computed radiography introduction

1. COMPUTED RADIOGRAPHY (CR) - AN
OVERVIEW AND EVALUATION OF PIPE
WALL THICKNESS BASED ON CONTRAST
MEASUREMENT
C H YADHUKRISHNA (213118008)
ILLYAS M K (213118010)
MUHAMED MUSTHAFA PULIKKAL (213118015)

2. INTRODUCTION
Computed radiography (CR), also commonly known as Photostimulable
phosphor (PSP) imaging.
It employs reusable imaging plates and associated hardware and software to
acquire and to display digital projection radiographs.
It is cassette based system like analog film and is more commonly considered to
be a bridge between classical radiography and digital radiography
CR is used almost exactly like conventional Imaging plate in a cassette which
must be processed in CR reader after X-ray exposure for conversion to digital
images.

3. CR system consists of
Image plate (photostimulable phosphor) PSP
X ray or gamma ray source
Image reader/digitizer/scanner
Photomultiplier tube (PMT), ADC (analoge to digital converter)
Computer console or workstation, software, monitors, and a printer.
Image reader can be
Single-plate readers (each cassette is loaded manually and read separately)
Multiple-plate readers (multiple plates—up to 10—can be stacked and
loaded automatically)

4. HISTORY
Cassette-based computed radiography (CR) systems have continued to
evolve in parallel with integrated, instant readout digital radiography
(DR) systems.
The fundamental innovations in the development of CR was by Kodak
(luckey in 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 BarFBR:𝐸𝑈2+
phosphor and a cassette-based approach.

5. PRINCIPLE OF OPERATION
In computed radiography, the imaging plates are inserted in a radiographic
table’s cassette holder and are exposed to X-rays or gamma rays.
The energy of the incoming radiation is stored in a special phosphor layer,
electrons in the phosphor plate are excited into a higher energy state,
forming a latent image.
Then a specialized machine known as a scanner is then used to read out the
latent image from the plate by stimulating it with a very finely focused laser
beam.
During which the trapped electrons absorb the laser energy and they emit
blue light with intensity proportional to the amount of radiation received
during the exposure as they return to their ground state.

6. The light is collected by a light guide and transmitted to a highly sensitive
analog device known as a photomultiplier tube (PMT) and converted to a
digital signal using an analog-to-digital converter (ADC).
The generated digital X-ray image can then be digitally stored and viewed
on a computer monitor and evaluated.
 After an imaging plate is read, it is erased by a high-intensity light source
that removes residual radiation and can immediately be re-used.
Imaging plates can typically be used up to 1000 times or more depending
on the application.

7. LIMITATIONS OF CONVENTIONAL RADIOGRAPHY
Films cannot tolerate a wide range in radiation exposure (OD - log E curve)
One has to choose between good contrast and good latitude
Image cannot be adjusted once taken
Require space for dark room and film/chemicals storage..
Price of films and chemicals is constantly going up (4-10% per year)
Need several exposures to show different tissues (bone vs soft tissue)
Image developing time is more (manual about 60 minutes)

8. STEPS INVOVLVED
X ray image received on phosphor plate subjecting to X ray exposure
After the image has been captured on an image plate by a standard x-ray system, the plate is taken
to an image-plate reader to extract the image.
The cassette is loaded (manually or automatically) into the reader.
Image extracted from phosphor plate by laser (Raw image)
Raw image processed for quality improvement
The digital image is then produced in 30-120 seconds and downloaded to an image-processing
system, usually a computer workstation, for display and manipulation.
Digital image can be either printed burned on CD, or sent to PACS(Picture Archiving and
Communication System)

10. COMPONENTS OF COMPUTED RADIOGRAPHY
1. IMAGING PLATES
The CR plate has a thin layer of phosphor grains, known as a photostimulable
phosphor. The plates are usually 1 mm thick and are coated with Europium
activated fluorohalide compounds in crystalline formation embedded in organic
binding material.
 Layers of Imaging Plate
i. Protective Layer: A thin layer of transparent film that protects the phosphor.
ii. Barium Fluorohalide Phosphor Layer: a closely dispersed of fine-grained,
Photostimulable phosphor plate is coated with europium-activated, barium
Fluoro-halide crystals, that store the latent image until released when re-
stimulated during processing (Thickness is about 0.3 mm).

