mri physics i: spins, excitation, relaxation
TRANSCRIPT
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MRI Physics I:Spins, Excitation, Relaxation
Douglas C. NollBiomedical EngineeringUniversity of Michigan
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Michigan Functional MRI Laboratory
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Outline
• Introduction to Nuclear Magnetic Resonance Imaging– NMR Spins– Excitation– Relaxation– Contrast in images
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MR Principle
Magnetic resonance is based on the emission and absorption of energy in
the radio frequency range of the electromagnetic spectrum
by nuclear spins
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Historical Notes• In 1946, MR was discovered independently by
Felix Bloch and Edward Purcell• Initially used in chemistry and physics for studying
molecular structure (spectrometry) and diffusion• In 1973 Paul Lauterbur obtained the 1st MR image
using linear gradients• 1970’s: MRI mainly in academia• 1980’s: MRI was commercialized • 1990’s: fMRI spread rapidly• 2000’s: era of new fast imaging methods• 2010’s: technology continues, standardization
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Important Events in the History of MRI• 1946 MR phenomenon - Bloch & Purcell• 1950 Spin echo signal discovered - Erwin Hahn• 1952 Nobel Prize - Bloch & Purcell• 1950 - 1970 NMR developed as analytical tool• 1963 Doug Noll born• 1972 Computerized Tomography• 1973 Backprojection MRI - Lauterbur• 1975 Fourier Imaging - Ernst (phase and frequency encoding)• 1977 MRI of the whole body - Raymond Damadian
Echo-planar imaging (EPI) technique - Peter Mansfield • 1980 Spin-warp MRI demonstrated - Edelstein• 1986 Gradient Echo Imaging NMR Microscope • 1988 Angiography – O’Donnell & Dumoulin• 1989 Echo-Planar Imaging (images at video rates = 30 ms / image)• 1991 Nobel Prize - Ernst• 1992 BOLD Functional MRI (fMRI)• 1994 Hyperpolarized 129Xe Imaging• 1997 Parallel MRI• 2003 Nobel Prize – Lauterbur & Mansfield• 2007 Sparse sampling/compressed sensing• 2010 Multiband (simultaneous multislice) MRI• 2017 The machine learning craze
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MR Physics• Based on the quantum mechanical
properties of nuclear spins
• Q. What is SPIN? • A. Spin is a fundamental property of nature
like electron charge or mass. Spin comes in multiples of 1/2 and can be + or –.
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Properties of Nuclear SpinNuclei with:
• Odd number of Protons• Odd number of Neutrons• Odd number of both
exhibit a net MAGNETIC MOMENT(e.g. 1H, 3He, 31P, 23Na, 17O, 13C, 19F )
Pairs of spins take opposing states, cancelling the observable effects.(e.g. 16O, 12C)
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Common NMR Active NucleiSpin % natural elemental
Isotope I abundance MHz/T abundanceof isotope in body
1H 1/2 99.985% 42.575 63%2H 1 0.015% 6.53 63%13C 1/2 1.108% 10.71 9.4%14N 1 99.63% 3.078 1.5%15N 1/2 0.37% 4.32 1.5%17O 5/2 0.037% 5.77 26%19F 1/2 100% 40.08 0%23Na 3/2 100% 11.27 0.041%31P 1/2 100% 17.25 0.24%
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Bar Magnet
Bar Magnets “North” and
“South” poles
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A “Spinning” Proton
A “spinning” protongenerates a tinymagnetic field
Like a littlemagnet
+angular
momentum
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NMR SpinsB0 B0
In a magnetic field, spins can either align with or against the direction of the field
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Protons in the Human Body• The human body is made up of many individual
protons.
• Individual protons are found in every hydrogen nucleus.
• The body is mostly water, and each water molecule has 2 hydrogen nuclei.
• 1 gram of your body has ~ 6 x 1022 protons
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Spinning Protons in the Body
Spinning protonsare randomly
oriented.
No magnetic field - no net effect
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Protons in a Magnetic FieldSpinning protonsbecome alignedto the magnetic
field.
