Medical Imaging Flashcards
what is the process of x-ray production
- external power supply is used to create a p.d between electrodes
- cathode (filament) is heated and produces electrons through thermionic emission
- the electrons are accelerated towards the anode
- x-ray photons are produced when the electrons are decelerated by hitting the anode
- kinetic energy of electrons is transferred into x-ray photons
- the energy output is less than 1% of the kinetic energy of the incident electrons, the rest heats up the anode
- the anode stays cool either by spinning or by having a coolant such as oil behind it
why is the x-ray tube a vacuum
so electrons do not lose energy through interacting with gas particles
what is the anode in an x ray tube made of and what is its main property
this is the target metal, such as tungsten, which has a large melting point
what does attenuation mean
attenuation is used to describe the decrease in the intensity of EM radiation as it passes through matter
what are the four attenuation mechanisms
- simple scatter
- photoelectric effect
- compton scattering
- pair production
what is simple scattering (x ray interaction with matter)
- x ray photon is scattered elastically by an electron
- x ray photon interacts with an electron in the atom, but has less energy than the energy required to remove the electron
- occurs for x ray photons between 1 - 20 keV
what is the photoelectric effect (x ray interaction with matter)
- x ray photon is absorbed by one of the electrons in the atom
- the electron uses this energy to escape from the atom
- x ray photons with less than 100keV
common in x ray imaging
what is compton scattering
- incoming x ray photon interacts with an electron within an atom
- electron is ejected from atom, but the x ray photon does not disappear completely, it is just scattered with less energy
- x ray photons with energy of 0.5 - 5MeV
what is pair production (x ray interaction with matter)
- x ray photon interacts with the nucleus of the atom
- it disappears and the EM energy of the photon is used to create an electron-positron pair
- only occurs when x ray photon has energy equal to or greater then 1.02MeV
what does a higher attenuation coefficient mean
it absorbs x ray photons more than a material with a lower attenuation coefficient
what is a contrast medium
used to improve visibility of internal structures in x ray images
what are the two most common contrast mediums and what are each used for
- iodine
- injected into blood vessels to view certain liquids such as blood flow
- barium sulfate
- ingested to view the digestive system
what is the attenuation coefficient proportional to
the cube of the atomic number
what is the process of a CAT scan
- patient lying on back slides into gantry
- x ray tube on one side and x ray detectors on the other
- x ray tube produces a fan shaped beam of electrons which irradiates a thin slice of the patient
- x rays are attenuated by different amounts by different tissues
- intensity of x rays is recorded by detectors
- sends electrical signals to a computer
- x ray tube and detectors rotate 360 degrees around patient creating 2D image of thin slice of patient
- table moves forward, new image is created
- images are manipulated to create a £D image of patient
what are the advantages of a CAT scan
- shape, size and position of tumours can be determined
- can distinguish between soft tissues of similar attenuation coefficients
- non-invasive
what are the disadvantages of a CAT scan
- slower and more expensive than a single x ray scan
- subject to ionising radiation
- patients must remain very still for a long period of time
why are gamma emitting sources ideal for medical tracers
- least ionising
- can penetrate through the patient to be detected externally
why must radioisotopes used in medical imaging have a short half life
- to ensure high activity from the source so that only a small amount is required to form the image
- so that patient is subjected to the radiation for a long period of time
describe the gamma camera
- gamma photons travel towards the collimator, a honeycomb of long, thin tubes made from lead
- any photons arriving at an angle to the axis of the tubes are absorbed by the tubes, so only those travelling parallel can reach the scintillator
- gamma photons that reach the scintillator material produces thousands of visible light photons
- the chance of a gamma photon interacting with the scintillator is only 1 in 10
- the photons of visible light travel to the photomultiplier tubes
- photons of light are converted into voltage
- the outputs are connected to a camera
- these electrical signals are processed to locate the impacts of the gamma photons on the scintillator
- these impact positions are used to construct a high-quality image that shows the concentrations of the medical tracer within the body
why must gamma photons be travelling parallel to the collimator tubes
to produce a high-quality image that isnt blurred
describe the process of a PET scan
- patient lies on table and is surrounded by a ring of gamma detectors
- FDG (fluorodeoxyglucose) is injected
- decays, giving off a positron
- positron annihilates and produces two gamma photons travelling in opposite directions
- photons picked up by gamma camera
- computer can determine the point of annihilation from the time difference in the arrival of the two gamma photons
what are the advantages of a PET scan
- non-invasive
- diagnoses different types of cancers
- can observe the function of the brain
what are the disadvantages of PET scans
- very expensive
- only found at larger hospitals
- not very common so only patients with complex health problems are recommended
what is an ultrasound transducer
- a device that changes electrical signals into sound and sound into electrical signals
- generates and receives ultrasound
- uses the piezoelectric effect
