For Examiner’s Use 21 9 (a) State what is meant by acoustic impedance. product of density (of medium) and speed of sound (in medium) .......................................................................................................................................... ......................................................................................................................................[1] (b) Explain why acoustic impedance is important when considering reflection of ultrasound at the boundary between two media. difference in acoustic impedance .......................................................................................................................................... determines fraction of incident .......................................................................................................................................... intensity ......................................................................................................................................[2] (c) Explain the principles behind the use of ultrasound to obtain diagnostic information about structures within the body. .......................................................................................................................................... pulse of ultrasound (directed into body) .......................................................................................................................................... reflected at boundary (between tissues) .......................................................................................................................................... (reflected pulse is) detected and processed .......................................................................................................................................... time for return of echo gives (information on) depth .......................................................................................................................................... amount of reflection gives information on tissue .......................................................................................................................................... .......................................................................................................................................... ......................................................................................................................................[5] © UCLES 2007 9702/04/O/N/07 [Turn over 20 10 Outline briefly the main principles of the use of magnetic resonance to obtain information about internal body structures. ................................................................................................................................................. large / strong (constant) magnetic field ................................................................................................................................................. nuclei rotate about direction of field / precess radio frequency / r.f. pulse ................................................................................................................................................. causes resonance in nuclei , nuclei absorb energy (pulse) is at the Larmor frequency ................................................................................................................................................. on relaxation / nuclei de-excite emit (pulse of) r.f. detected and processed ................................................................................................................................................. non-uniform field (superimposed) ................................................................................................................................................. allows for position of nuclei to be determined ................................................................................................................................................. and for location of detection to be changed ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ............................................................................................................................................. [8] © UCLES 2008 9702/04/M/J/08 For Examiner’s Use 21 11 (a) Distinguish between the images produced by CT scanning and X-ray imaging. .......................................................................................................................................... CT image: (thin) slice (through structure) .......................................................................................................................................... e.g. built up from many ‘slices’ / 3-D image .......................................................................................................................................... X-ray image: ‘shadow’ image (of whole structure) / 2-D image ...................................................................................................................................... [3] (b) By reference to the principles of CT scanning, suggest why CT scanning could not be developed before powerful computers were available. .......................................................................................................................................... X-ray image of slice taken from many different angles these images are combined .......................................................................................................................................... (and processed) repeated for many different slices .......................................................................................................................................... to build up a 3-D image .......................................................................................................................................... 3-D image can be rotated .......................................................................................................................................... computer required to store and process huge quantity of data .......................................................................................................................................... ...................................................................................................................................... [5] © UCLES 2008 9702/04/O/N/08 For Examiner’s Use 18 11 (a) Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures. .......................................................................................................................................... pulse of ultrasound .......................................................................................................................................... reflected at boundaries / boundary .......................................................................................................................................... received / detected (at surface) by transducer .......................................................................................................................................... signal processed and displayed .......................................................................................................................................... time between transmission and receipt of pulse gives (information about) depth of boundary .......................................................................................................................................... reflected intensity gives information as to nature of boundary .......................................................................................................................................... .................................................................................................................................... [4] (b) Data for the acoustic impedances and absorption (attenuation) coefficients of muscle and bone are given in Fig. 11.1. muscle bone acoustic impedance / kg m–2 s–1 absorption coefficient / m–1 1.7 × 106 6.3 × 106 23 130 Fig. 11.1 The intensity reflection coefficient is given by the expression (Z2 – Z1)2 . (Z2 + Z1)2 The attenuation of ultrasound in muscle follows a similar relation to the attenuation of X-rays in matter. A parallel beam of ultrasound of intensity I enters the surface of a layer of muscle of thickness 4.1 cm as shown in Fig. 11.2. muscle beam of ultrasound 4.1 cm Fig. 11.2 © UCLES 2009 9702/04/M/J/09 bone For Examiner’s Use 19 The ultrasound is reflected at a muscle-bone boundary and returns to the surface of the muscle. For Examiner’s Use Calculate (i) the intensity reflection coefficient at the muscle-bone boundary, coefficient = (Z2 – Z1) 2 / (Z2 + Z1) 2 = (6.3 – 1.7) 2 / (6.3 + 1.7) 2 = 0.33 0.33 coefficient = ............................................... [2] (ii) the fraction of the incident intensity that is transmitted from the surface of the muscle to the surface of the bone, fraction= exp (–µx) = exp(–23 × 4.1 × 10–2) =0.39 0.39 fraction = ............................................... [2] (iii) the intensity, in terms of I, that is received back at the surface of the muscle. intensity = 0.33 × 0.392 × I =0.050 I 0.05 intensity = ............................................ I [2] © UCLES 2009 9702/04/M/J/09 [Turn over 19 11 Outline briefly the main principles of the use of magnetic resonance to obtain diagnostic information about internal body structures. For Examiner’s Use ................................................................................................................................................. large / 1 T magnetic field applied along body (allow ‘across’) ................................................................................................................................................. r.f. pulse applied ................................................................................................................................................. causes hydrogen nuclei / protons ................................................................................................................................................. to resonate ................................................................................................................................................. (nuclei) return to equilibrium state / after relaxation time ................................................................................................................................................. r.f. (pulse) emitted ................................................................................................................................................. pulses detected, processed and displayed ................................................................................................................................................. resonant frequency depends on magnetic field strength ................................................................................................................................................. calibrated non-uniform field enables nuclei to be located ................................................................................................................................................. ................................................................................................................................................. ........................................................................................................................................... [6] © UCLES 2009 9702/41/O/N/09 [Turn over 22 10 (a) A typical spectrum of the X-ray radiation produced by electron bombardment of a metal target is illustrated in Fig. 10.1. intensity 0 wavelength Fig. 10.1 Explain why (i) a continuous spectrum of wavelengths is produced, e.m. radiation / photons is produced whenever a charged particle is accelerated .................................................................................................................................. wavelength depens on magnitude of acceleration .................................................................................................................................. electrons have a distribution of accelerations .................................................................................................................................. so continuous spectrum ............................................................................................................................ [3] (ii) the spectrum has a sharp cut-off at short wavelengths. .................................................................................................................................. when electron loses all its energy in one collision ............................................................................................................................ [1] © UCLES 2009 9702/42/O/N/09 For Examiner’s Use 23 (b) The variation with photon energy E of the linear absorption coefficient ! of X-rays in soft tissue is illustrated in Fig. 10.2. For Examiner’s Use 300 µ / cm–1 200 100 0 10 30 50 70 E / keV Fig. 10.2 (i) Explain what is meant by linear absorption coefficient parallel beam (in matter) .................................................................................................................................. I = I0 exp(-µx) .................................................................................................................................. I, I0, (µ) and x explained .................................................................................................................................. ............................................................................................................................ [3] (ii) For one particular application of X-ray imaging, electrons in the X-ray tube are accelerated through a potential difference of 50 kV. Use Fig. 10.2 to explain why it is advantageous to filter out low-energy photons from the X-ray beam. low-energy photons (far) less penetrating .................................................................................................................................. low energy photons could cause tissue damage .................................................................................................................................. .................................................................................................................................. ............................................................................................................................ [3] © UCLES 2009 9702/42/O/N/09 [Turn over 18 10 (a) Briefly explain the principles of CT scanning. .......................................................................................................................................... X-ray taken of slice / plane / section .......................................................................................................................................... repeated at different angles .......................................................................................................................................... images / data is processed .......................................................................................................................................... combined / added to give (2-D) image of slice .......................................................................................................................................... repeated for successive slices .......................................................................................................................................... to build up a 3-D image .......................................................................................................................................... image can be viewed from different angles / rotated .......................................................................................................................................... ...................................................................................................................................... [6] © UCLES 2010 9702/41/M/J/10 For Examiner’s Use 19 (b) A simple section through a body consists of four voxels, as illustrated in Fig. 10.1. For Examiner’s Use section directions of viewing Fig. 10.1 An X-ray image of the section is obtained by viewing along each of the directions shown in Fig. 10.1. The detector readings for each direction of viewing are summed to give the pattern of readings shown in Fig. 10.2. 