On average -body become magnetized. M
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Magnetization of Tissue
M
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A Top in a Gravitational Field
L
L
F=mgr
A spinning top in a gravitational field is similarto a nuclear spin in a magnetic field
(classical description)
A digression…
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A Top in a Gravitational Field
L
L
F=mgr
Gravity exerts a force on top that leads to a Torque (T):
gLLT
Lrm
dtd
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A Top in a Gravitational Field
This causes the top to precess around g at frequency:
y
x
z
L
Lmgr
L
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Spins in a Magnetic Field
M, L
B0
Spins have both magnetization (M)and angular momemtum (L):
0BMLT dtd
Applied magnetic field (B0) exerts a force on the magnetization
that leads to a torque:
LM F
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Spins in a Magnetic FieldThis can be rewritten to yield the
famous Bloch Equation:
0BMM dt
d
which says that the magnetization will precess around the applied
magnetic field at frequency:M
B0
0
00 B “Larmor Frequency”
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Common NMR Active NucleiSpin % natural elemental
Isotope I abundance MHz/T abundanceof isotope in body
1H 1/2 99.985% 42.575 63%2H 1 0.015% 6.53 63%13C 1/2 1.108% 10.71 9.4%14N 1 99.63% 3.078 1.5%15N 1/2 0.37% 4.32 1.5%17O 5/2 0.037% 5.77 26%19F 1/2 100% 40.08 0%23Na 3/2 100% 11.27 0.041%31P 1/2 100% 17.25 0.24%
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So far …
• At the microscopic (quantum) level: spins have angular momentum and magnetization– We use spin, proton, hydrogen nucleus interchangeably
• The magnetization of particles is affected by magnetic fields: torque, precession
• At the macroscopic level: They can be treated as a single magnetization vector (makes life a lot easier)
• Next: NMR uses the precessing magnetization of water protons to obtain a signal
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Spins in a Magnetic Field
Three “spins” with different applied magnetic fields.
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The NMR Signal
The precessing magnetization generates the signal in a coil we receive in MRI, v(t)
M
B
y
x
z
0
v(t)v(t)
t
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Frequency of Precession
• For 1H, the frequency of precession is:– 63.8 MHz @ 1.5 T (B0 = 1.5 Tesla)– 127.6 MHz @ 3 T – 300 MHz @ 7 T
00 B “Larmor Frequency”
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M
Excitation
• The magnetizationis initially parallelto B0
• But, we need it perpendicular in orderto generate a signal
M
B
y
x
z
0
v(t)
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The Solution: Excitation
RF Excitation(Energy into tissue) Magnetic fields
are emitted
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Excitation
• Concept 1: Spin system will absorb energy at E corresponding difference in energy states– Apply energy at 0 = B0 (RF frequencies)
• Concept 2: Spins precess around a magnetic field.– Apply magnetic fields in plane perpendicular
to B0.
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Resonance Phenomena• Excitation in MRI works when you apply
magnetic fields at the “resonance” frequency.
• Conversely, excitation does not work when you excite at the incorrect frequency.
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Resonance Phenomena
• Wine Glass• http://www.youtube.com/watch?v=JiM6AtNLXX4
• Air Track• http://www.youtube.com/watch?v=wASkwB8DJpo
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Excitation
Try this: Apply a magnetic field (B1)rotating at 0 = B0
in the plane perpendicular to B0
Magnetization will tip into transverse
plane
Applied RF
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Off-Resonance Excitation• Excitation only works
when B1 field is applied at 0 = B0(wrong E)
• We will see that this allows us the select particular groups of spins to excite (e.g. slices, water or fat)
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Flip Angle
• Excitation stops when the magnetization is tipped enough into the transverse plane
• We can only detect the transverse component: sin(alpha)
• 90 degree flip angle will give most signal (ideal case)
• Typical strength isB1 = 1-2 x 10-5 T
• 90 degree tip takesabout 300-600 s
α
B1
Courtesy Luis Hernandez
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M
What next? Relaxation
M
B
y
x
z
0
v(t)
Spins “relax” back to their equilibrium state
Excitation
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Relaxation
• The system goes back to its equilibrium state
• Two main processes:– Decay of traverse (observable) component– Recovery of parallel component
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T1 - relaxation• Longitudinal magnetization (Mz) returns to
steady state (M0) with time constant T1• Spin gives up energy into the surrounding
molecular matrix as heat• Factors
– Viscosity– Temperature– State (solid, liquid, gas)– Ionic content– Bo– Diffusion– etc.
