Voxel-Based Morphometry with Unified Segmentation

Ged Ridgway

Centre for Medical Image ComputingUniversity College London

Thanks to:

John Ashburner and the FIL Methods Group.

Preprocessing in SPM

• Realignment – With non-linear unwarping for EPI fMRI

• Slice-time correction• Coregistration• Normalisation• Segmentation• Smoothing

SPM8b’s unified tissue segmentation and spatial normalisation procedure

But first, an introduction toComputational Neuroanatomy

Aims of computational neuroanatomy

• Many interesting and clinically important questions might relate to the shape or local size of regions of the brain

• For example, whether (and where) local patterns of brain morphometry help to:? Distinguish schizophrenics from healthy controls? Understand plasticity, e.g. when learning new skills? Explain the changes seen in development and aging? Differentiate degenerative disease from healthy aging? Evaluate subjects on drug treatments versus placebo

Alzheimer’s Disease example

Baseline ImageStandard clinical MRI1.5T T1 SPGR1x1x1.5mm voxels

Repeat image12 month follow-uprigidly registered

Subtraction image

SPM for group fMRIfMRI time-series

Preprocessing Stat. modellingspm TImage

Results query

fMRI time-series

Preprocessing Stat. modelling“Contrast”

ImageResults query

fMRI time-series

Preprocessing Stat. modelling“Contrast”

ImageResults query

Group-wisestatistics

“Contrast”Image

SPM for structural MRIHigh-res T1 MRI

Group-wisestatistics

?

?

?

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High-res T1 MRI

High-res T1 MRI

The need for tissue segmentation

• High-resolution MRI reveals fine structural detail in the brain, but not all of it reliable or interesting– Noise, intensity-inhomogeneity, vasculature, …

• MR Intensity is usually not quantitatively meaningful (in the same way that e.g. CT is)– fMRI time-series allow signal changes to be analysed

statistically, compared to baseline or global values

• Regional volumes of the three main tissue types: gray matter, white matter and CSF, are well-defined and potentially very interesting

Examples ofsegmentation

GM and WM segmentations overlaid on original images

Structural image, GM and WM segments, and brain-mask (sum of GM and WM)

Segmentation – basic approach

• Intensities are modelled by a Gaussian Mixture Model (AKA Mixture Of Gaussians)

• With a specified number of components• Parameterised by means, variances and mixing

proportions (prior probabilities for components)

Non-Gaussian Intensity Distributions

• Multiple MoG components per tissue class allow non-Gaussian distributions to be modelled– E.g. accounting for partial volume effects– Or possibility of deep GM differing from cortical GM

Tissue Probability Maps

• Tissue probability maps (TPMs) can be used to provide a spatially varying prior distribution, which is tuned by the mixing proportions– These TPMs come from the segmented images of

many subjects, done by the ICBM project

Class priors

• The probability of class k at voxel i, given weights γ is then:

• Where bij is the value of the jth TPM at voxel i.

K1j ijj

ikki b

b)|kc(P γ

Aligning the tissue probability maps

• Initially affine-registered using a multi-dimensional form of mutual information

• Iteratively warped to improve the fit of theunified segmentationmodel to the data– Familiar DCT-basis

function concept, asused in normalisation

MRI Bias Correction

• MR Images are corupted by smoothly varying intensity inhomogeneity caused by magnetic field imperfections and subject-field interactions– Would make intensity distribution spatially variable

• A smooth intensity correction can be modelled by a linear combination of DCT basis functions

Summary of the unified model

• SPM8b implements a generative model– Principled Bayesian probabilistic formulation

• Combines deformable tissue probability maps with Gaussian mixture model segmentation– The inverse of the transformation that aligns the

TPMs can be used to normalise the original image

• Bias correction is included within the model

Segmentation clean-up

• Results may contain some non-brain tissue (dura, scalp, etc.)

