medical physics Flashcards
1
Q
What do lenses do
A
Refract light in order to change its direction
2
Q
What is a converging (convex) lense
A
- curved outwards on both sizes
- causes parallel light rays to converge at a point
3
Q
What type of images can converging lenses form
A
Real and virtual
4
Q
What type of image can be projected onto a screen
A
Real images
5
Q
What is a real image
A
Image formed when the object is further than the focal length
6
Q
What is a virtual image
A
Image formed when the object is closer than the focal length
7
Q
What is a diverging (concave) lense
A
- curved inwards on both sides
- cause parallel light rays to diverge
- always a virtual image
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
9
Q
What is principal focus in diverging lenses
A
The point from which the light rays appear to come from
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
11
Q
What is focal length
A
The distance from the centre of the lens to the principal focus
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
12
Q
What is accomodation
A
The ability of a lens to change its focal length to focus on an object
13
Q
What can the eye focus on when the ciliary muscles are relaxed
A
Objects far away - 5m to infinity
14
Q
What are the two types of photodetectors
A
Rods and cones
15
Q
What light levels are rods activated by
A
Low levels
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
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
17
Q
What is rhodospin
A
A substance that is destroyed by light
18
Q
What light levels are cones activated by
A
High light levels
19
Q
Why do cones produce a greater image detail than rods
A
One cone is connected to one nerve fibre
20
Q
How many types of light sensitive material do cones have
A
3
20
Q
What is the choroid
A
A tissue which supplies the eye with food and oxygen and contains dark pigment
21
Q
What happens to the iris in low light and why
A
Dilates to allow as much light to enter as possible
22
Why do cones have 3 types of light sensitive material
Each one detects one of red green and blue
23
What is the sclera
The tough protective layer of the eye
24
What is the sclera lined with
Choroid
25
What does the dark pigment in the choroid do
Reduces reflection meaning blurring of images is reduced
26
What is the iris
A ring of muscle which controls the amount of light which enters the pupil
27
What happens to the iris in high light and why
The iris contracts so the image can be focused on the fovea since outermost rays are removed
27
What is the back of the eye
The retina
28
What is the retina
The area where images are focussed
29
What is the retina formed of
Two types of photodetectors
30
what is the fovea (yellow spot)
a point directly behind the pupil made only of cones
31
What is spatial resolution
What you use to observe small details
31
Why do you see an object you are looking directly at in great detail
The density of cones is greatest on the fovea and decreases as you move away
31
What is the eye known as (refraction)
Optical refracting system
32
Why cant the fovea be used in low light levels
The cones aren’t activated so you notice greater detail in peripherals of your vision
33
What are the conditions necessary for two details/images to be distinguished
At least one rod or cone between the light from each of them
34
What is the near point
The closest distance which the eye can focus
35
Why is the eye known as an optical refracting system
Light refracts through it at several points
36
What is power of a lense
Ability to bend light
37
What is the far point
The furthest distance which the eye can focus
38
What type of lens does the eye act as
A converging lens
39
What happens if the image is closer than 5m
The ciliary muscles will contract causing the lens to be more curved causing its focal length to decrease and power to increase
40
Power equation for lenses
1/focal length
41
What is myopia
Eye is unable to focus on objects which are far away
41
Magnification equation
Size of image / size of object
42
What causes myopia
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
43
What corrects myopia
Diverging lenses
44
What is hypermetropia
Unable to focus on objects which are close to it
45
What causes hypermetropia
When the power of the lenses is too small or flexibility is reduced
46
What corrects Hypermetropia
Converging lenses
47
What is astigmatism
Where the cornea is not spherical so different planes have different curvatures so light is unevenly focused on the retina
48
What corrects astigmatism
Cylindrical lenses
49
what does the prescription for astigmatism include
power and axis angle of the lens
50
What is the near point of a healthy eye
25cm
51
What is the focal length of diverging lenses
Focal length = -far point
52
What is the outer ear formed of
The pinna and external auditory canal
53
What is the pinna's function?
