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Intensity-Based Registration
CAMP II – IN2022
Christian Wachinger, Nassir Navab
Computer Aided Medical Procedures (CAMP)
Technische Universität München, Germany
Intensity-Based Registration - C. Wachinger, N. Navab 2
Agenda
Introduction
Classification
Dimensionality / Modalities
Transformation
Registration Basis
Intensity-Based Registration
Method
Similarity Measures
Interpolation, Multi-Resolution
Intensity-Based Registration - C. Wachinger, N. Navab 3
Motivation
• Today, medical images are to a large extent digital
• Different images contain complementary information
• To use this information => Registration
Intensity-Based Registration - C. Wachinger, N. Navab 4
Registration: Definition
Bring two images into spatial alignment, or establish a common geometric reference frame.
Intensity-Based Registration - C. Wachinger, N. Navab 5Source: Xenios Papademitris
Intensity-Based Registration - C. Wachinger, N. Navab 6
Medical Imaging Modalities
MRICT PET 3D-US X-Ray US Video
2D3D
Registration
Visualization of fused data
Measurement of changes Computer aided diagnosis / therapy
Intraoperative Navigation …
Intensity-Based Registration - C. Wachinger, N. Navab 7
Agenda
Introduction
Classification
Dimensionality / Modalities
Transformation
Registration Basis
Intensity-Based Registration
Method
Similarity Measures
Interpolation, Multi-Resolution
Intensity-Based Registration - C. Wachinger, N. Navab 8
Classification Scheme
Dimensionality:
Registration Basis:
Transformation:
Modalities:
Subject/Object:
3D-3D, 2D-3D, 3D-4D, …
Extrinsic (e.g. markers) vs. Intrinsic
Rigid, affine, projective, deformable
mono-modal / multi-modal
Intra- vs. Intersubject, which anatomy
Number of Images: Pair-wise, Group-wise
Intensity-Based Registration - C. Wachinger, N. Navab 9
Dimensionality: 2D-2D / 3D-3D
• Digital Subtraction Angiography:X-Ray or CT scan image is subtracted from a second image with contrast agent >> highlighting of blood vessels
3D CT DSA:blood vessels
in the brain
2D X-Ray DSA
• Even if the same imaging device is acquiring the two images, registration is necessary in order to compensate for small shift of the patient position and movement of organs.
Intensity-Based Registration - C. Wachinger, N. Navab 10
Dimensionality / Modalities: 3D-3D
PET-MRMR-CT
CT Scan
MR Scan
Intensity-Based Registration - C. Wachinger, N. Navab 11
Dimensionality / Modalities: 2D-3D
MR Scan
C-Arm Fluoroscopy
Registration →intraop. Navigation
Intensity-Based Registration - C. Wachinger, N. Navab 12
Dimensionality / Modalities: 2D-3D
CT Scan
Linear Accelerator
Registration →Patient Positioning
Intensity-Based Registration - C. Wachinger, N. Navab 13
Dimensionality / Modality : 2D – 3D
CT Scan
Ultrasound
Registration →intraop. Navigation
Intensity-Based Registration - C. Wachinger, N. Navab 14
US segmentation with registration to segmented CT
Ultrasound CT US-Segmentation
Intensity-Based Registration - C. Wachinger, N. Navab 15
Registration Transformation
Image Y
T(Y)
Rigid Affine Projective Deformable
⎥⎦
⎤⎢⎣
⎡vTvtA
⎥⎦
⎤⎢⎣
⎡10tA
T⎥⎦
⎤⎢⎣
⎡10tR
T ?H4x4
Intensity-Based Registration - C. Wachinger, N. Navab 16
Registration Basis - Extrinsic
Stereotactic Frame>> invasive method!
