ROENGTENOGRAPHY

Introduction:
The basic properties of any wave are: 1. wavelength 2. frequency and 3. speed.
Xrays are electromagnetic waves and they obey the relation:
velocity = frequency times wavelength

Xray Generation:
In an atom the electrons are arranged in discrete enery levels called orbits. The electrons nearer the nucleus have lower energies whereas the electrons further away have higher energies. If an electron form a lower orbit jumps to a higher orbit, then a vacancy is created. Some other electron form a higher orbit jumps to fill that vacancy. In this de-excitation process, a single photon is emitted of just the right frequency, so that it falls in the category of an Xray photon.

A Practical Xray Machine:
1. evacuated Xray tube 2. cathode 3. rotating anode
Formation of space charge due to thermionic emission is followed by attraction of the electrons from the anode. The anode is coated with a hard metal like tungsten. When the electrons are suddenly stopped by the target. Xrays will be produced. The anode is rotated at speeds of 3000-4000 rpm to avoid over-heating.

Interaction of Xrays with matter:
1. attenuation 2. initiation of chemical reactions

Attenuation: absorption a) photoelectric effect b) Pair production and c)
photodisintegration of the nucleus

Attenuation Constants:
1. linear attenuation constants
2. mass attenuation constants
3. relative attenuation constants

Relative Attenuation Coefficients for different biological tissues: