Download - Principle of MRI Physics. Shafiee
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Basic Physical PrinciplesBasic Physical Principlesofof MRIMRI
Present by M.Shafiee. MScRSO at Urmia Nuclear Medicine Center
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Medical Imaging Modalities: HistoryMedical Imaging Modalities: History
• 1895: X-ray†
• ~1950: Ultrasound• ~1955: Radionuclide• 1972: CT†
• ~1980: MRI†
†Nobel Prize
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History of NMRHistory of NMRNMR = nuclear magnetic resonance
Felix Block and Edward Purcell1946: atomic nuclei absorb and re-emitradio frequency energy1952: Nobel prize in physics
nuclear: properties of nuclei of atomsmagnetic: magnetic field requiredresonance: interaction between magneticfield and radio frequency Bloch Purcell
NMRNMR MRI: Why the name change?MRI: Why the name change?
most likely explanation:nuclear has bad connotations
less likely but more amusing explanation:subjects got nervous when fast-talking doctors suggested an NMR
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Strengths of MRI
••Images of softImages of soft--tissue structures of the body, such as the heart,tissue structures of the body, such as the heart,
lungs, liver, are clearer and more detailedlungs, liver, are clearer and more detailed
••MRI can help evaluate theMRI can help evaluate the functionfunction as well as the structureas well as the structure
••Invaluable tool in early evaluation of tumorsInvaluable tool in early evaluation of tumors
••MRI contrast materials are less harmful than those used in XMRI contrast materials are less harmful than those used in X--rayray
or CTor CT
••Fast, nonFast, non--invasive angiographyinvasive angiography
••Exposure to radiation is minimal (nonExposure to radiation is minimal (non--ionizing)ionizing)
Compare the detail:Compare the detail:CT (left) vs. MRI TCT (left) vs. MRI T22 (right)(right)
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Risks and Weaknesses:Risks and Weaknesses:Risks and Weaknesses:Risks and Weaknesses:
• Metal implants may cause problems• Problems with claustrophobia• MRI is to be avoided during the first 12 weeks
of pregnancy• Bone is usually better imaged with X-rays• MRI typically costs more than CT
Necessary Equipment
Magnet Gradient Coil RF Coil
Source: Joe Gati, photos
RF Coil
4T magnet
gradient coil(inside)
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x 80,000 =
4 Tesla = 4 x 10,000 0.5 = 80,000X Earth’s magnetic field
Robarts Research Institute 4T
The Big MagnetVeryVery strong!strong!
Continuously on
1 Tesla (T) = 10,000 Gauss
Earth’s magnetic field = 0.5 Gauss
Main field = B0
B0
MRI Clinical Safety Practice• Pacemaker• Aneurysm clip• Metal in the eye• Credit card
• Bone implant• Denture
X
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Magnet SafetyThe whopping strength of the magnet makes safety essential.Things fly – Even big things!
Screen subjects carefullyMake sure you and all your students & staff are aware of hazzardsDevelop stratetgies for screening yourself every time you enter the magnet
Open Bore MRI Scanner
• Avoid claustrophobia• Lower image quality
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Orthopedic MRI
Orthoscan MRI
Synopsis of MRI
1) Put subject in big magnetic field
2) Transmit radio waves into subject [2~10 ms]
3) Turn off radio wave transmitter
4) Receive radio waves re-transmitted by subject0
5) Convert measured RF data to image
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Many factors contribute to MR imaging
• Quantum properties of nuclear spins• Radio frequency (RF) excitation properties• Tissue relaxation properties• Magnetic field strength and gradients• Timing of gradients, RF pulses, and signal
detection
What kinds of nuclei can be used for NMR?
• Nucleus needs to have 2 properties:– Spin– charge
• Nuclei are made of protons and neutrons– Both have spin ½– Protons have charge
• Pairs of spins tend to cancel, so only atoms withan odd number of protons or neutrons have spin– Good MR nuclei are 1H, 13C, 19F, 23Na, 31P
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Hydrogen atoms are best for MRI
• Biological tissues are predominantly 12C, 16O, 1H, and14N
• Hydrogen atom is the only major species that is MRsensitive
• Hydrogen is the most abundant atom in the body• The majority of hydrogen is in water (H2O)• Essentially all MRI is hydrogen (proton) imaging
A Single Proton
++++
++
There is electric chargeThere is electric chargeon the surface of the proton,on the surface of the proton,thus creating a small currentthus creating a small currentloop and generating magneticloop and generating magneticmomentmomentmm..
The proton also hasThe proton also hasmass which generatesmass which generatesananangular momentumangular momentumJJ when it is spinning.when it is spinning.
JJµµ
Thus proton “magnet” differs from the magnetic bar in that itThus proton “magnet” differs from the magnetic bar in that italso possesses angular momentum caused by spinning.also possesses angular momentum caused by spinning.
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Polarization• Spins are normally oriented randomly.• In an applied magnetic field, the spins align with the
applied field in their equilibrium state.• Excess along B0 results in net magnetization.
No Applied Field Applied Field
B0
Precession• Spins precess about applied magnetic field, B0, that is
along z axis.• The frequency of this precession is proportional to the
applied field:
B
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Magnetic Moment
II
BB
FFLL
F = IBLF = IBL
BB
LLWW
ττ = IBLW = IBA= IBLW = IBA
µµ = τ= τmaxmax / Β/ Β= Ι Α= Ι Α
tt==mmBB= µ Β= µ Βsinsinθθ
ForceForce TorqueTorque
Angular Momentum
JJ = m= mωω==mmvvrr
mm
vvrr
JJ
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mm==ggJJ
ggis theis the gyromagneticgyromagnetic ratio. unit: MHz/Tratio. unit: MHz/Tggis a constant for a given nucleusis a constant for a given nucleus
The magnetic moment and angularmomentum are vectors lying along thespin axis
How do protons interact with amagnetic field?
