Radiation Physics Guide

RADIATION PHYSICS
Guided by – Presented by –
Dr. Shalu Rai Dr. Priyanka
Dr. Deepankar Misra
Dr. Sahil Kidwai
Dr. Suman Bisla

Contents
 Basic consideration
 Composition of matter
 Nature of radiation
 X- Ray machine
 Production of x-rays
 Factors controlling the x-ray beam
 Effect of interaction of x-rays with matter
 Photoelectric absorption
 Reference

BASIC CONSIDERATION
 The world is composed of matter & energy.
 MATTER- is the substance of which all physical things are composed,
it occupies space, has inertia, mass can exert force and can be acted upon by force.
 occurs in 3 state,
-Freny 2nd edition
solid
liquid
gas

 ATOM- is the basic unit and consists of nucleus containing protons and neutrons ,
and electrons that are bound to the nucleus by electrostatic forces.
- White And Pharaoh’s Second South Asia Edition
 ELEMENT - is made up of an accumulation of a single species of atoms.
- Freny 2nd edition
 MOLECULE – sufficiently stable , electrically neutral group of atleast 2 atoms in definitie arrangement
held together by strong chemical bond .
- freny 2nd edition
 COMPOUND - is made up of recurring units of atoms, in a definite arrangement with at least
two of the atoms being different.
- Freny 2nd edition

Atomic structure (consists of two parts, a central nucleus and orbiting electrons).
 Nucleus
In all atoms (except hydrogen),
the nucleus - positively charged protons and neutral neutrons.
 A hydrogen nucleus contains a single proton.
 The number of protons in the nucleus - atomic number (Z), (unique to each
element).
 The total number of protons and neutrons - atomic mass
(A).
White and Pharaoh 2nd South Asia Edition

The ratio of neutrons to protons determines the stability of the nucleus
 Electron orbitals
Electrons - negatively charged particles
- exist in the extranuclear space
- bound to the nucleus by electrostatic attraction.

COMPOSITION OF ATOM
 Neil Bohr Model, ( classic view of atom )
- considers the structure of atoms like a solar system, with negatively charged electrons
- travel in discrete orbits around a central, positively charged nucleus
 Quantum Mechanical Model, ( contemporary view of atom )
assigns electrons into complex three dimensional orbitals with energy sublevels

The Bohr model,
 electrons exist in discrete orbits or “shells” denoted as K, L, M, N, O, and P (K-shell
being closest to the nucleus )
 The shells are also described by a quantum number 1, 2, 3 …, with 1 being the quantum number
for the K-shell.
 Each shell can hold a maximum of 2n2 electrons, where n is the quantum number of the shell.

The quantum mechanical model,
describes the electrons within three dimensional orbitals.
 The electron orbitals are described based on
their distance from the nucleus (principal quantum number; n = 1, 2, 3 …) and
their shape (designated s, p, d, f, g, h, and i).
 The electron orbitals in order of filling are 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f … and so forth.
 The Bohr model and the quantum mechanical model - understand diagnostic x-ray production and
interactions.

 The energy needed to overcome the electrostatic force (binds an electron to the nucleus) - ELECTRON
BINDING ENERGY. (eV or KeV)
 The electron binding energy is related to the atomic number and the orbital type.
Ionization
When the number of electrons = the number of protons in its nucleus, the atom is electrically NEUTRAL.
 a neutral atom loses an electron, it becomes a positive ion, and the free electron becomes a negative ion.
This process of forming an ion pair is termed IONIZATION.

 Ionizing radiations - have sufficient energy to displace electrons from their orbitals and ionize atoms.
Ex- High-energy particles,
x-rays,
ultraviolet
 Nonionizing radiations- do not have sufficient energy to remove bound electrons from their orbitals
Ex - visible light,
infrared
microwave radiations,
radio waves

NATURE OF RADIATION
 Radiation is the transmission of energy through space and matter.
 It may occur in two forms:
ELECTROMAGNETIC
RADIATION
PARTICULATE
RADIATION

Electromagnetic radiation
 According To White And Pharaoh ,
Electromagnetic radiation is the movement of energy through space as a combination of electric
and magnetic fields
 According To Freny ,
Electromagnetic radiation can be defined as the propagation of wave like energy (without mass)
through space or matter. The energy that is propagated is accompanied by oscillating electric and
magnetic fields positioned at angles to one another.
 man made or occur naturally.
 Electromagnetic radiations are arranged according to their energies - electromagnetic spectrum.

