(c) doc b5. Uses of Radioactive-isotopes emitting alpha, beta, positron or gamma ionising radiation

Doc Brown's Chemistry - KS4 science GCSE/IGCSE Physics and Chemistry Revision Notes

 IDEAS: These revision notes on how to use ionising radiation in a variety of industrial, medical and environmental situations should help with GCSE/IGCSE/O Level physics courses and A/AS level physics courses. How do we use radioisotopes for? How can we use alpha particle radiation, beta particle radiation and gamma radiation rays? How do we relate the use of ionising radiation with its physical properties e.g. it penetration into material or the half-life of the radioactive source. Gamma and beta emitting radioisotopes are extensively used in medical diagnostic and treatment procedures. Radioisotopes are used in a variety of ways in industry to improve productivity and, in some cases, to gain information that cannot be obtained in any other way. Radioisotopes are used in radiography, gauging applications and mineral analysis. Short-lived radioisotopes are used to trace flow and fluid mixing systems. Gamma ray sterilisation procedures are used in preparing medical supplies packaged food preservation. How does PET scanning work? The world of medical physics uses various diagnostic techniques to investigate blood flow and metabolic functions.

The technicalities of the 'half-life' of a radioisotope are dealt with in detail in section 6.

But to fully understand this page you specifically need to know

(i) Relative penetrating power of the ionising radiations: gamma > beta- > alpha

(ii) The half-life of a radioisotope is the time taken for half of the radioactive atoms of a specific isotope to decay

and (iii) an understanding of nuclear equations is assumed, refer to section 7.


An introduction to the use of radioisotopes and nuclear radiation in medicine

The medical uses of nuclear radiations and radioisotopes are expanding all the time as technology improves and hopefully become cheaper so more patients can be diagnosed and treated. However, as with the 'humble' X-ray, there are serious health and safety issues that must be addressed. You have to weigh the possible positive outcomes versus the possible dangers!

Ionisation caused by radiation can kill cells completely or damage it so it can't divide, whichever happens, the effect is damage to tissue. Although harmful in itself, if the radiation damages and alters the genetic material in the cell (DNA) it can cause mutations. Some mutations can cause the cell to divide and multiply out of control producing cancer tumours. Hence the need in any ionising radiation treatment to limit the patient's exposure to it.

Like it or not, any extra exposure to nuclear radiation or X-rays increases the risk of tissue damage and cancer. In any radiation treatment or diagnostic procedure the patient should be given the lowest possible effective dose and experience the shortest possible exposure time. Lead shielding can be used to protect areas of the body not being treated and wherever possible to radiation focussed onto the part of the body under examination or treatment.

What goes for patients must also apply to medical personnel carrying out the treatment or examinations using potentially harmful nuclear radiation procedures. Examples of protective measures include ...

Since radiation intensity decreases with distance from the source, personnel stand as far away as possible and even better (and now standard practice), operate equipment by remote control from separate cubicle.

Some kind of protective barrier also reduces the intensity of radiation so wearing lead-lined protective clothing.

Medical personnel also wear radiation badges to monitor the dose that they receive, as in the nuclear power industry, and this keeps a check to make sure they do not get exposed to potentially harmful dose of radiation.

Nuclear radiation sources can be used internally and externally to treat cancer tumours.

Internal techniques: For an internal radiation therapy procedure a radioisotope source is injected or implanted  into the body, preferably as near as possible to the tumour. This provides a high dose over a small area minimising potential damage to surrounding healthy tissue. It also has the advantage of not requiring as many hospital visits (and waiting times!) as external radiation therapy treatments. This shorter treatment times also enables the more efficient application of follow-up treatments like chemotherapy.

Internally treated patients may still be emitting radiation for a few days after each dose and limited contact with other people is advised (the source may only be removed later, it will depend on some extent as to value of the half-life of the radioisotope used). In the case of external radiation therapy where each session only lasts a few minutes, the patient does NOT emit radiation.

External techniques: High energy X-rays (NOT a nuclear radiation) or gamma rays (even higher energy) can be directed onto the tumour to kill the cancer cells from outside the body and so inevitably the radiation must pass through healthy cells.

