MRI - Basic Physics

Dr. Ulas BagciCenter for Research in Computer Vision (CRCV), University of

Central Florida (UCF), Orlando, FL.bagci@ucf.edu

May 20161

Looking Inside the Body (c. 1632)

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Looking Inside the Body (c. 2016)

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Brief History of MRI

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Nobel Prizes for MRI• 1944: Rabi

Physics (Measured magnetic moment of nucleus)

• 1952: Felix Bloch and Edward Mills Purcell Physics (Basic science of NMR phenomenon)

• 1991: Richard Ernst Chemistry (High-resolution pulsed FT-NMR)

• 2002: Kurt WüthrichChemistry (3D molecular structure in solution by NMR)

• 2003: Paul Lauterbur& Peter Mansfield Physiology or Medicine (MRI technology)

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How MRI Works (in 60 seconds)

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MRI Hardware Setup

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The Magnet

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Recipe for MR Imaging• 1) Put subject in big magnetic field (leave

him/her there)• 2) Transmit radio waves into subject [about 3

ms]• 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 scanner

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MRI Hardware Setup - Details

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MRI Hardware Setup - Details

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MRI measures the movement of H atoms

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• Hydrogen is abundant in the water molecules in human tissue. • The nuclei of Hydrogen can act like a “compass needle” in a

magnetic 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.

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B

N

S

W E

Earth’sField

protons

compass

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Compass needles

N

S

W E

υ

x

y

zMainFieldBo

42.58 MHz/T

Earth’sField North

Freq=γ B

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Gyroscopic motion

MainFieldBo

Larmor precession freq. = 42.58 MHz/Tx

y

z North • Protonhasmagneticmoment

• Protonhasspin(angularmomentum)

>>gyroscopicprecession

υ =γ Bo

M

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EXCITATION : Displacing the spinsfrom Equilibrium (North)

Problem: It must be moving for us to detect it.

Solution: knock out of equilibrium so it oscillates

How? 1) Tilt the magnet or compass suddenly

2) Drive the magnetization (compass needle)with a periodic magnetic field

Summary of MRI Physics in one slide!

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Fundamental Three Steps in MR

0)Equilibrium(magnetizationpointsalongBo)

1)RFExcitation(tipmagn.awayfromequil.)

2)Precessioninducessignal,dephasing(timescale = T2,T2*).

3)Returntoequilibrium(timescale= T1).

MRI Hardware Setup - Details

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Basic Physics of MRI• Nuclei lineupwithmagneticmomentseither inaparalleloranti-parallelconfiguration.• InbodytissuesmorelineupinparallelcreatingasmalladditionalmagnetizationM in

thedirectionofB0.

Nuclear magnetic moments precess

about B0.

Nuclei spin axis not parallel to B0 field

direction.

Basic Physics of MRI• FrequencyofprecessionofmagneticmomentsgivenbyLarmorrelationship

γ ~ 43 mHz/Tesla

Larmor frequencies of RICs MRIs

3T ~ 130 mHZ7T ~ 300 mHz

11.7T ~ 500 mHz

f = γ x B0

f = Larmor frequency (mHz)γ = Gyromagnetic ratio (mHz/Tesla)B0 = Magnetic field strength (Tesla)

Energy Differences

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NMRableNucleiBasicPhysicsofMRI

§ Body 1H content is high due to water (>67%)§ Hydrogen protons in mobile water are primary

source of signals in fMRI and aMRI

• Magnetism◆ Weexertacoupleonthenuclearspinbyplacinga

magneticfieldacrossthesample.

B

Whathappensinascan?Magneticfieldsandresonance

m

PrecessionSpin

◆ Iftheparticlewerenotspinning,itwouldalign likeacompass,but,becauseofthespin,itprecesses instead.

• Resonance◆ Anelectromagnetic fieldthatoscillatesatexactlythe

samefrequencyasthenucleuswillbeabsorbed.

◆ Undercertainconditions,thenucleuswillalsoemitaquantumofRFenergy

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TRLong

Short

Short LongTE

ProtonDensity

T1 poor!

Imagecontrastsummary:TR,TE

T2

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Sources• Must watch lecture (basic):

https://www.youtube.com/watch?v=jWRIKNeCXjI• http://www.medicine.uodiyala.edu.iq/uploads/lectures/phy

sics/MRI_Physics.pdf• MRI Made Easy (free, easy book):

http://www.stat.columbia.edu/~martin/Tools/MRI_Made_Easy.pdf

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