Unit thickness. Unit area. σ = NΔX = ΔI / I 0

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2 Unit thickness I 0 ΔI I σ = ΔI I 0 NΔX = ΔI / I 0 NΔX Unit area Δx Average probability of reaction with atom for the incident photons at unit area with the thickness of Delta-X Atom number at unit area with the thickness of Delta-X ΔI / I = Nσ = ΔX Σ 0 Average probability of reaction with atom for the incident photons at unit area with the thickness of Delta-X Thickness of Delta-X

3 ! Linear attenuation coefficient Definition: Ratio of reaction photons to incident photons after traverse material with unit thickness, denoted as µ. Unit thickness µ = di Idx ln I I 0 = µ d HVL = ln2 µ I 0 di µ m µ ρ = Unit area Δx ΔI / I0 Σ = Nσ = ΔX

4 <30 kev 30 kev-30 MeV >30 MeV Low hv X-ray photon hv>e b Binding Energy High hv X-ray photon λ 0 hv>>w Ultra-high hv X-ray 10, 000 times photon hv>1.022 MeV 1, 000 kvp hoelectron absorbed Charact. X-ray <0.5 kev, absorbed Recoil(Campton) Photoelectric effect All photon energy is absorbed Electron ϕ θ Compton effect λ Scattered photon Positive electron Mev photon Electric pair effect - Negative electron

5 Low energy High energy

6 4. Quality evaluation parameter Reference: P39-58

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11 X-ray fluoroscopy Fluorescent screen Dynamic/ real time Low resolution image Image can not be kept Patient exposed to X-ray for a few seconds or minutes ED 0.05mSv ED 0.94mSv

12 Digital X-ray Imaging CR - DR

13 Part II Computed Radiography CR! 1982! (imaging plate, IP) IP! IP IP

14 Computed Radiography physics X-ray First excitation laser Second excitation Photon/electric Imaging reader A/D Digital image

15 Imaging plate X Photo-stimulated luminescence, PSL BaFX: Eu 2+ Protective layer Fluorescence layer(psl) Support layer Protective layer Structure of IP 20cm*25cm X-ray -> IP -> latent image () -> image reader h Fading of PSL Deposit time should less than 8h

16 Imaging reader CCD " Six components: Laser optic system collector A/D IP transport erase

17 Imaging processing techniques! Standard & gray scale adjustment! Dynamic compression! Frequency adjustment - = Original smooth High freq. + Boundary sharpen An example of boundary enhancement technique

18 CR s Features Merits " Low dose " IP is repetitive " Good compatibility " No black room Drawbacks " Complicated process " Low time resolution (6 mins) " Spatial resolution 2.8lp/mm < 6-7lp/mm(film) " With image postprocessing " Digital storage Example: IP size is 35.56cm*43.18cm, and the digitized image is 2000*2510. Then, what is the pixel size? Rx=35.56/2000=0.18mm, Ry=43.18/2510=0.18mm. Line pairs: 1/(2*0.18)=2.8lp/mm

19 Part III Digital radiography -DR 1. DR introduction 2. DR classification 3. a-si, a-se plate detector 4. Line scanner X-ray imaging 5. Evaluation of DR

20 DR introduction Digital radiography is a form of x-ray imaging, where digital X-ray sensors are used instead of traditional photographic film. " Fast imaging process, <1.0 s " Digital display (Computer monitor) improves workflow efficiency. " Large-area digital image receptor, 35cm*43cm, 43cm*43 cm, 1:1 with low distortion. " As high as 65% detective quantum efficiency (DQE), low dose & early diagnose. " Wide dynamic imaging range, above 14 bit gray scale. " Good reliability, suit for the large turnover. " 5f/s, be able to realize DSA X-ray radiography: Preparation, Exposure, Developing, Quality control, Finish; 6 min CR: Preparation, Exposure, Scan, Finish; 7 min DR: Preparation, Exposure, Preview; Finish; 1.3 min