For a given overall radiation dose, the more accurately focussed the beam the more effective the treatment and less healthy cell damage. External radiation therapy requires multiple doses over a period of several weeks and so the total radiation dose tends to be higher for external procedures compared to internal procedures (described above). Unfortunately you cannot prevent damage to some healthy cells in the tissue surrounding the tumour.

Sometimes internal and external radiation therapies are used in conjunction with each other.

Decisions: Generally speaking internal treatments have no side-effects other than maybe discomfort from the implant procedure, but external treatments like radiotherapy can have both short-term and long-term effects which may be immediate or show up months or years later.

Side effects of apply radiotherapy to cancer tumours include sickness and feeling weak, skin irritation and most obvious of all, hair loss, BUT all of these changes are reversible over time after treatment is complete. However, unfortunately, there can be in a minority of cases, some long term side effects that are life changing e.g. infertility or organ damage e.g. the bowel.

You may consider what the quality of life might be with, or without, accepting treatment for the cancer. In older people cancers grow more slowly and it might be better to live with the tumour than risk surgical or radiotherapy procedures and this kind of decision boils down to life expectancy and quality of life. BUT these aspects of our healthcare do raise important social and ethical issues. For example a patient with terminal cancer might still be given treatment to make their final days of life less painful, treatment to reduce suffering is called palliative care.

With medical advice, you may go ahead with treatment or refuse it, considering the side-effects are not worth the risk - difficult decision! For example, a pregnant women diagnosed with cancer might refuse treatment until after the baby is born so as not risk harm to the foetus, but in doing so, she puts her own health, and even life, in danger.

Developing new treatments

The medical uses of nuclear radiation, are like any other branch of science and technology, are always developing with new techniques, radioisotopes and safer procedures.

The long term effects of new procedures are not fully understood so e.g. new radioisotopes can be tested on cultured cells grown in the laboratory. This minimises risks before testing their use in procedures on real people.

If a patient is to undergo a new procedure, whose outcome carries some risks, it is morally - ethically correct, to inform the patient that this technique carries particular risks which must be weighed up against the potential benefits outcome of the treatment. Unfortunately there may be other risks which the doctors may unaware of.

When it is felt that the treatment is ready to be tested on cancer patients a limited number will then be put on a medical trial.

So, who gets on the trial and who is left out? Is it safe to operate to anyone? If it is 'relatively' safe, how long before it is made available to the public? How much does it cost? Issues to consider and 'financial cost' now overlaps with ethical and social policy issues!

Some examples of radioactive isotopes used in medicine

Note: (i)  (t = half-life) and (ii) some of these examples are discussed in more detail in the next sections 5a-5d.

Technetium-99 (t = 6 hours): 99Tc,  a beta emitter used in to image the skeleton and heart muscle in particular, but also for brain, thyroid, lungs (perfusion and ventilation), liver, spleen, kidney (structure and filtration rate), gall bladder, bone marrow, salivary and lachrymal glands, heart blood pool, infection and numerous specialised medical studies.

Cobalt-60 (10.5 mins/5.29 years): 60Co, beta/gamma emitter once used for external beam radiotherapy.

Iodine-123 (t = 13 hours): 12353I Increasingly used for diagnosis of thyroid function, it is a gamma emitter without the beta radiation of I-131.

Iodine-125 (t = 60 days): 125I, a gamma emitter used to diagnostically evaluate the filtration rate of kidneys and to diagnose deep vein thrombosis in the leg. It is also widely used in radioimmuno- assays to show the presence of hormones in tiny quantities.

Iodine-131 (t = 8 days): 131I a beta and gamma emitter, is widely used in treating thyroid cancer and in imaging the thyroid gland also in diagnosis of abnormal liver function, renal (kidney) blood flow and urinary tract obstruction.

Iridium-192 (t = 74 days): 192Ir, a gamma emitter can be supplied in wire form for use as an internal radiotherapy source for cancer treatment and after use it can removed.

