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Presentation about basics of image deblurring, two popular approaches, and two computed tomography applications.
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Kriti Sen Sharma Graduate Research Assistant
Deblurring &
Applications in Computed Tomography
Outline
1st Half (15min) Deblurring Basics
2nd Half (15min)
Deblurring in CT
Blurring
Imaging Defects
What if we cannot improve imaging process anymore!!!
Solution
= DEBLURRING (Deconvolution)
Invert the imaging defects MATHEMATICALLY
Examples
Real life photography Acquired Image After Deconvolution
Examples
Astronomical Imaging Acquired Image After Deconvolution
Examples
Microscopic Imaging Acquired Image After Deconvolution
Mathematical Model-1
Imaging Defects
x A
b
b = Ax + n xd = A-1 b
Mathematical Model-2
Imaging Defects
!
"(x)
!
"(x) = l(x)# g(x)!
p(x)
!
g(x)
!
g(x) = p(x)" #(x) + n(x), x $ R2
!
l(x) inverse filter of p(x)
Mathematical Model-2
PSF: Point Spread Function
Deblurring Example-1
Noise = 10-10
Deblurring Example-2
Noise = 10-5
ill-posedness of the Inverse problem
Deblurring Example-3
Noise unknown
Solutions-1
Imaging Defects
x A
b
b = Ax + n xd = A-1 b
Recap
Solutions-1
Truncated Singular Value Decomposition
• A = U Σ VT = [u1 … uN] diag(s1… sN) [v1 … vN]T
• Truncation
Ak* = [v1 … vN] diag(1/s1… 1/sN) [u1 … uN] T
• xk = Ak* b
Solutions-1
Using k = 53 i.e. 53 major singular values used
Visible now?
Solutions-2
Imaging Defects
!
"(x)
!
"(x) = l(x)# g(x)!
p(x)
!
g(x)
!
g(x) = p(x)" #(x) + n(x), x $ R2
!
l(x) inverse filter of p(x)
Recap
Solutions-2
Wiener Filter
!
L(u) =P(u*)
P u( )2
+Snu( )
S" u( )
=P(u*)
P u( )2
+1
SNR(u)
!
P(u)P(u*) = P u( )2
L(u) = P(u)"1 =P(u*)
P u( )2
!
Snu( ) : PSD of noise
S" u( ) : PSD of object
!
L u( ) : PSD of inverse filter l(x)
P u( ) : PSD of blurring filter p(x)
End of Deblurring Basics!!
Now to discuss some real applications of
Deblurring in CT
Jing Wang, Ge Wang, Ming Jiang Blind deblurring of spiral CT images
Based on ENR and Wiener filter Journal of X-Ray Science and Technology – 2005 [previous: IEEE Trans. on Medical Imaging 2003]
Blind Deconvolution
1st Problem: Finding P(u) PSD of p(x) = ? 2nd Problem: Finding SNR(u) SNR at different u = ?
!
L(u) =P(u*)
P u( )2
+Snu( )
S" u( )
=P(u*)
P u( )2
+1
SNR(u)
Solution to 1st Problem
Assume p(x) → Gaussian with σ = ?
Deblur at multiple σ
Find σ that gives best deblurring
How to find best σ: Use ENR
Solution to 2nd Problem
Assume SNR(σ) = k
Find k by phantom studies
ENR
• Edge to Noise Ration
• in terms of I-divergence (Information Theoretic approach)
• Noise effect
• Edge effect
• ENR = Edge effect / Noise effect
ENR Maximization Principle
maximize ENR(σ,k)
to get optimal σ
Rollano Hijarrubia et. al. Selective Deblurring for Improved Calcification
Visualization and Quantification in Carotid CT Angiography: Validation Using Micro-CT IEEE Transactions on Medical Imaging 2009
Wiener Filter
• Two problems to be solved: – 1. Point Spread Function (PSF) = ? – 2. Signal to Noise Ratio (SNR) = ?
Solution to 2nd Problem
• Phantom designed
• Scanned • Reconstructed • Deblurred at various SNR • Optimum SNR value chosen
Solution to 1st Problem
• By measuring PSF of a bead image1
• Resolution of scanner: 0.3 - 0.4mm Bead size: 0.28mm
[1]. Meinel JF, Wang G, Jiang M, et al. Spatial variation of resolution and noise in multi-detector row spiral CT. Acad Radiol. 2003;10:607– 613.
Selective Deblurring
Axial MIPs (Maximum Intensity Projection) of the original, deconvolved, and restored images of the phantom.
MicroCT Reference
Thanks!
Questions?
Summary of J. Wang et.al. Given g(x)
Wiener Filter at various σ to get λ(x,σ)
ENR(σ) calculated
Max ENR(σ): σOPT
λ(x,σOPT)
Wiener filter SNR parameter chosen from
phantom studies
Edge to Noise Ratio
• I-divergence
• Noise effect
• Edge effect
• ENR
!
I(u,v) = u(x)logu(x)
v(x)" u(x) " v(x)[ ]
x
#x
#
!
N(",k) = I g,G" # $k g,k,"( )( )
!
E(",k) = I #k g,k,"( ),G" $ #k g,k,"( )( )
!
ENR(",k) =E(",k)
N(",k)