Image receptors/ dental crown & bridge courses

Image receptors
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Any medium or device by which the various levels of energy of radiation
can be captured, which after suitable processing or manipulation would
create a manifest image is called an image receptor.
An image receptor may be a radiographic film and cassette, a
phosphorescent screen.

• The X-ray film is the image receptor system used in dental radiology.
• The image reception is modified by the composition of the film and
also the use of intensifying screens and grids.
• The x-ray films, helps to record the information regarding the object
through which x-rays pass and hence they help in diagnosis and
treatment.

Types of image receptors in conventional radiography-
Direct-action or non-screen film( wrapped or packet film)-
sensitive primarily to X-ray photons.
Indirect-action or screen film, it is used in combination with
intensifying screens in a cassette.
This type of film is sensitive primarily to light photons, which are
emitted by the adjacent intensifying screens

Composition of intraoral x-ray film-
Base-
o transparent supporting material upon which emulsion is coated.
o Polyethylene terephthalate(polyester)0.2mm thickness.
2.substratum(subcoating)-
 Thin adhesive material on both sides of base
 It ensures good adhesion b/n sensitive emulsion and film base.
3.emulsion-
o Coated on both sides of base.
o Silver bromide crystals (silver iodide)suspended in gelatin

Silver halide crystals-
o 99%of AgBr and 1% of AgI.
o Diameter of AgBr crystals- 0.7-0.75mm
o Ag I-adds sensitivity of film emulsion, there by reducing
radiation dose.
o Photosensitivity is further increased by incorporation of sulphur
contamination during manufacturing.

Gelatin matrix-
o Supports the Ag halide crystals.
o It is made from cattle bone and helps to keep Ag Br crystals
evenly dispersed.
Super coat-
o Protective, transparent , nonabrasive layer over emulsion.
o This is an additional layer of gelatin acts as protective barrier.

Lead foil
sheet
Paper film
wrapper
X ray film
Inner contents of xray film packet
Label side

To increase sensitivity to x-ray radiation iodide, sulfur containing
compounds and some trace elements like gold are added. sulphur
compounds create the sensitivity sites that contribute latent image
formation.
The function of these latent image sites is to begin the process of
image formation by trapping the electrons generated when the
emulsion is irradiated

A silver bromide crystal in the emulsion of an x-ray film contains
mostly silver and bromide ions with small amounts of iodide ions in a
crystal lattice. Free interstitial silver ions and areas of trace chemicals
serve as sensitivity sites.

When the silver halide crystals are irradiated by X-ray photons
release of electrons usually by the bromide ions bromine atoms
Then moves in the crystal and strikes the image site, imparting a negative charge,
Free positively charged interstitial silver ions are attracted to the negative latent
image site and neutralize the image site with the result that an atom of metallic
silver is deposited at the site.
After exposure of a film to radiation, the aggregate of silver atoms at the latent
image sites, comprises the latent image
Latent image formation-

Image receptors in extra-oral radiography:
In extraoral radiography 2 types of image receptors are used.
Those that are placed outside the mouth during x-ray exposure.
Two types-
o Non-screen films
o Screen films

Non-screen films-
Used without intensifying screens
Emulsion is sensitive to direct exposure rather than fluorescent light.
Disadvantages-
Much slower
Long exposure time
But images are much sharper

Screen films-
• Used in combination with intensifying screens, that emit visible light.
• Screen film is different from dental intraoral film in that it is designed
to be particularly sensitive to visible light rather than to x-radiation
because this film is placed between two intensifying screens when an
exposure is made.
• The intensifying screens absorb x-rays and emit visible light, which
exposes the screen film.
• An intensifying screen is a smooth plastic sheet coated with minute
fluorescent crystals known as phosphors
• When exposed to x-rays the phosphors fluoresce and emit x-rays visible
light in the blue or green spectrum

Dif b/n intraoral and extra oral film-
The silver halide emulsion is designed to be sensitive primarily to
light rather than X-rays.
• Different emulsions are manufactured which are sensitive to the
different colours of light emitted by different types of intensifying
screens.
— Standard silver halide emulsion sensitive to BLUE light
— Modified silver halide emulsion with ultraviolet sensitizers
sensitive to ULTRAVIOLET light
— Orthochromatic emulsion sensitive to GREEN light
— Panchromatic emulsion sensitive to RED light

Digital Imaging-
1.Direct Digital Imaging
2. Indirect Digital Imaging
3. Storage Phosphor Imaging

Image receptors in digital radiography-
a) Solid state detectors (digital radiography)
1. Charged couple device
2. Complementary metaloxide conductor
3. Flat panel conductor
b) Photostimuable phosphor technology (computed radiography)

