Option I - Medical physics Flashcards
Label a diagram of the ear
What are the ossicles?
Small bones in the middle ear, malleus, incus, and stapes
What are the two ways in which sound pressure is increased in the ear?
- Lever action of the ossicles
- Difference in the sizes of the eardrum and the oval window
How do the ossicles amplify the sound pressure in the ear?
- 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%
How does the oval window amplify the sound pressure in the ear?
- 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
What would happen without the mechanism for pressure transformation in the ear?
Most sound would be reflected
What is the range of audible frequencies experienced by a person with normal hearing?
20 to 20 000 Hz
What is the change in observed loudness a response of?
A response to a change in intensity of the ear
What is loudness?
The way we perceive sound intensity
What is the intensity of a sound?
- 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
What is the relationship between loudness and sound intensity?
Logarithmic:
What is the relationship between intensity and amplitude?
What is sound intensity level?
- A way of measuring loudness
- More closely related to the loudness than the intensity(?)
I0 = thershold of hearing
What is the rule for adding sounds?
When there are two sounds at the same time their sound intensities (I) can be added up but not the sound intensity levels (IL).
What is the approximate magnitude of sound intensity level at which discomfort is experienced?
120 dB or 1 Wm–2
What are the effects on hearing of short-term exposure to loud noise?
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)
What are the effects on hearing of long-term exposure to loud noise?
Long exposure to sounds over 90 dB can lead to permanent hearing loss as the hair cells in the cochlea begin to die
What are the different types of hearing defects?
- Conductive loss
- Sensory loss
- Selective frequency loss
What is meant by conductive loss of hearing?
- 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
What are the causes for conductive loss of hearing?
- Blockages - wax or fluid
- Accidents - damaged eardrum or middle ear
- Diseases - the ossicles can be prevented from moving
- Age - the ossicles become less flexible
What is meant by sensory loss of hearing?
- Air conduction thresholds show a hearing loss
- Bone conduction thresholds show a hearing loss
- The problem is usually in the cochlea
What are the causes for sensory loss of hearing?
- Ageing
- Exposure to excessive noise over long periods of time
What is meant by selective frequency loss?
- Conductive loss particularly in the low and mid frequency range
- Can lead to loss in speech discrimination
Audiodiagrams of normal hearing, conductive hearing loss, and sensory hearing loss
What is attenuation coefficient?
A constant that allows us to calculate the intensity of X-rays given any thickness of material
What is half-value thickness?
The thickness required to reduce the intensity to one half of the original
Derive the relation between attenuation coefficient and half-value thickness
What is the equation for the intensity of the radiation after it has passed through an absorber?
I = I0e–µx
What is attenuation?
A reduction in intensity of radiation when it passes through a solid
Why does attenuation occur?
- 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
What is the relationship between radiation intensity and the distance travelled through an absorber?
Exponential (decreasing)
What is the basic principle of X-ray imaging?
- 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)
How is an intensifying screen used to enhance the effect of X-rays?
- 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
How can digital images be produced in X-ray imaging?
By using a CCD that is sensitive to X-rays
What are the limitations of X-ray imaging?
- Sharpness and contrast is lost
- Soft tissue is hard to identify
What are two ways of improving X-ray imaging?
- Barium meal
- Tomography
What is the principle of function of barium meal?
- 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
What is the function of tomography?
- 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
What is computer comography (CT scan)?
- 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
What is the piezoelectric effect?
- 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
How are ultrasounds produced and detected?
- 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
Why is a gel used on the surface of the skin when ultrasound is used?
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.
What is acoustic impedance?
- 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
What are attenuation and resolution of ultrasound dependent on?
Frequency
Why does the frequency of the ultrasound need to be specific?
- 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
What are the two types of ultrasound scans?
A- and B-scans
What is an A-scan ultrasound?
- 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
What is a B-scan ultrasound?
- 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
What is the basic principle of nuclear magnetic resonance (NMR) imaging?
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
What is the function of NRM imaging?
- 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
How is an image constructed from NMR and which factors affect it?
- 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
What are the use of lasers in clinical diagnosis and therapy?
- Pulse oximetry
- Endoscopes
- Scalpels and coagulators
How are lasers used in pulse oximetry?
- 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
How are lasers used in endoscopy?
- 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
How are lasers used as scalpels and coagulators?
- 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
What is radiation dosimetry?
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.
What does radiation dosimetry depend on?
- The type of the radiation
- The energy of the radiation
- How much is actually absorbed
What is the activity of a radioactive isotope?
The number of disintegrations per second
What is exposure (X)?
- 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
What is absorbed dose (D)?
A measure of the energy absorbed by the actual tissue
Unit: Jkg–1 or gray (Gy)
E = total energy absorbed
m = mass of tissue
What is dose equivalent (H)?
The absorbed dose multiplied by the radiation’s quality factor:
H = QD
- Unit is also Jkg–1 or sievert (Sv)
What is quality factor (Q)?
- Different types of radiation have different biological effects
- Each radiation has a factor that is dependent on its damaging effect
What are the three ways to minimise the exposure to radiation?
- Distance
- Intensity of all radiation decreases with distance - Shielding
- By wearing protective clothing or standing behind a shield, the dose can be reduced - Time
- The dose received from a source is proportional to the time spent exposed to the radiation, reducing the time will reduce the dose
What is the function of a film badge?
- 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
What is meant by the concept of balanced risk?
- 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
What is effective half-life, TE?
The time taken for the amount of a substance to be reduced by half in the body
What is the biological half-life, TB?
The chemical removal of the isotope from the body
(decrease due to biological processes)
What is the physical half-life, TP?
The decrease of a radioactive isotope due to radioactive decay
What are the two types of radiation therapy?
- External irradiation
- Internal irradiation
How does external irradiation work?
- 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
How does internal irradiation work?
- 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
What factors affect the choice of isotope in radiotherapy?
- Energy
- Type (α, β, γ)
- Chemical properties
- Physical half-life
- Biological half-life
How can the blood volume be measured using a radioactive tracer?
- A small amount of radioactive isotope is mixed with 1 litre of blood and its activity is measured
- The blood is put back and mixed up with the blood
- Another sample of 1 litre is taken and its activity is measured
- The ratio of original activity/diluted activity will give the volume:
What can radioactive tracers be used for in the body?
- Measuring blood volume
- Thyroid activity
- Calcium build-up in heart muscle
- Imaging of organs
- PET scan
How are PET scans done with the help of radioactive tracers?
- 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
