Xeroradiography is the production of visible image utilizing the charged surface of a photoconductor (amorphous selenium) as the detecting medium, partially dissipating the charge by exposure to X rays to form a latent image and making the latent image visible by xerographic processing.
2. āŖ Production of visible image
āŖ amorphous selenium-detecting medium
āŖ dissipating charge by X rays
āŖ Latent image visible by xerographic
processing.
2
7. PHOTOCONDUCTIVE LAYER
Three properties required:
1.good insulator
2.material must become electrically conducting
3.mechanical properties of durability & ease of
fabrication
7
10. ALUMINIUM SUBSTRATE
āŖ meticulously
cleaned with
exceedingly
smooth surface
10
INTERFACE LAYER
Heat treatment
of aluminum
substrate
thin layer of
aluminum oxide
insulator
SELENIUM COATING
amorphous
(vitreous form)
deposited onto
the aluminum
substrate
11. PROTECTIVE OVERCOATING
āŖ Cellulose acetate
āŖ 0.1Āµ thick layer
āŖ bonds well with selenium
āŖ high resistance prevent lateral conduction of
charges
āŖ It extends the life
11
12. PRINCIPLE
xray plate is charged by a corotron
Placed in a cassette
xray strikes the selenium
photoconduction occurs
charge image
12
13. Image is visible by adding toner/ developer
Resultant powder image is transferred to a paper and
fused
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18. EXPOSURE OF CHARGED PLATE
Exposure to Xray
electron- hole pairs
electrons migrate to plate surface and
discharge the positive charge originally
laid down
18
19. +ve holes migrate toward the substrate
neutralized by the induced negative charge
amt of discharge of +ve charge is ā to
intensity of Xrays
The remaining charge pattern on plate
surface is electrostatic latent image
19
20. Photoconductivity induced by Xrays vs light
āŖ Xray photons can
penetrate into
selenium
āŖ Absorption -
uniform
āŖ energy of Xray
photon is
transferred to a
photoelectron or
recoil electron
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22. 22
XERORADIOGRAPHIC UNDERCUTTING
āŖ D/t ionisation of air space btw selenium
surface & lid of cassette
āŖ Caused d/t interaction air molecules or
High speed electrons.
26. LIQUID DEVELOPMENT
āŖ Liquid toner particles are black
āŖ particle size 1.7 Āµ
āŖ Smaller particle + less charge on each
particle = more toner particles deposited
per image
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29. IMAGE TRANSFER AND FIXING
āŖ Image is transferred to paper & fixed
āŖ electrostatic transfer process used
āŖ paper coated with slightly deformable
layer of plastic
āŖ paper Pushed against the powder image
under high pressure
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30. PLATE CLEANING
āŖ All toner must be removed
āŖ plate is exposed to a light source
āŖ corotron exposes the plate to an
alternating current
āŖ cleaning brush then mechanically brushes
the residual toner
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31. RELAXATION
āŖ If the plate charged for new exposure
without rest, ghost image of previous
exposure will appear on development( long
as several hours)
āŖ rest period can be reduced to 2-3 min
If heated to 140ĖF for 150 sec.
31
32. STORAGE
The cleaned and relaxed plate is then held
in the storage compartment at 89Ė F until
needed for another exposure
32
33. ADVANTAGES:
āŖ High contrast
āŖ 1/3rd the exposure
āŖ Easy to use
āŖ Produces permanent dry images
āŖ Image is easily reversible
āŖ Useful in endodontics
33
34. DISADVANTAGES:
āŖ Edge enhancement artefacts.
āŖ Processors are expensive.
āŖ High radiation dose needed for extra oral
techniques.
āŖ 2.4-16.2 times larger than conventional
34
40. CONCLUSION:
xeroradiography has peculiar positive
properties which enable both detailed bony
and soft tissue examination, as well as
subtle visualisation of structures.