Potassium-42 (t = 12 hours): 42K, a beta minus and gamma emitter used for the determination of exchangeable potassium in coronary blood flow.

Sodium-24 (t = 15 hours): 24Na, is a beta- and gamma emitter is used to study of electrolytes in the body.

Xenon-133 (t = 5 days): 13354Xe, a beta- and gamma emitting gas used for pulmonary (lung) ventilation studies.

Krypton-81 (t = 13 seconds) made from Rubidium-81 (t = 4.6 hours), 81Kr gas provides images of pulmonary ventilation in the lung, e.g. in asthmatic patients, and provides early diagnosis of lung diseases and function.

Footnote: Although the above examples are all medical there are lots of industrial uses of using nuclear radiation from radioactive isotopes and many are described in sections 5a to 5d below.


Some general thoughts on the use of radioactive sources - radioisotopes

The lower the activity of a radioactive source, the safer it is to use.

The longer the half-life of a radioisotope, the longer it remains a hazard.

In medical applications you want a radioisotope with a short half-life to minimise the effects of potentially harmful radiation.

However, for some industrial applications, you might want a long half-life so that the source does not have to be changed too frequently.

Overall you seeking a balance between the source having the right activity and the right amount of time, but you also have to take into account the preparation of the radioisotope, its storage before and during use and disposal of the used source.



5. Examples of specific uses of radioactive isotopes emitting alpha, beta(+/-) or gamma radiation

The uses of radioactive isotopes usually depends on their penetrating power and the value of their half-life

Both specific industrial and medical applications of nuclear radiation sources are described.

(see properties of alpha, beta and gamma radiation)

5a (c) doc b Uses of alpha particle sources

  • (c) doc bBecause alpha particles are easily stopped, an alpha source is used in some smoke detectors.
    • A sealed weak alpha source of americium-241, with a long half-life of 458 years, it effectively produces a constant signal in a detector - formed of two electrodes close together with a potential difference across them.
    • It does this by sending a stream of alpha particles to the sensor across an air gap which causes ionisation of air molecules, this allows an electrical current flow of ions (charges) and hence a constant electrical signal.
    • The nuclear decay equation is:    +  
    • Any smoke particles present will block and absorb some of the alpha particles and change the sensor signal by reducing the amount of ionisation of air molecules, and this drop in signal triggers the alarm.
    • Beta and gamma radiation would be of no use because the smoke particles would not stop them, no change in signal, no alarm triggered!
    • Note:
      • Although gamma radiation is also emitted, the smoke particles have no effect on it.
      • This type of smoke detector can be safely used in the house because it is a very weak source using a tiny amount of the americium radioisotope.
      • An average smoke detector for domestic use contains about 0.29 micrograms of Am-241 (in the form of americium dioxide), and its activity is around 37000 Bq (37000 disintegrations/second).
      • It sounds a lot, but don't worry about it, non of the alpha particles can get out of the detector chamber and there are thousands of particles hitting or passing through your body every second with no ill-effect!
    • The alpha emitting americium-241 can be used to gauge and control the thickness of very thin metal foil sheet production (see beta radiation gauging for more details as to how this is done).
  • (c) doc bAlpha sources are too readily absorbed to show up with a Geiger counter or other detector and so are not suitable for 'tracer' applications.
    • However, an alpha particle emitting isotope of radium (radium-223, half-life 11.4 days) can be directly injected in tiny quantities into tumourous tissue to directly irradiate and kill cancer cells.
    • Its a rare but excellent medical use of an alpha emitter.
    • Since they are not very penetrating, there is less chance of damaging healthy cells surrounding the tumour.
    • This is an example of internal radionuclide therapy.
    • See also uses of beta radiation in cancer therapy.
  • more on the properties of alpha particles and nuclear equations for alpha decay




Reminders about the use of radioisotopes and radiation in medicine (medical physics)

A more detailed discussion is given near the top of the page - a quick reminder of why radioisotopes are so useful in diagnosing and treating medical conditions. Alpha emitting radioisotopes are usually too dangerous and not sufficiently penetrating to be of use in medicine. However despite the dangers, beta minus (electron emission), beta plus (positron emission) and gamma emitting radioisotopes are widely used in diagnostic medicine and treatments for dangerous medical conditions such as cancer.

