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9.6 Option – Medical Physics: 3. Radioactivity as a diagnostic tool
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3. Radioactivity can be used as a diagnostic tool
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Students learn to:
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Students: |
Extract from Physics Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
outline properties of radioactive isotopes and their half lives that are used to obtain scans of organs
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Your outline could take the form of a list of bulleted
points such as:
- Radioisotopes emit α,β or γ radiation spontaneously.
- This radiation is independent of the physical or chemical state of the radioisotope.
- The radiation emitted depends on the isotope.
- The radiation diminishes in intensity as time passes. The rate at which this happens is measured as the half-life of the isotope. This is a characteristic property for a specific isotope. One half-life is the time for the radioactivity to diminish by half. This is an exponential, non-linear process as it depends on probability.
- Radioisotopes may have half-lives ranging from very short time intervals to many millions of years.
- Radioisotopes vary in their ability to penetrate matter, with γ being the most penetrating.
- Isotopes can be identified by their mass number, A, and their atomic (proton) number, Z. The notation used for a nuclide (isotope) is: AZX, where X represents the element’s symbol.
- Radiation can be detected to indicate the position and quantity of radionuclide. They can thus be used as tracers.
- For diagnostic use radioisotopes can be incorporated into convenient chemical compounds and administered to the body.
- The radioactive tracer thus produced can be used to show the biochemical functioning of the body. This is different from other medical imaging.
- A number of radioisotopes can be used for diagnosis. They should only emit γ radiation, have a short half life, be easily incorporated into convenient compounds and be made readily available at high concentrations.
- Radioisotopes and their uses in medical physics are outlined in the pdf located at this link
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describe how radioactive isotopes may be metabolised by the body to
- Your description will be of a sequence of events or
process. It will start with the choice of radioactive
isotope. Radioisotopes need to be incorporated into a
suitable radiopharmaceutical. This is a chemical compound
which will carry the radioisotope to the target organ.
- Radioisotopes have the same chemical properties as
stable isotopes of the same element. Thus a suitable
chemical compound can be chemically synthesised or altered
to include the radioisotope. This ‘tagged’ or
‘labelled’ radiopharmaceutical will behave in
the same way as non-tagged compounds. When introduced to
the body, its emitted radiation can be detected by a gamma
camera. As the compound interacts with the patients’
biological processes it may accumulate in an organ as it is
metabolised.
- Radiopharmaceuticals that accumulate in an organ do so
because the organ has a particular affinity for that
compound. A good example is the accumulation of radioiodine
(123I) in the thyroid gland. A patient is given
a quantity of dilute tagged sodium iodide to drink. This
passes from the patient’s gastrointestinal tract into
their bloodstream and accumulates in the thyroid. The rate
at which this uptake occurs is measured using a gamma
camera. This information can be used to determine the
activity of the thyroid as a function of gland size.
- Another example is the use of technetium to determine bone disease. In this case, radioactive 99mTc is incorporated into phosphate molecules. These molecules preferentially bind with bone. Bone abnormalities have increased blood flow. The administered radiopharmaceutical thus accumulates at a greater rate in damaged areas, where the phosphates bind with the bone.
identify that during decay of specific radioactive nuclei positrons are given off
- You should recognise that there are a number of decay processes (á,â,ã) that give rise to different decay products. You should also recognise that a positron is an electron (â particle) with a positive charge (ie â+). Beta decay happens in order to gain a more stable ratio of protons to neutrons in the nucleus. If there are too many protons, the nucleus may adjust by converting a proton to a neutron and a positron. A neutrino is also produced by this variation of the beta decay process:
proton --> neutron + positron + neutrino
discuss the interaction of electrons and positrons resulting in the production of gamma rays
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To discuss this interaction you could provide a series a
points describing the process:
- One way a radioactive nucleus may gain stability is by converting a proton to a neutron. If this occurs the nucleus will also eject a positron (positive electron) and a neutrino.
- The emitted positron is also known as an anti-electron and is a form of anti-matter.
- If the positron collides with an electron, both will be annihilated. Each particle’s mass will be converted into a large amount of energy.
- As the particles have the same mass, the two amounts of energy will be the same size.
- These become two equal but opposite gamma rays.
