Summary: Medical Imaging
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1 PET
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1.1 Imaging prinicples
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Which particles of an atom are important for a PET scan?
- The number of protons and neutrons need to be about the same to be a stable atom.
- So when an atom is not stable, it will release a neutron.
- So it will gain a proton and a positively loaded electron = positron.
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What is the line of stability?
That the N = Z. So same amount of protons and neutrons. -
What happens with the energy levels when a proton is added?
It will drop and there will come energy free. So the beta particle (positrons) will gain energy. -
What happens when there is decay by beta positive particles?
- It meets an electron.
- annihilation (both get destroyed)
- The mass becomes light (electromechanic energy)
- A foton transports this energy; a foton is gamma ray.
- The fotons will go in opposite directions.
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How is the transmission through tissue?
The equation represents that the fractional change in intensity (ΔI/I\Delta I / IΔI/I) is proportional to:- The thickness of the absorber (Δx\Delta xΔx).
- The attenuation coefficient (μ\muμ), which determines how strongly the material absorbs photons.
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When a photon occurs. What can a photon do? (2)
- Photonelectric effect
- Compton scattering.
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What can compton scattering cause? (3)
- Decrease in image quality
- Higher dose in patient, because it effect more tissue
- More background radiation
- Decrease in image quality
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What type of photons cause the photoelectric effect and what type the compton scattering?
- PE => low energy
- Compton => high energy
- PE => low energy
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How does a scintilator work?
- Gamma ray goes in
- Transformation in visible light
- Photocathode transforms it in electrons
- Guiding the electrons via dynodes
- They arrive at the anode = signal output
- Gamma ray goes in
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What is coincidence detection?
When both are detected from the two sides, within a certain time period. This does not always go well.
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