5b Uses of beta minus radiation sources

  • (c) doc b Most Beta particles are stopped by a few mm or cm of solid materials.
    • Beta emitting radioisotopes can be used to monitor the thickness (gauge) of a sheet of material i.e. used in continuous gauging situation especially in fast moving production line situations.
    • The thicker the layer the more beta radiation is absorbed, so by measuring the beta radiation signal it can form the basis of an automatic thickness control.
    • A beta source is placed on one side of a sheet of material.
    • A detector (e.g. a Geiger counter) is put on the other side and can monitor how much radiation gets through.
    • The signal size depends on thickness of the sheet and it gets smaller as the sheet gets thicker.
    • Therefore the signal can be used to monitor the sheet thickness.
    • However, the radioisotope must give a stable and constant emission to give create a stable constant signal from the detector.
    • Therefore the half-life must be quite long so that any change in the signal does not result from rapid decay but only from change in the thickness of the sheet material passing through beta particle beam.
    • You can't use gamma radiation because it is too penetrating and unaffected by the sheet of material, and alpha radiation sources are no good either, because alpha particles wouldn't even penetrate the material sheet.
  • (c) doc b This idea is used to control production lines of paper, plastic or steel sheeting (diagram on right).
    • (c) doc bAfter the sheet material passes through 'flattening' rollers, it passes between a beta source and detector.
    • The detector signal is checked against that for a preset thickness.
    • The signal controls the position of the rollers producing the sheet of material.
    • If the signal is too big, the sheet is too thin, and the rollers are moved apart to thicken the sheet.
    • If the signal is too small, the sheet is too thick, and the rollers are moved closer together.
    • You can use gamma emitting radioisotopes to do the same thing.
  • (c) doc bRadioactive tracers can be used to diagnose some medical conditions.
    • Radioisotopes can be injected into the human body or taken in a tablet and then what happens to the this radioactive 'tracer' isotope as it is moved around the person's body can be monitored from outside with a suitable external detection system.
    • A computer takes the multiple readings from scanning the emissions and builds up an accurate picture of where the radioisotope has gone in the body.
    • The more concentrated the tracer the stronger the reading so the system produces a image ('map') of where the tracer has gone and where it may concentrate from the strongest radiation readings.
    • The technique can be used to detect and diagnose medical conditions including cancers and blood stream circulation problems.
    • The radiation emitted must pass out of the body to reach the detector, alpha sources can't be used, the radioisotope applied must be a beta or gamma emitter.
    • It is important that a low dose of the radioisotope is used AND has a relatively short half-life of a few hours or a few days to minimise the risk of cell damage from the emitted beta or gamma radiation.
      • A short half-life means the radioactivity in the body will rapidly disappear to almost zero.
    • A computer can analyse the detector signals from either beta of gamma radiation to build up on a screen a picture of e.g. blood circulation in the body can be followed.
    • Another example is the use of iodine-131 to check the functioning of the thyroid gland. If the thyroid gland is functioning normally its expected uptake of iodine can be 'raced' using this radioisotope - an example of a diagnostic scan.
      • Lack of a concentrated signal from the thyroid gland would indicate it is malfunctioning and not processing iodine as it should. Iodine-131 emits beta and gamma radiation and both radiations can pass out of the body to a detector.
      •  +
      • I've read that iodine-123 is now used, which gives a more pure and safer gamma emitting radioisotope.
      • Iodine-131 has a half-life of ~8 days, so with a low dosage used, after a few weeks all the radioactivity would disappear.
    • (c) doc b or (c) doc b  Alpha or beta emitters can be used to treat tumours
      • The technique of internal radiation therapy involves placing (injecting or implanting) the radioisotope inside the body, either into, or near, the tumour (cancer growth).
        • Alpha emitters can be injected near the tumour. Alpha radiation is highly ionising and will do great damage to nearby cancer cells. Since alpha particles have a low penetration, they will do little damage to healthy cells around the tumour.
        • Beta emitters are used in implants which are placed in or near the tumour. Beta particles are more penetrating and, unlike the less penetrating alpha particles, will pass through the implant casing to damage the adjacent cancer cells. However, using more penetrating beta emitters, risks damaging healthy cells beyond the cancer cells of the tumour.
        • The half-lives of radioactive sources used for internal treatments, should have short half-lives to limit the time healthy cells are exposed to radiation.
      • Gamma emitters can be used in the technique of external radiation therapy by aiming the highly penetrating gamma rays at the tumour in the body. See uses of gamma radiation for more details.
  • more on the properties of beta particles and nuclear equations for beta decay



5c (c) doc b Uses of gamma radiation sources

  • (c) doc b Gamma radiation is highly penetrating and so gamma sources are used where the radiation must be detected after passing through an appreciable thickness of material.
    • This is used in various tracer situations and usually the half-life should be relatively short to avoid any health hazards if used in detecting and diagnosing medical conditions.
  • (c) doc b A gamma emitting tracer can be added to the flow of water in a pipe and the outside of the pipes monitored with a Geiger counter.
    • Any leaks would be detected by an increase in radiation reading where the leak is.
    • Tracer monitoring can be used in industry and out in the general environment.
    • The flow of water in underground streams or pipes can be followed in a similar way.
  • (c) doc b Radiotherapy (radiation therapy)
    • Gamma emitters can be used in the technique of external radiation therapy by aiming the highly penetrating gamma rays at the tumour in the body.
    • It does seem ironic that the very radiation which causes cancer, can also be used to treat it.
    • Radiotherapists direct a beam of gamma radiation is directed onto the tumor site to kill the cancer cells, but it must be an appropriate dose to minimise damage to healthy cell tissue surrounding the cancer tissue.
    • High does of radiation will kill living cells and the idea is to focus a beam of radiation onto the cancer cells.
    • Unfortunately the radiation passes through the 'good' tissue too and kills or damages 'good' cells and this damage can cause sickness, but, if the cancer cells are all killed, surely its worth it.
    • Modern techniques use multiple rotating gamma sources that are focused on to the tumor.
    • This means the surrounding 'good cells' are less frequently hit and minimises potential harmful side-effects on the rest of the body (e.g. sickness or other mutations).
    • Radiotherapy also avoids the need for intrusive surgery which has its own risk factors.
    • The gamma emitters used should have relatively long half-lives to give the instrument a good working life without having to replace the radioactive sources too frequently.
    • Lots of shielding is required in a specially designed room to protect the radiographer, patients and medical staff from any radiation.
  • (c) doc bGamma radiation can be used in a non-destructive way to test the structure of a material.
    • In a sense it is an alternative to X-ray photography for more dense materials e.g.
    • It is used test the structure and quality of pipe welds e.g. in pipelines used in the oil industry.
      • A gamma source is placed inside the pipe and photographic paper wrapped around the weld.
      • If there is any gap or flaw in the weld, more gamma radiation gets through and shows up as increased exposure on the 'gamma-ray picture'.
      • Its better to find out the fault now, rather than later when it fractures, and has to be 'dug up' or retrieved from the bottom of the sea!
      • Gamma rays are used to test for flaws in jet engines in a similar way, any flaw allows more gamma rays to pass through so any minute cracks will be detected. Analogy - this is a similar technique to having an X-ray to detect fractures in bones or examination of luggage for airport security.
  • (c) doc bGamma radiation can be used to measure the thickness of materials.
    • This technique has already been described in uses of beta radiation, where the signal from transmitted nuclear radiation gives a measure of thickness, a technique described as gauging.
    • It is used in the automobile industry to measure the thickness of steel or aluminium in car body production
  • (c) doc b Because gamma radiation is so deadly and penetrating it can be used to sterilise surgical equipment or packaged food.
    • This irradiation is done with a strong gamma emitter like cobalt-60, with a long half-life which means the source can last for many years with out replacement.
    • The radiation is deadly for bacteria even in the most microscopic pockets of apparently smooth and shiny stainless steel of surgical instruments.
      • A high does of gamma radiation will kill any microbe-bacteria cells on surgical equipment.
      • Gamma radiation will penetrate microscopic cracks or pores in medical equipment that might harbour harmful bacteria.
      • It has the advantage over old fashioned 'boiling in water' of not requiring heating and even plastic instruments can be sterilised at room temperature without any damage.
    • It is very convenient for packaged 'convenience' food!
      • Again, a high does of gamma radiation will kill bacteria and prevent the food decaying and shouldn't involve any degradation.