To collect sound and direct it down the auditory canal towards the tympanic membrane
54
Where are wax glands
Auditory canal
55
What do wax glands do
Secrete a substance to protect the tympanic membrane and keep it flexible
56
What is the middle ear formed of
Ossicles, Eustachian tube, oval window and round window, tympanic membrane
57
What are ossicles
A group of 3 connected bones - malleus. Incus, stapes
58
What is the malleus connected to
Tympanic membrane
59
What is the stirrup connected to
Oval window
60
What do the ossicles act as
- A lever which amplify vibrations from tympanic membrane
- transmit the vibrations to the inner ear
- reduce amount of energy reflected from inner ear
61
What is the Eustachian tube connected to
The throat
62
Why is the Eustachian tube connected to the throat
So the middle ear can remain at atmospheric pressure
63
What size is the oval window compared to the tympanic membrane
Very small area
64
What does the fact the oval window is small do
Increases of pressure variations
65
What is the pressure detected by tympanic membrane increased by
A factor of 20
66
What is the inner ear formed of
Cochlea, auditory nerve and balance organs
67
What is the inner ear filled with
A fluid called perilymph
68
What does perilymph do
Allows transmission of vibrations
69
What do the balance organs do
Detect orientation and changes in velocity
70
What is the cochlea
A spiral shaped organ filled with a fluid called endolymph
71
What does the endolymph do
Allow vibrations to pass through to the basilar membrane
72
What is the basilar membrane covered in
Hair cells
73
What does the hair on the basilar membrane do
Produce electric signals when caused to vibrate at large amplitudes
74
What does the oval window transmit vibrations to
The cochlea
75
What are the vibrations passed through the cochlea as
Pressure waves
76
What causes the vibrations on the basilar membrane
It has different regions with different natural frequencies. Certain regions experience resonance. These areas vibrate at a large amplitude
77
What happens as sound waves move through the ear
Amplitude decreases but frequency is constant
78
What is intensity (ear)
Amount of energy arriving at the ear per second per unit area
79
Intensity equation
Intensity = Power / Area
80
What is intensity proportional to
Amplitude squared
81
What is the threshold of heating
Minimum intensity of a sound that a human ear can detect (1pWm^-2 at 1 kHz)
82
What is the frequency of sound perceived as
Loudness
83
What is the threshold of pain
Maximum intensity of a sound that a human ear can detect without extreme discomfort 1Wm^-2
84
What is the range of sound intensities
10^12
85
Why is there not a linear response between sound frequency and perceived loudness
The ear automatically decreases the amplification of high intensity sounds
86
What is the perceived loudness proportional to
Change in intensity over initial intensity
87
What is the change in loudness detected proportional to
Log of intensity change
88
Relative intensity level equation
10(log ( intensity / threshold of hearing)
89
What does the sensitivity of our heating depend on
Frequency of sound detected
90
When is the ear most sensitive at detecting sounds
2 kHz to 5 kHz, most sensitive at 3 kHz
91
What is an equal loudness curves (audiograms)
They show the required relative intensity level to detect a sound at a number of frequencies and vary person to person
92
How is an equal loudness curve produced
The loudness of a standard sound is compared to the loudness of a second sound
93
How is the equal loudness curves sound test actually done
By adjusting the intensity of the second sound until it is the exact same perceived loudness as the standard sound
94
What do equal loudness curves demonstrate
Sound intensity required for each frequency to have the same perceived loudness
95
What is a weighting filter used in
What is a weighting filter used in
96
What do weighting filters do
Mimic human ears response to different frequencies of sound
97
What is sound intensity level measured in
decibels
98
What is hearing loss
Where hearing has become less sensitive and needs to experience higher intensity sound waves in order to have the same perceived loudness
99
What can hearing loss be caused by
Deterioration with age or injury
100
How is an equal loudness curve created
- 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
101
If intensity is measured in decibels then what is the loudness of the sound given in
Phons
102
What is more affected when hearing is deteriorated with age
Higher frequencies
103
What is more affected when hearing is deteriorated by injury
The certain frequency that caused the injury will be affected most
104
What is more affected when heating is deteriorated with excessive noise
4 kHz
105
How to equal loudness curve detect the cause of hearing loss
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
106
what does both sides of the heart have
chamber called atrium and a ventricle which are separated by a valve
107
how does blood move from atria to ventricles then the body
Atria contract, ventricles contract which pushes blood to the body
108
What are the contractions of the atria and ventricles controlled by
Electrical signals produced in the sino atrial node (S-A )
109
How do the electrical signals move
Move across atria, short delay in the atrioventricular ( A-V) node, move across the ventricles
110
How are the electrical signals in the heat measured
An electrocardiograph which forms an ECG showing the change in PD over time
111
Why is there a pause when electrical signals move through the heart
So the atria can finish contracting otherwise the muscles would contract at the same time
112
