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Physics > Option I - Medical physics > Flashcards

Option I - Medical physics Flashcards

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1
Q

Label a diagram of the ear

A
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2
Q

What are the ossicles?

A

Small bones in the middle ear, malleus, incus, and stapes

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3
Q

What are the two ways in which sound pressure is increased in the ear?

A
  1. Lever action of the ossicles
  2. Difference in the sizes of the eardrum and the oval window
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4
Q

How do the ossicles amplify the sound pressure in the ear?

A
  • They act as levers
  • Increase the force in the same way a screwdriver works when used to get the lid off a tin of paint
  • Increases the force by roughly 50%
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5
Q

How does the oval window amplify the sound pressure in the ear?

A
  • The window is 15 times smaller than the eardrum
  • A given force will result in a higher pressure that is passed on to the cochlea
  • Increase in pressure ∆P1 causes a force F = ∆P1A1 on the eardrum
  • The pressure increases on the oval window
  • The amplification factor is A1/A2
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6
Q

What would happen without the mechanism for pressure transformation in the ear?

A

Most sound would be reflected

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7
Q

What is the range of audible frequencies experienced by a person with normal hearing?

A

20 to 20 000 Hz

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8
Q

What is the change in observed loudness a response of?

A

A response to a change in intensity of the ear

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9
Q

What is loudness?

A

The way we perceive sound intensity

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10
Q

What is the intensity of a sound?

A
  • The amount of energy that a sound wave brings to a unit area every second
  • The power delivered per unit area

P = power generated by the source

r = distance between the source and the receiver

Unit is Wm–2

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11
Q

What is the relationship between loudness and sound intensity?

A

Logarithmic:

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12
Q

What is the relationship between intensity and amplitude?

A
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13
Q

What is sound intensity level?

A
  • A way of measuring loudness
  • More closely related to the loudness than the intensity(?)

I0 = thershold of hearing

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14
Q

What is the rule for adding sounds?

A

When there are two sounds at the same time their sound intensities (I) can be added up but not the sound intensity levels (IL).

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15
Q

What is the approximate magnitude of sound intensity level at which discomfort is experienced?

A

120 dB or 1 Wm–2

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16
Q

What are the effects on hearing of short-term exposure to loud noise?

A

Short-term exposure to sounds over 100 dB can damage the hairs in the chochlea, leading to temporary deafness and/or tinnitus (ringing in the ears)

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17
Q

What are the effects on hearing of long-term exposure to loud noise?

A

Long exposure to sounds over 90 dB can lead to permanent hearing loss as the hair cells in the cochlea begin to die

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18
Q

What are the different types of hearing defects?

A
  1. Conductive loss
  2. Sensory loss
  3. Selective frequency loss
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19
Q

What is meant by conductive loss of hearing?

A
  • Air conduction thresholds show a hearing loss
  • Bone conduction thresholds are normal
  • Sounds are being processed correctly in the inner ear, but the vibrations are not reaching it
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20
Q

What are the causes for conductive loss of hearing?

A
  1. Blockages - wax or fluid
  2. Accidents - damaged eardrum or middle ear
  3. Diseases - the ossicles can be prevented from moving
  4. Age - the ossicles become less flexible
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21
Q

What is meant by sensory loss of hearing?

A
  • Air conduction thresholds show a hearing loss
  • Bone conduction thresholds show a hearing loss
  • The problem is usually in the cochlea
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22
Q

What are the causes for sensory loss of hearing?

A
  1. Ageing
  2. Exposure to excessive noise over long periods of time
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23
Q

What is meant by selective frequency loss?

A
  • Conductive loss particularly in the low and mid frequency range
  • Can lead to loss in speech discrimination
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24
Q

Audiodiagrams of normal hearing, conductive hearing loss, and sensory hearing loss

A
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25
Q

What is attenuation coefficient?

A

A constant that allows us to calculate the intensity of X-rays given any thickness of material

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26
Q

What is half-value thickness?

