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medical physics Flashcards

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1
Q

What do lenses do

A

Refract light in order to change its direction

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2
Q

What is a converging (convex) lense

A
  • curved outwards on both sizes
  • causes parallel light rays to converge at a point
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3
Q

What type of images can converging lenses form

A

Real and virtual

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4
Q

What type of image can be projected onto a screen

A

Real images

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5
Q

What is a real image

A

Image formed when the object is further than the focal length

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6
Q

What is a virtual image

A

Image formed when the object is closer than the focal length

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7
Q

What is a diverging (concave) lense

A
  • curved inwards on both sides
  • cause parallel light rays to diverge
  • always a virtual image
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8
Q

What is principal focus in converging lenses

A

The point at which light rays which are parallel to the principal axis are focused

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9
Q

What is principal focus in diverging lenses

A

The point from which the light rays appear to come from

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10
Q

What is the power of a lense

A

The measure of a lens’ ability to bend light

  • positive in converging
  • negative in diverging
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11
Q

What is focal length

A

The distance from the centre of the lens to the principal focus

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12
Q

What happens when the ciliary muscles contract

A

The lense becomes more curved so the focal length decreases, allows close up objects to be saw

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12
Q

What is accomodation

A

The ability of a lens to change its focal length to focus on an object

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13
Q

What can the eye focus on when the ciliary muscles are relaxed

A

Objects far away - 5m to infinity

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14
Q

What are the two types of photodetectors

A

Rods and cones

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15
Q

What light levels are rods activated by

A

Low levels

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16
Q

Why is the image produced by rods not clear

A

Many rods are connected to one nerve fibre and they don’t differentiate between wavelengths of light

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17
Q

What happens in very low light levels to rhodopsin

A

Enzymes fully reform the rhodopsin allowing the eye to reach its maximum sensitivity

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17
Q

What is rhodospin

A

A substance that is destroyed by light

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18
Q

What light levels are cones activated by

A

High light levels

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19
Q

Why do cones produce a greater image detail than rods

A

One cone is connected to one nerve fibre

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20
Q

How many types of light sensitive material do cones have

A

3

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20
Q

What is the choroid

A

A tissue which supplies the eye with food and oxygen and contains dark pigment

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21
Q

What happens to the iris in low light and why

A

Dilates to allow as much light to enter as possible

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22
Q

Why do cones have 3 types of light sensitive material

A

Each one detects one of red green and blue

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23
Q

What is the sclera

A

The tough protective layer of the eye

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24
Q

What is the sclera lined with

A

Choroid

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25
Q

What does the dark pigment in the choroid do

A

Reduces reflection meaning blurring of images is reduced

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26
Q

What is the iris

A

A ring of muscle which controls the amount of light which enters the pupil

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27
Q

What happens to the iris in high light and why

A

The iris contracts so the image can be focused on the fovea since outermost rays are removed

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27
Q

What is the back of the eye

A

The retina

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28
Q

What is the retina

A

The area where images are focussed

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29
Q

What is the retina formed of

A

Two types of photodetectors

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30
Q

what is the fovea (yellow spot)

A

a point directly behind the pupil made only of cones

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31
Q

What is spatial resolution

A

What you use to observe small details

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31
Q

Why do you see an object you are looking directly at in great detail

A

The density of cones is greatest on the fovea and decreases as you move away

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31
Q

What is the eye known as (refraction)

A

Optical refracting system

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32
Q

Why cant the fovea be used in low light levels

A

The cones aren’t activated so you notice greater detail in peripherals of your vision

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33
Q

What are the conditions necessary for two details/images to be distinguished

A

At least one rod or cone between the light from each of them

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34
Q

What is the near point

A

The closest distance which the eye can focus

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35
Q

Why is the eye known as an optical refracting system

A

Light refracts through it at several points

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36
Q

What is power of a lense

A

Ability to bend light

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37
Q

What is the far point

A

The furthest distance which the eye can focus

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38
Q

What type of lens does the eye act as

A

A converging lens

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39
Q

What happens if the image is closer than 5m

A

The ciliary muscles will contract causing the lens to be more curved causing its focal length to decrease and power to increase

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40
Q

Power equation for lenses

A

1/focal length

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41
Q

What is myopia

A

Eye is unable to focus on objects which are far away

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41
Q

Magnification equation

A

Size of image / size of object

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42
Q

What causes myopia

A

The image is focussed before the retina, this is caused by the eye being too long, lenses power is too high, far point less than infinity