What is it called when muscles in the heart are in their relaxed state
They are polarised
113
What are muscles in the heart when they experience a potential
Depolarised so contract
114
Why do muscles become polarised and depolarised
Change in ion imbalances in the cells
115
Labelled ecg
116
what does the p wave correspond to
contraction of atria
117
what does the qrs wave correspond to
contraction of ventricles
118
what does the t wave correspond to
relaxation of ventricles
119
What steps are taken to ensure best connection of electrodes for ecg
Electrodes are attached firmly, dead skin cells and hair removed, conducting gel used
120
What does the electrodes being placed on firmly do
Reduce the effect of noise on the reading
121
What does removing dead skin cells and hair do
Contact resistance is decreased
122
What does conducting gel do
Remove air so electrical contact is improved
123
How is electrical noise reduced
Make sure patient is relaxed, shield the electrocardiograph, electrodes made out of a material that doesn’t react, electrodes attached firmly
124
Why does making sure patient is relaxed reduce electrical nouse
To avoid signals produced by other muscle movement
125
Why does shielding the ecg reduce electrical noise
Remove sources of alternating currents
126
How to find the period of the wave in ecg
Measure the R - R distance and use formula
127
Pulse rate per minute formula
60 / period in seconds
128
What is the frequency used for medical ultrasounds
Between 1MHz and 20MHz
129
What is an ultrasound wave
A longitudinal wave with a frequency greater than 20kHz
130
How are ultrasound waves produced
PD applied to a piezoelectric material, it will experience mechanical deformation (piezoelectric effect)
131
What is a piezoelectric material (type)
Crystal
132
What happens when an alternating potential difference is applied to a piezoelectric material
The material will vibrate at the same frequency as the applied pd
133
What happens if the frequency of the alternating pd is equal to the natural frequency of the piezoelectric material
There is resonance and the vibrations reach their maximum amplitude
134
What happens when a piezoelectric material is hit by an ultrasound wave
It will deform, producing a potential difference which can be amplified and displayed
135
What is used to transmit and detect ultrasound waves
Transducer
136
How can you increase the resolution of the transducer
It is heavily damped to produce short pulses of ultrasound waves so signals do not overlap
137
What happens when an ultrasound reaches a boundary between two mediums
It is reflected
138
What does the amount of reflection of ultrasound waves depend on
The difference in acoustic impendance of the two mediums
139
What is acoustic impedance (Z)
A measure of how difficult it is for an acoustic wave to travel through a medium
140
What is the intensity reflection coefficient
The proportion of the incident ultrasound signal that is reflected when moving between two mediums
141
What happens if the acoustic impedance of both materials is the same
Wave is not reflected
142
What happens if the acoustic impedance of the second material is much larger
Most of the wave is reflected back
143
What is a coupling medium used for
Removing air since large difference in acoustic impedance between air and soft tissue
144
Why is a coupling medium used
To prevent the ultrasound being mostly reflected before entering the body
145
What is a coupling medium usually
Oil or gel
146
Why is it not possible to image structures of the body which are behind regions of air
Due to the large difference in acoustic impedance between air and soft Tissue
147
What is attenuation
Ultrasounds moving through the body being absorbed and scattered so amplitude decreases
148
When is the degree of attenuation increased
If acoustic impedance of the material the ultrasound is travelling through increases
149
Why must ultrasound waves that have travelled further In the body be amplified
They have experienced a high degree of attenuation so signal received is lower
150
What is used to amplify ultrasound waves
Swept gain amplifier
151
What are the two types of ultrasound scans used in medicine
A-scan (amplitude scan) and B-scan (brightness scan)
152
What occurs A-scans
- 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
What is the horizontal displacement signify in A-scans
The time taken for ultrasound to travel through to the boundary and back
154
What are A-scans useful for
Distances in the eye and foetus development
155
How is a B-scan formed
Moving ultrasound beam across patient
156
When can signals be received in B-scans
If transducer is normal to reflected beam
157
What is the amplitude of reflected signal used to determine in B-scans
The brightness of a spot on an oscilloscope
158
What is B-scans usd for
Find midline structure of brain in foetus, determine position of placenta, genetic testing on foetus
159
Advantages of ultrasounds
Non invasive, no known hazards, produces real time image, cheap, portable, no discomfort
160
Disadvantages of ultrasounds
Can’t view regions behind bone or lungs, resolution is poor, operator must be skilful and experienced, no specific details about solid masses
161
What is an optical fibre
Thin tubes of plastic or glass which light can travel though
162
What is the difference in RI of core and cladding
Core is more optically dense allowing TIR to occur
163
What does cladding to in optical fibres
Protects core from damage, signal degradation, light escaping core
164
What are the two bundles of optical fibres
Coherent and incoherent
165
What are coherent bundles
Relative positions of al optical fibres are kept the same
166
What are incoherent bundles
