Entrance Syllabus
Technical subject and organizational knowledge
1. Nuclear and Radiation Physics
1.1. Nuclear Physics
Radioactivity
General properties of alpha, beta and gamma rays
Alpha particle
Beta particle
Gamma particle
 It is a helium atom and It is an electron or a It is an energetic photon or light wave.
contains two neutrons and positron emitted by the It is a wave unlike alpha and beta
particles.
two protons
decay of nucleus
 Heavier than beta and Much lighter than alpha Highest penetrating energy. Gamma
waves can be stopped by a thick or
gamma particles
particles
 Least
penetrating They can be stopped, for dense enough layer material, with
energy. A sheet of paper instance, by an aluminium high atomic number materials such
or a 3-cm layer of air is sheet a few millimetres thick as lead or depleted uranium being
the most effective form of shielding.
sufficient to stop them.
or by 3 metres of air.
Laws of radioactivity
Natural radioactive series
The basic natural radioactive elements are included into four radioactive series: Thorium
series, neptunium series, uranium series and uranium-actinium series.
Radioactive equilibrium
Radioactive equilibrium is when the decay rate of a radioactive isotope equals the
production rate of that isotope by another source. If we multiply the decay constant ( λ
), which is radioactive-activity-per-second, by the number of atoms available to decay
(N) we get a radioactive decay rate.
Theory of beta decay
Beta decay is a radioactive decay in which a beta ray is emitted from an atomic nucleus.
During beta decay, the proton in the nucleus is transformed into a neutron and vice
versa. If a proton is converted to a neutron, it is known as β+ decay.
Similarly, if a neutron is converted to a proton, it is known as β– decay. Due to the change
in the nucleus, a beta particle is emitted. The beta particle is a high-speed electron when
it is a β– decay and a positron when it is a β+ decay.
Beta particles are used to treat health conditions such as eye and bone cancer and are
also used as tracers
Beata minus decay
Beta plus decay
Electron capture
Electron capture is the radioactive decay process by which an atom's inner orbital
electron is absorbed within the nucleus followed by conversion of a proton to a neutron
and emission of a neutrino (ve)
Internal conversion
Internal conversion is a non-radioactive, atomic decay process where an
excited nucleus interacts electromagnetically with one of the orbital electrons of an
atom. This causes the electron to be emitted (ejected) from the atom
Nuclear isomer
A nuclear isomer is a metastable state of an atomic nucleus, in which one or
more nucleons (protons or neutrons) occupy higher energy levels than in the ground
state of the same nucleus
Nuclear isomers are atoms with the same mass number and atomic number, but with
different states of excitation in the atomic nucleus. The higher or more excited state is
called a metastable state, while the stable, unexcited state is called the ground state.
Artificial radioactivity/ Induced radioactivity
The phenomenon by which even light elements are made radioactive by artificial or
induced methods is called artificial radioactivity
Examples of artificial radioactivity include medical imaging technology, which uses
artificial radionuclides for diagnosis, and other medical technology used for therapy.
Other examples include nuclear reactors and particle accelerators, where high energy
processes cause artificial radioactivity
Nuclear Cross-section
The nuclear cross section of a nucleus is used to describe the probability that a nuclear
reaction will occur. The concept of a nuclear cross section can be quantified physically
in terms of "characteristic area" where a larger area means a larger probability of
interaction. The standard unit for measuring a nuclear cross section (denoted as σ) is
the barn, which is equal to 10−28 m2, 10−24 cm2 or 100 fm2. Cross sections can be measured
for all possible interaction processes together, in which case they are called total cross
sections, or for specific processes, distinguishing elastic scattering and inelastic
scattering; of the latter, amongst neutron cross sections the absorption cross
sections are of particular interest.
