Direct Digital Radiography or Direct Capture Radiography

1. Direct Digital Radiographyor Direct Capture Radiography Bushong Ch. 26 & 29 & Carter Ch. 6

2. Late 1990’s • A new approach to imaging appeared • DR or DDR or Direct Capture imaging • At the moment departments can’t go all DR they must still have F/S or CR. Do you know why?

3. Directed Digital Radiography(DDR) Directed digital radiography, a term used to describe total electronic imaging capturing. DR is hard-wired to the image processing system.Eliminates the need for an image plate altogether.

5. DDR Systems

6. IMAGE CAPTURE CR • PSP – photostimulable phosphor plate • REPLACES FILM IN THE CASSETTE DR – NO CASSETTE – LIGHT or E- • Captured directly • On to a transistor, photodiode or charge-coupled device • Sent directly to a monitor

7. DIRECT RADIOGRAPHY • uses a TFT, CCD, or photodiode to receive image data (like bucky) • that captures and converts x-ray energy • directly into digital signal • seen immediately on monitor • then sent to PACS/ printer/ other workstations FOR VIEWING

8. CR imaging plate processed in a Digital Reader Signal sent to computer Viewed on a monitor DR CCD, TFT or photodiode receiver (like bucky) directly into digital signal seen immediately on monitor – CR vs DR

9. Digital Radiography DDR CR Direct Capture Indirect Capture Computed Radiography (CR) - PSL Direct-to-Digital Radiography (DDR)-Selenium Direct-to-Digital Radiography Silicon Scint. Laser Scanning Digitizers

10. Digital Radiography Fundamentals of Digital Radiography

11. Flat-Panel Detectors • Flat-panel detectors consist of a photoconductor, amorphous selenium (a-Se), which holds a charge on its surface that can then be read out by a TFT. This category also includes silicon and CCD detectors.

12. Capture Element • Where the remnant photons are captured. • DR = Cesium iodide (CsI), Gadolium oxysulfide (GdOS), or Amorphous selenium (a-Se). • And for CR? What is the name of the compound?

13. Direct vs Indirect Conversion • In direct conversion, x-ray photons are absorbed by the coating material and immediately converted into an electrical signal. The DR plate has a radiation-conversion material or scintillator, typically made of a-Se. This material absorbs x-rays and converts them to electrons, which are stored in the TFT detectors.

14. Collection element • Collects converted x-ray signal. • Types: Photodiode, A charge-coupled device (CCD), or A thin-film transistor (TFT). • Photodiode & CCD collect light. TFT is charge sensitive and collects E-.

15. TFT • The thin-film transistor (TFT) is a photosensitive array made up of small (about 100 to 200μm) pixels. Each pixel contains a photodiode that absorbs the electrons and generates electrical charges.

16. Active Matrix Array (AMA)Pixels are read sequentially, one at a time • Each TFT or CCD detector represents a pixel • DEL = charge collecting detector element

17. DR • A field-effect transistor (FET) or silicon TFT isolates each pixel element and reacts like a switch to send the electrical charges to the image processor.

18. Amorphous Selenium • No scintillation phosphor is involved • The image-forming x-ray beam interacts directly with amorphous selenium (a-Se), producing a charged pair.

19. Amorphous Selenium • The a-Se is both the capture element and the converting element. • a-Se is a direct DR process by which x-rays are converted to electric signal

20. DDR only using amorphous selenium (a-Se) • The exit x-ray photon interact with the a-Si (detector element/DEL). Photon energy is trapped on detector (signal) • The TFT stores the signal until readout, one pixel at a time

21. Indirect Conversion • Indirect conversion is a two-step process: x-ray photons are converted to light, and then the light photons are converted to an electrical signal. • A scintillator converts x-rays into visible light. The light is then converted into an electric charge by photodetectors such as amorphous silicon photodiode arrays or charge-coupled devices (CCDs).

22. CCD Array with a scintillation phosphor

23. Direct vs Indirect DR

24. Charge-Coupled Device • CCD, which is the light-sensing element. • The CCD is a silicon-based semiconductor • has three principal advantageous imaging characteristics: sensitivity, dynamic range, and size.

25. Sensitivity • is the ability of the CCD to detect and respond to very low levels of visible light • This sensitivity is important for low patient radiation dose in digital imaging.

26. Dynamic range • is the ability of the CCD to respond to a wide range of light intensity, from very dim to very bright • DR should lower patient dose

27. Size • A CCD is very small, and this makes it highly adaptable to uses in radiology • The CCD itself measures approximately 1 to 2 cm, but the pixel size is an exceptional 100 × 100 μm!

28. Coupling Element • Transfers the x-ray signal to the collection element. • Ex: A lens or fiber optic assembly, a contact layer, or amorphous selenium.

29. DEL Digital Value • Digital Value depends on: • Charge collected by DEL. • Bit depth • 10 bit = 1 – 1024 • 12 bit =1 - 4096

30. DEL collects x-raysignal

31. Spatial Resolution Should be best with DR. DR is limited by pixel size

32. Image Resolution

33. CR & DR 4000 x 4000 image only as good a monitor* 525 vs 1000 line more pixels = more memory needed to store resolution dependent on pixel size DR 4 lp/mm CR 6 lp/mm RAD 8 lp/mm Mammo 15 lp/mm IMAGE APPEARS SHARPER BECAUSE CONTRAST CAN BE ADJUSTED BY THE COMPUTER – (DIFFERENCES IN DENSITY) Image Resolution –(how sharply is the image seen)

34. Pixel Pitch • Spatial resolution determined by pixel pitch. • Detector element (DEL) size • 140 μm = ~3.7 lp/mm • 100 μm = ~ 5.0 lp/mm

35. Fill Factor

36. F/S & DDR imaging systems

37. Unlike CR plates, only the exposed pixels contribute to the image data base. • One exposure = Detector Readout

38. TFT Array Detectors • Detector is refreshed after exposure • If no exposures are produced. . . detector refreshed every 30 – 45 sec • Built in AEC, An ion chamber between grid and detector

39. Advantages/Disadvantages • CsI phosphors have high detective quantum efficiency (DQE) = lower patient dose • DQE = % of x-rays absorbed by the phosphors • a-Se only: there is no spreading of light in the phosphor = better spatial resolution

40. Dynamic range • is the ability of the CCD to respond to a wide range of light intensity, from very dim to very bright • DR should lower patient dose

41. DR • Initial expense high • very low dose to pt – due to the high DQE over CR and F/S. Fewer photons required to produce and image. • image quality of 100s using a 400s technique • Therfore ¼ the dose needed to make the image

42. Patient Dose • Important factors that affect patient dose • DQE: when using CsI systems • Both systems “fill factor” • The percentage of the pixel face that contains the x-ray detector. • Fill factor is approximately 80%

43. Viewing the Digital Image Ch. 29 Review pg 34 in carter***

44. Photometry • The science of the response of the human eye to light • The basic unit of photometry is the lumen (lm).

45. Illuminance • describes the intensity of light incident on a surface • Luminance intensity is a property of the source of light, such as a viewbox or a digital display device

46. Cosine Law • Is important when one is describing the luminous intensity of a digital display device. When a monitor is viewed straight on, the luminous intensity is maximum. When a monitor is viewed from an angle, the contrast and the luminous intensity are reduced.

47. When a digital display device is viewed from the side, illumination and image contrast are reduced.

48. Hard CopySoft Copy Radiology 1895Radiology 2001

49. Soft copy viewingdigital cathode ray tube (CRT)

50. active matrix liquid crystal display (AMLCD)