Individual fibres are arranged randomly so an image cant be viewed from other end
167
What are incoherent bundles used for
Transit light to hard to each areas
168
What does the resolution of the image formed by a coherent bundle of fibres dependant on
Size of the fibres and distance between them
169
What is the resolution if fibres have a smaller diameter and are packed tighter
High
170
Cost in producing thin fibres
Expensive
171
How can images be magnified in optical fibres
If the diameter of th fibre gradually increases
172
What are endoscopes
Instruments made for viewing inside of the body
173
What are endoscopes formed of
Incoherent bundles of fibres which transmit light and coherent bundles with an objective lense for transmitting images
174
What other features may an endoscope have
Water channels and co2 channels
175
What is a water channel for in endoscopes
To clear lens
176
What is a co2 channel for in endoscopes
Make room for endoscope in the body
177
What are tool apertures in the endoscope for
Key hole surgery
178
What is keyhole surgery
Where instruments are passed through the endoscope to perform surgery without large incisions
179
What are advantages of keyhole surery
Minimally invasive, lower risk of infection, quicker recovery time, cheaper
180
What are endoscopes most commonly used for
Diagnosis, looking for tumours and taking biopsies
181
What does mri stand for
Magnetic resonance scanner
182
What do mris use
Superconducting magnet that forms a uniform magnetic field
183
How is the superconducting magnet in mris cooled
Liquid helium
184
How does the uniform magnetic field affect protons
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
What is precession
A wobbling motion as protons become aligned parallel to the magnetic field, also precess around magnetic field lines
186
What are smaller electromagnets used for in mris
To form a gradient of magnetic field strength across the patents body by overlapping magnetic fields
187
What are the small electromagnets in mri scanners called
Gradient coils
188
What occurs as different areas of the body experience different magnetic field strengths
Protons have different precession frequencies so will absorb different frequencies of radiation
189
What are radio frequency coils used for
To produce pulses of radio waves causing protons with the same precession frequency as frequency of radiowave to become excited
190
What happens when protons become excited
They change their spin state
191
How does protons changing spin state occur
In small successive regions of the body
192
Why does protons changing spin state occur in small successive regions of the body
Protons in different regions of the body have different precession frequencies due to gradient of the field
193
What happens when protons de-excite
Emit EM radiation in form of radio frequency signals at their precession frequency
194
What happens to the em radiation emitted when protons de excite
Processed by a computer to form an image as protons positions can be calculated from the frequency of the signal they emit
195
How can the contrast in mri images be adjusted
by changing the time between pulses of radio waves
196
What are tissues formed of large molecules best imaged with (pulse )
Short time between pulses
197
Advantages of mri
Non invasive
no known hazards
high quality images
contrast can be adjusted to examine in more depth
imaging can be real time
198
Disadvantages of mri
Expensive
imaging of bones is poor
noisy
takes a while potentially causing discomfort
may feel claustrophobic
cant be used with metal implants
199
What is thermionic emission
Where a metal is heated until the free electrons on its surface gain enough energy and are emitted
200
Where is thermionic emission used
Production of X-rays
201
How are x rays produced
- 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
What is a characteristic spectrum ( x ray radiation)
Energy of x rays release depends on difference in energy levels of the anode
203
What is the characteristic spectrum
A line spectra as only photons at specific energies can be emitted
204
What is the maximum x ray photons energy equal to
Product of charge on electron and the accelerating voltage
205
What is maximum x ray photons energy
Value of the kinetic energy of the electrons as they hit the target
206
What happens if you combine the continuous spectrum and the characteristic spectrum
You can see the energy spectrum of x rays produced
207
What happens as the accelerating voltage is increased
Intensity of emitted x ray photons increases, peak photon energy increases, minimum wavelength decreases, more characteristic lines may appear
208
What is the intensity of emitted x ray photons approximately equal to
Accelerating voltage squared
209
In practice, what % of electrons kinetic energy is converted into x ray photons
1%
210
What does most of the electrons kinetic energy do in x rays
Increases internal energy of metal target, increasing temperature
211
What should the metal target (anode) be made of
A good thermal conductor with high SHC and melting point
212
What is the most common metal used for the target
Tungsten
213
What prevents overheating of x rays
The anode/target is rotated at high speeds so electron beam heats different areas
214
Why does the target have bevelled edges
To allow electrons to be focused on a large area whilst source area is small to produce a sharper image
215
What does the rotation of target allow
For the target to absorb higher quantities of heat before melting, higher voltage and current can be used so larger x ray outputs
216
What is the intensity of an x ray beam
Total energy emitted per second per unit area passing through a surface
217
What methods can be used to control the beam intensity of x rays
