X-Rays Have Several Basic Properties

Chapter 3 The Production of X-rays

X-rays have several basic properties:  Are unaffected by gravity (they have no mass)  Are unaffected by electric fields  Are unaffected by magnetic fields  Travel in straight lines  Are exponentially attenuated by matter  Cannot be focused

Collision Interactions: produce characteristic radiation  Elastic: momentum and kinetic energy are conserved (brakes off)  Inelastic: momentum is conserved, but kinetic energy is not. (brakes on)

Excitation: an increase in electron energy that raises an electron to a higher energy level.

Ionization: an increase in energy great enough to remove an electron from the atom; the basis for chemical changes and therefore biological effects.

Ion pair: negative electron and positively charged atom

Incoming electron that creates ion pair is deflected with less energy than it had before the collision.

Characteristic Radiation: the potential energy difference between where the outer orbital electron that moves starts and where it ends is given off as a photon.  Large: x-ray photon  Small: light photon

The exact energy of the emitted photon is dependant on the atoms (electron shell energy spacings of the target material- characteristic radiation/ characteristic x- rays) that initially lost the electron.

Energy spectrum: a graph displaying the number of photons in small energy intervals versus photon energy.  Characteristic radiation from excited or ionized atoms produces a series of photons with discrete energies (multiple lines)  Bremsstruhlung: produce a continuous spectrum

Radiative Interactions: produce bremsstrahlung radiation

Bremsstrahlung: when an electron interacts with the positive charge of an atomic nucleus, the nucleus’s electric field exerts force on the electron and causes it to change direction or velocity this change causes a loss in kinetic energy x-ray photon.

Photon produced with an energy equal to the energy loss of the electron.

In bremsstrahlung production the maximum energy of the x-ray equals the kinetic energy of the incoming electrons.

The closer the electron comes to the nucleus, the stronger the electrostatic attraction and the more the change of the electrons velocity.

Filters: used in medical x-ray beams to change their spectral characteristics, reduce the fraction of low-energy x-rays that contribute unnecessary radiation to the patient. Filters are chosen in order to deposit maximum energy in the diseased tissues.

Hardening: to increase the average energy of photons.

Orthovoltage range: 150-400 kVp

Thoraeus filter: tin, copper and aluminum (example of orthovoltage beam filter)

Megavoltage beams use a transmission filter that serves to remove low energy photons.

Beam direction depends on the energy of the incoming electrons.  Low energies: most x-rays produced are emitted from the target at almost right angles to the initial electron, have a great deal of angular spread.  High energies: x-rays emitted in a more forward direction with less angular spread, as energy increases- beam gets narrower and more intense in forward direction.

Targets are made of high atomic number (Z) materials, which produce x-rays when hit by high energy electrons.

Reflection/reflectance target: used so that the x-rays come off the target unimpeded in directions useful for imaging.

Thick target: all incoming electrons are stopped in the target.

Electron kinetic energytargetelectromagnetic photon energy

Transmission target: photon energy comes off the target in the same direction as the electrons enter.

In diagnostic tubes 99% energy transformed to heat. An estimate of the fraction of the electron energy converted to photon energy: f = 3.5 x 10-4 ZE Z = atomic number of target E = maximum energy of electrons in MeV

The minimum requirements for x-ray production are high speed electrons, means to decelerate them rapidly (high voltage source, source of electrons and an x-ray target)

The forms of energy involved in the production of an x-ray beam:  Electrical energy: voltage  Electrical potential energy: electrons at cathode  Kinetic energy: high speed electrons before striking target  Electromagnetic energy: photons produced in target  Heat: in target and tube housing- due to loss of kinetic energy