introducing the new usc graphic identity program · !e primary logotype for the overarching...
TRANSCRIPT
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Sparse sampling in MRI:From basic theory to clinical application
R. Marc Lebel, PhDDepartment of Electrical Engineering
Department of Radiology
The University Identity!is document provides the basic guidelines for the University of Southern California’s graphic identity program. !e key to the program is the typographic system of wordmark, monogram and contrasting color. To maintain consistency throughout the system, the new graphic identity program prohibits the use of any additional iconography, marks or artwork outside of the ap-proved University seal, shield and Trojan head (the latter to be used for spirit-related applications only). To obtain a full version of the new style guide, contact USC Purchasing or go online to: usc.edu/identity
The Seal and Shield!e primary logotype for the overarching University identity has been redesigned to combine the new USC monogram with the Adobe Caslon Pro wordmark and the shield. Regular- and small-use versions of both the seal and shield have been redrawn to ensure the best quality reproduction. Size restrictions for each are detailed above. !e shield exists as part of the o"cial University seal and will be used as an additional brand icon throughout the new identity program. !e only times the shield should be locked up with the wordmark or monogram are in the primary combinations shown at left. For additional questions about which logotype to use, email: [email protected]
Color!e o"cial colors of the University of Southern California, USC Cardinal and USC Gold, are equal in importance in identifying the university. Precise matching of all USC colors is essential in all color applications. Do not use maroon and yellow, or red and ochre as a substitute for the USC colors. Correct and consistent use of USC’s o"cial colors helps reinforce the university’s identity.
Introducing the New
Primary Logotype Combination
Informal Logotype (Horizontal)
Informal Logotype (Vertical)
Primary Monogram Combination
Formal Monogram Combination
Formal Logotype Combination
USC Graphic Identity Program
1-color USC monogram reproduces as 50% of the color being used.
Cardinal Background When the lockup appears on a cardinal background it reverses to white and the monogram runs gold.
Gold Background When the lockup appears on a gold background it can either reverse to white or run black while the monogram runs cardinal.
Black Backgrounds When the lockup appears on a black background it reverses to white and the monogram runs either cardinal, gold or 50% black.
The minumum required space around all logotypes is half the logotype’s height on all sides as shown above.
The Small-use Seal should be used for any application
between 1.25 inches and 0.75 inches wide.Do not reproduce
small-use version smaller than 0.75 inches.
0.75"1.25" wide
The Regular-use Seal should be used for any application
wider than 1.25 inches.For applications smaller
than 1.25 inches wide, use the small-use version.
1" wide 0.25"
The Regular-use Shield should be used for any
application wider than 1 inch.For applications smaller than 1 inch wide, use the
small-use version.
The Small-use Shield should be used for any application
between 1 inch and 0.25 inch.Do not reproduce small-use
version smaller than 0.25 inches wide.
Approved Color CombinationsBelow are the approved color combinations for all USC logotypes. !e logotypes shown here have been speci#-cally created for use across all media. Always use approved artwork. Do not attempt to typeset or create any logos on your own.
Do not center the shield over the wordmark.
Do not place the shield to the right of
the monogram.
Do not string wordmark in one line whencombining with monogram.
Do not place monogram beneath
wordmark.
Do not change the proportion of the
monogram to the seal. Unapproved colors and alignment.
Unapproved color combina-tion.
Light Gray30% Black
Hex: #CCCCCC
PMS 201
PMS 123
Dark Gray70% Black
Hex: #777777
Direct CMYK Conversion
C31 M88 Y51 K22DO NOT USE
Direct CMYK Conversion
C0 M24 Y94 K0DO NOT USE
Black100% Black
Hex: #000000
Proper Bridge CMYK ConversionC7 M100 Y65 K32
Proper Bridge CMYK ConversionC0 M27 Y100 K0
WhiteHex: #FFFFFF
Approved RGB Conversion
R153 G27 B30Approved HEX:
#990000
Approved RGB Conversion
R255 G204 B0Approved HEX:
#FFCC00PMS 123
Secondary Web Colors
PMS 201
Primary logotypes are to be used most often throughout USC publications and collateral.
