Image Registration John Ashburner
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Transcript of Image Registration John Ashburner
Image Registration
John Ashburner
Pre-processing OverviewfMRI time-series
Motion Correct
Anatomical MRI
Coregister
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24232221
14131211
mmmmmmmmmmmm
Deformation
Estimate Spatial Norm
Spatially normalised
Smooth
Smoothed
Statistics or whatever
Template
Contents* Preliminaries
* Smooth* Rigid-Body and Affine Transformations* Optimisation and Objective Functions* Transformations and Interpolation
* Intra-Subject Registration* Inter-Subject Registration
Smooth
Before convolution Convolved with a circleConvolved with a Gaussian
Smoothing is done by convolution.
Each voxel after smoothing effectively becomes the result of applying a weighted region of interest (ROI).
Image Registration• Registration - i.e. Optimise the parameters
that describe a spatial transformation between the source and reference (template) images
• Transformation - i.e. Re-sample according to the determined transformation parameters
2D Affine Transforms* Translations by tx and ty
* x1 = x0 + tx
* y1 = y0 + ty
* Rotation around the origin by radians* x1 = cos() x0 + sin() y0
* y1 = -sin() x0 + cos() y0
* Zooms by sx and sy
* x1 = sx x0
* y1 = sy y0
*Shear*x1 = x0 + h y0
*y1 = y0
2D Affine Transforms* Translations by tx and ty
* x1 = 1 x0 + 0 y0 + tx
* y1 = 0 x0 + 1 y0 + ty
* Rotation around the origin by radians* x1 = cos() x0 + sin() y0 + 0
* y1 = -sin() x0 + cos() y0 + 0
* Zooms by sx and sy:* x1 = sx x0 + 0 y0 + 0
* y1 = 0 x0 + sy y0 + 0
*Shear*x1 = 1 x0 + h y0 + 0*y1 = 0 x0 + 1 y0 + 0
3D Rigid-body Transformations* A 3D rigid body transform is defined by:
* 3 translations - in X, Y & Z directions* 3 rotations - about X, Y & Z axes
* The order of the operations matters
1000010000cossin00sincos
10000cos0sin00100sin0cos
10000cossin00sincos00001
1000Zt100Y010X001
rans
trans
trans
ΩΩΩΩ
ΘΘ
ΘΘ
ΦΦΦΦ
Translations Pitchabout x axis
Rollabout y axis
Yawabout z axis
Voxel-to-world Transforms* Affine transform associated with each image
* Maps from voxels (x=1..nx, y=1..ny, z=1..nz) to some world co-ordinate system. e.g.,
* Scanner co-ordinates - images from DICOM toolbox* T&T/MNI coordinates - spatially normalised
* Registering image B (source) to image A (target) will update B’s voxel-to-world mapping* Mapping from voxels in A to voxels in B is by
* A-to-world using MA, then world-to-B using MB-1
* MB-1 MA
Left- and Right-handed Coordinate Systems
* NIfTI format files are stored in either a left- or right-handed system* Indicated in the header
* Talairach & Tournoux uses a right-handed system* Mapping between them sometimes requires a flip
* Affine transform has a negative determinant
Optimisation* Image registration is done by optimisation.* Optimisation involves finding some “best”
parameters according to an “objective function”, which is either minimised or maximised
* The “objective function” is often related to a probability based on some model
Value of parameter
Objective function
Most probable solution (global
optimum)Local optimumLocal optimum
Objective Functions* Intra-modal
* Mean squared difference (minimise)* Normalised cross correlation (maximise)* Entropy of difference (minimise)
* Inter-modal (or intra-modal)* Mutual information (maximise)* Normalised mutual information (maximise)* Entropy correlation coefficient (maximise)* AIR cost function (minimise)
* Nearest neighbour* Take the value of the
closest voxel* Tri-linear
* Just a weighted average of the neighbouring voxels
* f5 = f1 x2 + f2 x1
* f6 = f3 x2 + f4 x1
* f7 = f5 y2 + f6 y1
Simple Interpolation
B-spline Interpolation
B-splines are piecewise polynomials
A continuous function is represented by a linear combination of basis
functions
2D B-spline basis functions of degrees 0, 1,
2 and 3
Nearest neighbour and trilinear interpolation are the same as B-spline interpolation with degrees 0 and 1.