• This can be removedautomatically usingsimple morphologicalfiltering operations– Erosion– Conditional dilation

Lower segmentationshave been cleaned up

Limitations of the current model

• Assumes that the brain consists of only GM and WM, with some CSF around it.– No model for lesions (stroke, tumours, etc)

• Prior probability model is based on relatively young and healthy brains– Less appropriate for subjects outside this population

• Needs reasonable quality images to work with– No severe artefacts– Good separation of intensities– Good initial alignment with TPMs...

Extensions (possible or prototype)

• Multispectral modelling– (New Segment Toolbox)

• Deeper Bayesian philosophy– E.g. priors over means and variances– Marginalisation of nuisance variables– Model comparison

• Groupwise model (enormous!)• Combination with DARTEL (see later and new seg tbx)• More tissue priors e.g. deep grey, meninges, etc.• Imaging physics

– See Fischl et al. 2004, as cited in A&F introduction

}{,, skkkk σμ

Voxel-Based Morphometry

• In essence VBM is Statistical Parametric Mapping of segmented tissue density

• The exact interpretation of gray matter concentration or density is complicated, and depends on the preprocessing steps used– It is not interpretable as neuronal packing density or

other cytoarchitectonic tissue properties, though changes in these microscopic properties may lead to macro- or mesoscopic VBM-detectable differences

A brief history of VBM

• A Voxel-Based Method for the Statistical Analysis of Gray and White Matter Density… Wright, McGuire, Poline, Travere, Murrary, Frith, Frackowiak and Friston. NeuroImage 2(4), 1995 (!)– Rigid reorientation (by eye), semi-automatic scalp editing and

segmentation, 8mm smoothing, SPM statistics, global covars.

• Voxel-Based Morphometry – The Methods. Ashburner and Friston. NeuroImage 11(6 pt.1), 2000– Non-linear spatial normalisation, automatic segmentation– Thorough consideration of assumptions and confounds

A brief history of VBM

• A Voxel-Based Morphometric Study of Ageing… Good, Johnsrude, Ashburner, Henson and Friston. NeuroImage 14(1), 2001– Optimised GM-normalisation (“a half-baked procedure”),

modulation of segments with Jacobian determinants

• Unified Segmentation. Ashburner and Friston. NeuroImage 26(3), 2005– Principled generative model for segmentation using

deformable priors

• A Fast Diffeomorphic Image Registration Algorithm. Ashburner. Neuroimage 38(1), 2007– Large deformation normalisation to average shape templates

• …

VBM overview

• Unified segmentation and spatial normalisation• Optional modulation with Jacobian determinant• Optional computation of tissue totals/globals• Gaussian smoothing• Voxel-wise statistical analysis

VBM in pictures

Segment

Normalise

VBM in pictures

Segment

Normalise

Modulate (?)

Smooth

VBM in pictures

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Normalise

Modulate (?)

Smooth

Voxel-wise statistics

Segment

Normalise

Modulate (?)

Smooth

Voxel-wise statistics

VBM in pictures

VBM Subtleties

• Whether to modulate• Adjusting for total GM or Intracranial Volume• How much to smooth• Limitations of linear correlation• Statistical validity

Modulation

• Multiplication of the warped (normalised) tissue intensities so that their regional or global volume is preserved– Can detect differences in

completely registered areas

• Otherwise, we preserve concentrations, and are detecting mesoscopic effects that remain after approximate registration has removed the macroscopic effects– Flexible (not necessarily

“perfect”) registration may not leave any such differences

Native

intensity = tissue density

Modulated

Unmodulated

“Globals” for VBM

• Shape is really a multivariate concept– Dependencies among

volumes in different regions

• SPM is mass univariate– Combining voxel-wise

information with “global” integrated tissue volume provides a compromise

– Using either ANCOVA or proportional scaling

Above: (ii) is globally thicker, but locally thinner than (i) – either of these effects may be of interest to us.

Figures from: Voxel-based morphometry of the human brain… Mechelli, Price, Friston and Ashburner. Current Medical Imaging Reviews 1(2), 2005.

