Essay About X-Ray Fundamentals And Radio Waves

X-Ray Fundamentals
Essay title: X-Ray Fundamentals
Basic physics of X-ray imaging
Carl A Carlsson and Gudrun Alm Carlsson
Department of Radiation Physics
Faculty of Health Sciences
Linköping university
Sweden
REPORT
LiH-RAD-R-008
Second edition 1996
TABLE OF CONTENTS
1. Introduction ……3
2. The physics of the X-ray source: the X-ray tube ………3
3. The energy spectrum of X-rays ………7
4. The interactions of X-rays with matter ………12
5. Contrast ………19
6. Energy absorption of X-rays ………22
7. Stochastics in the X-ray image ………27
8. Appendix ………28
9. References ………29
Basic physics of X-ray imaging
1. INTRODUCTION
In X-ray diagnostics, radiation that is partly transmitted through and partly absorbed in
the irradiated object is utilised. An X-ray image shows the variations in transmission
caused by structures in the object of varying thickness, density or atomic composition. In
Figure 1, the necessary attributes for X-ray imaging are shown: X-ray source, object
(patient) and a radiation detector (image receptor).
Figure 1. The necessary attributes for X-ray imaging: X ray source, object (patient) and
radiation detector
After an introductory description of the nature of X-rays, the most important processes in
the X-ray source, the object (patient) and radiation detector for the generation of an X-ray
image will be described.
2. THE PHYSICS OF THE X-RAY SOURCE: THE X-RAY TUBE
a. The nature of X-rays
X-rays are like radio waves and visible light electromagnetic radiation. X-rays, however,
have higher frequency, ν, and shorter wavelength, λ, than light and radio waves. The
radiation can be considered as emitted in quanta, photons, each quantum having a well
defined energy, hν, where h is a physical constant, Plancks constant, and ν is the
frequency. The energy of X-ray photons are considerably higher than those of light.
A number of the phenomena, which are observed with X-rays are most conveniently
described by the wave properties of the radiation while other phenomena can be more
easily understood if the X-rays are considered as being composed of particles (photons)
with well defined energies and momentum. The rest mass of a photon is zero. This means
that photons can never be found at rest. All photons move at the same velocity, c, in a
vacuum, given by c = 2.998 108 m/s.
b. Relationship between wave length and frequencyThe wave length multiplied with the frequency (number of wave lengths per unit time)
equals the velocity of light
λ⋅ν=c (1)
c. The propagation of X-rays
Similarly to visible light, X-rays propagate linearly. The rays from a point source form a
divergent beam. The number of photons passing per unit area perpendicular to the
direction of motion of the photons is called the fluence, Φ. The fluence in a vacuum
decreases following the inverse square law, given by
Φ(r)=Φ(1)⋅1