PET (positron emission tomography) uses a positron-emitting tracer, usually fluorine-18 in FDG, to make 3D images of active tissue such as tumours or regions of the brain.
2. Each positron travels less than about a millimetre, meets an electron and they annihilate.
3. Their mass becomes two gamma photons of equal energy that travel in opposite directions. Each photon has energy equal to the rest energy of an electron:E = mec2 = 9.11 × 10−31 × (3.00 × 108)2 = 8.2 × 10−14 J = 511 keV 4. The patient lies inside a ring of detectors. Two detectors on opposite sides that register photons at the same moment (in coincidence) show that the annihilation happened on the straight line joining them.
5. The small difference in arrival times Δt shows where on that line it happened. The point is a distance cΔt / 2 from the midpoint, towards the detector hit first.
6. A computer combines millions of events into a 3D image of tracer concentration.Worked example: one photon arrives 0.20 ns before the other.distance from midpoint = cΔt / 2 = 3.00 × 108 × 0.20 × 10−9 / 2 = 0.030 m = 3.0 cm
2. Each positron travels less than about a millimetre, meets an electron and they annihilate.
3. Their mass becomes two gamma photons of equal energy that travel in opposite directions. Each photon has energy equal to the rest energy of an electron:
5. The small difference in arrival times Δt shows where on that line it happened. The point is a distance cΔt / 2 from the midpoint, towards the detector hit first.
6. A computer combines millions of events into a 3D image of tracer concentration.Worked example: one photon arrives 0.20 ns before the other.