Medical Imaging Flashcards
How is an X-Ray created?
Deceleration of electrons at the anode (target metal) converts their KE into high energy photons
Basic structure of X-ray
Electrons accelerated from cathode due to high temperature (thermionic emission)
Vaccinated glass tube
Target metal attached to spinning motor to prevent overheating
Max energy of x-ray photon
Voltage x charge
Efficiency of X-ray
1% of energy converted to photons
Attenuation of X-rays
I=I₀e^-μx
what are I, I₀, μ, x
Intensity
Initial intensity
Attenuation coefficient
Distance from surface
Describe simple scatter
Simple scattering occurs when an X-ray photon interacts with an atom and is scattered with no change in energy
Describe Compton effect
Compton scattering occurs when an X-ray photon removes an electron from an atom and is scattered with reduced energy
Describe pair production
Photon absorbed and creates electron-positron pair (1.1MeV)
Describe photoelectric effect
Photon absorbed by an electron which its ejected from the atom. gap in electron is filled by another electron which emits a photon.
what is attenuation coefficient proportional to
μ∝Z³
properties of contrast media
high proton number
iodine/ barium
Define thermionic emission
when electrons are released from a material by heating it
what is a radiopharmaceutical
medical tracer; radioisotope (fluorine-18/ technetium-99m) combined with other elements to target specific organ
features of technetium-99m
emits gamma radiation
half life of 6 hours
decays to stable isotope
features of fluorine-18
used in PET scans
undergoes beta plus decay
half life of 110 minutes
what is a collimator
hollow lead tubes which only let gamma rays pass if they are directly bellow them
whats is a scintilator
above the collimator, prudes photons of visible light when gamma ray hits it
what are photomultiplier tubes
photons incident at the photocathode produce cascade of electrons which produces pulse of voltage at the end of the tube
how does transducer emit ultrasound
crystal supplied with alternating potential difference to expand/contract it which causes sound waves to be produced at high frequencies.
how does reflection of ultrasound at boundary depend on the physical property of the material
there will be no reflections if both materials have the same impedance.
Large difference in impedance will provide with most reflection
fraction of reflected intensity= (z2-z1)^2 / (z2+z1)^2
how are ultrasounds used to measure blood flow
ultrasound waves reflected at angle to the blood flow
work out speed from change in frequency of the reflected wave caused by doppler shift