. Ex- γ-Rays,
x-rays,
ultraviolet rays,
visible light,
infrared radiation (heat),
microwaves,
radio waves

ELECTRIC WAVES
longest wavelength, 1015Å.
 HERTZIAN WAVES
used by high altitude transmission satellites,
wavelength ( 1016Å to 1013Å).
 COMMUNICATION WAVES OR RADIOWAVES
can pass through most materials except that of great bulk.
wavelength (1013Å to 108 Å)
Freny 2nd edition

USES
i. Medicine:
Radiowaves are used to transmit the pattern of the heartbeat through a monitor at a patient’s
home to a near by hospital.
ii. Others:
The prime purpose of radiowaves is to convey information from one place to another through
the intervening media ( radios, tv, cellular phone and radio compass )
Freny 2nd edition

 SHORT WAVE DIATHERMY OR MICROWAVES
wave lengths from 3 × 10-2 m to 3 × 10-4 m,
uses –
i. transmit information to satellites.
ii. Mobile phones
iii. microwave ovens.
iv. These waves can penetrate tissues and cause moisturized molecules in cells to vibrate resulting in
internal friction and increase in temperature of affected cells.
v. The therapeutic uses are:
• Treatment of sinusitis.
• reduce postoperative swelling and trismus arising from traumatic procedures.
Freny 2nd edition

 INFRARED WAVES "below the red."
wavelengths (40,000Å to 1,00,000Å).
results from molecular vibration and excitation of outer electrons of atoms.
comprises of that portion of sunlight which the body feels as heat.
emitted by an electric bulb or heating coil.
USES
tooth vitality testing,
surgical diathermy,
altering properties of dental materials like wax, gutta-percha, and for curing acrylic.
snakes in the pit viper family, like rattle snake, have sensory "pits", which are used in image infrared
light and this allows the snake to detect warm blooded animals, even in dark burrows.

 VISIBLE LIGHT
 this ranges from 4,000 Å to 7,700 Å.
 The range of color is often called VIBGYOR, with red having the longest wavelength and violet
having the shortest wavelength.
R-red, O-orange, Y-yellow, G-green, B-blue, I-indigo, V-violet.
USES
dental photography and operative field illumination.
Freny 2nd edition

 ULTRAVIOLET LIGHT
this ranges from 1,000Å to 2,000Å
 In the 19th century, Neil Finsen proved that UV rays caused sun burn
 Finsen et al in 1902 recognized the beneficial and bactericidal effects of UV on TB bone.
 They can also be produced from incandescent lamps.
 These rays have a slight penetrating power
Freny 2nd edition

 can penetrate living tissues for a depth of a few millimeters and cause biological effects like:
1. Photo erythema (sun burn).
2. Photo pigmentation (sun tan).
3. Is an agent in the production of vitamin D.
4. Bactericidal effect.
5. Aging of skin.
6. Eyes are sensitive to UV rays which can cause keratitis.
Freny 2nd edition

 The ultraviolet spectrum can be divided into three groups based on the wave lengths:
1. Between 200 and 290 nm, short or germicidal UV rays or UVC, these can cause genetic
mutations, altered reproductive cycles and cell death.
2. Between 290 and 320 nm, middle or erythemal UV rays or UVB, these can cause skin erythema
and are commercially available as sun or mercury vapor lamps.
3. Between 320 and 380 nm, long or black light UV rays or UVA, on its own it is not damaging but
when used with sensitizing chemicals, it can cause extensive biological damage.
Freny 2nd edition

