What are radioisotopes?

How are radioisotopes used? 

Radioisotopes are an essential part of radiopharmaceuticals. In fact, they have been used routinely in medicine for more than 30 years. Every Australian is likely to benefit from nuclear medicine and, on average, will have at least two nuclear medicine procedures in their lifetime^[1]. 

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Some radioisotopes used in nuclear medicine have short half-lives, which means they decay quickly and are suitable for diagnostic purposes; others with longer half-lives take more time to decay, which makes them suitable for therapeutic purposes.

Industry uses radioisotopes in a variety of ways to improve productivity and gain information that cannot be obtained in any other way.

Radioisotopes are commonly used in industrial radiography, which uses a gamma source to conduct stress testing or check the integrity of welds. A common example is to test aeroplane jet engine turbines for structural integrity.

Radioisotopes are also used by industry for gauging (to measure levels of liquid inside containers, for example) or to measure the thickness of materials.

Radioisotopes are also widely used in scientific research and are employed in a range of applications, from tracing the flow of contaminants in biological systems to determining metabolic processes in small Australian animals.

They are also used on behalf of international nuclear safeguards agencies to detect clandestine nuclear activities from the distinctive radioisotopes produced by weapons programs.

What is a radioactive source?

A sealed radioactive source is an encapsulated quantity of a radioisotope used to provide a beam of ionising radiation. Industrial sources usually contain radioisotopes that emit gamma rays or X-rays.

What are some commonly-used radioisotopes?

Radioisotopes are used in a variety of applications in medical, industrial, and scientific fields. Some radioisotopes commonly-used in industry and science can be found in the tables below. Medical radioisotopes are described in the next section.

Radioisotopes in medicine

Nuclear medicine uses small amounts of radiation to provide information about a person's body and the functioning of specific organs, ongoing biological processes, or the disease state of a specific illness. In most cases the information is used by physicians to make an accurate diagnosis. In certain cases radiation can be used to treat diseased organs or tumours.

How are medical radioisotopes made?

Medical radioisotopes are made from materials bombarded by neutrons in a reactor, or by protons in an accelerator called a cyclotron. ANSTO uses both of these methods. Radioisotopes are an essential part of radiopharmaceuticals. Some hospitals have their own cyclotrons, which are generally used to make radiopharmaceuticals with short half-lives of seconds or minutes.

What are radiopharmaceuticals?

A radiopharmaceutical is a molecule that consists of a radioisotope tracer attached to a pharmaceutical. After entering the body, the radio-labelled pharmaceutical will accumulate in a specific organ or tumour tissue. The radioisotope attached to the targeting pharmaceutical will undergo decay and produce specific amounts of radiation that can be used to diagnose or treat human diseases and injuries. The amount of radiopharmaceutical administered is carefully selected to ensure the safety of each patient.

Common radiopharmaceuticals

About 25 different radiopharmaceuticals are routinely used in Australia's nuclear medicine centres.

The most common is technetium-99m, which has its origins as uranium silicide sealed in an aluminium strip and placed in the OPAL reactor's neutron-rich reflector vessel surrounding the core. After processing, the resulting molybdenum-99 precursor is removed and placed into devices called technetium generators, where the molybdenum-99 decays to technetium-99m. These generators are distributed by ANSTO to medical centres throughout Australia and the near Asia Pacific region.

A short half-life of 6 hours, and the weak energy of the gamma ray it emits, makes technetium-99m ideal for imaging organs of the body for disease detection without delivering a significant radiation dose to the patient. The generator remains effective for several days of use and is then returned to ANSTO for replenishment.

Another radiopharmaceutical produced in OPAL is iodine-131. With a half-life of eight days, and a higher-energy beta particle decay, iodine-131 is used to treat thyroid cancer. Because the thyroid gland produces the body's supply of iodine, the gland naturally accumulates iodine-131 injected into the patient. The radiation from iodine-131 then attacks nearby cancer cells with minimal effect on healthy tissue.

Other commonly-used radiopharmaceuticals can be found in the lists below.

Nuclear imaging

Nuclear imaging is a diagnostic technique that uses radioisotopes that emit gamma rays from within the body.

How is nuclear imaging different to other imaging systems?

There is a significant difference between nuclear imaging and other medical imaging systems such as CT (Computed Tomography), MRI (Magnetic Resonance Imaging) or X-rays.

The main difference between nuclear imaging and other imaging systems is that, in nuclear imaging, the source of the emitted radiation is within the body. Nuclear imaging shows the position and concentration of the radioisotope. If very little of the radioisotope has been taken up a ‘cold spot’ will show on the screen indicating, perhaps, that blood is not getting through. A ‘hot spot’ on the other hand may indicate excess radioactivity uptake in the tissue or organ that may be due to a diseased state, such as an infection or cancer. Both bone and soft tissue can be imaged successfully with this system.

How does nuclear imaging work?

A radiopharmaceutical is given orally, injected or inhaled, and is detected by a gamma camera which is used to create a computer-enhanced image that can be viewed by the physician.

Nuclear imaging measures the function of a part of the body (by measuring blood flow, distribution or accumulation of the radioisotope), and does not provide highly-resolved anatomical images of body structures.

What can nuclear imaging tell us?

The information obtained by nuclear imaging tells an experienced physician much about how a given part of a person’s body is functioning. By using nuclear imaging to obtain a bone scan, for example, physicians can detect the presence of secondary cancer ‘spread’ up to two years ahead of a standard X-ray. It highlights the almost microscopic remodelling attempts of the skeleton as it fights the invading cancer cells.

Other types of imaging

Positron Emission Tomography (PET) scans

A widely-used nuclear imaging technique for detecting cancers and examining metabolic activity in humans and animals. A small amount of short-lived, positron-emitting radioactive isotope is injected into the body on a carrier molecule such as glucose. Glucose carries the positron emitter to areas of high metabolic activity, such as a growing cancer. The positrons, which are emitted quickly, form positronium with an electron from the bio-molecules in the body and then annihilate, producing a pair of gamma rays. Special detectors can track this process, enabling the detection of cancers or abnormalities in brain function.

Computed Tomography (CT) scans

A CT scan, sometimes called CAT (Computerised Axial Tomography) scan, uses special X-ray equipment to obtain image data from hundreds of different angles around, and 'slices' through, the body. The information is then processed to show a 3-D cross-section of body tissues and organs. Since they provide views of the body slice by slice, CT scans provide much more comprehensive information than conventional X-rays. CT imaging is particularly useful because it can show several types of tissue - lung, bone, soft tissue and blood vessels - with greater clarity than X-ray images.

Though a CT scan uses radiation, it is not a nuclear imaging technique, because the source of radiation - the X-rays - comes from equipment outside the body (as opposed to a radiopharmaceutical inside the body).

PET scans are frequently combined with CT scans, with the PET scan providing functional information (where the radioisotope has accumulated) and the CT scan refining the location. The primary advantage of PET imaging is that it can provide the examining physician with quantified data about the radiopharmaceutical distribution in the absorbing tissue or organ.