40
Xeroradiography is the production of visible image utilizing the charged surface of a photoconductor (amorphous selenium) as the detecting medium, partially dissipating the charge by exposure to X rays to form a latent image and making the latent image visible by xerographic processing.
The imaging method was discovered by an American
physicist, Chester Carlson in 1937. Later, the Xerox Company followed with laboratory
investigations of the technique and its potential applications in medical sciences. Others like Binnie et al, Grant et al and White et al worked on phantoms and cadavers, using the 125 system. Their
works led to the development and marketing of intraoral xeroradiographic system. Xeroradiography
may be new in dentistry, but in medicine, it had long been used in the diagnosis of breast
diseases, imaging of the larynx, and respiratory tract for foreign bodies, temporomandibular joint,
skull, and Para osseous soft tissues.
The image receptor plates are size no 1 and 2 films and fit well in oral cavity
Over the past 40 years, 125 system became applicable in medical sciences. A prototype
Xeroradiographic imaging system specific for intraoral use was later developed. Following these
clinical trials showed that xeroradiography is superior for imaging of dental structures necessary for
successful periodontal and endodontic therapy. Xeroradiographic radiation of 90% of more than that
for silver halide radiograph has been reported, while others found that Xeroradiographic radiation is
one third to half of that for halide radiograph.
The valance band is the highest energy band in which electrons normally exist at a very low temperature.
The next highest permissible band is called the conduction band.
These two bands are separated by a forbidden gap.
Solids that have many electrons in the conduction level are known as conductors. Conductors do not have forbidden region.
In insulators the forbidden gap is so large that electrons seldom absorb enough energy to bridge the gap.
A semiconductor is a solid that contains a small forbidden gap. Addition of appropriate quantity of energy will allow some electrons to bridge the forbidden gap and enter the conduction band.
In photoconductor, this energy is provided by absorption of visible electromagnetic radiation (light).
Photoconductors are a special type of semiconductors.
In Xeroradiography, the photoconductor used is selenium.
Three properties required by the photoconductor to be used in Xeroradiography are.
The electrical conductivity in the dark must be that of a good insulator so that a charge pattern present on the surface will be retained long enough to complete the steps of development.
The material must become electrically conducting during exposure to X rays so that an electrostatic image can be formed on its surface by the exposure .
It must have mechanical properties of durability and ease of fabrication
It is a sheet of aluminum approximately 9 Ā½ x 14 inches in size on which a layer of amorphous selenium has been deposited. There is an interface layer between the selenium and the aluminum and an over coating protecting the selenium layer
The aluminum plate on which selenium is deposited is made up of meticulously cleaned aluminum with an exceedingly smooth surface
Surface defects affect the Xeroradiographic plateās sensitivity by giving rise to changes in the electrostatic charge in the photoconductor.
Heat treatment of aluminum substrate serves to form a thin layer of aluminum oxide.
Although aluminum is a conductor, aluminum oxide is an insulator.
The purpose of interface is to prevent negative charges induced in the aluminum from migrating into selenium and dissipating the positive charge induced on the selenium surface.
Highly purified selenium in the amorphous (vitreous form) is deposited onto the aluminum substrate by condensation of vaporized liquid selenium in a high vacuum.
The thickness of selenium layer is 150Āµ for powder toner development while it is 320Āµ for liquid toner development plates.
Cellulose acetate is used . It is applied by dip- coating from solution to form about 0.1Āµ thick layer.
It bonds well with selenium.
It has high resistance enough to prevent lateral conduction of charges, which would degrade the electrostatic latent image.
It extends the life of Xeroradiographic plate by a factor of about 10.
xray plate is charged to a high positive potential by a corotron
Placed in a cassette just like conventional film
When xray strikes the selenium, photoconduction occurs and this produces a charge image of the part examined
Image is made visible by adding toner/ developer
Resultant powder image is transferred to a paper and fused
Conventional X-ray source is used in the production of xeroradiographs. The film, however, is
replaced by a selenium-coated photoreceptor (Xerox Plate), which has a uniformly distributed
electrostatic charge.