 USES
• Disclose plaque or unclean surfaces by fluorescence.
• Photo polymerization of composite used for:
– Fissure sealing. – Fixing orthodontic brackets.
- Restoring teeth. – Placing pontics in temporary bridge work.
It is interesting to note that ultraviolet rays are not visible to the human eye but certain insects like the
bumble bee can see them!!!
Freny 2nd edition

 X-RAYS
 1895 by Wilhelm Conard Roentgen, a German scientist who found them quite by accident,
 X-rays have a wave length from 1 to 2 Å to 0.001Å
 produced in the X-ray tube by removal of the inner electrons, and are detected by photographic
film, ionization chamber
X-rays are divided into four type depending on their wave lengths:
• 1-2 Å-Grenz or Super Soft X-rays, - (treat superficial lesions)
• 1-0.5Å Soft X-rays,- (contact therapy)
• 0.5-0.1 Å-Medium X-rays, - (diagnostic and superficial therapy)
• 0.1 Å-Hard X-rays,- (deep X-ray therapy)
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 GAMMA RAYS
 high powered X-rays,
 wavelength - 0.001Å.
 These are electromagnetic radiations, but their source is from the radioactive decay process.
 They have shorter wave length and greater penetrating power
 used in the treatment of tumors, e.g. Radon needles or seeds which are implanted at the tumor
site,
Freny 2nd edition

LASER (Light Amplification by Stimulated Emission of Radiation)
is a device which can operate in the infrared, visible or ultraviolet region of the spectrum and
which amplifies electromagnetic waves by stimulated emission of radiation.
Laser light has four characteristics that distinguish it from the light produced from other sources,
like the electric bulb and the sun.
• Laser light is highly directional and travels in a narrow beam, the sides of which stay almost
parallel.
• Laser produces coherent light, that is it has only one frequency.
• Laser light is of a single color.
• Laser light is very bright, powerful with very high intensity
Freny 2nd edition

 In dentistry two types of lasers are used:
1. Soft Tissue Laser (800-990 nm), e.g. Argon soft tissue laser(blue), CO2 Laser
2. Hard Tissue Laser (2500-3000 nm), e.g.) Erbium hard tissue laser

 Dental Applications
1. Surgical excision of benign tumors
2. Frenectomy.
3. Cavity detection.
4. Treatment of Temporomandibular joint for reduction of pain and inflammation.
5. Treatment of ulcerative lesions.
6. Oral biopsies
7. Treatment of gummy smile
8. Treatment of tooth sensitivity
9. Treatment of melanin pigmented gingiva.
10. Local anesthesia free cavity preparation.
11. Enameloplasty, excavation of pits and fissures for placement of sealants.
12. For early detection of dysplastic cells
Freny 2nd edition

Properties of Electromagnetic Radiations
1. Travel through space in a wave motion along a straight line.
2. Neither mass nor weight nor electrical charge.
3. Travel at the speed of light, in a vacuum, i.E. 3 × 108 m/sec or 186,000 miles/second.
4. Give off an electric field at right angles to the path of propagation and a magnetic field at right angles
to both (the path of propagation and electric field).
5. Transfer energy from place to place in quanta (photons)
6. Have a measureable frequency and wavelength but different temperature and energy.
7. Invisible to the naked eye, with the exception of those falling within the range of the visible spectrum

 Some properties of electromagnetic radiation are best explained by quantum theory,
 whereas others are most successfully described by wave theory
Quantum theory
considers electromagnetic radiation as small discrete bundles of energy called photons.
Photons are bundles of energy with no mass or weight that travel as waves at the speed of light and move
through space in a straight line, "carrying the energy" of electromagnetic radiation.
The unit of photon energy is eV or E,
where, E = hc/ λ
Where h is a plank’s constant, (6.61 × 10–34 Joules/second)
c is velocity, which is also a constant,
therefore E = 1/λ

Wave theory
 maintains that radiation is propagated in the form of waves, (waves resulting from a disturbance in
water)
 Such waves consist of electric and magnetic fields oriented in planes at right angles to one another
that oscillate perpendicular to the direction of motion
.

