Using isotopes as tracers
Radioactive isotopes have many useful applications in a wide variety of situations, for example, they can be used within a plant or animal to follow the movement of certain chemicals. In medicine, they have many uses, such as imaging, being used as tracers to identify abnormal bodily processes, testing of new drugs and conducting research into cures for disease.
Thyroid gland
The thyroid gland is situated in the neck region of the body, attached to the upper portion of the trachea. Its main function is to produce the hormone thyroxine, which has an important role in regulating body metabolism.
Phosphorus uptake by plants
Plants take up phosphorus-containing compounds from the soil through their roots. By adding a small amount of radioactive phosphorus-32 to fertiliser and then measuring the rate at which radioactivity appears in the leaves, it is possible to calculate the rate of uptake of phosphorus from the soil. The information gathered could help plant biologists to identify plant types that can absorb phosphorus quickly. These plants may give better yields resulting in more food or fibre at less expense.
Pesticide levels
To measure pesticide levels, a pesticide can be tagged with a radioisotope such as chlorine-36, and this is applied to a field of test plants. Over a period of time, radioactivity measurements are made. Estimates can then be made about how much accumulates in the soil, how much is taken up by the plant and how much is carried off in run-off surface water.
Medical tracers
Radioactive isotopes and radioactively labelled molecules are used as tracers to identify abnormal bodily processes. This is possible because some elements tend to concentrate (in compound form) in certain parts of the body – iodine in the thyroid, phosphorus in the bones and potassium in the muscles. When a patient is injected with a compound doped with a radioactive element, a special camera can take pictures of the internal workings of the organ. Analysis of these pictures by a specialist doctor allows a diagnosis to be made.
The thyroid gland, situated in the neck, produces a hormone called thyroxine, which regulates the rate of oxygen use by cells and the generation of body heat. Within each molecule of thyroxine, there are 4 iodine atoms. If a patient is made to drink a solution of sodium iodide that has been doped with radioactive iodine-131, most of it will end up in the thyroid gland. A special camera can capture the radiation emitted by the iodine-131, and an image of the gland can be constructed. An assessment can then be made about the shape, size and functioning of the gland.
Positron emission tomography (PET)
A positron emission tomography (PET) scan measures important body functions, such as blood flow, oxygen use and glucose use. The information gathered helps doctors find out how well organs and tissues are functioning.
Radionuclides used in PET scanning are isotopes with short half–lives, such as carbon-11 (~20 min), nitrogen-13 (~10 min), oxygen-15 (~2 min) and fluorine-18 (~110 min). These radionuclides are added into compounds normally used by the body such as glucose (or variations of glucose), water or ammonia. Such labelled compounds are known as radiotracers. In some situations, the patient is required to breath oxygen gas labelled with oxygen-15.
The radionuclides used in PET decay by a process called positron emission. A positron is the antimatter version of the electron. When a positron meets an electron, an annihilation event occurs, resulting in the production of two gamma rays. The two emitted gamma rays travel in opposite directions.
PET scanner
Positron emission tomography (PET) is different to other medical imaging techniques because it requires patients to be injected with a radioactive substance. The images a PET scanner produces shows where in the body the radioactive substance has been transported. This means a PET image is a picture of what is happening metabolically in the body compared to other medical imaging techniques that show the structure or parts of the body.
The scanning instrument picks up the location of these gamma rays and, with the aid of a powerful computer, generates a map of where these events are occurring. By combining the PET scan with a CT scan, a more complete picture of how well an organ is functioning can be made.
Due to the short half-lives of most radioisotopes, the radiotracers must be produced using a cyclotron (a type of particle accelerator) and radiochemistry laboratory that are close to the PET imaging facility. The half-life of fluorine-18 is long enough such that fluorine-18 labelled radiotracers can be manufactured commercially at an off-site location.
