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radionuclide Flashcards

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USMLE® Step 1 flashcards
1
Q

what is beta decay?

A

neutron decays to porton + electron

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2
Q

how are positrons emitted?

A

proton decays to neutron and positron

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3
Q

what is the decay equation?

A

N=N0exp(-lamdat)
Lambda = decay constant
N = number of nuclei after time t

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4
Q

why are gamma particles regularly used for nuclear medicine?

A

they have the energy to escape human tissue although they interact with matter (photoelectric and compton scattering)

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5
Q

why are alpha particle less preferable for nuclear medicine?

A

their coulomb interaction causes short range travel - imaging not possible

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6
Q

what are beta particles less preferable for nuclear medicine?

A

do not travel straight, undergo coulomb interaction, imaging impossible as their travel distance 1mm

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7
Q

what is the purpose of a cyclotron?

A

produces PET radionuclides by smashing protons together with high energy, in a circular path by B field and vacuum chamber

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8
Q

what are the two types of cyclotrons

A

1 ) shielded room (expensive but easy access for maintenance)
2) self shielded (compact, cheap, work near by but access difficult)

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9
Q

how do nuclear reactors produce radionuclides?

A

by fission: uncle of heavy elements absorb thermal neutron, fission products = neutrons and radionuclide

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10
Q

radionuclide generators produce what sort of radionuclides?

A
  • short lived radionuclides

- ‘carrier free product’ daughter nuclides distinctly different to parents for seperation

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11
Q

describe an ideal generator

A
  • simple
  • daughter yield high
  • compact with shielding
  • daughter stable, parent free
  • sterile
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12
Q

what is the decay equation for 99Tc?

A

99Mo -> 99mTc -> 99Tc

beta decay, than gamma decay 6 hours

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13
Q

properties of photon emitting radio tracers for SPECT scan

A
  • low energy gamma photons
  • heavy nuclides label larger molecules
  • majority use 99mTc
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14
Q

how are tracer synthesised?

A
  • with robotics and shielding due to radioactivity

- only small quantities desired

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15
Q

properties of positron emitting radio tracers for PET scan

A
  • short half lives (expensive)
  • low energy
  • cyclotron required
  • biological elements
    comm: 18FDG
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16
Q

what problems are there with tracer distribution?

A

short half lives need delivering quickly, or ideally a cyclotron on site making tracers expensive for PET

17
Q

what does PET stand for?

A

positron emission tomography

18
Q

what does SPECT stand for?

A

single photon emission computed tomography

19
Q

what are the principles of radionuclide imaging?

A

1) small amounts of molecules labelled with radionuclide
2) radiotracer administered to person
3) signal detected of radionuclide decay
4) convert signal to image

20
Q

describe how molecules are labelled with radionuclides?

A
  • radiotracer amounts are are v small, half life is low, pure (radionuclide at one molecule)
  • radionuclide is pure, no other ways of decay
21
Q

describe how radiotracer administered to biological system?

A

-all tracer molecules travel quickly to the abnormal area and stay

22
Q

how is the signal of the radionuclide decay detected?

A
  • emitted particle should pass through the body, arriving at the detector in a predictable manner
  • detector can discriminate each decay event
23
Q

how do the gamma photons go from patient to electrical pulse?

A
  • collimator absorb non parallel photons
  • scintillator crystal absorbs gamma photons
  • scintillation emits optical photons that are converted to electrical current by photocathode in PMT
24
Q

how does scintillation work?

A
  • gamma photons ejects electrons by e- (compton/photoelectric)
  • when electron is excited then relaxed, visible light photon emitted
  • NaCL doped with thallium crystal material
25
Q

describe a problem with light photons travelling from scintillation crystal to PMT photocathode

A

if their is air/solid interface TIR occurs so SI grease of similar optical index to crystal and cathode used

26
Q

How does the PMT (photomultiplier tube) work?

A

light hits photocathode producing e-, e- attracted to dynode gaining energy from ascending voltages. result is amplified electric signal

27
Q

what is a block detector?

A

cuts in scintillator crystal to distribute light between PMT, cost effective and solution to reading out scintillation light from large number of small crystals

28
Q

how does the pulse height analyser exclude scatter?

A

don’t want badly positioned counts, so an energy limit applied within a percentage of the radioisotope energy

29
Q

how does the collimator work?

A

parallel lead bars absorb photons that will negatively impact the image

30
Q

how does the septal thickness of the collimator effect the image

A

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

31
Q

how does the hole depth of the collimator effect image

A

increasing the hole depth is increases resolution but decreases the sensitivity

32
Q

how does the hole size of the collimator effect the image?

A
  • decreasing the hole size increases resolution

- smaller holes means more are of lead septa so sensitivity decrease

33
Q

how does the collimator to patient distance effect image?

A

larger collimator to patient distance increases the amount of noise effecting image

34
Q

what is the attenuation decay equation?

A

I=I0 exp(-ux)

u=attenuation coefficient

35
Q

describe image reconstruction with the analytical approach? adv and dis

A
  • projects backwards the data with filter
  • fast, linear, well known
  • low resolution, streak artefacts
36
Q

describe image reconstruction using the iterative approach? adv and dis

A
  • model measurement and integrating filtering
  • high resolution, visually better images
  • slow, stopping criteria, salt and pepper noise

Medical (6 decks)

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