Medical Physics 4 I3 Radiation in Medicine

Transcription

1 Name: Date: 1. This question is about radiation dosimetry. Medical Physics 4 I3 Radiation in Medicine Define exposure. A patient is injected with a gamma ray emitter. The radiation from the source creates an exposure in the body of C kg 1. The average energy required to singly ionize an atom in the human body is approximately 40 ev and the quality factor for gamma radiation is 1. Deduce that this corresponds to a dose equivalent of 340 msv. (Total 4 marks) 2. This question is about radiation used in medicine. Define the terms exposure and absorbed dose. Exposure: Absorbed dose: Explain, with reference to α and γ radiation, the distinction between absorbed dose and dose equivalent. Explain why, when using radioactive tracer elements in the treatment of cancer, it is better to use radioactive isotopes that have a long physical half-life and a short biological half-life. 1

2 (Total 7 marks) 3. This question is about dosimetry. Explain what is meant by quality factor (relative biological effectiveness). The radioactive isotope potassium-40 occurs naturally in the body. Use the data below to calculate the annual dose equivalent that the body receives from the decay of potassium-40 within the body. number of atoms of potassium-40 per kilogram of the body = decay constant of potassium-40 = year 1 energy absorbed by the body from the decay of one atom of potassium-40 = J quality factor of the radiation from decay of potassium-40 = 1 (Total 4 marks) 4. This question is about medical diagnosis. State and explain the use of a barium meal in X-ray diagnosis. a gel on the skin during ultrasound imaging. 2

3 a non-uniform magnetic field superimposed on a much larger constant field in diagnosis using nuclear magnetic resonance. (Total 7 marks) 5. This question is about radioactive isotopes of iodine. The isotope iodine-131 is used to treat malignant growths in the thyroid gland. The isotope has a physical half-life of 8 days and a biological half-life of 21 days. Explain the term biological half-life. Calculate the effective half-life of the isotope. The isotope iodine-123 has a physical half-life of 13 hours. Suggest why it is preferable to use this isotope for imaging the thyroid rather than iodine-131 (1) (Total 5 marks) 6. This question is about dosimetry. Describe what is meant by the term relative biological effectiveness (quality factor). The whole body of a person of mass 70 kg is exposed to monochromatic X-rays of energy 200 kev. As a result of this exposure, the person receives a dose equivalent of 500 µsv in 2.0 minutes. 3

4 Deduce that the person absorbs about X-ray photons per second. (4) (Total 6 marks) 7. This question is about the use of radiation in medicine. When referring to radiation dosage, α-radiation and γ-radiation have different quality factors. (i) State which type of radiation has the larger quality factor... (1) (ii) Explain why, for the same absorbed dose, the radiations have different effects The risk factor attached to any particular dose equivalent depends not only on the total dose but also on the dose rate. Explain why the risk factor depends on dose rate. Iodine-131 is used to label human serum albumin. This isotope has a physical half-life of 8.0 days and a biological half-life of 21 days. Determine the time taken for the activity within the body of a particular dose of this isotope to be reduced to 4 1 of its initial activity. (Total 10 marks) 4

5 8. This question is about X-ray absorption. The diagram below shows a parallel beam of X-rays incident on a section of bone of thickness d. d intensity I 0 intensity I The incident intensity is I 0 and the transmitted intensity is I. The graph below shows the I variation with bone thickness d of the ratio. The incident intensity I 0 is constant. I 0 I 1.0 I d / mm (i) Estimate the half-value thickness of the bone. (1) 5

6 (ii) Use your answer in (i) to calculate the attenuation coefficient of X-rays of this sample of bone. I For X-rays of different frequency, the fraction for a given thickness of bone is I 0 greater than shown on the graph. Explain the effect of this change on the attenuation coefficient and on the half-value thickness calculated in. Explain by reference to attenuation coefficients why barium meals may be used to assist in the X-ray imaging of the stomach. (4) (Total 10 marks) 6