Biomedical Engineering Reference
In-Depth Information
electric and magnetic fields change together forming an EM wave, which travels
at the speed of light. The oscillation can come from atoms being energized either
thermally or nonthermally or from an alternating current. The frequency of the
emission is directly related to the traveling velocity of the electrons. The longer
the electrons stay in the magnetic field, the more energy they lose. As a result, the
electrons make a wider spiral around the magnetic field, and emit EM radiation at
a longer wavelength. This can be related to the initial velocity of the electron, or it
can be due to the strength of the magnetic field. A stronger magnetic field creates
a tighter spiral and therefore greater acceleration. The frequency of the radiation
is usually controlled by taking advantage of the natural oscillating frequencies of
tuned circuits or quartz crystals. This method is used for generating EM radiation
in microwave ovens, TV signals, and garage door openers.
X-rays are generated by accelerating a beam of electrons through a high volt-
age gradient and then colliding them into a metal plate (Figure 8.3). X-rays are
produced when the electrons are suddenly decelerated—these waves are called
bremsstrahlung (German word meaning braking) radiation—and when the elec-
trons make transitions between lower atomic energy levels in heavy elements. The
X-rays that emerge have a range of energies, however the maximum energy (i.e.,
the shortest wavelength) is determined entirely by the voltage difference through
which the beam of electrons is accelerated (Figure 8.3). For example, a beam of
electrons accelerated through a potential difference of 50 keV will produce X-ray
photons up to but never exceeding 50 keV. Typically, a heated cathode tube serves
as the source of the electrons, which are accelerated by applying a voltage of about
Figure 8.3
Schematic showing the assembly for generating X-rays nonthermally and intensity at-
tenuation through a tissue of various densities.