- 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
What does increasing current in x rays change
Only intensity, range of energies is constant
219
How to increase sharpness of x ray
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
What does the dose of radiation depend on
Intensity and exposure time
221
Does longer exposure time increase or decrease resolution in x ray
Increase
222
What can exposure time be reduced by but resolution still high
Intensifying the image or using a more sensitive detector
223
Do x rays with higher energies from higher or lower contrast
Higher
224
What happens to cathode current to decrease intensity of the x ray beam
It is decreased
225
How else can the dose of x ray be decreased
Bu filtering out low energy x ray photons
226
What is used to filter out low energy x ray photons
Aluminium filter
227
What is the attenuation of x rays
The reduction of the intensity of the x ray beam when interacting with a substance caused by x rays being absorbed and scattered
227
What does the patient wearing a lead lined apron do
protect parts of the body not being x - rayed
228
What does attenuation depend on
The thickness, density and proton number of the material being passed through and the energy of the photons
228
How does the intensity of a narrow monoenergetic beam of x rays decrease due to attenuation
Exponentially
229
What does linear attenuation coefficient depend on
Density of material
229
What is linear attenuation coefficient
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
What is the half value thickness
Thickness of a material at which the intensity is reduced to half of the initial value
231
What is mass attenuation coefficient
Describes rate of energy loss per unit mass
232
Why is the attenuation being dependant on the density of the material important
Because the degree of attenuation allows materials to be differentiated
233
How much of the x ray energy is absorbed by the photographic film
0.1
234
Half value thickness formula
X (1/2) = ln2/ mu
234
Why must exposure time for x rays be long
To give a clear image
235
What scans produce 2D images
MRI ultrasound and x ray
235
What negatives does a long exposure time have
Increases radiation dose and increase chance of blurring due to movement
236
What can x rays be detected digitally by
A flat panel dectector
237
What do ct scanners produce
High contrast images of a cross section of the body
237
Is ct 2D or 3d
3d
238
How does a ct scanner work
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
Disadvantages of ct scanners
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
what is a scintillator
a material that emits light photons when struck by high energy photons
240
Advantages of ct scanners
Produce high quality images of complicated bone fractures and organs,
non invasive, higher quality than ultrasound,
full cross sectional area image is formed
241
how can x rays be detected digitally
with a flat panel detector (FTP)
242
how do FTP's work
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
what does each photodiode act as in FTP
a single pixel
244
what determines the resolution of the FTP
amount of photodiodes
245
how is the charge on each photodiode found
via electronic scanning and using Thin film transistors, converted to digital signals which are sent to an image processor to form digital image
246
advantages of FTP
- 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
what can be used to reduce exposure time required for photographic detection in x rays
intensifying screen
247
how can contrast be enhanced in x rays
place a contrast medium in area examined - made of a material with high proton number
248
what should the proton number difference be between the material examined and surroundings be to have a clear x ray
large
249
what is a barium meal used for and how
contrast medium for gastrointestinal tract
patient swallows barium meal, barium lines tract so x ray can see it
250
what does x ray opaque mean
absorb more x ray radiation than surroundings so appear opaque white on x ray images ( contrast media are this)
251
why do the crystals in intensifying screens fluoresce
they absorb x ray photons and their electrons are excited, they then de excite emitting visible light photons
251
what are intensifying screens formed of
crystals which fluoresce when exposed to x rays
252
where is the film usually placed in intensifying screens and why
between two intensifying screens in close contact so maximises light captured by film to produce sharp image
253
how is exposure time reduced by having film between two intensifying screens
film more sensitive to visible light than x ray so exposure time and dose reduced
254
what forms moving x ray images
fluoroscopic image intensification - fluorescent screen used instead of film
254
what is the intensity of the output of light from the initial collision with fluorescent screen
low - image intensifier must be used
254
process to form moving X - ray images
- 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
what Is the side of fluorescent viewing screen closest to photocathode covered in
thin layer of aluminium
255
what are focussing electrodes also used for
to decrease size of image to further increase brightness therefore lower intensity x rays used
256
why is the brightness of viewing screen much higher than light emitted by initial screen
kinetic energy has been increased of the electrons
257
what is a gamma camera used for
a device to detect gamma rays and form 2d image
258
why is one side of the fluorescent viewing screen covered in aluminium
allows electrons to pass thought but prevents light produced at viewing screen moving back to photocathode.
