Resident Physics Lecture

Electromagnetic Radiation

Prof. J.K Tonui, PhD

School of Medicine,
Department of Radiology & Imaging
 Objectives
 At the end of this lecture, you will be able to:
a) Define electromagnetic (EM) radiation.
b) Know the different types of EM radiation/spectra.
c) Describe the wave and particle characteristics
(wave-particle duality) of EM radiation.
d) Calculate the energy, frequency, and wavelength of
EM radiation.
e) Identify the properties associated with radiation and
the ability to cause ionization.

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 Introduction
 Diagnostic imaging employs the following forms of energy :
 X-rays,
 Gamma rays,
 Radiofrequency and
 Sound.
 Human body (matter)
 Is partly but not completely transparent to the energies above
but
 All them are used for diagnosis (imaging) and therapeutic
(treatment) applications in radiology departments.

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 Energy and Matter
 Energy and matter are two important and related quantities
in nature, where:
 Matter
o Describes “staff” contained by any object (e.g. human body),
o Has a definite size, shape and physical state (solid, liquid and gas), and
o Is characterized by 2-physical quantities - mass and volume.
 Energy
o Describes the ability (effort) of radiation to do work, and
o Make matter visible or invisible, hence
o Produces images of the patient’s internal tissues in medical imaging, and
o Energy has different forms but no size nor shape.

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 Matter - Mass
 Matter
 Is characterized by mass, which in turn
 Is composed of collection of smallest particles called an atom.
 A combination of two or more atoms
 Is called a molecule, which
 Is sometimes used as the smallest unit of mass for some substances,
especially gaseous substances, which
 Can exists as diatomic, e.g. H2, O2, N2 or multi-atomic, e.g CH4.
 An atom consists of even smaller particles
 Called subatomic particles, which are
 Electron, proton and neutrons.

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 Mass
 The units of mass, i.e. kilogram (kg) or gram (g)
 Are too large when used to measure masses of subatomic
particles, hence
 A smaller unit was introduced to measure masses at the
subatomic level, which
 Is called the atomic mass unit (amu), defined as:

1amu  1.66 1024 g  1.66 1027 kg

 The masses of subatomic particles in amu are:

 Mass of a proton, mp = 1.00783 amu
 Mass of a neutron, mn = 1.00866 amu
 Mass of an electron, me = 0.00055 amu

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 Energy
 Energy
 Is defined as the capacity (effort expended) to do work and
 Can exist in several forms:
o Chemical- ability of a substance to undergo transformation,
o Kinetic – energy due to motion of an object,
o Potential – energy due to position of an object in a force
field,
o Binding – energy holding particles or atoms/molecules of
an object together,
o Nuclear – energy released when nucleus disintegrate or
breaks.

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 Relation btw energy and mass
 Mass and energy are related by Einstein equation:

E  mc2
 where E is energy in Joules (J), m is the mass in kilograms (kg), and c is
the velocity of light in a vacuum, c = 3 × 108 m/s.
 The Einstein equation above
 States that everything around us can be classified as either matter
or energy,
 One can be converted to the other!

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 Kinetic & Potential Energy
 Kinetic Energy (K.E)
 Is energy of an object by virtue of its speed
and is given by:
1
K .E  m 2
2

o Where m = mass (kg), and v = velocity (m/s)

 Potential Energy (P.E)

 Energy of an object by virtue of its position in
a force field, e.g. gravitational force:
P.E  mgh

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 Conservation Laws
 Both energy and mass
 Are conserved quantities, which means that
 Energy and mass are neither created nor destroyed.
 Energy conservation law
 States that energy can’t be created nor destroyed but can be converted from
one form to another.
 Mathematically,
 It states that the sum of K.E and P.E is constant:

E  K.E  P.E
 Einstein equation is also mathematical form of energy conservation.

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 Radiation

 Radiation
 Is a name given to energy that is capable of travelling thro’ space
and matter, and
 This radiation is always around us as either natural or man made!
 Radiation sources can be
 A collections of matter
o Store energy within them e.g. sun or radioactive material
 A device that convert other forms of energy into radiation
o e.g. a bulb converts electrical energy into light, x-ray tube convert
electricity to x-rays etc.

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 Types of Radiation
 Two types of radiation is used in radiology:
 Electromagnetic (EM) radiation
o e.g. radio waves, visible light, heat, X- and  rays
 Particles radiation
o e.g. beta, neutrons, alpha and protons.

