Medical X-ray Film Processing K Thayalan
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Basic ConceptsCHAPTER ONE

Physics is a science dealing with nature. It is very important to study the properties of material bodies in nature, under different physical conditions. Measurement of physical quantities such as length, mass and time are essentitial to understand the properties of material bodies. Any physical quantity is measured accurately only in terms of a standard of its own kind. The standards which are defined and accepted by all are called the units. For example, distances are measured in meters, masses in kilograms and time in seconds.
1.1.1 SI System
The system of units, called the SI system has six fundamental units. They are:
1. Length
meter (m)
2. Mass
kilogram (kg)
3. Time
second (s)
4. Electric current
ampere (A)
5. Temperature
kelvin (K)
6. Luminous intensity
candela (cd)
1.1.2 Units of Length, Mass and Time
Meter: One meter is the length equal to 1650763.73 wavelength of the orange-red line of krypton = 86 discharge lamp kept at 15°C and 76 cm of mercury.
Kilogram: One kilogram is the mass of the platinum-iridium cylinder of diameter equal to its height kept at the international bureau of weights and measures near Paris.
Time: The second is defined as the duration of 9192 631 770 periods of the radiation corresponding to the transition between two specified energy levels of Cesium-133 atom.
Matter may be defined as that which occupies space and affect our senses continually. A limited portion of our matter is called material body. A material body may be considered to be made up of very large number of particles.
1.2.1 Motion and Speed
A body is said to be in motion if it changes its position continuously with respect to its surroundings. The speed of a body is the rate at which it describes its path. Here the direction of motion is not taken into account. Thus, speed has only magnitude.
1.2.2 Displacement, Velocity and Acceleration
Displacement of a moving body is its change of position measured by a straight line joining the initial and final positions of the body. It has both magnitude and direction. The velocity of a moving body is the rate of change of displacement. The unit of velocity is m/s. Acceleration of a moving body is its rate of change of its velocity. The unit of acceleration is m/s2.
The acceleration exerted by the earth on the body is called the acceleration due to gravity of the earth. It is denoted by the letter g and the value of g varies from place to place. For practical calculations, the approximate value of g is 9.8 m/s2.
1.2.3 Inertia and Force
Newton has showed that all material bodies have inertia. It is the inability of any object to change itself its state of rest or of 3uniform motion. Force is the action exercised on a body so that it changes the state of rest or of uniform motion of the body. It is measured by the product of mass of the body and the acceleration produced by the force on the body (F = ma). The unit of force is newton. It is the force acting on a body of mass 1 kilogram producing an acceleration of one m/s2.
1.2.4 Mass and Weight
The mass of a body is the quantity of matter contained in it. A weight of a body is the force of attraction exerted by the earth on it. If m is the mass of a body and g is the acceleration due to gravity at a given place, then weight = mg newtons.
1.2.5 Work
If a force acts on a body and the point of application of the force moves, then work is said to be done by the force. If the force F is applied and it moves the body through a distance s, in its direction then work done (W) = F × s. The unit of work is joule. One joule is the amount of work done when the point of application of a force of 1 newton moves a body through a distance of 1 m.
1.2.6 Power
The rate of doing work is called power. It is measured by the amount of work done in unit time. Power = W/t. The unit of power is watt. One watt = 1 Joule/s. The power consumed at the rate of 1 kilowatt for 1 hour is called 1 kilowatt-hour, which is = 36 × 105 J.
1.2.7 Energy
The energy of a body is its capacity to do work. It is measured by the amount of the work that it can perform. The unit of energy is joule. Usually, radiation energy is measured by the unit electron volt (eV). There are many forms of energy such as mechanical energy, heat energy, light energy, electrical energy and atomic energy.
There are two forms of mechanical energy, such as (i) potential energy and (ii) kinetic energy. Potential energy of a body is the energy it possess by virtue of its position. It is calculated by the relation mgh, where m is the mass of the body, g is the acceleration due to gravity and h is the height from the earth. The kinetic energy of a body is the energy possessed by it due to its motion. Kinetic energy = mv2/2, where v is the velocity and m is the mass of the body. Energy can neither be created nor destroyed, but can be transformed from one form to another.