 Waves are described in terms of their wavelength (λ, meters) and frequency (ν, cycles per second,
hertz)
Velocity (c)
 refers to the speed of the wave.
All electromagnetic radiations travel as waves or at the speed of light [3 × 108 meters per second
(186,000 miles per second)] in a vacuum

Wavelength (λ)
defined as the distance between the crest of one wave and the next .
 determines the energy and penetrating power of the radiation; the shorter the distance between the
crests, the shorter the wavelength and higher the energy and ability to penetrate matter.
measured in nanometers (1 × 10–9 meters or one-billionth of a meter) for short waves and in
meters for long waves

Frequency
is the number of waves that pass a given point in a certain amount of time.
 Frequency and wave length are inversely related;
if the frequency of the wave is high, the wave length will be short,
if the frequency is low the wavelength will be long.
The unit of measurement of frequency is Hertz

Particulate radiation
 These are tiny particles of matter that possess mass and travel in straight lines and at high speeds.
Particulate radiations transmit kinetic energy by means of their extremely fast-moving, small masses.
Small atoms
have approximately equal numbers of protons and neutrons,
 larger atoms
tend to have more neutrons than protons.
RADIOACTIVITY
 Larger atoms are unstable (unequal distribution of protons and neutrons,)
 they may break up,
 releasing α (alpha) or β (beta) particles or γ (gamma) rays.

 Four types of particulate radiations are recognized:
• ELECTRONS - may be classified as beta particles or cathode rays.
They differ in origin only.
a. Beta particles are fast moving electrons emitted from the nucleus of radioactive atoms.
b. Cathode rays are streams of high speed electrons that originate in an X-ray tube.
• ALPHA PARTICLES - emitted from the nuclei of heavy metals and exist as two protons and
neutrons, without electrons.
• PROTONS - accelerated particles, especially hydrogen nuclei, with a mass of 1 and a charge of +1.
• NEUTRONS - accelerated particles with a mass of 1 and no electric charge.
freny 2nd edition

RADIATION PHYSICS PART- II
 X- Ray machine
 Production of x-rays
 Factors controlling the x-ray beam
 Effect of interaction of x-rays with matter
 Photoelectric absorption
 Reference

X-RAY MACHINE
 X-ray machines produce x-rays that pass through a patient's tissues and strike a digital receptor or
film to make a radiographic image.
Dental x-ray machines can be used to expose intraoral and extra oral films .
Dental x-ray machine is made of three components :
Control panel
Extension arm
Tube head

Control panel
An exposure button
An on and off switch and an indicator light
Control devices (time, kilo voltage, milliamperage selectors) to regulate the x-ray beam.
Control panel is plugged into an electrical outlet.

Extension arm
 It suspends the x-ray tube head and houses the electrical wires that extends from control panel to the
tube head.
 It also allows movement and positioning of tube head.

Tube head
 It is a tightly sealed ,heavy metal housing that contains x-ray tube that produces dental x-rays
 Components
• Metal housing : protects x-ray tube and grounds high voltage.
• Insulating oil : surrounds x-ray tube and prevents its over heating.
• Tube head seal : seals the oil in tube head and acts as a filter to x-rays.
• X-ray tube : main x-ray generating system.
• Aluminum disks : used for added filtration b/w collimator and tube head seal.
• Lead collimator : restricts the shape and size of x-ray.
• Position indicating device.

 The primary components of an x-ray machine - x-ray tube and its power supply, (positioned within the
tube head)
 For intraoral x-ray units- the tube head is typically supported by an arm (mounted on a wall).
 operator can adjust duration of the exposure
energy
exposure rate
 An electrical insulating material(oil), surrounds the tube and transformers.
 the tube is recessed within the tube head to increase the source-to-object distance and minimize
distortion

X-Ray Tube
An x-ray tube is composed of a cathode and an anode situated within an evacuated glass envelope or
tube .
To produce x-rays, electrons stream from the filament in the cathode to the target in the anode, where
the energy from some of the electrons is converted into x-rays

Cathode
 consists of a filament and a focusing cup.
 The filament - source of electrons within the x-ray tube.
 It is a coil of tungsten wire approximately 2 mm in diameter and 1 cm or less in length.
 Filaments typically contain approximately 1% thorium, which greatly increases the release of
electrons from the heated wire.
 The filament is heated to incandescence with a low-voltage source and emits electrons at a rate
proportional to the temperature of the filament.