11. iii. Light reflective Layer: This layer increases the intensity of light being
emitted from the crystals by reflecting it back toward the reader, instead of
it being absorbed.
iv. Conductive Layer: This is a light absorbing layer, made up of conductive
needle-like crystals that absorb any unreflected light as well as any
electrostatic charges.
v. Polyester Support Layer: Made from a polyester material, this layer gives
structural rigidity and a base for the coating of all of the other layers.
Polyester is used because of its excellent stabilities well as its durability and
flexibility.
vi. Light Shielding Layer: This is a carbon particle layer that prevents the light
from leaking from the rear of the imaging plate.
vii.Backing Layer: This is a protective layer made from a soft polymer that
prevents scratching when the plates are stacked during the manufacturing
process.

12. PROTECTIVE LAYER
BARIUM FLUOROHALIDE PHOPHOR LAYER
REFLETIVE LAYER
CONDUCTIVE LAYER
POLYSTER BASE SUPPORT LAYER
LIGHT SHIELDING LAYER
BACKING LAYER

13. X-RAY EXPOSURE
• When the X-ray is absorbed by the material, absorbed energy excites the
europium atoms, causing ionization of Eu atom.
• The electrons are raised to higher energy state in the conduction band.
Where electrons travel freely until they are trapped in a so called F-centre
in a metastable state with an energy level slightly below that of conduction
band but higher than that of the valence band.
• The number of trapped electrons per unit area is proportional to intensity
X-rays incident at each location. These trapped electrons constitute the
latent image.
• Due to the thermal motion the trapped electrons will slowly be liberated
from the traps, and so at room temperature the image should, however, be
readable up to 8 hours after exposure

14. IMAGE SCANNING
Plate is scanned by the laser causing the emission of blue light from the plate which is then
detected by the photomultiplier. Plate is erased with light (not laser) and returned to the cassette

15. PHENOMENON OF LASER LIGHT
The cassette is placed in the reader where the Image Plate is extracted and raster scanned
with a highly focused and intense laser light of low energy (~2 eV).
Laser light is absorbed at the F-center (Farbzentren center) and, thus, stimulates the
trapped electrons up to conduction band where they are free to move to Europium atom
thereby leaving high energy conduction band to lower energy valence band.
When these electrons become reabsorbed by trivalent Europium, trivalent Europium is
transferred back into divalent Europium atom. This involves the liberation of high energy
(~3 eV) and this is done by emission of green light. Since, there is a larger energy
difference between conduction band, valence band and F- center, the green light emitted
has a higher energy than the laser light needed to stimulate the trapped electrons.

16. The difference in wavelength between the two light is critical for the detection of emitted light.
Incident x-rays form an “electron” latent image in a metastable “F” center site that can be stimulated
with a low energy laser beam (~2.0 eV), producing the desired luminescent signals (~3.0 eV).

17. PHOTOMULTIPLIER TUBE
• A photomultiplier tube (PMT) useful for light detection of very weak signals, is a
photo emissive device in which the absorption of a photon results in the emission of
an electron.
• These detectors work by amplifying the electrons generated by a photocathode
exposed to a photon flux.

18. ERASING PROCESS
• Residual latent image information is erased by using an intense light
(consisting of wavelengths that remove electrons in traps without
stimulating further electron trapping), and the image plate is reinserted into
the cassette for reuse.
• Exposure to a bright fluorescent light removes the remaining information in
10-15 sec.
• Ghosting - if the residual latent image remained so it should be removed by
flooding with intense white light.

20. ADVANTAGES
• CR technology can be considered as the digital replacement of conventional X-ray
film.
• Post-processing, manipulation & storage of images is easy.
• Repeat examinations are reduced due to wide exposure latitude.
• Image Plate is reusable.
• Ability to produce consistent high quality images.
• Ability to deliver images quickly to those who need to make critical decisions.

21. • Decreased time to acquire images thereby increasing patient throughput in
medical diagnosis.
• Increased Radiologist reading capacity and the ability to have instant
comparison images along with previous reports.
• Ability to print on paper or make CD media instead of film, economically
profitable.
• Increased savings: no film, chemicals, dark room and storage room
required
• Computer processing of raw image brightness, contrast, sharpness
enhancement, zooming, measurements…
• Reduction in exposure and processing time
• Software based evaluation and reporting
• Digital magnification allows better detail viewing
• Special analysis tools: e.g. for automated wall thickness measurement
• Global data exchange via a local network and/or the Internet

22. DISADVANTAGES
• Initial cost
• Additional cost (Service and maintenance)
• Plate is sensitive to fogging: use grid, need to be erased daily
• Dose-creep: since, exposure latitude is wide, high exposure technique
is used which increases the patient dose which is called dose-creep.