This document provides an overview of digital radiography and compares computed radiography (CR) and direct digital radiography (DR) systems. It discusses the limitations of traditional film screen radiography including limited dynamic range and inability to manipulate images. For CR, it describes the use of storage phosphor plates which capture x-ray information for later readout and digitization. For DR, it explains direct and indirect conversion panels used to directly convert x-rays to electrical signals. Key advantages of digital systems include immediate image viewing, manipulation, and storage without chemical processing.

1. DIGITAL RADIOGRAPHY- AN UPDATE
BY: DR. SHIVAM BATRA

2. FILM SCREEN RADIOGRAPHY
 In this, the image is represented as a negative, with white representing low
dose and black high.
 This convention is retained in DR, with pixels having high values being
displayed dark and those with low values light.

3. LIMITATIONS OF FILM SCREEN RADIOGRAPHY
 Radiographic film has a defined latitude (range of densities) that can be recorded simultaneously.
This problem is seen in chest radiography where 40 percent of lung area is obscured by heart,
mediastinum and diaphragm. Mediastinum and retro cardiac portion of lung remain under penetrated
when exposure is given for optimal lung details.
 Time consuming.
 Film – screen systems are intolerant to exposure errors, with under/over exposure leading to loss of
radiographic contrast.
 Films cannot be transmitted or duplicated without loss of quality.
 Acquisition, display and storage of image are non-separable.

4.  Radiographic film has a
defined latitude (range of
densities)

5. UNDER/ OVER EXPOSURE

6.  Another limitation- noise inherent
in these images: Radiography uses
area beams, i.e. large rectangular
beam of X-rays. The Compton
scattered portion of the remnant
X-ray beam increases with
increasing field size which inturn
increases the noise of the image.
 Also, owing to this the patient
increases.

7. ALSO,
 Film storage is a problem.
 Film quality can deteriorate with time, especially if chemical
processing is suboptimal.
 Important limitation- image cannot be manipulated before it is
displayed and image quality is therefore not necessarily the same in all
the images.

8. DIGITAL RADIOGRAPHY
 The process wherein digital detectors are used to capture
information of an object is termed as digital radiography.
 In this, the image is divided into a matrix of individual cells and
pixels. Each pixel has an assigned value that is related to the
intensity of the signal in the corresponding part of the image.

9. DIGITAL IMAGING CONSIST OF FOUR SEPARATE STEPS:
 Image generation
 Image processing
 Image archiving
 Presentation of the image

10. PRINCIPLE OF DIGITAL RADIOGRAPHY
 The digital detector is exposed to X-rays generated by a standard
tube.
 The energy absorbed by the detector is transformed into electrical
charges, which are then recorded, digitized and quantified into a
gray scale.
 After sampling, post-processing software is required for organizing
the raw data into a clinically meaningful image.

11. DIGITAL RADIOGRAPHY SYSTEMS
In digital radiography, a digital detector replaces films and intensifying screens. There are
two basic types of digital radiography systems depending upon the types of detectors used to
capture the radiographic information.
 Computed radiography (CR) system use a storage Phosphor image plate (photostimulable
phosphor plate) enclosed in a light tight cassette. CR utilizes a two stage process with image
capture and image readout done separately.
 Direct digital radiography (DR) uses detectors that have a combined image capture and
image read out process. DR systems can be further divided to direct and indirect conversion
groups, depending on the type of X-ray conversion used

13. COMPUTED RADIOGRAPHY
 It is a process of capturing radiographic data from a conventional
X-ray machine and processing the data digitally to produce crisp
and high quality radiographic images.

16. IMAGING PLATES
• The Imaging Plate looks like
the intensifying screens
found in Conventional film-
screen cassettes
• They are made of
photostimulable phosphor

18. When the storage phosphor
image plates (IP) are exposed to
X-rays, the X-ray energy is
absorbed and temporarily
stored by the crystals (detective
layer of the plate) by bringing
electrons to higher energy
levels.
X-ray energy can be stored for
several hours depending on the
specific physical properties of
the phosphor crystals used.

19.  Storage phosphors are unique because they respond to a very wide range of
X-ray exposures
 This latitude gives the flexibility in selecting X-ray technique and takes care
of under or over exposure
 Regardless of the exposure, the image can be displayed correctly
 As a consequence, retakes due to inappropriate exposures are drastically
reduced

20. STORAGE PHOSPHOR
PRINCIPLE

22.  The imaging plate is coated with photostimulable phosphor, also
called storage phosphor
 The phosphor material is generally a kind of Bariumfluorohalide
 The Imaging Plate contains not only the phosphor layer, but also a
protective coat, a conductive layer, support and laminate layers

23. STORAGE PHOSPHOR PLATE EXPOSURE
 The storage phosphor plate fits inside a standard size cassette and is exposed
to X-rays exactly like film.
 The X-ray energy is stored on the plate in the form of latent energy.