Typical nuclear radii are of the order 10−14 m. Assuming spherical shape, we therefore
expect the cross sections for nuclear reactions to be of the order of 𝜋𝑟 2 or 10−28 m2 (i.e.,
1 barn). Observed cross sections vary enormously: for example, slow neutrons absorbed
by the (n, γ) reaction show a cross section much higher than 1,000 barns in some cases
(boron-10,
cadmium-113,
and
xenon-135),
while
the
cross
sections
for transmutations by gamma-ray absorption are in the region of 0.001 barn.
Nuclear Fission
When the nucleus of an atom splits into lighter nuclei through a nuclear reaction the
process is termed as nuclear fission.
1.2 Particle Accelerator
Particle accelerators for industrial, medical and research applications
Particle accelerators produce and accelerate beams of charged particles, such as
electrons, protons and ions, of atomic and sub-atomic size. They are used not only in
fundamental research for an improved understanding of matter, but also in plethora of
socioeconomic applications related to health, environmental monitoring, food quality,
energy and aerospace technologies, and others.
Particle accelerators can be linear (straight) or circular in shape and have many
different sizes. They can be tens of kilometers long or fit in a small room, but all
accelerators feature four principal components.
What are the different types of particle accelerators?
Ion implanters
Ion implanters are widely used in industry to, for example, make materials more
resistant to damage from wear and usage. Around 12 000 ion implanters around the
world help fabricate semiconductors for cell phones and solar panels. They are also
used in metal, ceramic and glass finishings to harden surfaces, make them more durable
and improve their longevity. Ion implanters can also improve the reliability of materials
used for medical implants, so they are safer to use in the body.
Electron beam accelerators
With almost 10 000 machines in operation globally, electron beam accelerators are an
industry work horse. They, for example, help make materials more durable in extreme
temperatures or resistant against chemicals. Electron beams are also widely used for
sterilizing medical products and foods, and to disinfect sewage water. They are used
widely in the automotive and aerospace industries, machine construction and medical
product manufacturers.
Linacs
Linear accelerators (linacs) may vary in length, from a couple of meters to a few
kilometres. Many of them are used in scientific research. The most widely known are the
medical linacs installed at hospitals, which create bursts of X-rays that are guided
towards tumour cells to destroy them. There are about 1000 medical linacs operating
worldwide.
Cyclotrons
More than 1200 cyclotrons around the world create proton or deuteron beams for
medical uses. They produce radioisotopes that are used for medical imaging to diagnose
and subsequently treat cancers. Many cyclotrons are located at hospitals to produce
life-saving radiopharmaceuticals containing short-lived radioisotopes for patients.
A cyclotron is a type of particle accelerator which repeatedly propels a beam of charged
particles (protons) in a circular path. Medical radioisotopes are made from nonradioactive materials (stable isotopes) which are bombarded by these protons. When
the proton beam interacts with the stable isotopes, a nuclear reaction occurs, making
the stable isotopes radioactive isotopes (radioisotopes).
Technetium-99m (Tc-99m) and Gallium-68 (Ga-68), are now also being produced in
cyclotrons
It makes use of the magnetic force on a moving charge to bend moving charges into a
semicircular path between accelerations by an applied electric field. The applied electric
field accelerates electrons between the "dees" of the magnetic field region. The field is
reversed at the cyclotron frequency to accelerate the electrons back across the gap.
Synchrotrons
The more than 70 synchrotrons are the giants among particle accelerators. They are
used for scientific research and are best known for helping us understand the
fundamental laws of our universe but also for numerous applications. Scientists use
synchrotrons to study chemistry, biomedicine, natural and cultural heritage, the
environment, and much more.
Electrostatic accelerators
Electrostatic accelerators, notably tandems accelerators, are less expensive, and
scientists use them to investigate material properties, monitor the environment, support
biomedical research, study cultural heritage objects and more. With recent boosts in
their capacity, experts expect the current 300 machines to grow in numbers in the
coming years.
