PHYSICS 2: HSC COURSE 2 nd edition (Andriessen et al) CHAPTER 20 Radioactivity as a diagnostic tool (pages 394-5) 1. (a) A radioisotope is an isotope that is unstable and will emit particles from the nucleus until it becomes stable. (b) Radioactive decay is the emission of particles from the nucleus of a radioactive element. (c) Emissions from radioactive nuclei are alpha or beta particles or gamma radiation which come out of the nuclei as they decay. 2. (a) The half-life of a radioisotope is the time it takes for half the radioactive material in the sample to decay. For example, the half-life of thallium-201 is 3.05 days. This means that if we start with 2 grams of thallium-201, in 3.05 days we will have 1 gram of thallium-201, the other 1 gram having decayed into a new element. (b) Exposure to the radioactive emissions could be reduced by limiting the dose to the patient to the minimum needed for the scan. The doctor or radiographer performing the test using the thallium-201 should not stay in the same room as the patient except when it is necessary. They should handle the radioisotope only while it is in a leadlined container or syringe. Because they are exposed to radioisotopes in the course of their work, they should wear a detector to monitor their exposure to radioactive emissions, to ensure it does not exceed safe limits. 3. Technetium-99m is attached to a pharmaceutical containing polyphosphate ions. This process is called labelling and the labelled pharmaceutical is called a radiopharmaceutical. The radiopharmaceutical is injected into the patient. It travels through the bloodstream and within an hour accumulates in the bone. Where the blood flow is greater, more radioisotope accumulates and the amount of emission from the decay of the radioisotope is greater. These areas are called hot spots and are frequently regions where there are tumours. 4. This radioisotope is not suitable for use in medical diagnosis as the half-life is too long. After waiting a suitable time for the radiopharmaceutical to accumulate in the target organ, the rate of emission will be low, because the half-life is 100 days. Hence Medical Physics Chapter 20 1
a clear image will not be able to be made. A larger dose of radioisotope could be given to increase the rate of emission but this will mean more radioisotope will either remain in the patient s body or be excreted into the sewage system. Issue For Against Amount of emissions produced Will be small (unless a larger dose is given) Image will not be clear enough Size of dose (will need to be large to give high enough rate of emissions) Image will be clearer More radioisotope may remain in the patient s body harmful for patient; or more may be excreted to sewage system environmental problem 5. The amount of emissions produced may have decreased below a useful level for producing an image before the radiopharmaceutical has accumulated in the target organ. Once the radiopharmaceutical has accumulated in the target organ, the emissions may need to be monitored for a period of time and a short half-life will mean that the emission rate will be too low by the time the examination has finished. 6. (a) In 1 half-life the activity will drop to 2.0 MBq and in another half-life the activity will drop to 1 MBq. Hence the isotope will take 2 half lives, or 4.0 minutes, to reach an activity of 1.0 MBq. (b) Halving the activity each half-life, we see it will take 4 half-lives for the activity to reach 0.25 MBq. The time involved will be 4 x 2.0 minutes or 8.0 minutes. 7. Xenon should not be used in preference to krypton for investigations of lung function. In the investigation, the radioactive gas would be inhaled and accumulate in the lungs. The γ rays produced as the radioisotope decays would be detected with a gamma camera. As the radioisotope collects in the lungs very quickly, krypton-81m with a short half-life of 13 seconds would be adequate. A small dose would produce significant activity as the half-life is very small. If xenon-133 were used, a larger dose would be needed to produce the same activity and any residual radioisotope, which was not excreted, would remain in the body for many days because the half-life is 5.3 Medical Physics Chapter 20 2
days. Xenon-133 would also produce β particles as it decayed and these would penetrate the tissue with which they were in contact and cause damage to the tissue. 8. (a) Isotope Where it is used Justification Iodine-123 To examine the thyroid gland It is taken up by the thyroid gland. The amount of γ radiation given out is a measure of whether or not the thyroid gland is functioning normally because the uptake of a normal thyroid gland is known. Technetium-99m To examine bone Polyphosphate ions labelled with technetium-99m accumulate in bone and the gamma rays produced show the flow of blood. High blood flow and hence high gamma ray activity are called hot spots and are often associated with disease. (b) Radioisotopes used in medical imaging are taken into the body and the γ rays which are produced are detected outside the body. If α particles are produced, they will penetrate some of the internal tissue before being absorbed and causing ionisation which is damaging. They will not be detected outside the body as they will be absorbed before they emerge. Hence they would be useless for imaging; and they are also harmful. 9. One factor is the half-life of the radioisotope and the other is the organ that is to be studied because some organs are taken up by a particular radioisotope while others are not. Medical Physics Chapter 20 3
