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Bioengineering 280A Principles of Biomedical Imaging Assume z=d/2 10 cm 10 cm Fall Quarter 2013 X-Rays Lecture 2 z z 5 cm 10 cm d d

Assume z=d/2 Assume z=d/2 TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

Signals and Images Kronecker Delta Discrete-time/space signal /image: continuous valued function with a discrete time/space index, #1 for n = 0 δ[n] = $ denoted as s[n] for 1D, s[m,n] for 2D , etc. %0 otherwise n n δ[n] m € n Continuous-time/space signal /image: continuous 0 valued function with a continuous time/space index, δ[n-2] denoted as s(t) or s(x) for 1D, s(x,y) for 2D, etc. n 0 t y x

TT Liu, BE280A, UCSD Fall 2013 x TT Liu, BE280A, UCSD Fall 2013

1 Kronecker Delta Function Discrete Signal Expansion ∞ # 1 for m = 0,n = 0 g[n] = g[k]δ[n − k] δ[m,n] = $ ∑ % 0 otherwise k=−∞ ∞ ∞ g[m,n] = ∑ ∑ g[k,l]δ[m − k,n − l] δ[m,n] δ[m-2,n] k=−∞ l=−∞ € δ[n] g[n] n 0 € -δ[n-1] δ[m,n-2] δ[m-2,n-2] n n 0 1.5δ[n-2] n 0

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013 n

2D Signal Image Decomposition 1 0 0 1 c +d a 0 0 b c d 0 0 0 0 + = + a b 0 0 0 0 a b a 0 0 0 0 + +b = 1 0 0 1 c d 0 0 0 0 + c 0 0 d g[m,n] = aδ[m,n]+ bδ[m,n −1]+ cδ[m −1,n]+ dδ[m −1,n −1] 1 1 = ∑∑g[k,l]δ[m − k,n − l] k= 0 l= 0

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

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2 1D Dirac Delta Function ∞ δ(x) = 0 when x ≠ 0 and δ(x)dx =1 ∫−∞ Can interpret the integral as a limit of the integral of an ordinary function that is shrinking in width and growing in height, while maintaining a Notation : constant area. For example, we can use a shrinking rectangle function δ(x) - 1D Dirac Delta Function ∞ ∞ such that ∫ δ(x)dx =lim ∫ τ −1Π(x /τ)dx. δ(x,y) or 2δ(x,y) - 2D Dirac Delta Function −∞ τ →0 −∞ δ(x,y,z) or 3δ(x,y,z) - 3D Dirac Delta Function  δ (r ) - N Dimensional Dirac Delta Function € τ→0 1 €

x -1/2 1/2

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

2D Dirac Delta Function Generalized Functions ∞ ∞ Dirac delta functions are not ordinary functions that are defined by their δ(x,y) = 0 when x 2 + y 2 ≠ 0 and δ(x,y)dxdy =1 ∫−∞ ∫−∞ value at each point. Instead, they are generalized functions that are defined where we can consider the limit of the integral of an ordinary 2D function by what they do underneath an integral. that is shrinking in width but increasing in height while maintaining constant area. ∞ ∞ ∞ ∞ δ(x,y)dxdy =lim τ −2Π(x /τ,y /τ)dxdy. The most important property of the Dirac delta is the sifting property ∫−∞ ∫−∞ τ →0 ∫−∞ ∫−∞ ∞ Useful fact : δ(x,y) = δ(x)δ(y) δ(x − x )g(x)dx = g(x ) where g(x) is a smooth function. This sifting ∫−∞ 0 0 property can be understood by considering the limiting case

∞ −1 lim τ Π(x /τ)g(x)dx = g(x0 ) € τ →0 ∫−∞

g(x)

€ Area = (height)(width)= (g(x0)/ τ) τ = g(x0) τ→0 x0

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

3 Representation of 1D Function Representation of 2D Function

Similarly, we can write a 2D function as ∞ ∞ g(x,y) = g(ξ,η)δ(x −ξ,y −η)dξdη. From the sifting property, we can write a 1D function as ∫−∞ ∫−∞ ∞ g(x) = g(ξ)δ(x −ξ)dξ. To gain intuition, consider the approximation To gain intuition, consider the approximation ∫−∞ ∞ ∞ 1 + x − nΔx . 1 + y − mΔy. ∞ 1 * x − nΔx - g(x,y) ≈ ∑ ∑ g(nΔx,mΔy) Π- 0 Π- 0Δ xΔy. g(x) ≈ ∑ g(nΔx) Π, /Δ x. m=−∞ n=−∞ Δx , Δx / Δy Δy n=−∞ Δx + Δx . , /

€ g(x) €

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

Impulse Response

Intuition: the is the response of a system to an input of infinitesimal width and unit area.

