medical

equations:
J09 11b)
J10 41 10b)
N10 41 10
N10 43 11a)
J11 41 10c)
J12 41 12
J12 43 10
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Explain the principles behind the use of X-rays for imaging internal body structures 4M
  X-ray beam directed through body onto detector (plate)
  different tissues absorb/attenuate beam by different amounts
  giving ‘shadow’ image of structures
  any other detail e.g. comment re sharpness or contrast
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Describe how the image produced during CT scanning differs from that produced by X-ray imaging 5M
  X-ray image is flat OR 2-dimensional
  CT scan takes many images of a slice  at different angles
  these build up an image of a slice through the body
  series of images of slices is  made
  so that 3D image can be built up
  image can then be rotated
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 By reference to the principles of CT scanning, suggest why CT scanning could not be developed before powerful computers were available 5M
  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
  (any five, 1 each to max 5)
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 Explain what is meant by linear absorption coefficient 3M
    parallel beam (in matter)
    I  = I 0  exp(-µx)
    I, I 0 , (µ) and x explained
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 Briefly explain the principles of CT scanning 6M
    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
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 Explain  the  principles  behind  the  use  of  ultrasound  to  obtain  diagnostic  information about structures within the body 5M
   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 structures
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  Explain  the  main  principles  behind  the  generation  of  ultrasound  to  obtain  diagnostic information about internal body structures 6M
   either  quartz  or  piezo-electric crystal
   opposite faces /two sides coated (with silver) to act as electrodes
   either  molecular structure indicated     or    centres of (+) and (–) charge not coincident
   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
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 Outline briefly the main principles of the use of magnetic resonance to obtain information about internal body structures 8M
  large / strong (constant) magnetic field                  B1
  nuclei rotate about direction of field / precess         (1)
  radio frequency / r.f. pulse                              B1
  causes resonance in nuclei , nuclei absorb energy        (1)
  (pulse) is at the Larmor frequency                       (1)
  on relaxation / nuclei de-excite  emit (pulse of) r.f.    B1
  detected and processed                                    B1
  non-uniform field (superimposed)                          B1
  allows for position of nuclei to be determined            B1
  and for location of detection to be changed              (1)
  B6 plus any two extra details, 1 each, max 2)
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 Explain the principles of the generation and detection of ultrasound waves 6M
  quartz/piezo-electric crystal
  p.d. across crystal causes  either  [centres of (+) and (–) charge to move]   or  [crystal to change shape]
  alternating p.d. (in ultrasound frequency range) causes crystal to vibrate crystal cut to produce resonance
  when crystal made to vibrate by ultrasound wave
  alternating p.d. produced across the crystal
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 This technique involves the use of two superimposed magnetic fields.  Describe the functions of these two magnetic fields 4M
  strong uniform (magnetic) field
  either     aligns nuclei     or    gives rise to Larmor/resonant frequency in r.f. region
  non-uniform (magnetic) field
  either     enables nuclei to be located     or       changes the Larmor/resonant frequency
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(N09 42 10a graph page22) Explain why  a continuous spectrum of wavelengths is produced 3M
      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
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(N09 42 10a graph page22)the spectrum has a sharp cut-off at short wavelengths 1M
      either  when electron loses all its energy in one collision
      or   when energy of electron produces a single photon
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Explain what is meant by linear absorption coefficient 3M
     parallel beam (in matter)
      I  = I 0  exp(-µx)
      I, I 0 , (µ) and x explained
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(N09 42 10b graph page23)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. 3M
      either  low-energy photons absorbed (much) more readily     or  low-energy photons (far) less penetrating
      low-energy photons do not contribute to X-ray image
      low energy photons could cause tissue damage
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(N10 41 10c page 21)By reference to your answers in (b)(ii), explain the use of a gel on the surface of skin during ultrasound diagnosis 3M
       either  very little transmission at an air-skin boundary
        (almost) complete transmission at a gel-skin boundary
        when wave travels in or out of the body  A1  [3]
     or    no gel, majority reflection
            with gel, little reflection
            when wave travels in or out of the body  (A1)
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 Suggest why, on an X-ray plate, the contrast between bone and muscle is much greater than that between fat and muscle 3M
    attenuation (coefficients) in muscle and in fat are similar
    attenuation (coefficients) in bone and muscle / fat are different
    contrast depends on difference in attenuation
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Describe the effect on the transmission of ultrasound through a boundary where there is a large difference between the acoustic impedances of the two media 3M
    alpha would be nearly equal to 1
    either  reflected intensity would be nearly equal to incident intensity
    or   coefficient for transmitted intensity = (1 – a)
    transmitted intensity would be small
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 what is meant by the hardness of an X-ray beam 2M
  hardness measures the penetration of the beam
  greater hardness, greater penetration
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 how hardness is controlled 2M
  controlled by changing the anode voltage
  higher anode voltage, greater penetration/hardness
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 State what is meant by acoustic impedance 1M
  product of density (of medium) and speed of sound (in medium)
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 Explain why acoustic impedance is important when considering reflection of ultrasound at the boundary between two media 2M
  difference in acoustic impedance
  determines fraction of incident intensity
  that is reflected/amount of reflection
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