past paper notes- medical physics Flashcards
explain the principles behind the generation of ultrasound waves for diagnosis in medicine. /5
piezoelectric is used as transducer
pd across crystal makes the crystal distort
alternating pd across crystal is applied which causes crystal to vibrate
when applied frequency is natural frequency, crystal resonates
natural frequency of crystal is in ultrasound range
Ultrasound frequencies as high as 10MHz are used in medical diagnosis. suggest one advantage of the use of high frequency ultrasound rather than lower frequency ultrasound. /1
smaller structures can be resolved
Suggest why the ultrasound from the transducer is pulsed? /2
- transducer both transmits and receives
- it needs to be pulsed in order to measure depths
- it needs to be pulsed to determine the nature of boundaries
Explain the significance of the magnitudes of Z1 and Z2 on the ratio IT/Io. /2
- if the values of Z1 and Z2 are equal then IT/Io is equal to 1
- if values of Z1 and Z2 are different then IT/Io is equal to O
Outline the principles of computed tomography (CT). /6
- X-ray image of a single slice is taken
- from different angle
- computer is used to form
- 2D image of slice
- repeated for many slices
- to build up 3D image
Explain why there is a continuous distribution of wavelengths in the graph of variation with wavelength of the intensity of the X-ray beam? /3
- X ray photon is producrd when electron is accelerated suddenly
- the photon released has range of accelerations in target
- hence there is distribution of waves
Explain why there is a sharp cut- off at short wavelength in the graph of variation with wavelength of the intensity of the X-ray beam? /2
- electron gives all its energy to one photon
- electron stopped in single collision
Explain why there is a series of peak superimposed on the continuous distribution of wavelengths in the graph of variation with wavelength of the intensity of the X-ray beam? /1
de excitation of electrons in target
In the X-ray imaging of body structures, longer wavelength photons are frequently filtered out
of the X-ray beam.
(i) State how this filtering is achieved.
alumunium filter
In the X-ray imaging of body structures, longer wavelength photons are frequently filtered out
of the X-ray beam.
(ii) Suggest the reason for this filtering.
long wavelength X-rays do not pass through the body
In an X-ray tube, the hardness of an X-ray beam may be controlled. (i) State what is meant by the hardness of the beam.
the penetration of the beam
In an X-ray tube, the hardness of an X-ray beam may be controlled.
State how the hardness of the beam may be decreased.
decreasing the accelerating voltage
State one advantage and one disadvantage of producing a CT scan image of a person rather than a standard X-ray image.
advantage: image is 3D
disadvantage: more risk to health
Outline the use of ultrasound to obtain diagnostic information about internal body structures? /5
pulses of ultrasound are generated and directed into the body
they reflect at each boundary layer attenuate in every layer
gel is used to minimise reflection at skin
the time delay between impulses gives the dept of the boundary
while the intensity of the received signal gives information of the nature of the boundary layer
Describe the function of quartz crystal? /3
quartz crystal generates ultrasound
detects reflected ultrasound
vibration cause crystals to generate emf
Explain why ultrasound used in medical diagnosis is emitted in pulses? /2
allows the reflected signal to be distinguished from the emitted signal
detection occurs in the time between emitted pulses
reflection of ultrasound detected by same probe/transducer
cannot emit and detect at the same time (hence pulses)
Explain the principles of the detection of ultrasound waves in medical diagnosis? /3
piezo electric crystal are used to produce ultrasounds
ultrasound makes crystal vibrate
vibration produces emf across crystal
By reference to specific acoustic impedance, explain why there is very little transmission of ultrasound waves from air to skin?/3
specific acoustic impedance of air and skin is very different
intensity reflection coefficient depends on difference between acoustic impedance
most ultrasound reflected so little transmission
Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures? /6
pulses of ultrasound
reflected at boundaries
reflected pulses detected by ultrasound generator
reflected signal processed and displayed
time delay gives information about the depth of boundary
intensity of reflected pulse gives information about the body
gel used to minimise reflection at skin
degree of reflection depends upon impedances of two media
State the names of the particles involved in the annihilation process. /1
electron and positron
Explain how the gamma photons are used to produce an image. /3
the gamma photons travel in opposite directions
gamma photons detected outside body
gamma photons arrive at different times at detector
determine location of production of gamma
Ultrasound is used to produce diagnostic information about internal body structures.
(a) Explain how ultrasound waves are detected. /3
piezo electric crystal
ultrasound causes vibrations of crystal
vibrations causes emf which is detected
Give two reasons why some stars appear brighter than others. /2
1) brighter star has greater luminosity
2) brighter star is moving closer to earth
State the purpose of CT. /1
to produce a 3-dimensional image of structure/body