21 DR classification # Plane and line scan # Indirect digital radiography(idr) and direct digital radiography(ddr)

22 Cesium iodide CsI/ Gd2O2S:Tb/ DR classification INDIRECT CONVERSION X-ray Scintillator/ Visible light Photodiode/or CCD Thin film transistor(tft) / Electrical signal A/D DIRECT CONVERSION X-ray Photoconductor/ Electrical charge Thin film transistor(tft) / Electrical signal A/D Digital signal Digital signal a-si) CCD a-se) CMOS From the viewpoint of exposure, DR system can be divided into! Plane exposure, Flat Plane Detector! Line scan, the detectors own three types: a-se) PbI 2 ) CMOS

23 a-si plate detector (IDR) " Scintillator Cesium iodide CsI/ " Photoelectric effect Visible light s peakvalue is 540 nm, very close to the optimized response wavelength of a-si photodiode, nm. ( ) Perfect match " Photo transfer function Needle-shaped CsI(TI)crystal, 6-10 micro dia., height " a-si photodiode( Every photodiode is a pixel, 70~200 micro. GE TM a-si plate detector Field: 41cm*41cm Pixel: 2050*2050 Pixel size: 200 micro DQE: 77% Imaging process: 3s Space resolution: 2.5 linepair/mm Pixel (A/D): 14 bit

24 a-se plate detector (DDR) X-ray a-se

25 a-se plate detector (DDR)! Reflective mode: Reflective mirror! Direct mode: Lens! Fiber mode: Fiber! Translation mode

26 Line scanner X-ray imaging 0.16mm*8 column 2048 detector form a row with the width of 41 cm Three types MWPC (Multi wires proportional chamber) /CMOS (CdZnTe, CZT)

27 Line scanner X-ray imaging

28 Linear scanner X-ray imaging features Merit 1. Direct conversion avoid energy and information loss during the conversion. 2. Reduce the disturbance of scattered ray. 3. High sensitivity reduces the dose. The dose of 0.01mGy for chest scan is much lower than the personal protection standard of 0.3 mgy. Drawback 1. Cannot acquire real-time images. 2. Relative low spatial resolution 1.6lp/mm.

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30 Part IV-5 Detector Quality Evaluation Parameters 1. Pixel & spatial resolution 2. Dynamic range 3. Modulation transfer function 4. Detective quantum efficiency

31 Pixel & Spatial resolution Pixel- In digital image, a pixel (or picture element) is the smallest item of information in an image. Pixels are normally arranged in a 2-dimensional grid, and are often represented using dots, squares, or rectangles. Example 1 Calculate the pixel size & spatial resolution of DR system where one Flat Plane Detector (14*17 inch) is digitized into a matrix size of 2000*2510. Spatial resolution Example 2 Calculate the total number of bytes stored for this DR system with a 2048*2048 matrix if an 8-bit ( analog-to digital converter is used.

32 Pixel & Image quality Store & display cost Mammo radiography General Noise radiography limit Resolution limit High 70~ ~200 Quality Low Pixel (Micron)

33 Detector dynamic range! DR X Example Low limit: 1micro-Gy High limit: 10 mili-gy e ~14 6bit8bit 10bit

34 Modulation transfer function, MTF ( I I )/( I I ) M = + max min here I max indicates the maximum signal, I min is the minimum.! MTF is located in the region of [0, 1];! Higher MTF, higher contrast, and lower information transfer lose;! If MTF=0 the image can t be obtained;! Human can t tell the difference with MTF less than 0.025;! MTF is a function of the spatial resolution. So we must refer MTF@ known spatial resolution. max min Modulation Transfer

35 Detective Quantum Efficiency, DQE!! DQE DQE! X DQE! X DQE

36 DQE importance! DR-65%; CR-35%; Radiography (Screen-film) 25%.! Because of the higher DQE, flat-panel detectors are expected to use radiation dose more efficiently, and they could theoretically provide lower noise image over the ranger of spatial frequencies encountered in medical imaging.

37 Conclusion! Classification of X-ray radiography, CR, and DR! CR workflow and main components! DR classification( DDR & IDR, Plane & Line scan)! Quality evaluation parameter: DQE, MTF, dynamic range, Pixel & spatial parameters

38 Questions?