DIAGNOSTIC X RAY BASICS

1. Presenter – Dr. Prajwith K J Rai

2.  Wilhelm Conrad Roentgen discovered x rays on
November 8, 1895
 Properties of x rays - December 28, 1895
 Was awarded the first Nobel Prize for Physics in
1901
 International day of Radiology – November 8

3. X rays belong to a group of radiations called electromagnetic radiation,
which is the transport of energy through space as a combination of electric and
magnetic fields.
Radio waves, radiant heat, visible light and gamma radiation
Electromagnetic radiation is produced by a charged particle being accelerated
(type of energy).

4.  Electromagnetic radiation acts as a
1. wave
2. particle.

6. Nucleus of an atom – Protons(Z) + neutrons
Electrons orbit in specific shells (K, L, M, N, etc.) around the nucleus
Orbiting electrons – Properties of X rays and its interaction with matter

7. Ionizing radiation – Higher energy; emits electrons or other
particles from atom when they collide.
Eg. Alpha, gamma and x rays
Non ionizing radiation – Excites electrons from a lower level to a
higher level
Eg. UV, visible, infrared, microwave, radio waves

8. Wave concept
 Electromagnetic radiation is propagated through space in the form of waves.
 atom absorbs energy  The absorbed energy causes one or more electrons to
change their location within the atom.
 When the electron returns to its original position, an electromagnetic wave is
produced.
 Depending on the kind of atom and the amount of energy, this electromagnetic
radiation can take the form of heat, light, ultraviolet, or other electromagnetic
waves.

9.  Electromagnetic waves do not require a medium; they can be propagated
through a vacuum.
 Distance b/w two successive crests - wavelength of the wave (λ).
 Number of waves passing a particular point in a unit of time - frequency (ν).
 Velocity of the wave, V = λ x ν.

10. Electromagnetic radiation travels at the same velocity in a vacuum
(3 x 108 meters per second)
Frequency inversely proportional to wavelength.
Electromagnetic radiation differs commonly in wavelength.
Wavelength of diagnostic x rays is extremely short and expressed in
angstrom units (Å) or nanometers.

11. Particle concept
Short electromagnetic waves (X rays) may react with matter as if they were
particles, which are usually discrete bundles of energy (quantum/ photon).
Describes the interactions between radiation and matter.
energy of quantum or photon ∝ frequency of radiation.
E = hν, where E = photon energy, h = Planck's constant and v = frequency

13.  X rays are produced by energy conversion when a fast moving stream of electrons is
suddenly decelerated in the target anode of an x ray tube.
 X ray tube - Pyrex glass that encloses vacuum containing two electrodes (diode tube).
 Electrons produced at cathode (negative electrode or filament) can be accelerated by a
high potential difference towards anode (positive or target electrode).
 Electrons produced by a heated tungsten filament and accelerated across the tube to hit
the tungsten target, where x rays are produced.

15. Two sources of electrical energy are required –
Filament heating voltage (10V) and current (10A)
Accelerating voltage (30-150kv) between anode and cathode – drives the current
of electrons flowing between anode and cathode (tube current)
Small increase in temperature produces large increase in tube current.

16.  Fast moving electrons are suddenly stopped by impact on metal target
 Electric charge of electron does not change  the increasing voltage across the x ray tube
increase the kinetic energy of electron (E = ev)
 High speed electrons lose energy at target by 2 processes –
Reaction of electrons with nucleus of tungsten atoms
Collision between high speed electrons and electrons in the shell of target tungsten
atom

17. Bremsstrahlung radiation
 When an electron pass near the nucleus of a tungsten atom, the positive charge of
the nucleus acts on negative charge of electron.
 The electron is thus deflected from its original position.
 Electron may lose energy and slows down when its direction changes.
 The kinetic energy lost by electron is directly emitted in the form of photon
radiation.

18.  The electron only gives up part of its energy in
the form of radiation - braking radiation.
 Braking phenomenon (wide distribution in
the energy of radiation) -
Electron undergoes many reactions before
coming to rest
Electron beam that strikes the target have
widely different energy.

19. Wavelength of the radiation produced depends on energy of the electron
(keV) and the potential difference (kVp) .
Deceleration of the electrons in the electric field of a nucleus depends on
how close the electron passes to the nucleus,
the energy of the electron and
the charge of the nucleus.

20. Characteristic radiation
The electrons bombarding the target ejects
electrons from the inner orbit of the target
atoms.
Removal of an electron from the tungsten atom
causes the atom to have an excess positive
charge → a positive ion.

21. In the process of returning to its normal state, the ionized atom of
tungsten may get rid of excess energy in two ways:
An additional electron (Auger electron) may be expelled by the atom and
carry excess energy [there is no x ray production in this way].
The atom emits radiation that has a wavelength within the range of
diagnostic x rays [characteristic x rays.

22. Properties of X rays Highly penetrating, invisible rays.
 Liberate minute amounts of heat on passing through matter.
 Behave both as waves and as particles.
 Are not deflected by electric or magnetic fields. (electrically neutral).
 Poly energetic, having wide spread of energies and wavelengths,
useful energy range 25 to 120 kVp.