Markerse.g. invasive bone fiducials,noninvasive skin markers
Results in 3D point sets available for registration
Intensity-Based Registration - C. Wachinger, N. Navab 17
Registration Basis - Intrinsic
anatomical landmarks>> 3D point sets to register
segmented objects>> 3D surfaces to register
full image content>> pixel/voxel intensities
Intensity-Based Registration - C. Wachinger, N. Navab 18
Number of Images
• Pair-wise– Aligning two images
• Group-wise– Mosaicing
– Population studies, Atlas construction
Intensity-Based Registration - C. Wachinger, N. Navab 19
Number of Images
• Pair-wise– Aligning two images
• Group-wise– Mosaicing
– Population studies, Atlas construction
Source: S. Joshi
Before Alignment
After Alignment
Source: L. Zöllei
Atlas
Atlas
Intensity-Based Registration - C. Wachinger, N. Navab 20
Agenda
Introduction
Classification
Dimensionality / Modalities
Transformation
Registration Basis
Intensity-Based Registration
Method
Similarity Measures
Interpolation, Multi-Resolution
Intensity-Based Registration - C. Wachinger, N. Navab 21
Intensity- vs. Feature-Based Registration
• Next class: Registration based on Features
• This class: Registration based on image intensities
Corresponding Points Point Cloud ↔ Shape
Intensity-Based Registration - C. Wachinger, N. Navab 22
Intensity-Based 3D-3D Registration
X Y X,T(Y)• Define Transformation T on one of the images
• Compare X, T(Y) using the full image content• Refine T and repeat, until convergence criteria reached.
MRI fMRI
Intensity-Based Registration - C. Wachinger, N. Navab 23
Intensity-Based 3D-3D Registration
Similarity MeasurementTransformation T
Volume Y Volume X
T(Y)
Initial T
Intensity-Based Registration - C. Wachinger, N. Navab 24
Intensity-Based 3D-3D Registration
Interpolation
Similarity Measurement
Optimization
Transformation T
Volume Y Volume X
T(Y)
Value
Iterative Execution
Initial T
Intensity-Based Registration - C. Wachinger, N. Navab 25
Source: Xenios Papademitris
Interpolation
Similarity Measurement
Optimization
Transformation T
Volume Y Volume X
T(Y)
Value
Iterative Execution
Initial T
Intensity-Based Registration - C. Wachinger, N. Navab 26
Intensity-Based 3D-3D Registration
• In each iteration, compute a similarity measure sim(X, T(Y)) using the full image content of X and Y, i.e. all voxels!
• Using T(Y) requires voxel interpolation. Consideration of speed / quality!
• Maximize sim(X, T(Y)) by varying transformation→ optimization algorithm on transformation parameters!
• 3D Rigid transformation has 6 DOF
– 3 Translation Tt
– 3 Rotation Tr
Interpolation
Similarity Measurement
Optimization
Transformation T
Volume Y Volume X
T(Y)
Value
Iterative Execution
Initial T
How to represent the rotation?
T = Tt * Tr
T̂ = argmaxTsim(X,T(Y ))
Intensity-Based Registration - C. Wachinger, N. Navab 27
Representations of 3D Rotations
• Rotation group SO(3) (special orthogonal group)
• 3 degrees of freedom
• Rotation matrix
• Axis angle
• Quaternion
• Euler angles
• …
SO(3) = { Q ∈ R3×3 : QQ> = I, det(Q) = 1}
Source: www.euclideanspace.com
Tr =
⎡⎣cos γ − sin γ 0sin γ cos γ 00 0 1
⎤⎦⎡⎣1 0 00 cosβ − sinβ0 sinβ cosβ
⎤⎦⎡⎣cosα − sinα 0sinα cosα 00 0 1
⎤⎦
Intensity-Based Registration - C. Wachinger, N. Navab 28
Intensity-Based 3D-3D Registration
• In each iteration, compute a similarity measure sim(X,T(Y)) using the full image content of X and Y, i.e. all voxels!
• Using T(Y) requires voxel interpolation. Consideration of speed / quality!
• Minimize sim(X, T(Y)) by varying transformation→ optimization algorithm on transformation parameters!
Intensity-Based Registration - C. Wachinger, N. Navab 29
Similarity Measures: SSD / SAD
Volume X Volume T(Y)
∑ −=i
ii yxN
SSD 2)(1
Sum of Squared Differences:Simple measure, can be optimized
with special algorithms
∑ −=i
ii yxN
SAD 1
Sum of Absolute Differences:Less sensitive on large intensity
differences than SSD
→ Only for intra-modal Registration, e.g. CT-CT
Volume X Volume T(Y)
Intensity-Based Registration - C. Wachinger, N. Navab 30
Similarity Measures: SSD / SAD
• Images slightly tilted:
• However: We encounter problems already if the images have different contrast or window/level values
- =
- =
Intensity-Based Registration - C. Wachinger, N. Navab 31
Correlation Coefficient (Normalized Cross Correlation)
Volume X Volume T(Y)
∑ −−=i
iiyx
yyxxNCC ))((1σσ
Normalized Cross Correlation:Expresses the linear relationship between voxel
intensities in the two volumes
→ Already looser dependence on intensities!
