- Center for Magnetic Resonance and Optical Imaging
Download
Report
Transcript - Center for Magnetic Resonance and Optical Imaging
Principles of MRI
A r i B o r t h a k u r, P h D
As s o c i ate Di r e c to r, Ce nte r for Ma gne ti c Re s ona nc e & O pti c a l I m a gi ng
Department of Radiology
Perelman school of Medicine, University of Pennsylvania
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?
slide 2
Outline
Slide
#3
Hardware:
MRI scanner
RF coil
Gradients
Image contrast
Software:
Pulse sequences
Fourier Transform
slide 3
How does it work?
http://www.med.harvard.edu/AANLIB/cases/case17/mra.mpg
slide 4
Original Concept*
1.
2.
Rotating Gradient
Filtered Back-projection
*Lauterbur, Nature (1973)
slide 5
Fourier Imaging*
1.
Three orthogonal gradients:
2.
Slice Selection
Phase Encoding
Frequency Encoding
k-space
Representation of encoded signal
*Kumar, et al. J. Magn. Reson. (1975)
slide 6
MRI scanners
19 Tesla
4.7 Tesla
1.5 Tesla
To generate B0 field and create polarization (M0)
slide 7
MRI coils
Mouse MRI platform
Torso coil
Head coil
To transmit RF field (B1) and receive signal
slide 8
Gradients
To spatially encode the MRI signal
slide 9
Image Contrast
Contrast in MRI based on differences in:
Relaxation times (T1, T2, T2*, T1r)
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
slide 10
Magnetization
“spin”
“spin ensemble”
Bitar et al. RadioGraphics (2006)
spin-up
or
spin-down
=
Boltzmann
distribution
Net
Magnetic
Moment
slide 11
Energy levels
Boltzmann distribution
N+/N- = exp (-E/kBT)
Energy gap
Larmor Equation
slide 12
MR experiment
RF pulse
applied in x-y
“transverse”
plane
M0
aligned
along B0
Mz=M0
RF off
M
“nutates”
down to
x-y plane
Mz=M0cos
and
Mxy=M0sin
Detect Mxy
signal
in the same
RF coil
slide 13
Equation of motion
Bloch Equation (simple form)
slide 14
Effect of RF pulse
The 2nd B field
Choosing a frame of reference rotating at rf,
makes B1 appear static:
slide 15
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/
slide 16
After RF pulse-Relaxation
Spin-Lattice relaxation
time=return to thermal
equilibrium M0
Spin-spin relaxation
without “reversible”
dephasing
Spin-spin relaxation
with reversible
dephasing
T1>T2>T2*
slide 17
Signal detection
http://www-mrsrl.stanford.edu/ ~brian/mri-movies/
slide 18
How does MRI work?
NMR signal
MR Image
?
=
Water
Fat
4.7 ppm 1.2 ppm
1) Spatial encoding gradients
2) Fourier transform
slide 19
Pulse Sequence
(timing diagram)
Less MRI time/low image quality
Less MRI time/low image quality
90
180
RF
TE
slice
90
freq
freq
TR
Spin-echo
(<90)
TE
slice
RF
slice
phase
freq
freq
Gradient-echo
Fast spin-echo
90
phase
TR
180
slice
phase
RF
180
RF
phase
TR
180
TE
Echo planar imaging
slide 20
MRI
B0
M0
Gz
B1
Gy
Gx
slide 21
Slice Selection*
Gradient-echo
RF
TE
slice
phase
freq
TR
*Mansfield, et al. J. Magn. Reson. (1976)
*Hoult, J. Magn. Reson. (1977)
slide 22
Slice Selection
Gz•z
B0
Mz
z{
Mxy
z
RF
Mz
BWRF= Gz•z
slide 23
Frequency Encoding*
a.k.a “readout”
Gradient-echo
RF
TE
slice
phase
freq
TR
*Kumar, et al. J. Magn. Reson. (1975)
slide 24
Frequency Encoding
Gx•x
Mz
FOVx
BWread= Gx•FOVx
slide 25
Phase Encoding*
Gradient-echo
RF
TE
slice
phase
freq
pe
TR
*Kumar, et al. J. Magn. Reson. (1975)
*Edelstein, et al. Phys. Med. Biol. (1980)
slide 26
Phase Encoding
Gy2•y
FOVy
Gy1•y
Gy0•y
Each PE step imparts a 1/
different
phase twist to the magnetization along y
pe= Gy•FOVy
slide 27
Traversing k-space
Spin-warp imaging*
ky
Gradient-echo
RF
slice
TE
phase
kx
freq
TR
*Edelstein, et al. Phys. Med. Biol. (1980)
slide 28
Fourier Transform
k-space
image
FT
freq
phase
slide 29
Traversing k-space
Spiral
Radial
Echo-planar
freq
phase
freq
freq
Spiral
Echo planar
freq
freq
Radial
slide 30
Fourier Transform
slide 31
Typical FT pairs
slide 32
K-space
The signal a coil receives is from the whole object:
S( tx ,ty ) =
replace:
K-space signal:
ig ( G t
r
x,y
e
(
)
òò
x x
×x+ Gy ty ×y
) dxdy
kx, y = gGx, y tx, y
S( kx ,ky ) =
ig ( k ×x+ k ×y )
r
x,y
e
dxdy
(
)
òò
x
y
The image is a 2D FT of the k-space signal*:
r( x,y) =
òò
-ig k ×x+ k ×y
S( kx ,ky ) e ( x y ) dkx dky
*Kumar, et al. J. Magn. Reson. (1975)
slide 33
K-space weighting
High-frequency
data=edges
Low-frequency
data=contrast/signal
slide 34
MRI Examples
slide 35
3D Neuro MRI
slide 36
Susceptibility-Weighted Imaging (SWI)
slide 37
BOLD fMRI
slide 38
Phase Contrast MR Angiography
slide 39
TSE
TE = 25 ms
TSE
Fat Saturated
Resolution:200x200µm2
slide 40
7T Project: Hyper-polarized 3He MRI
K Emami, et al., Mag Reson Med 2009
slide 41
7T Project: ASL-Perfusion MRI
TAG
Image
Control
MRI “head coil”
MRI
Arterial Spin Labeling (ASL) technique
Williams et al., PNAS 1994
slide 42
ASL MRI & PET of Alzheimer’s Disease
ASL MRI Scan
PET Scan
CONTROL
AD
Detre et al., Neuroimage 2009
slide 43
Take home…
Slide
# 44
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
slide 44