Lec 2 Production of X-rays, Factors Controlling X-ray Beam Flashcards
2 types of radiation produced during x-ray production?
Bremsstrahlung Radiation
Characteristic Radiation
The X-ray photons emitted by the rapid deceleration of the bombarding electrons passing close to the nucleus of the tungsten atom.
Bremsstrahlung or braking radiation
The amount of deceleration and degree of deflection determine the amount of energy lost by the bombarding electron and hence the energy of the resultant emitted photon. A wide range or spectrum of photon energies is therefore possible and is termed _________?
Continuous spectrum
Small deflections of the bombarding electrons are the most common, producing many __________?
Low-energy photons
- Have little penetrating power and most will not exit from the X-ray tube itself.
- They will not contribute to the useful X-ray beam
Low-energy photons
Removal of low-energy photons from the beam is known as?
Filtration
Large deflections are less likely to happen so there are relatively few ________?
High-energy photons
The ______________ is directly elated to the size of the potential difference (kV) across the Xray tube.
Maximum photon energy possible (E max)
This involves electron ‘jumps’ from one energy level (shell) to another, and results in the emission of X-ray photons with specific energies.
Characteristic radiation
Contributes only a small fraction of the photons in an x-ray beam.
Characteristic radiation
The production of this radiation has no practical implications for dentomaxillofacial radiography.
Characteristic radiation
What are the 6 factors controlling the x-ray beam?
- Exposure Time (s)
- Exposure Rate/ Milliamperage (mA) Setting
- Tube Voltage Peak (kVp)
- Filtration
- Collimation (shape)
- Inverse Square Law (distance)
It is desirable to keep the exposure time as _______ as possible to minimize _______ from patient motion.
- Short
- Minimize blurring from px motion
Also known as exposure rate.
Milliamperage setting
Affects the quantity of radiation produced by the X-ray tube
Milliamperage setting (exposure rate)
Affects image density or darkness
Milliamperage setting (exposure rate)
Refers to the number of photons in an x-ray beam.
Beam quantity
Increases the kinetic energy of the electrons to the target
Tube voltage peak (kVp)
Increasing the kVp of an X-Ray machine increases:
a. The _______ of photons generated.
b. The __________ of the photons.
c. The _________ of the photons
d. Affects __________ and differences in ________.
- number of photons
- mean energy
- maximal energy
- image contrast and differences in density
Refers to the mean energy of an x-ray beam.
Beam quality
Beam quantity =
Beam quality =
Beam quantity = milliamperage setting
Beam quality = tube voltage peak
A metal disk (aluminum) filters harmful low-energy (soft) X-rays.
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. - - - E.g. x-ray tube glass wall, insulating oil
Inherent filtration
May be supplied in the form of aluminum disks placed over the port in the head of the x-ray machine.
Added filtration
- It is the sum of the inherent and added filtration.
- Is to be equal to the equivalent of 1.5 mm of aluminum for a machine operating at up to 70 kVp and 2.5 mm of aluminum for machines operating at higher voltages.
Total filtration
- It is a metallic barrier with an aperture in the middle used to shape and restrict the size of the x-ray beam and the volume of tissue irradiated.
- Rectangular or round
Collimator
- Improve image quality
- Reduces the exposed volume and thereby the number of scattered photons reaching the image receptor, resulting in reduced patient exposure and improved images.
Collimator
The intensity of an x-ray beam (the number of photons per cross-sectional area per unit of exposure time) varies with distance from the focal spot.
Inverse square law
Changing the distance between the x-ray tube and the patient, such as by switching from a machine with a short aiming tube to one with a long aiming tube, has a marked effect on beam intensity. Such a change requires a corresponding modification of the kVp or mA to maintain the same intensity at the image receptor.
Inverse square law
