Topic 1 - Radiation and Treatment Flashcards
How can X-rays be used in medical imaging?
- High energy EM waves, absorbed by dense materials (e.g. bone, metal)
- Radiographers: use these properties for photographs for diagnoses e.g. bone fractures
- Ionising radiation so health risk
How can X-rays be used in CAT scanning?
- CAT, computerised axial tomography
- Use X-rays to produce 2D slice through body
- Use intense beams of X-rays that are strongly ionising
- Can image soft tissue, not just bones
How do endoscopes work?
- Use visible light, not ionising
- Reflect light along optical fibres
- See inside patient to investigate problems without invasive surgery
- Can be used for keyhole surgery
What are the three ways ultrasound can be used medically?
- Breaking down kidney stones
- Pre-natal scanning of a foetus
- Measuring speed of blood flow
What is ultrasound?
- Sound waves with frequencies above 20kHz, above range of human hearing
- Non-ionising so safer than ionising
How is ultrasound used to break down kidney stones?
- Kidney stones: hard masses that can block urinary tract, painful
- Ultrasound beam concentrates high energy waves at kidney stone
- Turns it into sand like particles
- Pass out patient’s body in urine
- No surgery and relatively painless
How is ultrasound used in pre-natal scanning of a foetus?
- Ultrasound imaging: diagnose soft tissue problems and perform pre-natal scans of foetus in womb
- U.sound waves reflected off the different tissue boundaries
- Times/distributions of echoes processed by computer to form image on a screen
How is ultrasound used in measuring the speed of blood flow?
- Works in real time
- Can show things moving/changing
- Can investigate blood flow, particularly in heart/liver
- Can measure the speed of blood flow by special u.sound machines
- Can identify blockages in veins/arteries
What is radiation?
Energy emitted from a source
Can be in the form of a wave or particle
What two factors affect the intensity of radiation?
- Distance from source
- What it’s passing through: some radiation absorbed by medium passing through (unless in vacuum)
What is the general rule of how much radiation is absorbed by a medium it passes through?
Denser mediums absorb more energy, so decrease intensity more
If radiation has a greater intensity, what must also be greater?
More energy it carries per second or higher power (higher power=more energy transferred per second when hits an object)
How does surface area affect the amount of radiation it absorbs?
Greater surface area means more radiation hits it, so catches more
What is the equation for intensity?
Intensity=power/area
What is intensity measured in?
W/m^2
What is refraction?
When radiation changes direction as it enters a different medium
What causes refraction?
The change in density from one medium to another, therefore changing the speed
How does radiation refract when it slows down?
Bends towards the normal
What happens if radiation hits a boundary at 90⁰?
Does not change direction but does slow down
What happens to a light when it hits a different medium?
Some light reflected, and some will pass through (refraction), depending on the angle of incidence, i
What are the two main types of lenses?
Converging and diverging
What shape is a converging lens and how does it affect the light?
- Convex
- Cause parallel rays to converge (move together) to focus at the lens focal point, F
What shape is a diverging lens and how does it affect the light?
- Concave (caves inwards causing parallel rays to diverge [spread out])
- Focal point is point where rays hitting the lens parallel to the axis appear to come from, trace them back until they all meet up at point behind the lens
- Focal length: distance between middle of lens and focal point
What are the three rules for refraction in a converging lens?
- An incident ray parallel to the axis refracts through the lens and passes through the focal point on the other side
- An incident ray passing through the focal point before entering the lens will refract through the lens and travel parallel to the axis
- An incident ray passing through the centre of the lens carries on in the same direction
What are the three rules for refraction in a diverging lens?
- An incident ray parallel to the axis refracts through the len, and travels in line with the focal point (so it appears to have come from the focal point)
- An incident ray passing towards the focal point refract through the lens and travels and parallel to the axis
- An incident ray passing through the centre of the lens carries on in the same direction
What type of image does a diverging lens always create?
Virtual
Right way up, smaller than object on same side of lens, despite object location
What is the correlation between power and focal length?
The more powerful the lens, the more strongly it converges, so shorter focal length
What is the equation for the power of a lens?
Power of lens = 1 / focal length
What is the difference between the power of a converging lens and the power of a diverging lens?
A converging lens’ power is always positive, a diverging lens’ power is always negative
In the equation 1/f=(1/u)+(1/v), what do f, u and v represent?
f=focal length
u=object distance
v=image distance
What is the equation to show the relationship between the position of the object, position of the image and the focal length?
1/f=(1/u)+(1/v)
In the equation 1/f=(1/u)+(1/v), what does it mean if v is positive/negative?
- Positive: real image
- Negative: virtual image
What is the iris and what is its function?
- Coloured part of the eye
- Made of muscles controlling size of pupil (hole in middle of iris)
- Controls the amount of light entering the eye
What is the cornea and what is its function?
- Transparent ‘window’ with a convex shape
- Focuses the light
How do the lens and the cornea work together to focus light to retina?
