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© 2020 IJRAR August 2020, Volume 7, Issue 3 www.ijrar.org (E-ISSN 2348-1269, P- ISSN 2349-5138)

IJRAR19W1493 International Journal of Research and Analytical Reviews (IJRAR) www.ijrar.org 924

A REVIEW ON THE PHYSICS OF X-RAY

RADIATION IN DIAGNOSTIC IMAGING: AN

IMPLICATION TO IMPROVE AWARENESS ON

ITS SAFETY USAGE

1Gidey Gebrehiwot

1Department of Physics, 1Adigrat University, Adigrat, Ethiopia

Abstract

Background: Medical x-ray exposures have the largest man made source of population exposure to ionizing radiation in different countries.

Recent developments in medical imaging have led to rapid increases in a number of high dose x-ray examinations performed with

significant consequences for individual patient doses and for collective dose to the population as a whole. Studies showed that patients,

healthcare professionals, medical students and other medical staffs are poorly aware of about the physics of radiation exposure and

radiation risks in diagnostic imaging. As a result writing a review on the described title was necessary.

Objective: The objective of this study was to review and provide an overview about the basic concepts of radiation, x-ray generation and

imaging, radiation risks and protection during diagnostic imaging.

Method: A review of available book references and different studies on the physics of x-ray radiation basics, its generation, imaging,

biological effects and radiation protection was conducted. Book references and articles that met the criteria were considered. An overview

narrative type of reviewing was performed by using different electronic search engine, personal experience and hand search of the book

references and articles.

Results: This review demonstrated that fundamental knowledge and awareness in relation to x-ray radiation, radiation protection,

biological effects and doses used for radiological applications are insufficient or poor among patients and healthcare professions. It also

showed that, lack of communication between healthcare professionals and patients with respect to radiation exposure. Finally, this review

document result was realized that can fill all the gaps.

Conclusion: Despite the benefits of x-ray radiation in diagnostic imaging, we learned that there are a lot of biological risks arising from it.

To make its benefit over weighs its risk, applying the ALARA principle effectively based on adequate knowledge and experience is a must.

To achieve the ALARA principle, continues training on basic concepts of radiation, its biological effect and radiation safety principles for

the concerned bodies and necessity of careful consideration of curriculum is thus recommended.

Keywords: X-ray, x-ray tube, X-ray imaging, stochastic effects and non-stochastic effects, Dosimetry, Radiation protection.

1. INTRODUCTION

X-ray imaging procedures are an important tool to help doctors save lives through quick and accurate diagnoses. Medical x-ray exposures

have the largest man made source of population exposure to ionizing radiation in different countries. Recent developments in medical

imaging have led to rapid increases in a number of high dose x-ray examinations performed with significant consequences for individual

patient doses and for collective dose to the population as a whole. One-third of all successful cancer treatments involve radiation. Carefully

targeted radiation beams destroy cancerous cells while limiting damage to nearby healthy cells. Current science suggests that there is some

risk from any exposure to radiation [1]. Concerns are growing over the risks associated with these high levels of exposure, particularly the

potential increased lifetime risk of cancer [2]. This issue has been addressed by the European Council Eurotom directive of 1997, which

made a number of recommendations. Among the recommendations were that radiation protection should be integrated into the curriculum

of medical schools [3]

Radiologists, medical physicists, registered radiologist assistants, radiologic technologists, and all supervising physicians have a

responsibility for safety in the workplace by keeping radiation exposure to staff, and to society as a whole, “as low as reasonably

achievable” (ALARA) and to assure that radiation doses to individual patients are appropriate, taking into account the possible risk from

radiation exposure and the diagnostic image quality necessary to achieve the clinical objective. All personnel that work with ionizing

radiation must understand the physics of x-ray imaging. Studies showed that patients, healthcare professionals, medical students and other

medical staffs are poorly aware of about the physics of radiation exposure and radiation risks in diagnostic imaging[4-13]

As a result writing a review on the described title was necessary. The objective of this study was to review and provide an overview about

the basic concepts of radiation, x-ray generation and imaging, radiation risks and protection during diagnostic imaging.

2. METHODOLOGY

A review of available book references and different studies on the physics of x-ray radiation basics, its generation, imaging, biological

effects and radiation protection was conducted. Book references and articles that met the criteria were considered. An overview narrative

type of reviewing was performed by using different electronic search engine, personal experience and hand search of the book references

and articles.

3. MAIN BODY DISCUSSION

3.1. Basic concepts of radiation

Radiation is the transport of energy by electromagnetic waves or atomic particles. Radiation can be classified into electromagnetic

radiation(EMR) and particle radiation. Radiations are very important in diagnostic imaging.

http://www.ijrar.org/



3.1.1. Electromagnetic and particle radiations

Electromagnetic radiation: is an energy propagation by oscillation of Electric and magnetic fields. They have no mass, have a speed of

𝑐 = 3𝑥108𝑚/𝑠 in vacuum, they have dual characteristics(wave-particle duality). According to Einstein’s theory, the energy of a photon is

proportional to the frequency of the electromagnetic wave (equation 1). Electromagnetic waves can, like all waves, be characterized by

their amplitude(A), wavelength (λ), frequency (𝑣) and speed(c). The speed of the wave is equal to the product of the frequency and the

wavelength, and its magnitude depends upon the nature of the material through which the wave travels and the frequency of the radiation.

In a vacuum, however the speed for all electromagnetic waves is a constant, usually denoted by 𝑐, and in which case: 𝑐 = 𝜆𝑣, for X rays,

wavelength is usually expressed in nanometres (nm) and frequency is expressed in hertz (hz). When interactions with matter are

considered, electromagnetic radiation is generally treated as series of individual particles, known as photons. The energy of each photon is

given by the Planck-Einstein relation;[14]

𝐸 = ℎ𝑣 (3.1)

Where the constant of propertionality ℎ = 6.63𝑥10−34𝐽. 𝑠 is Planck’s constant.

The wave properties of electromagnetic waves are diffraction, interference, and reflection.

The various types of electromagnetic waves are listed in figure 1, which shows the electromagnetic spectrum. Note the wide ranges of

frequencies and wavelengths. No sharp dividing point exists between one type of wave and the next. Remember that all forms of the

various types of radiation are produced by the same phenomenon— accelerating charges[15].

Figure 1: the electromagnetic spectrum. [15]

X-rays have wavelengths in the range from approximately 10-8 m to 10-12 m. The most common source of x-rays is the stopping of high-

energy electrons upon bombarding a metal target. X-rays are used as a diagnostic tool in medicine and as a treatment for certain forms of

cancer.

Particle radiation: is energy propagation by travelling corpuscles that have a definite rest mass and within this limit they have definite

momentum and definite position at any time. These are alpha particles, protons, electrons, positrons and neutrons as shown in the table

below(table 1).

table 1. particle radiations

Particles Symbol Relative charge Mass(amu)

Approximate energy equivalence (Mev)

Alpha α/or H++ +2 4.0028 3727

Proton P/or H+ +1 1.007593 938

Electron e-/or B- -1 0.000548 0.511

Positron e+/or B+ +1 0.000548 0.511

Neutron n0 0 1.008982 940

According to the famous Einstein’s mass-energy equivalence relationship, we have the expression;

𝐸 = 𝑚𝑐2 (3.2)

Where E is energy, m is mass of the particle and c is speed of light. This mathematical expression tells us that mass can be changed to

energy and vise versa.




3.1.2. Ionizing and non-ionizing radiations Depending on its ability to ionize matter, radiation, can be classified into two main categories: Ionizing and non-ionizing radiations.