1.The first step in the Xeroradiographic process is to sensitise the photoconductor by applying a uniform electrostatic charge to its surface in the dark.
Because selenium is an insulator in the dark, during charging, the xeroradiograhic plate can be considered to be a parallel plate capacitor in which the outer layer of selenium surface and the aluminum backing (substrate) acts as parallel plates and the selenium itself acts as dielectric.
A capacitor or condenser consists of two sheets of conducting material with a sheet of insulating material between them . One plate is charged positive and the other is negative. So a potential difference or voltage exists between the two plates. The insulator material in between is known as dielectric
If a positive charge is deposited on the surface of selenium, the remainder of the selenium atoms are distorted into the configuration of induced dipoles. The negative pole of each atom are attracted towards the positive charge, causing the atomic layer adjacent to the aluminum substrate to present a positive charge at the selenium- aluminum interface.
Polarization of the selenium atoms will attract negative charges in the aluminum substrate towards the back of selenium layer. In other words negative charge has been induced in the aluminum substrate by the presence of positive charge on the surface of selenium.
The interface layer reduces the negative charge leakage from the substrate into the selenium.
The device used to produce a charge on the surface of the selenium operates on the principle of corona discharge.
If a sufficiently high potential difference called corona threshold voltage is applied between a fine wire and ground, the air near the wire becomes ionised.
If the voltage is positive, the free electrons of the gas near the wire will move towards the wire. These electrons will be of high energy and instead of going straight to the wire, they will interact with many molecules or atoms in the air and create many additional ions.
The positive ions thus created will move away from the wire.
This movement of ions is called corona current.
When such a wire is placed near the surface of Xeroradiographic plate, some positive ions repelled from the wire will be deposited on the surface of the plate.
After the plate is sensitized by corona charging it must be enclosed in a cassette that is light tight and rigid enough to provide mechanical protection for the fragile plate.
Exposure to Xray.
electron- hole pairs.
electrons migrate to plate surface and discharge the positive charge originally laid down.
he positive holes migrate through the selenium toward the substrate, where they are neutralized by the induced negative charge.
The amount of discharge of the positive charge is proportional to the intensity of Xrays that penetrate the patient.
The remaining charge pattern on the plate surface is called electrostatic latent image.
Photoconductivity induced by Xrays and that by light differ in two important ways,.
1. The Xray photons can penetrate further into the selenium and their absorption may be uniform throughout the selenium layer.
2. The energy of Xray photon is transferred to a photoelectron or a recoil electron and each of these electrons will usually have enough kinetic energy to produce many more charged carriers (electron- hole pairs) along its tract.
Caused by ionisation of the air in the space between the selenium surface and the lid of the cassette.
Ionisation may be caused by interaction of Xrays directly with air molecules or it may be caused by high speed electrons being able to escape from the selenium and enter the air space.
To avoid this, a DC voltage is applied between the cassette lid and the aluminum backing. If the cassette lid is made positive, negative air ions can be attracted away from the plate surface.
Development consists of attracting small charged particles called toner to the electrostatic latent image.
The toner used is a pigmented thermoplastic material of average diameter 4 Āµ.
The powder cloud development or the aerosol development is fast, simple and it provides edge enhancement.
The exposed plate is placed on top of a dark box into which an aerosol of charged toner particles is sprayed through a nozzle.
Aerosol is created when a pressurized gas ( usually nitrogen) is used to force toner through small bore tube. An aerosol is a suspension of small liquid or solid particles in a gaseous medium.
involves attracting negatively charged toner particles to the remaining positive charge on the surface of selenium.
To get positive development, positive voltage is applied to the aluminum backing to attract only negatively charged particles. With positive development, unexposed or minimally exposed areas of the plate will be dark blue. ( calcification will appear as blue).
