Image Processing Reference
In-Depth Information
How, then, are x rays made? For the most part, they are made the same way they
were made by their discoverer, Prof. Wilhelm Konrad Roentgen (1845-1923), a
German physicist. Roentgen's method is not incandescent—rather it uses electrons
as x-ray generating “bullets.” Roentgen used an evacuated tube with two metal
electrodes inside connected to a high voltage power supply. The electric field
generated by the power supply accelerates electrons emitted from a hot wire
filament inside the tube and smashes them into a metal plate. The electrons
penetrate into atoms in the metal surface and are sharply deflected by the electric
fields near the nuclei of the atoms. This deflection produces x rays. The energy of
the resulting x rays is described in units that include the acceleration voltage of
the x-ray tube. We thus speak of x rays as having energies of kilo-electron volts,
or keV. One keV is the kinetic energy of an electron that has dropped through
a potential difference of 1000 volts (1 kV). This same method of generating x
rays is still used today. Typical medical x-ray tube voltages are 100 kV; we would
describe the resulting x rays as having energies of 100 keV, corresponding to a
wavelength of about 0.01 nm (10 11 m), or about a tenth of the radius of an atom.
Electron-volt units are a convenient way to describe their energy, even when the
source has nothing to do with voltage, as in the case of thermal x rays. In fact,
this descriptive convention applies to all sorts of high-energy particles produced
by particle accelerators, radioactive decay, or astrophysical sources.
Gamma rays are electromagnetic waves with extremely short wavelengths, shorter
than x rays, and with correspondingly higher photon energies. They are not caused
by the rearrangement of electrons within an atom; rather, they are generated by
changes in the nuclei of atoms, changes due to nuclear decay processes where
a nucleus changes internally and releases energy. Their energies correspond to
binding energies ofnucleonsin the nucleus. We can generate gamma rays by
assembling concentrated masses of radioactive material or by using powerful
particle accelerators to accelerate electrons to very high energies and then smash
them into a target material, causing nuclear processes to occur. The gamma-ray
band encompasses all photons with energies above about 100 keV, corresponding
to wavelengths of 0.01 nm(10 11 m) and shorter. What is the difference between x
rays and gamma rays? There is no particular boundary between the two (since the
naming convention is somewhat arbitrary), but most scientists consider x rays to
be produced by electron-atom interactions, whereas gamma rays are in the million
electron-volt or higher energy range and are produced by nuclear processes such
as radioactive decay.
X-ray and gamma-ray imaging technology is quite different from imaging
technology described in previous chapters. The wavelike properties of light at
medium wavelengths (visible, UV and IR) make it possible to image this light
onto detectors, much like our own eye, which like a standard film camera consist
of a detector material in a focal-plane array (the retina) and an imaging optic (the
crystalline lens) that focuses a scene onto the retina. X rays and gamma rays do
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