      • After air-tight packing and sealing, the food is quite safe to eat on opening later (days-months), and is NOT radioactive.
      • After cooking and sealing in a plastic packet, you don't need to reopen to complete the sterilization to give it a long shelf-life!
    • Gamma radiation is used to sterilise male insects as a method of pest control.
    • Gamma radiation is used to sterilise blood for transfusion.
    • Radiation pellets of gamma emitters are used in grain elevators to kill insects and rodents in the same way radiation prolongs the shelf-life of foods by destroying bacteria, viruses, and moulds.
  • (c) doc bUsing gamma emitter tracers in medicine
    • By injecting a radioactive tracer into a patients body (or swallowing a drink/tablet), its movement in the blood stream around the body can be monitored with an external detector system.
    • This enables clinicians to detect and diagnose certain medical conditions.
    • The radioisotope used must be a beta or gamma radiation emitter to penetrate the body and be externally detected.
    • The radioisotope must have a short half-life for safety reasons.
  • The gamma emitting radioisotope sodium-24, can be used in tracer studies of animal blood circulation, an important diagnostic tool in clinical medicine.
    • It undergoes beta decay with a half-life of 15 hours, a safe time for medical use.
      •    +  
      • The emitted beta or gamma radiation can be detected outside of the body.
      • The tracer can be injected into the body in a sodium chloride (saline) solution.
  • (c) doc b (c) doc b Technetium-99 is a gamma emitter (half-life 6 hours) and is used in medicine as a tracer.
    • In medical applications, in a suitable chemical form, the radioisotope is injected into the body and its 'movement' can be followed by a suitable external detector system.
    • Time is allowed for the radioactive tracer to spread and its progress tracked with a detector outside the body.
    • The patient can be placed next to a 'detection screen' that shows where the radioactive tracer is.
    • The effective function of organs like the liver and digestion system can be checked.
    • The half-life must be relatively short so it does not linger in the body increasing the harmful effects of cell damage.
    • Technetium atoms can be incorporated into many organic chemicals called radiopharmaceuticals which can be used to monitor biochemical aspects of the bodies chemistry e.g. the functioning and performance of a particular organ.
  • (c) doc bSimilarly, a patient can breathe in air with a gaseous gamma emitter in it, and the effectiveness and structure of the lungs can be checked.
    • The detector system can be focussed on rib cage and lung area of the body once the gaseous radioactive compound has been breathed in.
    • The gas must be a molecule containing a suitable radioactive atom.
  • (c) doc bIodine-131, another gamma emitter (half-life = 8 days), can be used to check on the functioning of a thyroid gland. The body needs iodine to make the hormone thyroxine and so the take up of iodine can be monitored by measuring the gamma radiation from the thyroid gland. Gamma radiation, being the most penetrating, it passes out through the body and so readily be detected outside the body by some suitable detector e.g. with a special camera or fluorescent screen.
    • The half-life should be long enough to allow good detection BUT NOT too long to be dangerous to the body over a period of time!
      • Short half-lives minimise the risk of damaging healthy cells by reducing the amount of radioactivity circulating in the body.
    • One method of treating thyroid cancer is to inject Iodine-131 into the body in a soluble salt form e.g. potassium iodide, so that it deliberately concentrates in the thyroid gland and the gamma radiation kills the thyroid cancer cells.
    • This is another example of 'medical physics' and important diagnostic technique in clinical medicine.
      • (c) doc bBeta sources can be used, though not as penetrating as gamma and have an increased risk of cell damage.. 
      • (c) doc bAlpha sources are too readily absorbed to show up via a detector and so are not suitable for these 'tracer' applications.
      • However, an alpha particle emitting isotope of radium can be directly injected in tiny quantities into tumourous tissue to directly irradiate and kill cancer cells (see uses of alpha radiation).
      • See also uses of beta radiation in cancer therapy.
  • more on the properties of gamma radiation and nuclear origin of gamma radiation


5d Uses of positron radiation sources (beta plus decay nuclides) - PET scan use

One of the most important uses of beta plus (positron) emitters is PET Scanning in medicine.