Van de Graaff generator
A Van de Graaff generator is an electrostatic generator which uses a moving belt to
accumulate electric charge on a hollow metal globe on the top of an insulated column,
creating very high electric potentials. It produces very high voltage direct current (DC)
electricity at low current levels. It was invented by American physicist Robert J. Van de
Graaff in 1929. The potential difference achieved by modern Van de Graaff generators
can be as much as 5 megavolts
Betatron
Betatron is a type of particle accelerator and is used to accelerate electrons. It uses the
electric field induced by a varying magnetic field to accelerate electrons to high speed
in a circular orbit. The word betatron derives from the fact that high-energy electrons
are often called β-particles
Applications of Betatron
Some applications are stated below:

Provides high energy beam electrons of about 300 MeV.

Used as a source of X-rays and gamma rays if the electron beam is made to direct on
a metal plate.

The X-rays produced with the help of betatron can be used in industrial and medical
fields.

High energy electrons can be used in particle physics.

Possible solar flare mechanism.
Limitations of Betatron
This type of particle has maximum energy, and can be limited by the strength of
the magnetic field due to the saturation of iron and by the practical size of the magnet
core.
It is actually a transformer, which acts as the secondary coil of the transformer. This
accelerates the electrons in the vacuum tube around a circular path. It works under
constant electric fields and variable magnetic fields.
Synchro-Cyclotron
A synchrocyclotron is a special type of cyclotron, patented by Edwin McMillan in 1952, in
which the frequency of the driving RF electric field is varied to compensate for
relativistic effects as the particles' velocity begins to approach the speed of light. This is
in contrast to the classical cyclotron, where this frequency is constant.
In the synchrocyclotron, only one dee (hollow "D"-shaped sheet metal electrode) retains
its classical shape, while the other pole is open
Components of modern linacs
● Gantry; ● Gantry stand or support; ● Modulator cabinet; ● Patient support assembly
(i.e. treatment table); ● Control console.
— The length of the accelerating waveguide depends on the final electron kinetic energy,
and ranges from ~30 cm at 4 MeV to ~150 cm at 25 MeV.
— The main beam forming components of a modern medical linac are usually grouped
into six classes:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
Injection system;
RF power generation system;
Accelerating waveguide;
Auxiliary system;
Beam transport system;
Beam collimation and beam monitoring system
Injection system
Two types of electron gun are in use as sources of electrons in medical linacs:
— Diode type;
— Triode type.
Radiofrequency power generation system
The microwave radiation used in the accelerating waveguide to accelerate electrons to
the desired kinetic energy is produced by the RF power generation system, which
consists of two major components:
● An RF power source; ● A pulsed modulator.
The RF power source is either a magnetron or a klystron. Both are devices that use
electron acceleration and deceleration in a vacuum for the production of high power RF
fields. Both types use a thermionic emission of electrons from a heated cathode and
accelerate the electrons towards an anode in a pulsed electrostatic field; however, their
design principles are completely different.
Accelerating waveguide
Two types of waveguide are used in linacs: RF power transmission waveguides and
accelerating waveguides.
The simplest kind of accelerating waveguide is obtained from a cylindrical uniform
waveguide by adding a series of discs (irises) with circular holes at the centre, placed
at equal distances along the tube. These discs divide the waveguide into a series of
cylindrical cavities that form the basic structure of the accelerating waveguide in a linac.
The accelerating waveguide is evacuated to allow free propagation of electrons. The
cavities of the accelerating waveguide serve two purposes:
— To couple and distribute microwave power between adjacent cavities;
— To provide a suitable electric field pattern for the acceleration of electrons.
Two types of accelerating waveguide have been developed for the acceleration of
electrons:
(i) Travelling wave structure;
(ii) Standing wave structure.
In the travelling wave structure the microwaves enter the accelerating waveguide on
the gun side and propagate towards the high energy end of the waveguide, where they
either are absorbed without any reflection or exit the waveguide to be absorbed in a
resistive load or to be fed back to the input end of the accelerating waveguide. In this
configuration only one in four cavities is at any given moment suitable for electron
acceleration, providing an electric field in the direction of propagation.
