X-rays production

X-rays production

Learning objectives

• At the end of this session the student should be able to:
• Describe the process of x-rays production
• Explain the general characteristics of X-radiation
• Describe the types of x-rays

Understand the X-ray spectrum

X-ray production

The concept of atomic orbitals, energy levels and electron shells

For the better understanding of X-rays production, it is important to have good knowledge of the atomic structure.

In previous session we saw how electrons revolve around the nucleus in their paths called orbits.

Possible electron orbits are grouped into different shells.

X-rays production

Electrons within the same shell have the same quantity of binding energy and the energy decreases with increasing shell number/ energy levels Number of electrons within a shell vary according to the type of shell i.e 2n² Shell number (n) /Quantum number

• Shell symbol
• Number of electrons
• K
• L
• M
• N
• O
• P

Q

Production of (X) radiation

Electric current is passed through the tungsten filament and heats it up. As it is heated up the increased energy enables electrons to be released from the filament through thermionic emission. The electrons are attracted towards the positively charged anode and hit the tungsten (W) target (atomic number Z=74) with a maximum energy determined by the tube potential (voltage).

NB: The atomic number of target affects both quantity and effective energy (quality) of x-rays

X-rays production

As the electrons bombard the target they interact via Bremsstrahlung and characteristic interactions which result in the conversion of energy into heat (99%) and x-ray photons (1%). The x-ray photons are released in a beam with a range of energies (x-ray spectrum) out of the window of the tube and form the basis for x-ray image formation.

….

• Cathode (-)
• Filament
• Made of thin (0.2 mm) tungsten wire because tungsten:
• has a high atomic number (A =184, Z=74)
• is a good thermionic emitter (good at emitting electrons)
• can be manufactured into a thin wire

has a very high melting temperature (3422°c) Filament is a conducting wire with a high melting point, forming part of a thermionic valve and heated by an electric current

….

!The size of the filament relates to the size of the focal spot. Some cathodes have two filaments for broad and fine focusing.

• Focusing cup
• Made of molybdenum as:

high melting point

poor thermionic emitter so electrons aren’t released to interfere with electron beam from filament Negatively charged to focus the electrons towards the anode and stop spatial spreading Focusing cup

concentrates the electron beam towards the focal spot of the anode

…..

Filament current:

The current (usually 10 A) heats up the filament to impart enough energy to the electrons to be released i.e. it affects the number of electrons released.

• Tube current:
• This is the flow of electrons to the anode and is usually 0.5 – 1000 mA

!It affects the energy and number of electrons released.

…..

• Anode
• Target made of tungsten for same reasons as for filament
• Rhenium added to tungsten to prevent cracking of anode at high temperatures
• Positively charged to attract electrons

Set at angle to direct x-ray photon beam down towards patient. Usual angle is 5° – 15° Rhenium is used in X-ray tubes to improve the strength, toughness, heat resistance, and precision of the target material

Focal spot and Anode angle

Anode angle increases the surface area of the focal spot which increases the ability of the anode to absorb heat Anode angle causes variation of the beam intensity across the x-ray field

Anode angle increases the surface area of the focal spot which increases the ability of the anode to absorb heat

Anode angle causes variation of the beam intensity across the x-ray field.

Types of electron interactions and resulting radiations

At the anode, electrons can interact with the atoms of the anode in several ways to produce x-ray photons.

(a)Outer shell interaction: low energy EM released and quickly converted into heat energy (b)Inner shell interaction: This involves K or L shells produces characteristic radiation (useful x-rays) ( c)Nucleus field interaction: Produces Bremsstrahlung radiation

1. Characteristic radiation

The characteristic radiation are x-rays produced by interaction of highly energetic incident electrons and the target electrons in the K or L shell.

At a specific photoenergy there are peaks where more x-rays are released. These are at the characteristic radiation energies and are different for different materials.

Only K-characteristic x-rays are useful for imaging

Production of Characteristic x-radiation

2. Bremsstrahlung radiation

Bremsstrahlung radiation(German word=slowed down).

Generated when a high-speed electron is deflected by the nucleus of a target atom. This deceleration releases energy as X-rays.

They represent wide part of the graph in which photons with a range of energies are produced.

Bremsstrahlung accounts for the majority of x-ray photon production.

The word Bremsstrahlung is retained from the German language to describe the radiation that is emitted when electrons are decelerated

Bremsstrahlung radiation

X-ray Spectrum

Quantity and quality of x-radiation

X-ray intensity

X-ray intensity (I) refers to the x-ray photon energy passing through a unit area in a unit time. It represents the amount of x-rays coming off the target.

The intensity of x-rays depends on the applied tube current (mA)

Intensity (I) of X-rays….

Effect of mA and mAs (Tube Current and Time) The product of tube current in milliamperes and exposure time in seconds (mA*sec) describes the total number of electrons bombarding the target.

!!Inverse square law: The intensity (I) of x-rays varies inversely proportional to the square of the distance (d) between the Source (focal spot) and the subject/film i.e. Doubling the distance will reduce intensity/exposure to One quarter (1/4)

Inverse square law

quantity of x-rays

X-Ray Quality

The term quality describes the penetrating power of the radiation.

The penetrability of an x-ray beam is called the x-ray quality and it is determined by tube Voltage (KV). X-rays with high penetrability are termed high-quality x-rays High-energy x-rays are able to penetrate tissue more deeply than low- energy x-rays.

X-rays with high penetrability are termed high-quality x-rays

Factors that affects the Quantity and quality of X-rays

A number of factors under the control of radiographers influence the size and shape of the x-ray emission spectrum and therefore the quality and quantity of the x-ray beam. These include:- Target Material X-ray

• Tube Current (Miliamperage- mA)
• Tube voltage (kVp)
• Exposure time (s)
• Filtration
• Collimation

Source to film distance

Characteristics of X-rays

X-rays have shorter wavelength of electromagnetic spectrum They are highly energetic radiations capable of penetrating opaque materials and breaking up molecules, hence damage living cells They are charge less particles

• They are not affected by magnetic /electric fields

They are produced

https://www.youtube.com/watch?v=KEASC8UVAmM

https://www.youtube.com/watch?v=fREyzdwxCjs