Pairs of Parameters - Medical Radiation Physics - Past Exam, Exams of Medical Physics

Download Pairs of Parameters - Medical Radiation Physics - Past Exam and more Exams Medical Physics in PDF only on Docsity! S1801(a) Semester 1, 2011 Page 1 of 12 THE UNIVERSITY OF SYDNEY MRTY 1031 - MEDICAL RADIATION PHYSICS JUNE 2011 Time allowed: TWO HOURS Total marks: 80 MARKS INSTRUCTIONS • All questions are to be answered. • Use the answer sheet provided for section A and the answer book for section B. • Hand in the answers to section A and B separately at the end of the examination. DATA Free fall acceleration at earth's surface g = 9.8 m.s-2 Speed of sound in dry air (0°C) v = 331 m.s-1 Speed of light in vacuum c = 2.998 × 108 m.s-1 Permittivity of free space ε0 = 8.854 × 10 -12 C2.N-1.m-2 Permeability of free space µ0 = 4π × 10 -7 T.m.A-1 Elementary charge e = 1.602 × 10-19 C Electron volt 1 eV = 1.602 × 10-19 J Speed of light in vacuum c = 2.998 × 108 m.s-1 Planck’s constant h = 6.626 × 10-34 J.s Boltzmann constant k = 1.381 × 10-23 J.K-1 Stefan-Boltzmann constant σ = 5.671 × 10-8 W.m -2 .K-4 Rydberg constant RH = 1.097 × 107.m-1 Atomic mass unit u = 1.66054 × 10-27 kg Rest masses - electron me = 9.10938 × 10 -31 kg - proton mp = 1.67262 × 10 -27 kg - neutron mn = 1.67493 × 10 -27 kg - hydrogen atom mH = 1.67353 × 10 -27 kg S1801(a) Semester 1, 2011 Page 2 of 12 Formula Sheet Fgravity = G mEarthm r2 V = U q0 or U = q0V F = mg Vb −Va = − Wa→b q0 = Fed q0 = ED F = ma R = V I W = Fd P = VI = I 2R = V 2 R P = W t Rtotal = R1 + R2 + ... KE = 1 2 mv2 = p 2 2m 1 Rtotal = 1 R1 + 1 R2 + ... GPE = mgh λmin nm[ ] = 1.24 kVp T = 1 f hfmax = KEfinal = eVp v = fλ E joule[ ] e = E eV[ ] E = hf = hc λ hfmax eV[ ] = Vp volt[ ] λmax = 2.898 ×10−3 T I ∝ 1 x2 or I1x1 2 = I2x2 2 F = σT 4 I = I0e −µx 1 λ = RH 1 nlower 2 − 1 nupper 2 ⎛ ⎝⎜ ⎞ ⎠⎟ µ = − 1 x loge I I0 ⎛ ⎝⎜ ⎞ ⎠⎟ p = h λ or λ = h p H = 1000 µmedium µwater −1 ⎛ ⎝⎜ ⎞ ⎠⎟ Fe = k q1q2 d 2 = 1 4πε0 q1q2 d 2 M.A.C. = µ ρ E = Fe q0 or Fe = qE Δλ = λ '− λ = h mec 1− cosφ( ) S1801(a) Semester 1, 2011 Page 5 of 12 Question 7 Five identical light bulbs are used to construct the three simple circuits 1, 2 and 3 as shown below. Which of the following listings of the circuits correctly ranks their total light output in decreasing order of brightness – from brightest to dimmest? (a) 1 – brighter than – 2 – brighter than – 3 (b) 1 – brighter than – 3 – brighter than – 2 (c) 2 – brighter than – 1 – brighter than – 3 (d) 3 – brighter than – 2 – brighter than – 1 Question 8 The diagram below shows a cross-section view of a high power X-ray tube. Various ‘items’ or ‘regions’ within the tube are labelled with numbers. Which of the following statements would be false during normal operation of the tube? (a) Item 2 ensures that region 6 remains under vacuum (b) Thermionic emission takes place in item 3 (c) Item 1 rotates to prevent the focal spot becoming too hot (d) Items 4 & 5 are more positive than item 3 S1801(a) Semester 1, 2011 Page 6 of 12 Question 9 Each of three X-ray photons (of identical wavelength) is Compton scattered by a single collision with nearly stationary electrons. • Photon X is deviated through an angle of 90° from its original direction. • Photon Y is scattered backwards towards the direction it came. • Photon Z is slightly deviated by approximately 1° from its original direction. Which of the following sequences correctly lists the photons in order of increasing wavelength after Compton scattering? (a) X, Y, Z (b) Z, X, Y (c) Z, Y, X (d) Y, X, Z Question 10 Which of the following statements is completely correct? (a) A mammography tube is different from most other X-ray tubes because it is designed to produce lower energy (i.e. higher quality) X-ray photons. (b) In the photon spectrum of an X-ray tube, the sharp characteristic peaks are caused by sudden deceleration of incident electrons by the nucleus of target atoms. This process is called "Bremsstrahlung". (c) Increasing the filament current in a normally-operating X-ray tube increases the filament temperature and space charge around the filament and therefore increases the tube current. (d) An X-ray tube is normally operated in the ‘saturation region’ because that way the tube voltage barely changes even if the current fluctuates and so the tube's behaviour is more stable. Question 11 Consider a beam of X-ray photons with an energy range from 10 keV to 1.02 MeV. The beam is being attenuated by human tissue. Which of the following statements is most correct? (a) The lowest energy photons have a higher probability of being scattered by the Compton effect than of being absorbed by the photoelectric effect. (b) The lowest energy photons have a higher probability of being absorbed by pair production than of being attenuated by the photoelectric effect. (c) The lowest energy photons have a higher probability of being absorbed by the photoelectric effect than of being scattered by the Compton effect. (d) The highest energy photons have a higher probability of being absorbed by the Compton effect than of being scattered by the photoelectric effect. S1801(a) Semester 1, 2011 Page 7 of 12 Question 12 An X-ray photon travelling through a material interacts with an atom. The photon is absorbed completely, but most of its energy carried away by an electron that was knocked out of the atom. What is this process called? (a) Bremsstrahlung (b) Compton