Evacuated
glass envelope
Cathode with
filament
Anode with
focal spot
STEP-DOWN
TRANSFORMER
STEP-UP
TRANSFORMER
Electron
cloud
Radiation is emitted
Window

 The filament lies in a focusing cup, a negatively charged concave molybdenum bowl.
 The parabolic shape of the focusing cup electrostatically focuses the electrons emitted by the
filament into a narrow beam directed at a small rectangular area on the anode called the focal spot
 The electrons move to the focal spot because they are both repelled by the negatively charged
cathode and attracted to the positively charged anode.
 The x-ray tube is evacuated to prevent collision of the fast-moving electrons with gas molecules,
which would significantly reduce their speed.
 The vacuum also prevents oxidation, or “burnout,” of the filament.

Anode
 It consists of a tungsten target embedded in a copper stem .
 Purpose of target is to convert kinetic energy of the electrons generated from filament into x-ray
photons.
 conversion of the kinetic energy of the electrons into x-ray photons is an inefficient process, with more
than 99% of the electron kinetic energy converted to heat.
 Tungsten is selected as target material because of:
1. High atomic number-efficient in producing x rays.
2. High melting point is required because heat is generated at anode.
3. Low vapor pressure-maintain vacuum in tube.
4. High specific heat-to dissipate heat into copper stem.

 The tungsten target is typically embedded in a large block of copper which functions as a thermal
conductor to remove heat from the tungsten, reducing the risk of the target melting.
 The focal spot is the area on the target to which the focusing cup directs the electrons and from
which x-rays are produced.
 The size of the focal spot is an important technical parameter of image quality—a smaller focal
spot yields a sharper image.
 A limitation to reducing focal spot size is the heat generated.

 To overcome this limitation, x-ray tubes use one of the two anode configurations.
Stationary anode
Rotating anode
Stationary anode
 the target is placed at an angle to the electron beam (20 degrees )
 When viewed through the aiming ring, the area from which the photons of the useful x-ray beam
originate appears smaller, making the effective focal spot smaller than the actual focal spot size.
 This allows production of x-rays from a larger area, allowing better heat distribution while
maintaining the image quality benefits of a small focal spot.

Rotating anode:
 the tungsten target is in the form of a beveled disk that rotates during the period of x-ray production
 As a result, the electrons strike successive areas of the target disk, distributing the heat over this extended
area of the disk.
. X-ray tubes with rotating anode can be used with longer exposures and with higher tube currents of 100 to
500 milliamperes (mA), which is 10 to 50 times that possible with stationary targets
. Such rotating anodes are not used in intraoral dental x-ray machines
 occasionally used in cephalometric units; cone beam machines; and multidetector computed tomography x-
ray machines, which require high radiation output for longer, sustained exposures.

Power supply
 The x-ray tube and two transformers lie within an electrically grounded metal housing called the
head of the x-ray machine.
 The primary functions of the power supply transformers of an x-ray machine are to:
• Provide a low-voltage current to heat the x-ray tube filament
• Generate a high potential difference to accelerate electrons from the cathode to the focal spot on
the anode

X-Ray Tube Controls
Tube Current (Milliamperes, mA)
During x-ray production, electrons produced at the filament are attracted to the anode.
 This flow of electrons from the cathode to the anode generates a current across the x-ray tube and
is called the tube current.
The magnitude - regulated by the milliampere control
For many intraoral dental x-ray units - the mA setting is fixed, (7 to 10 ma)

Tube Voltage (Kilovoltage, kV)
A high voltage is required between the anode and cathode to give electrons sufficient energy to
generate x-rays.
 The kilovolt peak (kVp) selector - adjusts the high-voltage transformer to boost the peak voltage
of the incoming line current (110 or 220 V).
Typically, intraoral, panoramic, and cephalometric machines operate between 50 and 90 kVp
(50,000 to 90,000 V),
whereas computed tomographic machines operate at 90 to 120 kVp, and higher