In the standing wave structure each end of the accelerating waveguide is terminated
with a conducting disc to reflect the microwave power, resulting in a buildup of standing
waves in the waveguide. In this configuration, at all times, every second cavity carries
no electric field and thus produces no energy gain for the electrons. These cavities
therefore serve only as coupling cavities and can be moved out to the side of the
waveguide structure, effectively shortening the accelerating waveguide by 50%
Microwave power transmission
The microwave power produced by the RF generator is carried to the accelerating
waveguide through rectangular uniform S band waveguides that are either evacuated
or, more commonly, pressurized with a dielectric gas (Freon or sulphur hexafluoride,
SF6) to twice the atmospheric pressure
Auxiliary system
The linac auxiliary system comprises four systems:
● A vacuum pumping system producing a vacuum pressure of ~10–6 torr in the
accelerating guide and the RF generator;
● A water cooling system used for cooling the accelerating guide, target, circulator and
RF generator;
● An optional air pressure system for pneumatic movement of the target and other beam
shaping components;
● Shielding against leakage radiation.
Electron beam transport
● 90º bending; ● 270º bending (achromatic); ● 112.5º (slalom) bending
Linac treatment head
The linac head contains several components that influence the production, shaping,
localizing and monitoring of the clinical photon and electron beams.
● The important components found in a typical head of a fourth or fifth generation linac
include:
—Several retractable X ray targets;
—Flattening filters and electron scattering foils (also called scattering filters);
—Primary and adjustable secondary collimators;
—Dual transmission ionization chambers;
—A field defining light and a range finder;
—Optional retractable wedges;
—Optional MLC.
Beam collimation
In a typical modern medical linac, the photon beam collimation is achieved with two or
three collimator devices:
● A primary collimator;
● Secondary movable beam defining collimators;
● An MLC (optional).
● Clinical electron beams also rely on electron beam applicators (cones) for beam
collimation.
Dose monitoring system
It deals with standards for the type of radiation detectors, display of monitor units (MUs),
termination of radiation and monitoring of beam flatness and dose rate.
● Most linacs use sealed ionization chambers to make their response independent of
ambient temperature and pressure.
● The customary position of the dose monitor chambers is between the flattening filter
or scattering foil and the photon beam secondary collimator.
1.3 X-ray Generators
 discovery of X rays by Roentgen in 1895
 typically range in energy between 10 kVp and 50 MV
Characteristic X rays
-
-
result from Coulomb interactions between the incident electrons and atomic
orbital electrons of the target material (collision loss).
The energy difference between the two shells may either be emitted from the
atom in the form of a characteristic photon (characteristic X ray) or transferred
to an orbital electron that is ejected from the atom as an Auger electron.
The photons emitted through electronic shell transitions have discrete energies
that are characteristic of the particular target atom in which the transitions have
occurred; hence the term characteristic radiation.
Bremsstrahlung (continuous) X rays
-
result from Coulomb interactions between the incident electron and the nuclei of
the target material.
Photons with energies ranging from zero to the kinetic energy of the incident
electron may be produced, resulting in a continuous bremsstrahlung spectrum;
The bremsstrahlung spectrum produced in a given X ray target depends on the
kinetic energy of the incident electron as well as on the thickness and atomic
number Z of the target.
GAMMA RAY BEAMS AND GAMMA RAY UNITS
Basic properties of gamma rays
For use in external beam radiotherapy, γ rays are obtained from specially designed
and built sources that contain a suitable, artificially produced radioactive material.
● The parent source material undergoes a β decay, resulting in excited daughter nuclei
that attain ground state through emission of γ rays (γ decay).
● The important characteristics of radioisotopes in external beam radiotherapy are:
— High γ ray energy;
— High specific activity;
— Relatively long half-life;
— Large specific air kerma rate constant ГAKR.
