What are radioisotopes?
Radioisotopes are widely used in medicine, industry and scientific research, and new applications for their use are constantly being developed.
Radioisotopes are radioactive isotopes of an element. Different isotopes of the same element have the same number of protons in their atomic nuclei but differing numbers of neutrons. They can also be defined as atoms that contain an unstable combination of neutrons and protons.
How do radioisotopes occur?
The combination can occur naturally, as in radium-226, or by artificially altering the atoms. In some cases, a nuclear reactor is used, in others, a cyclotron.
Atoms containing this unstable combination regain stability by shedding radioactive energy, hence the term radioisotope. The process of shedding the excess radioactive energy is called radioactive decay. The radioactive decay process of each type of radioisotope is unique and is measured with a time period called a half-life.
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. On average, one in every two Australians can expect at some stage in his or her life to undergo a nuclear medicine procedure that uses a radioisotope for diagnostic or therapeutic purposes.
Some radioisotopes used in nuclear medicine have very short half-lives, which means they decay quickly; 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 distinctive radioisotopes produced in 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 or X-rays.
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 of the patients illness. 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 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 each patient’s safety.
About 25 types of 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 placed in the OPAL reactor's neutron-rich reflector vessel surrounding the core. After processing, the resulting molybdenum-99 is removed and placed into devices called technetium generators where it changes to technetium-99m. These generators are distributed by ANSTO to medical centres throughout Australia and the near Asia Pacific region.
The short half-life of 6 hours and the weak energy 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.
One of the shorter half-life (eight days) radiopharmaceuticals is iodine-131, used to fight thyroid cancer. Because the thyroid gland produces the body's supply of iodine, the gland naturally accumulates iodine-131 injected into patient. Its radioactivity attacks nearby cancer cells with minimal effect on healthy tissue.
Nuclear imaging is a 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 (computerised 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 (by measuring blood flow, distribution or accumulation of the radioisotope) of a part of the body and does not provide highly resolved anatomical images of body structures.
Types of imaging equipment
Positron Emission Tomography (PET) scans
A widely used 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 gamma rays. Special detectors can track this process and enables 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, or '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.
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.
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.
View more information about radioisotopes and their uses.