1. Direct Digital Radiography or Direct Capture Radiography Bushong Ch. 27

2. Late 1990’s A new approach to imaging appeared DR or DDR or Direct Capture imaging Too early to tell which system will prevail

3. Directed Digital Radiography (DDR) Directed digital radiography, a term used to describe total electronic imaging capturing. 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 – PHOTONS CAPTURED DIRECTLY ONTO A TRANSISTOR SENT DIRECTLY TO A MONITOR

7. DIRECT RADIOGRAPHY uses a transistor receiver (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 vs DR CR imaging plate processed in a Digital Reader Signal sent to computer Viewed on a monitor DR transistor receiver (like bucky) directly into digital signal seen immediately on monitor –

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

10. Two types of DDR systems Both are based on the thin-film transistor as an active matrix array (AMA) Built the size of a conventional S/F receptor

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

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

13. DEL collects e-

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

15. DDR using cesium iodide scintillation phosphors CsI is coated over an active matrix array (AMA) of amorphus silicon (a-Si) photodiodes Amorphus means without shape Photodiodes are used to detect light or measure its intensity also called a charge coupled device (CCD)

16. DDR steps using cesium iodide Exit x-rays interact with CsI scintillation phosphor to produce light The light interact with the a-Si to produce a signal The TFT stores the signal until readout, one pixel at a time

17. CsI phosphor light detected by the AMA of silicon photodiodes

18. 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

19. Active matrix array of silicon photodiodes

20. 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

21. F/S & DDR imaging systems

23. Image Resolution – (how sharply is the image seen) 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 CR 2 - 5 lp/mm RAD 3-6 lp/mm DR 3 - 5 lp/mm IMAGE APPEARS SHARPER BECAUSE CONTRAST CAN BE ADJUSTED BY THE COMPUTER – (DIFFERENCES IN DENSITY)

24. Image Resolution

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

26. Signal Sampling Frequency Good sampling under sampling

27. DR Initial expense high very low dose to pt – image quality of 100s using a 400s technique Therfore ¼ the dose needed to make the image

28. Flat Panel TFT Detectors Have to be very careful with terminology One vendor claims: “Detector has sharpness of 100 speed screen” May be true: TFT detectors can have very sharp edges due to DEL alignment But ! Spatial resolution is not as good as 100 speed screen. TFT detector = 3.4 lp/mm 100 speed screen = 8 – 10 lp/mm

29. 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

30. 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%

31. Fill Factor

32. DDR has all the advantages of CR imaging techniques Post processing & PACS Questions ?