PET is the acronym for Positron Emission Tomography and uses radioactive isotopes that emit positrons in the beta plus mode of decay.

PET scanning is a technique used to show the effective functioning, or otherwise ('malfunctioning') of various tissues and organs enabling diagnosis of certain medical conditions.


Introduction to PET scans:

Positron emission tomography (PET) scans are used in medicine to produce highly detailed three-dimensional images of the inside of the human body. PET images can clearly show the part of the body being investigated, including any abnormal areas, and can highlight how well certain functions of the body are working. PET scans are often combined with computerised tomography (CT) scans to produce even more detailed 3D images, known as a PET-CT scan. PET scans may also occasionally be combined with a magnetic resonance imaging (MRI) scan, known as a PET-MRI scan.

Why are PET scans are used?

An important advantage of a PET scan over other diagnostic techniques is that it can show how well certain parts of your body are working, rather that showing what a particular part of the body looks like. PET scans are particularly helpful for investigating confirmed cases of cancer, to determine how far the cancer has spread and how well it's responding to treatment i.e. keep track of the behaviour of a tumour. Sometimes PET scans of blood vessel function are used to help plan operations, such as a coronary artery bypass graft of the heart or brain surgery for epilepsy. They can also help diagnose some conditions that affect the normal workings of the brain, such as dementia.

How do PET scans work?

PET scanners work by detecting the gamma radiation given off by a substance called a radiotracer as it collects in different parts of your body. In most PET scans a radiotracer called fluorodeoxyglucose (FDG) is used, which is similar to the naturally occurring sugar, so your body treats it in a similar way in its energy releasing metabolic chemistry. By analysing the areas where the radiotracer does and doesn't build up (varying concentration), it's possible to work out how well certain body functions are working and identify any abnormalities. For example, a concentration of FDG in the body's tissues can help identify cancerous cells because cancer cells use glucose at a much faster rate than normal cells because of an increase in rate of cell division.

  • How is the procedure carried out?
    • The patient is injected into a vein of the arm or hand with a substance that is normally present and used in the body e.g. a special compound like glucose with a positron emitting isotope of fluorine in it (e.g. fluorodeoxyglucose, FDG).
      • Remember that glucose is an important molecule in the bodies metabolism, so monitoring its concentration is a way of monitoring metabolic activity.
    • The radioisotope must have a short half-life (19F 110 min, < 2 hours) to minimise radiation exposure to the patient and the molecule carrying the radioisotope then spreads around the body of the patient into tissues and organs over the next hour and acts as a tracer.
      • The explanation outlined below applies to any positron emitter used in PET scanning.
    • The fluorine-19 decays by positron emission (beta+ disintegration)
    • 189F  ===> 188O  +  0+1e
      • fluorine-18 decays to oxygen-18 plus a positron.
      • BUT a positron (positive electron, antiparticle) interacts with the nearest electron (particle) and is destroyed immediately and in the process two high energy gamma photons are released in this annihilation,
        • e+ + e   ==> 2

        • The two gamma photons shoot out of the body in opposite directions (its a momentum effect) and both gamma beams can be detected and used in the final scan analysis.

        • The PET scanner itself is circular and wide enough for the patient to moved through it slowly and this enables the body to be scanned in 'thin slices' and a computer builds up an accurate 3D picture of where the radioisotope has gone.

        • The 'all-round' monitor around the body detects each pair of gamma rays and the resulting 3D analysis (3D triangulation) of the gamma bursts allows you, from their intersection, to pin-point various features e.g. the accurate location of a tumour from increased metabolic activity.