1.3.1 Charge (q)
The term electric is derived from the Greek word electron. Electric bodies are said to possess electric charge. There are two types of charges namely; i. positive charge, and ii. negative charge. Two like charges repel each other and two unlike charges attract each other. Electric charge in nature comes in units of one magnitude (e) only. Hence anybody can have charges only in multiples of e. The charges can neither be created nor be destroyed. Materials which allow charges to pass through them are called conductors, e.g. metals, human body, earth, etc. Bodies which do not allow the charge to pass through are called insulators, e.g. glass, mica, plastic, etc.
The unit of charge is coulomb. One coulomb is the charge which when placed at a distance of 1 m in air or vacuum from an equal and similar charge experiences a repulsive force of 9 × 10−9 N.
1.3.2 Potential (V)
The space around a charge in which its influence is felt is known as electric field. The electric potential at a point in an electric field is the work done in taking a unit positive charge from infinity to that point. The unit of potential is volt. The potential difference between two points is said to be 1 volt, if the work done in moving a unit charge from one point to other is 1 J.5
1.3.3 Current (I)
The flow of electric charge in a conductor is called an electric current. It is defined as the rate of flow of charge through any section of a wire. The unit of current is ampere (A). The electric current through a wire is called one ampere, if one coulomb of charge flows through the wire in one second.
Every matter is made up of atoms and molecules. The atoms are always in movement and hence possess kinetic energy. This kinetic energy is responsible for the hotness and coldness of a body. Temperature is the measure of the hotness or coldness of a body. Temperatures are measured in degrees with the help of thermometers. Temperatures are measured either in fahrenheit (°F) or celsius (°C) or in kelvin (K).
All thermometers have lower fixed point and upper fixed point. The temperature of melting point of ice is taken as the lower fixed point. The temperature of the steam is taken as the upper fixed point. In the Fahrenheit scale, the lower fixed point is 32 and the upper fixed point is 212. The interval between the two points is divided into 180 equal points. In the Celsius scale the lower fixed point is 0 (zero) and the upper fixed point is 100. The interval between the two points is divided into 100 equal points. Zero degree centigrade (Celsius) equal to 273 Kelvin. The relation between Celsius and Fahrenheit is given by C/100 = (F–32)/180.
Heat is form of energy which can be transformed from one place to another. There are three methods of heat transfer namely, conduction, convection and radiation. The conduction and convection process require a medium, whereas the radiation process need no medium for tranfer. Heat is measured in joule or calorie. One calorie = 4.2 joule.
Everything in the world is made up of one or more of the basic substance called elements, e.g. carbon, iron, mercury and oxygen. Elements consist of tiny bits of matter called atoms. 6An atom is defined as the smallest indivisible particle of an element that can take part in a chemical reaction. Atom can join together to form larger chemical units called molecule. For example, two atoms of hydrogen combines with one atom of oxygen and forms the water molecule (H2O). Thus, molecules are the smallest part of an element or a compound which exist in a free and separate state.
More recent discoveries proved that the atoms contain still small particles such as protons, neutrons and electrons (Fig. 1.1). These particles are called subatomic particles. Proton is a positively charged particle, whereas the electron is a negatively charged particle. Neutrons has no charge. The protons and neutrons are clustered at the center of the atom, called nucleus. The electrons are arranged outside the nucleus.
