Gamma-emitting radioactive tracers

A radioactive tracer is introduced into the body so that its distribution can be detected. As the tracer’s radioactive atoms decay, they emit gamma photons. Some of these photons escape the body and reach a detector outside it, providing information about where the tracer is located.

The radioisotope can be incorporated into a compound that is taken up by a particular organ or tissue. The detected radiation then provides information about the tracer’s distribution there.

Properties of a useful gamma tracer

A tracer’s half-life is the time taken for half of its radioactive nuclei to decay. It must be long enough for the tracer to be prepared and for imaging to take place, but short enough to limit prolonged radiation exposure.

The emitted gamma photons must also have sufficient energy to escape the body and be detected outside it. Not every photon needs to escape, but enough must reach the detector to provide useful information.

Tracer radioisotopes

• Technetium-99m has a half-life of about 6 hours and emits gamma photons of approximately $$140\ \text{keV}$$. It can be attached to different compounds to target different organs.
• Iodine-131 has a half-life of about 8 days and emits beta particles as well as gamma radiation, including photons of approximately $$364\ \text{keV}$$. Radioactive iodide is taken up by the thyroid, and iodine-131 can also be used to label compounds. Its beta emission contributes to radiation exposure inside the body.
• Indium-111 has a half-life of about 2.8 days and emits gamma photons of approximately $$171\ \text{keV}$$ and $$245\ \text{keV}$$. It can be used to label compounds that target particular tissues.

Molybdenum-technetium generator

A molybdenum-technetium generator provides technetium-99m for use as a radioactive tracer. It contains molybdenum-99, which decays to technetium-99m. Molybdenum-99 is the longer-lived parent and has a half-life of about 66 hours; technetium-99m is the shorter-lived daughter.

Separating technetium-99m

The molybdenum-99 is retained on a column inside the generator. As it decays, technetium-99m is produced on the column. Saline, a salt solution, is passed through the column and washes out the technetium-99m while the molybdenum remains in place.

The collected solution contains technetium-99m in pertechnetate ions, $$\ce{^{99m}TcO4^-}$$. Washing a substance from a column in this way is called elution.

Reusing the generator

After elution, molybdenum-99 remains on the column and continues to decay. Further technetium-99m accumulates, so it can later be eluted again. The generator cannot be used indefinitely because the amount of parent molybdenum-99 decreases as it decays.

The parent’s longer half-life makes the generator practical to transport and store. The shorter-lived technetium-99m can then be obtained locally when needed.

PET scanning

Positron emission tomography (PET) produces an image showing where a radioactive tracer is distributed in the body. The tracer contains a radionuclide that emits positrons. It is incorporated into a compound that is taken up by the tissue, or involved in the biological activity, being investigated.

From tracer to gamma photons

After the tracer has entered the body, an emitted positron travels a short distance before it meets an electron. The positron and electron then annihilate, producing two gamma photons. Each photon has an energy of approximately $$511,\text{keV}$$, and the photons travel in approximately opposite directions.

Detecting photon pairs

Detectors arranged around the body record pairs of gamma photons that arrive at nearly the same time. A detected pair indicates that the annihilation occurred along the line joining the two detectors; it does not identify the exact point on that line.

Many photon pairs are detected along different lines. These detections are used to reconstruct an image of the tracer distribution. Because each positron travels a short distance before annihilation, the annihilation occurs close to, but not necessarily exactly at, the point where the tracer emitted it.