        • Or you can detect the progress of the radioisotope moving through the blood stream OR its build-up in any part of the body.

      • AND it is this gamma radiation passing out of the patient's body that is detected by the scanner.

        • Incidentally, fluorine-18 is made by bombarding oxygen-18 with protons in a cyclotron - see section 7).
          • 188O  +  11H  ===> 189F10n
          • Some hospitals actually have a cyclotron to make the radiotracer on the spot and enables radioisotopes of quite short half-lives to be used. A PET-CT scanner costs ~2.5 million pounds and cyclotron facility costs ~3.5 million pounds, so there aren't too many of them around at the moment.
      • The radioactive isotopes used must have short half-lives so the radioactivity doesn't last too long and be harmful to the patient, but this presents a supply problem.
      • With such short half-lives, the PET isotopes must be made near to the hospital to have a high activity, or not enough would be left if manufactured a large distance away. Some large hospitals have their own cyclotron and make them on the spot, thereby maximising the radioactivity.
      • In the UK your background radiation dosage is around 2.2 millisieverts/year, though this can vary and a single PET scan is about 7 millisieverts, so each time you have a PET scan (or any other nuclear radiation treatment or X-ray) there is always a slightly greater risk of cell damage.
  • What can you find out from a PET scan?
    • The distribution of the radiated gamma rays from the radioactivity will match up with the bodies metabolic activity i.e. some of the bodies biochemistry which involves the energy releasing glucose.
    • Therefore the cells which are working hardest e.g. dividing cancer cells, using more of radioisotope 'labelled' molecule, will give out more gamma radiation and will show up as a more intense area are on the scan.
      • Therefore PET scanning is used to diagnose some types of cancer.
    • PET scans can also show areas of damaged tissue in the heart by detecting decreased blood flow, so is a diagnostic method for coronary artery heart disease. Dead or damaged heart muscle can cause a heart attack.
    • PET scans can plot blood flow and activity in the brain which can help diagnose conditions like epilepsy.
    • Active cancer tumours can be detected by PET scanning showing the relative metabolic activity in tissue, which is greater in cancer cells because they are growing more rapidly than healthy cells, so you get a stronger signal from the cancer cells.
  • -
  • See section 7. How positron emitting radioisotopes are made in a cyclotron


Atomic structure, radioactivity and nuclear physics revision notes index

Atomic structure, history, definitions, examples and explanations including isotopes gcse chemistry notes

1. Atomic structure and fundamental particle knowledge needed to understand radioactivity gcse physics revision

2. What is Radioactivity? Why does it happen? Three types of atomic-nuclear-ionising radiation gcse physics notes

3. Detection of radioactivity, its measurement and radiation dose units, ionising radiation sources - radioactive materials, background radiation gcse physics revision notes

4. Alpha, beta & gamma radiation - properties of 3 types of radioactive nuclear emission & symbols ,dangers of radioactive emissions - health and safety issues and ionising radiation gcse physics revision

5. Uses of radioactive isotopes emitting alpha, beta (+/) or gamma radiation in industry and medicine gcse notes

6. The half-life of a radioisotope - how long does material remain radioactive? implications!, uses of decay data and half-life values - archaeological radiocarbon dating, dating ancient rocks gcse physics revision

7. What actually happens to the nucleus in alpha and beta radioactive decay and why? nuclear equations!, the production of radioisotopes - artificial sources of radioactive-isotopes, cyclotron gcse physics revision notes

8. Nuclear fusion reactions and the formation of 'heavy elements' by bombardment techniques gcse physics notes

9. Nuclear Fission Reactions, nuclear power as an energy resource gcse physics revision notes

(c) doc bRADIOACTIVITY multiple choice QUIZZES and WORKSHEETS

Easier-Foundation Radioactivity Quiz

or Harder-Higher Radioactivity Quiz

 (c) doc b five word-fills on radioactivity * Q2 * Q3 * Q4 * Q5and ANSWERS!

crossword puzzle on radioactivity and ANSWERS!

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