Ionizing radiations are radiations that carry enough energy per quantum to remove an electron from an atom or a molecule, thus introducing

a reactive and potentially damaging ion into the environment of the irradiated medium. The ionization potential of atoms, i.e. the minimum

energy required to ionize an atom, ranges from a few electron volts for alkali elements to 24.6 eV for helium which is in the group of noble

gases. Ionization potentials for all other atoms are between the two extremes. Ionizing radiation can ionize matter either directly or

indirectly because its quantum energy exceeds the ionization potential of atoms. Both directly and indirectly ionizing radiation can traverse

human tissue, thereby enabling the use of ionizing radiation in medicine for both imaging and therapeutic procedures [16]. Directly

ionizing radiation consists of charged particles, such as electrons, protons, α- particles and heavy ions. It deposits energy in the medium

through direct coulomb interactions between the charged particle and orbital electrons of atoms in the absorber. Indirectly ionizing

radiation consists of uncharged (neutral) particles which deposit energy in the absorber through a two-step process; in the first step, the

neutral particle releases or produces a charged particle in the absorber which, in the second step, deposits at least part of its kinetic energy

in the absorber through coulomb interactions with orbital electrons of the absorber in the manner discussed above for directly ionizing

charged particles. X rays, γ rays, energetic neutrons, electrons, protons and heavier particles are examples of ionizing radiation. Indirectly

ionizing photon radiation consists of three main categories: (i) Ultraviolet, (ii) x- ray and (iii) γ- ray. Ultraviolet photons are of limited use

in medicine. Radiation used in imaging and/or treatment of disease consists mostly of photons of higher energy, such as x- rays and γ- rays.

The commonly accepted difference between the two(x- rays and γ rays) is based on the radiation’s origin. The term ‘γ -ray’ is reserved for

photon radiation that is emitted by the nucleus or from other particle decays. The term ‘x-ray’, on the other hand, refers to radiation emitted

by electrons, either orbital electrons or accelerated electrons (e.g. bremsstrahlung type radiation). Non-ionizing radiations refers to a

radiative energy that, instead of producing charged ions when passing through matter, has sufficient energy only for excitation. Non-

ionizing radiation cannot ionize matter because its energy per quantum is below the ionization potential of atoms. Near ultraviolet

radiation, visible light, infrared photons, microwaves and radio waves are examples of non-ionizing radiation[17].

3.1.3. Structure of an atom The atom is the smallest division of an element in which the chemical identity of the element is maintained. The constituent particles

forming an atom are protons, neutrons and electrons. Protons and neutrons are known as nucleons and form the nucleus of the atom.

Protons have a positive charge, neutrons are neutral and electrons have a negative charge mirroring that of a proton. In comparison to

electrons, protons and neutrons have a relatively large mass exceeding the electron mass by a factor of 1836.The following general

definitions apply to atomic structure:

—atomic number Z is the number of protons in an atom.

—atomic mass number A is the number of nucleons in an atom, i.e. the number of protons Z plus the number of neutrons N in an atom: A =

Z+ N.

There are different theories/models that explain the structure of an atom. These are Thomson’s, Rutherford’s, and Bohr’s Models. The

accepted model is the Bohr’s Model.

Bohr’s theory/model of an atom

In 1913, Bohr proposed an atomic model, which explained with amazing accuracy of the main features of the spectra of hydrogenic atoms.

His model was based on the following postulates[17]:

(i) The electron in hydrogen atom moves in circular orbit around the nucleus.

(ii) In contrast to classical physics where the radius of electronic orbit can assume any magnitude, Bohr asserted that only those orbits are

allowed in which the angular momentum of electron is integral multiple of ( ℎ/2𝜋), i.e., 𝑛( ℎ/2𝜋)where n is an integer, called principal

quantum number. n = 1, 2, 3….. label the first, second, third ….. orbits of the electron and h is Planck's constant.

(iii) Since the revolving electron around the nucleus is not a stable system under the laws of classical electrodynamics, Bohr assumed that

the classical laws do not apply, at least, to the atomic phenomena. That is, the electron revolving in any one of the allowed orbits does not

radiate. These non-radiating orbits are called stationary orbits. However, while making transition from a stationary orbit of higher energy

to that of lower energy it does radiate. The electron may also go over from orbit of lower energy to that of higher energy by absorbing

energy.

If 𝐸𝑖 and 𝐸𝑓 are the energies of electron in the initial and final orbits respectively, then the energy of the emitted photon of frequency 𝑣

while the electron translates(as shown in figure 2) is given by:

ℎ𝑣 = 𝐸𝑓 − 𝐸𝑖 = 13.6 (1

𝑛𝑓2 −

1

𝑛𝑖2) (3.3)

figure 2. bohr’s model of the hydrogen atom [17]

The closer the electron orbit is to the positively charged nucleus, the greater the attractive force on the electron and the greater binding

energy. The total binding energy EB of a nucleus can, thus, be defined as the energy liberated when Z protons and (A – Z) neutrons are

brought together to form the nucleus. An electron may be moved to an orbit farther from nucleus or at position outside the atom (i.e.




ionization) only if sufficient energy is added to overcome the difference in binding energies between the original and final location of the

electron. The binding energy of electrons in a particular orbit increase with the number of proton in the nucleus.

3.1.4. Radiation –Matter interaction

Particle radiation-Matter interaction

Particle radiations interact with matter and lose their kinetic energy via; excitation, ionization and radiative losses.

1. Excitation: When the particle transfer energy to the electron in the absorbing material and causing promotion of electron orbital (i.e.

energy level) farther from the nucleus.

2. Ionization: When the energy transferred to the electron from the particles is larger than the binding energy where the electrons is ejected

from the atom completely. If the ejected electron causes further ionization this is called secondary ionization. The number of primary and

secondary ion pairs produced per unit path length of the incident radiation is called specific ionization(S.I)(expressed in ion pairs per

cm).The amount of energy deposited per unit length is called linear energy transfer. LET is directly proportional to square of charge and

inversely proportional to kinetic energy. LET is the product of S.I(IP/cm) and average energy deposited per ion pair(ev/IP). High LET

particles are like alpha particles, protons, these cause much damage to the tissue. Low LET particles are like electron/beta minus, beta plus

and EM radiations.

3. Radiative lose: Radiative lose or scattering is an interaction where the radiation is deflected from the original direction. This scattering

can be either elastic or inelastic. Elastic scattering is when the total kinetic energy of the colliding particle is unchanged other wise

inelastic. The processes of ionization can be considered as elastic scattering if binding energy of electron is negligible compared to K.E of

incident electron.

Electromagnetic radiation-matter interaction:

There are four major types of interaction of EM with matter. These are; Rayleigh scattering, Compton scattering, Photoelectric absorption

and Pair production

1). Rayleigh scattering: This is formed when the incident photon is re-emitted with only as light change in direction. As a result of no

energy transfer from photons to charged particles in the absorber, Rayleigh scattering is of no importance in radiation dosimetry [16].

2). Compton scattering: It can be described as an interaction b/n a photon as well as stationary electron. The interacting electron is not

free, rather it is bounded to a nucleus of an absorbing atom, but the photon energy ℎ𝑣 is much larger than the binding energy 𝐸𝐵 of the

electron, so that the electron is said to be loosely bounded or essentially ‘free and stationary’. In this scattering, the photon loses part of its

energy to the recoil(compton) electron and is scattered as a photon of energy ℎ𝑣 ′ through a scattering angle θ, while ϕ is the recoil angle

(figure 3). Scattered photons have less energy and larger wavelength than the incident. This interaction occurs between photon and

outer(valance) electron. The relationship between their energies can be expressed as;

𝐸0 = 𝐸𝑠𝑐 + 𝐸𝑒 (3.4)

Where 𝐸0 is energy of incident photon, 𝐸𝑠𝑐 , is energy of scattered photon and 𝐸𝑒 is energy of scattered electron. Compton scattering is the

predominant interaction of x-rays and gamma rays with soft tissue in the diagnostic range. The change in wave length of the scattered and

incident radiation is given by the equation:

𝜆2 − 𝜆1 =ℎ

𝑚0𝑐(1 − 𝑐𝑜𝑠𝛳) = 𝜆𝑐(1 − 𝑐𝑜𝑠𝛳) (3.5)

Where 𝜆2 and 𝜆1 are the wave lengths of the scattered and incident photon respectively,