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 Electromagnetic (EM) Radiation
 EM radiation is now accepted as having dual character:
It behaves both as a wave and a particle, which is
Is usually referred as wave-particle duality.
Compare with human beings – body & spirit!
 The Physicists
 Use whichever form that describes experimental
observation well.
 This radiation
 Travels with the speed of light in vacuo or air, i.e.
 c = 3.0 x 108 m/s.

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 EM radiation - Photon
 As a particle,
 EM radiation is thought of as consisting of little energy packet, called
o A photon or Quanta.
 Photons
 Can be emitted or absorbed when atoms or molecules change energy
levels or move from one level to another and
 Each photon carries energy given by

E  h
 Where E is energy (J), h is plank’s constant (h = 6.62 x 10-34 Js) and 
is frequency (Hz).

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 EM radiation - Photon
 As a wave,
 EM radiation obeys wave equation, which relates its velocity (c),
wavelength () and frequency () as:

c  
 Where  is wavelength (m), which is visualized by the color of
light (rainbow colors have different wavelengths).
 Hence, photon energy can also be given as:
hc
E


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 EM Radiation - Spectrum
 EM radiation consists of a family of radiation that
 Differ only in the amount of energy packaged in each photon.
 Examples are:
1. x-rays
2. Radio waves
3. Microwaves 2
1
4. Light

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 EM Radiation
 EM radiation
 Has no mass but posses energy,
 Is not deflected by either electrical or magnetic fields,
 Propagates through matter as well as vacuum and
 Has a constant speed in a given medium,
o Has maximum speed in vacuum of 3.0 x 108 m/s and
o Lower in other media other air, and is given by:

c

n
o Where n is called index of refraction of the substance or medium.

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 Properties of EM radiation
 EM radiation
 Travels in straight lines but its trajectory can be altered when
it interacts with matter.
 This interaction can occur either by
 Absorption - removal of the radiation from the beam or
 Scattering - change in direction of the trajectory.
 EM radiation is characterized by
 The wavelength (), frequency (v), and energy per photon (E),
which related by:
hc
E  h 


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 Categories of EM wave
 The categories of EM radiation
 Is called EM spectrum and is given in next slide.
 EM radiation used in radiology are:
 -rays
o Which emanate from within the nucleus of radioactive atom and
o Are used in nuclear medicine for imaging and therapy
 X-rays
o Are produced outside the nucleus, or at the orbits of an atom
and
o Are used in radiography modalities

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 EM Spectrum

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 The EM Spectrum

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 The EM Spectrum

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 EM Radiation Used in Radiology
 Visible light
 Is perceived by our eye, but cannot penetrate our bodies, and
 Is used in detection of x- and -rays and
 For examination of x-ray images in view boxes.
 Radiofrequency (RF) radiation
 Are used for excitation and detection of signals for image
production in MRI systems.
 X-rays and -rays
 Are ionizing radiation used for medical imaging (diagnosis) and
treatment (therapy).

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 Wave-Particle Duality of EM radiation
 As mentioned, EM radiation
 Sometimes acts like particles or waves.
 The particle nature explains
 Radiation interaction (collision) with
matter.
 Wave concepts explains
 Refraction, diffraction and polarization
properties of EM waves.

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 Wave Characteristic
 Waves
 Can be mechanical or electromagnetic and
 Are characterized by amplitude (A), wavelength (), frequency (v),
and period (T).
 A (m) – gives intensity or energy of the wave (E  A2).
  (m) - distance between any two identical points on
adjacent cycles.
 T (s) - time required to complete one cycle of a wave.
  (Hz) - number of complete cycles per second

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 Wave Nature

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 Wave Nature
 Wavelengths of x- and  rays are typically measured
 In nanometers (nm),
 Where 1 nm = 10-9 m.
 Frequency is related to period by
1

T
 Frequency is expressed in hertz (Hz),
 where 1 Hz = 1 cycle/sec = 1 s-1.

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 Wave Nature
 EM radiation propagates
 As a pair of perpendicular electric and magnetic fields, as shown in
below, and notice
 E- and B-field are  to each other and direction of travel

Electric field

Magnetic field

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 Worked Examples
 Question: Find the frequency of the blue light with a wavelength of 400
nm.
 Solution: From the wave equation given above , we make the frequency
the subject to get
3.0 10 8
c
   7.5  1014
Hz
 400 10 9

 Question: Sound waves are disturbances in the air and travel at a

velocity of 344 m/s. Middle C has a frequency of 264 Hz. Calculate the
wavelength of this note.
 Solution:  344
   1.3 m
 264

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 Particles Characteristic
 Particle radiation are used normally in NM for therapeutic
applications and such radiation have
 Mass and charge, except neutrons, which have zero
charge and
 Their velocities depends on their kinetic energy.
 The source of these particles are:
 Radioactive decay,
 Cosmic rays, and
 Nuclear reactions, etc.

