short presentation about CR

1. COMPUTED
RADIOGRAPHY

2. Contents
1. Introduction
2. History of CR
3. How Does Computed Radiography Work ?
4. Image Generation of Computed Radiography
5. Advantages and Disadvantages of Computed Radiography

3. 1. Introduction
• Computed radiography is emerging as a digital imaging modality for use in
conventional radiography. It is based on photostimulable phosphor image
plate technology. The image plate (IP) is housed in a cassette similar to a
standard radiographic cassette. The IP phosphor retains a latent image of the
energy pattern to which it was exposed. This latent image is "read" as it is
released from the phosphor by laser light exposure.

4. 1. Introduction
• The image plates have a detective layer of
photostimulable crystals that contain different
halogenides such as bromide, chlorine, or iodine (eg,
BaFBr:Eu2+)
• Phosphor crystals are cast into plates of resin
material in unstructured scintillators.
• Image plates replace the conventional films in the
cassette.
• Exposure process with storage-phosphor image
plates:

5. 2. History of CR
The fundamental innovation in the
development of CR was by Kodak
(Luckey 1975) who conceived the
storage of an x-ray image in a
phosphor screen.
It required significant technical steps
and conceptualization of the
application by Fuji (Kotera et al
1980) to produce the first medical x-
ray images. Fuji, the main developer
of CR in the eighties, used
BaFBr:Eu2+ phosphor and a
cassette-based approach.

6. 3. How Does CR Systems Works ?
Image plate is
exposed to X-ray
energy
The image plate is
scanned with a
laser beam.
The stored energy
is set free and light
is emitted.
An array of
photomultipliers
collects the light.
Light is converted
to electrical charges
by an analog-to
digital converter

7. Image Plates Image Plate Scanner

8. 4. Image Generation and Processing of CR
 Protective layer:
• A very thin, tough, clear plastic for protection
of the phosphor layer
 Phosphor, or active, layer :
• A layer of photostimulable phosphor that
“traps” electrons during exposure
• Usually made of phosphors from the barium
fluorohalide
• May also contain a dye that differentially
absorbs the stimulating laser light to prevent as
much spread as possible
Image Plate Modeling

9. 4. Image Generation and Processing of CR
Reflective layer:
• A layer that sends light in a forward
direction when released in the cassette
reader
• May be black to reduce the spread of
stimulating light and the escape of
emitted light
• Some detail lost in this process
Conductive layer:
• A layer of material that will absorb and
reduce static electricity
Color layer:
• Newer plates may contain a color layer,
located between the active layer and the
support that absorbs the stimulating light
but reflects emitted light.
Support layer:
• A semi-rigid material that gives the
imaging sheet some strength.
Backing layer: A soft polymer that
protects the back of the cassette.

10. 4. Image Generation and Processing of CR
X-Ray
Electron
Trap
ABSORPTION
Electron
Trap
EMISSION
Laser Stimulation
light
• Image plates exposed by
conventional X-ray
equipment.
• Eu+2  Eu+3 + e-
• Electrons are trapped in F-
centre
• Latent image generated as a
matrix of trapped electrons
in the plate.
Conduction band
Valenced band

11. An energy diagram of the excitation and PSL
processes in a BaFBr:Eu2+ phosphor

12. 4. Image Generation and Processing of CR
Read-out Process by Scanner
• The extracted imaging plate is scanned with
red laser and gives energy to trapped electron
and energizes them.
• Extra energy allows the trapped energy to
escape active layer where they emit blue light
at energy of 3eV.
• Laser scans IP multiple times.
• Scanning produces lines of light intensity
information that are detected by
photomultiplier tube that amplifies and send it
to digitizer.

13. 4. Image Generation and Processing of CR
In the process of digitizing the light signal, each phosphor storage center is scanned .
The light is directed to the photomultiplier/charged coupled device that converts the light to an
electronic signal.
That electronic signal is digitized by an analog to digital converter (ADC).
The ADC assigns each picture element or pixel a value that corresponds to a level of brightness.
The entire image is divided into matrix of pixels based on the brightness of each pixel.
Number of photons detected determine the gray level.
Each pixel can have a gray level between 2 and 4096.
Digitizing the Signal

14. 4. Image Generation and Processing of CR
• Spatial resolution
Dependent on IP size
Less than corresponding speed screen-film
• Contrast sensitivity
Dependent on exposure and SNR
• Exposure
Variable speed detector
Image Performance Measures

15. 4. Image Generation and Processing of CR
• Not all electrons have came back to the initial phase after reading.
• Exposure to a bright fluorescent light removes the remaining information in 10-
15 sec and the IP is reinserted into the cassette for reuse.
• Ghosting may appear in the IP if the residual latent image remained so it should
be removed by flooding with intense white light.
Erasing

16. 5.Advantages and Disadvantages of CR
systems
Advantages of CR systems
• Reduced rates of failed x-ray exposure.
• Because CR systems are cassette based, they can
easily be integrated into existing radiographic
devices, are highly mobile, and are easy to use for
bedside examinations and immobile patients.
• Flexible in routine clinical use.
• Can easily be replaced by the radiographer with no
need for specialized equipment or service personnel
if a single image plates shows defects.
• Imaging plates are reusable.
Disadvantage of CR systems
• Spatial resolution is usually lower than that with
conventional screen-film combinations.
• Long time to view image.
• Risk of over-exposure.
• High maintenance.