Medical Imaging Flashcards
How an x-ray tube works
Electrons are emitted by the hot filament cathode, which is heated by a current. The electrons are directed at a tungsten anode and are accelerated across the p.d. and smash into it. When they collide, some of the electrons kinetic energy is converted into x rays.
Maximum energy of an emitted x ray
W=QV
Energy is equal to the charge of an electron * the p.d. of the x ray tube
The other way that x rays appear in an x ray tube
An x ray knocks a low energy electron out of the atom and a high energy electron will then take its place and emit its lost energy as an x ray
Intensity of an x ray beam
Power per unit area
X ray attenuation
When x rays pass through matter they are absorbed and scattered
How are X rays attenuated (3)
Photoelectric effect - Photon absorbed by electron which is ejected and gap is filled by another electron which emits a photon
Compton scattering - A photon knocks an electron out of an atom which causes the photon to lose energy and be scattered
Pair production - High energy photon decays into an electron-positron pair
How to distinguish materials with similar attenuation coefficients using x rays
Artificial contrast media with high attenuation coefficients are used (often swallowed)
Examples of artificial contrast media (2)
- Barium
- Iodine
How a CAT scan works
A x ray tube creates a fan shaped tube. This is then rotated around a patient and will therefore read a thin ‘slice’. The x-rays are detected by a ring of detectors and visualised with computer software
Advantage of CAT scan over normal x-rays
It provides a much more accurate picture and can be used to build up a 3d image from which a doctor can observe and remove certain densities. They are also more sensitive to changes in density
Collimated
When waves are made to be parallel
Medical tracer
A radioactive substance that is injected/ingested into the patient
Examples of medical tracers (2)
- Technetium-99m (Tc-99m)
- Fluorine-18
How a gamma camera works
Gamma rays emitted by the tracer pass through the collimater, which is made of lead and stops photons that have been scattered from being detected. The photons then hit the scintillator, which is a large crystal of sodium iodide. This flashes whenever a photon hits it (it emits many photons of visible light). This visible light is then incident on an array of photomultiplier tubes which emit an electron for every photon via the photoelectric effect. These electrons cause a current which can be detected and stored and viewed on a computer
Use of x rays vs medical tracers
x rays show the structure of organs whereas tracers show the function of organs