Medical physics

Diagram showing how X-rays are produced

Key elements:

• High voltage power supply

• Hot filament (cathode)

• target metal (anode)

What is the purpose of the high voltage power supply when producing X-rays?

To create a large p.d. between the cathode and anode to accelerate electrons towards the anode.

What is the purpose of the hot filament / heater when producing X-rays?

To produce electrons by thermionic emission

What is the purpose of the target metal when producing X-rays?

To decelerate the electrons and cause them to produce X-ray photons

What is the purpose of the led shield around an X-ray tube?

To shield the radiographer from X-rays by aborbing X-rays not passing through the window

How is a collimated beam of X-rays produced?

• The cathode is a heater which produces electrons by thermionic emission

• These electrons are accelerated towards the anode by a large p.d. from the high voltage power supply

• The anode is the target metal which decelerates the electrons, causing them to produce X-ray photons

• The lead shielding aborbs all X-rays other than those passing through the window, producing a cillmated beam of electrons

What range of potential difference is used to generate X-rays for imaging?

30 to 120 kV

Why is the X-ray tube evacuated?

So that electrons pass through the tube without interacting with gas atoms.

What are the two ways that electron energy is released on hitting the target anode?

• X-rays (<1%)

• Increasing the thermal energy of the anode

How is the anode cooled?

• Often oil is circulated to cool the anode

• Alternatively the anode is rotated to spread the heat over a large surface area

How are X-rays collimated?

• The anode is shaped to emit X-rays in the desired direction

• A lead shield absorbs any X-rays emitted in other directions

Sketch a typical X-ray spectrum of intensity vs wavelength for an X-ray tube running at a particular supply voltage

Identify the bremsstrahlung on this X-ray spectrum

Identify the K lines on this X-ray spectrum

Describe how K-lines are produced

• The electrons incident on the anode can remove electrons in the metal atoms close to the nuclei, creates gaps in the lower energy levels.

• These gaps are quickly filled by electrons dropping from higher energy levels.

• These transitions release photons of specific energies and therefore wavelengths, creating spikes in intensity at these wavelengths

What are the four X-ray attenuation mechanisms?

• Simple scatter

• Photoelectic effect

• Compton scattering

• Pair production

Simple scatter of X-rays (diagram and description)

The X-ray photon interacts with electron in the atom, but has less energy than the work function, so the X-ray photon simply bounces off without any change to its energy.

What is the range of energies over which simple scattering occurs?

X-ray photons with 1-20 keV

Photoelectric effect with X-rays (diagram and description)

The X-ray photon is absorbed by one of the electrons in the atom. The electron uses this energy to escape from the atom.

What is the range of energies over which the photoelectric effect occurs?

X-ray photons with 20-100 keV

Compton scattering (diagram and description)

The incoming X-ray photon interacts with an electron within the atom. The electron is ejected from the atom, and the X-ray photon is scattered with reduced energy.

What is the range of energies over which compton scattering occurs?

X-ray photons with 0.1-5 MeV

Pair production from X-rays (diagram and description)

An X-ray photon interacts with the nucleus of the atom. It disappears and the electromagnetic energy of the photon is used to create an electron and its antiparticle, a positron.

What is the range of energies over which pair production occurs?

X-ray photons with >= 1.02 MeV

Which attenuation mechanism is most likely in a hospital X-ray machine?

• Photoelectric effect

• Hospital X-ray machines typically used 30-100 kV producing X-ray photons with <100 keV

• Hence the photoelectric effect is dominant

Which attenuation mechanism is least likely in a hospital X-ray machine?

Pair production as this requires a p.d. > 1 MV which is ten times higher than the typical p.d.s used in hospital X-ray machines.

State the equation for transmitted intensity of X-rays in a given substance. Identify each quantity

I = I₀ e^-μx

• I is the transmitted intensity

• I₀ is the innitial intensity before absorption

• μ is the attenuation coefficient of the substance

• x is the thickness of the substance

What is the attenuation coefficient also known as?

The absorption coefficient

The higher the attenuation coefficient, the ___ X-rays a material will aborb per unit length

more

Draw a sketch of transmitted intensity of X-rays I versus thickness of substance x

What is Z?

Atomic number - the number of protons in a nucleus

How is μ related to Z?

μ ∝ Z³

What is the average Z of soft tissue?

7

What is the average Z of bone?

14

What is the Z of iodine?

53

What is the Z of barium?

56

What is a contrast medium?

A subtance that improves the visibility of a material (like soft tissue) in an X-ray image