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43
Q

What corrects myopia

A

Diverging lenses

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44
Q

What is hypermetropia

A

Unable to focus on objects which are close to it

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45
Q

What causes hypermetropia

A

When the power of the lenses is too small or flexibility is reduced

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46
Q

What corrects Hypermetropia

A

Converging lenses

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47
Q

What is astigmatism

A

Where the cornea is not spherical so different planes have different curvatures so light is unevenly focused on the retina

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48
Q

What corrects astigmatism

A

Cylindrical lenses

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49
Q

what does the prescription for astigmatism include

A

power and axis angle of the lens

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50
Q

What is the near point of a healthy eye

A

25cm

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51
Q

What is the focal length of diverging lenses

A

Focal length = -far point

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52
Q

What is the outer ear formed of

A

The pinna and external auditory canal

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53
Q

What is the pinna’s function?

A

To collect sound and direct it down the auditory canal towards the tympanic membrane

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54
Q

Where are wax glands

A

Auditory canal

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55
Q

What do wax glands do

A

Secrete a substance to protect the tympanic membrane and keep it flexible

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56
Q

What is the middle ear formed of

A

Ossicles, Eustachian tube, oval window and round window, tympanic membrane

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57
Q

What are ossicles

A

A group of 3 connected bones - malleus. Incus, stapes

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58
Q

What is the malleus connected to

A

Tympanic membrane

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59
Q

What is the stirrup connected to

A

Oval window

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60
Q

What do the ossicles act as

A
  • A lever which amplify vibrations from tympanic membrane
  • transmit the vibrations to the inner ear
  • reduce amount of energy reflected from inner ear
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61
Q

What is the Eustachian tube connected to

A

The throat

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62
Q

Why is the Eustachian tube connected to the throat

A

So the middle ear can remain at atmospheric pressure

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63
Q

What size is the oval window compared to the tympanic membrane

A

Very small area

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64
Q

What does the fact the oval window is small do

A

Increases of pressure variations

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65
Q

What is the pressure detected by tympanic membrane increased by

A

A factor of 20

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66
Q

What is the inner ear formed of

A

Cochlea, auditory nerve and balance organs

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67
Q

What is the inner ear filled with

A

A fluid called perilymph

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68
Q

What does perilymph do

A

Allows transmission of vibrations

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69
Q

What do the balance organs do

A

Detect orientation and changes in velocity

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70
Q

What is the cochlea

A

A spiral shaped organ filled with a fluid called endolymph

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71
Q

What does the endolymph do

A

Allow vibrations to pass through to the basilar membrane

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72
Q

What is the basilar membrane covered in

A

Hair cells

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73
Q

What does the hair on the basilar membrane do

A

Produce electric signals when caused to vibrate at large amplitudes

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74
Q

What does the oval window transmit vibrations to

A

The cochlea

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75
Q

What are the vibrations passed through the cochlea as

A

Pressure waves

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76
Q

What causes the vibrations on the basilar membrane

A

It has different regions with different natural frequencies. Certain regions experience resonance. These areas vibrate at a large amplitude

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77
Q

What happens as sound waves move through the ear

A

Amplitude decreases but frequency is constant

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78
Q

What is intensity (ear)

A

Amount of energy arriving at the ear per second per unit area

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79
Q

Intensity equation

A

Intensity = Power / Area

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80
Q

What is intensity proportional to

A

Amplitude squared

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81
Q

What is the threshold of heating

A

Minimum intensity of a sound that a human ear can detect (1pWm^-2 at 1 kHz)

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82
Q

What is the frequency of sound perceived as

A

Loudness

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83
Q

What is the threshold of pain

A

Maximum intensity of a sound that a human ear can detect without extreme discomfort 1Wm^-2

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84
Q

What is the range of sound intensities

A

10^12

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85
Q

Why is there not a linear response between sound frequency and perceived loudness

A

The ear automatically decreases the amplification of high intensity sounds

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86
Q

What is the perceived loudness proportional to

A

Change in intensity over initial intensity

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87
Q

What is the change in loudness detected proportional to

A

Log of intensity change

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88
Q

Relative intensity level equation

A

10(log ( intensity / threshold of hearing)

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89
Q

What does the sensitivity of our heating depend on

A

Frequency of sound detected

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90
Q

When is the ear most sensitive at detecting sounds

A

2 kHz to 5 kHz, most sensitive at 3 kHz

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91
Q

What is an equal loudness curves (audiograms)

