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Medical Tracers

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Medical Tracers

How do doctors look inside our bodies when we are scanned by a machine? This has to do with the radioactivity of so-called medical tracers. Some radioactive substance is inserted into our body, and as a result, the radiation emitted by the substance is visible from outside our body by special equipment. Doctors can then base their diagnosis on where they see radioactivity inside our bodies. You can learn the basics of medical tracers by reading this article.

Definition of medical tracers

A medical tracer is a chemical compound containing a radioactive isotope that is used by doctors to see inside bodies by detecting radiation it emits.

So exactly how do doctors use medical tracers to diagnose us?

Diagnosis with medical tracers

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.

This is why the radioactive substance is called a (medical) tracer: it traces the path that the non-radioactive version of the substance takes within a body.

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 thanthrough 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.

If someone is suspected to have a dangerously swollen kidney, then we can find the biochemical agent that is absorbed by kidneys, and attach a radioactive isotope to it such that the substance emits beta or gamma radiation: it is now a medical tracer. In practice, the isotope technetium-99m is often used in medical tracers for kidney scans. We then inject the medical tracer into the person's body and wait a bit until the kidneys have absorbed most of it. We can now point beta or gamma radiation detection devices toward our patient, effectively see their kidneys, and either diagnose them with a swollen kidney or not.

Medical Tracers PET scan of kidneys and the brain StudySmarterPET scan showing the brain, kidneys, and bladder of a person, Wikimedia Commons CC BY-SA 4.0.

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

Medical Tracers PET scan brain StudySmarterPET scan showing radioactivity in the brain, Wikimedia Commons CC BY-SA 3.0.

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.

Technetium-99

A variant of technetium-99 that is in an excited state (technetium-99m) is the most widely used medical radioactive isotope because it has a lot of good properties. It emits low-frequency gamma radiation in the process of going to its lower energy level, has a half-life of 6 hours, and it is not toxic, so it checks all the boxes of being used in a good medical tracer.

Gallium-68

Gallium-68 emits beta radiation, has a half-life of 68 minutes, and it is not toxic, so this is also a good medical tracer. This isotope is mostly used for detecting some variants of cancer.

Fluorine-18

Fluorine-18 emits beta radiation, has a half-life of 110 minutes, and it is not toxic. It is widely used in PET scans to let us observe brains, hearts, thyroid glands, and bones among other things.

Rubidium-82

Rubidium is an alkali metal that has characteristics that are very similar to those of potassium. Therefore, the beta decay of rubidium-82 can be used to trace the movement of potassium: it can take the place of potassium in many biological processes. It has a half-life of only just over one minute, so the diagnosis should be done quickly, but the patient will only experience serious radiation levels for a few minutes.

Example of the use of a medical tracer

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

It is known that a dead or damaged heart muscle cell does not retain potassium very well, while healthy heart muscle cells do. To find out if someone's heart is (partially) damaged, we can use a medical tracer as in the following step-by-step procedure.

  1. We combine rubidium-82 isotopes with an appropriate biological substance to create a medica tracer.
  2. We inject this contaminated substance into the patient.
  3. We wait for a few minutes. In general, the wait time depends on how easily the substance is absorbed and the half-life of the medical tracer.
  4. At this point, the radioactive rubidium-82 should have taken the place of some potassium atoms in the heart muscle cells.

If we see radiation coming from heart tissue, we know that this tissue contains rubidium-82, so it contains potassium, which means that it is a healthy bit of heart tissue. If we do not see radiation coming from some part of the heart muscle tissue, we know that this part must be damaged or dead. Thus, we can diagnose our patient based on our radiation observations made possible by the 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.

Uses of radioactive tracers: Fracking

Fracking is a process in which you create fractures in rock formations. You can inject these cracks with a radioactive tracer such that you can observe the profile of the fractures you created.

Uses of radioactive tracers: Radiocarbon dating

Medical Tracers A fossil of a mammoth in a museum StudySmarterRadiocarbon dating can determine how long ago this Columbian mammoth died, Wikimedia Commons CC BY-SA 3.0.

The carbon of every living organism consists of the same percentage of carbon-14 isotopes, but once an organism dies, the carbon atom interchange with its environment stops. This means that the percentage of carbon-14 isotopes decreases slowly but steadily, because carbon-14 is radioactive, with a half-life of nearly 6000 years. Thus, by measuring the percentage of carbon-14 isotopes in an organism, we can figure out how long ago it died. In this process, you could regard the carbon-14 as a tracer that is naturally present in every organism, and not the precise location of the radioactive isotopes matters, but the quantity of the total radiation matters in "diagnosing" how long an organism has been deceased.