effect (c) Pair production (d) Photoelectric effect Question 13 As an X-ray beam containing a range of photon energies passes through a material, it experiences ‘beam hardening’. How does this affect the X-ray beam? (a) Beam quality increases because average photon energy decreases. (b) Beam quality increases which also results in an intensity increase. (c) Average photon energy increases, however beam intensity decreases. (d) Average photon energy decreases, therefore beam intensity decreases. Question 14 When does the conversion of a latent image into a visible image occur in film? (a) When an electron activates the sensitivity centre (b) When electrons are added to the silver ions at the sensitivity centre (c) When the silver bromide crystals interact with a light photon (d) When the silver ions migrate to the sensitivity centre Question 15 An advantage of digital imaging over film imaging is the ability to “post-process” the image. What is post processing? (a) Altering the appearance of the image once it has been formed. (b) Changing the speed of the developing cycle (c) Use of a daylight processor to develop the image (d) Reading out the latent image using a laser Question 16 A magnification mammogram is taken with the compressed breast raised on a platform 29 cm above the image receptor. The SID is 65 cm. A lump is seen on the image with a diameter of 32 mm. What is the diameter of the lump within the breast? (a) 14.3 mm (b) 16.0 mm (c) 17.7 mm (d) 57.6 mm S1801(a) Semester 1, 2011 Page 10 of 12 Question 2 In 1913 Niels Bohr proposed a modification to Rutherford’s nuclear model of the atom. He suggested that electrons occupy discrete energy levels (or ‘shells’) surrounding the nucleus. (a) Explain what is meant by “discrete energy levels” in this context. (b) Describe the experimental evidence that indicates there are discrete energy levels in atoms. (c) Would you expect the energy levels of a neutral helium atom to be the same, similar or very different to a neutral hydrogen atom? Explain your reasoning. (d) Would you expect the energy levels of a singly-ionised helium atom (i.e. with only one electron) to be the same, similar or very different to a neutral hydrogen atom? Explain your reasoning. (10 marks) Question 3 The graph above is a spectrum of photon energies from a filtered X-ray tube. Consider the four quantities indicated in the graph: the size of the area under the curve N and the horizontal positions of features labelled M, O and P. (a) For each of the following conditions listed below, identify which of the four quantities will change. Where possible, state if the change is an increase or decrease. If possible, quote values for the new quantities (with any relevant units). You are not required to provide an explanation why. (i) You increase the acceleration voltage to 120 kV. (ii) You reduce the thickness of the filter. (iii) You decrease the tube current to one quarter (1/4) of its original value. (b) (i) What is the name given to the features labelled O? (ii) Name any changes that would need to be made to the tube to affect the horizontal positions of the features labelled O. (10 marks) S1801(a) Semester 1, 2011 Page 11 of 12 Question 4 (a) A 300 kV X-ray tube with a 6 mm aluminium filter, produces a beam with an linear attenuation coefficient in copper of 0.707 mm-1. Calculate the half value thickness (HVT) in copper for this beam. Identify the main assumptions or approximations you employed. (b) You have used a plastic phantom to test an X-ray CT machine. By analysing the CT image, you find that the CT number for the plastic is -40 HU. For the same X-ray beam, calculate the linear attenuation coefficient for the plastic expressed as a percentage of the linear attenuation coefficient of water. (c) For each of the following two tissue types (i) and (ii), identify which attenuation mechanism (Compton or photoelectric) is most responsible for radiographic contrast and very briefly explain why. Give one example of each kind of tissue. (i) Variable density tissues of roughly uniform atomic number Z (ii) Roughly uniform density tissues with high Z compared to soft tissue (10 marks) Question 5 In mammographic imaging the breast tissue is compressed. This means that during imaging the tissue is put under pressure between two plates. (a) (i) One of the things this does is reduce the thickness of the tissue. How does this affect the image quality? (ii) How does this affect the patient dose? (b) Compressing the tissue also reduces the distance between more of the tissue and the image receptor. How does this affect the image quality? (c) Compressing the tissue also reduces the movement of the tissue. How does this affect the image quality? (10 marks) S1801(a) Semester 1, 2011 Page 12 of 12 Question 6 The penumbra is the partial shadow outside the darkest portion of the shadow of an opaque object. In an X-ray image the penumbra is caused by the size of the focal spot. (a) Draw a diagram to illustrate how the size of the penumbra of an object is affected by the size of the focal spot. (b) What effect does the size of the focal spot have on the quality of an image? Explain your answer. (c) What effect does the size of the focal spot have on the magnification in an image? Explain your answer. (d) Describe one imaging situation in which you would use a fine (small) focal spot. (10 marks) This is the end of your questions.