It d tiIntroduction to MtiR I i(MRI)Phi Magnetic Resonance...
Transcript of It d tiIntroduction to MtiR I i(MRI)Phi Magnetic Resonance...
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I t d tiI t d tiIntroduction Introduction to to
M ti R I i (MRI) Ph iM ti R I i (MRI) Ph i
David C. Zhu, Ph.D.
Magnetic Resonance Imaging (MRI) PhysicsMagnetic Resonance Imaging (MRI) Physics
Cognitive Imaging Research CenterDepartments of Psychology and Radiology
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Reading assignment for next three lectures:
(1) Read Chapters 1 5(1) Read Chapters 1-5.
(2) Watch “MRI-Made Easy” video.
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~ 1937 Began the concept of magnetic resonance by Isidor Rabi
Nobel price in 1944
Felix Block and Edward Purcell discovered magnetic resonance
Nobel price in 1952~1945
Paul Lauterbur introduced spatial gradients~1972 Beginning of MRIspatial gradients to provide spatial information
g g
I t d ti fNobel price in 2003
~1976Introduction of echo-planar imaging (EPI)by Peter Mansfield
Fast MRI
First human-body 1.5T by GE1982 MRI technologyRapid growth
Discovery of BOLD contrastEarly 1990s Basis of fMRI
p g
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GoalsGoalsGoalsGoals
1. Basic concepts of MRI2. Basic meanings of TE, TR, T1, T2, T2*, k space, EPI
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An atom
Nucleusp+
e- p+
n
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C l t d i MRICommon elements used in MRI:
1H, 13C, 23Na, 31P
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Hydrogen
Nucleus
e- p+
( we have a lot of H2O)!
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Spin Physics1H ( t ) classically:1H (proton)
Angular Momentum (spinspin)N
classically:
Magnetic dipole
Magnetic field B
S
Magnetic field B
Quantized to lower and higher energy states with a Boltzmann distribution: ~ 3 ppm/T excess in lower energy.
dM M B
Bloch Equation:
Bdt
M B
M = magnetization = net magnetic moment for all spins in a sample
= Larmor frequency= 42.58 MHz/T for proton
128 MHz at 3 TE = h , h = 6.626 x 10-34 J S
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JT Bushberg, JA Seibert, EW Leidholdt Jr., and JM Boone. The Essential Physics of Medical Imaging
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A happy volunteer after surviving a fMRI session
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Magnet
Superconducting electromagnets
Magnet
Gradient coils -261C
RF coilsZero i t
Subject body
resistance
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Coil for the static magnetic field
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Gradient Coil
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RF coils (Transmit and Receive)
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surface coil volume coil phased-array coil
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Magnetic Resonance Imaging Hardware Interface in Control RoomInterface in Control Room
fMRI stimulusfMRI stimulus presentation system
3T magnet Room
Equipment Room:Equipment Room:Gradient amplifiers
RF amplifierP l tPulse sequence generator
Image reconstruction
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Spin-Lattice (T1) and Spin-Spin (T2) Relaxation Processes
(T2 becomes T2* if local field is inhomogeneous)
Z
M S i f t (dephasing)Z
M
Y
B0
Spins fan out (dephasing)
T2 decay
Longitudinal magnetization YInitial 90
XT1 recovery
re-growth
X
T deca and
T2 decay andT1 recovery
RF excitation
vector summation
Z
T2 decay andT1 recoverycontinueBack to
equilibrium state
1 ycontinue
TE = time of echo
TR = time of repetition
YRF
TE time of echo
X
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T2* Decay and T1 Recovery Movie 1
http://www.stanford.edu/class/ee369b/Site/Movies.html
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T2* Decay and T1 Recovery Movie 2
*2|)0(M||)(M| xyxyT
t
et
)1(M)(M 10z
Tt
et
*21 )1(0TTE
TTR
eekMS
Courtesy of Brian Hargreaves. http://www-mrsrl.stanford.edu/~brian/mri-movies/
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TR = 3sGradient Echo
TR = 3s
TE = 6.9 ms TE = 45 ms
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Spin Echo Techniques(Obtain the effect of T2 instead of T2* )
Spins fan out (dephasing) ZZ
M
Initial 90
p ( p g)
T2* decay
Y
180 RF excitation
TE/2
M
Y
B0
Initial 90RF excitation
X
TE/2
X
ZZ
YY
TE/2XX
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Z
Y
X 128 MHzX
