Workbook #1 Flashcards
What year were X-rays discovered and by who?
Roentgen had discovered X-rays on November 8, 1895
Xray properties
- Penetrate matter
- Produce a latent image on film or an image receptor
- Produce fluorescence in certain materials
- Produce ionization of matter
photon
- an X-ray as a particle or bundle of energy
- has no mass or charge
- moves in straight lines at the speed of light.
Ionization
- Interactions of X-rays with the electrons within the tube head or the patient
- is the process where electrons are removed from atoms due to collisions with x-ray photons
Bremsstrahlung Radiation
-is produced when an electron passed near the nucleus of an atom. The negatively charged (-) electron is deflected by the positively charged (+) nucleus. The energy lost by the deceleration of the electron is emitted in the form of a photon of radiation
Characteristic radiation
- results when an electron within an atom is ejected from the inner orbit of the atom by an incoming high-speed electron.
- The radiation is specific to, or characteristic of, the particular atom in which the interaction occurs.
- Characteristic radiation makes up only a small portion of the X-rays produced in a dental X-ray unit
Examples of electromagnetic radiation
radio waves, microwaves, cosmic rays, and visible light
Relationship of wavelength and frequency
-inverse relationship
High Wavelength low frequency and vice versa
What type of rays are more penetrating?
High frequency short wavelengths
Volt (V)
is a unit of electrical potential that can be considered a measure of work capacity
Xray units range for kVp
70 kVp is a common setting
although it may be as low as 60 kVp or as high as 90 kVp
A high kVp setting means
will produce X-rays that are more energetic, have greater penetrating power, and has a greater capacity to pass through matter.
15 kV rule
Increasing the kilovoltage by 15 would require decreasing the exposure time by one half to keep the image density (darkness) the same.
-For every 15% increase in kV, the density (or darkness) on the resulting image will double
Ampere (A)
unit of electrical current, or the number of electrons flowing in an electrical circuit.
Units Ampere are measured in X-rays
milliamperes (mA)
Thermionic Emission
- The current in the filament circuit in an X-ray machine is used to head a very thin wire (a filament) made of tungsten, similar to the filament in a light bulb. Heating the filament agitates the tungsten atoms; some of the electrons in these atoms escape from their orbits
- The tungsten electrons that have been boiled off form a cloud around the filament, and it is these “free” electrons that are ultimately responsible for generating X-rays.
-Think of boiling off of tungsten electrons
milliamperes
The milliamperage setting in the x-ray unit influences the current flowing through the filament and therefore the number, or quantity, of X-rays that will be produced in the x-ray tube.
What are the factors that influence the number of X-rays produced
milliamperage
exposure time
Components of dental X-ray tube
Cathode Focusing Cup Anode Filter Tube head housing Oil Collimator
Cathode
- negatively charged end of an x-ray tube
- consists of the tungsten filament and a focusing cup
Focusing Cup
- has a negative electrostatic charge and is usually made of molybdenum.
- shape of the cup and its negatively charge repel the electrons from the filament and keep them suspended in a “cloud” around the filament.
- When the high voltage circuit is activated, the focusing cup’s charge and shape help direct the electrons toward the anode and prevent from spreading out and missing the target.
Anode
- positive end of the x-ray tube
- made of tungsten and copper
- a small block of tungsten is embedded in a large copper stem or sleeve to act as the target for the electrons
Why is tungsten used in the anode
Tungsten is used because it has a high atomic number and thus can produce many X-rays
-has a high melting point and a low vapor pressure; it will not melt or vaporize, unless extremely long exposures are made or unless many exposures are made in a short period of time.
Why is copper used in the anode
Copper is used around the tungsten target to conduct heat rapidly away from the target, thus reducing the wear by heat on the target.
Filter
- aluminum filter in the form of a disk is placed at the port, or opening, of the open-ended cylinder
- small fraction of X-rays that are allowed to escape (1%) must pass through this filter
What type of filter is used for X-ray machines operating below 70 kV?
1.5 mm of added aluminum filtration
What type of filter is used for X-ray machines operating above 70 kV?
2.5 mm of added aluminum filtration
Tubehead housing
- all structures within tube head are surrounded by a glass envelope in a vacuum environment.
- Glass envelope is housed by metal (lead)
- the metal insulates the tube head components and reduces the amount of radiation that can leak
Oil
Within the tube head, the glass tube is immersed in oil to help absorb the heat created by x-ray production.
Collimator
-disk of metal, usually lead, that has a small aperture that restricts the size or shape of the x-ray beam as it exits the tube head
Collimation
the restriction of the x-ray beam size by a lead diaphragm
Production of X-rays
1) The operator turns on the x-ray machine
2) the operator selects the kilo voltage and milliamperage setting, if appropriate. Many machines do not allow for such adjustments
3) The operator selects the exposure time. The exposure time is the length of time the X-rays are actually produced. In dental units, this time is usually less than 1 second.
4) The operator pushes the exposure button.
5) Electrical current passing through the filament boils off electrons through thermionic emission.
6) The potential difference between the cathode and the anode of the tube is activated. The electrons, which have a negative charge, are attracted to the positive anode at extremely high speeds. Nothing impedes their progress because the x-ray tube is a vacuum tube; there are no air molecules inside it.
7) The electrons from the filament collide with the target at the anode end of the tube. The collision of the high-speed electrons against the target process energy in the form of x-rays.
8) X-rays are generated in all directions (360 degrees) from the target. The X-rays travel at the speed of light.
9) Most of the X-rays produces are absorbed by the leaded glass x-ray tube and the metal housing surrounding the X-ray tube.
10) A small portion of the X-rays generated escape the x-ray tube through an unleaded area in the glass (the window).
11) These escaped X-rays then pass through a filter, which absorbs the less useful, lower energy X-rays in the beam.
12) The x-ray beam then passes through a lead collimator, which restricts the spread of the beam. The X-rays that pass through the opening in the collimator (the port) travel through the X-ray cone to reach the patient and the image receptor.