- Cornea has fixed power
- Lens can change focusing power by changing shape
What are the ciliary muscles and what are their function?
- Control the lens
- When contract, lens has fat, spherical shape
- When relax, lens become thin, flatter shape
What is the retina and what is its function?
- Covered in light sensitive cells
- Detect light and send signals to brain via the optic nerve
What is the far point of an eye?
The furthest distance the eye can focus comfortably
If you have normal sight, what is the far point and near point of your eye?
infinity
around 25cm
What is the near point of an eye?
The closest distance that the eye can focus on
What does it mean if you are short sighted?
- Can’t focus on things far away (far point is closer than infinity)
- Images of distant object brought into focus in front of retina
What can cause short sightedness?
- Cornea and lens aren’t powerful enough
- Eyeball is too long
- Ciliary muscles are unable to relax enough to change the shape of the lens to focus the light on the retina
What does it mean if you are long sighted?
- Can’t focus clearly on near objects
- Near point is further than normal (25cm+)
What can cause long sightedness?
- Cornea and lens too weak
- Eyeball is too short
- Ciliary muscles not able to contract enough to change the shape of lens to focus light on the retina
Why do lots of people become long sighted as they get older?
Eye lens becomes stiffer or ciliary muscles become weaker so eye loses some focussing power
How can short sightedness be corrected?
- Glasses with diverging lenses
- Correcting lens must have focal point at eye’s fault far point
- Means objects at infinity, which were out of focus, now seem to be in focus at the far point
How can long sightedness be corrected?
- Glasses with converging lenses
- Converging lens used to produce virtual image of objects 25cm away at eye’s near point
- Close objects which were out of focus now seem in focus at near point
How can contact lenses be used to correct vision?
- Cornea does most of eye’s focussing
- When wrong shape can be too weak/powerful (responsible for long and short sight)
- Contacts sit on top of cornea
- Shaped to compensate for fault
- Can be converging or diverging
How can laser eye surgery be used to correct vision?
- Vaporise tissue, changing cornea’s shape
- Slimming it: less powerful, improving short sight
- Changing so more powerful, improving long sight
- Surgeon can precisely control amount of tissue removed
What are the advantages and disadvantages of using glasses to correct vision?
+Can be cheapest
+Can even treat severe eye lenses as can be very thick
+Can be adjusted to correct vision changes
-Often heavy and uncomfortable
-Not the best looking
What are the advantages and disadvantages of using contact lenses to correct vision?
\+Convenient \+Relatively cheap \+Lightweight \+Almost invisible \+Can be adjusted to correct vision changes -Uncomfortable -Can fall out of eye -Can get eye infections if not looked after properly
What are the advantages and disadvantages of using laser eye surgery to correct vision?
+Easier than having to wear contacts/glasses
- Expensive
- Risk of complications e.g. infection, eye responding to make vision worse
- Can not be adjusted to correct vision changes
What affects the amount of light refracted?
Refractive index
What is the refractive index of:
air?
water?
glass?
1
- 33
- 5
What is the equation relating angle of incidence, angle of refraction and refractive index?
n=(sin i)/(sin r)
In the equation n=(sin 1)/(sin r), what does i, r and n represent?
i=angle of incidence
r=angle of refraction
n=refractive index
What happens to the light passing out of a glass prism if the angle of incidence is less than the critical angle?
Most light passes out but a little bit is internally refelcted
What happens to the light passing out of a glass prism if the angle of incidence if equal to the critical angle?
The emerging rays come out along the surface. Quite a bit of internal reflection
What happens to the light passing out of a glass prism if the angle of incidence is greater than the critical angle?
No light comes out, all internally reflected, total internal reflection
What is TIR (total internal reflection)?
When not light passes out of a prism and it is all reflected
What is the equation to work out the critical angle?
sin C=n~r / n~i
In the equation sin C=n~r / n~i, what do n~r and n~i represent?
n~r=refractive index of stuff light is travelling towards
n~i=refractive index of stuff light is travelling from
What is the correlation between refractive index and critical angle?
The higher the refractive index, the lower the critical angle
What can optical fibres be used for?
Medical diagnosis and communications technology
How do optical fibres work?
- Total internal reflection
- Light waves reflect off side of thin inner core of plastic/glass
- Wave enters one end of fibre and is reflected repeatedly until it emerges at the other end
- Fibre must be narrow enough to keep angles above the critical angle + fibre mustn’t be bent too sharply
What is an endoscope?
A thing tube containing optical fibres that lets surgeons examine inside the body
What is an endoscope made up of?
Consist of 2 bundles of optical fibres: one to carry light to area of interest, one to bring image back for viewing
How can the image from an endoscope be viewed?
Through and eyepiece or displayed as full colour moving image on TV screen
What is the big advantage of using endoscopes?
Can now do keyhole surgery (only making very small holes in the body) which couldn’t be done before optical fibres
