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AN INTRODUCTION TO DIGITAL RADIOGRAPHY

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1. 1. Lecturer : Seyedeh Shokoofeh Mousavi Gazafroudi 3/6/2016 Isfahan University of Medical Sciences 2
2. 2.  Introduction to digital radiography(DR)  Process  System  classification  Computed Radiography (CR)  photostimulable phosphor (PSP)  Exposure & readout  Imaging plates  Charged Couple Device (CCD)  Indirect Digital Radiography (IDR)  Structure & function  Direct Digital Radiography (DDR)  Structure & function 3
3. 3. Introduction to digital radiography(DR) Process System classification
4. 4. Introduction  Historically, digital radiography referred to specialized modalities that produced digital images  Digital images can be numerically processed This is not possible in conventional radiology.  Digital images can be easily transmitted through networks and archived  Since the early 1990s, Digital Radiography has grown to include Computed Radiography(CR) and ‘true’ Digital Radiography(DR) or Direct Radiography. 5
5. 5. What is digital radiology?  In conventional radiographic images, spatial position and blackening are analogue values  Digital radiology uses a matrix to represent an image  A matrix is a square or rectangular area divided into rows and columns. The smallest element of a matrix is called ”pixel”  Each pixel of the matrix is used to store the individual grey levels of an image, which are represented by positive integer numbers  The location of each pixel in a matrix is encoded by its row and column number (x,y) 6
6. 6. Digital Radiology Process  Image acquisition  Image processing  Image display  Image archiving (PACS)  Image retrieving 7
7. 7. Digital Radiography Systems 8
8. 8. 9
9. 9. Computed Radiography (CR) photostimulable phosphor (PSP) Exposure & readout Imaging plates Charged Couple Device (CCD)
10. 10. Computed Radiography (CR)  Uses same radiographic equipment  No change in X-ray machine  Uses an imaging plate  Contains a photo stimulator phosphor  Need a cassette reader  Images can be sent to a PACS 11
11. 11. Computed Radiography  Is a marketing term for photostimulable phosphor (PSP) detector systems.  Re-usable metal imaging plates replace film & cassette  Uses conventional bucky & x-ray equipment  When x-rays are absorbed some light is promptly emitted, but much of the absorbed x-ray energy is trapped in the PSP screen and can be readout later.  CR imaging plates are made of barium fluorohalid. 12
12. 12. The principle of PSP 13 Excitation Storage Emission CB Trap ADC PMT
13. 13. CR readout processing steps  After the cassette is exposed by the x-ray beam, the cassette is loaded into a reader.  The reader removes the phosphor plate and exposes it to a laser, stimulating the phosphors.  The light emitted from the plate is collected, quantified, and digitized.  after read-out, plate erased using a bright light 14
14. 14. CR Exposure & Readout 15
15. 15. Reading Imaging Plate  Reader scans plate with laser light using rotating mirror  Film pulled through scanner by rollers  Light given off by plate measured by PM tube & recorded by computer 16
16. 16. PMT Beam deflector Laser Source Light channelingguide Plate translation: Sub-scan direction Laser beam: Scandirection Output Signal Reference detector Beam splitter Cylindrical mirrorf-theta lens Amplifier ADC Toimage processor 17
17. 17. Laser & Emitted Light are Different Colors  Phosphor stimulated by laser light  Intensity of emitted light indicates amount of radiation incident on phosphor at each location  Only color of light emitted by phosphor measured by PMT 18
18. 18. Reading Imaging Plate  plate is photostimulable phosphor  radiation traps electrons in high energy states  higher states form latent image Higher Energy Electron State Lower Energy Electron State - - - - - - - - - - - - - - - - - - - - - - - - - - - X-Ray Photon - Photon pumps electron to higher energy state 19
19. 19. Reading Imaging Plate  reader scans plate with laser  laser releases electrons trapped in high energy states  electrons fall to low energy states  electrons give up energy as visible light  light intensity is measure of incident radiation Laser Beam Higher Energy Elect ron St at e Lower Energy Elect ron St at e - - - - - - - - - - - - - - - - - - - - - - - - - - - - Lower Energy Electron State 20
20. 20. 21
21. 21. CR Throughput  Generally slower than film processing  CR reader must finish reading one plate before starting to read the next  Film processors can run films back to back 22
22. 22. Charged Couple Device (CCD)  Is an integrated circuit made of crystalline silicon  Forms images from visible light  For linear CCD detectors charge pocket at the very bottom of linear array spills onto a transistor, and produce an electronic signal that is digitized. The entire line of pixels and readout in a shift-and-read process.  For 2D CCD detectors , the charges on each column are shifted onto the bottom row of pixels, that entire row is read out horizontally, and then next charges from all columns are shifted down one pixel. 23
23. 23. CCD detectors function 24
24. 24. 25
25. 25. Indirect & Direct Digital Radiography (IDR & DDR)
26. 26. Digital Radiography (DR)  Receptor provides direct digital output  Potentially lower patient dose than CR  High latitude as for CR  No processor / reader required  Images available in < 15 seconds  Much less work for technologist 27
27. 27. Flat panel detector 28
28. 28. Indirect digital radiography  X-ray strike scintillator producing light  Photodiode array converts light to electrons  The intensifying screen is made up of cesium-iodide crystals and the photodetector is made up of amorphous silicon. Light 29
29. 29. Indirect digital radiography  Electronic sensor are replaces the light sensitive film emulsion  X-rays are absorbed in the screen and the absorbed energy is then relayed to the photodetector by visible light photons  The ratio of the light sensitive area to the entire area of each detector element is called fill factor 30
30. 30. Flat Panel Imaging Arrays (indirect conversion) 31
31. 31. Direct Digital Radiography  Is made from a layer of photoconductor materials on top of a TFT array  The electrons released in the detector layer from x-ray interaction are used to form the image directly 32
32. 32. Direct DR  the photoconductor is made up of amorphous selenium. selenium has higher atomic number than silicon, but it is still quit low compared with conventional x-ray intensifying screen phosphorus.  Due to electric field ,selenium direct detectors can be made much thicker. 33
33. 33. Flat Panel Imaging Arrays (direct conversion) 34
34. 34. Direct Vs. Indirect 35
35. 35. 36
36. 36. Digital mammography  it uses a mosaic of CCD and CsI scintillator.  As scanning x-ray beam detector move to the right , the charge packet in the CCD array are moving to the left 37
37. 37. Patient dose considerations  The exposure necessary to produce good images are directly related to the detective quantum efficiency of the detector (DQE)  CR systems require about twice the exposure of a corresponding 400- speed screen-film detector for comparable image quality 38
38. 38. Copy display  Hard copy display refers to displaying image on film, and soft copy display refers using video monitors. 39
39. 39. Digital image corrections  Dead pixel correction  Column defect correction  Dark noise correction 40
40. 40. Processing  Is performed by altering the relation between digital number in the image and displayed brightness  Windowing, leveling & reversing the contrast are simple procedures performed routinely. 41
41. 41. Image Processing based on Convolution  The science of manipulating digital images often involves the mathematical operation called “Convolution”.  𝑔 𝑥 = −∞ +∞ 𝐼 𝑥’ ℎ 𝑥 − 𝑥’ 𝑑𝑥’ 42
42. 42. DR properties  Adaptive histogram equalization (AHE)  Contrast & spatial resolution 43
43. 43. I'm so glad I live in a world where there are “Autumns”