Medical tracers such as technetium-99m and fluorine-18 can be used as diagnostic tools.
- Inject tracer (into bloodstream)
- Taken up by organ or shows blockage
- By emitting radiation detected by camera
More specifically, tracers may be used to:
- Monitor blood flow in brain
- Monitor bone growth
- Monitor functioning of organs like heart and lungs
To ensure that tracers reach the correct organ or tumour, the radioisotope is chemically combined with elements that will target the desired tissues to make a radiopharmaceutical. Tracers must be selected carefully, with specific characteristics:
- radiation: used in preference to because it is sufficiently penetrating to pass through patient without being absorbed, permitting detection, and is least ionising so causes little damage to cells.
- Short half-life: ensures high enough activity to produce image and that patient is not subjected to high dosage of
- radiation long after the procedure.
To detect the photons emitted by the tracer, a camera is used. This consists of:
- Collimator: photons travel along axis of lead tubes
- Scintillator: photons produce many photons of visible light
- Photomultiplier tube: electrons produced. Used to generate image.
- Computer: signals used to produce image
- To improve image quality, increase scan time or use longer collimator
Positron emission tomography (PET) is a further diagnostic tool used to image internal structures.
- Brain is surrounded by ring of detectors
- Positrons annihilate electrons
- Produces 2 identical photons travelling in opposite directions
- Delay time allows identification of position of annihilation
- Computer connected to cameras produces image
PET provides the benefit of being a non-invasive technique which can provide valuable diagnostic information about organ functionality and internal structure. But PET scanners are very expensive to run due to the facilities required to produce the medical tracers.