Image Intensifiers & Digital Radiography

1. Image Intensifiers & Digital Radiography based on Canon

3. Why Digital? • Store and retrieve without loss of quality • Processing to optimize and improve image • Rapid storage and retrieval • Rapid long distance transmission • Improved image management • Economics

4. Considerations • Image Structure • Matrix of pixels each containing a numerical value • Ranges from 8 to 14 bits (256 to 16384) • Bits determines dynamic range • shades of gray displayed Storage Transmission (PACS)

5. Stages of digital radiography? Traditional Radiography X-ray generator Fluorescent screen Film Digital Radiography X-ray generator Phosphorescent screen Intensifier CCD detector SEC vidicon Readout Silicon detector + fluorescent material Electronic readout Photostimulable luminescence plate Laser scan readout Indirect Indirect Direct Indirect

6. Computed RadiographyElectrontraps in phosphor layer Electromagnetic energy stored until processing Laser beam (red or infrared) releases stored energy causing the emission of light Light emissions read by photodiode scanning the imaging plate plateDigital Radiography–Indirect X-ray photon generates light emission from phosphor layer Array of photodiodes measure light emitted and generate data signal–Direct X-ray photons generate flow of electrons in dielectric plate Electrode collection plate as an array of transistors generate data signal Summary

7. Fluoroscopy and Image Intensifiers

8. Fluoroscopy Geometry • Tube as far below the table as practical • Image Intensifier above patient, at convenient height, but minimized air gap • Automatic exposure control provides kVp and mA change for constant monitor image intensity

9. Fluoroscopy Used to create continuous sequence of images Usually of cardiac, vascular or alimentary systems Direct fluoroscopy Unintensified image is very faint: 3 times fainter than viewing box Intensifier used to produce brighter image S/N depends on x-ray flux Intensifier does not improve S/N Intesifier introduces noise. After Heggie

10. Flouroscopy Uses Designed to allow realtime imaging/ viewing • Direct Fluoroscopy: • view shoe fittings • insert catheter Screens were 200 microns thick and backed by 2mm lead glass Output was yellwo/green Spatial resolution 3 lp/mm Screens - CWO4 until 1933 ZnCdS: Ag or ZnS:Ag (doped) Quantum efficiency = 30% Conversion efficiency =10%-18%

11. Intensifier Development

12. A typical intensifier

13. Indirect Fluoroscopy + intensifier Input phosphor: CsI:Na t lifetime > tframe Photocathode: SbCs3 in contact with phosphor QE <10% Electrostatic optics: Acelerating voltage 30 kV Potential focuses electrons on output phosphor Output phosphor P20 Green Demagnification = output diameter2/ input diameter2 Intensity gain due to demagnification = input diameter2/ ouput diameter2

14. The Faceplate

15. Indirect Fluoroscopy + intensifier • Tube gain = demagnification gain * Flux gain • Demagnification gain = input diameter2/ ouput diameter2 • For 23 cm tube with o/p diameter 2.0cm • Demagnification gain = 132 • Flux gain: • one electron for every 25 photons from input phosphor • electron gains 30keV • electron produces 2000 photons at output phosphor • 75% of photons are transmitted through output window • Flux gain = 60 • Tube gain = 132* 60 = 8000 • NOTE: S/N is not increased by flux gain nor demagnification • S = FG * X-ray flux Noise = FG *(X-ray Flux)1/2

16. Contrast: 20:1 or 30:1 • Scattering of x-rays at • entrance window • phosphor • Scattering of light in • input phosphor • output phosphor • Scattering of electrons in • intensifier • Resolution: 3 lp/mm • phosphor thickness • electron optics • space charge effects at high x-ray intensity • output phosphor thickness • S/N • NOTE: S/N is not increased by flux gain nor demagnification • S = FG * X-ray flux Noise = FG *(X-ray Flux)1/2 Image Quality

17. Recording the Intensified Image • The output of the intensifier may be recorded on • cine film ( old technique) • vidicon • linear CCD array

18. In a conventional, digitized R&F imaging chain, the signal degradation that occurs with each component consumes more than 60% of the original x-ray signal.

