radionuclide Flashcards
what is beta decay?
neutron decays to porton + electron
how are positrons emitted?
proton decays to neutron and positron
what is the decay equation?
N=N0exp(-lamdat)
Lambda = decay constant
N = number of nuclei after time t
why are gamma particles regularly used for nuclear medicine?
they have the energy to escape human tissue although they interact with matter (photoelectric and compton scattering)
why are alpha particle less preferable for nuclear medicine?
their coulomb interaction causes short range travel - imaging not possible
what are beta particles less preferable for nuclear medicine?
do not travel straight, undergo coulomb interaction, imaging impossible as their travel distance 1mm
what is the purpose of a cyclotron?
produces PET radionuclides by smashing protons together with high energy, in a circular path by B field and vacuum chamber
what are the two types of cyclotrons
1 ) shielded room (expensive but easy access for maintenance)
2) self shielded (compact, cheap, work near by but access difficult)
how do nuclear reactors produce radionuclides?
by fission: uncle of heavy elements absorb thermal neutron, fission products = neutrons and radionuclide
radionuclide generators produce what sort of radionuclides?
- short lived radionuclides
- ‘carrier free product’ daughter nuclides distinctly different to parents for seperation
describe an ideal generator
- simple
- daughter yield high
- compact with shielding
- daughter stable, parent free
- sterile
what is the decay equation for 99Tc?
99Mo -> 99mTc -> 99Tc
beta decay, than gamma decay 6 hours
properties of photon emitting radio tracers for SPECT scan
- low energy gamma photons
- heavy nuclides label larger molecules
- majority use 99mTc
how are tracer synthesised?
- with robotics and shielding due to radioactivity
- only small quantities desired
properties of positron emitting radio tracers for PET scan
- short half lives (expensive)
- low energy
- cyclotron required
- biological elements
comm: 18FDG
what problems are there with tracer distribution?
short half lives need delivering quickly, or ideally a cyclotron on site making tracers expensive for PET
what does PET stand for?
positron emission tomography
what does SPECT stand for?
single photon emission computed tomography
what are the principles of radionuclide imaging?
1) small amounts of molecules labelled with radionuclide
2) radiotracer administered to person
3) signal detected of radionuclide decay
4) convert signal to image
describe how molecules are labelled with radionuclides?
- radiotracer amounts are are v small, half life is low, pure (radionuclide at one molecule)
- radionuclide is pure, no other ways of decay
describe how radiotracer administered to biological system?
-all tracer molecules travel quickly to the abnormal area and stay
how is the signal of the radionuclide decay detected?
- emitted particle should pass through the body, arriving at the detector in a predictable manner
- detector can discriminate each decay event
how do the gamma photons go from patient to electrical pulse?
- collimator absorb non parallel photons
- scintillator crystal absorbs gamma photons
- scintillation emits optical photons that are converted to electrical current by photocathode in PMT
how does scintillation work?
- gamma photons ejects electrons by e- (compton/photoelectric)
- when electron is excited then relaxed, visible light photon emitted
- NaCL doped with thallium crystal material
describe a problem with light photons travelling from scintillation crystal to PMT photocathode
if their is air/solid interface TIR occurs so SI grease of similar optical index to crystal and cathode used
How does the PMT (photomultiplier tube) work?
light hits photocathode producing e-, e- attracted to dynode gaining energy from ascending voltages. result is amplified electric signal
what is a block detector?
cuts in scintillator crystal to distribute light between PMT, cost effective and solution to reading out scintillation light from large number of small crystals
how does the pulse height analyser exclude scatter?
don’t want badly positioned counts, so an energy limit applied within a percentage of the radioisotope energy
how does the collimator work?
parallel lead bars absorb photons that will negatively impact the image
how does the septal thickness of the collimator effect the image
septa must be thick enough to stop radiation with energy of imaged isotope. increasing thickness increases the imaging energy of photons that can pass, however this decreases sensitivity
how does the hole depth of the collimator effect image
increasing the hole depth is increases resolution but decreases the sensitivity
how does the hole size of the collimator effect the image?
- decreasing the hole size increases resolution
- smaller holes means more are of lead septa so sensitivity decrease
how does the collimator to patient distance effect image?
larger collimator to patient distance increases the amount of noise effecting image
what is the attenuation decay equation?
I=I0 exp(-ux)
u=attenuation coefficient
describe image reconstruction with the analytical approach? adv and dis
- projects backwards the data with filter
- fast, linear, well known
- low resolution, streak artefacts
describe image reconstruction using the iterative approach? adv and dis
- model measurement and integrating filtering
- high resolution, visually better images
- slow, stopping criteria, salt and pepper noise