𝜆𝑐 =ℎ

𝑚0𝑐 is Compton wave length, 𝑚0 is rest mass of the electron and c is speed of light.

figure 3: compton scattering [16]

3). Photoelectric effect: In the photoelectric effect, the photon interacts with a tightly bounded orbital electron of an absorber atom, the

photon disappears and the orbital electron is ejected from the atom (figure 4) with a kinetic energy;

𝐸𝑘 = ℎ𝑣 − 𝐸𝐵 . Where 𝐸𝑘 is kinetic energy of the electron, ℎ𝑣 is energy of the incident photon and 𝐸𝐵 is binding energy of the orbital

electrons. The incident photon transfers all of its energy to inner shell electrons. For photoelectric effect to happen, the photon energy ℎ𝑣

must exceed the binding energy 𝐸𝐵 of the orbital electron to be ejected.




figure 4:photoelectric effect[16]

4). Pair production: This happens when a very high energy photon interacts near the nucleus of the atom. When the incident photon ℎ𝑣

exceeds 2𝑚0𝑐2, with 𝑚0𝑐2 being the rest enrgy of the electron and positron, the production of an electron-positron pair in conjunction with

a complete absorption of the incident photon by the absorber atom becomes energetically possible. For the effect to occur, three quantities

must be conserved: energy, charge and momentum. To conserve the linear momentum simultaneously with total energy and charge, the

effect can not occur in free space; it can only occur in the coulomb electric field of a collision partner(atomic nucleus or orbital electron)

that can take up a suitable fraction of the momentum carried by the photon. Two types of pair production are known: nuclear pair

production and electronic pair production. If the collision partner is an atomic nucleus of the absorber, the pair production event is called

nuclear pair production and is characterized by a photon energy threshold slightly larger than two electron rest masses(2𝑚0𝑐2 =1.022𝑀𝑒𝑉). Less probable, but nonetheless possible, is pair production in the coulomb field of an orbital electron of an absorber atom. The

event is called electronic pair production or triple production and its threshold photon energy is 4𝑚0𝑐2 = 2.044 𝑀𝑒𝑉.

3.2. Photon beam description and Dosimetry

3.2.1. Photon beam description

An x-ray beam or gamma ray consists of a large number of photons usually with a variety of energies. These photon beams can be

described by different parameters. Such as:

1). Fluence(∅): of a photon is the quotient of the number of photons (𝑑𝑁) to that enter an imaginary shape of cross-sectional area (𝑑𝑎),

∅ =𝑑𝑁

𝑑𝑎

2). Fluence rate or flux density (φ): is fluence (∅) per unit time (𝑡), φ =𝑑∅

𝑑𝑡

3). Energy fluence(ψ): is the quotient of the sum of the energies of all photons(𝑑𝐸) to that enters a sphere of cross-sectional area 𝑑𝑎, ψ=𝑑𝐸

𝑑𝑎. For a monoenergetic beam 𝑑𝐸 is just the number of photons 𝑑𝑁 times energy ℎ𝑣 carried by each photon.

4). Energy fluence rate(I): is the energy fluence per unit time. 𝐼 =𝑑𝛙

𝑑𝑡

Attenuation of electromagnetic radiation

The most important parameter used in characterization of X-ray or gamma ray penetration into absorbing media is the linear attenuation

coefficient µ. This coefficient depends on the energy ℎ𝑣 of the photon and the atomic number Z of the absorber, and may be described as

the probability per unit path length that a photon will have an interaction with the absorber. Attenuation is the reduction of the number of

primary photons in a beam of radiation (by absorption and scattering). At low photon energies <30keV, photoelectric interaction dominates

the attenuation process. When a higher energy photons interact with low Z materials (ex: soft tissue) Compton scattering dominates. The

probability that a photon will be attenuated per thickness of the attenuator is called the linear attenuation coefficient. The relationship

between the initial number of photons(𝑁0) and the final number of photons (𝑁(𝑥)) after they pass through an attenuator whose thickness is

x and linear attenuation coefficient µ, is given by the following equation [18].

𝑁(𝑥) = 𝑁0𝑒−µ𝑥 (3.6)

The thickness of the material required to reduce the x-ray to one half is known as Half value layer(HVL). It is indirect way of measuring

photon energy.When the thickness is equal to one HVL, then the final number of photons(𝑁) will reduce to 𝑁0

2 and we can get the

relationship between HVL and µ as;

𝐻𝑉𝐿 = 0.693/µ (3.7)

3.2.2. Fundamentals of Dosimetry: Radiation quantities and units

Determination of the energy imparted to matter by radiation is the subject of dosimetry. The imparted energy is responsible for the effects

that radiation causes in matter, for instance, a rise in temperature, or chemical or physical changes in the material properties. Several of the

changes produced in matter by radiation are proportional to the absorbed dose, giving rise to the possibility of using the material as the

sensitive part of a dosimeter. Also, the biological effects of radiation depend on the absorbed dose. As a result, accurate measurement of

radiation is very important in all medical uses of radiation, be it for diagnosis or treatment of disease. The use of dosimetric quantities is

important in many aspects of the application of radiation. In diagnostic radiology, radiation protection of staff and patients is the most

important application of the dosimetric quantities [19]. Because x-rays damage or destroy living tissues and organisms, care must be taken

to avoid unnecessary exposure or overexposure[20]. In diagnostic imaging procedures, image quality must be optimized, so as to obtain the

best possible image with the lowest possible radiation dose to the patient to minimize the risk of morbidity. The risk of morbidity includes

acute radiation effects (radiation injury) as well as late radiation-induced effects, such as induction of cancer and genetic damage [17].

Several quantities and units were introduced for the purpose of quantifying radiation and the most important of these are listed below.

Exposure(X): It is defined as the quantity of x-ray required to liberate 2.58x10-4 c of charge per kg. It is the amount of charge produced by

ionizing EMR per mass of air. But we can not express non-ionizing radiation by exposure. Its unit is c/kg, other unit is Roetengen(R).




Dose/Absorbed dose(D): is the energy deposited by ionizing radiation(particle or EM) per unit mass of any material. Its SI unit is

Gray(Gy)=1J/kg. Its conventional unit is rad(=radiation absorbed dose). 1 rad=0.01J/kg.

Integral dose: is the total amount of energy imparted to matter. It is a product of the absorbed dose(D) and the mass over which the energy

is imparted. Its SI unit is Gray(Gy)

Equivalent dose[H]: it is the product of dose(D) and the radiation weighing factor(WR). H=WRxD. The SI unit is Seivert(Sv). Radiation

weighing factor(WR) is a factor used to reflect the relative effectiveness of the form of radiation.

Effective dose(HE)[Sv]: of radiation is defined as the equivalent dose, HT, multiplied by a tissue weighting factor WT, 𝐻𝐸 = ∑ 𝑊𝑇𝑥𝐻𝑇𝑇 ,

where 𝑊𝑇is tissue weighing factor(related to relative sensitivity of the tissue), 𝐻𝑇 is equivalent dose to tissue. The following table shows

different tissues have different tissue weighing factor.

Table 2: Tissue weighing factor of different tissues according to ICRP 2007 recommendations[21]

Tissue/organ ICRP tissue weighing factors

Gonads 0.08

Bone marrow 0.12

Lower large intestine 0.12

Lung 0.12

Stomach 0.12

Bladder 0.04

Breast 0.12

Liver 0.04

Oesophagus 0.04

Thyroid 0.04

Bone surface 0.01

Skin 0.01

Brain 0.01

Salivary glands 0.01

Remainder organs 0.12

Kerma (K)( Kinetic energy released in matter): is defined for indirectly ionizing radiation (photons and neutrons) as energy transferred

to charged particles per unit mass of the absorber.

