Different isotopes of the same element have the same number of protons in their atomic nuclei but differing numbers of neutrons.
Radioisotopes are radioactive isotopes of an element. They can also be defined as atoms that contain an unstable combination of neutrons and protons, or excess energy in their nucleus.
How do radioisotopes occur?
The unstable nucleus of a radioisotope can occur naturally, or as a result of artificially altering the atom. In some cases a nuclear reactor is used to produce radioisotopes, in others, a cyclotron. Nuclear reactors are best-suited to producing neutron-rich radioisotopes, such as molybdenum-99, while cyclotrons are best-suited to producing proton-rich radioisotopes, such as fluorine-18.
The best known example of a naturally-occurring radioisotope is uranium. All but 0.7 per cent of naturally-occurring uranium is uranium-238; the rest is the less stable, or more radioactive, uranium-235, which has three fewer neutrons in its nucleus.
Atoms with an unstable nucleus regain stability by shedding excess particles and energy in the form of radiation. The process of shedding the radiation is called radioactive decay. The radioactive decay process for each radioisotope is unique and is measured with a time period called a half-life. One half-life is the time it takes for half of the unstable atoms to undergo radioactive decay.
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.
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.
Naturally-occurring radioisotopes in industry and science
|Hydrogen-3 (tritium)||12.32 years||Used to measure the age of ‘young’ groundwater up to 30 years old.|
|Carbon-14||5,700 years||Used to measure the age of organic material up to 50,000 years old.|
|Chlorine-36||301,000 years||Used to measure sources of chloride and the age of water up to 2 million years old.|
|Lead-210||22.2 years||Used to date layers of sand and soil laid down up to 80 years ago.|
Artificially-produced radioisotopes in industry and science
|Hydrogen-3 (tritium)||12.32 years||Used as a tracer in tritiated water to study sewage and liquid wastes.|
|Chromium-51||27.7 days||Used to trace sand to study coastal erosion.|
|Manganese-54||312.12 days||Used to predict the behaviour of heavy metal components in effluents from mining waste water. Produced in reactors.|
|Cobalt-60||5.27 years||Used in gamma radiography, gauging, and commercial medical equipment sterilisation. Also used to irradiate fruit fly larvae in order to contain and eradicate outbreaks, as an alternative to the use of toxic pesticides. Produced in reactors.|
|Zinc-65||243.66 days||Used to predict the behaviour of heavy metal components in effluents from mining waste water. Produced in cyclotrons.|
|Technetium-99m||6.01 hours||Used to study sewage and liquid waste movements. Produced in 'generators' from the decay of molybdenum-99, which is in turn produced in reactors.|
|Caesium-137||30.08 years||Used as a radiotracer to identify sources of soil erosion and depositing, and also used for thickness gauging. Produced in reactors.|
|Ytterbium-169||32.03 days||Used in gamma radiography.|
|Iridium-192||73.83 days||Used in gamma radiography. Also used to trace sand to study coastal erosion. Produced in reactors.|
|Gold-198||2.70 days||Used to trace sand movement in river beds and on ocean floors, and to trace sand to study coastal erosion. Also used to trace factory waste causing ocean pollution, and to study sewage and liquid waste movements. Produced in reactors.|
|Americium-241||432.5 years||Used in neutron gauging and smoke detectors. Produced in reactors.|
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.
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.
Reactor-produced medical radioisotopes
|Phosphorus-32||14.26 days||Used in the treatment of excess red blood cells.|
|Chromium-51||27.70 days||Used to label red blood cells and quantify gastro-intestinal protein loss.|
|Yttrium-90||64 hours||Used for liver cancer therapy.|
|Molybdenum-99||65.94 hours||Used as the ‘parent’ in a generator to produce technetium-99m, the most widely used radioisotope in nuclear medicine.|
|Technetium-99m||6.01 hours||Used to image the brain, thyroid, lungs, liver, spleen, kidney, gall bladder, skeleton, blood pool, bone marrow, heart blood pool, salivary and lacrimal glands, and to detect infection.|
|Iodine-131||8.03 days||Used to diagnose and treat various diseases associated with the human thyroid.|
|Samarium-153||46.28 hours||Used to reduce the pain associated with bony metastases of primary tumours.|
|Lutetium-177||6.65 days||Currently in clinical trials. Used to treat a variety of cancers, including neuroendocrine tumours and prostate cancer.|
|Iridium-192||73.83 days||Supplied in wire form for use as an internal radiotherapy source for certain cancers, including those of the head and breast.|
Cyclotron-produced medical radioisotopes
|Carbon-11||20.33 minutes||Used in Positron Emission Tomography (PET) scans to study brain physiology and pathology, to detect the location of epileptic foci, and in dementia, psychiatry, and neuropharmacology studies. Also used to detect heart problems and diagnose certain types of cancer.|
|Nitrogen-13||9.97 minutes||Used in PET scans as a blood flow tracer and in cardiac studies.|
|Oxygen-15||2.04 minutes||Used in PET scans to label oxygen, carbon dioxide and water in order to measure blood flow, blood volume, and oxygen consumption.|
|Fluorine-18||1.83 hours||The most widely-used PET radioisotope. Used in a variety of research and diagnostic applications, including the labelling of glucose (as fluorodeoxyglucose) to detect brain tumours via increased glucose metabolism.|
|Copper-64||12.7 hours||Used to study genetic disease affecting copper metabolism, in PET scans, and also has potential therapeutic uses.|
|Gallium-67||78.28 hours||Used in imaging to detect tumours and infections.|
|Iodine-123||13.22 hours||Used in imaging to monitor thyroid function and detect adrenal dysfunction.|
|Thallium-201||73.01 hours||Used in imaging to detect the location of the damaged heart muscle.|
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.