A

They show the required relative intensity level to detect a sound at a number of frequencies and vary person to person

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92
Q

How is an equal loudness curve produced

A

The loudness of a standard sound is compared to the loudness of a second sound

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93
Q

How is the equal loudness curves sound test actually done

A

By adjusting the intensity of the second sound until it is the exact same perceived loudness as the standard sound

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94
Q

What do equal loudness curves demonstrate

A

Sound intensity required for each frequency to have the same perceived loudness

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95
Q

What is a weighting filter used in

A

What is a weighting filter used in

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96
Q

What do weighting filters do

A

Mimic human ears response to different frequencies of sound

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97
Q

What is sound intensity level measured in

A

decibels

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98
Q

What is hearing loss

A

Where hearing has become less sensitive and needs to experience higher intensity sound waves in order to have the same perceived loudness

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99
Q

What can hearing loss be caused by

A

Deterioration with age or injury

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100
Q

How is an equal loudness curve created

A
  • control frequency of 1000Hz at a specific intensity
  • another sound at different frequency generated. Volume varied till appears to have same loudness. Measure intensity level
  • repeat for different intensity levels
  • change intensity of control and repeate
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101
Q

If intensity is measured in decibels then what is the loudness of the sound given in

A

Phons

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102
Q

What is more affected when hearing is deteriorated with age

A

Higher frequencies

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103
Q

What is more affected when hearing is deteriorated by injury

A

The certain frequency that caused the injury will be affected most

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104
Q

What is more affected when heating is deteriorated with excessive noise

A

4 kHz

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105
Q

How to equal loudness curve detect the cause of hearing loss

A

For deterioration with age, the curve will be higher at all frequencies

for injury it’ll have a peak at a range of most affected

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106
Q

what does both sides of the heart have

A

chamber called atrium and a ventricle which are separated by a valve

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107
Q

how does blood move from atria to ventricles then the body

A

Atria contract, ventricles contract which pushes blood to the body

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108
Q

What are the contractions of the atria and ventricles controlled by

A

Electrical signals produced in the sino atrial node (S-A )

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109
Q

How do the electrical signals move

A

Move across atria, short delay in the atrioventricular ( A-V) node, move across the ventricles

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110
Q

How are the electrical signals in the heat measured

A

An electrocardiograph which forms an ECG showing the change in PD over time

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111
Q

Why is there a pause when electrical signals move through the heart

A

So the atria can finish contracting otherwise the muscles would contract at the same time

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112
Q

What is it called when muscles in the heart are in their relaxed state

A

They are polarised

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113
Q

What are muscles in the heart when they experience a potential

A

Depolarised so contract

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114
Q

Why do muscles become polarised and depolarised

A

Change in ion imbalances in the cells

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115
Q

Labelled ecg

A
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116
Q

what does the p wave correspond to

A

contraction of atria

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117
Q

what does the qrs wave correspond to

A

contraction of ventricles

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118
Q

what does the t wave correspond to

A

relaxation of ventricles

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119
Q

What steps are taken to ensure best connection of electrodes for ecg

A

Electrodes are attached firmly, dead skin cells and hair removed, conducting gel used

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120
Q

What does the electrodes being placed on firmly do

A

Reduce the effect of noise on the reading

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121
Q

What does removing dead skin cells and hair do

A

Contact resistance is decreased

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122
Q

What does conducting gel do

A

Remove air so electrical contact is improved

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123
Q

How is electrical noise reduced

A

Make sure patient is relaxed, shield the electrocardiograph, electrodes made out of a material that doesn’t react, electrodes attached firmly

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124
Q

Why does making sure patient is relaxed reduce electrical nouse

A

To avoid signals produced by other muscle movement

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125
Q

Why does shielding the ecg reduce electrical noise

A

Remove sources of alternating currents

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126
Q

How to find the period of the wave in ecg

A

Measure the R - R distance and use formula

127
Q

Pulse rate per minute formula

A

60 / period in seconds

128
Q

What is the frequency used for medical ultrasounds

A

Between 1MHz and 20MHz

129
Q

What is an ultrasound wave

A

A longitudinal wave with a frequency greater than 20kHz

130
Q

How are ultrasound waves produced

A

PD applied to a piezoelectric material, it will experience mechanical deformation (piezoelectric effect)

