This presentation in the conference medprai'16 ( the 1st Mediterranean Conference on Pattern Recognition and Artificial Intelligence) in Tebessa, Algeria on November 22-23, 2016.

1. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
1th Mediterranean Conference on Pattern Recognition and Artificial Intelligence
EL-Hachemi Samy Dominique Ramdane
Guerrout Ait-Aoudia Michelucci Mahiou
Hidden Markov Random Field model and BFGS
algorithm for Brain Image Segmentation
LMCS Laboratory, ESI, Algeria & LE2I Laboratory, UB, France
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2. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
1 Introduction
2 Hidden Markov Random Field
3 BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm
4 Experimental Results
5 Conclusion & Perspective
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3. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
Problematic & Solution
1 Nowadays, We face a huge number of medical images
2 Manual analysis and interpretation became a tedious task
3 Automatic image analysis and interpretation is a necessity
4 To simplify the representation of an image into items meaningful
and easier to analyze, we need a segmentation method
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4. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
A segmentation methods
Segmentation methods can be classified in four main categories :
1 Threshold-based methods
2 Region-based methods
3 Model-based methods
4 Classification methods
1 HMRF - Hidden Markov Random Field
2 etc
We have chosen HMRF as a model to perform segmentation
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5. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
Hidden Markov Random Field
1 HMRF provides an elegant way to model the segmentation
problem
2 HMRF is a generalization of Hidden Markov Model
3 Each pixel is seen as a realization of Markov random variable
4 Each image is seen as a realization of set or family of Markov
random variables
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6. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
Hidden Markov Random Field
The image to segment y = {ys}s∈S
into K classes is a realization of Y
1 Y = {Ys}s∈S is a family of
random variables
2 ys ∈ [0...255]
The segmented image into K classes
x = {xs}s∈S is realization of X
1 X = {Xs}s∈S is a family of
random variables
2 xs ∈ {1,...,K}
An example of segmentation into
K = 4 classes
The goal of HMRF is looking for x∗
x∗ = argx∈Ω max {P[X = x | Y = y]}
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7. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
Hidden Markov Random Field
1 This elegant model leads to the optimization of an energy function
Ψ(x,y) = ∑s∈S ln(σxs
)+
(ys−µxs )2
2σ2
xs
+ β
T ∑c2={s,t} (1 −2δ(xs,xt ))
2 Our way to look for the minimization of Ψ(x,y) is to look for the
minimization Ψ(µ), µ = (µ1,...,µK ) where µi are means of gray
values of class i
3 The main idea is to focus on the means adjustment instead of
treating pixels adjustment
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8. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
Hidden Markov Random Field
1 Now, we seek for u∗



µ∗ = argµ∈[0...255]K min{Ψ(µ)}
Ψ(µ) = ∑K
j=1 f(µj )
f(µj ) = ∑
s∈Sj
[ln(σj )+
(ys−µj )2
2σ2
j
]+ β
T ∑
c2={s,t}
(1 −2δ(xs,xt ))
2 To apply optimization techniques, we redefine the function Ψ(µ)
for µ ∈ RK
instead µ ∈ [0...255]K
. For that, we distinguish two
forms Ψ1
(µ) and Ψ2
(µ).
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9. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
Hidden Markov Random Field
Form 1
Ψ1
(µ) =



Ψ(µ) if µ ∈ [0...255]K
+∞ otherwise
Ψ1
treats all points outside
[0...255] in the same way
Form 2



Ψ2
(µ) = ∑K
j=1 F(µj ) where µj ∈ R
F(µj ) =



f(0)−uj if µj < 0
f(µj ) if µj ∈ [0...255]
f(255)+(uj −255) if µj > 255
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10. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm
1 BFGS is one of the most powerful methods to solve
unconstrained optimization problem
2 BFGS is the most popular quasi-Newton method
3 BFGS is based on the gradient descent to reach the local
minimum
4 Main idea of descent gradient is :
1 We start from the initial point µ0
2 At the iteration k +1, the point µk+1
is calculated from the point µk
according to the following formula : µk+1
= µk
+αk dk
- αk is the step size at the iteration k
- dk the search direction at the iteration k
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11. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm
Summary of BFGS algorithm
1 Initialization : Set k := 0,
Choose µ0
close to the solution
Set H0 := I, Set α0 = 1
Choose the required accuracy ε ∈ R,ε>0
2 At the iteration k :
Compute Hessian matrix approximation Hk
Compute the inverse of Hessian matrix
Compute the search direction dk
Compute the step size αk
Compute the point µk+1
3 The stopping criterion : If Ψ (µk
) <ε then ˆµ := µk
4 k := k +1
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12. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
DC - The Dice Coefficient
The Dice coefficient measures how much
the segmentation result is close to the
ground truth
DC =
2|A ∩B|
|A ∪B|
1 DC equals 1 in the best case
(perfect segmentation)
2 DC equals 0 in the worst case
(every pixel is misclassified)
FIGURE – The Dice Coefficient
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13. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
BFGS in practice
1 We used the Gnu Scientific Library implementation of the BFGS
2 To apply BFGS, we need at least the first derivative
3 In our case, computing the first derivative is not obvious
4 We have used the centric form to compute an approximation of
the first derivative
Centric form of the first derivative



Ψ (µ)) = ( ∂Ψ
∂µ1
,..., ∂Ψ
∂µn
)
∂Ψ
∂µi
= Ψ(µ1,...,µi +ε,...,µn)−Ψ(µ1,...,µi −ε,...,µn)
2ε
5 Good approximation of the first derivative relies on the choice of
the value of the parameter ε
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14. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
BFGS in practice - Results
1 Through the numerous tests conducted we have selected
ε = 0.01 as the best value for a good approximation of the first
derivative
2 We have tested two functions Ψ1
and Ψ2
, Ψ1
treats all points
outside [0...255] in the same way
3 In practice, Ψ1
and Ψ2
give nearly the same results
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15. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
HMRF-BFGS VS Classical MRF, MRF-ACO-Gossiping &
MRF-ACO
Methods
Dice coefficient
GM WM CSF Mean
Classical-MRF 0.763 0.723 0.780 0.756
MRF-ACO 0.770 0.729 0.785 0.762
MRF-ACO-Gossiping 0.770 0.729 0.786 0.762
HMRF-BFGS 0.974 0.991 0.960 0.975
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16. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
Results with (N-Noise,I-Intensity non-uniformity)
(N,I)
The initial Dice coefficient
Time(s)
point GM WM CSF Mean
(0 % , 0 %) µ0,1
0.974 0.991 0.960 0.975 27.544
(2 % , 20 %) µ0,2
0.942 0.969 0.939 0.950 15.630
(5 % , 20 %) µ0,3
0.919 0.952 0.920 0.930 84.967
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17. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
Example of segmentation using HMRF-BFGS
(0%,0%) (3%,20%) (5%,20%)
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18. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
Conclusion & Perspective
1 We have presented a combination method HMRF-BFGS
2 Through the tests conducted,
1 We have figured out good parameter settings
2 HMRF-BFGS method shows a good results
3 We conclude that HMRF-BFGS method it is very promising
4 Nevertheless, the opinion of specialists must be considered in the
evaluation
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19. Introduction Hidden Markov Random Field BFGS (Broyden, Fletcher, Goldfarb and Shanno) algorithm Experimental Results Conclusion & Pers
Thank you
for your attention
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