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MRI - Basic PhysicsDr. Ulas BagciCenter for Research in Computer Vision (CRCV), University ofCentral Florida (UCF), Orlando, FL.bagci@ucf.eduMay 20161
Looking Inside the Body (c. 1632)2
Looking Inside the Body (c. 2016)3
Brief History of MRI4
Nobel Prizes for MRI 1944: RabiPhysics (Measured magnetic moment of nucleus) 1952: Felix Bloch and Edward Mills PurcellPhysics (Basic science of NMR phenomenon) 1991: Richard ErnstChemistry (High-resolution pulsed FT-NMR) 2002: Kurt WüthrichChemistry (3D molecular structure in solution by NMR) 2003: Paul Lauterbur & Peter MansfieldPhysiology or Medicine (MRI technology)5
How MRI Works (in 60 seconds)6
MRI Hardware Setup7
The Magnet8
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Recipe for MR Imaging 1) Put subject in big magnetic field (leavehim/her there) 2) Transmit radio waves into subject [about 3ms] 3) Turn off radio wave transmitter 4) Receive radio waves re-transmitted by subject– Manipulate re-transmission with magnetic fields during this readoutinterval [10-100 ms: MRI is not a snapshot] 5) Store measured radio wave data vs. time– Now go back to 2) to get some more data 6) Process raw data to reconstruct images 7) Allow subject to leave scanner10
MRI Hardware Setup - Details11
MRI Hardware Setup - Details12
MRI measures the movement of H atoms Hydrogen is abundant in the water molecules in human tissue. The nuclei of Hydrogen can act like a “compass needle” in amagnetic field. Some types of tissues prevent hydrogen from spinning as freely.For example, bone is more restrictive to movement than fat.– These differences can be detected to distinguish tissue type.13
BprotonsEarth’sFieldNEWWald, fMRI MRPhysicsScompass
Compass needlesEarth’sFieldυzMainFieldBoNorthNEWySxFreq γ BWald, fMRI MRPhysics42.58 MHz/T
Gyroscopic motionMainFieldBozNorth Proton has magnetic momentM Proton has spin (angular momentum) gyroscopic precessionyxLarmor precession freq. 42.58 MHz/TWald, fMRI MRPhysicsυ γ Bo
EXCITATION : Displacing the spinsfrom Equilibrium (North)Problem: It must be moving for us to detect it.Solution: knock out of equilibrium so it oscillatesHow? 1) Tilt the magnet or compass suddenly2) Drive the magnetization (compass needle)with a periodic magnetic fieldWald, fMRI MRPhysics
Summary of MRI Physics in one slide!18
Fundamental Three Steps in MR0) Equilibrium (magnetization points along Bo)1) RF Excitation (tip magn. away from equil.)2) Precession induces signal,dephasing (timescale T2, T2*).3) Return to equilibrium (timescale T1).Wald, fMRI MRPhysics
MRI Hardware Setup - Details20
Basic Physics of MRI Nuclei line up with magnetic moments either in a parallel or anti-parallel configuration.In body tissues more line up in parallel creating a small additional magnetization M inthe direction of B0.Nuclei spin axis notparallel to B0 fielddirection.Nuclear magneticmoments precessabout B0.
Basic Physics of MRI Frequency of precession of magnetic moments given by Larmor relationshipf γ x B0f Larmor frequency (mHz)γ Gyromagnetic ratio (mHz/Tesla)B0 Magnetic field strength (Tesla)γ 43 mHz/TeslaLarmor frequencies of RICs MRIs3T 130 mHZ7T 300 mHz11.7T 500 mHz
Energy Differences23
Basic Physics of MRINMRable Nuclei§ Body 1H content is high due to water ( 67%)§ Hydrogen protons in mobile water are primarysource of signals in fMRI and aMRI
What happens in a scan?Magnetic fields and resonance Magnetism We exert a couple on the nuclear spin by placing amagnetic field across the sample. mIf the particle were not spinning, it would align like acompass, but, because of the spin, it precesses instead. Resonance BAn electromagnetic field that oscillates at exactly thesame frequency as the nucleus will be absorbed.Under certain conditions, the nucleus will also emit aquantum of RF energyPrecessionSpin
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Image contrast summary: TR, TELongProtonDensityT2TRShortT1poor!ShortWald, fMRI MRPhysicsTELong
Sources Must watch lecture (basic):https://www.youtube.com/watch?v jWRIKNeCXjI es/physics/MRI Physics.pdf MRI Made Easy (free, easy book):http://www.stat.columbia.edu/ martin/Tools/MRI MadeEasy.pdf44
Basic Physics of MRI Nuclei line up with magnetic moments either in a parallel or anti -parallel configuration. In body tissues more line up in parallel creating a small additional magnetization M in the direction of B 0. Nuclear magnetic moments precess about B 0. Nuclei spin axis not parallel to B 0 field direction.
