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 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 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 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

MRI Image Formation• First you need a scanner:

– The first MRI scanner

MRI Image Formation• Modern Scanners

MRI Image Formation• Our Scanner

MRI Image Formation• Our Scanner

MRI Image Formation• Our Scanner

MRI Image Formation• Our Scanner

MRI Image Formation• MRI Image formation

– resonance frequency depends on field strength

– gradient coils alter resonance frequency over distance

– slight differences in the “echo” frequency indicate the location of each proton

– second-dimension of a slice is coded by the phase of the protons

Increasing Field Strength

field gradient = frequency gradient

Functional Imaging

• Functional Imaging must provide a spatial depiction of some process that is at least indirectly related to neural activity

• in most imaging (i.e. PET, fMRI) that process is change in blood oxygenation related to changes in regional cerebral blood flow

• Why should we measure blood oxygenation?

Functional Imaging• Why should we measure blood

oxygenation?

• Onset of a stimulus (or cognitive task) changes local blood oxygenation– first with a decrease– then with an “overshoot”

Functional Imaging• Why should we measure blood

oxygenation?

• Onset of a stimulus (or cognitive task) changes local blood oxygenation– first with a decrease– then with an “overshoot”

• How do we measure changes in blood oxygenation?

Measuring Blood Oxygenation in the Brain

Functional Imaging• Recall that precessing protons

give off a radio “echo” as they realign with the magnetic field

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

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• 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 Heme

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!

• Thus deoxygenated tissue gives of less MR echo because the protons de-phase quickly

Functional Imaging• blood flow overshoots

baseline after a brain region is activated

• More oxygenated blood in that region increases MR signal from that region (other regions de-phase faster)

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

• How do we create those pretty pictures?