X-ray Tube and X-ray Production Flashcards
What are X-rays?
Type of electromagnetic radiation
Form part of the electromagnetic spectrum
Photons of energy travelling through space at a specific frequency and wavelength
Wavelength of x-rays ranges from 0.01 to 10 nanometers
Fairly high frequency and short wavelengths so x-ray photons have high a penetrative power
Ionising Radiation
High enough energy to strip electrons from an atom, or in the case of very high-energy radiation, break up the nucleus of the atom
Ionisation - the process in which an electron is given enough energy to break away from an atom
Results in the formation of two charged particles or ions: the molecule with a net positive charge, and the free electron with a negative charge
Results in chemical changes by breaking chemical bonds. This effect can cause damage to living tissue
Ionisation
Before:
1 atom containing 6 electrons, 6 protons and 6 neutrons
After:
1 positively charged ion containing 5 electrons, 6 protons and 6 neutrons + 1 free electron
Modern X- Ray Tube - what does it contain
Glass tube containing a vacuum
Functions as an energy converter
Receives electrical energy and converts it into two other forms of energy: x-radiation and heat
Rotating Anode X-Ray Tube
anode disc
anode stem
rotor support
rotor
bearings
vacuum
focusing cup
filament
cathode
glass envelope
stationary anode xray tube
glass envelope
copper anode
cathode
window
tungsten target
Cathode
Coiled filament wire which acts as a source of electrons
(-ve side of tube)
Two main parts:
coiled tungsten filament
nickel focusing cup
Cathode – Tungsten Filament
Why Tungsten?
Has a low work function – is a good thermionic emitter at lower temperatures
Does not easily evaporate
Will stay the same diameter therefore provides constant thermionic emission
Can be wound into a spiral (flexible) and is tough (high tensile strength)
Cathode Assembly
Most x-ray tube cathodes have two tungsten filaments to provide a large and a small focal spot size
Cathode – Focusing Cup
Nickel Focusing Cup
Negatively charged to keep the cloud of electrons from spreading apart
Designed to condense the electron beam to a small area on a focal track
Anode
A tungsten alloy target disc for the electrons which produces the x-ray photons
+ve side of tube
Two types of anode:
Rotating (found in most x-ray tubes)
Stationary (less common
eg. in dental x-ray machines)
Components
Anode Disc
Anode Stem
Motor
Anode Disc
Target anodes discs are made of tungsten alloy
Why Tungsten?
Tungsten is sufficiently dense to stop the electrons abruptly on its surface and thus produce maximum conversion of kinetic energy to X-ray energy
The tungsten nucleus has a high positive charge in its nucleus (Z=74) which causes greater deceleration of the electrons passing in vicinity of the nucleus
Has a high melting point of over 3000oC (highest melting point of all metals)
Low vapour production
Can be machined to give a smooth target track
Anode Stem
Made of molybdenum
Has to support the anode rotating at +3000rpm
Needs to resist heat flowing to the bearings
Must conduct electricity
In most tubes, the stem is short but thin
Anode Motor
Induction Motor
When the exposure button is pressed, a current is applied to the tube that produces a electromagnetic field that starts the rotation of the anode
This will cause the anode to spin to 3400 RPM
Envelope
Can be glass or metal
Contains the vacuum
Housing
Designed to limit the x-ray beam through the primary window
Minimise leakage of radiation with steel and lead
Insulating oil for heat dissipation
Provides mechanical support and damage protection
Power Source
High voltage generator – supplies the necessary electrical energy required to operate the x-ray tube
The x-ray tube requires electric energy to
To ‘boil’ electrons from the cathode filament
To accelerate these electrons from the cathode to anode
High Voltage Generators
Main components:
Transformers
Autotransformer
Filament circuit
Rectifiers
Separate control/console panel circuit
All connected in a high voltage circuit
Three phase generator (combination of 3 single phase generators)
Used by heavy duty industrial, professional and medical equipment where an intensive, constant power supply is required
Control Panel
Allows the radiographer to select the appropriate kVp, mA and exposure time
The exposure button readies the x ray tube for exposure by heating the filament and rotating the anode
Pressing the button further starts the exposure
The timing mechanism terminates the exposure
Transformers
A device to either increase or decrease the voltage in a circuit through electromagnetic induction
Consists of two wire coils wrapped around a closed iron core
Step-up transformers
Step-down transformers
A transformer can not create energy
An increase in the voltage must be accompanied by a corresponding decrease in current
Rectifier
Changes alternating current output of high voltage transformer to direct current
Allows current flow in one direction only at a steady constant rate
AC power will supply x-ray units with sinusoidal currents, resulting in ‘peaks and troughs’, limiting an x-ray tube to produce x-rays only half of the time during a cycle
A single-phase high voltage generator converts this AC power into a half or full wave rectified supply
Half- Wave Rectification
Half wave rectification results in a peak voltage that will dip to zero in a reoccurring manner
This will consequently have an effect on the behaviour of radiation produced, hence the name kilovoltage peak (kVp)
Full-Wave Rectification
Full wave rectification is more efficient
Bothe the +ve and –ve half cycles of the high tension transformer are used
The circuit reverses the –ve half cycle and applies it to the x-ray tube
Constant Potential Generator
The advancement of high voltage generators from single-phase to three-phase to constant potential generators have overcome this ‘voltage ripple’ creating a continuous, uninterrupted voltage
Modern x-ray units that utilise constant potential generators, do not have ‘voltage ripple’ and consequently employ the term kV rather than kVp
How Does an X-Ray Tube Work?
