Radiation Physics

Matter may be described as the substances of which all physical things are composed; it is anything that occupies space.

Matter may be divided into:

  • Elements: Elemental matter is made up of accumulations of single species of atoms.
  • Compounds: Compounds are composed of recurring units of a specific number of atoms in a definite arrangement and at least two of the atoms are different.

Fundamental unit of any particular element.

The distinguishing number of proton (positive charges) in the nucleus of an atom is referred to as its atomic number and is designated by symbol Z.

The total number of protons and neutrons in an atom’s nucleus is called its atomic mass number, designated by symbol A.

  • Ionization is the process by which a neutral atom or molecule acquires either positive or negative charge.
  • When atom loses or gains an electron it is said to be ionized.

Two types:

  • Corpuscular or particulate radiation
  • Electromagnetic radiation
  • These are minute particles of matter that travel in straight lines at high speeds from their sources.
  • They are small but possess mass.
  • All are charged except neutrons.
  • They all move extremely fast.
  • Alpha rays
  • Beta rays.
  • Cathode rays.

The rate of loss of energy from a particle as it moves along its track through the irradiated material (tissue) is called as its linear energy transfer (LET).

  • It is movement of energy through space as combination of electric magnetic fields.
  • They are generated when velocity of electrically charged particle is altered.
  • Gamma rays.
  • X rays.
  • UV rays.
  • Visible light.
  • Infrared rays.
  • Television.
  • Radar.
  • Microwaves.
  • Radio waves.
  • Wave theory of electromagnetic radiation is based on conjecture that radiation is propagated in the form of waves.
  • The electrical and magnetic fields are in planes at right angles to one another and are oscillating perpendicular to direction of motion, that they move forward in much the same way as ripple moves over surface of water.
  • All electromagnetic waves travel at velocity of light (3.0X108 m/s).

C = λv

Where

C = Velocity of light

λ= Wavelength in meter.

v=Velocity in hertz.

  • It assumes that transfer of energy by EM radiation is not in form of waves, but as a flux of quanta of photons (finite bundles of energy).
  • Each photon travel with speed of light and has specific amount of energy.
  • The unit of photon energy is electron volt.

E= hc/λ

E is energy in kilo electron volt.

h is plancks constant(6.61 x10-34joule/sec).

c is velocity of light.

λ is wavelength in angstroms.

X rays are weightless packages of pure energy (photons) that have low electrical charge and travel in waves along a straight line with specific frequency at speed of 3.0×108m/s.

  • X rays are invisible and weightless.
  • X rays travel in straight line.
  • X rays travel at speed of light.
  • X rays have wide range of wavelengths, 0.01 to 0.05nm in length.
  • X rays cannot be focus to a point, over distance the beam diverges much like a beam of light.
  • Because of their extremely short wavelength, X rays are able to penetrate materials that absorbs or reflect visible light.
  • X rays are differentially absorbed by matter that depends upon atomic structure of matter and wavelength of x rays (this property produce image on photogenic film).
  • X rays cause certain substances to fluoresce to emit radiation of longer wavelength (visible and ultraviolet light).
  • X rays produce biological charges.
  • X rays can ionize gases (remove electron from atom to form ions).

It is basic apparatus for x ray production

Dental x-ray machine made up of:

  • Control panel.
  • Extension arm.
  • Tube head.
  • On and off switch and indicator light.
  • Exposure button and indicator light.
  • Control devices to regulate the x-ray beam.
  • Control panel is plugged into an electric outlet.
  • It suspends the x-ray tube head and houses the electrical wires that extend from control panel to tube head.
  • It allows the movement and positioning of tube head.
  • Cathode (source of electron).
  • Anode (target).

Anode and cathode encased within evacuated glass envelope or tube. X rays produced when electrons strike the target.

Composed of:

  • Filament.
  • Focusing cup.
  • Source of electron.
  • It is coil of tungsten wire having length of 1 cm or less and 0.2cm in diameter mounted on two stiff wires for support and electric current.
  • Mounting wires lead through the glass envelope and connected to high and low voltage electric sources.

Hot filament emits electrons that separated from outer orbit of tungsten atoms at the rate proportional to its temperature by process called as thermionic emission.

