1. Digital Radiographic Imaging 101

2. Terms Digital radiography (DR), image receptors, film digitizer,
noise, signal to noise ratio, area beam, xenon gas detectors,
scintillation detectors, fan beams, pre and post patient collimation,
slit radiography, translation, interrogation time, attenuation profile,
extinction time, Computed radiography (CR), photostimulable
phosphor image plate, barium fluorohalide, europium, IP reader,
photomultiplier (PM) tube, photocathode, photoemission,
dynode, CR workstation, linear energy response, duel image format,
charged couple devices (CCD), amorphous silicon & selenium
image receptors, thin film transistor (TFT), active matrix array (AMA),
direct radiography.

3. Digital Acquisition Methods
1. Digitize radiographs with a film digitizer
Similar to a
paper scanner,
only for film
Film is read by
a laser and the
image file is sent
to a designated
secondary device

4. * Converts films to digital files * Teleradiology * Teaching files and
Uses
* Converts films to
digital files
* Teleradiology
* Teaching files and
presentations
Advantages
* Inexpensive
* Easy to use
* Small
Disadvantages
* Conversion of analog
to digital adds a step
that degrades image
quality
* Impractical for
converting large
archives

5. 1. Digitize radiographs with a film digitizer
Digital Acquisition Methods
1. Digitize radiographs with a film digitizer
2. Digitize the video signal with an ADC
Advantages: Inexpensive
Easy to install
Disadvantages: Noisy cameras
Poor signal-to-noise ratio (SNR)
Area beam (Scatter)
Small matrix size (525)

6. Digital Acquisition Methods
1. Scan radiographic films
2. Digitize the video signal
3. Scan projection radiography (SPR)
* Greatly reduces the area of the beam, and scatter
* Replaces the camera with detectors
Fan shaped
beam
Depth of beam
may be a cm, or
smaller

7. Xenon Gas Detector From a CT Scanner
Xenon gas chamber

8. Ring of Xenon Detectors in a CT Scanner

9. SPR used for CT Scout Films
Stationary
X-ray tube
Detectors

10. CT Scout Views Acquired by SPR, to produce a digital radiograph
Scatter radiation is greatly reduced

11. Detectors
1. Xenon Gas
2. Scintillation

12. Detectors 1. Xenon Gas 2. Scintillation Photomultiplier (PM tube)
Crystal

13. Digital Acquisition Methods
1. Scan radiographic films
2. Digitize the video signal
3. Scan projection radiography (SPR)
4. Computed Radiography (CR)
Photostimulable image plate (IP) technology
Barium Fluorohalide doped with Europium

14. CR Advantages Uses existing radiographic hardware
So is relatively inexpensive to purchase
Reduced number of repeats
Increased latitude
Filmless capture
The CR IP looks
like a conventional
intensifying screen,
and is housed in a
conventional looking
cassette.

15. CR Facts 300 RSV Only one speed (no detail or high speed)
Standard film sizes
Laser Film – wet or dry processing

16. Step 1. Make the exposure like any other radiographic
Computed Radiography
Step 1. Make the exposure like any other radiographic
exposure, only use an IP instead of film.
*Remnant photons strike plate
*Photoelectric interaction causes
barium fluorohalide to fluoresce
as electron is ejected.
*Electrons (that are of no more
use in film radiography) are
trapped in the energy traps
created by the europium IP

17. Reading the IP Converting the stored energy to an
electric current, point by point.

18. CR Workstations

19. Posterior bases obscured
Problems Inherent to Conventional Chest Radiography
Under-
exposed
Retrocardiac clear-
space overexposed
Posterior bases obscured
by diaphragm on PA

20. CR Film

21. Latitude Logarithmic response of film Linear response of CR Maxed out
Yet to respond
Linear response
of CR

22. Analog is continuous. Image is fixed in film Digital is discrete
Analog is continuous. Image is fixed in film Digital is discrete. Image may be manipulated 1 15 15 1

