Chapter 17: Ionising Radiation Martin Caon Examination Questions and Answers in Basic Anatomy and Physiology Third Edition

Martin Caon Examination Questions and Answers in Basic Anatomy and Physiology Third Edition Chapter 17: Ionising Radiation 17.1 Medical Imaging with X-Radiation X-ray photons directed at a patient do one of three things: 1. Pass straight through the patient (who is mostly empty space) without deviating from their trajectory, to strike the detector and form the medical image. 2. Are stopped within the patient by ionising some of the patient’s atoms, leaving their energy in the patient’s tissues. This absorbed energy contributes to the patient’s “absorbed dose” of radiation. 3. Are deflected from their path so do not reach the detector. These “scattered” photons do not contribute to the image. They may increase the radiation dose to the patient and will irradiate people nearby. Hence radiographers wear protective “lead” aprons which absorb this (relatively low energy) scattered X-radiation. A diagnostic procedure uses the X-rays that pass, undeflected, through the body to be detected by a device on which produces a “shadow picture” of internal structure. Bones, being dense tissue (containing calcium Ca—element number 20, and phosphorus P—element number 15), absorb most of the X-rays that fall on them so cast a shadow on the detector. Soft tissue is easily penetrated by X-rays, so they strike the measuring surface. Soft tissue contains mostly hydrogen, carbon and oxygen atoms which are less dense than Ca and P. The higher is the energy of the X-ray photon, the more penetrating it is. Low-energy photons (less than 20 keV) will be stopped—even by soft tissue—before they get out of the body. These photons will never influence the radiograph so should not be allowed into the patient in the first place. They are filtered out of the X-ray beam by passing the beam through a sheet of aluminium or copper before the X-rays enter the patient. X-radiation is a form of electromagnetic radiation that is produced by an electrical machine. That is, when the machine is switched off, no radiation is present. X-ray photons have enough energy to remove electrons from their atoms, that is, to create ions. Some X-rays have sufficient energy to break the chemical bonds within molecules. This breaks the molecule into fragments that may undergo unintended chemical reactions. This ability to deposit energy within a living cell can cause harm to that cell. On the other hand, because atoms are mostly empty space, the majority of X-ray photons pass through human tissue without colliding with anything. The ability of most X-rays to pass through tissue and on emerging, to be detected by some device, while some X-rays are absorbed within the tissue, is the basis for the medical imaging modalities known as mammography, X-ray pictures (radiographs) and CT scanning. Mammography uses X-rays below about 30 keV of energy, while CT scanning uses X-rays with energies up to 140 keV. While ionising radiation has the potential to cause damage to living cells, cells have mechanisms to repair the damage. The radiation dose that is received by someone undergoing an imaging procedure with X-rays (less than 10 mSv for CT) is medically acceptable considering the diagnostic information it provides and the benefit of this for the patient. The ability of X-radiation to damage living cells is utilised when a hospital linear accelerator is used to produce the very high energy X-rays (4–20 MeV) that are used to kill cancerous cells. These high energy X-rays are more penetrating than those with lower energy, so can be used to target tumours deep within the body. 1. Which statement about the differences between medical imaging using X-rays and a nuclear medicine scan using gamma rays is correct? a. An X-ray procedure leaves the patient with residual radioactivity while nuclear medicine does not. b. A gamma ray source can be switched off after which no gamma radiation is produced while an X-ray source will continue to produce radiation until the source decays. c. X-rays produce an image of internal anatomy while a nuclear medicine scan provides information about the functioning of an organ or tissue. d. A beam of gamma rays is fired at the patient and detected on the other side, while X-rays are produced by the nucleus of a radionuclide incorporated in the patient’s body. Answer is C: X-rays that pass through the body without being absorbed by the body are used to produce an image of internal structure. In nuclear medicine, a radioactive material is incorporated into the body, travels to certain organs from where a gamma ray is emitted. If physiology is altered sufficiently to affect the way the radioactive material moves about the body, the resulting image provides information about how much alteration there has been. 2. Conventional radiography—such as a chest X-ray (CXR)—differs from com-puted tomography (CT) in what respect? a. CT produces an image of all internal anatomy while in CXR, overlying anatomical structures obscure the view of underlying structures. b. In CT the patient is left with some residual radioactivity, but not with CXR. c. CT produces a lower absorbed dose of radiation to the patient than does a CXR. d. CT involves the use of ultrasound while a CXR results from X-rays. Answer is A: CT mages are unobstructed by the “shadow” of overlying structures. 