Formal logotypes are to be used in the most formal aca-demic applications such as certificates, legally sanctioned documents and formal invitations.
Informal logotypes are to be used in any informal applications that require more flex-ibility in reproduction, particularly when there are size constraints (small book bind, fabric, embroidery).
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Objective
• Provide an intuitive overview of compressed sensing as applied to MRI
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Objective
• This is not a research talk
• I will use my data to illustrate points
• This is an emerging technology
• There are unanswered questions
• Some of this talk will be speculative
• Provide an intuitive overview of compressed sensing as applied to MRI
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Outline
• MR Physics
• k-space and sampling requirements
• Compressed sensing
• Constrained reconstruction, sparsity, and random sampling
• Applications
• Neuroimaging
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Magnetic Resonance ImagingMain magnet, ~1 Tesla
(magnetize the sample)Gradient coils, mT/m(spatial localization)
Radio-frequency coil(s), ~uT(transmission and reception)
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Magnetic Resonance Imaging
α
Echo time
Repetition time
αRadio-frequency
Readout
Phase encode
Slice selectSpat
ial e
ncod
ing
grad
ient
s
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Magnetic Resonance Imaging
• Non invasive
• High resolution
• Multiple intrinsic contrast mechanisms
• Arbitrary slice orientation
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Magnetic Resonance ImagingAcquisition time
• Serially acquire all of the points in an image
• Solutions
• Adjust resolution and field-of-view to require fewer points
• Undersample the data
Spatial encoding Low sensitivity
• Very few spins contribute signal; lots contribute noise
• Solutions
• Adjust resolution
• Scan longer
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Magnetic Resonance ImagingAcquisition time
• Serially acquire all of the points in an image
• Solutions
• Adjust resolution and field-of-view to require fewer points
• Undersample the data
Spatial encoding Low sensitivity
• Very few spins contribute signal; lots contribute noise
• Solutions
• Adjust resolution
• Scan longer
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Magnetic Resonance ImagingAcquisition time
• Serially acquire all of the points in an image
• Solutions
• Adjust resolution and field-of-view to require fewer points
• Undersample the data
Spatial encoding Low sensitivity
• Very few spins contribute signal; lots contribute noise
• Solutions
• Adjust resolution
• Scan longer
Undersample withoutnoise amplification
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Magnetic Resonance ImagingSampling requirements
k-space* image-space
FourierTransform
dy
FOVy
dky
FOVky
FOVy = 1/dky
dy = 1/FOVky*All k-space images are logarithmically scaled
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Magnetic Resonance ImagingSampling requirements
FourierTransform
dy
FOVy
dky
FOVky
FOVy = 1/dky
dy = 1/FOVky
image-spacek-space*
*All k-space images are logarithmically scaled
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Magnetic Resonance ImagingSampling requirements
FourierTransform
dy
FOVy
dky
FOVky
FOVy = 1/dky
dy = 1/FOVky
image-spacek-space*
*All k-space images are logarithmically scaled
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Magnetic Resonance ImagingUndersampling
FourierTransform
dky
dkx
FOVx
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Magnetic Resonance ImagingUndersampling
dky
dkx
FourierTransform
sub-Nyquist sampling
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Magnetic Resonance ImagingUndersampling
a1 pm
Aliased signal
Coil sensitivity at ‘m’ True MRI signal at ‘m’
Coil sensitivity at ‘n’
True MRI signal at ‘n’
pn
a1 = S1,m pm + S1,n pn
True image
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Magnetic Resonance ImagingParallel Imaging
a2 = S2,m pm + S2,n pn[A] = [S][P] P = (S*S)-1S*A
In matrix form ...with solution:
SENSE: Sensitivity EncodingPruessmann KP, et al. MRM 1999
a1 = S1,m pm + S1,n pn
a1 pm pn
True image
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Magnetic Resonance ImagingParallel Imaging
a2 = S2,m pm + S2,n pn[A] = [S][P] P = (S*S)-1S*A
In matrix form ...with solution:
SENSE: Sensitivity EncodingPruessmann KP, et al. MRM 1999
a1 = S1,m pm + S1,n pn
a1 pm pn
True imageCoil 1
a2
Coil 2
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Magnetic Resonance ImagingParallel Imaging
1 2 3 4 5 6 7 8Number of receiver coils
1
2
3
4
5
6
7
8
Acce
lera
tion
fact
or