Charged couple device-
Introduced to dentistry in 1987
Thin wafer of silicon chips
CCDs are more sensitive to light than x-rays
• Scintillator – coated directly CCD or coupled to surface by fibre optics
Scintillator increases the x-ray absorption efficiency
• Scintillators
– Gadolinium Oxybromide
– Cesium Iodide
• In case of extra oral digital imaging
– CCD is fixed in position opposite to x-ray source with long axis of
CCD parallel to the fan shaped x-ray beam

Exposure to radiation
Breakage of covalent bonds between silicon crystals
Formation of electron hole pairs
Electrons are attracted towards most positive potential
Formation of charge packets
Each packet corresponds to one pixel
Charge pattern represent the latent imagc

Charged Couple Device (CCD)

Basis for typical consumer grade video cameras
Silicon based semiconductor
Each pixel is isolated from neighbouring pixel and is directly
connected to transistor
The charge is generated after the x-ray exposure is transferred to
the transistor as a small voltage

Flat panel detectors-
Flat panels are being used for medical imaging but also been used
in several extra oral imaging devices
Larger matrix areas with pixel size less than 100µm
Allows direct digital imaging of larger areas of body
Two methods
Indirect using intensifying screens
Direct using photoconducting material (selenium,gadolinium,cesium )
Used in cone beam imaging

Phosphostimuable phosphor imaging-
PSP plates absorb and store energy from xrays and then release this
energy as light (phosphorescence) when stimuated by another
light of an appropriate wavelength
The phosphorscence is quantified as a measure of the amount of xray
energy that the material has absorbed
Consists of reusable imaging plate coated with phosphor instead
of sensor
Flexible, fits into the mouth as a conventional film

Phosphor material
Europium-doped Barium halide (fluoride/iodide/chloride)
When exposed, valence electron in europium will absorb energy
Move into conduction band
Migration into halogen vacancies in fluorohalide crystals
Trapping in metastable state

Stimulation with red light of 600nm
Release of trapped electron by flourohalide crystals
Return of electrones to europium
Release of energy in green spectrum between 300-500 nm
Conduction of light by fibre optics
From psp to photomultiplier tube
Conversion of light into electrical energy

Mechanism-Exposure to x-ray source
Records diagnostic data on plates
Plate removed from mouth
Placed into an electronic processor
Laser scans the plate
Produces an image
Image transferred to computer

Flat pannel detectors

Computed tomography-
• The word "tomography" is derived from the Greek tomos (slice)
and graphein (to write)
• Linear tomography is a process by which an image layer of the
body is produced while structures above and below the image
layer are made invisible by blurring.

o The detector system situated opposite the X-ray tube.
o It converts the incident x-ray photos of varying intensity to electric
signals.
o These analog signals are amplified by downstream electronic
components and converted to digital pulses.
o The purpose of the detectors is to collect the data transmitted from
the patient and transfer to the computer

Mechanism-
X-ray tube consists of x-ray tube
emits a finely collimated, fan-shaped x-ray beam
Directed to series of scintillation crystals or ionization chambers
These signals are used for further reconstruction of images by
computer

Detectors are also classified as-
1. Scintilization detectors –
E.g.-sodium iodide and caesium iodide
2. Gas ionization-
Xenon gas may be pressurized to between 20-30 atm

• Scintillation detectors utilize a crystal that fluoresces when struck
by an x-ray photon which produces light energy.
• A photodiode I attached to the scintillation portion of the detector.
The photodiode transforms the light energy into electrical or
analog energy.
• The strength of the detector signal is proportional to the number
of attenuated photons that are successfully converted to light
energy and then to an electrical or analog signal.
• The most frequently used scintillation crystals are made of
Bismuth Germinate (Bi4Ge3012) and Cadmium Tungstate
(CdWO4). Earlier designs utilized Sodium and Cesium Iodide as
the light producing agent.

 long thin tungsten plates act as electron collection plates.
When attenuated photons interact with the charged plates and the
xenon gasionization occurs.
 The ionization of ions produces an electrical current.
 Xenon gas is the element of choice because of it's ability to remain
stable under extreme amounts of pressure.
 Utilizing more gas in a detector increases the number of molecules
that can be ionized therefore, the strength of the detector signal or
response is increased.
 The xenon gas detectors are generally fixed with the position of
the x-ray tube which occurs with 3rd generation scanner geometry
designs.