Alternating Current X-ray Generators:
 For an incoming line with alternating current (AC), the polarity of the line current alternates and
the polarity of the x-ray tube alternates at the same frequency.
When the polarity of the voltage applied across the tube causes the target anode to be positive and
the filament to be negative, the electrons around the filament accelerate toward the positive target,
and x-rays are produced
 when using a power supply with AC, x-ray production is limited to half the AC cycle.
Such x-ray units are referred to as self-rectified or half-wave rectified.
Many conventional dental x-ray machines are self-rectified.

Constant Potential (Direct Current) X-ray Generators:
Some dental x-ray manufacturers produce machines that replace the conventional 60-cycle AC,
half-wave rectified power supply with a high-frequency power supply that provides an almost
direct current.
This results in an essentially constant potential between the anode and cathode (see Fig. 1.11E),
and x-rays are produced through the entire cycle.
 Practical implications
 Because x-ray production occurs during the entire voltage cycle, constant potential units require
shorter exposure times to produce the same number of x-ray photons, minimizing patient motion.
 The intensity of x-ray photons produced is more consistent and reliable, especially with short
exposure times. This is of practical importance when using digital receptors that require less
radiation

Timer
A timer is built ino the high-voltage circuit to control the duration of the x-ray exposure
Before the high voltage is applied across the tube, the filament must be brought to operating
temperature to ensure an adequate rate of electron emission
To minimize filament damage, the timing circuit first sends a current through the filament for
approximately half a second to bring it to the proper operating temperature and then applies power
to the high-voltage circuit
A continuous low-level current passing through the filament maintains it at a safe low temperature,
further shortening the delay to preheat the filament.
 For these reasons, an xray machine may be left on continuously during working hours.

Tube Rating and Duty Cycle
Each x-ray machine comes with a tube rating chart that describes the longest exposure time the tube
can be energized for a range of voltages (kVp) and tube current (mA) values without risk of damage
to the target from overheating.
These tube ratings generally do not restrict tube use for intraoral radiography.
Duty cycle relates to the frequency with which successive exposures can be made without
overheating the anode.
The interval between successive exposures must be long enough for heat dissipation.
This characteristic is a function of the size of the anode, the exposure kVp and mA, and the method
used to cool the tube.

PRODUCTION OF X RAYS
 electron's kinetic energy is converted into x-ray photons by the formation of bremsstrahlung
radiation and characteristic radiation.
Bremsstrahlung Radiation
the primary source of radiation from an x-ray tube.
“braking radiation” in German,
produced by the sudden stopping or slowing of high-speed electrons by tungsten nuclei in the target

 The electron while passing near a nucleus may be deflected from it’s path by the action of
Coloumb forces of attraction and lose energy as bremsstrahlung , a phenomena predicted by
Maxwell’s general theory of electromagnetic radiation
 According to this theory energy is propagated through space by electromagnetic fields. As the
electron with it’s associated electromagnetic field, passes in the vicinity of nucleus, it suffers a
sudden deflection and acceleration .
 as a result a part or all of it’s energy is dissociated from it and propagated in space as
electromagnetic radiation
 Since an electron may have one or more bremsstrahlung interactions in the material and
interactions may result in partial or complete loss of electron energy, the resulting Bremsstrahlung
photon may have any energy up to the initial energy of electrons .

 the direction of Bremsstrahlung photon depend on the energy of the incidence electrons. At electron
energies below about 100 Kev , X rays are emitted in more or less equally in all direction .
 As the kinetic energy of the electron increases , the direction of X ray emission becomes
increasing forward
 Therefore transmission types of targets are used in megavoltage X-ray tubes ( accelerators ) in which
electrons bombard the target from one side and the X-ray beam is obtained on the other side .
 In low voltage X-ray tube it is advantageous to obtain the X ray on the same side of target i.e. 900 with
respect to the electron beam direction .

