principles of mri. some terms: –nuclear magnetic resonance (nmr) quantum property of protons...
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Principles of MRI
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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
![Page 3: Principles of MRI. Some terms: –Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency](https://reader036.vdocument.in/reader036/viewer/2022062515/56649ceb5503460f949b608c/html5/thumbnails/3.jpg)
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
![Page 4: Principles of MRI. Some terms: –Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency](https://reader036.vdocument.in/reader036/viewer/2022062515/56649ceb5503460f949b608c/html5/thumbnails/4.jpg)
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
![Page 5: Principles of MRI. Some terms: –Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency](https://reader036.vdocument.in/reader036/viewer/2022062515/56649ceb5503460f949b608c/html5/thumbnails/5.jpg)
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
![Page 6: Principles of MRI. Some terms: –Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency](https://reader036.vdocument.in/reader036/viewer/2022062515/56649ceb5503460f949b608c/html5/thumbnails/6.jpg)
Principles of NMR
• Protons are like little magnets– they orient in magnetic fields like
compass needles– what way do they normally point?
![Page 7: Principles of MRI. Some terms: –Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency](https://reader036.vdocument.in/reader036/viewer/2022062515/56649ceb5503460f949b608c/html5/thumbnails/7.jpg)
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
![Page 8: Principles of MRI. Some terms: –Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency](https://reader036.vdocument.in/reader036/viewer/2022062515/56649ceb5503460f949b608c/html5/thumbnails/8.jpg)
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)
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Principles of NMR
• Protons are like little magnets– Radio Frequency pulse will knock
protons at an angle relative to the magnetic field
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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
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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)
![Page 12: Principles of MRI. Some terms: –Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency](https://reader036.vdocument.in/reader036/viewer/2022062515/56649ceb5503460f949b608c/html5/thumbnails/12.jpg)
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
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Principles of NMR
• Protons are like little magnets– The following simple equation
explains MRI image formation
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MRI Image Formation
• First you need a scanner:
– The first MRI scanner
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MRI Image Formation
• Modern Scanners
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MRI Image Formation
• Our Scanner
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MRI Image Formation
• Our Scanner
![Page 18: Principles of MRI. Some terms: –Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency](https://reader036.vdocument.in/reader036/viewer/2022062515/56649ceb5503460f949b608c/html5/thumbnails/18.jpg)
MRI Image Formation
• Our Scanner
![Page 19: Principles of MRI. Some terms: –Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency](https://reader036.vdocument.in/reader036/viewer/2022062515/56649ceb5503460f949b608c/html5/thumbnails/19.jpg)
MRI Image Formation
• Our Scanner
![Page 20: Principles of MRI. Some terms: –Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency](https://reader036.vdocument.in/reader036/viewer/2022062515/56649ceb5503460f949b608c/html5/thumbnails/20.jpg)
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
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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?
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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”
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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?
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Measuring Blood Oxygenation in the Brain
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Functional Imaging• Recall that precessing protons
give off a radio “echo” as they realign with the magnetic field
![Page 26: Principles of MRI. Some terms: –Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency](https://reader036.vdocument.in/reader036/viewer/2022062515/56649ceb5503460f949b608c/html5/thumbnails/26.jpg)
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
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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
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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
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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
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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
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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)
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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?