Safety features in X-ray tubes
Some of the X-ray equipment safety features include:






Beam shutter (prevents the primary beam from passing when closed)
Failsafe (turns off X-ray generator and closes the beam shutter)
Beam stop (absorbs the primary beam)
Interlock (shuts down the beam if any switch in a series is open)
Warning lights (indicate X-ray beam is on)
Warning signs (caution that radiation-producing equipment is present)
Faults in X-ray tubes
1. Normal Aging
a) Normal Filament Burn Out
b) Accelerated Filament Burn Out
c) Slow Leaks
d) Inactivity
e) Glass Crazing
f) Arcing
g) Target Micro-cracking
h) Accidental Damage
i) Bearings
2. Deficiencies in Manufacturing
a) Immediate Failures
i) Weed Out by Test
ii) Hold Period
iii) Improper Materials
iv) Process Failures
b) Latent Failures
i) Process Optimization
ii) Marginal/Poorly Understood Processes
iii) Failure Analysis/Untraceable Causes
3. Application Mismatch
a) Low kV/High mA Emission
b) Temperature/Life
4. Improper Drive by the Power Supply
a) Supply Impedance
b) DC/AC Filament
c) High Frequency
d) Rotational Speed/Brake
e) Filament Boost
f) Logic Circuits
g) Filament Limit/Filament Preheat Settings
5. Tube Enclosure Considerations
a) Dielectric (oil) Leak
b) Overheating
c) Ambient Temperatures
d) Housing Attitude
e) Cable/Ground Connections
f) Dielectric Expansion Requirements
g) Rating Discipline
Interaction of Radiation with Matter
Radiation quantities and units
PHOTON FLUENCE AND ENERGY FLUENCE
PHOTOELECTRIC EFFECT
COMPTON EFFECT
PAIR PRODUCTION
Dosimetry & Standardization of Xray and gamma ray beams
Errors Vs Uncertainty
Error is the difference between the ‘measured value’ and the ‘true value’.
Uncertainty is a quantification of the doubt about the measurement result.
Whenever possible we try to correct for any known errors: for example, by applying
corrections from calibration certificates. But any error whose value we do not know is
a source of uncertainty.
Spread…standard deviation (σ)
The standard deviation is a measure of the dispersion of randomly occurring events
around a mean.
The usual way to quantify spread is standard deviation. The standard deviation of a set
of numbers tells us about how different the individual readings typically are from the
average of the set.
As a ‘rule of thumb’, roughly two thirds of all readings will fall between plus and minus
(±) one standard deviation of the average. Roughly 95% of all readings will fall within two
standard deviations. This ‘rule’ applies widely although it is by no means universal.
Types of Errors
➢ Systematic Errors
• uncertainties in the bias of the data, such as an unknown constant offset, instrument
mis-calibration
• implies that all measurements are shifted the same (but unknown) amount from the
truth.
• measurements with a low level of systematic error, or bias, have a high accuracy.
➢ Random Errors
• arise from inherent instrument limitation (e.g. electronic noise) and/or the inherent
nature of the phenomena (e.g. biological variability, counting statistics)
• each measurement fluctuates independently of previous measurements, i.e. no
constant offset or bias – measurements with a low level of random error have a high
precision.
Probability Distribution Function
These functions, characterize random errors with a distribution Statistical Models:
1. Binomial Distribution Random independent processes with two possible outcomes.
2. Poisson Distribution Simplification of binomial distribution with certain constraints.
3. Gaussian or Normal Distribution Further simplification if average number of
successes is large (e.g. >20).
The Effect of Background
❑ Background count (600 s) = 380 counts
❑ Sample count (600 s) = 650 counts
❑ Net counts = 650 – 380 = 270 counts
▪ Standard deviation of the difference, σ = √(650 + 380) = 32.1 counts
▪ 95 % conf. level (1.96 σ ) = 62.9 counts
▪ The net count may be expressed as 270 ± 63, at 95 % confidence level
▪ To get the count rate, divide by 600 s: 0.45 ± 0.11 cps
LC, LD and MDA
* LC = critical level
* LD = limit of detection
* MDA = minimum detectable concentration
* The critical level (LC) is the level, in counts, at which there is a predetermined
statistical probability of incorrectly identifying a measurement system background value
as “greater than background”
* Any response above the LC is considered to be greater than background
▪ The limit of detection (LD) is an a priori estimate of the detection capability of a
measurement system, and is also reported in units of counts.