131
Q

What is a piezoelectric material (type)

A

Crystal

132
Q

What happens when an alternating potential difference is applied to a piezoelectric material

A

The material will vibrate at the same frequency as the applied pd

133
Q

What happens if the frequency of the alternating pd is equal to the natural frequency of the piezoelectric material

A

There is resonance and the vibrations reach their maximum amplitude

134
Q

What happens when a piezoelectric material is hit by an ultrasound wave

A

It will deform, producing a potential difference which can be amplified and displayed

135
Q

What is used to transmit and detect ultrasound waves

A

Transducer

136
Q

How can you increase the resolution of the transducer

A

It is heavily damped to produce short pulses of ultrasound waves so signals do not overlap

137
Q

What happens when an ultrasound reaches a boundary between two mediums

A

It is reflected

138
Q

What does the amount of reflection of ultrasound waves depend on

A

The difference in acoustic impendance of the two mediums

139
Q

What is acoustic impedance (Z)

A

A measure of how difficult it is for an acoustic wave to travel through a medium

140
Q

What is the intensity reflection coefficient

A

The proportion of the incident ultrasound signal that is reflected when moving between two mediums

141
Q

What happens if the acoustic impedance of both materials is the same

A

Wave is not reflected

142
Q

What happens if the acoustic impedance of the second material is much larger

A

Most of the wave is reflected back

143
Q

What is a coupling medium used for

A

Removing air since large difference in acoustic impedance between air and soft tissue

144
Q

Why is a coupling medium used

A

To prevent the ultrasound being mostly reflected before entering the body

145
Q

What is a coupling medium usually

A

Oil or gel

146
Q

Why is it not possible to image structures of the body which are behind regions of air

A

Due to the large difference in acoustic impedance between air and soft Tissue

147
Q

What is attenuation

A

Ultrasounds moving through the body being absorbed and scattered so amplitude decreases

148
Q

When is the degree of attenuation increased

A

If acoustic impedance of the material the ultrasound is travelling through increases

149
Q

Why must ultrasound waves that have travelled further In the body be amplified

A

They have experienced a high degree of attenuation so signal received is lower

150
Q

What is used to amplify ultrasound waves

A

Swept gain amplifier

151
Q

What are the two types of ultrasound scans used in medicine

A

A-scan (amplitude scan) and B-scan (brightness scan)

152
Q

What occurs A-scans

A
  • short pulse ultra sound transmitted
  • electron beam on cathode ray oscilloscope begins to move at the same time as first pulse emitted
  • pulse reaches boundary between two mediums
  • some of pulse reflected back
  • reflected ultrasound causes pd to be generated in transducer
  • displayed by a CRO by vertical displacement of electron beam
  • Beam moves back
153
Q

What is the horizontal displacement signify in A-scans

A

The time taken for ultrasound to travel through to the boundary and back

154
Q

What are A-scans useful for

A

Distances in the eye and foetus development

155
Q

How is a B-scan formed

A

Moving ultrasound beam across patient

156
Q

When can signals be received in B-scans

A

If transducer is normal to reflected beam

157
Q

What is the amplitude of reflected signal used to determine in B-scans

A

The brightness of a spot on an oscilloscope

158
Q

What is B-scans usd for

A

Find midline structure of brain in foetus, determine position of placenta, genetic testing on foetus

159
Q

Advantages of ultrasounds

A

Non invasive, no known hazards, produces real time image, cheap, portable, no discomfort

160
Q

Disadvantages of ultrasounds

A

Can’t view regions behind bone or lungs, resolution is poor, operator must be skilful and experienced, no specific details about solid masses

161
Q

What is an optical fibre

A

Thin tubes of plastic or glass which light can travel though

162
Q

What is the difference in RI of core and cladding

A

Core is more optically dense allowing TIR to occur

163
Q

What does cladding to in optical fibres

A

Protects core from damage, signal degradation, light escaping core

164
Q

What are the two bundles of optical fibres

A

Coherent and incoherent

165
Q

What are coherent bundles

A

Relative positions of al optical fibres are kept the same

166
Q

What are incoherent bundles

A

Individual fibres are arranged randomly so an image cant be viewed from other end

167
Q

What are incoherent bundles used for

A

Transit light to hard to each areas

168
Q

What does the resolution of the image formed by a coherent bundle of fibres dependant on