24.  Incident X-rays excite electrons into a higher energy level (electron
traps)
 A latent image is created in the form of “stored energy”
 Stimulation with a scanning laser beam releases electrons
 Typical wavelength of the stimulating laser is 633 nm
 Falling back, electrons emit luminescent light
 Typical wavelength of the emitted light is 390 nm

25.  The emitted light intensity is proportional to the original incident X-ray
intensity.
 The emitted light is captured with an optical array and a photomultiplier and
is digitized.
 The residual image is erased from the plate by an intense light source, which
returns all electrons to their original state. This makes the plate ready to be
reused for new exposures.

28. PATIENT ID STATION
 Before exposing the cassette, the patient
demographic and exam data is stored on the
microchip attached on cassette
 This is done by inserting the cassette in a slot of ID
station and entering the data with the help of
keyboard
 When cassette is inserted in digitizer after X-ray
exposure, the digitizer reads both patient data as well
as X-ray exposure data
 The two data are combined to display images along
with patient data

29. DIGITIZER
 The plate is inserted into the digitizer where it is
scanned with a high power laser
 The laser light causes the storage phosphors to
release the energy they have captured in the form
of blue light
 In the digitizer, this blue light energy is converted
to electrical signals which are then digitized to
produce digital images

31. WHAT HAPPENS TO A STORAGE PHOSPHOR PLATE AFTER IT IS
SCANNED?
 After exposure and scanning, the phosphor plate is "erased" by
exposing to a bright light exposure within the digitizer
 The previous image stored in the phosphors is removed and the
plate is ready to be exposed again

32. WORKSTATION
 The digitized image data is
processed on a processing server
and is displayed on its monitor

35. ADVANTAGES OF
CR SYSTEM
Being cassette based, CR systems can easily be integrated into existing radiographic equipment.
Single CR systems can convert multiple radiography rooms to digital technology.
They have a wide dynamic range leading to reduced rates of failed X-ray exposure.
They are easy to use for bed side examinations and immobile patients.
CR cassettes can be placed in any position thereby enabling flexibility for positioning for difficult views.
Multiple cassette sizes are available.
In case of defect in the image plate, it can easily be replaced by the radiographer with no need for specialized
equipment or service personnel.

36. DRAWBACKS OF CR SYSTEM
It is a time consuming technique.
Image reader takes time before the image can be displayed, with time taken being
comparable to that required for film processing.
Spatial resolution is lower than that of film screen radiography.
Radiation dose required is same or more than film screen radiography.

37. DIRECT DIGITAL RADIOGARAPHY
 Digital radiography (DR) or direct digital radiography is a
way of converting X-ray into electrical charges by means
of a combined image capture and image readout process.

38. DR SYSTEMS CAN BE FURTHER DIVIDED INTO DIRECT AND INDIRECT CONVERSION GROUPS
DEPENDING ON THE TYPE OF X-RAY CONVERSION USED.
 Direct conversion detectors have an X-ray photoconductor such as amorphous
selenium that directly converts X-rays photons into an electric charge.
 Indirect conversion detectors have a two-step process for X-ray detection
– A scintillator is the primary material for X-ray interaction. When X-rays strike the
scintillator the X-ray energy is converted to visible light.
– The visible light is then converted into an electric charge by means of
photodetectors such as amorphous silicon photodioide arrays or CCDs (charged couple
device).

39.  In both direct and indirect conversion detectors, the
electric charge pattern that remains after X-ray exposure is
sensed by an electric readout mechanism, and analog-to-
digital conversion is performed to produce the digital
image.

40. DIRECT CONVERSION
 Uses amorphous selenium as photoconductor that converts X-rays photons
into electrical charges by setting electrons free.
 Others are: amorphous selenium, lead iodide, thallium bromide and
gadolinium compounds. All these elements have a high intrinsic spatial
resolution.

41.  Selenium-based direct conversion DR systems are
equipped with either (a) a selenium drum or (b) a
flat panel detector.