▪ The minimum detectable concentration (MDA) is the LD in counts multiplied by an
appropriate conversion factor to give units consistent with a required measurement,
such as Bq/cm2 or Bq/m3 .
▪ MDA is the lowest activity value that can be achieved when a sample is measured with
a detection system.
➢ Gamma-ray spectrometry: MDA depends on the background (statistical nature),
counting time, detector and sample properties, measurement geometry, nuclear 2 and
sample properties, measurement geometry, nuclear decay data of the considered
radionuclide.
➢ The MDA depends on the
▪ Background of counting system
▪ Counting efficiency of a counting system
▪ Counting time
The critical level and the detection limit can be calculated by using the following
formulae:
where LC = critical level (counts)
LD = detection limit (counts)
k = corresponds to (1-α) and (1-β) probability levels of the standardized normal
distribution (assuming α and β are equal)
B = number of background counts that are expected to occur while performing an actual
measurement.
If values of 0.05 for both α and β are considered acceptable, then k = 1.645 (from lookup tables) and the equations can be written as:
Where C is the conversion factor
MDA for Air Sampling Systems*
Charged particle equilibrium
• Charged particle equilibrium (CPE) exists for the volume v if each charged particle of
a given type and energy leaving v is replaced by an identical particle of the same energy
entering (in terms of expectation values)
• If CPE exists,
• RE condition is sufficient for CPE to exist
• In many practical cases RE condition is not satisfied, but cab be adequately
approximated if CPE condition exists
• Consider two general situations:
1. distributed radioactive sources
2. indirectly ionizing radiation from external sources
Transient charged-particle equilibrium
• TCPE is said to exist at all points within a region in which D ~ Kc , with the constant of
proportionality >1 • Consider two cases of a broad “clean” beam of indirectly ionizing
radiation (no contamination by charged particles) falling perpendicularly on a slab of
material:
1. Negligible radiative losses from secondary charged particles
2. Significant radiative losses; photons are allowed to escape from the phantom
Free Air Ion Chamber (FAIC)
Free-air ionisation chambers are the primary standard for air kerma in air for superficial
and orthovoltage X rays (up to 300 kV).
Principle: The reference volume (blue) is defined by the collimation of the beam and by
the size of the measuring electrode. Secondary electron equilibrium in air is fulfilled.
CAVITY THEORY
• In order to measure the absorbed dose at point P in the medium, it is necessary to
introduce a radiation sensitive device (dosimeter) into the medium
• the sensitive medium of the dosimeter is frequently called a cavity
• the sensitive volume of the dosimeter is in general not made of the same material as
the medium
The main interests of the cavity theory are:
• to study the modifications of charge and radiation distribution produced in the medium
by the cavity
• to establish relations between the dose in the sensitive volume of the dosimeter and
the dose in the medium
Bragg-Gray Cavity Theory
A cavity can be of small, intermediate or large size compared to the range of the charged
particles in the cavity.
The Bragg-Gray theory deals with small cavities
W. H. Bragg began the development of the theory, in 1910, but it was L. H. Gray, during
his PhD work, co-supervised by Bragg, who formalized the theory
The main assumptions of this theory are:
• the cavity dimensions are so small compared to the range of charged particles within
it so that the fluence of charged particles inside the cavity is not perturbed by the
presence of the cavity
• there are no interactions of uncharged particles in the cavity so that the absorbed dose
deposited in the cavity is due to the charged particles that cross the cavity
Fano Theorem
The conditions required by the Bragg-Gray theory are better accomplished if the
composition (atomic number) of the cavity is similar to that of the medium This was
observed in experiments with cavities filled with different gas compositions, and in
1954, U. Fano proved the theorem:
In a medium of given composition exposed to a uniform field of primary radiation, the
field of secondary radiation is also uniform and independent of the density of the
medium, as well as of the density variations from point to point
The Fano theorem is important because it relaxes the requirements on the size of the
cavity, which are very hard to meet, for instance, when the photon beam is of low
energy The theorem is valid only for infinite media and in conditions where the
stopping-power is independent of density
Burlin Cavity Theory
In Burlin’s theory:
• cavities have intermediate sizes
• cavity and medium are in CPE
• elemental compositions of both are similar
IAEA Code of Practice TRS 398 (2000)
1. Principles of the calibration procedure Measurement at other qualities
o The chamber is now to be used in a beam with a another quality Q such as
• high energy photons
• high energy electrons
that differs from the 60Co quality used in the chamber calibration at the
standards laboratory
o Then the formula for the determination of absorbed dose to water is changed
General properties of kQ:
• Values for kQ are dependent on the quality of radiation (type, energy, machine).