3.3. X-ray Production and Imaging

3.3.1. Fundamentals of X-ray production

X-Rays consists of high energy photons that, by definition, are manmade. The most obvious source of X-ray radiation is the X-ray

machine, which produces these energetic photons as a result of the bombardment of certain heavy metals — i.e., tungsten, iron, etc. — with

high energy electrons.The differential absorption of X-rays in tissues and organs, owing to their atomic composition, is the basis for the

various imaging methods used in diagnostic radiology. The principles in the production of X-rays have remained the same since their

discovery. However, much refinement has gone into the design of X-ray tubes to achieve the performance required for today’s radiological

examinations. The production of X-rays involve the bombardment of a thick target with energetic electrons. These electrons undergo a

complex sequence of collisions and scattering processes during the slowing down process, which results in the production of

bremsstrahlung and characteristic radiation. X-rays are produced in one or the other of the two separate and distinct processes described

below:

Bremsstrahlung radiation: When energetic electrons are slowed down or ‘negatively’ accelerated (decelerated) by interactions with other

charged particles in matter (e.g. by atomic nuclei), the attractive Coulomb forces cause a change of the electron’s trajectory and the kinetic

energy that they lose is converted to electromagnetic radiation, referred to as bremsstrahlung radiation meaning (braking radiation), and this

energy of the emitted photon is subtracted from the kinetic energy of the electron. The energy of the bremsstrahlung photon depends on the

attractive Coulomb forces and hence on the distance of electron from the nucleus. The energy spectrum of bremsstrahlung is non-discrete

(i.e. continuous) and ranges between zero and the kinetic energy of the initial charged particle. Bremsstrahlung plays a central role in

modern imaging and therapeutic equipment, since it can be used to produce x- rays on demand from an electrical energy source.

Radiation from electron transition:

When an electron is removed from its orbit, a vacancy or hole is created. An electron from an outer orbit usually fills this vacancy or hole

leaving a vacancy in the outer orbit, which in turn is filled by another transition. This series of transition is called electron cascade. The

energy released by each transition is equal to the difference in binding energy between the original and final orbits of the electron. This

energy may be released as EM or particulate radiation and is called characteristic radiation.

Characteristic radiation: A fast electron colliding with an electron of an atomic shell could knock out the electron, provided its kinetic

energy exceeds the binding energy of the electron in that shell. The binding energy is highest in the most inner K shell and decreasing for

the outer shells(L, M, etc.). The scattered primary electron carries away the difference of kinetic energy and binding energy. The vacancy

in the shell is then filled with an electron from an outer shell, accompanied by the emission of an X-ray photon with an energy equivalent to

the difference in binding energies of the shells involved. For each element, binding energies, and the monochromatic radiation resulting

from such interactions, are unique and characteristic for that element. The energy of this radiation is the characteristics of atomic species

original and final quantum number(n) states of the transitioning electron. Energetic X-ray photons produced in this manner are known as

“Characteristic X-rays” because their energies are always precisely known. The principal uses of X-ray radiations are in the areas of

medical and industrial radiological diagnostics. The majority of the overall public’s exposure to ionizing radiation occurs as a result of

exposure to X-rays. Like their γ-ray counterparts, X-rays are uncharged, energetic photons with substantial penetrating power, typically




requiring a substantial thickness of some shielding material [i.e., lead, iron, steel reinforced concrete, etc.] to protect individuals who might

otherwise be exposed. The electrons are slowed down and stopped in the target, within a range of a few tens of micrometers, depending on

the tube voltage. As a result, X-rays are not generated at the surface but within the target, resulting in an attenuation of X-ray beam.

Auger electron and fluorescent yield:

When the transition energy is transferred to an orbital electron (typically in the same shell as the cascading electron), an electron will be

ejected from the atom, this ejected electron from the atom is called Auger electron. The Auger electrons possess kinetic energy equal to the

difference between the transition energy and the binding energy of the ejected electron. The probability that the transition will result in

ejection of Auger electron is called fluorescent yield(W).

So, in general, X-rays originate either from characteristic deexcitation processes in the atoms (K, L transitions) (Characteristic X-rays) or

from energy loss of high energy charged particles (e.g. electrons) due to interaction with the atomic nucleus (bremsstrahlung). The kinetic

energy of electrons is converted into electromagnetic energy by this atomic reaction. X-rays are one of the main diagnostical tools in

medicine since its discovery by Wilhelm Roentgen in 1895 [17].

X ray tube:

The X-ray tube provides an environment for X-ray production via bremsstrahlimg and characteristic radiation mechanisms. The production

of both bremsstrahlung and chractersistic radiation requires energetic electrons hitting a target. The major components of a modern x-ray

tube are (figure 5);

1) cathode, 2) anode, 3) tube insert/glass envelop, 4) tube housing, 5) filters, and 6) collimators.

Figure 5 Principal components of X-ray tube

An X-ray tube works as follows:

The heated filament is negatively charged and the tungsten target is positive. Electrons are emitted from the heated filament towards the

tungsten target due to the very high potential difference between them. The tungsten target absorbs the electrons and releases some of the

energy in the form of X-rays. This process is very inefficient however and a lot of energy is released in heat. For this reason the tungsten

target has a copper mounting because it conducts heat and is cooled with by circulating oil through the mount. Spinning the tungsten target

at high speed also helps to stop it overheating. If high-speed electrons bombarded a substance, x-rays are produced by energy conversion,

when the fast moving stream of electrons suddenly decelerated in and around target atoms and produce bremsstrahlung x-ray, or when

they eject electrons from target atom resulting in production of characteristic x-rays. The combination of bremsstrahlung and characteristic

x-rays forms the x-ray emission spectrum. The flow of electrons ( measured in milli-ampere, mA) across the x-ray tube determines the

number of electrons that will interact with the target in the tube and thus determines the number of resulting x-ray photons that will be

produced and the maximum speed of the electron flowing across the x-ray tube determines the maximum energy of the x-ray photon (kilo

voltage peak, kVp) produced. The kinetic energy (𝐸𝑘)(equation 8) is converted into heat (99%) and only 1% is converted into x-rays.

𝐸𝑘 = ½𝑚𝑣2 (3.8) The amount of energy in the X-ray photon emitted depends on: the distance between nucleus and passing electron, the electrons’ initial

energy, the charge of the nucleus. The subatomic distance between bombarding electron and nucleus determines the energy lost by each

electron during bremsstrahlung process. Coulombic forces increases with inverse square of interaction distance. Higher 𝑘𝑉𝑝 yields higher

𝑘𝑒𝑉 which increases the electron energy and produces higher x-ray energies. High atomic number (Z) gives higher positively charged

nucleus that causes direct impact of electron with nucleus, results in loss of kinetic energy. Therefore, higher X-ray energies can be

produced.

Components of X-ray tube

1) Cathode

The arrangement of the filament, the focusing cup, the anode surface and the tube voltage generates an electric field accelerating the

electrons towards the focal spot at the anode. X-ray tubes with two focal spots usually employ two separate filament/cup

assemblies(cathode blocks). The degree of focusing depends on the potential difference or bias voltage between the filament and focusing

electrode. The focal spot will be largest if both are at the same potential. With an increasing negative bias voltage at the focusing cup, the

focus size will decrease and finally the electron current will be pinched off. The cathode is the source of electron in the x-ray tube. It is a

helical filament of tungsten wire surrounded by a focusing cup. Electrical resistance to electrons flow heats the filament to very high

temperature, releasing surface electron via a process called thermionic emission. Rate of emission of electrons depends on the temperature

which is determined by a filament current. Thermionic emission of electrons increases with temperature(Richardson’s law) and produce a

cloud of electrons (space charge) enclosing the filament. This space charge shields the filament from the anode voltage. Increasing the

filament temperature, increases the space charge, and at some point the electric field cannot remove all electrons produced, but the anode

current remains steady(space charge limited current). An attempt to increase the anode current by increasing the filament current might

eventually end in filament failure. Generator control usually prevents this situation. For high anode voltages, all electrons boiled off the




filament are accelerated to the anode, giving an anode current that is fairly independent of tube voltage (saturation current). The focusing

cup (cathode block) controls the width of the electron distribution and direct electrons towards the target. A negative potential is applied to

the focusing cup in order to control the electron spread, because electron-electron repulsion results in large focal spot. Length of the

filament determines the length of the distribution. 1 % to 2% of thorium can be added to tungsten filament to increase the efficiency of

electron emission and prolong its life. The electron from the filament travels through the vacuum tube when a positive voltage is applied to

anode. Adjustment in the filament current controls the tube current and also the filament temperature.