Contents* Preliminaries* Intra-Subject Registration
* Realign* Mean-squared difference objective function* Residual artifacts and distortion correction
* Coregister* Inter-Subject Registration
Mean-squared Difference
* Minimising mean-squared difference works for intra-modal registration (realignment)
* Simple relationship between intensities in one image, versus those in the other* Assumes normally distributed differences
Residual Errors from aligned fMRI* Re-sampling can introduce interpolation errors
* especially tri-linear interpolation
* Gaps between slices can cause aliasing artefacts* Slices are not acquired simultaneously
* rapid movements not accounted for by rigid body model
* Image artefacts may not move according to a rigid body model* image distortion* image dropout* Nyquist ghost
* Functions of the estimated motion parameters can be modelled as confounds in subsequent analyses
Movement by Distortion Interaction of fMRI*Subject disrupts B0 field, rendering it inhomogeneous
* distortions in phase-encode direction*Subject moves during EPI time series*Distortions vary with subject orientation
*shape varies
Movement by distortion interaction
Correcting for distortion changes using Unwarp
Estimate movement parameters.
Estimate new distortion fields for each image:
• estimate rate of change of field with respect to the current estimate of movement parameters in pitch and roll.
Estimate reference from mean of all scans.
Unwarp time series.
0B 0B
+
Andersson et al, 2001
Contents* Preliminaries* Intra-Subject Registration
* Realign* Coregister
* Mutual Information objective function
* Inter-Subject Registration
• Match images from same subject but different modalities:
–anatomical localisation of single subject activations
–achieve more precise spatial normalisation of functional image using anatomical image.
Inter-modal registration
Mutual Information
* Used for between-modality registration* Derived from joint histograms
* MI=ab P(a,b) log2 [P(a,b)/( P(a) P(b) )]* Related to entropy: MI = -H(a,b) + H(a) + H(b)
* Where H(a) = -a P(a) log2P(a) and H(a,b) = -a P(a,b) log2P(a,b)
Contents* Preliminaries* Intra-Subject Registration* Inter-Subject Registration
* Normalise* Affine Registration* Nonlinear Registration* Regularisation
* Segment* DARTEL
Spatial Normalisation - Procedure
Non-linear registration
* Minimise mean squared difference from template image(s)
Affine registration
EPI
T2 T1 Transm
PD PET
305T1
PD T2 SS
Template Images “Canonical” images
Spatial normalisation can be weighted so that non-brain voxels do not influence the result.
Similar weighting masks can be used for normalising lesioned brains.
Spatial Normalisation - Templates
Spatial Normalisation - Affine* The first part is a 12 parameter
affine transform* 3 translations* 3 rotations* 3 zooms* 3 shears
* Fits overall shape and size
* Algorithm simultaneously minimises* Mean-squared difference between template and source image* Squared distance between parameters and their expected values
(regularisation)
Spatial Normalisation - Non-linearDeformations consist of a linear combination of smooth basis functions
These are the lowest frequencies of a 3D discrete cosine transform (DCT)
Algorithm simultaneously minimises* Mean squared difference between template and
source image * Squared distance between parameters and their
known expectation
Targetimage
Affine registration.(2 = 472.1)
Non-linearregistration
withoutregularisation.(2 = 287.3)
Non-linearregistration
usingregularisation.(2 = 302.7)
Without regularisation, the non-linear spatial normalisation can introduce unnecessary warps.
Spatial Normalisation - Overfitting
Contents* Preliminaries* Intra-Subject Registration* Inter-Subject Registration
* Normalise* Segment
* Gaussian mixture model* Intensity non-uniformity correction* Deformed tissue probability maps
* DARTEL
Segmentation
* Segmentation in SPM5 also estimates a spatial transformation that can be used for spatially normalising images.
* It uses a generative model, which involves:* Mixture of Gaussians (MOG)* Bias Correction Component* Warping (Non-linear Registration) Component
Mixture of Gaussians (MOG)* Classification is based on a Mixture of Gaussians model
(MOG), which represents the intensity probability density by a number of Gaussian distributions.
Image Intensity
Frequency
Belonging Probabilities
Belonging probabilities are assigned by normalising to one.