• Moving (spinning) charged particle generatesits own little magnetic field– Such particles will tend to line up with external
magnetic field lines (think of iron filings around amagnet)
• Spinning particles with mass have angularmomentum– Angular momentum resists attempts to change
the spin orientation (think of a gyroscope)
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The energy difference between the two alignment statesdepends on the nucleus
DE =mz Bo/I DE = h ν
mz =ghI D E=gh Bo
ν = γ/2p Bo
known as Larmor frequency
γ/2p== 4242..5757 MHz / Tesla for protonMHz / Tesla for proton
B
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Resonance frequencies of common nuclei
Note: Resonance at 1.5T = Larmor frequency X 1.5
MRI
X-Ray, CT
Electromagnetic Radiation Energy
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MRI uses a combination of Magnetic andElectromagnetic Fields
• NMR measures the net magnetization of atomic nuclei in thepresence of magnetic fields
• Magnetization can be manipulated by changing the magneticfield environment (static, gradient, and RF fields)
• Static magnetic fields don’t change (< 0.1 ppm / hr):The main field is static and (nearly) homogeneous
• RF (radio frequency) fields are electromagnetic fields thatoscillate at radio frequencies (tens of millions of times persecond)
• Gradient magnetic fields change gradually over space and canchange quickly over time (thousands of times per second)
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Radio Frequency FieldsRadio Frequency Fields
• RF electromagnetic fields are used to manipulate the
magnetization of specific types of atoms
• This is because some atomic nuclei are sensitive to magnetic fields
and their magnetic properties are tuned to particular RF frequencies
• Externally applied RF waves can be transmitted into a subject to
perturb those nuclei
• Perturbed nuclei will generate RF signals at the same frequency –
these can be detected coming out of the subject
The Effect of Irradiation to the SpinSystem
Lower
Higher
Basic Quantum Mechanics Theory of MRBasic Quantum Mechanics Theory of MR
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Spin System After Irradiation
Basic Quantum Mechanics Theory of MRBasic Quantum Mechanics Theory of MR
Net magnetization is the macroscopic measure of many spins
BoM
T
BcM o
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Net magnetization
• Small B0 produces small net magnetization M• Larger B0 produces larger net magnetization M, lined
up with B0
• Thermal motions try to randomize alignment ofproton magnets
• At room temperature, the population ratio of anti-parallel versus parallel protons is roughly 100,000 to100,006 per Tesla of B0
Quantum vs Classical Physics
One can consider the quantum mechanicalproperties of individual nuclei, but to considerthe bulk properties of a whole object it is moreuseful to use classical physics to consider netmagnetization effects.
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To measure magnetization we mustperturb it
• We can only measure magnetization perpendicular tothe B0 field
• Need to apply energy to tip protons out of alignment• Amount of energy needed depends on nucleus and
applied field strength (Larmor frequency)• The amount of energy added (duration of the RF pulse
at the resonant frequency) determines how far the netmagnetization will be tipped away from the B0 axis
A Mechanical Analogy: A Swingset
• Person sitting on swing at rest is “aligned” with externallyimposed force field (gravity)
• To get the person up high, you could simply supply enoughforce to overcome gravity and lift him (and the swing) up– Analogous to forcing M over by turning on a huge static
B1
• The other way is to push back and forth with a tiny force,synchronously with the natural oscillations of the swing– Analogous to using a tiny RF B1 over a period of time to
slowly flip M over
gg
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If M is not parallel to B, then it precesses clockwise aroundthe direction of B.“Normal” (fully relaxed) situation has M parallel to B, and therefore does notprecess
This is like a gyroscope
Derivation of precession frequencyDerivation of precession frequency
This says that the precession frequency is the SAME as the larmor frequency
==mm×× BBo = dJ / dtJ =m/γ
dm/dt = γ (m× Bo)
mm(t) = ((t) = (µµxocoscos γγBBott ++ µµyosinsin γγBBott)) xx + (+ (µµyocoscos γγBBott -- µµxosinsin γγBBott)) yy ++ µµzozz
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Recording the MR signal
• Need a receive coil tuned to the same RF frequency asthe exciter coil.
• Measure “free induction decay” of net magnetization• Signal oscillates at resonance frequency as net
magnetization vector precesses in space• Signal amplitude decays as net magnetization
gradually realigns with the magnetic field• Signal also decays as precessing spins lose coherence,
thus reducing net magnetization
• T1 relaxation – Flipped nuclei realign with the magnetic field
• T2 relaxation – Flipped nuclei start off all spinning together, but
quickly become incoherent (out of phase)
• T2* relaxation – Disturbances in magnetic field (magnetic
susceptibility) increase the rate of spin coherence T2 relaxation
• The total NMR signal is a combination of the total number of
nuclei (proton density), reduced by the T1, T2, and T2* relaxation
components
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T2* decay
• Spin coherence is also sensitive to the fact that the
magnetic field is not completely uniform
• Inhomogeneities in the field cause some protons to
spin at slightly different frequencies so they lose
coherence faster
• Factors that change local magnetic field (susceptibility)
can change T2* decay
Different tissues have different relaxation times.These relaxation time differences can be used togenerate image contrast.
Different tissues have different relaxation times.These relaxation time differences can be used togenerate image contrast.
• T1 - Gray/White matter• T2 - Tissue/CSF• T2* - Susceptibility (functional MRI)
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