The reasons for this continuous spectrum are as follows:
 The continuously varying voltage difference between the target and filament causes the electrons
striking the target to have varying levels of kinetic energy.
 The bombarding electrons pass at varying distances around tungsten nuclei and are thus deflected
to varying extents.
 As a result, they give up varying amounts of energy in the form of bremsstrahlung photons.
 Most electrons participate in multiple bremsstrahlung interactions in the target before losing all
their kinetic energy.
 Consequently, an electron carries differing amounts of energy after successive interactions with
tungsten nuclei.

Characteristic Radiation
contributes only a small fraction of the photons in an x-ray beam.
produced when a high speed electron dislodges an inner shell electron from the tungsten atom and
causes ionization of the atom.
Once the electron is dislodged the remaining orbiting electrons are rearranged to fill the vacancy.
This rearrangement produces a loss of energy that results in the X-ray photon, with the energy equal
to the difference in the two orbital energy states.
The X-ray thus produced is called Characteristic Radiation

‘Direct hit’ Interaction ‘Near miss’ Interaction
Bremsstrahlung Radiation
Heterochromatic
Radiation
Characteristic
Radiation
Divergent
X-ray
Beam

FACTORS CONTROLLING THE X-RAY
BEAM
 Exposure time (s)
 Milliamperage setting (ma, tube current )
 tube voltage peak
 filtration
 collimation
 inverse square law

Exposure time
 When the exposure time is doubled, the number of photons generated at all energies in the x-ray
emission spectrum is doubled.
 The range of photon energies is unchanged.
 Practically, it is desirable to keep the exposure time as short as possible to minimize blurring from
patient motion.

Miliamperage
 the effects of exposure time, the quantity of radiation produced by an xray tube (i.e., the number of
photons that reach the patient) is directly proportional to the milliamperage setting (mA setting; )
 As the mA setting is increased, more power is applied to the filament, which heats up and releases
more electrons that collide with the target to produce radiation.
 Thus, as with exposure time, doubling the mA setting will double the number of photons
produced.
 The term beam quantity refers to the number of photons in an x-ray beam.
 Linearity and reproducibility of the mA and s settings are often included in the quality assurance
programs for x-ray units, including those used in dental and maxillofacial imaging.

Tube voltage peak
 Increasing the kVp increases the potential difference between the cathode and the anode,
increasing the kinetic energy of the electrons as they move toward the target.
 The greater the energy of an electron, the greater the probability it will be converted into x-ray
photons at the target.
 Increasing the kVp of an xray machine increases:
• The number of photons generated.
• The mean energy of the photons.
• The maximal energy of the photons

Filtration
 A filter preferentially removes low-energy photons from the beam but allows high-energy photons
that contribute to making an image to pass through
 Inherent filtration consists of the materials that x-ray photons encounter as they travel from the
focal spot on the target to form the usable beam outside the tube enclosure.
 These materials include the glass wall of the x-ray tube, the insulating oil that surrounds many
dental tubes, and the barrier material that prevents the oil from escaping through the x-ray port.
 The inherent filtration of most x-ray machines ranges from the equivalent of 0.5 to 2 mm of
aluminum.

 Added filtration may be supplied in the form of aluminum disks placed over the port in the head
of the x-ray machine.
 Total filtration is the sum of the inherent and added filtration.
 Federal regulations in the United States require the total filtration in the path of a dental x-ray
beam to be equal to the equivalent of 1.5 mm of aluminum for a machine operating at up to 70 kVp
and 2.5 mm of aluminum for machines operating at higher voltages

Collimator
 A collimator is a metallic barrier with an aperture in the middle used to shape and restrict the size
of the x-ray beam and the volume of tissue irradiated
 Round and rectangular collimators are most frequently used in intraoral radiography.
 Dental x-ray beams are usually collimated to a circle 2.75 inches (7 cm) in diameter at the patient's
face

 ROUND COLLIMATOR
is a thick plate of metal with a circular opening centered over the port in the x-ray head through
which the x-ray beam emerges.
round collimators are built into open-ended aiming cylinders
produces a cone shaped beam that is 2.75 inches in diameter and is considerably larger than the size
of two intraoral periapical films, and thus leads to an increased skin dose to the patient.