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T1 Recovery
• Tissue property (typically 1-3 seconds)• Spins give up energy into molecular matrix
• Differential Equation:
Mz
t
M0
M
B0
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T2 - relaxation• Transverse magnetization (Mxy) decay
towards 0 with time constant T2
• Factors– T1 (T2 T1)– Phase incoherence
» Random field fluctuations» Magnetic susceptibility» Magnetic field inhomogeneities (RF, B0, Gradients)» Chemical shift» Etc.
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T2 Decay
• Tissue property (typically 10’s of ms)• Spins dephase relative to other spins
• Differential Equation:
Mxy
tM
B0
0
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Steps in an MRI Experiment
0. Object goes into B0
1. Excitation2a. T2 Relaxation (faster)2b. T1 Relaxation (slower)3. Back to 1.
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Excitation
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Relaxation
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Resting State
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Excitation
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Excitation
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T2 Relaxation
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T2 Relaxation
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T2 Relaxation
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T1 Relaxation
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T1 Relaxation
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T1 Relaxation
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T1 Relaxation
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Typical T1’s, T2’s, and Relative “Spin Density” for Brain Tissue at 3.0 T
T1 (ms ) T2 (ms) R
Water 4000 2000 1CSF 4000 500 1Gray matter 1330 110 0.95White matter 830 80 0.8Fat 380 45 1
Several refs: Wansapura et al., JMRI, 1999; Bojoquez et al. MRI, 2017, and others. Note: Large variability based on technique, individual, area of brain, etc.
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The Pulsed MR Experiment
• MRI uses a repeated excitation pulse experimental strategy
RFpulses
90 90 90 90
Dataacquisition
time
TR(Repetition Time)
TE(Echo Time)
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Contrast
• TR mainly controls T1 contrast– Excitation or flip angle also contributes
• TE mainly controls T2 contrast
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T1 Contrast and TR
TR
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T1 Contrast and TR
TR
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T1 Contrast and TR
TR
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T1 Contrast and TR
TR
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T1 Contrast and TR
TR
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T1 Contrast and TR
TR
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T1 Contrast
• For short TR imaging, tissues with short T1’s (rapidly recovering) are brightest– Fat > brain tissue– White Matter > Grey Matter– Gray Matter > CSF
T1Weighting
SpinDensity
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T2 Contrast and TE
TE
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T2 Contrast and TE
TE
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T2 Contrast and TE
TE
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T2 Contrast and TE
TE
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T2 Contrast and TE
TE
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T2 Contrast
• For long TE imaging, tissues with short T2’s (rapidly recovering) are darkest– Fat < brain tissue– White Matter < Grey Matter– Gray Matter < CSF
T2Weighting
SpinDensity
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Contrast Equation
• For a 90 degree flip angle, the contrast equation is:
Signal 2/1/ )1( TTETTR ee
Spin Density T1-weighting T2-weighting
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Can the flip angle be less than 90? • Of course, but
the contrast equation is more complicated.
• Flip angle can be chose to maximize signal strength:
Ernst Angle
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Next Step
Making an image!!
First – some examples of MR Images and Contrast
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Supratentorial Brain Neoplasm
T2-weighted imageT1-weighted imagewith contrast
Brigham and Women’s Hospital Teaching Files
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Cerebral Infarction
T2-weighted imageMR AngiogramBrigham and Women’s Hospital Teaching Files
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Imaging Breast Cancer
http://tristans.com/services/breast-mri/
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Imaging any Orientation
https://sites.google.com/a/wisc.edu/neuroradiology/image-acquisition/the-basics
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Imaging Joints
http://wolverhamptonhipandkneeclinic.co.uk/services-view/anterior-cruciate-ligament-acl-surgery/
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Imaging Air Passages
Fain et al. J. Magn. Reson. Imaging 2007;25:910–923.
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Tagging Cardiac Motion
https://www.nature.com/articles/nrcardio.2009.189
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Calculated Images
Ganzetti M, Wenderoth N and Mantini D (2014) Whole brain myelin mapping using T1- and T2-weighted MR imaging data. Front. Hum. Neurosci. 8:671. doi: 10.3389/fnhum.2014.00671
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Diffusion and Perfusion Weighted MRI
https://www.wjgnet.com/1949-8470/full/v4/i3/WJR-4-63-g002.htm
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