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

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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
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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
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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:
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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/
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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
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MRI
B0
M0
Gz
B1
Gy
Gx
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Slice Selection*
Gradient-echo

RF
TE
slice
phase
freq
TR
*Mansfield, et al. J. Magn. Reson. (1976)
*Hoult, J. Magn. Reson. (1977)
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Slice Selection
Gz•z
B0
Mz
z{
Mxy
z
RF
Mz
BWRF= Gz•z
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Frequency Encoding*
a.k.a “readout”
Gradient-echo

RF
TE
slice
phase
freq
TR
*Kumar, et al. J. Magn. Reson. (1975)
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Frequency Encoding
Gx•x
Mz
FOVx
BWread= Gx•FOVx
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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)
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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)
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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)
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K-space weighting
High-frequency
data=edges
Low-frequency
data=contrast/signal
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MRI Examples
slide 35
3D Neuro MRI
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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