Original Image Blurred Image

Since any input can be thought of as the weighted sum of impulses, a linear system is characterized by its impulse response(s). Bushberg et al 2001

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

4 Impulse Response Full Width Half Maximum The impulse response characterizes the response of a system over all space to a (FWHM) is a measure of resolution. Dirac delta impulse function at a certain location.

h(x2;ξ) = L[δ(x1 −ξ)] 1D Impulse Response

h(x2,y2;ξ,η) = L[δ(x1 −ξ,y1 −η)] 2D Impulse Response

€ y2 y1 h(x2,y2;ξ,η) Impulse at ξ,η

x1 x2

TT Liu, BE280A, UCSD Fall 2013 Prince and Link 2005 TT Liu, BE280A, UCSD Fall 2013 € €

X-Ray Imaging (Addition) s(x) R(I) I1(x,y) K1(x,y) z 1 t(x) = δ(x) d M R(I) I2(x,y) K2(x,y)

€ I (x,y)+ I (x,y) K1(x,y) +K2(x,y) 1 2 R(I) 1 " x % s$ ' m # m&

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013 €

5 Linearity (Scaling) Linearity

A system R is linear if for two inputs

I1(x,y) and I2(x,y) with outputs R(I) I1(x,y) K1(x,y)

R(I1(x,y))=K1(x,y) and R(I2(x,y))=K2(x,y)

R(I) the response to the weighted sum of inputs is the a1I1(x,y) a1K1(x,y) weighted sum of outputs:

R(a1I1(x,y)+ a2I 2(x,y))=a1K1(x,y)+ a2K2(x,y)

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

Example Superposition Are these linear systems? g[m] = g[0]δ[m]+ g[1]δ[m −1]+ g[2]δ[m − 2] g(x,y) + g(x,y)+10 g(x,y) X 10g(x,y) h[m',k] = L[δ[m − k]]

10 10 y[m'] = L[g[m]] € = L[g[0]δ[m]+ g[1]δ[m −1]+ g[2]δ[m − 2]] g(x,y) Move up Move right g(x-1,y-1) = L[g[0]δ[m]] + L[g[1]δ[m −1]] + L[g[2]δ[m − 2]] By 1 By 1 € = g[0]L δ[m] + g[1]L δ[m −1] + g[2]L δ[m − 2] € [ ] [ ] [ ] = g[0]h[m',0]+ g[1]h[m',1]+ g[2]h[m',2] € 2 € = ∑g[k]h[m',k] k= 0

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

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6 Superposition Integral Space Invariance What is the response to an arbitrary function g(x1,y1)? If a system is space invariant, the impulse response depends only ∞ ∞ Write g(x ,y ) = g(ξ,η)δ(x −ξ,y −η)dξdη. on the difference between the output coordinates and the position of 1 1 ∫-∞ ∫-∞ 1 1 The response is given by the impulse and is given by h(x2,y2;ξ,η) = h(x2 −ξ,y2 −η)

I(x2,y2 ) = L[g1(x1,y1)] ∞ ∞ = L g(ξ,η)δ(x −ξ,y −η)dξdη [ ∫-∞ ∫-∞ 1 1 ] ∞ ∞ € = g(ξ,η)L δ(x −ξ,y −η) dξdη ∫-∞ ∫-∞ [ 1 1 ] ∞ ∞ = g(ξ,η)h(x ,y ;ξ,η)dξdη ∫-∞ ∫-∞ 2 2

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013 €

10 cm X-Ray Imaging z 5 cm s(x) d

z 1 t(x) = δ(x) Assume z=d/2 d M 10 cm 10 cm

€ z z 5 cm 10 cm d d 1 " x % s$ ' Is this a linear system? m # m& Is it a space invariant system? Assume z=d/2 Assume z=d/2 TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013 €