For diagnostic imaging, the accelerated electrons have the energy ranging from 20 to 150keV or peak voltage. The rate of electron flow

from cathode to anode is called tube current and measured in mili amperes(mA).

2) Anode

The anode is a metal target electrode that maintained at a positive potential difference relative to the cathode. Electrons which are deposited

to anode dissipates 99% of the energy into heat and only 1% into X-rays. For common radiographic applications, a high bremsstrahlung

yield is mandatory, requiring materials with high atomic numbers (Z). Additionally, because of the low effectiveness of X- ray production,

it is also essential that the thermal properties are such that the maximum useful temperature determined by melting point, vapour pressure,

heat conduction, specific heat and density is also considered. Tungsten (Z = 74) is the optimum choice here. For mammography, other

anode materials such as molybdenum (Z = 42) and rhodium (Z = 45) are frequently used. Therefore, Tungsten anode is used because of:

-its large atomic number (Z=74) leading to high X-ray efficiency.

-its high density, strength and mechanical durability.

-its high melting point compared with other metals.

-its ability to absorb heat and to conduct it away from the source.

Anode Configuration: Stationary and rotating anodes

Stationary anodes: For X-ray examinations that require only a low anode current or infrequent low power exposure(e.g. dental units,

portable X-ray units and portable fluoroscopy systems), an X-ray with a stationary anode is applicable. It is the simplest X-ray tube

configuration consisting of tungsten embedded in a massive copper block to dissipate the heat efficiently to the surrounding cooling

medium. As the focal spot is stationary, the maximum loading is determined by the anode temperature and temperature gradients. The

cupper block uses for physically supporting the tungsten and removes heat efficiently from the tungsten target because it is an excellent

heat conductor. The disadvantage of stationary anode is limitation of heat dissipation rate that limits the maximum tube current and

maximum X-ray flux.

Rotating anodes: In a tube with a rotating anode, a tungsten disk rotates during an exposure, thus effectively increasing the area

bombarded by the electrons to the circumference of a focal track. The energy is dissipated to a much larger volume as it is spread over the

anode disk. The rotating anode is attached to the rotor of an asynchronous induction motor. The rotor is mounted within the tube housing

on bearings(typically ball bearings). It is a solid tungsten disk connected to a bearing mounted shaft that spins during the production of X-

rays. A large anode surface is available to distribute the instantaneous thermal energy load and the large mass anode serves as a heat sink. It

provides greater heat loading and higher X-ray output capabilities.

Anode angle and focal spot size

The anode is inclined to the tube axis, typically with the central ray of the X-ray field perpendicular to the tube axis (figure 6). The

electrons hit the anode in the electronic focus, largely determined by the length of the cathode filament. The electronic focus appears

shortened in beam direction by sin ɵ as the effective focus. The reduction of the anode angles to achieve smaller effective focus size is

limited by the size of the field of view required as the X-ray beam is cut off by the anode. A further limit is given by the heel effect. Anode

angle is the angle of the target surface to the central axis of the X-ray tube (figure 6 a). Anode angle in general radiography range from 7 to

20 degrees but most commonly 12 to 15 degrees [17]. Actual focal spot size is the area on the anode that is struck by electrons (determined

by cathode filament and the width of the focusing cup slot). Effective focal spot size is the length and width of the emitted x-ray beam as

projected down the central axis of the X-ray tube. The angle can cause the effective focal spot length to be smaller than the actual length

and related as:

Effective focal length = Actual focal length x sin θ, (θ is anode angle).

Effective focal spot size < actual focal spot size, and the smaller the anode angle is, the smaller the focal spot size (figure 6 b).

a b

figure 6: a) anode angle and b) focal spot size [17]

Principle of line focus: For x-ray images of high quality the volume of the target from which X-rays emerge should be as small as

possible. To reduce the effective size of the focal spot, the target of an X-ray tube is mounted a steep angle with respect to the direction of

imaging electrons. This apparent reduction in size of focal spot is termed as line focus principle. For measurement purposes, the focal spot

size is defined along the central beam projection. For high anode currents, the area of the anode hit by the electrons should be as large as

possible, to keep the power density within acceptable limits. To balance the need for substantial heat dissipation with that of a small focal

spot size, the line focus principle is used (figure 6b).




Heel effect The intensity of X-ray beam is not uniformly distributed throughout the beam. In general, the beam consists of a central ray and a diverging

beam. The rays towards the cathode end of the tube have more intensity. This is because, in a diverging beam, the rays which are parallel or

near parallel to the inclined/angulated anode get absorbed by the anode itself. So, Heel effect refers to the reduction in the intensity of the

X-ray beam towards the anode side of the X-ray field. Therefore place the thicker part of the body on cathode side.

3)Tube insert/glass envelop

Tube insert contains the anode, cathode, rotor windings and support structure sealed in glass or metal enclosure in vacuum. In

mammography, the insert is made of same material as the tube enclosure (beryllium Z=4) to minimize absorption of low energy X-rays.

The tube envelope maintains the required vacuum in the X-ray tube. A failing vacuum, resulting from leakage or degassing of the

materials, causes increased ionization of the gas molecules, which slows down the electrons. The envelope is commonly made of glass but

high performance tubes increasingly have glass—metal or ceramic – metal envelopes. The X-ray beam exits the tube through a window in

the envelope. To reduce absorption, the thickness of the glass is reduced in this area. If low energy X-rays are used, as in mammography,

the exit port is a beryllium window, which has less absorption than glass because of its low atomic number.

4)Tube housing

The x-ray tube housing supports, insulates, and protects the X-ray tube from the environment (provides structural support). Between the

tube housing and X-ray tube insert(envelope) there is especial oil serving as electrical insulation and heat conduction for heat removal from

the envelope surface, which is heated by the infrared radiation from the anode. The oil expands as it gets hot during operation. The oil

carries the heat away to the housing by convection, sometimes enhanced by forced cooling with a ventilator or heat exchangers. The tube

housing is internally shielded to attenuate X-rays emitted in a direction other than at the tube window.

Typical tube housing has a bellow for oil expansion because of heat absorption, if it heats excessively; the expanded bellow activates a

micro switch that shuts down the system to cool it off. The inside of the housing is lined with lead sheets to minimize leakage radiation.

Lead shielding inside the housing attenuates the X-ray. The maximum acceptable exposure due to leakage radiation is limited by

regulation. Further, tube housings provide mechanical protection against the impact of envelope failure. The housing also provides

radiation shielding to prevent any radiation except the primary beam from leaving the housing.

5) Filters

As low energy photons do not contribute to the formation of an image, filters are used to reduce the low energy component. Again,

increasing filtration gives spectral hardening and reduction in the tube out put. X-ray contrast declines with spectrum hardness, which

should be considered in the selection of optimal exposure parameters. Anode roughness increases with total tube workload and increases

self-filtration. Hence, tubes tend to show a slight increase in X-ray hardness and a decrease in kerma output operational tube life. Filters for

X-ray tube have two categories:

a) Inherent filtration: X-rays generated in the anode pass various attenuating materials before leaving the tube housing. These materials

include the anode, tube envelope exit port (glass or metal), insulating oil and the window of the tube housing. This inherent filtration is

measured in aluminum equivalents(unit: mm Al). It is a filtration type due to all original components (such as glass metal envelop, housing

oil, light mirror at the collimator) of the X-ray tube. Filtration increases mean energy and degrades contrast.

b) Added filtration: Additional filter material is positioned between the tube window and collimation assembly as required. The effect of

added filtration on the X-ray output is an increase in the mean photon energy and half value layer(HVL) of the beam. As the X-rays

become more penetrating, less incident dose at the patient entrance is required to obtain the same dose at the image receptor, giving a

patient dose reduction. Since image contrast is higher for low energy X-rays, the addition of filters reduces image contrast and optimum

conditions must be established, depending on the type of examination. Added filtration also increases tube loading, as the tube output is

reduced and must be compensated for by an increase in mAs to obtain the image receptor dose required. When we place an attenuator

purposely to alter the effective energy of the output photon, the filtration type is called added filtration.

