6720 Lecture 6_analog imaging

6720 Lecture 6_analog imaging - Lecture VI 1. 2. 3. 4. 5....

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1 PHYS 6720 - Lecture VI 1 Lecture VI Radiographic Imaging - Analog Methods 1. Overview 2. Risk to patient – consideration of dose 3. Subject contrast 4. Radiographic film and intensifying screen 5. Anti-scatter grid and Mammography 6. Fluoroscopy and image intensifier 7. Other methods PHYS 6720 - Lecture VI 2 VI-1 Overview Æ Radiographic imaging is based on transmitted radiation signals with no refraction and non-dominant scattering (shadow-casting principle) Æ The resultant 2D images are projection of 3D distribution of attenuation coefficient onto the 2D image plane Æ Transmitted photons that have been scattered in the object contribute to the image noise and have to be reduced Æ Analog images are very difficult to be reconstructed as 3D images. Æ 3D Image reconstruction: requires digital methods for computer- based reconstruction. Introduction VI-1 Overview The shadow-casting principle Let L( r , s )=L c ( r , s ) + L s ( r , s ) and denote as 4 (, ) ( ) (, ) ( , ' ) (, ' ) ' sa s L Lp L d s π μμ μ ε =− + + Ω+ rs ss (,) c tc L L s 4 (, ' ){ (,' ) )} ' (,) s ts s c s L L L d s + + L s L ⋅∇ s A. Ishimaru, Wave propagation and scattering in random media , vol.1 (1978) PHYS 6720 - Lecture VI 4 VI-1 Overview The shadow-casting principle 0 (,,) ( , ) Dxy t xyzd l csc s LLLL A e L =+= + s 0 () cs IL d I I Ω = + rr s r r The formal solution of radiation radiance L( r , s ) is given by were A is the boundary from which the ray pathlength ( ) starts. L can be integrated over a fixed range of solid angle Ω 0 to obtain radiation intensity I( r ) 0 dl PHYS 6720 - Lecture VI 5 VI-1 Overview The shadow-casting principle D(x,y)=pathlength of the object corresponding to the (x,y) location of the image. 00 tt xyzd l l s Ixy Ie I −− ∫∫ =+ L 0 =L(A, s ) I(x,y) D(x,y) μ t (x,y,z) after removing scatters PHYS 6720 - Lecture VI 6 VI-2 Risk to patients General features of tissue interaction Æ mass attenuation coefficient of human tissues μ m = μ t / ρ μ m (cm 2 /g)
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2 PHYS 6720 - Lecture VI 7 VI-2 Risk to patients General features of tissue interaction ( σ inc /Z) independent of Z Mass density (g/cm 3 ) Electron density (1/kg) Effective atomic number Z eff p-e interaction for <50keV Incoherent scattering for >30keV Fat 0.92 3.34 × 10 26 5.9~6.3 Weak Strong Soft tissue (water) 1.00 ~ 1.04 3.34 × 10 26 7.42 Strong Strong Bone 1.3~1.8 3.00~3.19 × 10 26 11.6~13.8 Very strong Weak air 1.2 × 10 -3 3.01 × 10 26 7.64 negligible Negligible Ca 1.55 3.01 × 10 26 20 Very strong Negligible ( σ pe /Z) Z (3.0~3.8) PHYS 6720 - Lecture VI 8 VI-2 Risk to patients Example: electron density ρ e of Ca For Ca: Z=20, A=40 so the mass of 1 mole (contains N A atoms) is given by M mole =40g=0.0400(kg). For a system of Ca atoms with a mass M=1.00(kg), the total number of electrons contained in M is given by N e =ZxN A x(M/M mole )=20.0 × 6.022 × 10 23 × (25.0) thus ρ e = N e /M = 3.01 × 10 26 (1/kg) PHYS 6720 - Lecture VI 9 VI-2 Risk to patients Definition of f-factor Æ When absorption of x-ray intensity in tissue is concerned, it is often convenient to relate the dose in tissue with exposure measured in air Æ In air the absorbed dose D air is related to exposure X as D air (Gy)=33.85X(C/kg) Æ
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This note was uploaded on 04/25/2010 for the course PHYS 6720 taught by Professor Hu during the Spring '10 term at East Carolina University .

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6720 Lecture 6_analog imaging - Lecture VI 1. 2. 3. 4. 5....

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