X-ray Production

The first X-ray tube was the Crookes tube, a partially evacuated glass bulb containing two electrodes, named after its designer, the British chemist and physicist Sir William Crookes. When an electric current passes through such a tube, the residual gas is ionized and positive ions, striking the cathode, eject electrons from it. These electrons, in the form of a beam of cathode rays, bombard the glass walls of the tube and produce X rays. Such tubes produce only soft X rays of low energy.

Properties of X Rays

X rays affect a photographic emulsion in the same way light does . Absorption of X radiation by any substance depends upon its density and atomic weight. The lower the atomic weight of the material, the more transparent it is to X rays of given wavelengths. When the human body is X-rayed, the bones, which are composed of elements of higher atomic weight than the surrounding flesh, absorb the radiation more effectively and therefore cast darker shadows on a photographic plate. Another type of radiation, which is known as neutron radiation and is now used in some types of radiography, produces almost opposite results. Objects that cast dark shadows in an X-ray picture are almost always light in a neutron radiograph.

Fluorescence.

X rays also cause fluorescence in certain materials, such as barium platinocyanide and zinc sulfide. If a screen coated with such fluorescent material is substituted for the photographic films, the structure of opaque objects may be observed directly. This technique is known as fluoroscopy.

Ionization.

Another important characteristic of X rays is their ionizing power, which depends upon their wavelength. The capacity of monochromatic X rays to ionize is directly proportional to their energy. This property provides a method for measuring the energy of X rays. When X rays are passed through an ionization chamber, an electric current is produced that is proportional to the energy of the incident beam. In addition to ionization chambers, more sensitive devices, such as the Geiger-Müller counter( a device use to test the amount of radiation) and the scintillation counter, can measure the energy of X rays on the basis of ionization. In addition, the path of X rays, by virtue of their capacity to ionize, can be made visible in a cloud chamber.

X-ray diffraction.

X rays may be diffracted by passage through a CRYSTAL, (q.v.) or by reflection (scattering) from a crystal, which consists of regular lattices of atoms that serve as fine diffraction gratings . The resulting interference patterns may be photographed and analyzed to determine the wavelength of the incident X rays or the spacings between the crystal atoms, whichever is the unknown factor . X rays may also be diffracted by ruled gratings if the spacings are approximately equal to the wavelengths of the incident X rays.

Photoelectric Effect

When a quantum of radiation, or a PHOTON, (q.v.), in the X-ray portion of the electromagnetic spectrum strikes an atom, it may impinge on an electron within an inner shell and eject it from the atom. If the photon carries more energy than is necessWhen a quantum of radiation, or a PHOTON, (q.v.), in the X-ray portion of the electromagnetic spectrum strikes an atom, it may impinge on an electron within an inner shell and eject it from the atom. If the photon carries more energy than is necessary to eject the electron, it will transfer its residual energy to the ejected electron in the form of kinetic energy.

This phenomenon, called the photoelectric effect, occurs primarily in the absorption of low-energy X raysary to eject the electron, it will transfer its residual energy to the ejected electron in the form of kinetic energy.

This phenomenon, called the photoelectric effect, occurs primarily in the absorption of low-energy X rays

Pair production

In the third type of absorption, especially evident when elements of high atomic weight are irradiated with extremely high-energy X rays, the phenomenon of pair production occurs. When a high-energy photon penetrates the electron shell close to the nucleus, it may create a pair of electrons, one of negative charge and the other positive; a positively charged electron is also known as a POSITRON, (q.v.). This pair production is an example of the conversion of energy into MASS, (q.v.). The photon requires at least 1.2 MeV of energy to yield the mass of the pair. If the incident photon possesses more energy than is required for pair production, the excess energy is imparted to the electron pair as kinetic energy. The paths of the two particles are divergent.

Intensifying Screen

IS- used to compensate the high patient dose when using the Film alone.
-a device that converts the energy of the x-ray beam into visible light.
-amplifies the image- forming x-rays that reach the screen- film cassette.
Four Distinct Layers of IS
1. Protective Layer- the closest layer to the film. It is 10 to 20 µm thick and is applied to the face of the screen to make the screen resistant to abrasion and damage cause by rough handling. This layer also eliminates the build up of the static electricity and this is transparent to light.
2. Phosphor Layer- active layer of the IS. This emits light when struck by the radiation. This converts energy of the x-ray beam into visible light.
Properties of the IS Phosphor
1. High Atomic Number
2. X-ray Conversion Efficiency.
3. Good Spectral Matching Property
4. After glow property
5. High Resistance
3. Reflective Layer- it intercepts the light headed in other direction and redirects it to the film
Isotropic Emission- radiations are emitted equally in all direction.
4. Base- the farthest layer from the film. This is approximately 1mm thick and serves principally as a mechanical support for the active layer of phosphor.
Polyester- the material of choice of the IS base layer.
Luminescence- a light emitted in response to some outside stimulation.
Two Types of Luminescence
1. Fluorescence- the process of light emission only when the phosphor is stimulated.
2. Phosphorescence- the process of light emission even after the stimulation of phosphor.
Screen lag/ Afterglow- phosphorescence in the IS.
Screen Characteristics
Intensification Factor- the magnitude dose reduction property of the screen and this is a measure of the speed of the IS.
IF=exposure required without screen/ exposure required with screen
1. Screen Speed- a relative number that describes how efficient x-rays are converted into usable light. This conveys no useful information concerning patient dose.
Conditions that affect IS speed that are controlled by the RadTech.
a. Radiation Quality- as x-ray tube potential is increased, the IF increases also
b. Image Processing- excessive developing time results in a lowering IF because the emulsion nearest the base contains no latent image, yet it can be reduced to silver if the developer is allowed sufficient time to penetrate the emulsion to the depth.
c. Temperature- IS emits more light per x-ray interaction at low temperature than at high temperature.
2. Image Noise- occurs most often when fast screens and high kVp techniques are used.
3. Spatial Resolution- refers top how small an object can be image. When x-rays interact with the phosphor, the area of the film emulsion that is activated by the emitted light is larger than it would be with direct x-ray exposure, this results in reduced spatial resolution and image blur.
Screen Combination
Screen compatibility is essential; use only those films for which the screens are designed.
Cassette- rigid holder that contains the film and IS. The front cover is usually made with a carbon fiber material to minimize the attenuation of the x-ray beam.
Compression Device- placed between the each screen and cassette cover to maintain close contact when the cassette is closed and latched.
Carbon Fiber Material- consists of graphite fibers. This does not only reduces radiation but may also result in longer x-ray tube life because of lower demand radiographic techniques required.
Direct Exposure Film versus Screen Film
One reason why calcium tungstate is a useful screen phosphor because it emits light in a violet- to- blue region
Rare- earth Screens- its advantages are; faster, lower radiographic techniques required and therefore lower patient dose.
Characteristics of the Rare- earth Screens
1. Higher X-ray Absorption
2. Higher Conversion Efficiency- approximately 20%
3. Spectrum Matching
Care of Screens
Screens should handled when they are new and when they to install in a cassette and when they are being cleaned. When loading cassettes do not slide the film in. remove the film by rocking the cassette on the hinged edge and letting it fall into your fingers. Do not leave the cassettes open because the screen can be damaged by whatever might fall on them. Radiographic IS screens must be cleaned periodically.
Wire- mesh – used to test the screen contact using 50 kVp at 5 mAs and an SID of 100 cm.