Module 6 X-rays Flashcards
Typical wavelength of x rays?
1x10^-10 m
What are the four main components of a x ray tube?
Cathode, Acceleration tube, Target anode, Thin sheet of aluminium
What is the function of the cathode in generating x rays?
Emits a stream of electrons through thermionic emission
What is thermionic emission?
Where electrons are emitted from a metal by gaining KE from a PD
Describe how a cathode causes thermionic emission.
Cathode filament is heated by a low voltage. Electrons gain KE according to W=QV. And escape the filament by thermionic emission.
What is a typical voltage which is used in a cathode?
6V
What is the typical potential difference between the cathode and target anode?
50-100kV
Why is a vacuum used in an acceleration tube?
To prevent electrons colliding with air particles and losing energy
What proportion of colliding electrons cause the emission of x rays?
<1%
Explain why electrons accelerate in the acceleration tube.
Large potential difference between cathode and anode
Is an anode positive or negative?
Positive
Is a cathode negative or positive?
Negative
What is the target anode usually made of?
Tungsten
What are the properties of tungsten which make it useful for x ray emission?
High density, High melting point
Explain why the anode is sometimes rotated.
To distribute heat evenly and prevent damage
Explain what happens at the anode which causes emission of x rays.
Some collisions between electrons and tungsten atoms cause electrons to lose large amounts of KE by emitting an X ray photon.
What is the function of the thin sheet of aluminium?
Filters out low energy electrons
Why is it important that a thin sheet of aluminium is used when generating x rays?
Removes low energy x rays which do not contribute to the image produced and are potentially harmful to the patient.
Briefly describe the function of each of the four parts of an x ray generator.
Cathode: electrons gain KE through a low voltage and are emitted through thermionic emission. Acceleration: electrons are accelerated in a vacuum through a large potential difference by more KE = QV. Anode target: most electrons collide and KE is transferred to heat the target. Some electrons lose KE rapidly by emitting x rays.
What colour is a photographic plate originally?
White
What do x ray photons do to a photographic plate?
Darken it
Explain why when in air, the photographic plate turns black.
Air does not absorb x rays well.
Explain why something like bone is visible on x rays?
Strongly absorbs x rays. They can’t reach the plate so it remains white.
What are the benefits and risks to x rays?
Non invasive, Ionising
What does attenuation mean?
Reduction in intensity of x-rays as they pass through a material
What is μ?
Attenuation constant
What are the units for the attenuation constant?
m^-1 (can have any prefix next to it as long as they match x)
Name the four ways x rays can interact with matter.
Simple scatter, Photoelectric effect, Compton effect, Pair production
What is the energy range for x rays to interact with matter by simple scatter?
1-20keV
Describe what happens during simple scatter.
Low energy x ray photons (1-20keV) bounce off electrons orbiting an atom, changing direction but with the same energy.
Explain why simple scatter is considered an elastic collision.
Low energy photons bounce off electrons orbiting an atom and change direction but same speed and therefore same KE.
What is the energy range for x rays to interact with matter through the photoelectric effect?
50-100keV
Describe what happens during the photoelectric effect with x rays.
X ray photons of energy 50-100keV are absorbed by an orbiting electron. Photon disappears and orbiting electron gains its KE. Electron is emitted from the atom leaving it ionised.
Why during the photoelectric effect and Compton effect is momentum conserved?
It’s a typical collision.
In a conceptual sense, what is the Compton effect?
Same as photoelectric but photon is not completely absorbed and instead just loses some energy.
Outline the Compton effect.
X ray photons collide with an orbiting electron and change direction but at a lower energy (lower wavelength). Electrons gain PE to escape from the atom with KE.
What does ionisation mean?
Removal of electrons
What is the energy range of x ray photons for the Compton effect?
500keV-5MeV
Explain how pair production of x rays takes place.
X ray photon collides with a nucleus. Photon disappears and is converted to an electron positron pair.
Show how you find the minimum energy of a photon required for pair production.
E = mc^2 + mc^2 (where m is the mass of an electron)
What is the main process which contributes to the attenuation of x rays?
Photoelectric effect
What is the probability of the photoelectric effect taking place proportional to?
Z^3 (proton number cubed)
Why can x rays not differentiate between most soft tissues?
Most soft tissue have similar values of Z. Attenuation is similar producing minimal contrast in images.
What are the two main contrast media?
Barium meal (barium sulphate), Iodine
Explain why contrast media work.
Have a high Z value so increase the probability of photoelectric effect taking place. Produces large contrast as they absorb a lot of x ray photons.
Describe how a barium meal can be used as a contrast media.
Patient drinks a solution containing barium sulphate before an x ray of the digestive system. Produces very strong absorbance so the digestive system shows clear white images.
Describe how iodine can be used as a contrast media.
A solution of iodine is injected into the blood, producing very strong absorbance in the blood vessels which remain white against a grey background.
What does CAT stand for?
Computerised axial tomography
How are CAT scans also abbreviated?
CT scans
What is a gantry?
A part of a CT scanner that houses the X-ray tube and detector array. A circular machine or tube.
Describe how a CT scan works.
Patient lies on a gantry that has an x ray tube. The X ray tube and detectors rotate around the patient. X ray tube produces thin fan shaped beams which are attenuated by the patients tissues and the remaining x rays are detected. Detectors send electrical signals to a computer which produces a 3D image that can be viewed from any angle.
What are the advantages of CAT scans?
Can distinguish between different types of soft tissues of similar attenuation coefficients. 3D image produced can be viewed from any angle.
What are the disadvantages of CAT scans?
Expensive to buy and to run. Patients receive more ionising radiation from CAT scans as patients must be kept still for approximately 30 minutes for a complete scan.
Describe the structure of a CAT gantry.
A rotating circular tube with an X ray tube at one side and a detector array at the other.