A

Size of the fibres and distance between them

169
Q

What is the resolution if fibres have a smaller diameter and are packed tighter

A

High

170
Q

Cost in producing thin fibres

A

Expensive

171
Q

How can images be magnified in optical fibres

A

If the diameter of th fibre gradually increases

172
Q

What are endoscopes

A

Instruments made for viewing inside of the body

173
Q

What are endoscopes formed of

A

Incoherent bundles of fibres which transmit light and coherent bundles with an objective lense for transmitting images

174
Q

What other features may an endoscope have

A

Water channels and co2 channels

175
Q

What is a water channel for in endoscopes

A

To clear lens

176
Q

What is a co2 channel for in endoscopes

A

Make room for endoscope in the body

177
Q

What are tool apertures in the endoscope for

A

Key hole surgery

178
Q

What is keyhole surgery

A

Where instruments are passed through the endoscope to perform surgery without large incisions

179
Q

What are advantages of keyhole surery

A

Minimally invasive, lower risk of infection, quicker recovery time, cheaper

180
Q

What are endoscopes most commonly used for

A

Diagnosis, looking for tumours and taking biopsies

181
Q

What does mri stand for

A

Magnetic resonance scanner

182
Q

What do mris use

A

Superconducting magnet that forms a uniform magnetic field

183
Q

How is the superconducting magnet in mris cooled

A

Liquid helium

184
Q

How does the uniform magnetic field affect protons

A

Protons usually have spin, making them behave like small magnets, initially they move randomly, mri scanner makes the protons align parallel to the magnetic field lines

185
Q

What is precession

A

A wobbling motion as protons become aligned parallel to the magnetic field, also precess around magnetic field lines

186
Q

What are smaller electromagnets used for in mris

A

To form a gradient of magnetic field strength across the patents body by overlapping magnetic fields

187
Q

What are the small electromagnets in mri scanners called

A

Gradient coils

188
Q

What occurs as different areas of the body experience different magnetic field strengths

A

Protons have different precession frequencies so will absorb different frequencies of radiation

189
Q

What are radio frequency coils used for

A

To produce pulses of radio waves causing protons with the same precession frequency as frequency of radiowave to become excited

190
Q

What happens when protons become excited

A

They change their spin state

191
Q

How does protons changing spin state occur

A

In small successive regions of the body

192
Q

Why does protons changing spin state occur in small successive regions of the body

A

Protons in different regions of the body have different precession frequencies due to gradient of the field

193
Q

What happens when protons de-excite

A

Emit EM radiation in form of radio frequency signals at their precession frequency

194
Q

What happens to the em radiation emitted when protons de excite

A

Processed by a computer to form an image as protons positions can be calculated from the frequency of the signal they emit

195
Q

How can the contrast in mri images be adjusted

A

by changing the time between pulses of radio waves

196
Q

What are tissues formed of large molecules best imaged with (pulse )

A

Short time between pulses

197
Q

Advantages of mri

A

Non invasive

no known hazards

high quality images

contrast can be adjusted to examine in more depth

imaging can be real time

198
Q

Disadvantages of mri

A

Expensive

imaging of bones is poor

noisy

takes a while potentially causing discomfort

may feel claustrophobic

cant be used with metal implants

199
Q

What is thermionic emission

A

Where a metal is heated until the free electrons on its surface gain enough energy and are emitted

200
Q

Where is thermionic emission used

A

Production of X-rays

201
Q

How are x rays produced

A
  • electrons emitted from filament by thermionic emission in evacuated tube
  • electrons accelerated through a PD towards anode
  • when collided with anode they decelerate and emit energy as x ray photons
  • this is the bremsstrahlung and forms continuous spectrum of x- rat radiation
  • some electrons collide with orbital electrons of target atoms and ionise atoms
  • this causes electrons from higher levels to move down and occupy gaps releasing energy in x ray photons
202
Q

What is a characteristic spectrum ( x ray radiation)

A

Energy of x rays release depends on difference in energy levels of the anode

203
Q

What is the characteristic spectrum

A

A line spectra as only photons at specific energies can be emitted

204
Q

What is the maximum x ray photons energy equal to

A

Product of charge on electron and the accelerating voltage

205
Q

What is maximum x ray photons energy

A

Value of the kinetic energy of the electrons as they hit the target

206
Q

What happens if you combine the continuous spectrum and the characteristic spectrum

A

You can see the energy spectrum of x rays produced

207
Q

What happens as the accelerating voltage is increased

A

Intensity of emitted x ray photons increases, peak photon energy increases, minimum wavelength decreases, more characteristic lines may appear

