Option - Medical Physics Flashcards
1
Q
what are x-rays?
A
high energy electromagnetic waves
2
Q
what can x-rays do?
A
ionise
3
Q
what can x-rays do since they can ionise?
A
they can cause mutations and cancer
4
Q
how are x-rays produced?
A
by descelerating electrons and when electrons drop to inner energy levels of atoms
5
Q
how do x-rays tend to travel?
A
in straight lines
6
Q
what are x-rays absorbed by?
A
dense matter or high atomic number elemtns
7
Q
what do x-rays tend to pass through?
A
soft tissue
8
Q
what can x-rays be detected by?
A
photographic plates/film
9
Q
how are the majority of x-rays produced and how is this done?
A
by descelerating electrons, which require accelerating in order to attain high speeds
10
Q
how are electrons accelerated in order to attain high speeds?
A
by using pds of around 50kV
11
Q
explain in detail how the x-ray machine/x-ray tube works
A
the hot wire on the left of the diagram is heated using a small pd across the wire (using tungsten as the hot wire means that temperatures of 3400 degrees can be achieved)
this is such a high tempertaures that a small % of the electrons gain eenough energy to escape the hot wire
these electrons are then accelerated using a very large pd of around 50kV between the hot wire and the target (-ve electrons are attractived to the +ve metal target)
the electrons descelerate when they hit the metal target
descelerating electrons produce x-rays
fast electrons also knock out inner electrons from the metal target (atoms are ionised)
electrons from higher energy levels drop down to the gaps left (very big jump if electrons were knocked from lower levels in the atom = big photon)
these falling electrons produce x-rays of very specific wavelength
12
Q
what do electrons that stop immediately produce?
A
a photon
13
Q
what’s important about a photon?
A
max energy, minimum wavelength
14
Q
2 methods of x-ray production
A
descleerating electrons
electrons dropping to inner levels
15
Q
what do the methods of x-ray production lead to?
A
a distinctive x-ray spectrum
16
Q
what are the 2 lines to label on an x-ray specrum?
A
line spectrum
continuous spectrum
17
Q
what are along the axes of an x-ray spectrum?
A
x = wavelength
y = spectral intensity
18
Q
what causes the line spectrum on the x-ray spectrum?
A
electrons being knocked out of inner levels and outer electrons falling a big energy gap to fill the holes
19
Q
what causes the continuous spectrum on an x-ray spectrum?
A
electrons slowing down as they strike the target
20
Q
which parts of an x-ray spectrum is the line spectrum?
A
the peaks
21
Q
which parts of an x-ray spectrum is the continuous spectrum?
A
the flat lines
22
Q
what does a bigger voltage lead to in terms of light?
A
bigger voltage = higher intensity of lighth
23
Q
how do we obtain a higher intensity of light?
A
bigger voltage
24
Q
what does an x-ray spectrum show?
A
a line spectrum superimposed on a continuous spectrum
25
what happens to most energetic electrons in one strike and what does this cause?
lose all of their energy
causes the shortest possible wavelength
26
what are x-rays produced in?
an x-ray machine/x-ray tube
27
what does a graph of intensity against wavelength show?
a wide continuous spectrum with narrow spikes superimposed on it
28
what do we call the narrow spikes on the x-ray spectrum?
the line spectrum
29
what do the narrow spikes on the line spectrum of the x-ray spectrum correspond to?
transitions of electrons down to lower energy levels
30
what leads to the narrow spikes of the line spectrum?
the energy levels of the electrons are at very specific energies
31
what is the line spectrum very similar to?
the emission spectrum
32
what will a 60kV spectrum always have that's higher than a 50kV spectrum?
a higher intensity
33
what happens to the minimum wavelength as the tube pd increases in the x-ray tube?
minimum wavelength decreases
34
do the positions of the spikes in the line spectrum change?
no
35
how do we change the positions of the spikes in the line spectra?
by changing the metal of the target
36
what will changing the metal of the target in the x-ray tube do to the line spectrum?
change the positions of the spikes in the line spectra
37
how come we can calculate the highest energy x-rays?
because no x-ray can have more energy than the incoming electrons
38
what can an x-ray not have more energy than?
the incoming electrons
39
if an electron (by some incredibly coincidence) loses all its energy in only one collision with the metal target, which energies will be the same?
the energy of the x-ray photon produced will be the same as the energy of the electron
40
when will the energy of the x-ray photon produced be the same as the energy of the electron?
if the electron loses all its energy in only one collision with the metal target
41
equation for the highest energy x-rays + explain
hfmax = eV
hf max = the maximum energy of the photon
eV = the energy of the electron
42
give the full equation for the highest energy x-rays and explain
hfmax = eV
using c = flambda, we can replace fmax with fmax = c/lambdamin giving
hc/lambda min = eV
43
what is the equation for the highest energy x-rays almost identical to?
the equation used for photons from an LED
44
why is the equation for the highest energy x-rays almost identical to the equation for photons from an LED?
they both arise from
electron potential energy (eV) = photon energy (hf)
45
photon energy
hf
46
electron potential energy
eV
47
hf
photon energy
48
eV
electron potential energy
49
what is the intensity of x-rays proportional to?
the current in the x-ray tube
50
what is proprtional to the current in the x-ray tube?
the intensity of the x-rays
51
how can the current in the x-ray tube be varied?
by alternating the temperature of the hot wire (absorbers and filters can be used too)
52
what's bad about the process of changing the current in an x-ray tube by altering the tempature of a hot wire?
it's very inefficient - most of the input energy ends up as wasted heat
53
how do we work out the speed of electrons?