  • Milliampere control provides for fine adjustment of the voltage across the filament and in turn the flow of heating current through it.
  • It also controls the quantity of electron, which in effect controls the tube current.

Filament is located in focusing cup that is negatively charged, concave reflector cup of molybdenum.

It focuses the electron emitted by incandescent filament into a narrow beam directed at small rectangular area on anode called the focal spot.

  • To prevent the collision of moving electron with gas molecule.
  • To prevent oxidation and ‘burnout’ of filament.

It is composed of tungsten target and copper stem.

It converts the kinetic energy of electron from filament into x ray photons.

Due to:

  • High atomic number.
  • High melting point (33700 C).
  • Low vapor pressure at high temperature.

It is a good thermal conductor, dissipates heat from tungsten and copper anode.

  • It effects the sharpness of image.
  • Smaller the focal spot, sharper the x-ray image.
  • The projection of focal spot perpendicular to the electron beams is the effective focal spot.
  • Actual focal spot that is projected perpendicular from the target.
  • The target is inclined at 20 degree with respect to central ray.
  • This cause the effective focal spot about 1×1 mm in contrast to actual focal spot about 1×3 mm.
  • Use of anode with target angulated such that effective focal spot is smaller than actual focal spot size is known as “line focus principle”.
  • Stationary or fixed anode.
  • Rotating anode:
  1. Help to dissipate heat from small focal spot.
  2. Tungsten target is in the form of small bevel disc that rotates when the tube is in Operation.

Functions of power supply are to provide:

  • A current to heat the x-ray tube filament.
  • A potential difference between the anode and cathode.

These functions are accomplished by use of step down transformer and high voltage transformer respectively.

  • Transformer and the x-ray tube lie within electrically grounded metal housing called as head of x-ray machine.
  • An electrical insulating material, usually oil, surrounds the transformers.

Step up transformer:

  • Have more turns in secondary coil than primary coil.
  • In dental x-ray machine step up transformer takes 110 or 220 V current and increase this voltage to 65,000 to 90,000 V (65-90 kVp).
  • This provides high voltage necessary to accelerate cathode electrons toward the anode at high speed.

Step down transformer:  

  • The primary coil of step down transformer has more turns than secondary coils.
  • This decreases the voltage down from 110 or 220 V to 8 to 1 2 V in secondary coil which provides a high current of 3 to 6 A to heat filament.
  • Auto transformer:

The autotransformer varies the voltage input into the primary coil of step-up transformer, which in turn varies the kVp of the machine.

  • Tube voltage.
  • Exposure time.
  • Tube current.
  • Filteration.
  • Collimination.

Increase kVp results in:

  • Increases in the energy of each electron when they strike the target.
  • Increase efficiency of conversion of electron to x-ray photon.
  • Increases in the:
  1. Number of photon generated.
  2. Mean energy of photon.
  3. Maximum energy of photon.
  • Higher kVp produced higher mean energy of x-ray beam and greater penetration.
  • Low kVp produce low energy photon, unable to penetrate to hard tissue to expose the film but absorbed by soft tissues of face.
  • The quality or penetrating power of x-ray beam is controlled by kVp and is measured by its half value layer (HVL).
  • The HVL of x-ray beam is the thickness of absorbing material (aluminum) necessary to reduce the x-ray intensity to one half of its original value.
  • In dentistry, diagnostic x-ray beam has an HVL of 1.5 mm of aluminum for machines 69 kVp.
  • Machine capable of 70 to 90 kVp have HVL of 2.5 mm of aluminum.
  • Exposure time is the interval during which x rays are being produced
  • Increase exposure time, more x-rays produced.
  • The most popular exposure technique is “fixed kVp and mA technique” which varies only the exposure time.
  • The effect of changing time is simply to control the “quantity of exposure”.
  • The quantity of radiation produced by an x-ray tube is directly proportional to the tube current (mA) and the time the tube is operated.
  • As the mA setting increased more power is applied to filament, which heats up and releases more electrons that collide with the target to produce radiation.
  • Thus the quantity of radiation produced is proportional to the product of time and tube current.
  • Both have direct effect on quantity of photon in x ray beam.
  • These two factors (mA, sec) multiplies together so mAs is formed.
  • When impulses are used instead of sec for the exposure time the common factor (mAs) becomes MA impulses (mAi).
  • Milliampere second (mAs) or MA impulses (mAi) determine the total number of x-ray photons but do not indicate the energy of each photon.
  • Filtration is the process to increase the quality of x-ray beam by removing the less penetrating photons.
  • Low penetrating power photons (long wave length) contribute to patient’s exposure but not to the information of film.
  • Inherent filtration.
  • Total filtration.