Cone beam computed tomography
• CBCT introduces a more complex and accurate imaging with 3 –
dimensional visualization as compared to routinely used analog or digital
radiographs.
Principle-
o Imaging is accomplished by using a rotating gantry to which an x-ray source
and detector are fixed.
o A divergent pyramidal- or cone-shaped source of ionizing radiation is
directed through the middle of the area of interest onto an area x-ray
detector on the opposite side.
o During the rotation, multiple (from 150 to more than 600) sequential planar
projection images of the field of view (FOV) are acquired in a complete, or
sometimes partial, arc.

Because CBCT exposure incorporates the entire FOV, only one
rotational sequence of the gantry is necessary to acquire enough data
for image reconstruction.
Detectors –
Detectors were initially produced using a configuration of scintillation
screens, image intensifiers, and charge-coupled device (CCD) detectors.
More recently, high-resolution, inexpensive flat-panel detectors have
become available. Such flat detectors are composed of a large-area pixel
array of hydrogenated amorphous silicon thin-film transistors.

X rays are detected indirectly by means of a scintillator, such as
terbium-activated gadolinium oxysulphide or thallium-doped
cesium iodide, which converts X rays into visible light that is
subsequently registered in the photo diode array.
The configuration of such detectors is less complicated and offers
greater dynamic range and reduced peripheral distortion; however,
these detectors require a slightly greater radiation exposure

Magnetic resonance imaging-
It is a non-invasive method of mapping the internal structure and
certain aspects of function within the body using non-ionizing
electromagnetic radiation from the radiofrequency (RF) band of
the electromagnetic spectrum.

Radiofrequency coils-
They are used for transmitting & receiving signals at the resonant
frequency of protons within the patient.
Currently, whole body scanners use 0.02 – 4 T field strength & 0.85 –
170.3 MHz., frequency.
For commonly used 1.5 T scanners, the operating frequency is 63.86
MHz.
According to function types of RF coils-
1. Transmit receive coil
2. Receive only coil
3. Transmit only coil
4. Multiply tuned coil

Magnetic resonance (MR) is based upon the interaction between
an applied magnetic field and a nucleus that possesses spin.
Nuclear spin is one of several intrinsic properties of an atom and
its value depends on the precise atomic composition.
1H nucleus, consisting of a single proton, is a natural choice for
probing the body.
It has a spin of ½ and is the most abundant isotope for hydrogen.
1H MRI in vivo achieves excellent contrast between different
tissues.
Human body is composed of tissues that contain primarily water
and fat, both of which contain hydrogen

The concept of MRI is based upon the interaction between an
external magnetic field and nuclei which have a non-zero magnetic
moment.
For the hydrogen nucleons which consist of a solitary, unpaired
proton acts as a magnetic dipole. These magnetic dipoles, in the
absence of external influence, are randomly oriented and as such
have zero net Magnetization.

When an external magnetic field is applied to this sample, all the
hydrogen nuclear axes align either in the direction of the field i.e.,
which parallel the external magnetic field – spin up, or align anti-
parallel (opposite) with the magnetic field, spin down.
These orientations correspond to lower energy state and highly
energy states of the dipole respectively.
The combined effect of these two energy states is a weak net
magnetic moment, or magnetization vector (MV)

Nuclei can be made to undergo transition from one energy state to
another by absorbing or releasing certain quantity of energy.
This energy can be supplied or recovered in the form of
electromagnetic energy in RF portion of the electromagnetic
spectrum and this transition from one energy level to another is
called resonance.

• When an external magnetic field is applied, their N and S poles do
not align exactly with the direction of the magnetic field.
• The axes of spinning protons oscillate or wobble with a slight tilt
from a position which was parallel with the flux of external
magnet. This tilting or wobbling is called precession.

The rate or frequency of precession is called the Resonant or Larmor
frequency, which is proportional to the strength of the applied
magnetic field.
Larmor frequency of hydrogen is 42.58 MHZ in a magnetic field of 1
Tesla, where one Tesla is 10,000 times the earth’s magnetic field.
The magnetic field strengths used for MR imaging range from 0.1
to 4.0T

The motion of each proton can be described by a unique set of
coordinates perpendicular (x and y) and parallel (z) to external
magnetic field.
The perpendicular, or transverse, coordinates are nonzero and
vary with time as the proton precesses, but the z coordinate is constant
with time
For convention, external magnetic field and the axis of precession are
defined to be oriented in the z direction of a Cartesian coordinate
system. (This convention is not universally followed, but it is the
prevailing convention.)

• When RF from electromagnetic spectrum is applied Some of the
low energy nuclei gain energy to convert to high energy & result
in transverse magnetic vector

When hydrogen nuclei are subjected to the flux of an external magnetic field, two energy
states result: spin-up, which is in the direction of the field, and spin-down, which is in the
opposite direction of the field. The combined effect of these two energy states is a weak net
magnetic moment, or magnetization vector (Mv), parallel with the applied magnetic field.

