Medical Physics Semester 1

X-Ray Production

• X-rays are produced when highly energetic electrons interact with matter, converting some of their kinetic energy into electromagnetic radiation.
• The main components of the X-ray tube are:
  • A source of electrons (the cathode, filament)
  • An evacuated space in which the electrons are accelerated (glass envelope)
  • A high positive potential to accelerate the negative electrons (kV)
  • A target which the electrons strike (the anode)
• In the X-ray tube, up to 99% of accelerated electrons energy is converted to heat and approximately 1% is converted to X-ray photons.

X-Ray Characteristics

• X-rays are electromagnetic radiation (EMR) of very short wavelength (λ 1-0.1 A°) and very high penetrating power.
• The amount of energy carried by each photon depends on the frequency of radiation: E = h υ = h c /λ
• The higher the atomic number (Z) of the anode, the more intense X-ray beam is produced (e.g., Z of tungsten = 74)

Types of X-Rays

• Bremsstrahlung Spectrum (Continuous X-ray):
  • Produced when accelerated electrons interact with the target and are decelerated, emitting X-ray photons of equal energy
  • The amount of bremsstrahlung depends on the Z number of the target and the kV peak
• Characteristic X-ray:
  • Produced when an electron interacts with an atomic electron within the anode and ejects it from its shell, emitting X-ray photons of equal energy
  • The energy of the X-ray photon is equal to the binding energy difference between the two shells

X-Ray Absorption by Tissue

• X-ray attenuation is the reduction of the X-ray beam due to absorption and scattering.
• The attenuation of an X-ray beam can be measured using the equation: I = I˳ e –μx
• Linear attenuation Coefficient (μ) measures the probability that a photon interacts (absorbed or scattered) per unit length it travels in a specified material.
• μ depends on:
  • Energy of X-rays
  • Atomic number (Z) of the material
  • Density (ρ) of the material

Methods of X-Ray Interaction with Matter

• Photoelectric effect:
  • Occurs when low-energy X-ray photons transfer all of their energy to an atomic electron (more likely inner shell electron)
  • More common in materials with high Z number than those with low Z number

X-Ray Image Formation

• Film-screen technique:
  • X-ray film accommodated inside a special cassette is used as an image receptor
  • Screens (cardboard coated with phosphor layer) are used to convert X-ray photons to visible light photons, which form the image on the X-ray film
• Computed radiography (CR):
  • Phosphor detectors are used inside special cassettes as an image receptor
  • Exposing detector to X-rays results in an excitation process, forming a latent image
  • The exposed detector is scanned with a laser in the reader, converting the image information to electronic signals
• Digital radiography (DR):
  • Digital detectors made from amorphous selenium (a-Se) are used to detect X-rays
  • The uniform surface charge pattern is partially discharged when X-rays are absorbed, forming a latent image as an electronic signal

Radiographic Image Quality

• The main problem in obtaining good X-ray images is blurring.
• The blurred edge of an object in the X-ray image is called penumbra.
• Radiation interactions with tissue can be either direct or indirect.

Radiation Risk of X-Ray Examinations

• Adverse health effects of radiation can be classified into two groups:
  • Deterministic effects: high radiation doses result in relatively immediate damage
  • Stochastic effects: low radiation doses may result in cancer development, with a lag period of at least 5 years and up to 10 or 20 years