19. Cine camera

20. Vidicon Camera

21. 0V 250V 25V Vidicon Camera • Optical transfer system or fibreoptic faceplate • Target is Sb2S3 or PbO which store charge coated with a graphite signal plate • Electron beam is • focussed by magnetic field along tube axis • scanned by saddle coils • accelerated to 250eV by perforated anode • Electron beam reaches target with 25eV energy

22. Vidicon readout Image formation Target globule 1: absorbs light 2: emits electrons electrons attracted to anode globule +ve charge 3: Capacitor formed with signal plate Signal plate –vely charged Image readout 1: Electron beam discharges the +ve globule 2: The –ve charge on the signal plate flows back to the ground via a resistor This produces the signal voltage

23. Interventional Procedures with Long Fluoro Times Radiation Safety • Percutaneous Transluminal angioplasty -PCTA • Stent Placement • Radiofrequency Cardiac catheter Ablation • Transjugular intrahepatic portosystemic shunt • Neuroembolization • Pacemaker Harry M. Johnson, Ph.D. CancerCare Manitoba Winnipeg, Manitoba, Canada

24. Subject underwent 3 procedures. Total fluoroscopy time 120 min. tissue necrosis resulted Skin injury as it appeared 18-21 months post procedure Radiation Safety Risk of Multiple Cardiac Procedures Harry M. Johnson, Ph.D. CancerCare Manitoba Winnipeg, Manitoba, Canada

25. Influence of Pulsed Fluoro on Dose A B C D • A. Conventional: 30 pulses per second • B. 15 pulses per second; dose at 50% • C. 7.5 pulses per second; • dose at 25 % • D. 3.75 pulses per second; dose at 12.5 % • Example: 17 cm mode • - 30 pulses/s 2.3 cGy/min • - 15 pulses/s 1.4 cGy/min Harry M. Johnson, Ph.D. CancerCare Manitoba Winnipeg, Manitoba, Canada

26. CR (Computer Radiography) http://www.agfa.com/healthcare/clinicalsolutions/radiology/digitalradiography/computedradiography/marketview/

27. CR (Computer Radiography) Direct: Intensifier output screen is read out from CCD Array Indirect: • X-rays interact directly with screen which stores photon energy Readout by laser light stimulating emission in visible PM tube or CCD records visible intensity for each pixel scanned • Silicon panel ( + fluorescent material) interacts with X-rays Charge stored in each pixel is electronically read out

28. CR (Computer Radiography)

29. Pros and Cons

30. CR Record and ReadoutProcess

31. Storage Phosphor Plates • The CR MM2.0 Mammo plate for mammography. • Excellent homogeneity and short response time (the previous pixel is fully faded before the next one is stimulated). • Phosphor layer enables a low noise level. This improves the ability to view structures within the breast thanks to a higher contrast-to-noise ratio. • The anti-halo layer is a blue layer that forms a perfect barrier against laser light, while letting through the stimulated light.

32. Readout Image retention • Readout is recommended within 1 h after exposure. • Two hours after exposure 70% of the stored energy is still present with no visible loss of information upon readout. • Image retention still exceeds 45% after 24 h.

33. Typical Performance

34. Photostimulable Storage Phosphours • BaFBr compound, • Eu Activated Gadolinium oxysulfide phosphor The luminescent mechanism • Impurities in a crystal make luminescence possible • Divalent Europium ions create luminescence centres in PSP by replacing barium in the crystal • Number of impurity ions present affects the concentration of luminescence centres and therefore the energy stored in the crystal • Increasing the number of europium ions in the phosphor will increase the intensity of the photostimulated luminescence

35. Mechanism of Photostimulated Luminescence • Exact mechanism is not well understood • Two theories • monomolecular • bimolecular • Both theories try to explain the linear response of the phosphors over a wide range of exposures

36. Bimolecular • Bimolecular • electrons are released when x-rays change Eu+2 to Eu3+ by direct ionization or by trapping holes • scanning the phosphors with visible light stimulates electron-hole recombination and the F-centres release electrons to the conduction band • electrons can either combine with the newly formed F+centre or with the Eu3+ ions • electrons combining with the Eu3+ ions will result in photostimulable luminescence • because of the electrons in the conduction band, the photostimulable luminescence and photoconductivity should be temperature dependent

37. Monomolecular • Monomolecular • intensity of photostimulable luminescence is nearly temperature independent • hole is formed near Eu+2 forming a Eu+2-hole complex • F(Br-) centre is near this complex • when F(Br-) is excited by the laser, tunneling takes place causing the Eu2+ to become excited and luminesce • identity of hole trapping is thought to be an oxide impurity • enhance production of F(Br-) centres

38. The Readout System

39. Laser Readout

40. Transport Stage

41. Acceptance Optics

42. Detector

43. A/D Conversion

44. Response