23. Travels ordinarily in straight lines with same speed as light. (3x108 m/sec).
Cause fluorescence of certain crystals, making possible use in
fluoroscopy and
radiographic intensifying screens.
Produce biological and chemical changes by ionization and excitation in
substances through which they pass.
Cannot be focused by a lens.
Produce secondary and scattered radiation.
Ionize gases indirectly by ability to remove orbital electrons from atom.

25. X-ray photons may interact either with the
1. orbital electrons or with the
2. nucleus of the atom
In diagnostic energy range, the interactions are always with orbital electrons.
Interactions depend on the atomic makeup of the tissue and not its molecular
structure. E.g. oxygen atoms will stop the same number of x-ray photons
regardless of their physical state.

26. X ray photons may be either absored / scatter.
If absorbed  completely removed from the x ray beam  cease to exist.
If scattered  deflected into a random course  no longer carries information 
cannot portray an image (blackness)  as NOISE / FILM FOG.

27. ATTENUATION – A reduction in the intensity (energy) of the beam
ABSORPTION – The transfer of energy from the beam to the irradiated
material.
SCATTER – Radiation in a direction other than the primary beam, with or
without a loss of energy

28. Radiation interaction depends on :
Tissue electron density
Tissue thickness
Energy of the x ray (kVp).
The various structures of the body attenuate by differing amounts.

29. Coherent scattering
Photoelectric effect
Compton scattering
Pair production
Photodisintegration

30. Coherent scattering
When radiation undergoes a change in direction without a change in the
wavelength.
Occurs when low energy radiation encounters the electron of an atom and sets
them into vibration at the frequency of radiation.
Also termed as UNMODIFIED SCATERRING or CLASSICAL SCATTERING.

31. Absorption of radiation
Vibration of electron/atom
Emission of radiation as atom
returns to its undisturbed state

32. Two types –
Thompson scattering – single electron is involved.
Rayleigh scattering - cooperative interaction with all the
electrons of an atom.

33. Increases with low atomic number materials and lower photon energies.
Less than 5% of radiation undergoes coherent scattering.
Does not play a major role in diagnostic x rays.
This is the only type of interaction that does not cause ionization as there is
no energy transferred.

34. Photoelectric Effect The incident photon collides with the K shell electron  Gives up its energy to the
electron to overcome the binding energy  flies into the space as photoelectron
(negative ion)
 The vacant spot in the K shell is filled by an electron from the adjacent L or M shell
electrons → x ray photon is released (characteristic radiation).
 Atom deficient of one electron even if free electron from another atom fills the void→
end result is the same, a positive ion.

36. Probability of Occurrence
 Incident photon must have sufficient energy to overcome binding energy of the electron.
 Likely to occur when the photon energy and electron binding energy are nearly the same;
photoelectric effect is inversely proportional to energy.
 Tightly bound electrons are more likely to be involved in a photoelectric reaction; electrons are
more tightly bound in element with high atomic number.

37. Applications to diagnostic radiology
Advantages :
Produces good radiographic images
Does not produce scatter radiation
Enhances the natural soft tissue contrast
Film quality is good
Magnifies the difference in tissues composed of different elements
such as bone and soft tissue.
Disadvantages: Patient receives more radiation.

38. Compton scattering
 An incident photon with relatively high energy strikes a free outer shell
electron  ejects from its orbit  travels a new direction as scatter radiation
 Positive atom and negative electron “recoil” electron.
 Energy of incident photon goes to recoil electron (as kinetic energy) and retained in
the deflected photon.
 The amount of energy the photon retains depends on the initial energy and the angle of
deflection off the recoil electron.
 Photons also have a momentum, and the higher the energy of the photon, the difficult
they are to deflect.

39.  Probability of occurrence :
Depends on the total number and density of the electrons.
Higher the photon energy it is more likely to pass through the body than a low energy
photon
 For elements with low atomic number, all the electrons can be considered free, even k
shell.

40. Pair production The photon interacts with the nucleus in such a manner that its energy is converted to
matter namely an electron and positron.
 Both have the same mass and resting mass energy of 0.51 MeV.
 Not encountered in diagnostic procedures as it involved photons with
energies in excess of 1.02 MeV.

41. Photodisintegration
 Part of the nucleus of a atom is ejected by high energy photon.
 The ejected particle may be a neutron, proton, alpha particle or a cluster of particles.
 Incident photon must have sufficient energy to overcome nuclear binding energies of
7 – 15 MeV which is also far beyond diagnostic radiology and hence insignificant.

42. Only two interactions are important in diagnostic radiology, the
photoelectric effect and Compton scattering.
Coherent is numerically unimportant.
Pair production and photodisintegration = higher energies
The photoelectric effect is low energy interaction with high atomic number
absorbers produces high contrast.
Compton most common and is responsible for almost all scatter radiation
Summary