Medical Image Analysis

Transcription

1 Medical Image Analysis Instructor: Moo K. Chung Lecture 3. Deformation-based Morphometry (DBM) January 30, 2007

2 Deformation based Morphometry (DBM) It uses deformation fields obtained by nonlinear registration of brain images. Automated image registration (AIR) package ( uses polynomial basis. SPM pakage uses cosine basis.

3 Image Registration Process of transforming one image to another. Transformation types: linear (rigidbody), affine (non rigid-body), nonlinear. Type of data obtained: 3D vector fields (displacement, deformation).

4 Affine Transformation matrix for (rigid, non rigid) R: 3 x 3 matrix of rotation, scaling and shear p =Rp+c, where p'= # % % $ x " y " z " & (, p = ( ' # x& % y % $ z' (,c = ( # % % $ c x c y c z & ( ( '

5 Linear transform T is a linear transform if T(ap+bq) = at(p)+bt(q) for all numbers a, b. Checking if the affine transform is linear. Let T(p)=Rp+c. Note T(ap+bq)=R(ap+bq)+c=aT(p)+bT(q)+c(1-a-b). This shows the affine transform is nonlinear.

6 Rigid-body: Rotation and Translation only Original Image Shape doesn t change Source: A. Chowdhury, University of Georgia

7 Non rigid-body Scaling, Translation, Rotation, Reflection, Shear Original Image Non rigid-body

8 Nonlinear transform Anatomical variability is encoded in the nonlinear transform. Original image Target Image Result of warping

9 Piecewise Affine The Talairach normalization approach It uses a different matrix transformation for each of the 12 pieces of the Talairach Grid This is a really old technique based on an old subject. It provides an easy reference for comparing different study results. Other than for comparing results, it should not be used.

10 Talairach Definition Interhemispheric plane (3+ landmarks) 2 rotations and 1 translation Anterior and posterior commissure (AC, PC) 3rd rotation, 2 translations Scale to anterior, posterior, left, right, inferior, superior landmarks (7 parameters) Each cerebral hemispheres divided into six associated blocks (interhemispheric plane, AC-PC axial plane, 2 coronal planes through AC and PC.

11 Talairach Template

12 Affine vs. Nonlinear Affine 12 parameters Non-Rigid ~ 2000 parameters Mathematical details of nonlinear registration will be studied later. Source:Lawrence H. Staib Yale University

13 Deformation field The deformation field d is a transformation from a subject image to a target image: Example: AIR 3rd degree warping! 2! 5 2! 5! 5 2! 6 x ' =! 1.24! 0.47x! 1.02 " 10 y z " 10 x! 2.66 " 10 xy " 10 y " 10 xz " 10 yz " 10 z " 10 x " 10 x y! 9.3" 10 xy " 10 y! 5! 5 2! 8 3! 7 2! 10 2! 7 3! " + "! "! "! 8 2! 7! 7 2! 7 2! 8 2! x z xyz y z xz! " yz! " z

14 Visualizing Deformation Red: tissue expansion Blue: tissue shrinking Yellow: deformation change

15 Deformation field visualization SPM result

16 Displacement vector fields The vector difference between the final position and the original position. Relation between displacement and deformation

17 Visualizing Displacement Vector Field

18 Visualizing Displacement Vector Field Displacement is easier to model statistically than deformation

19 Real Example: modeling tissue growth Data: 28 normal subjects Images: 1.5T MRI, 2 scans per subjects First scan: 11.5±3.1 year Second scan: 17.8 ±3.2 year Problem: localize the regions of anatomical change over time.

20 Modeling on the rate of displacement change Why? Easier than modeling on deformation itself. : Covariance matrix : Gaussian random vector field

21 Estimating the rate of change For subject j, displacement is given by Finite difference:

22 Registration procedures in Chung et al. (2001) Is not optimal. How it should be done correctly? To reduce registration error, 1st scans to be registered to the 2nd scans.

23 Hotelling s T-square statistic Most efficient way of computing it in MATLAB Rate of displacement V j Sample mean Sample covariance V = n " V j j=1 " ˆ = 1 n #1 Hotelling s T-square (related to Mahalanobis distance) n $ (V j #V )( V j #V ) T j=1 H = V T " ˆ #1 V $ cf 3,n#3

24 Multiple comparison correction via random field theory We will study this topic later. Corrected P-value computation fmri-stat package by Keith Worsley has the MATLAB m code.

25 3D SPM Back These 3D blobs of signal are meaningless without superimposing them onto MRI. Right Corpus Callosum Front Tissue growth p < : Tissue loss p < ow: Structure displacement p < 0.05

26 Red: Tissue growth Blue: Tissue loss Yellow: Structure displacement

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28 3D SPM Back Right Corpus Callosum Front Red: Tissue growth p < Blue: Tissue loss p < Yellow: Structure displacement p < 0.05

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30 Hotelling s T-square for two sample test Group index j, subject index i V ij = µ j + " 1/ 2 e ij H 0 :µ 1 =µ 2 vs. H 1 :µ 1 "µ 2

31 Sample means:, V 1 V 2 Sample covariance: pool the variance across groups ˆ " = 1 n + m # 2 % n m ( ' $ (V i1 #V 1 )( V i1 #V 1 ) T + $ (V i2 #V 2 )( V i2 #V 2 ) T * & ) i=1 i=1 Hotelling s T-square statistic for two samples H = ( V 2 "V ) T ˆ 1 # "1 ( V 2 "V ) 1 $ cf 3,n +m"4

32 Lecture 4 topics Tensor based Morphometry (DBM) and Jacobian determinant