42. SELENIUM DRUM-BASED SYSTEM OF DIRECT CONVERSION
 A rotating selenium drum with a positive
electric surface charge is exposed to X-
rays. During exposure, a charged pattern
proportional to that of incident X-rays is
generated on the drum surface and is
recorded during rotation by an analog-to-
digital converter.
 Limitation- they are dedicated thorax
stand systems with no mobility at all.

43. NOTE
 Thin-film transistor arrays (TFTs) are used as active electronic elements in both
direct and indirect conversion, flat-panel detectors. Thin film transistor arrays
are typically deposited onto a glass substrate in multiple layers, beginning
with readout electronics at the lowest level and followed by charge collector
arrays at higher levels. Then, depending on the type of detector being
constructed, X-ray elements, light sensitive elements, or both, are deposited
to form the top layer of TFT array. The whole assembly is encased in a
protective enclosure with external casing for computer connection.

44. THIN FILM TRANSISTOR—DIRECT CONVERSION
 Direct conversion system based on thin-film
transistor arrays are constructed by adding an
X-ray photoconductor (amorphous selenium)
as the top layer of the electronic thin-film
transistor sandwich.
 It is sandwiched between two electrodes to
which high voltage is applied. When this layer
is exposed to X-rays, electrons and holes are
produced, proportional to the amount of X-
rays absorbed. Thus, the X-rays are directly
converted to electrical signal.
 Advantage: These detectors can be mounted
on thorax stands and bucky tables.

45. THIN FILM TRANSISTOR—INDIRECT CONVERSION
 Constructed by adding amorphous silicon
photodiode circuitry and a scintillator as the
top layers of the thin-film transistor sandwich.
These layers replace the X-ray photoconductor
layer that is used in direct conversion devices.
 Mechanism: When X-rays strike the scintillator,
visible light is emitted proportional to the
incident X-ray energy. Visible light photon are
then converted into an electric change by the
photodiode array, and the charge collected at
each photodioide is converted into a digital
value by using the underlying readout
electronics.

46. TYPES OF SCINTILLATORS: TWO TYPES
 Unstructured scintillator
 Structured scintillators: They reduce the problem of scatter.
 Thallium-doped cesium iodide (CsI) and Gadolinium oxysulphide or
Gadox (Gd2O2S) are commonly used as phosphor material. (CsI>
Gadox)

47. Advantages:
 The small size of the flat panel detectors is of
advantage as it allows integration into existing
bucky table or thorax stands.
 Image generation with flat panel detectors is
almost instant, with a time lapse less than 10
seconds.
 Also more number of patients can be imaged
in the same amount of time than with other
radiographic devices.
Disadvantages:
 CsI-based flat-panel detectors are highly
vulnerable to mechanical load because of their
fragile nature, these detectors are to be
handled with the utmost care. These systems
cannot be used outside fixed installations and
hence lack mobility.
 Portable flat panel detector systems make use
of Gd2O2S-based scintillators as they are
resistant to mechanical stress.

49. IMAGE PROCESSING
Improves image quality by reducing
noise, remove technical artifacts
and optimize contrast for viewing.
Direct radiography systems convert
X-ray image information into
electronic charges held by the TFT
array. As indirect-conversion
systems rely on light, substantial
scatter occurs before the energy is
converted to charge which reduces
signal-to-noise ratio.
Edge enhancement or high pass
filtering has the opposite effect.
Rather than display a weighted
average value of neighboring pixels,
a high pass filter adds in a
proportion of the difference
between the grayscale value of the
pixel and that of its neighbor. The
effect is to exaggerate the contrast
at the boundary between
structures, thus making the
structures more visible.

50. FEATURES OF DIGITAL X-RAY
1. Image enhancement
2. Annotation
3. Printing
4. Black border
5. Panoramic dental package
6. Full leg/ full spine

51. 7. UNDER OR OVER EXPOSURE

52. 8. SOFT TISSUE OR BONE WINDOWS

54. 9. DIGITAL IMAGE MANIPULATION
 Image pre-processing
 Scale the data to appropriate range
 Contrast enhancement – Anatomy specific grayscale manipulation
 Spatial frequency enhancement

55. 10. ZOOM

56. 11. COLLIMATION

58. 12.
MAGNIFYING
GLASS

59. 13. INVERT IMAGE

60. 14. VERTICAL FLIP

61. CR SYSTEM VS DR SYSTEM
DR
 transistor receiver (like bucky)
 directly into digital signal
 seen immediately on monitor –
CR
 imaging plate
 Processing is done in a Digital Reader
 Signal sent to computer
 Viewed on a monitor

62. Thank you