- The gamma rays move off in opposite directions.
describe how the positron emission tomography (PET) technique is used for diagnosis
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Your description needs to include:
- uses of the PET technique
- the radioisotopes used and
- how an image is recorded and developed.
- Positron emission tomography (PET) uses positron decay
to image and measure the functioning of organs. It is most
useful in the study of the brain. PET allows us to image
the functioning of the brain, which CAT scans or X-Rays
cannot do. The imaging is done without harming the patient.
Examples of its use include the investigation of
Parkinson’s disease, schizophrenia, strokes and brain
tumours.
- PET scanning is possible because some radionuclides
gain nuclear stability by converting a proton to a neutron,
at the same time releasing a positron. This antimatter
particle leaves the nucleus at high speed and may collide
with an electron in its surroundings (most do so within a
millimetre). The collision results in the formation of two
equal but opposite gamma rays. These can be detected and
their origin pinpointed by a gamma camera.
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The radionuclides which may be used for this must satisfy
a number of criteria. They must:
- produce positrons
- be readily incorporated into biological molecules
- have short half-lives
- be easily produced.
Examples include: carbon-11; oxygen-15; fluorine-18 and nitrogen-13. A cyclotron at the hospital produces the isotopes which are then quickly incorporated into radiopharmacueticals. Examples include oxygen-15 in oxygen gas, carbon-11 in carbon monoxide gas (in small amounts), and fluorine-18 replacing an oxygen in glucose to make fluorodeoxyglucose (FDG).
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The radiopharmacueticals are administered to the patient
and are metabolised. Examples include:
- tagged oxygen is inhaled and is used to study blood volume,
- FDG (tagged glucose) locates in the brain.
- As the radionuclide decays, streams of gamma rays leave
the patient. Each decay produces two gamma rays which are
detected as two scintillations on a circular ring-shaped
gamma camera surrounding the patient. Complex computing
backtracks these rays to their source. This process is
repeated for each decay. The data is used to build up a
computed map of the radionuclide’s locations. This
becomes the image or tomograph for the patient.
- Diagnosis involves comparing the image obtained with that from a healthy person.
Positron emission tomography Wikipedia
perform an investigation to compare an image of bone scan with an X-ray image
- You will need to obtain your two images by practising
efficient data collection techniques. Your images could
come from a variety of sources including texts, journals or
the Internet. Medical imaging facilities may also be of
assistance.
- To perform your investigation you
should first make a list of the properties of the images
for comparison. Image factors such as resolution, contrast,
colour and clarity could be included. Issues such as the
ability to show a range of tissues and the variability of
image with bone density or bone thickness should also be
considered. Include the ability to show a range of bone
abnormalities.
- To compare the images you could
tabulate your list and record your observations in order to
produce a summary paragraph. This should highlight the main
features of the comparisons by making a number of
statements such as:
“ A bone scan shows the presence of a fine stress fracture but the resolution can be less than an X-ray, which may not reveal the fracture as clearly.”
gather and process secondary information to compare a scanned image of at least one healthy body part or organ with a scanned image of its diseased counterpart
- Images may be obtained from a range of sources. For
example, an Internet search will reveal a number of sources
of scanned images. Other sources may include texts,
journals and medical imaging facilities.
- You should consider whether the images have been
enhanced in any way. This may be done to improve image
quality or may have been done for commercial reasons.
Although you are asked for at least one, using a number of
images will improve the reliability and validity of your
comparisons. Your assessment of the quality of your
gathered images should also include a consideration of the
sources of the material. Be aware that such material may
have issues of copyright or may have a commercial purpose
which should be considered.
- You should first make a list of the properties of the
images for comparison. Image factors such as resolution,
contrast, colour and clarity could be included. Include the
ability to show a range of disease. To compare the images
you could tabulate your list and record your observations
in order to produce a summary paragraph. This should
highlight the main features of the comparisons by making a
number of specific comparison statements. These could then
be given a conclusion such as:
“ A healthy brain has an evenly distributed pattern of radiation. This indicates that the blood flows to all parts and so the gamma camera detects evenly spread technetium 99-m. A brain that has suffered a stroke displays a lack of radiation from the site of the stroke. This indicates that the blood flow to this part is now diminished.”
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