Medical Physics Flashcards by Stephen Baylis

What is the function of pupil?

Controls the amount of light by dilating or contracting

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What is the function of the cornea?

Focuses light due to a high refractive index

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What is the function of the optic nerve?

Transmits electrical signals to the brain in the form of an inversed image

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What is the function of the choroid?

consists of blood vessels that supply oxygen to the cells in the eye
it is dark to reduce reflections within the eye

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What has the highest concentration of cone cells?

Fovea

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What is the blind spot?

retina doesn’t cover it so there are no cones or rods so it is not sensitive to light
if the image only falls on there you won’t see

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What is the aqueous humour?

watery, transparent liquid so it lets light from the cornea to the pupil
has a very low refractive index to reduce refraction

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What is the vitreous humour?

Black liquid that keeps the eyeball’s shape and stops interference

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What are the ciliary muscles?

Controls thickness and focus of the lens for accommodation

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What is the conjunctive membrane?

Protects and lubricates the eye by stopping microbes getting in

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What does the iris do?

Controls the size of the pupil and amount of light let in

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What is the function of fovea?

contains a high concentration of ‘cone’ cells which give the sharpest result for coloured images
found directly opposite from the pupil

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What is a retina?

A light sensitive layer that is focused on by the lens and when illuminated, produces electrical signals

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What is the function of the ear drum?

Transfers sound waves from the outer ear to the ossicles of the middle ear

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What is a name for the group of bones in the middle ear?

Ossicles

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What is the function of the ossicles?

System of levers which have a mechanical advantage of 1.5

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What are the functions of the oval and round window?

Allow sound vibrations to enter the fluid of the inner ear

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What is the function of the cochlea?

Convert pressure waves in the fluid into electrical signals

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What is a coherent bundle?

Fibres maintained in fixed positions relative to each other

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What is a non-coherent bundle?

Fibres have no fixed relative positions

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Give an application of a coherent bundle

Transit images of internal organs of the body

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State an application of a non-coherent bundle

Transmit light to inside the human body for illumination

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Give two advantages of a bundle consisting of fibres of very small diameter instead of using a bundles consisting of larger fibres

high resolution

- very flexible bundle

Give two reasons why a glass cladding is used around the central core of each fibre in a coherent bundle

so that scratches on the outer surface do not allow light to escape
to prevent mechanical damage to the surface of core

State the main components of a modern X-Ray tube

- glass tube - rotating anode - filament - lead housing

State the function of the glass tube in an X-Ray tube

- sealed so allows electrons to travel unimpeded

State the function of the rotating anode in an x-Ray tube

- rotation of anode to spread heated area | - target which emits X-rays when hit by energetic electrons

State the function of the filament in an X-ray tube

Heat source to release electrons from surface of cathode by thermionic emission

State the function of the lead housing in an X-ray tube

Prevent X-rays from escaping in unwanted directions

What does the threshold of hearing mean?

- the lowest level of sound (intensity) which the ear can detect

State the frequency at which the threshold of hearing has a value of 1.0 x 10^-12 Wm^-2

1kHz

Why is a logarithmic scale used for sound intensity?

- ear has a logarithmic response | - to accommodate very wide range of sound intensities to which ear can respond

Why is the dBA scale used?

The dBA scale takes account of the frequency dependence of the sensitivity of the ear

Explain how a piezoelectric crystal is caused to generate waves of ultrasound

- two faces of a thin slice of crystal act as electrodes - electrodes connected to high frequency source of e.m.f - as applied e.m.f alternates it applies alternating electric field across the slice of crystal between the electrodes - crystal expands and contracts at the same frequency as the applied e.m.f - the vibrations of the faces of the crystal slice produce ultrasound pressure waves

Why must the pulse of ultrasound be short in medical applications?

- transducer to be damped before the echo returns to allow the transducer to act as a receiver - emitted pulse must cease before echo arrives so there's no interference

Under what condition is ultrasound reflected strongly at boundaries between two types of material?

When there is a large difference in acoustic impedance

State two physical properties of the materials which determine the proportion of ultrasound which is reflected at a boundary

- tissue density | - tissue elasticity

Explain what a coupling medium/gel is and why and where it is used

- ultrasound is reflected back at boundaries with air - gel between transducer and skin - acoustically well-matched gel gives good transmission

Give two methods to reduce an X-ray dose and explain the effectiveness

- use of image intensifier screen, allows a low intensity X-rays can be used because image is enhanced by conversion of X-ray photons into light photons by fluorescent material - filters placed between source and patient, soft X-rat are absorbed by filters instead of patient

What is astigmatism caused by?

An irregularity in the curvature of the cornea

What is the image seen by a person with astigmatism?

Less well focused in one particular plane

How is astigmatism corrected?

Correctly orientated cylindrical lens

What is myopia?

Short-sighted

What is hypermyopia?

Long-sighted

What sign is the power of a converging lens?

Positive

What is the sign of a diverging lens?

Negative

What does u represent in the 1/f equation?

Object distance

What does v represent in the 1/f equation

Image distance

How to control the intensity of x-rays?

- control electron beam current

How to control maximum energy of X-ray photons

Anode pd

Units of mass attenuation coefficient?

m^2kg^-1

What energy of photons are absorbed by soft tissue and bone?

Below 30keV

What energies are absorbed more readily by bone than soft tissue?

30keV to 100keV

Uses of CT scanner?

Tumours in chest and abdomen

Uses of MR scanner

Imaging arteries in the brain,neck, chest

Which has the longest scan time?

MR scanner (1000s)

What's best at imaging organ movements?

CT scanner