Theoretical MaximumPractical Maximum• Acceleration rate ≤ number of
receiver coils
• Receiver coils must have a unique sensitivity along the accelerated dimensions
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Recap
• MRI is insensitive and very slow
• Data is sampled in k-space
• Uniform undersampling produces coherent aliasing
• Unwrapping is possible with multiple receiver coils (parallel imaging)
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Random Undersampling
FourierTransform
FourierTransform
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Random Undersampling
FourierTransform
FourierTransform
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Random Undersampling
FourierTransform
• T-e qu-st--n i-: h-w do w- s--pre-s -he inco--re-t -lias--g an- fi-l in th- -issing dat-?
• A--wer: c-nt-xt
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Constrained Reconstruction
FourierTransform
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Constrained Reconstruction
mask
FourierTransform
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Constrained Reconstruction
FourierTransform
mask
FourierTransform
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FourierTransform
FourierTransform
Replace
Constrained Reconstruction
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FourierTransform
FourierTransform
Replace
Constrained Reconstruction
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FourierTransform
FourierTransform
Replace
mask
Constrained Reconstruction
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FourierTransform
FourierTransform
Iterate...
Constrained Reconstruction
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Constrained Reconstruction
• Initial observations:
• Promote expected characteristic
• Enforce data consistency
• Iterate to improve the image
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Mathematical Model
• Two types of norms:
a) l0: counts the number of coefficients
b) l2: sum-of-squares
Data consistency
Sparsityminimize || m||0subject to ||Fm� y||22 < ✏
SparsityTransform m Image F Fourier
Transformy Acquired
k-space data✏ Noise
threshold
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Mathematical Model
Data consistency
Sparsity
• Choose an image that:
a) (mostly) conforms to the image model
b) is consistent with the acquired data
minimize || m||0subject to ||Fm� y||22 < ✏
SparsityTransform m Image F Fourier
Transformy Acquired
k-space data✏ Noise
threshold
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Transform Sparsity
• A mathematical operation that allows the image to be described with a small number of coefficients
minimize || m||0subject to ||Fm� y||22 < ✏Sparsity
Transform m Image
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• Images are ‘natural’ (adjacent pixels are often correlated)
• Wavelet Transform
Image domain Wavelet domain
|| m||0
Transform Sparsity
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Image domain Wavelet domain
• Images are ‘natural’ (adjacent pixels are often correlated)
• Wavelet Transform || m||0
Transform Sparsity
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Fully sampled Undersampled
• Images are ‘natural’ (adjacent pixels are often correlated)
• Wavelet Transform || m||0
Transform Sparsity
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• Dynamic images do not change very much
time frame: 1 2 3 4
Finite difference in time
Transform Sparsity
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Finite differences
||(T� I)m||0T Time shift I Identity
• Dynamic images do not change very much
Transform Sparsity
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Mathematical Model
Data consistency
Sparsity
• Choose an image that:
a) (mostly) conforms to the image model
b) is consistent with the acquired data
minimize || m||0subject to ||Fm� y||22 < ✏
SparsityTransform m Image F Fourier
Transformy Acquired
k-space data✏ Noise
threshold
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Mathematical Model
Data consistency
Sparsity
• This problem is very difficult to solve
• It is non-convex (may have local minima)
minimize || m||0subject to ||Fm� y||22 < ✏
SparsityTransform m Image F Fourier
Transformy Acquired
k-space data✏ Noise
threshold
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Mathematical Model
• Convert the l0 norm to an l1 norm
• Sum of absolute values
• Still promotes sparsity
SparsityTransform m Image F Fourier
Transformy Acquired
k-space data✏ Noise
threshold
minm
�||Fm� y||22 + � || m||1
� Regularization factor
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Mathematical Model
• This problem is convex (no local minima)
• Conjugate gradient
• Thresholding techniques
• Solution is iterative (very slow!)