Intensity-Based Registration - C. Wachinger, N. Navab 32
Correlation Coefficient – Example
• Two random variables X and Y
• What is their Correlation Coefficient CC(X,Y) ?
• X uniformly distributed in [-1;1], Y = X2, CC(X,Y) ?
X 3 5 7 9 11Y 11 19 27 35 43
Elements
X -1 -0,9 -0,8 -0,7 -0,6 -0,5 -0,4 -0,3 -0,2 -0,1 0Y 1 0,81 0,64 0,49 0,36 0,25 0,16 0,09 0,04 0,01 0
Elements
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 10,01 0,04 0,09 0,16 0,25 0,36 0,49 0,64 0,81 1
…
Intensity-Based Registration - C. Wachinger, N. Navab 33
Correlation Coefficient - Example
• Four datasets, two variables X,Y, NCC(X,Y) = 0.81 for each.
Source: wikipedia
Intensity-Based Registration - C. Wachinger, N. Navab 34
Correlation Coefficient - Example
• Point sets (x,y), with correlation coefficient of x and y.
Source: wikipedia
Intensity-Based Registration - C. Wachinger, N. Navab 35
Multi-modal 3D-3D Registration
No simple relationship between X and Y intensities anymore:
?
CT MR
Two approaches:
- Simulate one image from the other → reduce it to the mono-modal case
- Use more sophisticated similarity measures
Intensity-Based Registration - C. Wachinger, N. Navab 36
• How to measure the relationship between X and Y ?
• Measure the structure of the joint distribution
• How to measure the structure?
• Shannon Entropy
• Optimum at a highly structured (clustered) histogram
Information Theoretic Approach
X
Y
Intensity-Based Registration - C. Wachinger, N. Navab 37
Shannon Entropy, developed in the 1940s(communication theory)
∑−=i
ii ppH log
i
pi
i
piuniform distribution→ maximum entropy
any other distribution→ less entropy
Information Theoretic Approach - Entropy
Intensity-Based Registration - C. Wachinger, N. Navab 38
• Images X and Y are treated as random variables
• px(i) = probability of a pixel of X having the intensity value i
• px(i) and py(i) can be estimated from histograms
i=0…255
px(i)
i=0…255
py(i)
Information Theoretic Approach - Entropy
Intensity-Based Registration - C. Wachinger, N. Navab 39
Joint Entropy
• At each voxel location, we have a pair of intensity values, representing the combined image.
∑∑−=i j
xyxy jipjipYXH ),(log),(),(
Intensity-Based Registration - C. Wachinger, N. Navab 40
Joint Entropy
px(i)
py(i)
pxy(i,j)
px(i)
py(i)
pxy(i,j)
X and Y identical X and Y misaligned
Intensity-Based Registration - C. Wachinger, N. Navab 41
Joint Histogram
Intensity of Reference X
Intensity of TransformedTarget Y
SSD OptimumY = X
NCC OptimumY = a*X + b
Intensity-Based Registration - C. Wachinger, N. Navab 42
Joint Entropy
• Histogram for images from different Modalities
Joint HistogramTarget ImageSource Image
Not Aligned
Aligned
Source: PhD Thesis, L. Zöllei [7]
Intensity-Based Registration - C. Wachinger, N. Navab 43
Mutual Information
• Maximized if X and Y are perfectly aligned
• Any permutation on the intensity values of X or Y does not affect the measure → great for multimodal registration!