Liquid toner particles are black. The particle size of is 1.7 Āµ (blue powder toner ā 4Āµ). Also they have smaller charge per particle ( 150 elementary charge per particle, while in powder toner it is 1000 elementary charge per particle). Smaller particle plus less charge on each particle results in more toner particles being deposited per image on the imaging plate. Cancelling all the charge on the plate may mean that toner particles have to be stacked several layers deep on the plate. Toner particles stacked on top of each other will increase optical density in the final image recorded.
darkest area corresponds to most dense parts of anatomy
darkest area corresponds to least dense part of anatomy and dense parts appear white
The positive charge along the surface of selenium plate and the negative electrons I the base plate set up an electricstratic field. At the margins of the field the lines are not perpendicular. The fringing electrostatic field at 5he margins causes attraction of other toner particles at these interface. This results in enhancement of the edges and increased delineation of the margins
The image on the surface of Xeroradiographic plate is then transferred to paper and fixed to form a permanent image. A electrostatic transfer process is used. The paper is coated with a slightly deformable layer of plastic, such as a low molecular weight polyethylene material. When this paper is pushed against the powder image under relatively high pressure, the toner particles get embedded into it.
After the image is transferred, the paper is peeled off the plate and the loosely held powder image is made into a permanent bonded image by heating the paper to about 475Ė F. The heat softens the plastic coating on the paper and allows the toner particles to sink into and become bonded to the plastic.
All toner must be removed before the plate is to be reused . The plate is exposed to a light source (electroluminescent strip) that reduces the bond holding the residual toner to the plate. A preclean corotron exposes the plate to an alternating current that serves to neutralize the electrostatic forces holding toner to the plate. A cleaning brush then mechanically brushes the residual toner from the plate.
To prevent faint ghost images. Absorption of X-rays in the selenium produces an alteration in the physical property of selenium that causes some photoconductivity to persist for as long as several hours after exposure. If the plate were charged for a new exposure without allowing it to rest, a ghost image of the previous exposure will appear on development. This rest period can be reduced to 2-3 min if the plate is relaxed by heating it to 140ĖF for 150 sec.
STORAGE
The cleaned and relaxed plate is then held in the storage compartment at 89Ė F until needed for another exposure.
1. High contrast.
3. They require 1/3rd the exposure of conventional radiographs.
4. Easy to use.
5. Produces permanent dry images for viewing in about 20 seconds after the exposed
photoreceptor is returned to the machine.
6. Image is easily reversible.
7. For diagnostic purpose, as this method gives excellent definition of thin, fine structures due to edge enhancement,
b. Caries seen more readily.
c. Useful in endodontics.
d. Detection of cancer.
e. Imaging biomaterials.[
DISADVANTAGES:
1. Edge enhancement artefacts.
2. Processors are expensive.
3. High radiation dose needed for extra oral techniques. It was 2.4-16.2 times larger than
conventional film techniques
USES:
1. Mammography.
2. Cephalometry.
3. Evaluation of bone lesions in the mandible.
4. Sialography.
5. Study of variety of dental and non-dental structures.
6. TM joint tomography
The radius and ulna are dislocated posteriorly.
Note also a fracture of the radius. The relationship of the bones can be 3 udged a little better on the antero- posterior XR than on the usual film
(A and B) Metastasis to femur from bronchogenic carcinoma. Note that the ālucencyā of the de-
structive lesion is more apparent on the film image (A), whereas more of the over and underlying details
ofthe abnormal area arc seen on the XR (B).
(A and B) Metastasis to bones of pelvis from carcinoma of breast. The abnormally dense areas
representing the neoplastic disease are usually recorded in good local contrast Ofl the XR (B).
(A and B) Osteogenic sarcoma. The large sarcoma is evident on both modes. The XR ii. have more information in the deeper structures which cannot be seen quite as well even on multiple conventional roentgenographic film images.
