X-ray production Flashcards
How is X-radiation created?
Taking energy from electrons and converting it into photons with appropriate energies
Where does energy conversion take place?
Within x-ray tube
How can the quantity (exposure) and quality (spectrum) of the x-radiation produced be controlled?
Adjusting the electrical quantities (KV,MA) and exposure time,S, applied to the tube
What is an x-ray tube?
energy converter
Receives electrical energy and converts it into two other forms: x-radiation and heat
What is heat?
undesirable byproduct
What are x-ray tubes designed and constructed to maximize?
- X-ray production
2. Dissipate heat as rapidly as possible
What does x-ray tube consist of?
- it is a simple electrical device
2. two principle elements: a cathode and anode
What is the process of x-radiation?
As the electrical current flows through the tube from cathode to anode, the electrons undergo an energy loss
What is anode?
The component in which the x-radiation is produced
2, large piece of metal that connects to the positive side of the electrical circuit
What are two primary functions of anode?
- convert electronic energy into x-radiation
2. dissipate the heat created in the process
what does the fraction of the total electronic energy that is converted into x-radiation depend on?
- Atomic number (Z) of the anode material
2. Energy of the electrons
What do most x-ray tubes use as the anode material?
- tungsten
2. atomic number: 74
What are several characteristics of tungsten?
- Maintain strength at high temperatures
- High melting point
- Relatively low rate of evaporation
What is the shape of anode?
Beveled disks
Attached to the shaft of an electric motor that rotates them at relatively high speeds during x-ray production process
What is the focal spot?
Radiation produced in a very small area on the surface of anode
What is the dimension of the focal spot determined by?
Dimension of electron beam arriving from the cathode
In most x-ray tubes, what is the focal spot approx?
rectangular
What are the dimension of the focal spot?
Usually range from 0.1mm to 2mm
What is x-ray tubes designed to have?
Specific focal spot sizes
What does small focal spots produce?
less blurring
better visibility of detail
What does large focal spots have?
greater heat-dissipating capacity
What is the basic function of the cathode?
Expel electrons from the electrical circuit and focus them into a well-defined beam aimed at the anode
What does the typical cathode consist of?
small coil of wire (a filament) recessed within a cup-shaped region
What happens to electrons that flow through electrical circuit?
cannot generally escape from the conductor material and move into free space
Where is anode and cathode contained in?
Airtight enclose or envelope
What is referred to as the tube inset?
The envelope and its contents
what is the tube insert?
part of the tube that has a limited lifetime and can be replaced within the housing
What are the primary functions of the envelope?
provide support and electrical insulation for the anode and cathode assemblies and to maintain a vacuum in the tube
What does the presence of gases in the x-ray tube allow for?
electricity to flow through the tube freely, rather than only in the electron beam
- interfere with the x-ray production and possibly damage the circuit
What does the x-ray tube housing provide?
several functions in addition to enclosing and supporting the other components
What is the function of x-ray tube housing?
functions as shield and absorb radiation
What is the circuit connected to?
source of electrical energy
What does the generator receive?
Electrical energy from the electrical power system and converts it into the appropriate form to apply to the x ray tube
What does the generator provide?
The ability to adjust certain electrical quantities that control the x-ray production process
What are the three electrical quantities that can be adjusted?
- KV (voltage or electrical potential applied to the tube)
- MA (the electrical current that flows through the tube)
- S (duration of the exposure or exposure time)
What is the circuit?
A circulatory system for electrons
pick up energy as they pass through the energy and transfer to the x-ray tube anode
What happens as electrons pass through the x-ray tube?
undergo two energy conversion
- the electrical potential energy is converted into kinetic energy
- then converted into x-radiation and heat
What is the amount of energy carried by each electron determined by?
Voltage or KV between anode and cathode
What are two types of interactions that produce radiation?
- Interaction with electron shells produces characteristic x-ray photons
- Interactions with atomic nucleus produce Bremsstrahlung x-ray photons
What do electrons within atom each have a specific amount of?
Binding energy that depends on the size (atomic number, Z) of the atom and the shell in which the electron is located
What is the binding energy?
Energy that would be required to remove an electron from the atom
It is an energy deficit
What is the interaction that produces the most photons?
Bremsstrahlung process
‘’ braking radiation’’
What is the production process of Bremsstrahlung?
Electrons that penetrate the anode material and pass close to the nucleus are deflected and slowed down by the attractive force from the nucleus
What does KV (kilovoltage) establish?
Energy of the electrons as they reach the anode
No x-ray photon can be created with an energy greater than that of the electrons
What is the maximum photon energy determined by?
Maximum or peak voltage during the voltage cycle
What does characteristic radiation involve?
Collision between high-speed electrons and orbital electrons in the atom
How can the characteristic radiation occur
If the incoming electron has a kinetic energy greater than the binding energy of the electron within the atom
How can anode material give rise to several characteristic x-ray energies?
Electrons at the different energy levels can be dislodged by the bombarding electrons and the vacancies can be filled from different energy levels
What does subscript alpha (a) denote?
Filling with an L shell electron
What does subscript beta indicate?
Filling from either the M or N shell
What does the characteristic radiation produce?
Line spectrum with several discrete energies
What does the Bremsstrahlung produce?
Continuous spectrum of photon energies over a specific range
Why is the number of photons created at each characteristic energy different?
The probability for filling a K-shell vacancy is different from shell to shell
What does the Molybdenum anode tube produce?
- used for mammography
2. Produce two intense characteristic x-ray energies: K-alpha radiation (17.9 keV) and K-beta (19.5 keV)
Why is anodes that have dual surface areas, molybdenum and rhodium, useful?
Operator can select a spectrum that is more optimized for different breast sizes and densities
When is No characteristic radiation produced?
If KV is less than the binding energy of the K-shell electrons
What is the relative composition of an x-ray spectrum with respect to Bremsstrahlung and characteristic radiation depend on?
- Anode material
- KV
- Filtration
What is the efficiency of the x-ray production defined as?
Total x-ray energy expressed as a fraction of the total electrical energy imparted to the anode
What are the two factors that determine production efficiency?
- Voltage applied to the tube
- Atomic number of the anode, Z
2.Efficiency = KV x Z x 10-6.
What is the consequence of increasing KV?
The quantity of radiation produced per unit of heat is significantly increased
What is the x-ray efficacy of the x-ray tube defined as?
the amount of exposure, in milliroentgens, delivered to a point in the center of the useful x-ray beam at a distance of 1 m from the focal spot for 1 mAs of electrons passing through the tube
What does the efficacy value express?
The ability of a tube to convert electronic energy into x-ray exposure
Why is KV very useful?
Controlling the radiation output of an x-ray tube
What is radiation output?
proportional to the square of KV
What does waveform describe?
The manner in which KV changes with time during the x-ray production process because of the cyclic nature of the electrical supply
What is the most effective x-ray producer?
The waveform with the least KV variation during the exposure
