Practise analysing diagnostic ultrasound, X-ray imaging, CT and PET using their physical principles, calculations, image formation and clinical trade-offs.
Fluorine-18 (918 F) is a radioactive nuclide that is used as a tracer in positron emission tomography (PET scanning). Fluorine-18 decays to a nuclide of oxygen ( O ) according to
State what is meant by a tracer.
material introduced into the body
and (position in body) can be detected or absorbed by the tissue (being studied)
Explain how the radioactive decay of fluorine-18 results in the emission from the body of the gamma-ray photons that are detected during a PET scan.
positrons (emitted in the decay) and electrons annihilate
mass of particles becomes energy of gamma photons
Explain how the detection of the gamma-ray photons is used to produce an image of the tissue being examined.
arrival times of photons are processed
image built up of tracer concentration in the tissue
Positron emission tomography (PET scanning) involves the detection of gamma-radiation in order to identify the position of origin of positrons in the body.
Positrons are not naturally present in the body.
Explain how positrons come to be present in the body during PET scanning.
introduction of tracer (into the body)
containing a β+emitter
Positron emission tomography (PET scanning) makes use of a tracer containing a radioactive material that decays by positron emission.
State what is meant by a tracer.
substance introduced into the body
absorbed by tissues (being studied)
State the name of the particles that are emitted from the body and detected by the detectors during PET scanning.
gamma photons
Explain how the particles in (a)(ii) are created from positrons.
positrons (emitted) interact with electrons (in tissue)
positron is antiparticle of electron
(pair) annihilation (between the two) / matter turns into energy
Positrons can be artificially created by a process in the laboratory that is the reverse of the process in (b). This process creates both a positron and an electron moving at the same speed in opposite directions.
Suggest why two of the particles in (a)(ii) are needed to create one positron.
(same mass therefore) combined momentum of electron and positron is zero
photons have momentum so must be two of them (to conserve momentum)
In medical diagnosis, X-rays may be used to produce a single X-ray image or may be used in computed tomography (CT scanning).
Suggest an advantage and a disadvantage of CT scanning compared with single X-ray imaging for diagnosis.
advantage:
disadvantage:
advantage: produces 3-dimensional image
disadvantage: (much) greater exposure to radiation