(1) Read Chapters 1 5. (1) Read Chapters 1-5. (2) Watch “MRI-Made Easy” video
. ... echo-planar imaging (EPI) by Peter Mansfield. Fast MRI. First human-body ...
I t d ti Introduction to M Magnetic ti Resonance R Imaging I i (MRI) Physics Ph i David C. Zhu, Ph.D. Cognitive Imaging Research Center Departments of Psychology and Radiology
Reading assignment for next three lectures: (1) Read Chapters 1-5. 15 (2) Watch “MRI-Made Easy” video.
~ 1937
Began the concept of magnetic resonance by Isidor Rabi
~1945
Felix Block and Edward Purcell discovered magnetic resonance
~1972
Paul Lauterbur introduced spatial gradients to provide spatial information
~1976
1982 Early 1990s
IIntroduction t d ti off echo-planar imaging (EPI) by Peter Mansfield
Nobel price in 1944 Nobel price in 1952
Beginning g g of MRI Nobel price in 2003 Fast MRI
First human-body 1.5T by GE
MRI technology p ggrowth Rapid
Discovery of BOLD contrast
Basis of fMRI
Goals 1. Basic concepts of MRI 2. Basic meanings of TE, TR, T1, T2, T2*, k space, EPI
An atom
e-
p+ n
Nucleus
C Common elements l t usedd in i MRI: MRI 1H, 13C, 23Na, 31P
Hydrogen
Nucleus
e-
p+
( we have a lot of H2O)!
Spin Physics 1H
classically:
((proton) t )
N Angular Momentum (spin spin)
S Magnetic dipole Magnetic field B Quantized to lower and higher energy states with a Boltzmann distribution: ~ 3 ppm/T excess in lower energy. Bloch Equation:
B = Larmor frequency = 42.58 MHz/T for proton
128 MHz at 3 T
dM M B dt M = magnetization = net magnetic moment for all spins in a sample
E = h ,
h = 6.626 x 10-34 J S
JT Bushberg, JA Seibert, EW Leidholdt Jr., and JM Boone. The Essential Physics of Medical Imaging
A happy volunteer after surviving a fMRI session
Magnet Gradient coils RF coils Subject body
Superconducting electromagnets -261C Zero resistance it
Coil for the static magnetic field
Gradient Coil
RF coils (Transmit and Receive)
surface coil
volume coil
phased-array coil
Magnetic Resonance Imaging Hardware Interface in Control Room fMRI stimulus presentation system
3T magnet Room
Equipment Room: Gradient amplifiers RF amplifier P l sequence generator Pulse t Image reconstruction
Spin-Lattice (T1) and Spin-Spin (T2) Relaxation Processes (T2 becomes T2* if local field is inhomogeneous)
Z
Z M
S i fan Spins f outt (dephasing)
B0
T2 decay Y
Initial 90 RF excitation X
TR = time of repetition
Y
Longitudinal magnetization re-growth T1 recovery
T2 decay and T1 recoveryy continue
Back to equilibrium state
Z
X
vector summation
T2 decay deca and T1 recovery continue
TE = time of echo Y
RF X
T2* Decay and T1 Recovery Movie 1
http://www.stanford.edu/class/ee369b/Site/Movies.html
T2* Decay and T1 Recovery Movie 2
| M xy (t ) || M xy (0) | e
Tt * 2
S kM 0 (1 e
Tt
M z (t ) M 0 (1 e 1 )
TR T 1
)e
TTE* 2
Courtesy of Brian Hargreaves. http://www-mrsrl.stanford.edu/~brian/mri-movies/
Gradient Echo TR = 3s
TE = 6.9 ms
TE = 45 ms
Spin Echo Techniques (Obtain the effect of T2 instead of T2* )
Z
Z
Spins p fan out ((dephasing) p g)
M B0
T2 Initial 90 90 RF excitation
Y
180 RF excitation
