X-Radiation

Introduction X-rays were discovered by Roentgen in 1895. They are a form of high-energy electromagnetic radiation and are part of the electromagnetic spectrum, which also includes low energy radio waves, television and visible light.

*Radiography — the techniques involved in producing the various radiographic images *Radiology — the interpretation of these radiographic images. *X-rays are described as consisting of wave packets of energy. Each packet is called a photon which equivalent to one quantum of energy. X-ray beam is made up of millions of individual photons.

Atom is the fundamental unit of matter

X-ray production: X-rays are produced when high-speed electrons bombard a target material and are brought suddenly to rest. This happens inside the X-ray tube.

X-Ray machine

X-Ray tube

Dental X-Ray Machine The cathode (negative) :a heated filament of tungsten that provides the source of electrons. The anode (positive) :a target (a tungsten filament) set into the angled face of a large copper block to allow efficient removal of heat A focusing device(Molybdnum cup): aims the stream of electrons at the focal spot on the target. A high-voltage (kilovoltage, kV) connected between the cathode and anode accelerates the electrons from the negative filament to the positive target. A current (milliamperage, mA) flows from the cathode to the anode. This is a measure of the quantity of electrons being accelerated. A surrounding lead casing absorbs unwanted X-rays as a radiation protection measure since X-rays are emitted in all directions. Surrounding oil facilitates the removal of heat.

Filtration: to reduce patient dose, low-energy photons should be removed from the beam by placing an aluminum filter in the path of the beam. Inherent filtration consists of the materials that x-ray photons encounter as they travel from the focal spot on the target to form the usable beam outside the tube enclosure. These materials include: * The glass wall of the x-ray tube. * The insulating oil that surrounds many dental tubes. * The barrier material that prevents the oil from escaping through the x-ray port. The inherent filtration ranges from the equivalent of 0.5 to 2 mm of aluminum. Total filtration is the sum of the inherent filtration plus any added external filtration supplied in the form of aluminum disks placed over the port in the head of the x-ray machine.

The collimator : a metal disc or cylinder with central aperture designed to shape and limit the beam size to a rectangle or round with a maximum diameter of 6 cm. *Rectangular collimators further limit the size of the beam to just larger than the x-ray film, thereby further reducing patient exposure. *Collimation also improves image quality. When an x-ray beam is directed at a patient, the hard and soft tissues absorb about 90% of the photons and about 10% pass through the patient and reach the film. Many of the absorbed photons generate scattered radiation within the exposed tissues by a process called Compton scattering. Some of these scattered photons reach the film and degrade image quality.

The sequence of events of X-rays production : 1 The sequence of events of X-rays production : 1. The filament is electrically heated and a cloud of electrons is produced around the filament. 2. The high-voltage (potential difference) across the tube accelerates the electrons at very high speed towards the anode. 3. The focusing device aims the electron stream at the focal spot on the target. 4. The electrons bombard the target and are brought suddenly to rest. 5. The energy lost by the electrons is transferred into either heat (about 99%) or X-rays (about 1%). 6. The heat produced is removed and dissipated by the copper block and the surrounding oil. 7. The X-rays are emitted in all directions from the target. Those emitted through the small window in the lead casing constitute the beam used for diagnostic purposes.

Heat-producing collisions

X-ray-producing collisions

Definition of terms used in X-ray interactions Scattering: change in direction of a photon with or without a loss of energy. Absorption : deposition of energy, i.e. removal of energy from the beam. Attenuation : reduction in the intensity of the main X-ray beam caused by absorption and scattering Attenuation = Absorption + Scattering lonization : removal of an electron from a neutral atom producing a negative ion (the electron) and a positive ion (the remaining atom).

Interaction of X-rays with matter Four main interactions, depending on the energy of the incoming photon: NO Interaction: transmitted unchanged. Photoelectric effect: pure absorption (low-energy photons). Compton effect — scatter and absorption. (High-energy photons). Coherent scattering: pure scattering.

NO Interaction: It is possible for an x-ray photon to pass through matter or the tissues of patient without any interaction.

Photoelectric effect Low energy

Compton effect High energy

Coherent scattering unmodified scatter: The photon does not have enough energy to liberate the electron from its bound state (i.e. below the binding energy of the electron) so no energy transfer occurs. The only change is a change of direction (scatter) of the photon, hence 'unmodified' scatter. 

Dose units and dosimetry Dosimetry :is used to determine the quantity of radiation exposure or dose . Dose: is used to describe the amount of energy absorbed per unit of mass at a site of interest. Exposure: is a measure of radiation on the basis of its ability to produce ionization in air under standard conditions of temperature and pressure (STP). Absorbed dose :is a measure of the energy absorbed by any type of ionizing radiation per unit of mass of any type of matter. The equivalent dose :is used to compare the biologic effects of different types of radiation on a tissue or organ. The effective dose (E) is used to estimate the risk in humans. Radioactivity : describes the decay rate of a sample of radioactive material.

SI: The International System of Units