Medical Tracers

Diagnosis with medical tracers

Radioactive tracers are used to produce images of areas of the body during tests. Different parts of our bodies absorb different biochemical substances. By "attaching" a radioactive isotope to such a substance, we create a medical tracer, and we can observe how our body processes the medical tracer by looking at the radiation coming from it.

If this radiation is beta (β) or gamma (γ) radiation, then it can travel through a decent thickness of tissue (as opposed to alpha (α) radiation, the particles of which are too large to be able to travel much more than$0.1\mathrm{mm}$through tissue). Thus, if the medical tracer contains radioactive isotopes that emit beta or gamma radiation, and it is absorbed by an organ, then we can see this organ by observing the radiation coming from the organ.

The equipment that is used to detect gamma radiation is a specialized camera that is made to capture the gamma spectrum instead of the visible spectrum of light. To detect other types of radiation, so-called scintillation counters are used: these machines catch radiation particles inside a chamber that causes them to emit light. This process is called scintillation and we can use regular (but sensitive) cameras to detect this light.

Isotopes in medical tracers

Let's see what we require from a radioactive isotope that is used in a medical tracer.

1. The radiation emitted must be able to travel from the inside of a patient's body to a radiation detector outside the body.
2. The radiation must be able to be detected by the detector.
3. The radiation emitted must not be of such high energy that it damages the patient's body.
4. The radiation must be there long enough to be able to use it for a diagnosis, but short enough for the patient to only be exposed to the radiation for not much longer than necessary.
5. The isotope itself must not damage the patient's body in any other way than its radioactivity.

This means that the radioactive isotope inside the medical tracer must have the following characteristics.

1. The isotope emits beta or gamma radiation.
2. The energy per radiation particle must not be too low.
3. If the isotope emits gamma radiation, it must be of a low enough frequency as to not damage the patient's body.
4. The half-life of the isotope must be on the scale of minutes to days, and preferably a few hours.
5. The isotope must not be toxic.

Examples of medical tracers

A PET scan showing radioactivity in the brain.

The image above shows radiation levels inside someone's head after being injected with a medical tracer. The brighter the color in the image is, the higher the radiation level is in that place, and thus the higher the concentration of the medical tracer is. This gives doctors information about the health of the patient's head.

Examples of isotopes used in medical tracers

Different radioisotopes can be used in medical tracers. For the interested reader, we have collected some examples of radioactive isotopes that can be used in medical tracers.

Example of the use of a medical tracer

Let's look at an example of the process of using a medical tracer.

Uses of radioactive tracers

Radioactive isotopes can be used in tracers in other contexts as well, in which case they are called radioactive tracers. Below is a list of uses of radioactive tracers other than in medicine.