Radioactive Tracers and PET

X-rays show structure. Nuclear medicine shows function — which tissue is active, not merely where it is. That is the distinction the whole topic turns on.

A tracer is a radioactive isotope attached to a molecule the body uses, introduced into the patient and tracked from outside by the radiation it emits.

What makes a good tracer

A gamma emitter, since only gamma escapes the body to reach the detector. Alpha and beta would deposit their energy in tissue and cause harm without producing an image.

A short half-life, long enough to complete the scan but short enough to limit dose — typically hours.

Chemically suitable, so the body carries it to the organ under investigation.

Technetium-99m meets all three: a pure gamma emitter with a six-hour half-life, and it can be attached to a wide range of molecules. It is far and away the most-used medical isotope.

The gamma camera — a lead collimator admits only gamma photons travelling perpendicular to it, so each detector position corresponds to one point in the patient. Behind it, a scintillator converts each photon into a flash of light, and photomultiplier tubes convert the flash into an electrical pulse. A computer assembles the pulses into an image.

The collimator is why the image is spatially meaningful — without it, a photon arriving at a detector could have come from anywhere, and the picture would be a uniform blur.

PET — positron emission tomography

A positron-emitting tracer is used, most often fluorine-18 attached to glucose.

Each emitted positron travels a very short distance before meeting an electron, and the two annihilate.

The annihilation produces two gamma photons of 0.511 MeV travelling in exactly opposite directions, as momentum conservation requires.

A ring of detectors registers both arrivals. The tiny difference in arrival times locates the annihilation along the line between the two detectors.

Example: attaching the tracer to glucose is what makes PET so useful. Tissue that consumes glucose rapidly lights up, so active brain regions and fast-growing tumours both stand out. The image shows metabolism itself — biochemistry made visible from outside the body.

Why 0.511 MeV — it is the rest energy of an electron, from E = mc². The two annihilating particles have equal mass, and their energy is shared equally between the two photons.