protons begin to precess • protons gradually realign with field (relaxation)

Principles of NMR • Protons are like little magnets • Radio Frequency pulse will knock protons at an angle relative to the magnetic field • once out of alignment, the protons begin to precess • protons gradually realign with field (relaxation) • protons “echo” back the radio frequency that originally tipped them over • That radio “echo” forms the basis of the MRI image

Principles of NMR • Protons are like little magnets • The following simple equation explains MRI image formation

Functional Imaging • Recall that precessing protons give off a radio “echo” as they realign with the magnetic field • We pick up the combined echo from many protons that are in phase

Cognitive Neuroscience

Functional Imaging • Oxygenated hemoglobin is diamagnetic - it has no magnetic effects on surrounding molecules • Deoxygenated hemoglobin is paramagnetic - it has strong magnetic effects on surrounding molecules! Hemoglobin

Functional Imaging • blood flow overshoots baseline after a brain region is activated • More oxygenated blood in that region increases MR signal from that region

Functional Imaging • recall that the precession frequency depends on the field strength • anything that changes the field at one proton will cause it to de-phase

Functional Imaging • recall that the precession frequency depends on the field strength • anything that changes the field at one proton will cause it to de-phase • The de-phased region will give off less echo

Functional Imaging • It is important to recognize that fMRI “sees” changes in the ratio of oxygenated to deoxygenated blood - nothing more • BOLD: Blood Oxygenation Level Dependant contrast

Functional Imaging • How do we create those pretty pictures? • We ask the question “When the subject engages in this cognitive task, where does blood oxygenation change significantly?” “where does it change randomly?”

MRI Image Formation • First you need a scanner: • The first MRI scanner

MRI Image Formation • Modern Scanners

MRI Image Formation • Our Scanner

Experimental Design in fMRI • Experimental Design is crucial in using fMRI • Simplest design is called “Blocked” • alternates between active and “rest” conditions Active Rest Active Rest 60 sec 60 sec 60 sec 60 sec

Principles of MRI