Basics of Radiation Physics: X-Ray and Others

Basics of X-ray Physics

• X-rays are produced by the interaction of accelerated electrons with tungsten nuclei within the tube anode.
• Two types of radiation are generated: characteristic radiation and bremsstrahlung (braking) radiation.
• Changing the X-ray machine settings alters the properties of the X-ray beam.

X-ray Production

• X-rays are produced within the X-ray machine, with no external radioactive material involved.
• Radiographers can change the current and voltage settings on the X-ray machine to manipulate the properties of the X-ray beam produced.
• Different X-ray beam spectra are applied to different body parts.

The X-ray Tube

• A small increase in the filament voltage results in a large increase in tube current, which accelerates high-speed electrons from the very high temperature filament negative cathode within a vacuum, towards a positive tungsten target anode.
• The anode rotates to dissipate heat generated.
• X-rays are generated within the tungsten anode and an X-ray beam is directed towards the patient.

X-ray Generation

• X-rays are generated via interactions of the accelerated electrons with electrons of tungsten nuclei within the tube anode.
• There are two types of X-ray generated: characteristic radiation and bremsstrahlung radiation.
• Characteristic radiation is generated when a high-energy electron collides with an inner shell electron, both are ejected from the tungsten atom, leaving a 'hole' in the inner layer, which is filled by an outer shell electron with a loss of energy emitted as an X-ray photon.
• Bremsstrahlung radiation is generated when an electron passes near the nucleus, it is slowed, and its path is deflected, and energy lost is emitted as a bremsstrahlung X-ray photon.

The X-ray Spectrum

• A spectrum of X-ray energy is produced within the X-ray beam as a result of characteristic and bremsstrahlung radiation generation.
• The spectrum can be manipulated by changing the X-ray tube current or voltage settings, or by adding filters to select out low energy X-rays.
• Radiographers can apply different spectra of X-ray beams to different body parts.

The X-ray Beam

• X-rays travel in straight lines and a beam of X-rays diverges from its source.
• Structures the beam hits first will be magnified in relation to those that are nearer the detector.
• Occasionally, magnification can be helpful in localizing abnormalities.
• To reduce magnification, the X-ray source can be moved further away from the subject.
• Structures that need to be measured accurately should be placed closer to the detector.

Tissue Densities

• An X-ray image is a map of X-ray attenuation.
• Attenuation of X-rays is variable depending on density and thickness of tissues.
• Describing X-ray abnormalities in terms of density may help in determining the tissue involved.
• The denser the tissue, the more X-rays are attenuated.
• For example, X-rays are attenuated more by bone than by lung tissue.
• There are five different densities that can be useful to determine the nature of an abnormality: low density material (air), very dense material (metal or contrast material), and bodily tissues (varying degrees of grey, depending on density and thickness).

Nature of X-rays

• According to the quantum theory, electromagnetic radiation can also be considered as particles called photons.
• Each photon has an associated amount of energy.
• The relationship between wavelength and frequency is λ = c/ν, where c is the velocity of light.
• Most of the kinetic energy of the electrons striking the target is converted into heat, less than 1% being transformed into X-rays.

Properties of the Continuous Spectrum

• The continuous spectrum is a smooth, monotonic function of intensity vs wavelength.
• The intensity is zero up to a certain wavelength – short wavelength limit (λ SWL).

Acute Health Effects of Radiation Exposure

• At very high doses, radiation can impair the functioning of tissues and organs and produce acute effects such as nausea and vomiting, skin redness, hair loss, acute radiation syndrome, local radiation injuries (also known as radiation burns), or even death.

Learn about the basics of X-ray physics, including how X-rays are produced and the two types of radiation generated. This quiz is based on a lecture by Dr. Ziyad Shihab Ahmed Al-Sarraj from the College of Pharmacy at Ashur University.