• Each type of ionization chamber needs a particular kQ
• Values for kQ are given in protocol tables for a large variety of beam qualities and
chambers (e.g.in TRS 398)
3. Correction factors
Air temperature and air pressure
 T0 and P0 are the reference conditions for chamber air temperature (in °C) and
pressure.
 T and P are the actual air temperature (in °C) and pressure.
 Then in the user’s beam, the correction factor for air temperature and air
pressure kT,P is:
Polarity effect
• Under identical irradiation conditions the use of potentials of opposite polarity in an
ionization chamber may yield different readings. This phenomenon is called the
polarity effect.
• If the used polarity differs from that at calibration, the following correction factor
must be applied:
-
M+ is the chamber signal obtained at positive chamber polarity
M- is the chamber signal obtained at negative chamber polarity
M is the chamber signal obtained at the polarity used routinely (either positive
or negative).
Recombination effect
• In pulsed radiation (i.e. at any linear accelerator!), the dose rate during a pulse is
relatively high and general recombination is often significant.
• In the IAEA Code of Practice it is recommended, that the correction factor ks for
pulsed beams be derived using the two voltage method:
-
where the values of the collected charges M1 and M2 are measured at the
polarizing voltages V1 and V2, respectively.
V1 is the normal operating voltage and V2 a lower voltage.
The ratio V1/V2 should ideally be equal to or larger than 3.
the constants a0, a1 , and a2 are given as:
The three most import correction factors are:
• kT,P for air density
• kpol for polarity effects
• ksat for missing saturation effects
Determination of radiation quality Q
Definition of the quality index Q for HE photons
 For high energy photons produced by clinical accelerators the beam
quality Q is specified by the tissue phantom ratio TPR20,10
 This is the ratio of the absorbed doses at depths of 20 and 10 cm in a
water phantom, measured with a constant SCD of 100 cm and a field size
of 10 cm × 10 cm at the plane of the chamber.
 The most important characteristic of the beam quality index TPR20,10 is its
independence of the electron contamination in the incident beam.
MEDICAL IMAGING
Principles of Xray Diagnosis and conventional imaging
Projection X-ray Imaging
 conventional radiography
 Imaging modality that involves two-dimensional images of the body, either
statistically or dynamically variable in time.
X-Ray Image Quality Assurance
Contrast:
 fractional difference in the signal or brightness between the structure of interest
and its surroundings
 directly proportional to the atomic number, density, and tissue thickness
Dynamic Range:
 the range of various X-ray intensities that can be imaged by the detector
 The detectors with wide dynamic range will show very low or very high exposure
values in an image, and viewers can view the range of different visible intensities
Spatial Resolution:
 the imaging system's ability to distinguish the adjacent structures separate from
each other
 A bar pattern containing alternate radio-dense bars and radiolucent spaces of
equal width can be imaged to get the subjective measurement of spatial
resolution in units of line pairs per millimeter.
 The modulation transfer function (MTF) is an objective measurement of the spatial
resolution obtained by measuring the transfer of signal amplitude of various
spatial frequencies from object to image
Noise:
 the random or structured variations within an image that do not correspond to Xray attenuation variations of the object
 Controlling exposure factors is the best way to reduce quantum noise.
