bmb 601 mri - center for magnetic resonance & optical · pdf filebmb 601 mri slide 2 ......
TRANSCRIPT
2/23/12
1
A r i B o r t h a k u r, P h D
A s s i s t a n t P r o f e s s o r , D e p a r t m e n t o f R a d i o l o g y A s s o c i a t e D i r e c t o r , C e n t e r f o r M a g n e t i c R e s o n a n c e & O p t i c a l I m a g i n g
U n i v e r s i t y o f P e n n s y l v a n i a S c h o o l o f M e d i c i n e
BMB 601 MRI
slide 2
A brief history…
Isidor Rabi wins Nobel prize in Physics (1944) for for his resonance method for recording the magnetic properties of atomic nuclei.
Felix Bloch and Edward Purcell share Nobel Prize in Physics (1952) for discovery of magnetic resonance phenomenon.
Richard Ernst wins Nobel Prize in Chemistry (1991) for 2D Fourier Transform NMR.
His post-doc, Kurt Wuthrich awarded a Nobel Prize in Chemistry (2002) for 3D structure of macromolecules.
Nobel Prize in Physiology & Medicine awarded to Sir Peter Mansfield and Paul Lauterbur (2003) for MRI.
Who’s next? Seiji Ogawa for BOLD fMRI?
2/23/12
2
slide 3
Outline Slide
# 3
Hardware: ! MRI scanner ! RF coil ! Gradients
Image contrast Software:
! Pulse sequences ! Fourier Transform
slide 4
How does it work?
http://www.med.harvard.edu/AANLIB/cases/case17/mra.mpg
2/23/12
3
slide 5
Original Concept*
1. Rotating Gradient 2. Filtered Back-projection
*Lauterbur, Nature (1973)
slide 6
Fourier Imaging*
1. Three orthogonal gradients: ! Slice Selection ! Phase Encoding ! Frequency Encoding
2. k-space ! Representation of encoded signal
*Kumar, et al. J. Magn. Reson. (1975)
2/23/12
4
slide 7
MRI scanners
19 Tesla! 4.7 Tesla! 1.5 Tesla!
To generate B0 field and create polarization (M0)
slide 8
MRI coils
Head coil
Mouse MRI platform
Torso coil To transmit RF field (B1) and receive signal
2/23/12
5
slide 9
Gradients
To spatially encode the MRI signal
slide 10
Image Contrast
Contrast in MRI based on differences in: Relaxation times (T1, T2, T2
*, T1ρ) ! Local environment
Magnetization = spin density ! Concentration of water, sodium, phosphorous etc.
Macromolecular interactions ! Chemical-exchange, dipole-dipole, quadrupolar interactions
Contrast agents ! Gadolium-based, iron-oxide, manganese
2/23/12
6
slide 11
Magnetization
Bitar et al. RadioGraphics (2006)
“spin” “spin ensemble” spin-up or
spin-down =
Boltzmann distribution
Net Magnetic Moment
slide 12
Energy levels
Larmor Equation
Energy gap
N+/N- = exp (-ΔE/kBT) Boltzmann distribution
2/23/12
7
slide 13
MR experiment
M0 aligned along B0 Mz=M0
RF pulse applied in x-y “transverse”
plane
M “nutates” down to x-y plane
Mz=M0cosα and
Mxy=M0sinα
RF off
Detect Mxy signal
in the same RF coil
slide 14
Equation of motion
Bloch Equation (simple form)
2/23/12
8
slide 15
Effect of RF pulse
The 2nd B field
Choosing a frame of reference rotating at ωrf, makes B1 appear static:
slide 16
Nutation
Lab frame of reference e.g. B1 oscillating in x-y plane
Rotating frame of reference e.g. B1 along y-axis
http://www-mrsrl.stanford.edu/ ~brian/mri-movies/
2/23/12
9
slide 17
After RF pulse-Relaxation
Spin-Lattice relaxation time=return to thermal equilibrium M0
Spin-spin relaxation with reversible dephasing
Spin-spin relaxation without “reversible” dephasing
T1>T2>T2*
slide 18
Signal detection
http://www-mrsrl.stanford.edu/ ~brian/mri-movies/
2/23/12
10
slide 19
How does MRI work?