Z Z
128 MHz
Y Y
X X
127.9999 MHz 128.0001 MHz
Z
Y
X
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Explanation of T2* decay
3.000 T 3.000 T
After 3 ms 3.000 T 3.000 T
Vectorsum
3.000 T 3.000 T 3.000 T 3.000 T
Af3+10-6 T 3.000 T
After 3 ms
Vectorsum3+10-6 T 3.000 T
3+2×10-6 T 3-10-6 T 3+2×10-6 T 3-10-6 T
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Spin Echo Technique
Courtesy of Brian Hargreaves. http://www-mrsrl.stanford.edu/~brian/mri-movies/
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Spin Echo
TE 13 TE 90
Proton density weighted
T2 weighted T1 weightedTE = 13 msTR 900TE = 13 ms TE = 90 ms
TR = 3 s
TR = 900 ms
TR 3 s21 )1(0
TTE
TTR
eekMS
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Laboratory Frame
Courtesy of Brian Hargreaves. http://www-mrsrl.stanford.edu/~brian/mri-movies/
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Rotating Frame
Courtesy of Brian Hargreaves. http://www-mrsrl.stanford.edu/~brian/mri-movies/
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Long
T1
Relaxationtime
1
T2
Short
Molecular motion: slow intermediate fastllMolecular size:
Molecular interactions:
large intermediateintermediate
small
bound free
JT Bushberg, JA Seibert, EW Leidholdt Jr., and JM Boone. The Essential Physics of Medical Imaging
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B
B B G Zz 0
Slice Selection
Z
- Z1
Z1B0
B G Zz0 1
2 1G Zz
0 G Zz
B G Zz0 1
Z
- Z1
Z10
0 1 G Zz
G Z
2 1G Zz
(a)
0 1 G Zz
(b)
RF coil
M
Gradient coils
RF with a narrow bandwidth Slice-select gradient
Y Magnet
B0ZX
Y
Excite a slice of tissue
B0
X
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ZSpatial encoding using a gradient pulse
G
xYrot G X tx 1
Gx
0 G Xx
0 1 G Xx
Xrot
x 1
X
- X1
X1
0
G X
(b) At X1
Z 0 1 G XxZ
G X t(a)
Yrot
G X tx 1
Phase offset relative to rotating frame at 0Xrot
(c) At –X1
g 0
00 B
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TR (time of repetition)
TE (time of echo)
Xgradient
( )
Gx
Gx tTx/2 Tx/2
Tx/2
Ygradient
t = 0
Gy Gradient Echo Sequence
Zgradient
Ty
TGz
Sequence
RF
g Tz
Tz/2
RF
DataAcquisitionAcquisition
Data acquisitionwindow
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Acquire signal (Fourier Transform)
Frequency domain (k space)
Inverse Fourier Transform
Space domain
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Dr. Seiji Ogawa
cycles/millimeter
millimeter
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Transformation
Britney Spears on earth Britney Spears on Mars
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ky (ky = 1/yfov) K space (Spatial Frequency Domain)
1st ky line
2nd ky line
(yres-1)/2
Xgradient
Gx
GTx/2 Tx/2
Tx/2
3rd ky linegradient
Y
Gx t
t = 0
x x
Gy
kx (k = 1/xfov)
-(xres-1)/2 (xres-1)/2Y
gradient
Ty
Gy
kx (kx 1/xfov)
ykixkixyxy
yxTt
eeeyxMtyxM 222)0,,(),,(
(kymax -1)th ky line
(kymax-2)th ky line
t
t
xx
dGk
dGk02
(kymax)th ky line-(yres-1)/2
dxdytyxMktS xy ),,()( 0
yy dGk02
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http://www.revisemri.com/tutorials/what_is_k_space/
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EPI Pulse Sequence
X Grad
Y Grad
Z GradZ Grad
RF
Time
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Regular EPI Sequence
Xgradient gxep1 gxepdw
gxepw
g gxep1 gxepdw
gyep1
Ygradient gyepb
Zgradient
gzrf1
gz1
gzk
g
RF
gz1
Time
RF
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EPI Pulse SequenceK space
Typical 64 64
KyTE
X Grad
Y Grad
63rd Ky line
Z Grad33rd Ky line
Kx
RF
Kx
Time
1st Ky line2nd Ky line
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30 slices
Slice #30Sli #2Slice #1 Slice #3Slice #2
Slice #29
2 sec = TR
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2 sec
Repeat many titimes
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2 sec
Bimanual finger tapping motor study (P ≤ 10-7)(12 s resting and then 24 s finger tapping at 1 Hz, TR = 2 s)
2 sec
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GoalsGoalsGoalsGoals
1. Basic concepts of MRI2. Basic meanings of TE, TR, T1, T2, T2*, k space, EPI
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Artifacts due to back-and-forth trajectory in k space
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Susceptibility artifacts
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Image artifacts due to field variation
NormalNormal
Variation along X
Variation along Y
Variation l Zalong Z
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Another common technique for fMRI: spiral imaging