208
Q

What is the intensity of emitted x ray photons approximately equal to

A

Accelerating voltage squared

209
Q

In practice, what % of electrons kinetic energy is converted into x ray photons

A

1%

210
Q

What does most of the electrons kinetic energy do in x rays

A

Increases internal energy of metal target, increasing temperature

211
Q

What should the metal target (anode) be made of

A

A good thermal conductor with high SHC and melting point

212
Q

What is the most common metal used for the target

A

Tungsten

213
Q

What prevents overheating of x rays

A

The anode/target is rotated at high speeds so electron beam heats different areas

214
Q

Why does the target have bevelled edges

A

To allow electrons to be focused on a large area whilst source area is small to produce a sharper image

215
Q

What does the rotation of target allow

A

For the target to absorb higher quantities of heat before melting, higher voltage and current can be used so larger x ray outputs

216
Q

What is the intensity of an x ray beam

A

Total energy emitted per second per unit area passing through a surface

217
Q

What methods can be used to control the beam intensity of x rays

A
  • increase anode voltage so electrons gain more ke so photons have higher energies and can ionise deeper within the target
  • increase current so more electrons released per second so more x rays produced.
218
Q

What does increasing current in x rays change

A

Only intensity, range of energies is constant

219
Q

How to increase sharpness of x ray

A

Putting detection plate as close as possible while moving x ray source far away (to patient)

keeping patent still

using lead grid between patient and film to stop scattered x rays from reducing contrast

220
Q

What does the dose of radiation depend on

A

Intensity and exposure time

221
Q

Does longer exposure time increase or decrease resolution in x ray

A

Increase

222
Q

What can exposure time be reduced by but resolution still high

A

Intensifying the image or using a more sensitive detector

223
Q

Do x rays with higher energies from higher or lower contrast

A

Higher

224
Q

What happens to cathode current to decrease intensity of the x ray beam

A

It is decreased

225
Q

How else can the dose of x ray be decreased

A

Bu filtering out low energy x ray photons

226
Q

What is used to filter out low energy x ray photons

A

Aluminium filter

227
Q

What is the attenuation of x rays

A

The reduction of the intensity of the x ray beam when interacting with a substance caused by x rays being absorbed and scattered

227
Q

What does the patient wearing a lead lined apron do

A

protect parts of the body not being x - rayed

228
Q

What does attenuation depend on

A

The thickness, density and proton number of the material being passed through and the energy of the photons

228
Q

How does the intensity of a narrow monoenergetic beam of x rays decrease due to attenuation

A

Exponentially

229
Q

What does linear attenuation coefficient depend on

A

Density of material

229
Q

What is linear attenuation coefficient

A

A measure of how easily a beam of x rays can pass through a material and it describes the rate of energy loss per unit thickness

230
Q

What is the half value thickness

A

Thickness of a material at which the intensity is reduced to half of the initial value

231
Q

What is mass attenuation coefficient

A

Describes rate of energy loss per unit mass

232
Q

Why is the attenuation being dependant on the density of the material important

A

Because the degree of attenuation allows materials to be differentiated

233
Q

How much of the x ray energy is absorbed by the photographic film

A

0.1

234
Q

Half value thickness formula

A

X (1/2) = ln2/ mu

234
Q

Why must exposure time for x rays be long

A

To give a clear image

235
Q

What scans produce 2D images

A

MRI ultrasound and x ray

235
Q

What negatives does a long exposure time have

A

Increases radiation dose and increase chance of blurring due to movement

236
Q

What can x rays be detected digitally by

A

A flat panel dectector

237
Q

What do ct scanners produce

A

High contrast images of a cross section of the body

237
Q

Is ct 2D or 3d

A

3d

238
Q

How does a ct scanner work

A

X ray tube is rotated around patient

emits narrow monochromatic x ray Beam which passes through at different orientations

detectors arranged outside the path of x ray tube detecting intensity of x ray after passing through

detector opposite source beam will register intensity

recorded intensities sent to computer

239
Q

Disadvantages of ct scanners

A

Large dose of ionising radiation,
expensive,
contrast between materials of similar densities is small so images may be distorted, patient must be completely still