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Fig. 1.1: Atomic structure of hydrogen atom
1.5.2 Atomic Number and Mass Number
The number of protons in an atom is equal to the number of electrons. This is called the atomic number. The sum of the protons and the neutrons present in the nucleus of an atom is called the mass number. Atoms with same atomic number but with different mass numbers are called isotopes. Isotopes with unstable nuclei capable of performing radioactivity is called as radioisotopes. The electron has very little mass. The proton mass is roughly equal to that of 1836 electrons. The mass and charge of these particles are as follows:
1.66 × 10−27kg, 1.6 × 10−19 coulombs
1.7 × 10−27kg, nil
1/1840 of proton mass, −1.6 × 10–19 coulumbs
1.5.3 Arrangement of Electrons
The electrons revolve around the nucleus in circular orbits corresponding to different energy levels. These energy levels are called shells, namely K, L, M, etc. Each shell is at a definite distance from the nucleus. The maximum number of electrons that can be accommodated in any shell can be found by the formula 2n2, where n is the number of the shell. Thus, K, L and M shell can have 2, 8 and 18 electrons respectively. The arrangement of electrons in different shells, for the first 5 elements are given in the Table 1.1.
Table 1.1   Arrangement of eletrons in different shells
Mass number
Atomic number
No. of electrons in shells
Hydrogen (H)
Helium (He)
Lithium (Li)
Beryllium (Be)
Boron (B)
An electric charge is surrounded by an electric field. If the charge moves, a magnetic field is produced. When the charge undergoes an acceleration or deceleration, the magnetic and the electric fields of the charge will vary. The combined variation of the electric and magnetic fields results in loss of energy. The charge radiate this energy in a form known as electromagnetic radiation. The electromagnetic radiation moves in the form of sinusoidal waves (Fig. 1.2).
The electromagnetic wave possess wavelength (λ), frequency (ν) and velocity (c). The distance between two consecutive positive peaks is known as wavelength. The number of cycles of the wave which pass a fixed point per second is known as the frequency of the wave.8
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Fig. 1.2: Electromagnetic radiation
The velocity of the wave is the distance traveled per second by the wave. The relation between wavelength, frequency and velocity of the electromagnetic wave can be expressed as c = νλ. All electromagnetic waves, travel at the same velocity in a given medium. In vacuum the velocity is about 2.998 × 108 meter per second.
1.6.1 Quantum Nature of Radiation
Though electromagnetic radiation have the properties of waves, it also behaves like a stream of small bullets each carrying a certain amount of energy. This bundle of radiation energy is called a quantum or photon. The amount of energy carried by the photon depends upon the frequency of the radiation. The actual amount of energy (E) carried by a photon is given by the equation E =hν, where h is the Planck's constant = 6.63 × 10−34 Js. Substituting the value of ν = c/λ in the above equation, the energy
E = hc/λ = 124/λ
The product of the velocity of light (c) and Plank's constant (h) is 124. E is measured in keV and λ is in nanometers (nm). It is seen that the energy of the photon is inversely proportional to its wavelength. As the wavelength decreases, the energy increases.9
1.6.2 Fluorescence
When electromagnetic radiation falls on certain crystalline substance, visible or ultraviolet light is emitted from the crystal. This phenomenon is called fluorescence. Fluoroscence is a form of light emission within 10−8s of X-ray exposure. If the emission of light is delayed beyond 10−8s, it is called phosphorescence. Actually the crystal absorbs the radiation of shorter wavelength which produces excitation in the crystal. Due to subsequent electronic transition the crystal emits radiation of energy in the visible region, e.g. Zinc-cadmium sulfide and calcium tungstate.
1.6.3 Electromagnetic Spectrum
Electromagnetic spectrum includes radiation from very long radio waves to short penetrating gamma rays. All of them travel at a velocity c in a vacuum. The wavelength and photon energy of the whole range of electromagnetic radiations are summarized in Table 1.2.
Table 1.2   The wavelength and photon energy range of electromagnetic radiations
Photon energy
Radio, Television and
Radar waves
3 × 104 m -100 µm
4.1 × 10−11 eV - 1.2 × 10−2 eV
Infra red
100 µm-700 nm
1.2 × 10−2 eV - 1.8 eV
Visible light
700 nm-400 nm
1.8 eV – 3.1 eV
400 nm-10 nm
3.1 eV – 124 eV
X-rays, Gamma rays
10 nm-10−4 nm
124 eV - 124 MeV
Cosmic rays
10−4 nm-10−7 nm
107 eV - 1011 eV