Below: The two “cortices” on the right both have equal volume…

Total Intracranial Volume (TIV/ICV)

• “Global” integrated tissue volume may be correlated with interesting regional effects– Correcting for globals in this case may overly reduce

sensitivity to local differences– Total intracranial volume integrates GM, WM and

CSF, or attempts to measure the skull-volume directly• Not sensitive to global reduction of GM+WM

(cancelled out by CSF expansion – skull is fixed!)– Correcting for TIV in VBM statistics may give more

powerful and/or more interpretable results

Smoothing

• The analysis will be most sensitive to effects that match the shape and size of the kernel

• The data will be more Gaussian and closer to a continuous random field for larger kernels

• Results will be rough and noise-like if too little smoothing is used

• Too much will lead to distributed, indistinct blobs

Smoothing

• Between 7 and 14mm is probably best– (lower is okay with better registration, e.g. DARTEL)

• The results below show two fairly extreme choices, 5mm on the left, and 16mm, right

Nonlinearity

Circles of uniformly increasing area.

SmoothedPlot of intensity at circle

centres versus area

Caution may be needed when looking for linear relationships between grey matter concentrations and some covariate of interest.

VBM’s statistical validity

• Residuals are not normally distributed– Little impact on uncorrected statistics for experiments

comparing reasonably sized groups– Probably invalid for experiments that compare single

subjects or tiny groups with a larger control group• Need to use nonparametric tests that make less

assumptions, e.g. permutation testing with SnPM

VBM’s statistical validity

• Correction for multiple comparisons– RFT correction based on peak heights should be OK

• Correction using cluster extents is problematic– SPM usually assumes that the smoothness of the

residuals is spatially stationary• VBM residuals have spatially varying smoothness• Bigger blobs expected in smoother regions

– Toolboxes are now available for non-stationary cluster-based correction

• http://www.fmri.wfubmc.edu/cms/NS-General

VBM’s statistical validity

• False discovery rate– Less conservative than FWE– Popular in morphometric work

• (almost universal for cortical thickness in FS)– Recently questioned…

• Topological FDR in SPM8– See release notes for details and paper

Variations on VBM

• “All modulation, no gray matter”– Jacobian determinant “Tensor” Based Morphometry– Davatzikos et al. (1996) JCAT 20:88-97

• Deformation field morphometry– Cao and Worsley (1999) Ann Stat 27:925-942– Ashburner et al (1998) Hum Brain Mapp 6:348-357

• Other variations on TBM– Chung et al (2001) NeuroImage 14:595-606

Deformation and shape change

Figures from Ashburner and Friston, “Morphometry”, Ch.6of Human Brain Function, 2nd Edition, Academic Press

TemplateTemplateWarpedOriginal

Jacobian Matrix

Deformation fields and Jacobians

Deformation

vector field

Determinant of

Jacobian Matrix

encodes voxel’s

volume change

Longitudinal VBM

• Intra-subject registration over time much more accurate than inter-subject normalisation

• Imprecise inter-subject normalisation– Spatial smoothing required

• Different methods have been developed to reduce the danger of expansion and contraction cancelling out…

Longitudinal VBM variations

• Voxel Compression mapping separates expansion and contraction before smoothing– Scahill et al (2002) PNAS 99:4703-4707

• Longitudinal VBM multiplies longitudinal volume change with baseline or average grey matter density– Chételat et al (2005) NeuroImage 27:934-946

Late CSF - Early CSF Late CSF - modulated CSF

Smoothed

Late Early

Warped early Difference

Early CSFLate CSF

Relative volumesCSF “modulated”

by relative volume

Longitudinal VBM variations

Nonrigid registration developments

• Large deformation concept– Regularise velocity not displacement

• (syrup instead of elastic)

• Leads to concept of geodesic– Provides a metric for distance between shapes– Geodesic or Riemannian average = mean shape

• If velocity assumed constant computation is fast– Ashburner (2007) NeuroImage 38:95-113– DARTEL toolbox in SPM8b