10. Graph of activity on the vertical axis and time on the horizontal axis. Activity of carbon-11 drops off more quickly than bromine-75 showing the different half-lives. The graphs start from the same point on the vertical axis. The decay curve for each is exponential for C-11 the activity halves every 20 minutes while Br-75 s activity halves every 100 minutes. 11. (a) A study to measure the volume of blood in the body uses a radioactive tracer which mixes with the blood. (b) A study to detect blockage in the lungs uses a tracer which is trapped in the fine capillaries in the lung. If the tracer cannot become trapped it may be because the lung is blocked. 12. Sample of molybdenum-99 decays to technetium-99m saline solution added technetium-99m is flushed out and removed. 13. Technetium-99m has a relatively short half-life (6 hours), it emits γ rays only when it decays, and it readily attaches to different compounds to form radioactive tracers. These different compounds, when labelled, are metabolised by a number of different organs and hence technetium-99m can be used to image many organs. 14. The radioisotope fluorine-18 replaces a hydrogen atom on some molecules of β- D-glucose and the radiopharmaceutical so formed is called FDG. This radiopharmaceutical is injected into the bloodstream. The molecules are of a suitable size to reach the brain. Fluorine-18 decays, with a half-life of 109.8 minutes, emitting positrons. After travelling a short distance from their place of emission a positron encounters an electron and the pair of particles annihilate one another producing 2 gamma photons of energy 0.51 Mev. These travel in opposite directions from the site of annihilation and emerge from the head, to be detected by the gamma cameras surrounding the patient s head. The intensities of pairs of gamma rays is measured, and by comparison with known attenuation for gamma rays travelling through tissue, the site of the annihilation can be determined. About half a million gamma ray pairs are needed to make a useful image, and so a computer has a valuable role in analysing the collected data. The concentration of glucose in the brain for healthy brain function Medical Physics Chapter 20 4
is known. Tumours require more oxygen and hence more glucose. A site of a tumour could show up as a site where more gamma rays than expected were detected, because more positron emitting radioisotopes circulated there. It is important to give the patient a dose of the radiopharmaceutical that is large enough to last for the duration of the test but not so large that it will remain in the patient s body longer than necessary and expose the patient to unnecessary radiation. The radiographer should avoid contact with the gamma radiation produced or with the positrons emitted from the fluorine-18 by using a shield to absorb any harmful radiation and by moving away from the region where radiation would be produced. 15. (a) A positron is a positively charged beta particle. (b) Positrons may be obtained when a proton disintegrates into a neutron and a positron. (c) Positron-electron interaction results in the annihilation of the pair and the production of two gamma rays of energy 0.51 MeV travelling in opposite directions. In medical diagnosis, the pairs of gamma rays, produced inside the body, are detected and their location determined from knowledge about the attenuation of the gamma rays as they pass through tissue. By locating the source of gamma rays, the source of the radiopharmaceutical can be determined. Often a different amount of radiopharmaceutical from what is expected indicates disease. 16. The healthy kidney shows uniform colour throughout the kidney. It is assumed that the colour indicates release of gamma rays from a radioisotope in the urine, which is being filtered from the blood in the kidneys. In the diseased kidney, there is colour showing normal function at the top of the kidney, but in the lower section the colour is absent. We can deduce that the lower section is not functioning and the urine is not being filtered out. This could be due to a cancer in this part of the kidney. 17. (a) The top study shows both lungs because they are functioning normally. The radioisotope will reach the capillaries around the alveoli in the perfusion study and the radioisotope will reach the spaces in the alveoli in the ventilation study. The bottom studies show only the right lung in the perfusion study. Both lungs are visible in the ventilation study. Medical Physics Chapter 20 5
(b) In the bottom study, the radioisotope does not reach the capillaries because they are blocked in the left lung. (This is a front view of the lungs.) The perfusion study shows the blocked capillaries. The radioisotope can reach the spaces in the alveoli in the ventilation study and so both the lungs show up on the image. The ventilation study would show blocked alveoli, not blocked capillaries. 18. (a) The X-ray shows the bone as whiter on the outside. The inside of the bone is a similar shade to the surrounding tissue. The bone scan shows the bone brighter at the ends of the long bones in the legs - possibly due to these areas being where the bone grows. The X-ray shows a distinct break in one bone. The bone scan shows white patches on bones in the spine, ribs, shoulder and upper legs of the skeleton which has tumours. (b) The X-ray produces an image because X-rays are absorbed by bone and tissue through which they pass. The bone is denser than the surrounding tissue and so bone absorbs more X-rays. A shadow of the bone forms on the X-ray photo and this shadow shows a break in the bone. In the bone scan, a radiopharmaceutical is injected into the bloodstream and accumulates in bone. Where there is increased blood flow, more gamma radiation from the decaying radioisotope is detected. The areas of high blood flow show up as white spots on the scan and often indicate tumours. 19. A printed image of the organs should be made. The activity in the diseased organ may be greater than in the healthy organ, as the bone scan shows on page 388. The activity may be less in the diseased organ due to blockage of the radioisotope from the diseased area, as in figure 20.9, page 388. Make sure you compare the images you find. Medical Physics Chapter 20 6