medical physics Flashcards
how are x-rays produced
- rapidly accelerating or decelerating charged particles
- their kinetic energy is transferred into high energy photons
how can you differentiate between x-rays and gamma rays
- X-rays and gamma rays have frequencies that overlap so cannot distinguish by wavelength
- use their method of production
- gamma rays come from radioactive decay or particle collisions with a mass defect
- X-rays produced by accelerating charged particles
why are X-rays used in medical imaging
have energy that is lower than gamma rays
general structure of an x ray tube
heat filament (cathode) and tungsten anode with a potential difference between them and sealed in a vacuum tube
how does an x-ray tube tube work
- electrons are emitted from the heated filament via thermionic emission and drain towards the anode
- collide with the anode and some of. their kinetic energy is released as x-rays in all directions
- the rest is transferred to heat energy within the anode
why does the x-ray tube need a vacuum
prevents electrons from colliding with molecules of air before they gain enough energy to release x-rays
how is the anode prevented from overheating
- rotating so new section is in contact with the x-rays
- using water as a coolant, circulating it through the anode
how are x-rays focused into one beam
- vacuum tube is encased in a material that is thinner in one area, so only x-rays that pass through that section are released from the tube
- they pass through a collimator - a series of straight parallel tubes that absorb any rays that are not travelling parallel to the axis of the tubes
why is it better for x-rays ti be in a beam rather than emitted in all directions
allows them to be directed at specific areas (like a broken bone) and minimises the patients exposure to them
what is x-ray attenuation
when a material absorbs x-rays decreasing the intensity exponentially
how can you calculate the intensity of x-rays leaving a material
I = I0e^-Px
I0= initial intensity
P = attenuation
x = thickness
explain the process of taking an x-ray of a patient
- x-rays directed at area of patients body an pass through bine and soft tissue
- bone higher attenuation coefficient it absorbs more x-rays then soft tissue
- if photographic film is placed behind the patient the areas where bine is will not blacken as much as areas of soft tissue creating an image
- nowadays digital detectors are used in place if photographic film
the greater the attenuation (absorption) coefficient…
the more the material will absorb incident x - rays
simple scattering
- x-rays of energy between 1 and 20KeV are directed at a material
- x-rays will reflect off layers of atoms or molecules in the material because they have insufficient energy to undergo more complex processes (like photoelectric effect)
photoelectric effect
- x0rays of energy less than 100KeV are directed at a material
- x-rays can be absorbed by electrons in the material if they have the same energy as the ionisation energy of the atoms
- x-ray is absorbed, a photoelectron is released and another electron may de-excite, releasing another photo in the process
Compton effect
- x-ray of energy between 0.5 and 5MeV are directed at a material
- x-rays will lose a small amount of their energy to electrons in the absorbing materials due to an inelastic collision between the photon and electron
- scattered x-ray photon will have less energy than before ( greater wavelength)
- the Compton electron will be scattered in a different direction as momentum must be conserved
pair production process
- an x-ray of energy greater than 1.02MeV passes through the electric field of an atom
- electron-position pair is produced
- positron will annihilate with another electron and produce two photons
- process not very important in medical x-rays as the photon energies are usually not high enough to cause pair production
contrast media and give two examples
- high attenuation coefficient materials that have heavy atoms with a large proton number and therefore a large number of electrons
- easily identified in x-ray images as they absorb a lot of x-ryas
- examples (barium (Z=56) or iodine (Z=53) compared to soft tissue (Z=7)
relationship between attenuation coefficient and proton number
P = Z^3
what does the CAT stand for in CAT scan
computerised axial tomography
what is a CAT scan and how do they work
- 3D x-ray image of a patients body made up of lots of 2d images
- x-ray tube generates a fan - shaped beam that is directed onto a patent that is laying down
- ring of detectors behind the patient to detect the beam intensity
- tube and detectors rotate around the patient and up and down their body to create 3d image
compare CAT scans to conventional x-ray images
- CAT scans Gove a better resolution image and having a 3d image makes it easier to assess the injury
- however CAT scans take significantly longer than conventional x-rays so the patient is exposed for longer