Signal to Noise Ratio (SNR):
 combines the effects of contrast, resolution, and noise.
 Higher the signal and lower the noise, the better is the image quality.
Artifacts:


Artifacts contribute to poor image quality due to factors other than low
resolution, noise, and SNR.
These include unequal magnification, non-uniform image due to detector
problems, bad detector elements, aliasing, and improper use of grids.
Factors affecting image quality
Beam energy and kVp It is directly proportional to the atomic number of the anode target, peak
kilovoltage (kVp) of the X-ray generator, and amount of filtration in the beam.
 Higher energy beams cause greater X-ray penetration, less degree of attenuation
by the tissues, and more scatter radiation
 This results in lower contrast and lower dose
Tube current-exposure time product (mAs) Tube current determines the total photons impinging the patient to form an image.
 linear relationship between mAs and patient dose
 An increase in mAs leads to an increase in patient dose and reduction in noise
Acquisition geometry
 Image acquisition geometric factors affecting image quality include a source to
image receptor distance, orientation, the amount of magnification, and size of the
focal spot
Magnification
 An increase in the air gap or patient to image receptor distance leads to an
increase in magnification and a decrease in scatter radiation, resulting in
improved image contrast and noise
 the radiation dose is increased as the patient is closer to the X-ray tube
 As there is a fixed size of the focal spot, an increase in magnification can cause
an increase in a blur
The focal spot size
 A decrease in focal spot size leads to improved spatial resolution.
 However, an X-ray tube with a small focal spot has limited maximum output,
leading to increased exposure time that can cause increased patient motion and
motion blur.
Collimation
 defined as the confinement of the spatial extent of an X-ray beam that impinges
upon the region of interest in the patient and detector.
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Effective collimation causes a decrease in scattered radiation that reaches the
detector
This leads to the improvement of image contrast and noise and increased SNR.
It also causes less radiation exposure and a reduction in effective radiation dose
to the patient.
Anti-scatter grid Anti-scatter grid improves image quality by decreasing the scattered radiation.
 However, it can also negatively affect image quality by attenuating the primary Xray beam.
Applications of Radiation in industry, Agricultural and Research
Radioisotopes in Industry
 Industrial tracers
 to monitor fluid flow and filtration, detect leaks, and gauge engine wear
and corrosion of process equipment
 Small concentrations of short-lived isotopes can be detected whilst no
residues remain in the environment. By adding small amounts of
radioactive substances to materials used in various processes it is
possible to study the mixing and flow rates of a wide range of materials,
including liquids, powders, and gases and to locate leaks.
 Inspection
 to inspect metal parts and the integrity of welds across a range of
industries
 Instead of the bulky machine needed to produce X-rays, all that is needed
to produce effective gamma rays is a small pellet of radioactive material
in a sealed titanium capsule.
 Gauges
 Gauges containing radioactive (usually gamma) sources are in wide use in
all industries where levels of gases, liquids, and solids must be checked
 They measure the amount of radiation from a source which has been
absorbed in materials. These gauges are most useful where heat,
pressure, or corrosive substances, such as molten glass or molten metal,
make it impossible or difficult to use direct contact gauges.\
 Carbon dating
 Analyzing the relative abundance of particular naturally-occurring
radioisotopes is of vital importance in determining the age of rocks and
other materials that are of interest to geologists, anthropologists,
hydrologists, and archaeologists, among others.
Application of radio-isotopes in agriculture
 Plant nutrition - High crop production - Radioisotopes can be used to “label”
different fertilizers. By attaching radioactive tracers to known quantities and
varieties of fertilizers, it is possible to directly determine the associated nutrient
efficiencies as the labeled products are absorbed at critical locations in the plant.
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This technique can be used to substantially reduce the amount of fertilizer
required to produce robust yields, thereby reducing costs to the farmer and
minimizing environmental damage.
Insect control
Mutant varieties
Food preservation
Improve animal health