23. CR Postprocessing & Characteristic Curves

24. Histogram

25. Processing Algorithms
Poorly exposed image
plates may be corrected
by software to some extent

26. Patient Dose
Calculated and displayed
Total energy absorbed by IP
Fuji S number (200 ave) Low number = high exposure
Kodak ( ) Low number = low exposure

27. Cassetteless Readers
Chest units.
In table

28. Laser, Dry Image Hardcopy Devices

29. Digital Acquisition Methods
1. Scan radiographic films
2. Digitize the video signal
3. Scan projection radiography (SPR)
4. Computed Radiography (CR)
5. Charge Coupled Devices (CCD)

30. Charge-Coupled Device (CCD)
CCD’s have
replaced the
Vidicon tube
in camcorders

31. Charge-Coupled Device (CCD)
Read-out row
Photoelectric detectors
embedded in layers of
silicon
Each pixel is 6 to 25
microns in size, and
can store 10,000
to 50,000 electrons.

32. Charge-Coupled Device (CCD)
.. . … ……. . .
… …
Incident light creates
a charge in the pixels

33. Charge-Coupled Device (CCD)
.. . … ……. . .
… …
A shutter closes
to stop further
interaction of light
on the pixels
The frame is ready
for reading

34. Charge-Coupled Device (CCD)
Charges shift from
one pixel to another
as they move to
the readout row

35. Charge-Coupled Device (CCD)
Charges shift from
one pixel to another
as they move to
the readout row

36. Charge-Coupled Device (CCD)
Charges move
along the readout row, and exit the
chip.

37. Charge-Coupled Device (CCD)
Charges move
along the readout row, and exit the
chip.

38. Charge-Coupled Device (CCD)
Question: When the charges
leave the CCD chip, where
do they go?
Charges move
along the readout row, and exit the
chip.
Answer: If the CCD was
functioning as a camera,
they could be sent directly
to an analog monitor as
the video signal, or...

39. Charge-Coupled Device (CCD)
ALU CU
Primary
Memory
Secondary
(RAM)
ADC
DAC
DAC
ADC
or...they could be sent
to an ADC, on to RAM,
displayed on a digital
monitor, and stored in
a secondary memory
device

40. Digital Acquisition Methods
1. Scan radiographic films
2. Digitize the video signal
3. Scan projection radiography (SPR)
4. Computed Radiography (CR)
5. Charged Couple Devices
6. Flat Panels
Amorphous Silicon & Amorphous Selenium

41. Thin film transistors (TFT) in an Active Matrix Array (AMA), are incorporated in a “flat panel” detector that is used in place of a film cassette.

42. Thin Film Transistors (TFT)
139 microns (half
a hair)
Diodes connected
to rows
Current flows
out columns

43. Amorphous Silicon Cesium iodide (CsI) scintillator
converts X-rays to light
Light is converted to a charge
by a photodiode at a TFT junction.

44. Amorphous Selenium Photon in Interaction creates electron-hole pairs
(called Direct Radiography)
Positive charge
Electrode with a bias voltage
Photoconductor material
Photon in
Negative charge
TFT
Interaction creates
electron-hole pairs
Signal out

45. * Method by which the stored, latent electronic image is discharged
Digital Radiography (DR)
Receptor Reader* Energy Comment
Transformations
Video Target of camera Electron gun x-ray to light to Use limited by
charged globules noise of camera
to video signal
SPR Xenon Interrogations of x-ray to ionized Dedicated cxr
successive detectors electrons and CT scouts
Scintillation Interrogations of x-rays to light
successive detectors to current
CR Photostimulable Helium-neon x-ray to light to Only portable
phosphorIP laser to trapped electrons receptor
to light to current
CCD IC Point by point discharge x-ray to light to Potential next
of photoelectric trapped electrons generation of IIs
detectors (pixels) to current
Amorphous TFT AMA point by point discharge x-ray to light Called direct
silicon Flat panel of TFTs to current radiography
Amorphous TFT AMA point by point discharge x-ray to current Called direct
selenium Flat panel of TFTs radiography
* Method by which the stored, latent electronic image is discharged