Radiology Lecture 1: Physics of Radiation

Introduction to Radiology

• Radiology is the science that deals with diagnosis, therapeutic, and research applications of high-energy radiation.
• Dental radiography is a process of image production for an object through the use of x-radiation.
• Radiologic examination is an integral component of the diagnostic procedure.

Nature of Radiation

• Radiation is the transmission of energy through space and matter.
• It may occur in two forms: electromagnetic and particulate.
• Electromagnetic spectrum: x-ray (blue) have energies of 10 to 120 keV, and Magnetic resonance (MR) imaging uses radio waves (orange).
• X-ray was discovered by Roentgen in 1895 and travels in a wave motion called sine-wave.
• The basic unit is x-ray photon or quantum, and the distance between the crests of these waves is called wave-length measured in Angstrom units (Å).

Production of Radiation (X-ray Machine)

• The x-ray machine consists of a cathode and an anode situated within an evacuated glass envelope or tube.
• The glass of the tube is leaded to prevent the generated X-ray from escaping in all directions.
• The cathode consists of a filament and a focusing cup, and the filament is the source of electrons within the x-ray tube.
• The anode consists of a tungsten target embedded in a copper stem, which converts the kinetic energy of the colliding electrons into x-ray photons.
• The tungsten target has a high atomic number, high melting point, and high thermal conductivity, making it an ideal target material.
• The x-ray tube is evacuated to prevent collision of the fast-moving electrons with gas molecules, which would significantly reduce their speed.

X-ray Tube Components

• Focusing cup: a negatively charged concave molybdenum bowl that directs electrons into a narrow beam.
• Filament: a coil of tungsten wire that emits electrons when heated.
• Anode: a tungsten target embedded in a copper stem that converts kinetic energy into x-ray photons.

Types of Radiation

• X-ray photons can pass through the atom without any change.
• Coherent scattering (Thompson scattering): low-energy x-ray photon interaction with an atom, resulting in a change in direction but not energy loss.
• Compton effect: moderate-energy x-ray photon interaction with a free or loosely bound outer shell electron, resulting in ionization, energy reduction, and direction change.
• Photoelectric effect: high-energy x-ray photon interaction with an inner-shell electron, resulting in the ejection of a high-energy photoelectron.

X-ray Beam Control

• Added filtration: using an aluminum sheet as an extra filter to reduce the intensity of the x-ray beam.
• Collimation: a process used to control the size and shape of the x-ray beam using diaphragms, metal cylinders, cones, and rectangular tubes.

Collimators

• Diaphragms: metal plates or disks with a hole in the center that determines the shape of the x-ray beam.
• Collimator shapes: round or rectangular, controlling the diameter of the x-ray beam at the skin surface.

Half-Value Layer

• A method of monitoring the penetration quality of the x-ray beam.