6) Collimators

The limitation of the X-ray field to the size required for an examination is accomplished with collimators (figure 7). The benefits of

collimating the beam are twofold — reduction in patient dose and improvement of image contrast due to a reduction in scattered radiation.

A collimator assembly is typically attached to the tube port, defining the field size with adjustable parallel opposed lead diaphragms or

blades. To improve the effectiveness of collimation, another set of blades might be installed at some distance from the first blades in the

collimator housing.

Today, non-mobile X-ray equipments are equipped with automatic collimator called positive beam limiting (PBL) devices(motor-driven

shuttle).

figure 7:collimators in an x-ray tube

3.3.2. Basic physics of X-ray imaging

Introduction Knowledge of the structure of the atom, elementary nuclear physics, the nature of electromagnetic radiation and the production of X-rays is

fundamental to the understanding of the physics of medical imaging and radiation protection. In X-ray diagnostics, radiation that is partly

transmitted through and partly absorbed in the irradiated object is utilized. An X-ray image shows the variations in transmission caused by

structures in the object of varying thickness, density or atomic composition. In figure 8, the necessary attributes for X-ray imaging are

shown: X-ray source, object (patient) and a radiation detector (image receptor).




figure 11: necessary attributes for x-ray imaging

a. Penetration ability of X-rays

Previously we have seen that the number of monoenergetic photons that penetrate an object decreases exponentially with increasing object

thickness(equation 6). Generally, the penetration ability increases with increasing tube potential. By increasing the tube potential, a larger

fraction of the incident photons will reach the detector (image receptor). If the detector absorbs all of this radiation, exposure time may be

decreased resulting in lower patient absorbed doses (for imaging systems that require a certain amount of energy to be absorbed, e.g.,

conventional screen-film imaging). The reduction in exposure time achieved by increasing the tube potential will, however, mainly be due

to the increased bremsstrahlung yield at higher tube potentials.

b. The filtering effect of the patient

Almost all photons with energies below 30 keV are removed from the primary beam in passing through the water layer. Water is fairly

equivalent to most soft tissues regards attenuation, scattering and energy absorption of X-rays. After passage of 20 cm water, the

transmitted photons have energies above 30 keV. Above 20 keV, Compton/incoherent scattering is the dominating interaction process.

Since incoherent scattering does not vary much with photon energy, the X-ray image of a thick layer of body tissues mainly shows density

differences

between the tissues. Fat which has low density can be seen against other soft tissues. The

skeleton, which contains calcium is seen with good contrast due to the photoelectric

effect. In order to utilize photoelectric absorption to distinguish between soft tissues with

small differences in atomic composition and density, low energy photons, below 30 keV

, are needed as in, for instance, mammography.

c. Scattered radiation

The scattered radiation transmitted through the patient degrades image contrast and

contributes to the irradiation of organs distant from the primary beam as well as to

personel present in the examination room.

d. Radiographic Contrast/Radiographic densities As a beam of X-rays passes through the human body, some of the X-rays are absorbed or scattered producing reduction or attenuation of

the beam. Tissues of high density and/or high atomic number cause more X-ray beam attenuation and are shown as lighter grey or white on

a radiograph. Less dense tissues and structures cause less attenuation of the X-ray beam, and appear darker on radiographs than tissues of

higher density. For example, X-rays are attenuated more by bone than by lung tissue. A radiographic image is composed of a 'map' of X-

rays that have either passed freely through the body or have been variably attenuated (absorbed or scattered) by anatomical structures. For

descriptive purposes there are five different radiographic densities that can be useful to determine the nature of an abnormality. If there is

an unexpected increase or decrease in the density of a known anatomical structure then this may help determine the tissue structure of the

abnormality. Radiographic density is a measure of the degree of film darkening. The five principal radiographic densities that are

recognized on plain radiographs are listed here in order of increasing density;

1. Air/gas: black, e.g. lungs, bowel and stomach

2. Fat: dark grey, e.g. subcutaneous tissue layer, retroperitoneal fat

3. Soft tissues/water: light grey, e.g. solid organs, heart, blood vessels, muscle and fluid-filled organs such as bladder

4. Bone: off-white

5. Contrast material/metal: bright white.

X-ray Contrast Agent

Substances with high atomic numbers have high density which is useful for X-ray contrast, and appear bright white in X-ray exams. e.g.

Barium (atomic number 56) causes considerable attenuation of X-rays compared with the soft tissues of the body (used for barium meals

and barium enema’s for diagnosis in the gastrointestinal tract) (Barium sulfate - inert) used mainly for plain radiographs. Salts of iodine

(atomic no. 53) are used as water soluble CT contrast agents.

Process of Image Production

The produced X-rays may be either attenuated, absorbed, scattered or transmitted. The transmitted X-ray photons (+some scatter) reaches

the cassette and may interact with intensifying screens (produce light) or film then latent image (i.e. undeveloped) produced which is then

processed. Imaging techniques that use X-rays are: radiography, fluoroscopy and CT scans.

Radiography: This is the familiar X-ray where a beam of X-rays produced by an X-ray machine is directed at the part of your body that is

being examined and on to a special film to make a picture.

Fluoroscopy: This technique uses X-rays to produce a moving image on a TV screen. Individual “still” pictures can be chosen and saved

or the entire video may be saved. This technique is used to examine the intestine or to obtain images of flowing blood in blood vessels.

Computed tomography (CT) scan: This is a more sophisticated way of using X-rays. The patient lies on a narrow table which passes

through a circular hole in the middle of the scanner. Many tiny beams of X-rays pass through a slice of the body on to banks of detectors.




The X-ray sources and the detectors rotate around inside the machine. An image of the slice is formed by a computer and displayed on a

TV screen. The patient moves slowly through the hole to take pictures of different slices of the body and sometimes to produce 3D

pictures. If many slices are imaged, the radiation dose can be as high or higher than that for fluoroscopy.

Factors Controlling The X- Ray Beam

The X-ray beam emitted from an X-ray tube may be modified to suit the needs of the application

by altering the beam exposure length (timer), exposure rate (mA), beam energy (kVp and

filtration), beam shape (collimation), and target-patient distance (long or short cone).

I. Exposure Time: When the exposure time is doubled, the number of photons generated is doubled, but the range intensity of photons’

energies is unchanged . Therefore changing the time simply controls the “quantity” of the exposure, the number of photons generated. The

amount of radiation that a patient receives is determined by the mAs (mA x time).

II. Tube Current (mA): Illustrates the changes in the spectrum of photons that result from increasing tube current

(mA) while maintaining constant tube voltage (kVp) and exposure time. 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 reaction. A linear relationship exists between

mA and radiation output. The quantity of radiation produced (mAs) is expressed as the

product of time and tube current. The quantity of radiation remains constant regardless of

variations in mA and time as long as their product remains constant.

III. Tube Voltage (kVp): Increasing the kVp increases the potential difference between the cathode and anode, thus

increasing the energy of each electron when it strikes the target. The greater the potential

difference, the faster the electrons travel from the cathode to the anode. This results in an

increased efficiency of conversion of electron energy into X-ray photons, and thus an

increase in; i) The number of photons generated ii) Their mean energy. iii) Their maximal energy.

IV. Filtration: An X-ray beam consists of a spectrum of X-ray photons of different energies, but only photons with sufficient energy to

penetrate through anatomic structures and reach the image receptor (usually film) are useful for diagnostic radiology. Those that are of low

-energy (long wavelength) contribute to patient exposure but do not have enough energy to reach the film. The higher the kVp, the less

radiation is absorbed by the patient. Consequently, to reduce

patient dose, the less-penetrating photons should be removed. This can be accomplished

by placing an aluminum filter in the path of the beam.