When energy in the form of an electromagnetic wave in the radiofrequency (RF) range from
an RF antenna coil is directed to tissue with hydrogen nuclei that are aligned in the Z axis
by an external static magnetic field, the protons in the tissue that have a Larmor frequency
matching that of the electromagnetic wave absorb energy and shift or rotate away from the
direction induced by the imaging magnet.

MR signal
The precession of the magnetic moments of hydrogen nuclei in phase
in the transverse plane, induces a current flow in the receiver coil,
called MR signal.
Strength of the signal depends on the degree to which hydrogen is
bound within the molecule.
Loosely bound atoms such as in soft tissues & liquids react to RF
pulse & detect the signal.
After MR signal further process of image formation can be carried
out.sssssss

Ultrasound –
• A procedure in which high-energy sound waves are bounced of
internal tissues or organs and make echoes.
• The echo patterns are shown on the screen of an ultrasound
machine, forming a picture of body tissues called a sonogram

Ultrasound machine-
• Basically the ultrasound machine has a
pulse generator,
transducer and
an oscilloscope.
• As the ultrasonic beam passes through or interacts with tissues of
different acoustic impedance, it is attenuated by a combination of
absorption, reflection, refraction, and diffusion.

Ultrasound device prototype, probe shaped as a dental hand
piece and PZT transducer
Ultrasound device prototype, probe shaped as a dental hand
piece and PZT transducer

How it Works
• The ultrasound image is created by first transmitting sound waves
into the body and then interpreting the intensity of the reflected
echoes.
• This is achieved using a hand held probe which contacts the body
via a water based gel.
• The data collected is then processed within the body of the
scanner and displayed as a black and white image generally
referred to as grey scale.

Principle-
• Scanners used for sonography generate electrical impulses that are
converted into ultra-high frequency sound waves by a transducer, a
device that can convert one form of energy into another- in this case,
electrical energy into sonic energy
• The most important component of the transducer is a thin
piezoelectric crystal or material made up of a great number of dipoles
arranged in a geometric pattern.

• Currently, the most widely used piezoelectric material is
lead zirconate titanate(PZT).
Other materials used as piezoelectric materials are-
barium titanate (BaTi),
lead titanate (PbTi),
lead metaniobate (PbNb2O6) and
vismut titanate (Bi4Ti3O12).

Types of transducers-
• High frequency (7-15 mHz) (linear) transducer has higher
resolution but poor penetration.
• Low frequency (2-5 mHz) (curved array) transducer has lower
resolution but better penetration

Image construction-
• The probe contains a large number of transmitters set in a line
along its length.
• Typically up to five of these firing simultaneously generate a short
pulse of ultrasound that travels in a narrow column away from the
probe.
• The transmitters then act as receivers and record the intensity of
the reflected sound.

• The detected echoes may be represented in four ways:
A-mode.
B-mode,
M-mode and B-scan mode and
Color Doppler mode

Scintigraphy-
o Scintigraphy ("scint," Latin scintilla, spark) is a form of diagnostic
test used in nuclear medicine .
o Where in radioisotopes (radiopharmaceuticals) are taken
internally, and the emitted radiation is captured by external
detectors to form two-dimensional images.

Principle-
radiopharmaceutical has been administered, be distributed
throughout the body
The patient will now be emitting radiation in the form of gamma
rays (γ-rays), which escape from the body and permit external
detection and measurement.
gamma (or scintillation) camera, is used to detect the distribution of
the radioactivity within the patient’s body

collimator
Gamma rays that pass through collimator strikes the scintillation crystal
Crystal is made up of a sodium iodide (thallium trace amounts), fluoresces
when it absorb gamma rays.
These flashes of light are detected by photomultiplier tubes coupled to the
crystal with light pipes.
These tubes capture the flash and amplifies the signal.
The signals from tubes go through an analog to digital converter then pulse
height analyzer final image

Conclusion-
Type of radiograph Image receptors used
Conventional intra-oral and
extra oral radiography
X-ray film primarily it is made up
of a Ag halide crystals
Digital radiography CCD, Flat pannel detector,
complementary metal oxide
semiconductor.
CT Ionization chamber, xenon gas
detector
CBCT CCD, FPD
MRI Radiofrequency coil
USG Piezoelectric crystal
Scintigraphy Scintilization crystal (made up of
Sodium Iodide traces of thalium
element)

References
1. STUART C. WHITE , MICHAEL J.PHAROAH – ORAL
RADIOLOGY principles and interpretation
2. ERIC WHAITES- Essentials of dental radiography and
radiology , 3rd edition
3. Freny karjodkar, oral and maxillofacial radiology, 2nd edition

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