239
Q

what is a scintillator

A

a material that emits light photons when struck by high energy photons

240
Q

Advantages of ct scanners

A

Produce high quality images of complicated bone fractures and organs,

non invasive, higher quality than ultrasound,

full cross sectional area image is formed

241
Q

how can x rays be detected digitally

A

with a flat panel detector (FTP)

242
Q

how do FTP’s work

A

using a scintillator - energy of light photons emitted by scintillator is proportional to energy of x-ray photons it absorbs. photodiodes bound to scintillator absorb light photons and convert them to electric charge

243
Q

what does each photodiode act as in FTP

A

a single pixel

244
Q

what determines the resolution of the FTP

A

amount of photodiodes

245
Q

how is the charge on each photodiode found

A

via electronic scanning and using Thin film transistors, converted to digital signals which are sent to an image processor to form digital image

246
Q

advantages of FTP

A
  • more sensitive so low exposure time needed
  • faster
  • stored digitally so easily accessed
  • sent to professionals for evaluation instantly
  • higher resolution and less distortion
  • small and light weight
  • easily transported
247
Q

what can be used to reduce exposure time required for photographic detection in x rays

A

intensifying screen

247
Q

how can contrast be enhanced in x rays

A

place a contrast medium in area examined - made of a material with high proton number

248
Q

what should the proton number difference be between the material examined and surroundings be to have a clear x ray

A

large

249
Q

what is a barium meal used for and how

A

contrast medium for gastrointestinal tract

patient swallows barium meal, barium lines tract so x ray can see it

250
Q

what does x ray opaque mean

A

absorb more x ray radiation than surroundings so appear opaque white on x ray images ( contrast media are this)

251
Q

why do the crystals in intensifying screens fluoresce

A

they absorb x ray photons and their electrons are excited, they then de excite emitting visible light photons

251
Q

what are intensifying screens formed of

A

crystals which fluoresce when exposed to x rays

252
Q

where is the film usually placed in intensifying screens and why

A

between two intensifying screens in close contact so maximises light captured by film to produce sharp image

253
Q

how is exposure time reduced by having film between two intensifying screens

A

film more sensitive to visible light than x ray so exposure time and dose reduced

254
Q

what forms moving x ray images

A

fluoroscopic image intensification - fluorescent screen used instead of film

254
Q

what is the intensity of the output of light from the initial collision with fluorescent screen

A

low - image intensifier must be used

254
Q

process to form moving X - ray images

A
  • x ray photons hit fluorescent screen and emit light
  • light photons hit photocathode which releases electrons into vacuum tube
  • electrons accelerated by PD increasing KE
  • electrons focused using electrodes to preserve relative positions
  • collide with fluorescent viewing screen
  • image viewed directly or recorded for further evaluatio
255
Q

what Is the side of fluorescent viewing screen closest to photocathode covered in

A

thin layer of aluminium

255
Q

what are focussing electrodes also used for

A

to decrease size of image to further increase brightness therefore lower intensity x rays used

256
Q

why is the brightness of viewing screen much higher than light emitted by initial screen

A

kinetic energy has been increased of the electrons

257
Q

what is a gamma camera used for

A

a device to detect gamma rays and form 2d image

258
Q

why is one side of the fluorescent viewing screen covered in aluminium

A

allows electrons to pass thought but prevents light produced at viewing screen moving back to photocathode.

258
Q

what is the dose size of fluoroscopic image intensification and why

A

relatively high as constant beam of x ray

259
Q

why do gamma photons first pass through a lead collimator before being detected

A

only allows photons parallel to it to pass though allowing a sharper image to be formed

260
Q

how can a gamma camera form a 3d image

A

if multiple gamma cameras are used

260
Q

what happens after photons pass though lead collimator

A

collide with scintillator which causes visible light photons to be released, detected by photomultiplier tubes which convert to electric signals and sent to computer for processing

261
Q

what is a photomultiplier tube

A

evacuated tube containing a photocathode and electrodes called dynodes at increasingly higher positive potentials along tube

262
Q

what happens when light is incident on photocathode

A

electrons released, accelerated towards first dynode, when collides with first dynode more electrons released this is secondary emission. process repeates until num of electrons increased significantly. electrons reach end of photomultiplier tube collide with anode producing electrical signal

