X-ray Production

X-ray photons are produced when the high-speed electrons from the cathode strike an anode target.

  • These incoming electrons are called incident electrons and are represented in drawings by a solid arrow.

  • Photons = X-rays

The target interactions that produce the x-ray photons consist if less than 1% of the total kinetic energy of the incident electrons.

  • Over 99% of the kinetic energy of the incident electrons is converted to heat.

  • Instead, they transfer enough energy to excite the outer-shell electrons to the point where they will emit infrared radiation as heat.

  • 1% X-ray’s and 99% Heat

The interaction that will occur depends on the electron kinetic energy and the binding energy of the electron shells of the atom.

Brems interactions may occur only when the incident electrons interacts with the force field of the nucleus.

These emissions are called bremsstrahlung photons and their energy is exactly the difference between the entering and the exiting kinetic energy of the electron.

  • Subtract the entering and the exiting to get your answer of the photon energy which would be low, medium, or high.

  • “Driving a car and taking a turn and feeling the pull on you is the X-ray photons energy exiting from the Atom.”

Characteristics interactions may occur only when the incident electron interacts with an inner-shell electron. (K-shell)

  • The incident electron must have enough energy to knock an inner-shell electron from orbit, thereby ionizing the atom.

  • This dropping of an electron from an outer, higher-energy state into an inner, lower-energy state results in the energy difference between the two shells being emitted as an x-ray photon.

  • Subtract the binding energies to find the photons energy; 69.5-2.8=66.7keV

  • Subtract the binding energies to find the photons energy; 12.1-0.6=11.5 keV

Emission Spectrum

Within the diagnostic x-ray range, most photons are produced by bremsstrahlung target interactions.

  • Characteristic photons will not comprise any of the useful beam until the kVp is above 70 because removal of a k-shell electron from tungsten requires 69.5 keV.

As mentioned previously, the K-shell emissions are the only ones within the diagnostic x-ray range.

  • Both brems and characteristic emissions combine to form the complete primary beam spectrum.

  • The kilovoltage peak of the exposure is the maximum possible energy for any photon that exits the x-ray tube

  • The average primary beam photon has a keV energy of only about 30-40 percent of the kVp - take kVp and divide by 3.

When mA, time, or mAs is changed, all of which control the quantity (number) of electrons striking the target, the result is a change in the amplitude of the emission graph. 1 change.

When kVp, which controls the quality (energy) of electrons striking the target, is changed, the result is a change in the number of higher-energy photons as well as in the amplitude of the emission graph. 2 changes.

The rule states that a 15% increase in kVp is equivalent to doubling the mAs.

Filtration

As the x-ray beam passes through the filtering materials, some of the lower-energy photons are absorbed.

  • This decreases the intensity of the beam but at the same time increase the average photon energy.

Generator phasing

As generator phasing efficiency increases, the x-ray beam increases in intensity, and there is also an increase in the average photon energy.