7 1D Convolution g[m] = g[0]δ[m]+ g[1]δ[m −1]+ g[2]δ[m − 2] ∞ h[m',k] = L[δ[m − k]] = h[m $ − k] I(x) = g(ξ)h(x;ξ)dξ ∫-∞ ∞ = g(ξ)h(x −ξ)dξ y[m'] = L[g[m]] ∫-∞ g(x) h(x) = L[g[0]δ[m]+ g[1]δ[m −1]+ g[2]δ[m − 2]] = ∗ = L[g[0]δ[m]] + L[g[1]δ[m −1]] + L[g[2]δ[m − 2]] = g[0]L δ[m] + g[1]L δ[m −1] + g[2]L δ[m − 2] [ ] [ ] [ ] € Useful fact: = g[0]h[m'−0]+ g[1]h[m'−1]+ g[2]h[m'−2] ∞ g(x)∗δ(x − Δ) = g(ξ)δ(x − Δ −ξ)dξ 2 ∫-∞ = ∑g[k]h[m'−k] = g(x − Δ) k= 0

TT Liu, BE280A, UCSD Fall 2013 TT Liu,€ BE280A, UCSD Fall 2013

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2D Convolution Rectangle Function

For a space invariant linear system, the superposition integral 1 $0 x >1/2 becomes a convolution integral. Π(x) = % &1 x ≤1/2 ∞ ∞ x I(x ,y ) = g(ξ,η)h(x ,y ;ξ,η)dξdη 2 2 ∫-∞ ∫-∞ 2 2 -1/2 1/2 ∞ ∞ = g(ξ,η)h(x −ξ,y −η)dξdη ∫-∞ ∫-∞ 2 2 Also called rect(x) y = g(x2,y2 )**h(x2,y2) € 1/2 Π(x,y) = Π(x)Π(y) where ** denotes 2D convolution. This will sometimes be x € abbreviated as *, e.g. I(x2, y2)= g(x2, y2)*h(x2, y2). -1/2 1/2

-1/2 € TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

8 1D Convolution Examples 2D Convolution Example

1 g(x)= δ(x+1/2,y) + δ(x,y) h(x)=rect(x,y) * = ? y y x x -1/2 1/2 -3/4 1/4 x x -1/2 1/2 1 1

* = ? x x I(x,y)=g(x)**h(x,y) -1/2 1/2 -1/2 1/2 x

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

2D Convolution Example X-Ray Imaging s(x)

z 1 t(x) = δ(x) d M

€

1 " x % s$ ' m # m&

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013 €

9 X-Ray Imaging X-Ray Imaging For off-center pinhole object, the shifted source image can be written as s(x) # x − Mx & # x & 1 # x − Mx & s% 0 ( = s% ( ∗ δ% 0 ( $ m ' $ m' M $ M ' z 1 t(x) = δ(x − x ) # x & M 0 = s(x /m)* t% ( d $ M '

x0 x0 For the general 2D case, we convolve the magnified object with the impulse response € € " x y % 1 " x y % I(x,y) = t$ , ' ∗∗ s$ , ' # M M & m2 # m m& 1 # x − Mx & d d − z s% 0 ( M(z) = ; m(z) = − Mx0 m $ m ' z z Note: we have ignored obliquity factors etc.

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

€ € €

X-Ray Imaging X-Ray Imaging m =1;M = 2 s(x) = rect(x /10)

1 " x % " x % z s$ ' ∗ t$ ' = rect(x /10)∗ rect(x /20) € m # m& # M & € d = ??? t(x) = rect(x /10)

€ €

d d − z 1 " x % " x % M(z) = ; m(z) = − s$ ' ∗ t$ ' z z m # m& # M &

TT Liu, BE280A, UCSD Fall 2013 TT Liu, BE280A, UCSD Fall 2013

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10 Summary

1. The response to a linear system can be characterized by a spatially varying impulse response and the application of the superposition integral. 2. A shift invariant linear system can be characterized by its impulse response and the application of a convolution integral.

TT Liu, BE280A, UCSD Fall 2013

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