• Currently initialised from unified seg_sn.mat files

DARTEL exponentiates a velocity flow field to get a deformation field

Velocity flow field

Example geodesic shape average

Linear Average

Average on Riemannian manifold

(Not on Riemannian manifold)

DARTEL averagetemplate evolution

Grey matter average of 452 subjects – affine

471 subjects– DARTEL

Iterations

Questioning Intersubject normalisation

• Registration algorithms might find very different correspondences to human experts– Crum et al. (2003) NeuroImage 20:1425-1437

• Higher dimensional warping improves image similarity but not necessarily landmark correspondence– Hellier et al. (2003) IEEE TMI 22:1120-1130

Questioning Intersubject normalisation

• Subjects can have fundamentally different sulcal/gyral morphological variants– Caulo et al. (2007) Am. J. Neuroradiol. 28:1480-85

• Sulcal landmarks don’t always match underlying cytoarchitectonics– Amunts, et al. (2007) NeuroImage 37(4):1061-5

Intersubject normalisation opportunities

• High-field high-resolution MR may have potential to image cytoarchitecture

• Will registration be better or worse at higher resolution?– More information to use– More severe

discrepancies?– Need rougher

deformations– Non-diffeomorphic?

De Vita et al (2003) Br J Radiol 76:631-7

4.7T FSE

Intersubject normalisation opportunities

• Regions of interest for fMRI can be defined from functional localisers or orthogonal SPM contrasts– No obvious equivalent for single-subject structural MR

• Potential to include diffusion-weighted MRI information in registration ?– Zhang et al. (2006) Med. Image Analysis 10:764-785

Summary of key points

• VBM performs voxel-wise statistical analysis on smoothed (modulated) normalised segments

• SPM8b performs segmentation and spatial normalisation in a unified generative model

• Intersubject correspondence is imperfect– Smoothing alleviates this problem to some extent

• Also improves statistical validity

• Some current research is focussed on more sophisticated registration models

Unified segmentation in detail

An alternative explanation to the paper and to John’s slides from London ‘07

http://www.fil.ion.ucl.ac.uk/spm/course/slides07/Image_registration.ppt

Unified segmentation from the GMM upwards…The standard Gaussian mixture model

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Unified segmentation from the GMM upwards…Anatomical priors through mixing coefficients

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Unified segmentation from the GMM upwards…Aside: MRF Priors (A&F, Gaser’s VBM5 toolbox)

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Optimisationis tricky…

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Unified segmentation from the GMM upwards…Spatially deformable priors (inverse of normalisation)

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Unified segmentation from the GMM upwards…Spatially deformable priors (inverse of normalisation)

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Unified segmentation from the GMM upwards…Objective function with regularisation

Unified segmentation from the GMM upwards…Optimisation approach

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Iterated Conditional Modes is used – this alternately optimises certain sets of parameters, while keeping the rest fixed at their current best solution

is very difficult…

• EM used for mixture parameters• Levenberg Marquardt (LM) used for bias and

warping parameters– Note unified segmentation model with Gaussian

assumptions has a “least-squares like” log(objective) making it ideal for Gauss-Newton or LM optimisation

• Local opt, so starting estimates must be good– May need to manually reorient troublesome scans

Unified segmentation from the GMM upwards…Optimisation approach

Unified segmentation from the GMM upwards…Optimisation approach

Figure from C. Gaser

• Repeat until convergence…– Hold γ, μ, σ2 and α constant, and

minimise E w.r.t. b• Levenberg-Marquardt strategy,

using dE/dβ and d2E/dβ2

– Hold γ, μ, σ2 and β constant, and minimise E w.r.t. α

• Levenberg-Marquardt strategy, using dE/dα and d2E/dα2

– Hold α and β constant, and minimise E w.r.t. γ, μ and σ2

• Expectation Maximisation

Note ICM steps

Results of the Generative model

Key flaw, lack of neighbourhood correlation – “whiteness” of noise

Motivates (H)MRF priors, which should encourage contiguous tissue classes

(Note, MRF prior is not equivalent to smoothing each resultant tissue segment, but differences in eventual SPMs may be minor…)