V. Collimation: A collimator is a metallic barrier with an aperture in the middle used to reduce the size and shape of the X-ray beam and

therefore the volume of irradiated tissue within the patient. The round collimator is a thick plate of radio-opaque material (usually lead)

with a circular opening centered over the port in the X-ray through which the X-ray beam emerges. Typically, round collimators are built

into open-ended aiming cylinders. Rectangular collimators further limit the beam to a size just smaller than that of the X -ray film. The size

of the beam should be reduced to the size of the film being exposed to reduce further unnecessary patient exposure. Some types of film-

holding instruments also provide rectangular collimation of the X-ray beam. Use of collimation also improves image quality. 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 may degrade image quality. The detrimental effect of scattered radiation of the images can be minimized by

collimating the beam to reduce the number of scattered photons reaching the film.

Receptors for Projection Radiography

X-ray images are formed as shadows of the interior of the body. Since it is not yet practical to focus X-rays, an X-ray receptor has to be

larger than the body part to be imaged. The capture of an X-ray image may conceptually be divided into three stages.

First stage: is the interaction of the X-ray with a suitable detection medium to generate a measurable response.

Second stage: is the temporary storage of this response with a recording device.

Third stage: is the measurement of this stored response.

Many different types of image receptors are used in modern diagnostic radiology. They all have in common that they form an image by

absorption of energy from the X-ray beam (after transmitting through the body).

The blackening of the film after X-ray exposure is expressed in terms of its optical density(D):

𝐷 = log (𝐼0/𝐼) (3.9)

where I0 and I is the light intensities before and after passing through the exposed film material respectively. I and I0 are related as

𝐼(𝑥) = 𝐼0exp (−𝜇𝑥) (3.10)

3.4. Biological effects of ionizing radiation and Radiation Protection

3.4.1. Biological effects of ionizing radiation

After their discovery, X-rays were quickly applied to medical diagnostic use. Today X-rays remain a valuable tool in diagnosis and

treatment of many injuries and diseases. But the use of X-rays is not without risk. Just like other forms of high-energy radiation, X-rays can

cause damage to cells in the body, which in turn can increase the risk of developing cancer [22, 23]. One-third of all successful cancer

treatments involve radiation, if carefully targeted. Current science suggests there is some risk from any exposure to radiation [24]. Some of

the radiation risks are: mutations, damage DNA, can cause cancer, and can kill cells. The amount of damage depends upon: type, energy,

absorbed amount of radiation and sensitivity of cells to radiation. Exposures to high levels of radiation may also cause hair loss, skin

burns, sickness, and death [35].

All the methods of medical imaging(such as radiography, fluoroscopy and CT ) can bring very real benefits to patients. The overriding

concern of your doctor and of the hospital radiology department is to ensure that when radiation is used, the benefits from making the right

diagnosis, and consequently giving you the right treatment, outweigh any small risk involved. If treatment decisions depend on the

findings, then the risk to your health from not having the examination is likely to be much greater than that from the radiation itself. Before

going ahead, your doctor must be able to reassure you that there is no other way of providing new information that is essential for the




effective management of your medical problem. Make sure your doctor is aware of other X-rays or scans you have had, in case they make

additional examinations unnecessary.

A biological effect occurs when a change can be measured in a biological system after the introduction of some type of stimuli. However,

the observation of a biological effect, in and of itself, does not necessarily suggest the existence of a biological hazard or health effect. A

biological effect only becomes a safety hazard when it “ causes detectable impairment of the health of the individual or of his or her

offspring” [26 ]. Biological effects could be physiological, biochemical or behavioural changes induced in an organism, tissue or cell.

Radiation risks for older and younger patients

As you get older you are more likely to need an X-ray examination. Fortunately radiation risks for older people are lower than those for

young. This is because there is less time for a radiation-induced cancer to develop, so the chances of it happening are greatly reduced.

Children, however, with most of their life still ahead of them, may be at twice the risk of middle-aged people from the same X-ray

examination. This is why particular attention is paid to ensuring that there is a clear medical benefit for every child who is X-rayed. The

radiation dose is also kept as low as possible without detracting from the information the examination can provide.

The study of the biological effects of ionizing radiation started practically at the same time as the discovery of X-rays in 1895. Since the

techniques and methods accepted today to quantify radiation dose were absent at that time, first findings and studies were barely qualitative.

Nonetheless, harmful effects to man and to laboratory animal species from ionizing radiation were already observed in the early years of

the 20th century, when lack of data was a shared concern and there were no radiological protection standards. The first quantitative studies

in experimental radiobiology developed during the 1920s, with the results of the epidemiological studies on the survivors of the atomic

bombing of Hiroshima and Nagasaki, and data obtained from studies on patients exposed to radio therapeutic treatments, currently provide

a large amount of information on the health effect of ionizing radiation which is the base of safety standards for occupationally exposed

individuals. Biological effects of ionizing radiation are a consequence of the ionization of atoms of biomolecules, which might cause

chemical changes and alter or eradicate its functions. Due to ionization, proteins can lose the functionality of its amino groups and modify

its behavior, thus increasing its chemical responsiveness; enzymes would be deactivated; lipids will suffer peroxidation; carbohydrates will

dissociate; and nucleic acids chains will experiment ruptures and modifications of structure. But from all possible combined alterations,

DNA is the primary target for radiation because it contains genes/chromosomes that hold information for cell functioning and reproduction

that are critical to cell survival. The deposition of energy by ionizing radiation is a random process. Even at very low doses there is some

probability that enough energy may be deposited into a critical volume within a cell to result in cellular changes or cell death. Repair of

cellular damage, such as DNA repair, may take from minutes to hours after exposure depending on the type of damage.

Another possible result is mutation. The cell will survive but with modification in the DNA sequence of the cell’s genome. Mutated cells

are capable of reproduction and thus perpetuate the mutation. Ionizing radiation is more effective at producing biological damage when its

LET (linear energy transfer) is high, the dose rate is high and the period of time between consecutive exposures is short.

According to ICRP recommendations, adverse health effects from radiation exposure are grouped in two general categories, i.e., harmful

tissue reactions (deterministic effects) and stochastic effects (of random or statistical nature) [27]. Deterministic(non-stochastic) effects are

characterized by three qualities:

i. A certain minimum dose must be exceeded before the particular effect is observed;

ii. The magnitude of the effect increases with the size of the dose; and

iii. There is a clear causal relationship between exposure to radiation and the observed effect.

Harmful tissue reactions (deterministic effects) resulting from the killing/malfunctioning of cells is characterized by a certain dose called

“threshold.” The reason for the threshold is that a serious malfunction or death of a critical population of cells in a given tissue should be

sustained before injury is expressed in a clinically relevant form. Tissue reactions are also characterized by different periods of latency, so

it could be distinguished between early tissue reactions detected in a few days or weeks (on a timescale of hours to weeks), and late tissue

reactions, detected months to years after the irradiation. The most severe tissue reaction is death. Mortality after irradiation is generally the

result of severe cell depletion in tissues or other major dysfunction of one or more vital organs of the body.

The main concern of radiation safety at low doses has been radiation induced cancer and hereditary diseases. In the very early days of

radiation protection standards it was assumed that “genetic damage” from radiation (meaning hereditary effect), would accumulate across

generations and eventually have a marked impact on the health of human populations. Since recommendations adopted in the late 1970s

(ICRP 26) until present, it has been assumed that, stochastic or probabilistic effects may occur at low doses, and are generally considered to

be cancers (including leukemia) and genetic defects in the progeny [28]. This assumption has implied that there is a linear no-threshold

increase in genetic cell damage as a function of radiation dose, and that each unit of radiation would increase the risk. The potential

biological effects and damages caused by radiation depend on the conditions of the radiation exposure. It is determined by: quality of

radiation, quantity of radiation, received dose of radiation and exposure conditions (spatial distribution). In the context of radiation

protection, stochastic effects mean cancer and genetic effects [29]. The result of exposure to a carcinogen or a mutagen is an increase in the

probability of occurrence of the effect, with the increase in probability proportional to the size of dose. Thus according to Cember (1996),

people develop cancer whether or not they are exposed to carcinogens.