Imposed by the materials like:

  • Glass wall of the x-ray tube.
  • Insulating oil surrounds the dental tube.
  • Barrier material, that prevents the oil from escaping through the x-ray port.
  • It is the sum of inherent filtration plus any added external filtration supplied in the form of aluminum disks placed over the port in head of x-ray machine.
  • 5 mm of Al is required for 70kvp.
  • 2.5 mm of Al for higher voltage.
  • Collimating the x-ray beam reduces the formation of scattered radiation by placing a radiopaque barrier containing an aperture in the path of beam.
  • Collimation reduces the size of x-ray beam.
  • It also reduces the patient exposure and increases film quality. 
  • Diaphragm.
  • Tubular.
  • Rectangular.
  • General radiation (bremsstrahlung).
  • Characteristic radiation.
  • Most of the x-ray produced by dental x-ray machines are called as bremsstrahlung, beams, white or general radiation.
  • Bremsstrahlung composed of two German words: Bremse means “brake” and Strahl means “ray”.
  • It is “breaking radiation” because radiation because radiation is produced by braking or decelerating of high speed electrons.
  • Bremsstrahlung radiation produced by interaction of a cathode electron with a tungsten atom nucleus of the target (anode).
  • Cathode electron deflected and decelerated by the nucleus of tungsten atom producing an x ray photon.
  • Occasionally the total kinetic energy of a cathode electron is converted into x ray photon energy by a direct collision with nucleus of tungsten atom of target (anode).
  • Characteristics radiation contributes only a small fraction of photon in an x ray beam.
  • It occurs when incident electron ejects an inner electron from tungsten target.
  • When this happens an electron from outer orbital is quickly attracted to the void in the deficient inner orbital.
  • As outer orbital electron replaces the displaced electron a photon is emitted with an energy equivalent to the difference in the binding energies of two orbitals.
  • The energies of characteristic photons are discrete because they represent the difference of energy levels of specific electron orbitals and are characteristic of target atom
  • The relationship between distance and intensity of radiation is called inverse square law.
  • The intensity of radiation varies inversely as the square of source film distance.
  • When x-rays (incident photons or primary photons) strike any form of matter like body tissue scattered radiation produced that possess longer wavelength than primary radiation.
  • Scatter radiation in dental radiography comes from Compton scattering.
  • Unmodified (coherent) scattering.
  • Compton (incoherent) scattering.
  • Also known as Thompson scattering, classical, elastic scattering
  • It occurs when low energy x rays (below 10keV) interact with matter and undergo a change in direction without change in wavelength.
  • In this type interaction between x rays and matter no energy is transferred and no ionization occurs.
  • This interaction was first described by A H Compton in 1923.
  • An x-ray with relatively moderate to high energy strikes a loosely bound (free) outer-shell electron, ejecting its orbit.
  • An electron can be considered free when its binding energy is a great deal less than incident photon. This reaction ionizes the atom, producing an ion pair- a positive atom and a negative electron – called a Compton electron or recoil electron.
  • The incident photon is scattered in all directions (including backward) and retains most of its original energy.
  • Both scattered x-ray photon and Compton electron may have sufficient energy to undergo many more ionizing interactions before losing all their energy.
  • Compton scattering is the most common interaction between x-rays and body tissues and is responsible for almost all scatter radiation.
  • In dentistry approx. 62% of x-ray photons undergo Compton scattering that can be minimize by rectangular collimation, shorter exposure time and faster speed films.
  • It is a form of absorption of x-ray photons by matter. It occurs when an incident photon collides with a bound electron in an atom of the absorbing medium.
  • At this point the incident photon ceases to exit.
  • The electron is ejected from its shell and becomes a recoil electron (photoelectron).
  • The kinetic energy imparted to recoil electron is equal to the energy of photon minus that required to overcome the electron’s binding energy.