Water ! Fat!4.7 ppm 1.2 ppm!
NMR signal!
?!=!
MR Image!
1) Spatial encoding gradients 2) Fourier transform
slide 20
Pulse Sequence (timing diagram)
Less MRI time/low image quality!
RF!
phase!
slice!
freq!!
90°!
TE!
Echo planar imaging!
Less
MRI
tim
e/lo
w im
age
qual
ity!
RF!
phase!slice!
freq!
90°!
TE!
Spin-echo!TR!
180°!
RF!
phase!slice!
freq!!
α (<90°)!
TE!
Gradient-echo!TR!
RF!
phase!slice!
freq!!
90°!
Fast spin-echo!TR!
180°! 180°! 180°!
2/23/12
11
slide 21
MRI
B0 M0 B1
Gz
Gx
Gy
slide 22
RF!
phase!
slice!
freq!
α!
TE!
Gradient-echo!
TR!
Slice Selection*
*Mansfield, et al. J. Magn. Reson. (1976) *Hoult, J. Magn. Reson. (1977)
2/23/12
12
slide 23
Mz
Slice Selection
B0
Mz
Mz
Mxy γGz•z RF
BWRF= γGz•Δz
Δz{
slide 24
Frequency Encoding*
RF!
phase!
slice!
freq!
α!
TE!
Gradient-echo!
TR!
*Kumar, et al. J. Magn. Reson. (1975)
a.k.a “readout”
2/23/12
13
slide 25
Frequency Encoding
Mz
γGx•x
BWread= γGx•FOVx
FOVx
slide 26
RF!
phase!
slice!
freq!
α!
TE!
Gradient-echo!
TR!
Phase Encoding*
*Kumar, et al. J. Magn. Reson. (1975) *Edelstein, et al. Phys. Med. Biol. (1980)
τpe
2/23/12
14
slide 27
Phase Encoding
γGy2•y
1/τpe= γΔGy•FOVy
FOVy
γGy1•y
γGy0•y
Each PE step imparts a different phase twist to the magnetization along y
slide 28
Traversing k-space
Spin-warp imaging*
RF!
phase!
slice!
freq!
α!
TE!
Gradient-echo!
TR!
ky
kx
*Edelstein, et al. Phys. Med. Biol. (1980)
2/23/12
15
slide 29
FT
k-space
freq phase
image
Fourier Transform
slide 30
Spiral Radial Echo-planar
phase!
freq!!
Echo planar! freq!
freq!!
Radial!
!freq!
freq!! Spiral!
Traversing k-space
2/23/12
17
slide 33
K-space
The signal a coil receives is from the whole object:
€
S tx ,ty( ) = ρ x,y( )eiγ Gxtx ⋅x+Gyty ⋅y( )dxdy∫∫
€
kx, y = γGx, ytx, y
€
S kx ,ky( ) = ρ x,y( )eiγ kx ⋅x+ ky ⋅y( )dxdy∫∫
The image is a 2D FT of the k-space signal*:!
€
ρ x,y( ) = S kx ,ky( )e−iγ kx ⋅x+ ky ⋅y( )dkxdky∫∫
*Kumar, et al. J. Magn. Reson. (1975)
replace:!
K-space signal:!
slide 34
K-space weighting
High-frequency data=edges
Low-frequency data=contrast/signal
2/23/12
18
slide 35
What is wrong?
Original!
Freq. de-phaser!too large!!
*Pauly, http://www.stanford.edu/class/ee369b/tests/mt_sol.pdf
slide 36
What is wrong?
Original
ky step was too large or FOVy was too small!
2/23/12
19
slide 37
What is wrong?
Original!