3.4.2. Radiation protection

The Aim of Radiological Protection

The primary aim of radiological protection, as stated in ICRP Publication 60, is “to provide an appropriate standard of protection for

mankind without unduly limiting the beneficial practices giving rise to radiation exposure”. Several features of medical practice require an

approach to radiological protection that is slightly different from that in other practices [30]. Because;

(a) The exposure of patients is deliberate. Except in radiotherapy, it is not the aim to deliver a dose of radiation, but rather to use the

radiation to provide diagnostic information or to conduct interventional radiology. Nevertheless, the dose is given deliberately and cannot

be reduced indefinitely without prejudicing the intended outcome.




(b) The patient needs a special relationship with the medical and nursing staff. For this reason, the system of protecting the staff from the

source, e.g. shielding, should be designed to minimize the sense of isolation experienced by the patient.

(c) In radiotherapy, the aim is to destroy the target tissue. Some deterministic damage to surrounding tissues and some risk of stochastic

effects in remote non-target tissues are inevitable. (d) Hospital and radiology facilities have to be reasonably accessible to the public, whose

exposure is thus more difficult to control than it is in the industrial premises.

Radiologists, medical physicists, and all supervising physicians have a responsibility for safety by keeping radiation exposure to staff, and

to society as a whole to make it as low as reasonable achievable ( ALARA principle) and to assure that radiation doses to individual

patients are appropriate, taking into account the possible risk from radiation exposure and the diagnostic image quality necessary to achieve

the clinical objective. Responsible bodies must understand the key principles of radiation protection and proper management of radiation

dose to patients. In general poorly controlled medical imaging equipment, techniques and facilities can cause serious harm to the patient.

Whenever medical imaging procedures are used, it is vital that the maximum diagnostic benefit is obtained while using the minimum

possible radiation dose.

Basic principles of radiation protection

The means for achieving the objectives of radiation protection have evolved over many years to the point where, for some time, there has

been a reasonably consistent approach throughout the world — namely the ‘system of radiological protection’, as espoused by the

International Commission on Radiological Protection (ICRP). The ICRP then puts exposure of individuals into three categories: medical

exposure, occupational exposure and public exposure.

All medical exposures must be subject to the principles of justification and optimization of radiological protection, which are common to

all practices dealing with potential exposures of humans to ionizing radiation. Medical exposure refers primarily to exposure incurred by

patients for the purpose of medical diagnosis or treatment. It also refers to exposures incurred by individuals helping in the support and

comfort of patients undergoing diagnosis or treatment, and by volunteers in a programme of biomedical research involving their exposure.

Occupational exposure is the exposure of workers incurred in the course of their work. Public exposure is exposure incurred by members of

the public from all exposure situations, but excluding any occupational or medical exposure. An individual person may be subject to one or

more of these categories of exposure. The ICRP system has three fundamental principles of radiological protection, namely: Principle of

justification, principle of optimization and principle of limitation of dose.

The principle of justification: Any decision that alters the radiation exposure situation should do more good than harm. Most of the

assessments needed for the justification of a practice are made on the basis of experience, professional judgement, and common sense, but

quantitative decision-aiding techniques are available and, if the necessary data are accessible, should be considered. Justification of

medical exposures requires that all medical imaging exposures must show a sufficient net benefit when balanced against possible detriment

that the examination might cause. For patients undergoing medical diagnosis or treatment, there are different levels of justification. The

practice involving exposure to radiation must be justified in principle through the endorsement of relevant professional societies, as matters

of effective medical practice will be central to this judgement. Also, each procedure should be subject to a further, case by case,

justification by both the referring clinician who is responsible for the management of the patient and the radiologist who selects the most

appropriate imaging examination to answer the referrer’s question. Justification of medical exposures is the responsibility of both the

radiological medical practitioner and the referring medical practitioner. A medical exposure is justified if it provides a benefit to the patient

in terms of relevant diagnostic information and a potential therapeutic result that exceeds the detriment caused by the examination. Imaging

methods with lower patient effective dose should be considered if the same diagnostic information can be obtained. This is true for all

patients, but is especially important for younger patients. No new imaging modality should be established unless the exposed individual or

society receives a net benefit to offset the detriment. Justification of medical exposures should be made on three levels.

The principle of optimization of protection: The likelihood of incurring exposures, the number of people exposed and the magnitude of

their individual doses should all be kept as low as reasonably achievable (ALARA), taking into account economic and societal factors.

The optimization of protection is the most powerful of the components of the system of radiological protection. It should pervade all stages

of the use of radiation in medicine. The underlying idea of optimization can be expressed by the question: Are there any reasonable

steps that I can take to improve protection? The basic aim of optimization of protection is to adjust the protection measures relating to

the application of a source of radiation within a practice in such a way that the net benefit is maximized. As with justification, experience,

professional judgement, and common sense play major roles in the procedures of optimization, all of which are consistent with the good

practice of medicine. In addition to the requirements of optimization of radiological protection, the concept of optimization of clinical

practice in diagnostic radiology must also be considered. This is the process requiring a diagnostic outcome for a patient from an imaging

procedure while minimizing the factors that cause patient detriment.

The principle of limitation of doses: The total dose to any individual from regulated sources in planned exposure situations other than

medical exposure of patients should not exceed the appropriate limits recommended by the ICRP. In a nuclear medicine facility,

occupational and public exposures are subject to all three principles, whereas medical exposure is subject to the first two only. The

exposure of individuals resulting from the combination of all the relevant practices should be subject to dose limits as stipulated in the

Radiation Ordinances (Cap 303), Laws of Hong Kong. Individual dose limits have been set for occupational and public exposure so that a

continued exposure just above the dose limits would result in additional risks from the relevant practices that could reasonably be described

as ‘unacceptable’ in normal circumstances.

Notwithstanding the responsibilities outlined above, all persons working with radiation have responsibilities for radiation protection and

safety; they must follow applicable rules and procedures, use available protective equipment and clothing, cooperate with personnel

monitoring, abstain from willful actions that could result in unsafe practice, and undertake training as provided [17]. During the operation

of the X-ray machine, radiation warning sign should be turned on and the following safety conditions must be taken.

1. Before making an exposure, close the doors of the X-ray room.

2. Do not direct the x-ray beam at windows of the room or towards the control panel or darkroom wall.




3. During radiography all staff must stand behind the protected control panel and may observe the patient through the lead glass window.

4. Gonad shields must be used on patients whenever appropriate, and the field must be adjusted to the minimum size consistent with

adequate clinical diagnosis.

5. When films or patients require support, use mechanical supports whenever possible.

6. No patient should wait or change in the X-ray room while another patient is being radiographed.

7. If anyone is ever required to support a patient or film during an exposure, he/she must: (i) wear a protective apron and gloves and avoid

the direct beam by standing to one side and away from the X-ray tube (ii) record, in the notebook provided, his/her name, the date, the

number of exposures, and the radiographic techniques used.

We can use the radiation warning sign as shown in the figure below (figure 11).

figure 15. radiation warning sign [5]

4. RESULT AND CONCLUSION

Result: This review demonstrated that fundamental knowledge and awareness in relation to x-ray radiation, radiation protection, biological

effects and doses used for radiological applications are insufficient or poor among patients and healthcare professions. It also showed that,

lack of communication between healthcare professionals and patients with respect to radiation exposure. Finally, this review document

result was realized that can fill the knowledge gap to concerned bodies.

Conclusion: Despite the benefits of x-ray radiation in diagnostic imaging, we learned that there are a lot of biological risks arising from it.

To make its benefit over weighs its risk, applying the ALARA principle effectively based on adequate knowledge and experience is a must.

To achieve the ALARA principle, continues training on basic concepts of radiation, its biological effect and radiation safety principles for

the concerned bodies and necessity of careful consideration of curriculum is thus recommended.

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