258
what is the dose size of fluoroscopic image intensification and why
relatively high as constant beam of x ray
259
why do gamma photons first pass through a lead collimator before being detected
only allows photons parallel to it to pass though allowing a sharper image to be formed
260
how can a gamma camera form a 3d image
if multiple gamma cameras are used
260
what happens after photons pass though lead collimator
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
what is a photomultiplier tube
evacuated tube containing a photocathode and electrodes called dynodes at increasingly higher positive potentials along tube
262
what happens when light is incident on photocathode
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
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what does a gamma camera use
photomultiplier tube
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What can gamma emitting radioisotopes be used as in medicine
Tracers to investigate specific regions of the body
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What can the gamma radiation emitted by the tracers be detected with
A gamma camera
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How do medical tracers work
Bind to substances
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What does the path of the medical tracer depend on
Depends on the substance the tracer binds to
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What are the 3 medical tracers
Technetium 99m. Iodine 131, indium 111
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What does technetium 99m emit
Pure gamma
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Physical Half life of technetium 99m
6 hours
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How/where is technetium 99m prepared
On site at the hospital
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What does iodine 131 emit
Beta and gamma
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What does the 6 hour half life of technetium 99m mean in regards to exposure
Short enough to limit exposure but long enough for tests to be carried out
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What outweighs the potential hazard of iodine 131
Naturally absorbed by thyroid
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What is iodine 131 naturally absorbed by
The thyroid
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Why is it not ideal that iodine emits beta and gamma
Beta is potentially hazardous
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What does indium 111 emit
Pure gamma
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Physical half life of iodine 131
8 days
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Physical half life of indium 111
2.8 days
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What does pet scan stand for
Positron emission tomography
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What is indium 111 used for
To label antibodies and blood cells to detect infections
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Procedure for pet scans
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
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What type of images does pet scans form
3d and cross sections
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Why does image formed depend on metabolic activity
Cells with high metabolism break down more radionuclide so more annihilation would occur
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What does the image formed in pet scanners depend on
metabolic activity of the cells
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Advantages to pet scanners
Metabolic activity measured, tumours detected, can say if tumours spreading or malignant, brain activity investigated
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How is technetium 99m produced in hospitals
Molybdenum - technetium generator
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Disadvantages of pet scanners
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
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Half life of molybdenum
66 hours
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How do molybdenum - technetium generators work
Molybdenum combined with aluminium oxide, decays to technetium which doesn’t bond to aluminium oxide, technetium isolated by washing out with saline solution
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Why do you need to limit exposure of radiotherapy to healthy cells
The x rays kill cancerous and healthy cells
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What are high energy x rays used in
Radiotherapy to kill or contain malignant tumours
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Why does a rotating beam limit exposure to healthy cells
Rotates with tumour at centre of rotation so limits exposure of healthy cells
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How is exposure to healthy cells limited in radiotherapy
- 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
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Why does correct energy x rays reduce exposure to healthy cells
Maximise harm to tumour, minimise harm to healthy
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Why does multiple beams limit exposure to healthy cells
Overlap at the tumour meaning experiences most damage there
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What do radioactive implants emit
Beta
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Why are healthy cells not damaged further from site with radioactive implants
Beta looses most of its energy before reaching the cells
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Where are radioactive implants placed
Next to or inside of a tumour
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What should the half life of radioactive implants be
Long so treat tumour effectively
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What is used as a radioactive implant
Iridium 192
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Half life of iridium 192
74 days
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Pros of radioactive implants
Deliver high dose of radiation to small areas
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what does the rate of decay of radionuclides within the body depend on
physical decay process and rate of excretion
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what will the body do to the substances that radionuclides are attached to
metabolise
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what is physical half life
time taken for number of nuclei to halve
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what is biological Half life
time taken for half of a sample of material to be excreted by biological means
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what is the effective half life
time taken for the date of decay of the initial sample within the body to halve
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