• H(X) and H(Y) help to make the measure more robust
∑∑=
=−+=
i j yx
xyxy jpip
jipjip
YXHYHXHYXMI
)()(),(
log),(
),()()(),(
Intensity-Based Registration - C. Wachinger, N. Navab 44
Agenda
Introduction
Classification
Dimensionality / Modalities
Transformation
Registration Basis
Intensity-Based Registration
Method
Similarity Measures
Interpolation, Multi-Resolution
Intensity-Based Registration - C. Wachinger, N. Navab 45
Image Grid
Origin
Spacing in y
Spacing in x
An Image is sampling of a continuous field using a discrete grid
Source: ITK registration methods
Intensity-Based Registration - C. Wachinger, N. Navab 46
Image Grid
Pixel Value
Pixel Region
[0,0] [1,0] [2,0]
[0,1]
[0,2]
[0,3]
Partial Volume Effect: a single voxel contains a mixture of multiple tissue values
Source: ITK registration methods
Intensity-Based Registration - C. Wachinger, N. Navab 47
Fixed Image Grid
j
i
y
xFixed Image
Physical Coordinates
y’
x’Moving Image
Physical Coordinates
Moving Image Grid
j
i
Space Transform
Source: ITK registration methods
Intensity-Based Registration - C. Wachinger, N. Navab 48
Things I will not do …
I will not register images in pixel spaceI will not register images in pixel spaceI will not register images in pixel spaceI will not register images in pixel spaceI will not register images in pixel spaceI will not register images in pix
Source: ITK registration methods
Intensity-Based Registration - C. Wachinger, N. Navab 49
Quiz
256 x 256 pixels
MRI-T2
128 x 128 pixels
PET
Scaling Transform
What scale factor ?a) 2.0b) 1.0c) 0.5
Source: ITK registration methods
Intensity-Based Registration - C. Wachinger, N. Navab 50
Things I will not do …
I will not register images in pixel spaceI will not register images in pixel spaceI will not register images in pixel spaceI will not register images in pixel spaceI will not register images in pixel spaceI will not register images in pix
Source: ITK registration methods
Intensity-Based Registration - C. Wachinger, N. Navab 51
Interpolation Methods
• Essential Problem: Transforming an Image while keeping the rasterization grid
→ →
I1 T(I1) I2=T(I1)
Intensity-Based Registration - C. Wachinger, N. Navab 52
Interpolation – Forward Warping
I2(u’,v’) I2(u’+1,v’)
I2(u’,v’+1) I2(u’+1,v’+1)
→ Holes possible
Intensity-Based Registration - C. Wachinger, N. Navab 53
Interpolation – Backward Warping
⎟⎟⎠
⎞⎜⎜⎝
⎛′′
=⎟⎟⎠
⎞⎜⎜⎝
⎛ −
vu
Tyx 1
))(),((),( 12 yroundxroundIvuI =′′
I2(u’,v’)
I1(u,v)
I1(u+1,v)
I1(u+1,v+1)
I1(u,v+1)
(x,y) Nearest Neighbor Interpolation:
Bilinear Interpolation:
)1)(1)(1,1()1)()(1,())(1)(,1(
))()(,(),(
1
1
1
12
yvxuvuIyvuxvuIvyxuvuI
vyuxvuIvuI
−+−++++−+−++−−++
+−−=′′
Intensity-Based Registration - C. Wachinger, N. Navab 54
Interpolation by Convolution
• Expression as convolution with a filter kernel h
Nearest:
Bilinear:
-2 -1 0 1 2-0.2
0
0.2
0.4
0.6
0.8
1 nearestlinearcubic
Bicubic:
Intensity-Based Registration - C. Wachinger, N. Navab 55
Interpolation in 3D
• Extension to 3D: Trilinear Interpolation
Vxyz = V111 · (x− 1)(y − 1)(z − 1)+V110 · (x− 1)(y − 1)z+V101 · (x− 1)y(z − 1)+V011 · x(y − 1)(z − 1)+
V100 · (x− 1)yz+V010 · x(y − 1)z+V001 · xy(z − 1)+
V000 · xyz
Intensity-Based Registration - C. Wachinger, N. Navab 56
Multi-Resolution (multi-scale) Approach
Registration
Registration
Registration
Fixed Image Moving Image
Source: ITK registration methods
Intensity-Based Registration - C. Wachinger, N. Navab 57
Student Project : Simultaneous Deformation
• Combination of registration of
– multiple images: simultaneously
– deformable registration
Intensity-Based Registration - C. Wachinger, N. Navab 58
The end …
Thank you for your attention!