Lecture 3 - Basic principles of physics relevant to medical imaging Flashcards
Define electron binding energy
Amount of energy required to remove an electron from a particular shell
Electron binding energy is highest for the K shell and goes down (L,M,N). It also increases with atomic number (more +ve nucleus attracting e- = more difficult to remove them from shells)
Define Radioisotope
An element with more neutrons than normal, becoming unstable. It is able to give off particles as decay (radioactive).
For nuclear medicine, radioisotopes with relatively short half lives are used in order to reduce harm to the patient
Define atomic number
Z - the number of protons in the nucleus
Define atomic mass
A - The total number of nucleons in the nucleus
Define isotope
Any nucleus which contains the same number of protons but different mass number (more neutrons)
How does increasing wavelength affect energy?
As wavelength increases energy decreases
as wavelength increases frequency decreases
Which level of energy waves are not capable of being reflected or refracted?
High energy
Define attenuation
The reduction of the intensity of an electromagnetic beam as it travels through matter. It can be due to photoelectric absorption, compton scattering and pair production
Define luminescence
Ability to produce light - Fluorescence and phosphorescence (electron transitions within the material being irradiated, cause the emission of photons that have less energy than the incident photons)
Define and name some types of ionising radiation
Xrays and gamma.
Have sufficient energy to remove electrons from atoms (ionisation)
Produce highly reactive positively or negatively charged ions
These ions can cause damage to DNA and proteins, with long term biological effects
Define and name some types of non ionising radiation
Sound waves (ultrasound), radio waves (MRI)
They have less energy and hence do not cause ionisation (less harmful)
They can agitate body tissues as they produce heat
They don’t cause long term biological effects
What is the deterministic effect of radiation?
With every increase in radiation dose you increase cell death.. There is therefore a different level of effect at each particular dose.
What is the stochastic effect of radiation?
Radiation has an ongoing effect, but at a certain level disease such as cancer will be induced (like a threshold.) You cannot be certain at which dodge point this will happen.
Define acceleration
A change in velocity, (given as the change in velocity per minute).
If the accelerating force is in the opposite direction to the direction of moving body then it will cause a loss of velocity (deceleration). e.g. throwing a ball upward in the air will cause it to decelerate.
Define energy
Capacity to do work.
Define potential energy
The energy possessed by the body
Define kinetic energy
energy that a body possesses by virtue of its motion.
What property on an X-ray machine can you alter to change the potential difference?
kVp.
Potential difference is the driving force which moves the electron along a conductor, it has an effect on kinetic energy, measured in kVp.
What property on an X-ray machine can you alter to change the electric current?
mAs.
Electric current measures the amount of current, which determines the amount of thermionic emission, measured in mAs.
When producing X-rays, electrons collide with target atoms in order to rapidly decelerate and consequently produce X-rays.
What are the 3 ways they might collide with target atoms?
- interaction between them and outer electrons in the atom. This produces heat but no X-rays.
- Interaction between them and the nuclei of the atom. This produces X-rays and heat. (bremsstrahlung)
- Interaction between them and individual electron in the inner orbits. This produces X-rays and heat. (characteristic)
What happens in Bremsstrahlung production of X-rays?
Energetic electrons from the filament interact with the nuclei of target atoms and are slowed down, thus giving off energy in the form of an X-ray. (‘Breaking’ radiation).
Bremsstrahlung radiation forms a continuous spectrum of radiation.
What happens in characteristic production of x-rays?
Energetic electrons remove electrons from inner orbitals and the vacancies created are filled by the electrons from the outer orbitals, giving off energy in the form of X-rays.
What does the quantity of the radiation an X-ray beam measure?
The number of photons in the beam.
What does the quality of the radiation in the X-ray beam measure?
It’s penetrating power.
