6.5 Medical Physics (X Rays)

Question 1

How are X-rays produced?

Answer 1

When charged particles are rapidly decelerated (or accelerated) and their kinetic energy is transformed into high frequency photons

Question 2

X rays and gamma rays have overlapping frequency spectra. What might we see as a result of this?

Answer 2

X rays and gamma rays with the same frequency, and therefore equal wavelength and energy.

Question 3

What is the only difference between x-rays and gamma rays that have identical frequency?

Answer 3

Gamma rays are produced from radioactive decay
X-rays are produced by Bremsstrahlung or braking radiation

Question 4

What is Bremsstrahlung (braking radiation)?

Answer 4

When radiation is given off by charged particles due to their acceleration.

Question 5

What can x-rays used in medical imaging be referred to as?

Answer 5

Soft X-rays (as they usually have lower energy than gamma rays)

Question 6

How do X-ray tubes produce X-rays?

Answer 6

By acccelerating electrons in a high voltage electric field, then rapidly decelerating them via collision with hard metal anode.

Question 7

Where are electrons first emitted from in the process of producing x-rays?

Answer 7

A heater or filament (at the negative electrode) via thermionic emission

Question 8

What is thermionic emission?

Answer 8

The process by which electrons are emitted from a heated source (such as a high resistance coiled wire).

Question 9

Why are electrons emitted into a vacuum tube during the process of x-ray emission?

Answer 9

To prevent electrons from colliding with air molecules before they have acquired enough energy to emit x-rays

Question 10

What is the maximum voltage used in the electric field accelerating the electrons during the process of x-ray emission?

Answer 10

200kV

Question 11

What is the maximum gain of kinetic energy of electrons during the process of x-ray emission? (where the maximum voltage is 200kV)

Answer 11

200keV
(Using W=QV)

Question 12

How does the motion of an electron change when it collides with the hard metal anode during the process of x-ray emission?

Answer 12

It decelerates rapidly

Question 13

During x ray emission, how does the energy of an electron change once they collide with the hard metal anode?

Answer 13

Some of their kinetic energy is emitted as X-rays. The rest is lost to thermal energy in the anode.

Question 14

During x ray emission, there is a risk of the anode overheating. What are two ways to prevent this?

Answer 14

• Rotate the anode. This way, a new area is constantly exposed to the electron beam.
• Cool it with a circulating water supply.

Question 15

X rays are emitted in every direction from the anode in an X-Ray tube. How would we turn this into a collimated beam?

Answer 15

Encase the vacuum tube in lead shielding, except in one spot, the window, so that the X-rays emerge in one place outside the tube.
We can direct the beam into a collimator to further collimate the beam.

Question 16

What is a collimator?

Answer 16

A series of straight and parallel metal tubes. Any rays which aren’t parallel to the tubes are absorbed.

Question 17

Why is a collimated beam of x rays much more useful?

Answer 17

It can be directed at specific parts of a patient.

Question 18

[Breaking Radiation Intensity Profile] There are a few sharp lines of characteristic radiation which is not due to decelerating electrons. What are they caused by?

Answer 18

Incident electrons knocking out bound low energy level electrons in the anode atoms. Higher energy electrons will then transition down to the unoccupied shell, and their excess energy will be emitted as radiation.

Question 19

X rays can ionise matter. How can we minimise the damage to living tissue?

Answer 19

It must be exposed to only low intensity beams for short periods of time.

Question 20

What is attenuation?

Answer 20

The gradual decrease in energy/intensity of X rays as they pass through matter.

Question 21

Does attenuation change depending on the material?

Answer 21

Yes. For example, bone attenuates X rays to a greater extent than soft tissues.

Question 22

In what way does the intensity of a collimated beam of X-rays decrease?

Answer 22

Exponentially.

Question 23

State equation for attenuation of an X-Ray beam

Answer 23

I = I₀ e^-μx

I : Attenuated intensity after passing through medium
I₀: Initial Intensity
μ: Attenuation Coefficient
x: Thickness of medium

Question 24

What is another name for the attenuation coefficient?

Answer 24

Absorption coefficient

Question 25

List the absorption mechanisms, from lowest energy to highest energy.

Answer 25

Simple Scattering (1-20keV)
Photoelectric Effect (<100keV)
Compton Effect (0.5-5MeV)
Pair Production (>1.02MeV)

Question 26

Absorption Mechanism: Simple Scattering

Answer 26

X-rays of energy 1-20keV will reflect off layers of atoms in the material, as they don’t have enough energy to undergo more complex processes.

Question 27

Absorption Mechanism: Photoelectric Effect

Answer 27

• X-rays of energy less than 100keV can be absorbed by electrons in the material, as they have the same energy as the ionisation energies of the atoms.
• When an X-ray is absorbed by an atom, a photoelectron is released and another electron may transition down to the lower energy level, emitting a scattered photon in the process.

Question 28

Absorption Mechanism: Compton Effect

Answer 28

• X-rays of 0.5-5MeV lose only a fraction of their energy to electrons in the absorbing materials.
• This is due to an inelastic interaction between the photon and electron.
• The scattered X-ray photon will have less energy than before, and so its wavelength will be greater.
• The compton electron will be scattered in a different direction, as momentum must be conserved.

Question 29

Absorption Mechanism: Pair Production

Answer 29

• When X-ray photons with energy greater than 1.02MeV pass through the electric field of an atom, it will spontaneously produce an electron-positron pair via the mass energy relation.
• The positron goes on to collide with another electron and annihilate, producing photons.

Question 30

How significant is pair production in medical X-rays?

Answer 30

Not very significant. The photon energies are usually not high enough to produce an electron-positron pair.

Question 31

What is contrast media?

Answer 31

High attenuation coefficient materials. They have heavy atoms (large number of protons and electrons)

Question 32

Describe the relationship between attenuation coefficient and proton number.

Answer 32

μ ∝ Z³