Medical Tracers - Key takeaways

  • A medical tracer is a substance containing a radioactive isotope that is used by doctors to see inside bodies.
  • By "attaching" a radioactive isotope to a biochemical substance, we can observe that substance by looking at the radiation coming from it. If this substance is absorbed by an organ, then we can see this organ by observing the radiation coming from the organ.
  • Medical tracers 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 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.
  • The principle of seeing things using radioactive tracers is not limited to only medical applications.

Frequently Asked Questions about Medical Tracers

A medical tracer is a substance containing a radioactive isotope that is used by doctors to see inside bodies.

Medical tracers are used to diagnose patients. In general, radioactive tracers are used to observe things that we can otherwise not observe because there is a visual obstruction in our way.

A good example of a radioactive isotope used in a medical tracer is technetium-99m. This excited state of the isotope technetium-99 emits low-energy gamma radiation and has a half-life of 6 hours, making it excellent for use in a medical tracer.

Medical tracers emit detectable radiation, so if part of the tracer is absorbed by an organ, we can see this organ through the radiation emitted by the tracer.

The radiation used in medical tracers is beta or gamma radiation, because those types of radiation can penetrate the multiple centimetres of tissue that are between the organ and the doctor.

Final Medical Tracers Quiz

Question

Which statement is true?

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Answer

A radioactive tracer is always a medical tracer.

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Question

How are medical tracers used to observe how the body processes a substance? 

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Answer

By using a medical tracer, we can observe a substance by looking at the radiation coming from it. If this medical tracer is absorbed by an organ, then we can see this organ by observing the radiation coming from the organ.

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Question

What type of radiation should medical tracers emit, and why?

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Answer

Beta or gamma radiation, because these are the types of radiation that can penetrate multiple centimetres of tissue such that doctors can detect it.

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Question

Should the energy of the radiation emitted by medical tracers be low or high?

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Answer

The energy should be low, in general, because high-energy radiation can be damaging to living tissue.

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Question

What is the optimal half-life duration of a medical tracer, and why?

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Answer

A few hours, so the radiation is present long enough for a diagnosis but goes away reasonably quickly after that so the patient isn't exposed to unnecessary radiation.

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Question

Are medical tracers limited to be used only in medicine?

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Answer

Yes, by definition. Radioactive tracers in general can be used in other contexts as well.

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Question

Why can't we use alpha radiation emitters as medical tracers? Can we use them as radioactive tracers in other contexts?

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Answer

Alpha radiation does not penetrate through liquids and solids deeper than 0.1 mm, so it is undetectable in the medical context but also in almost all other contexts.

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Question

Name a reason why mercury-208 should not be used as a medical tracer. It emits beta particles and has a half-life of about 40 minutes.

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Answer

Mercury is highly toxic.

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Question

Why do doctors often protect themselves during a scan that uses medical tracers?

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Answer

Doctors will be exposed to the radiation from the medical tracers every day, which in the long run means they are exposed to a significant amount of radiation.

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Question

If we want to observe the iodine intake of the thyroid, what medical tracer would be suitable to do this?

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Answer

An isotope of iodine that emits low-energy beta or gamma radiation and that has a half-life of a few hours. An example of this is iodine-123.

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Question

Why would an examination using a medical tracer not be advised to people who are suspected to be perfectly healthy?

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Answer

Patients receive a small dose of radiation during such an examination, coming from the medical tracer. If this examination is not necessary at all, the benefits do not outweigh the costs.

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Question

In what circumstance would a long half-life of a medical tracer not be a problem?

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Answer

In the circumstance that the medical tracer would exit the body of the patient in some form after a couple of hours.

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Question

Give an argument for why the energy of the radiation of the medical tracer should not be too low.

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Answer

If the energy per radiation particle is too low, the detector will not be able to detect the radiation.

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Question

Give an argument for why the energy of the radiation of a medical tracer should be low.

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Answer

High energy radiation particles can harm the patient.

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Question

Explain why a half-life of 1 second is too short for a medical tracer.

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Answer

The transport of substances through organisms is on the scale of minutes to hours, so once the medical tracer reaches its 'destination', the radioactive isotopes will have largely decayed by then.

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