A

The thickness required to reduce the intensity to one half of the original

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27
Q

Derive the relation between attenuation coefficient and half-value thickness

A
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28
Q

What is the equation for the intensity of the radiation after it has passed through an absorber?

A

I = I0e–µx

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29
Q

What is attenuation?

A

A reduction in intensity of radiation when it passes through a solid

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30
Q

Why does attenuation occur?

A
  • The photons interact with the atoms of the medium
  • Energy (photons) is lost in every interaction
  • The number of interactions between the photons and solid depends on how many photons there are initially
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31
Q

What is the relationship between radiation intensity and the distance travelled through an absorber?

A

Exponential (decreasing)

32
Q

What is the basic principle of X-ray imaging?

A
  • Some body parts attenuate the X-ray beam much more than other body parts
  • Photographic film darkens when it is exposed to X-ray but remains light if the X-ray does not reach the paper (is absorbed)
33
Q

How is an intensifying screen used to enhance the effect of X-rays?

A
  • A sheet of fluorescent material that gives out visible light when X-ray photons land on it is used
  • The sheet is placed on either side of the photographic film
  • The light emitted causes the light areas to become lighter on the film without increasing intensity or exposure time
34
Q

How can digital images be produced in X-ray imaging?

A

By using a CCD that is sensitive to X-rays

35
Q

What are the limitations of X-ray imaging?

A
  • Sharpness and contrast is lost
  • Soft tissue is hard to identify
36
Q

What are two ways of improving X-ray imaging?

A
  1. Barium meal
  2. Tomography
37
Q

What is the principle of function of barium meal?

A
  • A barium meal is drunk
  • The stomach and the gut are filled with barium sulphate
  • This makes the gut attenuate X-rays more and the places are distinguishable on the film
38
Q

What is the function of tomography?

A
  • Used when a shadow picture isn’t good enough
  • The X-ray beam is focused on a certain region
  • All other regions are blurred out of focus
  • The film and the source of X-rays are moved around the body to achieve an image from all sides
39
Q

What is computer comography (CT scan)?

A
  • A more sophisticated version of tomography
  • The X-ray source and the detectors are rotated around the body
  • X-ray images from different angles are taken and gathered together to form a complete 3D image
  • The detectors and X-ray source are moved along the body to get a picture of the whole body
  • Computers build the 3D model
40
Q

What is the piezoelectric effect?

A
  • When a quartz crystal is compressed or stretched a potential difference is induced across it
  • Happens because the atoms in the crstal are arranged in such a way so that when the crystal is deformed they become polarised
  • If a p.d. is applied across the crystal, the dipoles are made to line up with the electric field, resulting in expansion of the crystal
41
Q

How are ultrasounds produced and detected?

A
  • An alternating p.d. of frequency >20 kHz is applied to quartz crystal, causing it to vibrate
  • To detect, the p.d. induced when a sound when a sound wave causes a crystal to vibrate can be used
42
Q

Why is a gel used on the surface of the skin when ultrasound is used?

A

Because the difference in the acoustic impedance between the air and skin is very large and would cause the ultrasound to reflect on the surface of the skin.

43
Q

What is acoustic impedance?

A
  • When ultrasound waves are incident on the boundary between two media, part of the wavefront is reflected and part refracted
  • The percentage reflected depends on the relative acoustic impedance of the two media:

Z = ρc

(density of a substance ant the speed of sound in that substance)

ρ = density of the medium

c = velocity of the ultrasound

44
Q

What are attenuation and resolution of ultrasound dependent on?

A

Frequency

45
Q

Why does the frequency of the ultrasound need to be specific?

A
  • When ultrasound is diffracted, it spreads out and will not be reflected back to the detector
  • The wavelength used must be short enough so that it is not diffracted
  • A wavelength a bit less than a few millimetres is often used
  • Higher frequencies would give better resolution but are absorbed more by the body, resulting in higher attenuation
46
Q

What are the two types of ultrasound scans?