263
Q

what does a gamma camera use

A

photomultiplier tube

264
Q

What can gamma emitting radioisotopes be used as in medicine

A

Tracers to investigate specific regions of the body

265
Q

What can the gamma radiation emitted by the tracers be detected with

A

A gamma camera

265
Q

How do medical tracers work

A

Bind to substances

266
Q

What does the path of the medical tracer depend on

A

Depends on the substance the tracer binds to

267
Q

What are the 3 medical tracers

A

Technetium 99m. Iodine 131, indium 111

268
Q

What does technetium 99m emit

A

Pure gamma

269
Q

Physical Half life of technetium 99m

A

6 hours

270
Q

How/where is technetium 99m prepared

A

On site at the hospital

270
Q

What does iodine 131 emit

A

Beta and gamma

270
Q

What does the 6 hour half life of technetium 99m mean in regards to exposure

A

Short enough to limit exposure but long enough for tests to be carried out

271
Q

What outweighs the potential hazard of iodine 131

A

Naturally absorbed by thyroid

271
Q

What is iodine 131 naturally absorbed by

A

The thyroid

272
Q

Why is it not ideal that iodine emits beta and gamma

A

Beta is potentially hazardous

273
Q

What does indium 111 emit

A

Pure gamma

274
Q

Physical half life of iodine 131

A

8 days

275
Q

Physical half life of indium 111

A

2.8 days

275
Q

What does pet scan stand for

A

Positron emission tomography

276
Q

What is indium 111 used for

A

To label antibodies and blood cells to detect infections

277
Q

Procedure for pet scans

A

Inject with positron emitting radionuclide which attaches to substance under investigation, patient left for an hour to allow radionuclide to move to region of interest, positrons annihilate with electrons in body releasing 2 gamma photons moving in opposite directions which is detected. Image of the radioactivity in region is formed

277
Q

What type of images does pet scans form

A

3d and cross sections

278
Q

Why does image formed depend on metabolic activity

A

Cells with high metabolism break down more radionuclide so more annihilation would occur

279
Q

What does the image formed in pet scanners depend on

A

metabolic activity of the cells

280
Q

Advantages to pet scanners

A

Metabolic activity measured, tumours detected, can say if tumours spreading or malignant, brain activity investigated

281
Q

How is technetium 99m produced in hospitals

A

Molybdenum - technetium generator

282
Q

Disadvantages of pet scanners

A

Ionising radiation, take a long time, have to stay still which is uncomfortable, claustrophobic, scanners themselves are expensive and large so patient may need to travel to specific hospital

283
Q

Half life of molybdenum

A

66 hours

283
Q

How do molybdenum - technetium generators work

A

Molybdenum combined with aluminium oxide, decays to technetium which doesn’t bond to aluminium oxide, technetium isolated by washing out with saline solution

284
Q

Why do you need to limit exposure of radiotherapy to healthy cells

A

The x rays kill cancerous and healthy cells

284
Q

What are high energy x rays used in

A

Radiotherapy to kill or contain malignant tumours

285
Q

Why does a rotating beam limit exposure to healthy cells

A

Rotates with tumour at centre of rotation so limits exposure of healthy cells

285
Q

How is exposure to healthy cells limited in radiotherapy

A
  • scan locate tumours accurately
  • correct energy x ray
  • shielding healthy tissues
  • narrow beam
  • passed through collimated so photons parallel
  • multiple beams used that overlap
  • rotating beam
286
Q

Why does correct energy x rays reduce exposure to healthy cells

A

Maximise harm to tumour, minimise harm to healthy

287
Q

Why does multiple beams limit exposure to healthy cells

A

Overlap at the tumour meaning experiences most damage there

288
Q

What do radioactive implants emit

A

Beta

289
Q

Why are healthy cells not damaged further from site with radioactive implants

A

Beta looses most of its energy before reaching the cells

289
Q

Where are radioactive implants placed

A

Next to or inside of a tumour

290
Q

What should the half life of radioactive implants be

A

Long so treat tumour effectively

291
Q

What is used as a radioactive implant

A

Iridium 192

291
Q

Half life of iridium 192

A

74 days

292
Q

Pros of radioactive implants

A

Deliver high dose of radiation to small areas

293
Q

what does the rate of decay of radionuclides within the body depend on

A

physical decay process and rate of excretion

293
Q

what will the body do to the substances that radionuclides are attached to

A

metabolise

294
Q

what is physical half life

A

time taken for number of nuclei to halve

295
Q

what is biological Half life

A

time taken for half of a sample of material to be excreted by biological means

296
Q

what is the effective half life

A

time taken for the date of decay of the initial sample within the body to halve

297
Q
A

physics (4 decks)

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