25 22 34 31 Fig. 10.2 For any one direction, the total of the detector readings is 16. (i) For the pattern of readings of Fig. 10.2, state the magnitude of the background reading. 16 background reading = ................................................ [1] (ii) © UCLES 2010 On Fig. 10.1, mark the pattern of pixels for the four-voxel section. 3 2 6 5 9702/41/M/J/10 [2] [Turn over 19 11 Explain the main principles behind the generation of ultrasound to obtain diagnostic information about internal body structures. For Examiner’s Use ................................................................................................................................................. quartz or piezo-electric crystal ................................................................................................................................................. opposite faces /two sides coated (with silver) to act as electrodes ................................................................................................................................................. potential difference across crystal causes crystal to change shape ................................................................................................................................................. alternating voltage (in US frequency range) applied across crystal ................................................................................................................................................. causes crystal to oscillate / vibrate ................................................................................................................................................. (crystal cut) so that it vibrates at resonant frequency ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ........................................................................................................................................... [6] © UCLES 2010 9702/42/M/J/10 [Turn over 20 10 (a) (i) State what is meant by the acoustic impedance of a medium. density × speed of wave (in the medium) .................................................................................................................................. .............................................................................................................................. [1] (ii) Data for some media are given in Fig. 10.1. medium speed of ultrasound / m s–1 acoustic impedance / kg m–2 s–1 330 1500 1600 4100 4.3 × 102 1.5 × 106 1.6 × 106 7.0 × 106 air gel soft tissue bone Fig. 10.1 Use data from Fig. 10.1 to calculate a value for the density of bone. ρ = (7.0 × 10 6) / 4100 = 1700 kgm–3 1700 density = .................................... kg m–3 [1] (b) A parallel beam of ultrasound has intensity I. It is incident at right-angles to a boundary between two media, as shown in Fig. 10.2. boundary incident intensity I transmitted intensity IT reflected intensity IR acoustic impedance Z1 acoustic impedance Z2 Fig. 10.2 The media have acoustic impedances of Z1 and Z2. The transmitted intensity of the ultrasound beam is IT and the reflected intensity is IR. (i) State the relation between I, IT and IR. I = IT + IR .............................................................................................................................. [1] © UCLES 2010 9702/41/O/N/10 For Examiner’s Use 21 (ii) The reflection coefficient a is given by the expression a= For Examiner’s Use (Z2 – Z1)2 . (Z2 + Z1)2 Use data from Fig. 10.1 to determine the reflection coefficient a for a boundary between 1. gel and soft tissue, α = (0.1×10 6) 2 / (3.1×10 6) 2 = 0.001 0.001 a = .................................................. [2] 2. air and soft tissue. 1 a = .................................................. [1] (c) By reference to your answers in (b)(ii), explain the use of a gel on the surface of skin during ultrasound diagnosis. no gel, majority reflection .......................................................................................................................................... with gel, little reflection .......................................................................................................................................... when wave travels in or out of the body .......................................................................................................................................... ...................................................................................................................................... [3] © UCLES 2010 9702/41/O/N/10 [Turn over 19 Section B For Examiner’s Use Answer all the questions in the spaces provided. 9 (a) (i) State, with reference to X-ray images, what is meant by sharpness. edges can be (clearly) distinguished .................................................................................................................................. .............................................................................................................................. [1] (ii) Describe briefly two factors that affect the sharpness of an X-ray image. size of X-ray source / anode / target / aperture scattering of X-ray 1. ............................................................................................................................... beam .................................................................................................................................. 2. ............................................................................................................................... .................................................................................................................................. [3] (b) An X-ray image is taken of the skull of a patient. Another patient has a CT scan of his head. By reference to the formation of the image in each case, suggest why the exposure to radiation differs between the two imaging techniques. .......................................................................................................................................... X-ray image involves a single exposure .......................................................................................................................................... CT scan: exposure of a slice from many different angles .......................................................................................................................................... repeated for different slices .......................................................................................................................................... CT scan involves a (much) greater exposure ...................................................................................................................................... [4] © UCLES 2010 9702/43/O/N/10 [Turn over 22 11 The linear attenuation (absorption) coefficient µ for X-ray radiation in bone, fat and muscle is given in Fig. 11.1. µ / cm–1 bone fat muscle 2.9 0.90 0.95 Fig. 11.1 (a) A parallel X-ray beam of intensity I0 is incident either on some bone or on some muscle. The emergent beam has intensity I. Calculate the ratio (i) I for a thickness of I0 1.5 cm of bone, I / Io = exp (-1.5 x 2.9) = 0.013 0.013 ratio = ................................................ [2] (ii) 4.6 cm of muscle. I / Io = exp (-4.6 x 0.95) = 0.013 0.013 ratio = ................................................ [1] (b) Suggest why, on an X-ray plate, the contrast between bone and muscle is much greater than that between fat and muscle. .......................................................................................................................................... attenuation (coefficients) in muscle and in fat are similar .......................................................................................................................................... attenuation (coefficients) in bone and muscle / fat are different .......................................................................................................................................... contrast depends on difference in attenuation ...................................................................................................................................... [3] © UCLES 2010 9702/43/O/N/10 For Examiner’s Use
