1. Direct Digital Radiography or Direct Capture Radiography
Yu zixi
Radiological department
Taishan Medical College

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 DR – NO CASSETTE – PHOTONS
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. DDR CR Digital Radiography Direct Capture Indirect Capture Computed
(CR) - PSL
Direct-to-Digital
Radiography
(DDR)-Selenium
Direct-to-Digital
Radiography
Silicon Scint.
Laser
Scanning
Digitizers

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 =
12 bit =

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

19. 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

20. Active matrix array of silicon photodiodes

21. 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

22. F/S & DDR imaging systems

23. F/S & DDR imaging systems

25. 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)

26. Image Resolution

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

28. Signal Sampling Frequency
Good sampling
under sampling

29. DR Initial expense ---high very low dose to patient
image quality of 100s using a 400s technique
Therfore,¼ the dose needed to make the image

30. 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

31. 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

32. 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%

33. Fill Factor

34. DDR has all the advantages of CR imaging techniques
Post processing & PACS

41. Advantages of DDR Fast speed, large throughput High spatial resolution
Low patient radiation dose
Can be updated
Less noise
Powerful post-processing

42. THANK YOU!