Question

a. Explain why filtering of an X-ray beam causes a simultaneous decrease in beam quantity and increase in quality [a diagram may be useful]

b. The two main mechanisms by which X-rays interact with matter are via Compton scattering and absorption by the photoelectric effect. By referring to the dependence of these effects on X-ray energy, explain what happens to X-ray image quality when using excessively high tube voltages

Answer 1

Answer (a)

Appropriate filtration can be used to absorb low energy photons from the x-ray beam so that the filtered beam consists of a greater percentage of high energy photons. Hence, the average energy of the beam increases. This process is referred to as beam hardening.

Beam quantity or the number of x-ray photons in the beam decreases due to the removal of lower-energy photons by added filters.

Beam quality or overall energy of the beam increases as the filtered beam consists of a greater percentage of high energy photons.

Answer (b)

X-rays interact with tissue via photoelectric effect and compton scattering. Factor that contributes to contrast in x-ray imaging is the photoelectric effect and the factor that contributes to the noise in the image is the Compton scattering effect. Excessive tube voltage will increase the x-ray energy and the image contrast gets reduced.

Attenuation of photoelectric effect at k-edge (k-shell is the innermost shell of an atom) of materials like iodine and barium is very low. If x-ray energy is closer to K-edge, likelihood of photoelectric effect to occur is higher. Thus, lowering peak kilovoltage kV (X-ray energy) improves contrast.