A

A- and B-scans

47
Q

What is an A-scan ultrasound?

A
  • The simple way
  • A graph is plotted of the strength of the reflected beam against time
  • Makes it possible to see the position of changes in medium
  • To gain more information the probe can be moved up and down (reveals size, shape, and changes in thickness)
  • Does not produce an image
48
Q

What is a B-scan ultrasound?

A
  • Converts the signal into a dot
  • The brightness of the dot corresponds to the strength of the signal
  • If the probe is swept across the organ the dots can be converted into an image
  • Also a probe with many transmitters can be used
49
Q

What is the basic principle of nuclear magnetic resonance (NMR) imaging?

A

Instead of sending waves from an external source, the NMR works by getting the hydrogen nuclei in the body to give out radio waves. The radiation is analysed and a detailed image can be built.

Involves the use of a non-uniform magnetic field in conjunction with a large uniform field

50
Q

What is the function of NRM imaging?

A
  • Nuclei are charged and have spin
  • Nuclei with odd numbers of protons or neutrons behave like magnets
  • If a magnetic field is applied, they will line up
  • If it is pushed to one side, it will oscillate back and forth before eventually coming to rest at its starting position
  • If a hydrogen nucleus is placed in a strong magnetic field and displaced slightly, it will oscillate at a specific frequency
  • The nucleus emits radio waves, which can be detected and used to build up an image
51
Q

How is an image constructed from NMR and which factors affect it?

A
  • The radio waves are detected using a coil of wire and analysed with a computer

Frequency:

  • the frequency depends upon the strength of the magnetic field (stronger magnet = higher frequency)
  • if the magnetic field is made to vary from one place to another, the position of the source can be found

Relaxation time:

  • the time taken for the oscillation to die away is different for different types of tissue
  • by measuring the time it is possible to determine the tissue type
52
Q

What are the use of lasers in clinical diagnosis and therapy?

A
  1. Pulse oximetry
  2. Endoscopes
  3. Scalpels and coagulators
53
Q

How are lasers used in pulse oximetry?

A
  • Red and infrared laser is shone through a thin part of a body (earlobe or fingertip)
  • Red light is absorbed with blood cells with no oxygen
  • IR is absorbed with blood cells with oxygen
  • The relative absorption of the two wavelengths can be used to calculate the amount of oxygen in the blood
  • Allows measuring oxygen content and pulse without having to take a sample
54
Q

How are lasers used in endoscopy?

A
  • Endoscopes enable looking inside the body without surgery
  • A tube is inserted into the body
  • The tube has a collection of optical fibres
  • The optical fibres allow illumination of the insides from an outside source
  • Reflected light is collected using a lens system
  • Further optical fibres are used to allow an image to be viewed
55
Q

How are lasers used as scalpels and coagulators?

A
  • A laser focused on a small region can increase its temperature enough to cut through tissue
  • The heat ensures the site is kept free of germs and the risk of infection is reduced
  • Blood vessels and nerves are automatically sealed off
  • A defocused laser beam can stop bleeding by stimulating the blood to form a clot coagulator
56
Q

What is radiation dosimetry?

A

The calculation of how much radiation is absorbed as a result of exposure to different types of radiation, and the effect that this has on different parts of the body.

57
Q

What does radiation dosimetry depend on?

A
  1. The type of the radiation
  2. The energy of the radiation
  3. How much is actually absorbed
58
Q

What is the activity of a radioactive isotope?

A

The number of disintegrations per second

59
Q

What is exposure (X)?

A
  • A measure of how much ionising radiation one would be exposed to in a particular environment
  • Only used for X and γ radiation
  • Gives some indication of the potential danger of the environment
  • Doesn’t give a true measure of the amount of radiation a body will absorb

Q = total charge of all the positive ions produced

m = mass of air in the room

Unit is Ckg–1

60
Q

What is absorbed dose (D)?