Freq. gradient!amplitude was !too low!!
slide 38
MRI Examples
2/23/12
22
slide 43
TSE$Fat$Saturated$
TSE$TE$=$25$ms$
Resolution:200x200µm2
slide 44
7T Project: Hyper-polarized 3He MRI
K$Emami,$et$al.,$Mag$Reson$Med$2009$
2/23/12
23
slide 45
MRI
MRI “head coil”
Williams et al., PNAS 1994
7T Project: ASL-Perfusion MRI
Arterial Spin Labeling (ASL) technique
TAG
Imag
e
Con
trol
slide 46
ASL MRI & PET of Alzheimer’s Disease
ASL MRI Scan
18-FDG
PET Scan
CONTROL AD
Detre et al., Neuroimage 2009
2/23/12
24
slide 47
Sodium MRI of Inter-vertebral
Discs
slide 48
Motivation
Lower Back Pain (LBP) is the most common ailment of the working-age adult.
Affects > 4 million individuals each year in the US alone.
Economic burden of $100 billion.
2/23/12
25
slide 49
Diagnosis of LBP-Discogram
1. Inject saline to reproduce pain. 2. Introduce dye to visualize defects.
slide 50
Mechanical function of spine-Pole Vault
• Dynamic system"
• Pole temporarily stores energy"
• Releases energy delayed"
2/23/12
26
slide 51
Intervertebral Disc (IVD)
http://www.spineuniverse.com
slide 52
Hydrostatic System"
Mechanical function of IVD
2/23/12
27
slide 53
Disc Degenerative Disease
slide 54
Rationale for Sodium MRI
[PG] decreases with Degeneration
Urban et al Spine 1998
2/23/12
28
slide 55
Rationale for Sodium MRI
Urban and Maroudas Bioc. Biop. Acta 1979 Urban and Winlove, J. Magn. Reson. Imag. 2007
Sodium is a natural biomarker for PG
slide 56
How is [Na] related to [PG]?
Fixed Charge Density (FCD) ! Side chains of PG ! [Na] correlated with [PG] through [FCD]
[Na] [FCD] [PG] detect early DDD
Maroudas et al., in Adult Articular Cartilage 1979 Lesperance et al., J. Orthop. Res. 1992
2/23/12
29
slide 57
Sodium MRI
What do you need? 1. RF coil 2. Transmit/receive switch 3. Broadband capable scanner (all vendors) 4. A short echo-time MRI pulse sequence
slide 58
Sodium MRI Ex Vivo
Can [PG] be quantified in the NP non-destructively?
1. Calibrate sodium MRI signal* from IVD.
y = 1.4x + 60.5
R2 = 0.99
0
100
200
300
400
500
0 50 100 150 200 250 300
calibration phantom [Na] (mM)
Sodiu
m S
ignal
Inte
nsi
ty (
a.u
.)
Shapiro et al., J. Magn. Reson. 2000
2/23/12
30
slide 59
Donnan Equilibrium
2. Convert [Na] in tissue to fixed charge density:
300mM
0mM
Lesperance et al., J. Orthop. Res. 1992
3. Convert FCD to [PG]:
slide 60
Sodium MRI Ex Vivo
Wang et al., Spine (2010)
• Sodium maps in bovine discs
2/23/12
31
slide 61
Sodium MRI Ex Vivo
Wang et al., Spine (2010)
Sodium content vs. PG content
slide 62
Sodium MRI In Vivo
Wang et al., Spine (2010)
• Sodium mapping vs. T2 MRI
2/23/12
32
slide 63
New 7T MRI!
ultra-short echo pulse sequence
TE/TR=200µs/25ms 2mm isotropic 15 minute acquisition
slide 64
Sodium MRI Movies
Axial Coronal Sagittal
2/23/12
33
slide 65
Sodium MRI Montage
SNR ~26:1 in cartilage
slide 66
Take home… Slide # 66
Magnet ! Create B0 ! Produce M0
RF coil ! Transmit B1 field ! Detect Signal
Image contrast ! Relaxation, concentration, interactions etc.
Gradients ! Spatial encoding ! Signal in k-space
Fourier Transform ! From k-space to image space
Pulse sequences ! Traverse k-space ! Image Artifacts