Attenuation is the reduction in intensity of x ray beam as it transverses dental tissues by either the absorption (photo electric) or deflection (Compton scattering) of the photons from the x ray beam.

  • In photoelectric absorption and Compton scattering, electrons are ejected from their orbits in the absorbing material after interaction with x-ray photons.
  • These secondary electrons give up their energy in the absorber by either of two processes:
  1. Collisional interaction with other electrons, resulting in ionization or excitation of the affected atom.
  2. Radiative interaction, which produce bremsstrahlung radiation, resulting in the emission of low-energy x-ray photons.
  • Secondary electrons eventually dissipate all their energy mostly as heat by collisional interactions, and come to rest.

Handheld portable dental X-ray devices are used in dentistry for taking intraoral radiographs. The current handheld portable X-ray devices resemble a photographic camera or have a “shotgun” design. The devices are used as a replacement for wall-mounted or semi-mobile X-ray devices (on a tripod or mobile support) and differ in two major characteristics from those:

  • The operator holds the handheld portable X-ray devices during exposure of the patient, so the operator cannot stand back and therefore dedicated means of (scatter) radiation protection need to be provided.
  • The newer handheld devices have typically a lower output dose rate (set by current, waveform, filtration and cone length) than do the traditional wall-mounted direct current units.
  • NOMAD.
  • Non‑FDA–cleared device.
  • Tube‑based X-ray fluorescence (XRF).
Most accepted and cleared by FDA is NOMADTM.
  • In nursing homes.
  • In home health care.
  • Special needs patients (individuals who require assistance for disabilities such as medical, mental, or psychological).
  • Sedated patients who are unable to cooperate with the positional requirements of the radiograph.
  • Forensic work.
  • In outreach programs like dental camps.
  • Hand‑held X-ray machine offers portability in X-ray technology, featuring cordless operation, rechargeable 14.4‑V nickel–cadmium battery packs.
  • It provides more than 100–700 exposures on a single charge.
  • It weighs 8 pounds, has internal lead shielding, and an external lead–acrylic backscatter shield.
  • There is an automatic shut off and “Enable” feature that minimizes the risk of inadvertent exposure.
  • The hand‑held X-ray device uses direct current, operates at a fixed 60 kV, 2.3 mA, and has a 0.4 mm focal spot with a 20 cm source-to-skin distance.
  • The high-voltage power supply is a high-frequency DC unit, reduces the dose to the patient.
  • The X-ray tube is surrounded by compounds of heavy metals to reduce the leakage radiation from the tube.
  • The beam limiting device (exit cone) is lined with Pb.
  • Backscattered radiation is also absorbed in a Pb-filled acrylic shield attached at the end of the exit cone. This shield has a Pb-equivalent of 0.5 mm thickness, and protects the operators’ torso, hands, face, and gonads from backscattered radiation from the patient’s face and teeth.
  • Operator can stay with the patient during X-ray
  • Take radiographs twice as fast as with conventional X-ray
  • Reduce number of retakes by up to 50%.
  • Move hand‑held X-ray devices from room to room, eliminating the need for multiple units.
  • Carry anywhere – ideal for hospital, home-health, humanitarian, and out-of-office use.
  • The any high degree of flexibility makes it easy to take exposures while the patient is reclined, lying completely on their back, or sitting upright.
  • There are less chances of cone-cut and others errors in taking X-
  • No need to follow accurate angulations, usually 90° or perpendicular to the film or sensor is maintained.
  • Light weight, cordless, easily transportable.
  • No awkward arms to manipulate; no drift.
  • No need for special stands, cables, remotes.
  • Hundreds of diagnostic quality images from a single battery charge.
  • Works with sensors, film, phosphor plates.
  • Exceptionally valuable in dental surgery suites.
  • Perfect for use with sedated patients, children, geriatric or special needs.
  • It is rechargeable one so no need of current to operate the device.
  • The main drawback is that ALARA principal is not followed.
  • The device must be angled and the operator cannot be completely within the protection zone.
  • Compulsory lead apron is necessarily should be worn by the operator.
  • Some time the radiofrequency which is used to produce X-ray leads disturbs other operating machines.
  • An experienced operator is needed.
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  6. Ramesh DNSV, Wale M, Thriveni R, Byatnal A Hand-held X-ray device: A review 2018;30:153-57.
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