• Must choose
SparsityTransform m Image F Fourier
Transformy Acquired
k-space data✏ Noise
threshold
minm
�||Fm� y||22 + � || m||1
� Regularization factor
�
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Regularization factorminm
�||Fm� y||22 + � || m||1
� too small? � just right? � too big?
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Reconstruction Movie
k-space image-space wavelet-space
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Sampling Patterns• Avoid coherent aliasing in sparsity domain
• Pseudo-random sampling
• Undersample in domains/dimensions that take time to acquire
• Often phase-encode dimensions in k-space
• Undersample in as many dimensions as possible
2D → 1D undersampling
kx
(in/out of screen)ky
3D → 2D undersampling
ky
kz
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Sampling Patterns
• Avoid coherent aliasing in sparsity domain
• Pseudo-random sampling
• Can test several different patterns
M. Lustig et al. MRM 2007
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Sampling PatternsNot sparse!
• The center of k-space (describes the low resolution image) has a lot of energy
• It should be sampled more heavily
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Sampling Patterns
• Variable density sampling compensates for the lack of sparsity at low resolutions
Centrallines: 0 8 16 32 48 85
MSE: 24.2 14.2 8.98 4.22 4.06 8.44
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Compressed Sensing and Parallel Imaging
minm
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l1-SPIRiT(Iterative Self-consistent Parallel Imaging Reconstruction)Lustig M. MRM 2010
CS-SENSE
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Recent work from MREL
• We have been working to implement compressed sensing and parallel imaging on the scanner
• GE 3T scanners at HSC
• Fat/water separation
• Visualization of the upper airway
• Neuroimaging
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Phantom images
1.0x
2.0x
8.0x
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1.0x 2.0x 8.0x
Phantom images
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1x (7.6 minutes) 2x (3.8 minutes) 4x (1.9 minutes) 8x (0.96 minutes)
3D T1-w SPGR: 256 x 256 x 256 (0.78 x 0.78 x 0.78 mm3)
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1x (7.6 minutes) 2x (3.8 minutes)
4x (1.9 minutes) 8x (0.96 minutes)
3D T1-w SPGR: 256 x 256 x 256 (0.78 x 0.78 x 0.78 mm3)
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Dynamic Contrast Enhanced MRI3D T1-w SPGR: 256 x 186 x 10 (0.93 x 0.93 x 3.0 mm3)
35 time frames, 10 s temporal resolution
Retrospective undersampling
l1-SPIRiT with 4D wavelet, 3D finite differences, dynamic constraints
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Dynamic Contrast Enhanced MRI3D T1-w SPGR: 256 x 186 x 10 (0.93 x 0.93 x 3.0 mm3)
35 time frames, 10 s temporal resolution
Retrospective undersampling
l1-SPIRiT with 4D wavelet, 3D finite differences, dynamic constraints
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General Thoughts
• We’re going to see a lot more compressed sensing/constrained reconstruction in the future
• Vendors are watching the field very carefully
• Currently challenging to implement in a robust and reliable manner
• Collaborations are needed
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General Thoughts (fMRI)
Acquiredata
Spatial andtemporal
smoothing
Register
Impose hemodynamicmodel
Test for statisticalsignificance
Currently:
Acquire data(appropriate
sampling pattern)
Potentially:Reconstruct parametric maps
Regularize for:spatial smoothness
temporal drift
Impose hemodynamic
response
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Questions?