spatial preprocessing of fmri data
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Spatial preprocessing of fMRI data. Klaas Enno Stephan Laboratory for Social and Neural Systrems Research Institute for Empirical Research in Economics University of Zurich Functional Imaging Laboratory (FIL) Wellcome Trust Centre for Neuroimaging University College London. - PowerPoint PPT PresentationTRANSCRIPT
Spatial preprocessing of fMRI data
Methods & models for fMRI data analysis in neuroeconomics17 April 2010
Klaas Enno Stephan Laboratory for Social and Neural Systrems ResearchInstitute for Empirical Research in EconomicsUniversity of Zurich
Functional Imaging Laboratory (FIL)Wellcome Trust Centre for NeuroimagingUniversity College London
With many thanks for helpful slides to:John AshburnerMeike GrolGed Ridgway
Overview of SPM
Realignment Smoothing
Normalisation
General linear model
Statistical parametric map (SPM)Image time-series
Parameter estimates
Design matrix
Template
Kernel
Gaussian field theory
p <0.05
Statisticalinference
Functional MRI (fMRI)• Uses echo planar imaging (EPI) for
fast acquisition of T2*-weighted images.
• Spatial resolution:– 3 mm (standard 1.5 T scanner)– < 200 μm (high-field systems)
• Sampling speed:– 1 slice: 50-100 ms
• Requires spatial pre-processing and statistical analysis.
EPI(T2*)
T1
dropout
subjects
sessions
runs
single run
volume
slices
Terminology of fMRI
TR = repetition timetime required to scan one volume
voxel
Terminology of fMRI
Slice thicknesse.g., 3 mm
Scan Volume:Field of View
(FOV),e.g. 192 mm
Axial slices
3 mm
3 mm
3 mm
Voxel Size(volumetric pixel)
Matrix Sizee.g., 64 x 64
In-plane resolution192 mm / 64
= 3 mm
Standard space
The Talairach Atlas The MNI/ICBM AVG152 Template
Why does fMRI require spatial preprocessing?
• Head motion artefacts during scanning
• Problems of EPI acquisition: distortion and signal dropouts
• Brains are quite different across subjects
→ Realignment
→ “Unwarping”
→ Normalisation (“Warping”)→ Smoothing
Realignment or “motion correction”• Even small head movements can be a major problem:
– increase in residual variance– data may get completely lost if sudden movements
occur during a single volume– movements may be correlated with the task
performed• Therefore:
– always constrain the volunteer’s head– instruct him/her explicitly to remain as calm as
possible– do not scan for too long – everyone will move after
while !
minimising movements is one of the most important factors for ensuring good data quality!
Realignment = rigid-body registration
• Assumes that all movements are those of a rigid body, i.e. the shape of the brain does not change
• Two steps:Registration: optimising six parameters that
describe a rigid body transformation between the source and a reference image
Transformation: re-sampling according to the determined transformation
Linear (affine) transformations• Rigid-body transformations are a subset• Parallel lines remain parallel • Operations can be represented by:
x1 = m11x0 + m12y0 + m13z0 + m14
y1 = m21x0 + m22y0 + m23z0 + m24
z1 = m31x0 + m32y0 + m33z0 + m34
• Or as matrices:
1zyx
1000mmmmmmmmmmmm
1zyx
0
0
0
34333231
24232221
14131211
1
1
1
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 radiansx1 = cos() x0 + sin() y0 + 0y1 = -sin() x0 + cos() y0 + 0
• Zooms by sx and sy:x1 = sx x0 + 0 y0 + 0y1 = 0 x0 + sy y0 + 0
Shearx1 = 1 x0 + h y0 + 0y1 = 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
• Non-commutative: 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
Realignment
• Goal: minimise squared differences between source and reference image
• Other methods available (e.g. mutual information)
A special case ...
• If a subject remained perfectly still during a fMRI study, would realignment still be a good idea to perform?
• When could this issue be of practical relevance?
Coregistration
• also affine registration (like realignment)
• used to register a structural image to a (mean) functional one– allows more accurate anatomical localisation of activations– must be done before spatial normalisation (if warping
parameters are estimated from the T1 image)
• examples in SPM8 Manual, Chapters 28, 29
Practical demonstration: realignment & coregistration
Joint and marginal histograms
intensityfrequency
intensity
frequency
• Nearest neighbour– Take the value of the
closest voxel
• linear (2D: bilinear; 3D: trilinear)– Just a weighted average
of the neighbouring voxels– f5 = f1 x2 + f2 x1
– f6 = f3 x2 + f4 x1
– f7 = f5 y2 + f6 y1
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.
Why does fMRI require spatial preprocessing?
• Head motion artefacts during scanning
• Problems of EPI acquisition: distortion and signal dropouts
• Brains are quite different across subjects
→ Realignment
→ “Unwarping”
→ Normalisation (“Warping”)→ Smoothing
Residual errors after realignment
• Resampling can introduce interpolation errors• 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 included as confound regressors in subsequent statistical analyses.
Movement by distortion interactions
• Subject disrupts B0 field, rendering it inhomogeneous
→ distortions in phase-encode direction
• Subject moves during EPI time series
→ distortions vary with subject orientation→ shape of imaged brain
varies
Andersson et al. 2001, NeuroImage
Movement by distortion interaction
Movement by distortion interactions
after head rotationoriginal deformations
deformations after realignment mismatch in deformationsAndersson et al. 2001, NeuroImage
Different strategies for correcting movement artefacts
• liberal control:realignment only
• moderate control:realignment + “unwarping”
• strict control:realignment + inclusion of realignment parameters in statistical model
Why does fMRI require spatial preprocessing?