Non-Gaussian Intensity Distributions* Multiple Gaussians per tissue class allow non-Gaussian
intensity distributions to be modelled.* E.g. accounting for partial volume effects
Modelling a Bias Field* A bias field is modelled as a linear combination
of basis functions.
Corrupted image
Corrected imageBias Field
Tissue Probability Maps* Tissue probability maps (TPMs) are used instead of
the proportion of voxels in each Gaussian as the prior.
ICBM Tissue Probabilistic Atlases. These tissue probability maps are kindly provided by the International Consortium for Brain Mapping, John C. Mazziotta and Arthur W. Toga.
Deforming the Tissue Probability Maps* Tissue probability
images are deformed so that they can be overlaid on top of the image to segment.
Optimisation* The “best” parameters are those that minimise this
objective function.* Optimisation involves finding them.* Begin with starting estimates, and repeatedly change
them so that the objective function decreases each time.
Steepest DescentStart
Optimum
Alternate between optimising different
groups of parameters
Tissue probability maps of GM
and WM
Spatially normalised BrainWeb phantoms
(T1, T2 and PD)
Cocosco, Kollokian, Kwan & Evans. “BrainWeb: Online Interface to a 3D MRI Simulated Brain Database”. NeuroImage 5(4):S425 (1997)
Contents* Preliminaries* Intra-Subject Registration* Inter-Subject Registration
* Normalise* Segment* DARTEL
* Flow field parameterisation* Objective function* Template creation* Examples
A one-to-one mapping* Many models simply add a smooth displacement to an identity transform
* One-to-one mapping not enforced* Inverses approximately obtained by subtracting the displacement
* Not a real inverse
Small deformation approximation
DARTEL* Parameterising the deformation
* φ(0)(x) = x* φ(1)(x) = ∫ u(φ(t)(x))dt* u is a flow field to be estimated
* Scaling and squaring is used to generate deformations.
t=0
1
Scaling and squaring example
Forward and backward transforms
Registration objective function* Simultaneously minimize the sum of:
* Likelihood component* Drives the matching of the images.* Multinomial assumption
* Prior component* A measure of deformation roughness* Regularises the registration.
* ½uTHu
* A balance between the two terms.
Likelihood Model* Current DARTEL model is multinomial for matching
tissue class images.* Template represents probability of obtaining different
tissues at each point.
log p(t|μ,ϕ) = ΣjΣk tjk log(μk(ϕj))t – individual GM, WM and background
μ – template GM, WM and background
ϕ – deformation
Prior Model
Simultaneous registration of GM to GM and WM to WM
Grey matter
White matter
Grey matter
White matter
Grey matter
White matter
Grey matter
White matter
Grey matter
White matterTemplate
Subject 1
Subject 2
Subject 3
Subject 4
TemplateInitial
Average
After a few iterations
Final template
Iteratively generated from 471 subjects
Began with rigidly aligned tissue probability maps
Used an inverse consistent formulation
Grey matter average of 452 subjects – affine
Grey matter average of 471 subjects
Initial GM images
Warped GM images
VBM Pre-processing* Use Segment button for
characterising intensity distributions of tissue classes.
* DARTEL import, to generate rigidly aligned grey and white matter maps.
* DARTEL warping to generate “modulated” warped grey matter.
* Smooth.* Statistics.
References* Friston et al. Spatial registration and normalisation of images.
Human Brain Mapping 3:165-189 (1995).* Collignon et al. Automated multi-modality image registration based on
information theory. IPMI’95 pp 263-274 (1995).* Ashburner et al. Incorporating prior knowledge into image registration.
NeuroImage 6:344-352 (1997).* Ashburner & Friston. Nonlinear spatial normalisation using basis functions.
Human Brain Mapping 7:254-266 (1999).* Thévenaz et al. Interpolation revisited.
IEEE Trans. Med. Imaging 19:739-758 (2000).* Andersson et al. Modeling geometric deformations in EPI time series.
Neuroimage 13:903-919 (2001).* Ashburner & Friston. Unified Segmentation.
NeuroImage 26:839-851 (2005).* Ashburner. A Fast Diffeomorphic Image Registration Algorithm. NeuroImage
38:95-113 (2007).