past paper notes- medical physics

Question 1

explain the principles behind the generation of ultrasound waves for diagnosis in medicine. /5

Answer 1

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

Question 2

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

Answer 2

smaller structures can be resolved

Question 3

Suggest why the ultrasound from the transducer is pulsed? /2

Answer 3

• 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

Question 4

Explain the significance of the magnitudes of Z1 and Z2 on the ratio IT/Io. /2

Answer 4

• 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

Question 5

Outline the principles of computed tomography (CT). /6

Answer 5

• 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

Question 6

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

Answer 6

• 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

Question 7

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

Answer 7

• electron gives all its energy to one photon
• electron stopped in single collision

Question 8

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

Answer 8

de excitation of electrons in target

Question 9

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.

Answer 9

alumunium filter

Question 10

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.

Answer 10

long wavelength X-rays do not pass through the body

Question 11

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.

Answer 11

the penetration of the beam

Question 12

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.

Answer 12

decreasing the accelerating voltage

Question 13

State one advantage and one disadvantage of producing a CT scan image of a person rather than a standard X-ray image.

Answer 13

advantage: image is 3D

disadvantage: more risk to health

Question 14

Outline the use of ultrasound to obtain diagnostic information about internal body structures? /5

Answer 14

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

Question 15

Describe the function of quartz crystal? /3

Answer 15

quartz crystal generates ultrasound

detects reflected ultrasound

vibration cause crystals to generate emf

Question 16

Explain why ultrasound used in medical diagnosis is emitted in pulses? /2

Answer 16

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)

Question 17

Explain the principles of the detection of ultrasound waves in medical diagnosis? /3

Answer 17

piezo electric crystal are used to produce ultrasounds

ultrasound makes crystal vibrate

vibration produces emf across crystal

Question 18

By reference to specific acoustic impedance, explain why there is very little transmission of ultrasound waves from air to skin?/3

Answer 18

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

Question 19

Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures? /6

Answer 19

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

Question 20

State the names of the particles involved in the annihilation process. /1

Answer 20

electron and positron

Question 21

Explain how the gamma photons are used to produce an image. /3

Answer 21

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

Question 22

Ultrasound is used to produce diagnostic information about internal body structures.
(a) Explain how ultrasound waves are detected. /3

Answer 22

piezo electric crystal

ultrasound causes vibrations of crystal

vibrations causes emf which is detected

Question 23

Give two reasons why some stars appear brighter than others. /2

Answer 23

1) brighter star has greater luminosity

2) brighter star is moving closer to earth

Question 24

State the purpose of CT. /1

Answer 24

to produce a 3-dimensional image of structure/body