• Head motion artefacts during scanning
• Problems of EPI acquisition: distortion and signal dropouts
• Brains are quite different across subjects
→ Realignment
→ “Unwarping”
→ Normalisation (“Warping”)→ Smoothing
Individual brains differ in size, shape and folding
Spatial normalisation: why necessary?
• Inter-subject averaging– Increase sensitivity with more subjects
• Fixed-effects analysis– Extrapolate findings to the population as a
whole• Random / mixed-effects analysis
• Make results from different studies comparable by bringing them into a standard coordinate system– e.g. MNI space
Spatial normalisation: objective• Warp the images such that functionally
corresponding regions from different subjects are as close together as possible
• Problems:– Not always exact match between structure and
function– Different brains are organised differently– Computational problems (local minima, not enough
information in the images, computationally expensive)
• Compromise by correcting gross differences followed by smoothing of normalised images
Spatial normalisation: affine step
• The first part is a 12 parameter affine transform– 3 translations– 3 rotations– 3 zooms– 3 shears
• Fits overall shape and size
Spatial normalisation: non-linear step
Deformations consist of a linear combination of smooth basis functions.
These basis functions result from a 3D discrete cosine transform (DCT).
Spatial normalisation: Bayesian regularisation
Deformations consist of a linear combination of smooth basis functions set of frequencies from a 3D
discrete cosine transform.Find maximum a posteriori (MAP) estimates: simultaneously minimise
– squared difference between template and source image – squared difference between parameters and their priors
)(log)(log)|(log)|(log yppypyp MAP:
Deformation parameters
“Difference” between template and source image
Squared distance between parameters and their expected values
(regularisation)
Templateimage
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
Segmentation
GM and WM segmentations overlaid on original images
Structural image, GM and WM segments, and brain-mask (sum of GM and WM)
Segmentation & normalisation• Circular relationship between segmentation & normalisation:
– Knowing which tissue type a voxel belongs to helps normalisation.– Knowing where a voxel is (in standard space) helps segmentation.
• Build a joint generative model:– model how voxel intensities result from mixture of tissue type distributions– model how tissue types of one brain have to be spatially deformed to
match those of another brain
• Using a priori knowledge about the parameters: adopt Bayesian approach and maximise the posterior probability
Ashburner & Friston 2005, NeuroImage
Unified segmentation with tissue class priors
• Goal: for each voxel, compute probability that it belongs to a particular tissue type, given its intensity
• Likelihood model: Intensities are modelled by a mixture of Gaussian distributions representing different tissue classes (e.g. GM, WM, CSF).
• Priors are obtained from tissue probability maps (segmented images of 151 subjects).
Ashburner & Friston 2005, NeuroImage
p (tissue | intensity) p (intensity | tissue) ∙ p (tissue)
Normalisation options in practice
• Conventional normalisation:– either warp functional scans to EPI template directly– or coregister structural scan to functional scans and then
warp structural scan to T1 template; then apply these parameters to functional scans (“Normalise: Write”)
• Unified segmentation:– coregister structural scan to functional scans– unified segmentation provides normalisation parameters– apply these parameters to functional scans (“Normalise:
Write”)
Normalising structural image to T1 template
Estimate normalization parameters (T1 -> T1 template)
Apply the normalization parameters to the T1 image
And apply the normalization parameters to the (coregistered) functional images
T1 image
EPI time series
Normalising EPI images to EPI template
Calculate mean of EPI time series
Estimate normalization parameters (EPI -> EPI template)
EPI time seriesApply the normalization parameters to all EPI images
T1 image
Apply the normalization parameters to the (coregistered) T1 image
Smoothing• Why smooth?
– increase signal to noise– inter-subject averaging– increase validity of Gaussian Random Field theory
• In SPM, smoothing is a convolution with a Gaussian kernel.• Kernel defined in terms of FWHM (full width at half maximum).
Gaussian convolution is separable Gaussian smoothing kernel
Smoothing
Before convolution Convolved with a circleConvolved with a Gaussian
Smoothing is done by convolving with a 3D Gaussian which is defined by its full width at half maximum (FWHM).Each voxel after smoothing effectively becomes the result of applying a weighted region of interest.
Summary: spatial preprocessing steps
• Head motion artefacts during scanning
• Problems of EPI acquisition: distortion and signal dropouts
• Brains are quite different across subjects
→ Realignment
→ “Unwarping”
→ Normalisation (“Warping”)→ Smoothing
Thank you!
Supplementary slides
World coords(2, 4, -4) mm
(4, 4, -2)
(2, 2, -4)
Voxel index(45, 66, 35)
(44, 66, 36)
(45, 65, 35)
x 2 92y 2 -128z 2 -74
W V
W V
W V
xyz
World space & voxel space
7412892
zyx
200020002
zyx
V
V
V
W
W
W
1zyx
100074200
12802092002
1zyx
V
V
V
W
W
WBy moving to 4D, one can includetranslations within a single matrix multiplication
These 4-by-4 “homogeneous matrices” are the currency of voxel-world mappings, affine coreg. and realignment. In SPM5 & SPM8, they are stored in the .hdr files. In SPM2, they are stored in .mat files.
To find inverse mappings, or results of concatenating multiple transformations, we simply follow the rules of matrix algebra
Changing coordinate systems
74-2zz128-2y
922x
VW
VW
VW
yx
Representing rotations
0
0
1
1
yx
)θcos(θ)sin(θ)sin()θcos(
yx
0
0
0
1
1
1
zyx
1000)θcos()θsin(0)θsin()θcos(
zyx