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

r2