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 Particle Radiation
 The particle radiation used in medical imaging are
produced artificially using three common methods:
 Nuclear reactors,
 Particle accelerators ( linear or cyclotron) and
 Generators.
 The particle radiation used mainly in NM
 Is the beta and positron particles,
 Which are fast moving electrons and positively charged electrons
(hypothetical) respectively.

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 Particle Radiation
 The beta particles or electrons
 Are produce by radioactive atoms with excess neutrons, and
 Are normally used for therapy and not imaging because of their low
tissue penetration, and
 Electrons are produced when x-rays interact with matter, and
 The process is called ionization.
 The positrons
 Are positively charged electrons (hypothetical) and
 Are produced by radioactive atoms with excess protons, but
 Are immediately annihilated by combining with nearby electrons and
 Yield two -rays in opposite direction and are used for imaging in
NM technique called Positron Emission Tomography (PET).

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 Wavelengths & Frequency of EM
Highest wavelength = lowest frequency
Radio
Infrared
Visible light
Ultraviolet
Soft x-rays
Diagnostic x-rays
Therapeutic x-rays & gammas

Lowest wavelength = highest frequency

Velocity = Wavelength X Frequency

c=x
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 Photon Energy
 Usually the energy of the photon is calculated
 Using a simplified equation derived by substituting the values
of h = 6.625 × 10−34 Js, and c = 3 × 108 m/s into the energy
equation in the previous slide to get

1.24
E (keV) 
 (nm )

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 SI Unit of Energy - Joule (J)
 The SI unit of energy
 Is the joule (J), but
 Is a very large unit when used to express energies of radiation
used in radiology, hence
 A smaller unit, called electronvolt (eV), was introduced, and
 Is defined as the kinetic energy acquired by an electron which has been
accelerated by a potential difference of 1Volt (V) and
 Has numerical value:
E  qV  1.602 10-19 C x 1 V

1eV  1.602  10-19 J

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 Energy Unit - Electron Volt
 Hence,
 The electron volt (eV) is a small energy unit, and
 Is widely used to express photon energy together with its multiples:
o 1 keV = 1,000 eV and
o 1 MeV = 1,000,000 eV .
 The energy range of typical x- and  rays photons used in
radiology are:
 For imaging, it range from 15 keV (for mammography) to several
hundred keV (max. 200 keV), and
 For therapy, energy range is a few MeV.

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 Ionizing and non-ionizing Radiation
 Radiation are also classified according to their ability to ionize
matter into two categories:
1. Ionizing radiation
o Are radiation with enough energy to strip electrons (i.e. ionize or
remove) from atoms when they interact and
o Examples are x- and -rays, UV and fast moving particles.
2. Non-ionizing radiation.
o Are radiation without enough energy to strip electrons from atoms and
o Examples are microwaves, and visible light.

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 Ionizing and Non-ionizing Radiation
Region of ionizing radiation

Non-ionizing radiation

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 Ionization
 Ionization
 Removes bound electrons from atomic shells, hence
 Produced ionized atoms or molecules.
 The threshold energy for ionization depends on the type
of matter, e.g.
 The minimum energy necessary to remove an electron from H2 is
13.6 eV, H20 is 12.6 eV and C6H = 9.3 eV, and
 This energies are called ionization energy.

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 Typical Examples of MCQs
1. The energy equivalent rest mass of an electron is:
A. 981 MeV
B. 1.02 MeV
C. 0.51 MeV
D. 1.02 keV
E. 0.51 keV
2. In the electromagnetic spectrum, which of the following has
the highest frequency?
A. radio waves
B. ultra violet rays
C. visible light
D. infra red rays
E. Gamma rays

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 Typical Examples of MCQs
3. Photon energy _________ wavelength.
A. is directly proportional to
B. is inversely proportional to
C. does not depend on
D. increases (but not proportionally) with
E. decreases (but not proportionally) with

4. The relationship between photon speed c, wavelength  and the

frequency v is:
A. c =  . v
B. v = c . .
C. =v/c
D.  = v / c
E. None of the above

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