X-ray Tube and X-ray Production

Question 1

What are X-rays?

Answer 1

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

Question 2

Ionising Radiation

Answer 2

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

Question 3

Ionisation

Answer 3

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

Question 4

Modern X- Ray Tube - what does it contain

Answer 4

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

Question 5

Rotating Anode X-Ray Tube

Answer 5

anode disc
anode stem

rotor support
rotor

bearings

vacuum

focusing cup

filament

cathode

glass envelope

Question 6

stationary anode xray tube

Answer 6

glass envelope
copper anode
cathode
window
tungsten target

Question 7

Cathode

Answer 7

Coiled filament wire which acts as a source of electrons

(-ve side of tube)

Two main parts:
coiled tungsten filament
nickel focusing cup

Question 8

Cathode – Tungsten Filament

Answer 8

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)

Question 9

Cathode Assembly

Answer 9

Most x-ray tube cathodes have two tungsten filaments to provide a large and a small focal spot size

Question 10

Cathode – Focusing Cup

Answer 10

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

Question 11

Anode

Answer 11

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

Question 12

Anode Disc

Answer 12

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

Question 13

Anode Stem

Answer 13

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

Question 14

Anode Motor

Answer 14

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

Question 15

Envelope

Answer 15

Can be glass or metal
Contains the vacuum

Question 16

Housing

Answer 16

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

Question 17

Power Source

Answer 17

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

Question 18

High Voltage Generators

Answer 18

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

Question 19

Control Panel

Answer 19

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

Question 20

Transformers

Answer 20

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

Question 21

Rectifier

Answer 21

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

Question 22

Half- Wave Rectification

Answer 22

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)

Question 23

Full-Wave Rectification

Answer 23

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

Question 24

Constant Potential Generator

Answer 24

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

Question 25

How Does an X-Ray Tube Work?

Answer 25

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

Question 26

Thermionic Emission

Answer 26

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’

Question 27

X-Ray Production

Answer 27

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

Question 28

Heat Dissipation

Answer 28

Rotating anode
Tungsten
Focal spot
Glass Envelope
Insulating oil
Steel housing / lead casing

Question 29

X-Ray Tube Rating

Answer 29

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

Question 30

X-Ray Production at the Anode

Answer 30

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%)

Question 31

Characteristic Radiation

Answer 31

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

Question 32

Bremsstrahlung (Braking) Radiation

Answer 32

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

Question 33

Characteristic vs Bremsstrahlung

Answer 33

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

Question 34

X-Ray Spectrum

Answer 34

As a result of characteristic and bremsstrahlung radiation generation, a spectrum of X-ray energy is produced within the X-ray beam

Question 35

Altering the X-Ray Spectrum

Answer 35

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

Question 36

Altering the X-Ray Spectrum

Answer 36

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

Question 37

X-Ray Energies

Answer 37

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)

Question 38

Therapeutic Energies

Answer 38

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.

Question 39

Therapeutic Energies

Answer 39

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.

Question 40

Linear Accelerator

Answer 40

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)