Current flows along a wire = electrons moving
A potential difference is applied at both ends of the conductor
As current flows through there is an increase mechanical vibrational energy
Raises temperature of the conductor = increase in heat energy
Energy conversion: from electrical to heat energy
Thermionic Emission
Emission of electrons from heated metal
Current increases temperature of tungsten filament in the cathode to about 2000oC
Increased energy overcomes binding energy of atom
Electrons are freed forming a ‘cloud’
X-Ray Production
The current (mA) applied to the cathode filament and the duration (seconds) will determine the total number of electrons released (mAs)
The released electrons are charged with high velocity towards the positive anode by the tube voltage (kVp) applied across the tube
At the anode, 99% of the energy from the electrons is converted to heat, and only 1% is converted to
x-ray photons
The x-ray photons are targeted
perpendicularly out of the
x-ray tube through a window
and towards the patient producing
the x-ray beam
Heat Dissipation
Rotating anode
Tungsten
Focal spot
Glass Envelope
Insulating oil
Steel housing / lead casing
X-Ray Tube Rating
The parameters (kVp, mAs) that can be safely used during its operation without causing damage to the x-ray tube itself
Unique to each individual x-ray tube model
The amount of heat produced depends on:
Voltage (kV)
Current (mA)
Length of exposure (s)
Type of voltage waveform
Number of exposure taken in rapid sequence
Energy and heat are usually measured in Joules (J) but this is not normally used to measure x-ray tube heat
The heat unit (HU) was introduced when single-phase equipment was common to make it easy to calculate heat
X-Ray Production at the Anode
When the accelerated electrons hit the anode they interact with the atoms of the anode and produce x-ray photons via two mechanisms:
Characteristic Radiation (10%-20%)
Bremsstrahlung Radiation (80%-90%)
Characteristic Radiation
A high energy electron collides with an inner shell electron and both end up ejected from the tungsten atom leaving a ‘vacant’ electron space
An outer shell electron moves to fill this inner space emitting energy in the form of an x-ray photon in the process. The energy of the emitted x-ray photon is equivalent to the energy level difference between the outer and inner shell electron involved in the transition
This is called characteristic radiation because the energy of the emitted electrons is dependent upon the anode material, not on the tube voltage
Energy is released in specific values corresponding to the binding energies of different shells
Bremsstrahlung (Braking) Radiation
When high energy electrons penetrate the anode target, some electrons travel close to the nucleus due to the attraction of its positive charge and are subsequently influenced by its electric field
These electrons are slowed down or ‘braked’ and their path is deflected whilst loosing a portion of their kinetic energy
The energy lost is emitted as Bremsstrahlung radiation and this ranges over a spectrum of energies
Characteristic vs Bremsstrahlung
Characteristic radiation
Only accounts for small percentage of x-ray photons produced
Bombarding electron interacts with inner shell electron
Radiation released due to electron dropping down into lower energy state
Radiation released is of a specific energy
X-ray beam energy depends on element of target atoms not tube voltage
Bremasstrahlung
Accounts for 80% of photons in x-ray beam
Bombarding electron interacts with whole atom
Radiation released due to diversion of bombarding electron as a result of the atomic pull
Radiation released is of a large range of energies
X-ray beam energy depends on tube voltage
X-Ray Spectrum
As a result of characteristic and bremsstrahlung radiation generation, a spectrum of X-ray energy is produced within the X-ray beam
Altering the X-Ray Spectrum
There are different ways of altering the range of energies in an x-ray spectrum:
Changing the intensity of the x-ray beam – this can be achieved by altering the kilovoltage peak (kVp) which is the peak voltage applied to the x-ray tube and determines the acceleration of the electrons from the cathode to the anode
A low accelerating voltage will produce lower intensity x-ray photons across the spectrum whilst a high accelerating voltage will produce higher intensity x-rays including larger characteristic peaks
Altering the X-Ray Spectrum
Changing the amount of x-ray photons present – this can be achieved by altering the current over a set period of time at the cathode and across the x-ray tube
An increase in current (mA) results in a higher production of electrons that are inside the x-ray tube which will increase the quantity of x-ray photons produced
The amount of time (seconds) used for a set current will also determine the number of electrons produced
A combination of current and time known as milliampere-seconds (mAs) is a measure of radiation produced over a set amount of time via an x-ray tube
The number of x-ray photons increase but the size of the characteristic peaks stays the same
X-Ray Energies
Superficial X-rays: 35 to 60 kV (Mammography)
Diagnostic X-rays: 20 to 150 kV( General X-rays)
Orthovoltage X-rays: 200 to 500 kV (Superficial Therapy)
Supervoltage X-rays: 500 to 1000 kV (Superficial Therapy)
Megavoltage X-rays: 1 to 25 MV (Linac)
Therapeutic Energies
In radiotherapy, the required energy of the produced photons is much greater (usually 1-25MV)
There is still a negative cathode, positive anode and vacuum
The photons are produced by a linear accelerator
A linear accelerator is a device that uses high Radio‐Frequency (RF) electromagnetic waves to accelerate charged particles (electrons) to high energies in a linear path, inside a tube like structure called the accelerator waveguide.
Therapeutic Energies
In radiotherapy, the required energy of the produced photons is much greater (usually 1-25MV)
There is still a negative cathode, positive anode and vacuum
The photons are produced by a linear accelerator
A linear accelerator is a device that uses high Radio‐Frequency (RF) electromagnetic waves to accelerate charged particles (electrons) to high energies in a linear path, inside a tube like structure called the accelerator waveguide.
Linear Accelerator
Electrons are emitted from an electron gun and accelerated in a straight line (to about 40% of the speed of light)
Radiofrequency waves generated by a magnetron or klystron pass through a waveguide, establishing an electric field
The radiofrequency waves ‘carry’ the electrons and give them their energy
Electrons strike the target (at approximately 98% of the speed of light)