A

A measure of the energy absorbed by the actual tissue

Unit: Jkg–1 or gray (Gy)

E = total energy absorbed

m = mass of tissue

61
Q

What is dose equivalent (H)?

A

The absorbed dose multiplied by the radiation’s quality factor:

H = QD

  • Unit is also Jkg–1 or sievert (Sv)
62
Q

What is quality factor (Q)?

A
  • Different types of radiation have different biological effects
  • Each radiation has a factor that is dependent on its damaging effect
63
Q

What are the three ways to minimise the exposure to radiation?

A
  1. Distance
    - Intensity of all radiation decreases with distance
  2. Shielding
    - By wearing protective clothing or standing behind a shield, the dose can be reduced
  3. Time
    - The dose received from a source is proportional to the time spent exposed to the radiation, reducing the time will reduce the dose
64
Q

What is the function of a film badge?

A
  • A way of monitoring exposure to radiation at work
  • A piece of photographic paper
  • Ionising particles absorbed by the paper cause a chemical change in the grains of it
  • When the paper is processed the grains turn black
  • The number of the black grains is a measure of the amount of radiation received
  • The paper is separated into different regions to distinguish between different types of radiation
  • Each region has a filter of different thickness
  • The badge has to be handed in at regular intervals
65
Q

What is meant by the concept of balanced risk?

A
  • All exposure to ionising radiation is harmful
  • Sometimes the benefits outweight the possible harm
  • The risk of not ttreating a particular condition needs to be weighed against any extra risk that involves using the radiation
  • In general all exposure needs to be as low as possible and needs to show a positive overall benefit
66
Q

What is effective half-life, TE?

A

The time taken for the amount of a substance to be reduced by half in the body

67
Q

What is the biological half-life, TB?

A

The chemical removal of the isotope from the body

(decrease due to biological processes)

68
Q

What is the physical half-life, TP?

A

The decrease of a radioactive isotope due to radioactive decay

69
Q

What are the two types of radiation therapy?

A
  1. External irradiation
  2. Internal irradiation
70
Q

How does external irradiation work?

A
  • Cancer cells are irradiated with a beam of gamma radiation
  • Will also kill healthy cells but they are able to repair themselves
  • The beam is passed through the body from different directions that intersect at the site of the cancer
  • This way the cancer will get the highest dose
  • Can only be used when the cancer is localised
71
Q

How does internal irradiation work?

A
  • Used if the cancer is not localised
  • The body is irradiated from the inside
  • A solid radioactive is placed next to the tumour or a radioactive fluid is injected or ingested
  • If they’re in the bloodstream, they will follow the same path that the substance would normally follow → radioactive iodine can be used to treat thyroid cancers because that is were iodine collects
72
Q

What factors affect the choice of isotope in radiotherapy?

A
  1. Energy
  2. Type (α, β, γ)
  3. Chemical properties
  4. Physical half-life
  5. Biological half-life
73
Q

How can the blood volume be measured using a radioactive tracer?

A
  1. A small amount of radioactive isotope is mixed with 1 litre of blood and its activity is measured
  2. The blood is put back and mixed up with the blood
  3. Another sample of 1 litre is taken and its activity is measured
  4. The ratio of original activity/diluted activity will give the volume:
74
Q

What can radioactive tracers be used for in the body?

A
  1. Measuring blood volume
  2. Thyroid activity
  3. Calcium build-up in heart muscle
  4. Imaging of organs
  5. PET scan
75
Q

How are PET scans done with the help of radioactive tracers?

A
  • Carbon-11 tracer is inhaled through carbon monoxide
  • The carbon monoxide attaches to red blood cells and is carried around the body
  • Carbon-11 decays by sending two gamma ray photons in opposite directions
  • The gamma radiation is detected and used to construct an image of the brain
  • Areas with more blood flowing in (more radiation) have more activity

Physics (16 decks)

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