3. Which of the following imaging modalities does NOT involve the use of “ionising radiation”? a. Mammography b. Ultrasound c. A scintigram using technetium d. A chest X-ray Answer is B: Ultrasound produces an oscillation in the particles of the body, but does not use ionising electromagnetic radiation. 4. When inspecting an X-ray image, the order of densities from blackest to whitest is: a. Bone, water, fat, air b. Air, fat, water, bone c. Air, water, fat, bone d. Bone, air, water, fat Answer is B: The blackest part on an X-ray image is air, while the whitest is bone. 5. Which of the following imaging modalities uses X-rays? a. Computed tomography (CT) b. Single-photon emission computed tomography (SPECT) c. Positron emission tomography (PET) d. Nuclear medicine scan (scintigram) Answer is A: CT machines generate X-rays. The others are all nuclear medicine procedures. 6. Radiation which is “ionising” includes which of the following? a. X-rays and gamma rays b. Infrared radiation c. Radiation emitted by mobile phones d. Microwaves Answer is A: The other three choices, while also being forms of electromagnetic radiation, do not utilise energies that are sufficient to remove electrons from their atoms. 7. The lead aprons that are used for the protection of staff in diagnostic radiography procedures do not provide protection against the ionising radiation used in nuclear medicine or radiotherapy. Why is this? Because: a. Charged particles are much easier to stop (are less penetrating) than pho-tons are. b. Gamma rays are more penetrating than x-rays even if both have the same energy. c. Such aprons do not cover the arms, feet, head and neck. d. The shielding provided by aprons is not sufficient to stop photons with energies above 100 keV. Answer is D: Diagnostic radiography uses X-rays with an average energy of 70 keV or less, which can be stopped by relatively thin amounts of lead (or leadlike) material. In order to stop the gamma rays used in nuclear medicine imaging, the thickness of the “aprons” would make them prohibitively heavy to wear. 8. When compared to visible light, which is not very penetrating, why can radiation such as X-rays and gamma rays pass right through the human body? Because: a. The density of the human body is relatively low. b. They have no mass and no charge. c. Atoms in the body are mostly empty space. d. They have very high energy. Answer is D: X- and gamma rays are high energy radiation, which means high frequency, which means short wavelength, which means that many of the photons will pass through the body without interacting with any atoms. The other three choices do not distinguish between different types of radiation. 9. Which of the following is true? a. A patient exposed to diagnostic X-rays will emit X-rays for a short time after the procedure. b. A cancer patient treated with a megavoltage beam of X-rays will emit X-rays for a short time after the treatment. c. For a short time after having a bone scan using the radionuclide technetium- 99m, the patient will emit gamma rays. d. The human body does not contain any radioactive material unless it has been exposed to man-made radioactive material. Answer is C: A bone scan (any nuclear medicine scan) involves taking some radioactive materials into the body which then emits gamma rays to be detected outside the body. The body takes some time to excrete the material (and much of it decays) so until that happens, the body is more radioactive than is usually the case. 10. What does the term ionising radiation refer to? a. The radiation that is emitted by ionised atoms b. That part of the electromagnetic spectrum with wavelengths less than 300 nm which has enough energy to produce ions c. Alpha, beta and gamma rays spontaneously emitted from radionuclides d. Radiation with enough energy to produce ionisation in the material which absorbs it Answer is D: If the radiation produces ions when it interacts with any substance, then it is ionising radiation. Choice B is true but does not include particulate radiation. Choice C is true but does not include X-rays or cosmic rays. 11. Which of the following is a correct use of the unit known as the “electron volt” (eV)? a. One electron volt is the amount of radioactivity that results in one disinte-gration per second. b. Radiopharmaceuticals contain gamma photon emitting radionuclides whose energy is usually in the range 100–250 keV. c. One electron volt is equal to 1.9 × 1016 J of energy. d. A photon of visible light has energy of about 1.5 MeV. Answer is B: eV is a unit of energy (not radioactivity). It is equal to 1.9 × 10−16 J of energy. Visible photons have energy of 1.5 eV or less. 12. Which one of the statements about the penetrating ability of radiation is true? a. 750 keV gamma rays are more penetrating than 750 keV X-rays. b. 140 keV gamma rays are more penetrating than 60 keV X-rays. c. 2 MeV beta rays (electrons) are more penetrating than 1 MeV gamma rays. d. 1 MeV gamma rays are more penetrating than 2 MeV X-rays. Answer is B: Gamma rays and X-rays are indistinguishable, once they have travelled away from their site of production. So a 140 keV gamma or X-ray is more penetrating than a lower energy one. 13. What may the term “ionising radiation” be applied to? a. All electromagnetic radiation b. Radiation that produces ions when it interacts with matter c. Infrared radiation d. Radiation that is emitted by ions Answer is B: The production of ions (i.e. removal of electrons from an atom) is the sign of ionising radiation. Such a change in the medium through which radiation passes is significant.