 RECTANGULAR COLLIMATOR
restricts the size of the X-ray beam to an area slightly larger than a size 2 intraoral (normal adult intraoral
periapical films) and thus significantly reduces the patient exposure.
Some types of receptor-holding instruments also provide rectangular collimation of the x-ray beam

 Collimators also improve image quality.
 When an x-ray beam is directed at a patient, the hard and soft tissues absorb approximately 90% of
the photons and approximately 10% pass through the patient to reach the image receptor (film, or
digital receptor).
 Many of the absorbed photons generate scattered radiation within the exposed tissues by a process
called Compton scattering
These scattered photons travel in all directions, and some reach the receptor and degrade image
quality.
 Collimating the x-ray beam thus reduces the exposed volume and thereby the number of scattered
photons reaching the image receptor, resulting in reduced patient exposure and improved images.

Inverse Square Law
The intensity of an x-ray beam (the number of photons per cross-sectional area per unit of exposure
time) varies with distance from the focal spot.
 For a given beam, the intensity is inversely proportional to the square of the distance from the
source.
 The reason for this decrease in intensity is that an x-ray beam spreads out as it moves from its
source.


THIS RELATIONSHIPAS FOLLOWS ;
where I is intensity
D is distance.
If a dose of 4 Gy is measured at 1 m, a dose of 1 Gy would be found at 2 m and a dose of 0.25 Gy
would be found at 4 m.

INTERACTIONS OF X RAYS WITH
MATTER
 In dental and maxillofacial imaging, the x-ray beam enters the face of a patient, interacts with hard
and soft tissues, and strikes a digital sensor or film.
 The incident beam contains photons of many energies but is spatially homogeneous.
 That is, the intensity of the beam is essentially uniform from the center of the beam outward.
 As the beam goes through the patient, it is reduced in intensity (attenuated).
This attenuation results from absorption of individual photons in the beam by atoms in the tissues
or by photons being scattered out of the beam.

 In absorption interactions, photons interact with tissue atoms and cease to exist.
In scattering interactions, photons also interact with tissue atoms but then move off in another
direction.
The frequency of these interactions depends on the type of tissue exposed (e.g., bone vs. soft
tissue).
Bone is more likely to absorb x-ray photons, whereas soft tissues are more likely to let them pass
through.

 There are three means of beam attenuation in a diagnostic x-ray beam
Photoelectric absorption
• Compton scattering
• Coherent scattering

BEAM ATTENUATION
 The extent of beam - depends primarily on the energy of the beam and the thickness and density of
the attenuating material.
 High-energy x-ray photons have a greater probability of penetrating matter,
 lower-energy photons have a greater probability of being attenuated.
 The higher the kVp setting, the greater the penetrability of the resulting beam
 A useful way to characterize the penetrating quality of an x-ray beam is by its half-value layer
(HVL)
 The HVL is the thickness of an absorber, such as aluminum, that reduces the number of x-ray
photons by 50%.
 As the mean energy of an x-ray beam increases, so does the amount of material required to reduce
the beam intensity by half (its HVL).

when the energy of the incident photon is increased to match the binding energy of the 1s orbital
electrons of the absorber, the probability of photoelectric absorption increases sharply and the
number of absorbed photons is greatly increased. This is called K-edge absorption
 Photons with energy less than the binding energy of 1s orbital electrons interact by photoelectric
absorption only with electrons in the 2s or 2p orbitals and in orbitals even farther from the nucleus.
 Rare earth elements are sometimes used as filters because their 1s orbital binding energies, or K
edges (e.g., 50.24 keV for gadolinium), greatly increase the absorption of high-energy photons.
This is desirable because these high-energy photons degrade image contrast and are not as likely as
mid-energy photons that primarily contribute to a radiographic image.

 The reduction of beam intensity also depends on physical characteristics of the absorber.

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