* decay
Y
TE/2 X
X
Z
Z
Y
Y
TE/2 X
X
Z
Y
128 MHz
X
Z
Z
Y
Y
X
X
128.0001 MHz
127.9999 MHz Z
Y
X
Explanation of T2* decay
3.000 T
3.000 T
3.000 T
3.000 T
3+10-6
T
3+2×10-6 T
3.000 T 3-10-6 T
After 3 ms
After Af 3 ms
3.000 T
3.000 T
3.000 T
3.000 T
3+10-6 T
3.000 T
3+2×10-6 T
3-10-6 T
Vector sum
Vector sum
Spin Echo Technique
Courtesy of Brian Hargreaves. http://www-mrsrl.stanford.edu/~brian/mri-movies/
Spin Echo
Proton density weighted
T2 weighted
T1 weighted
TE = 13 ms
TE = 90 ms
TE = 13 ms TR = 900 ms
TR = 3 s
S kM 0 (1 e
TR T 1
)e
TE T 2
Laboratory Frame
Courtesy of Brian Hargreaves. http://www-mrsrl.stanford.edu/~brian/mri-movies/
Rotating Frame
Courtesy of Brian Hargreaves. http://www-mrsrl.stanford.edu/~brian/mri-movies/
Long T1 T2
Relaxation time
Short Molecular motion: Molecular size: Molecular interactions:
fast
slow large
intermediate intermediate
ll small
bound
intermediate
free
JT Bushberg, JA Seibert, EW Leidholdt Jr., and JM Boone. The Essential Physics of Medical Imaging
B
Slice Selection B B0 Gz Z B0 Gz Z1
2Gz Z1
- Z1
0 Gz Z B0
Z
Z1
0 Gz Z1
B0 Gz Z1
- Z1
2Gz Z1
0
Z1
0 Gz Z1
(a)
RF with a narrow bandwidth
(b)
Slice-select gradient
Gradient coils Y
M Magnet Z
X Excite a slice of tissue
B0
RF coil
B0
Z
Spatial encoding using a gradient pulse
Z
Gx x
Gx X 1 t Yrot
0 Gx X Xrot
0 Gx X1
(b) At X1
- X1
0
X1
X
0 Gx X1
Z
Gx X 1 t (a)
Yrot
Phase offset relative to rotating g frame at 0
0 B0
Xrot (c) At –X1
TR (time of repetition) TE ((time of echo)) X gradient
Tx/2 Gx
Tx/2
Tx/2 t
Gx
t=0 Gy
Y gradient
Z g gradient
Gradient Echo Sequence Ty
Gz Tz Tz/2
RF
Data Acquisition Data acquisition window
Acquire signal (Fourier Transform)
Frequency domain (k space)
Inverse Fourier Transform Space domain
Dr. Seiji Ogawa
cycles/millimeter millimeter
Transformation
Britney Spears on earth
Britney Spears on Mars
ky (ky = 1/yfov)
K space (Spatial Frequency Domain)
(yres-1)/2 1st ky line 2nd ky line
X gradient
Tx/2
Tx/2
Gx
3rd ky line
Gx
Tx/2 t t=0
(xres-1)/2
-(xres-1)/2
Y gradient
kx (kx = 1/xfov)
Gy
Ty Tt
M xy ( x, y, t ) M xy ( x, y,0)e 2 e i 2k x x e (kymax-2)th ky line (kymax -1)th ky line (kymax)th ky line -(yres-1)/2
kx ky
i 2k y y
t
G d 2 0
2
x
t
G d 0
y
S (t ) k0 M xy ( x, y, t )dxdy
http://www.revisemri.com/tutorials/what_is_k_space/
EPI Pulse Sequence
X Grad
Y Grad
Z Grad
RF
Time
Regular EPI Sequence gxepw
X ggradient
gxep1
gxepdw
gyep1
Y gradient
gyepb
gzk gzrf1
Z ggradient
gz1
RF Time
K space Typical 64 64
EPI Pulse Sequence
TE
Ky
X Grad
63rd Ky line Y Grad
33rd Ky line Z Grad
Kx RF
Time
2nd Ky line 1st Ky line
30 slices
Slice #30 Slice #1
Slice #3
Sli #2 Slice Slice #29
2 sec = TR
2 sec
Repeat many times ti
2 sec 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)
Goals 1. Basic concepts of MRI 2. Basic meanings of TE, TR, T1, T2, T2*, k space, EPI
Artifacts due to back-and-forth trajectory in k space
Susceptibility artifacts
Image artifacts due to field variation Normal
Variation along X
Variation along Y
Variation along l Z
Another common technique for fMRI: spiral imaging