KE of electrons = initital PE of electrons
1/2mv^2 = eV
(m = mass of an electron, which is in the data book)
54
what happens to the intensity of x-rays as they pass through a medium?
their intensity decreases exponentially according to:
I = Ioe^(μx)
55
equation for the exponential decrease of x-ray intensity as they pass through a medium
I = Ioe^(μx)
56
I = Ioe^(μx) + explain the symbols
equation for the exponential decrease of x-ray intensity as they pass through a medium
I = intensity of the x-rays
I0 = original intensity of the x-rays
μ = absorption (or attenuation) coefficient
x = distance travelled by the x-rays
57
what is the half value thickness for x-ray absorption given by?
x1/2 = ln2/μ
58
what is half value thickness?
the thickness that gives half the x-ray intensity
59
state and explain what the half value thickness equation is
the half value thickness is the thickness that gives half the x-ray intensity
i.e --> I/I0 = 1/2 hence 1/2 = e^(-μx1/2)
taking logs of both sides gives
ln (1/2) = -μx1/2 --> ln (2) = μx1/2
----> x1/2 = ln2/μ
60
which fact is image intensification based on?
the fact that a 30kV x-ray photon has 10,000x more energy than a 3eV visible photon
61
what do we use to absorb x-rays?
crystals called scintillators
62
what do crystals called scintillators do?
absorb x-rays
63
what do scintillators do once they've absorbed x-rays?
they convert the energy of one x-ray into multiple visible photons
64
why is it good that scintillators convert the energy of one x-ray into multiple, visible photons?
it's a lot easier to detect these visible photons than it is to detect one x-ray photon
65
what has happened when a scintillator has converted the energy of one x-ray into multiple visible photons?
the image is intensified
66
disadvantage of x-ray techniques
the exposure to ionising radiation
67
why is image intensification important?
it can reduce x-ray dosages by factors of between 50 and 1200
68
efficiency
output energy/input energy x100
69
give 2 examples of image intensification
the x-ray cassette
fluoroscopy
70
describe the x-ray cassette
2 outer layers = scintillators
centre "filling" = the photographic plate
71
what happens to most of the x-rays in the first scintillator in an x-ray cassette?
they pass straight through
72
what happens when we place a second scintillator on the opposite side of the plate in an x-ray cassette?
we can almost double the exposure
73
what does image intensification lead to in terms of x-ray dosage?
it means that the x-ray dosage for an x-ray is reduced by a factor of between 50 and 1200, depending on the resolution required for the image
74
what's good and bad about thicker scintillators?
provide more intensification but they also blur the image
75
what type of x-rays provide moving images of the patient's inwards?
fluoroscopy
76
what does fluoroscopy do?
provides moving images of the patient's inwards
77
explain exactly how fluoroscopy is done
the tube and collimater ensure we have a directed beam of x-rays
the x-ray filter gives us a beam with a suitable mean wavelength
the anti-scatter grid is similar to the collimator. it's a group of hollow cylinder making sure that we only detect x-rays travelling in the correct direction (we don't want x-rays that have been scattered in the wrong direction because they will blur the image)
the image is produced in the sctintullator screen with each absorbed x-ray giving around 1000 visible photons
the CCD video camera continually sends images of the screen to a monitor for the surgeon to carry out the operation
the scintillator and CCD camera are both kept in a dark container - we only want to detect light from the scintillator screen
78
what does an x-ray filter do?
gives us a beam with a suitable mean wavelength
79
what is an anti-scatter grid and what does ti do?
a group of hollow culinders maling sure that we only detect x-rays travelling in the corect direction
80
why don't we want x-rays travelling in the wrong direction?
they will blur the image
81
another disadvantage to x-ray imaging in addition to the exposure to ionising radiation
the low contrast between soft tissues
82
how can the low contrast between soft-tissues in x-ray imaging be overcome?
with the use of contrast media
83
contrast medium
a substance that includes a high atomic number element for increase x-ray absorption
84
2 most common contrast media
barium metal
iodine
85
explain how and why barium metal is a common contrast medium to use in x-ray imaging
this is uually barium sulfate mixed with water which is swallowed by the patient. Barium, having an atomic number of 56, is a good absorber of x-rays and assists greatly in taking images of all areas of the alimentary canal (throat, stomach, small intestine etc)
86
explain how and why iodine is a common contrast medium to use in x-ray imaging
liquids containing iodine can be injected into veins or arteries leading to pictures or even videos of blood flow. iodine has an atomic number of 53 which provides the extra absorption and contrast required for these images
87
CT scan
an x-ray image that utilises a fan shaped beam opposite a line of digital detectors
88
an x-ray image that utilises a fan shaped beam opposite a line of digital detectors
CT scan
89
how are the beam and detectors rotated around the patient during a CT scan?
in a helical pattern
90
what happens when the beam and detectors are rotated around the patient in a helical pattern during a CT scan?
it takes multiple "slices" of the patient to produce a 3D image
91
advantages of a CT scan over a normal x-ray image
it produces 3D images
it produces better soft-tissue contrast
92
disadvantages of CT scans
it requires a greater x-ray dosage
it is more expensive and can take longer for a detailed scan although a quick torso scan takes around 10 seconds
93
explain how radiotherapy is done
a beam of high energy x-rays are directed towards the tumour
the beam is then rotated around the patients with the tumour at the centre of rotation - this means that the tumour recieved the maximum dose while the dose to the surrounding tissue is kept as low as possible
94
what is the photon energy range usually used for x-ray imaging?
20kev - 150keV
95
what is the imaging x-rays usual half value thickness?
around 5cm
96
what do we require to be higher for radiotherapu?
we need more penetrating x-rays that have higher half value thicknesses
97
what is the range of radiotherapy photons usually?
1meV to 25MeV
98
half value thicknesses of radiotherapy photons
10-50cm
99
what do higher energies of the x-rays used for radiotherpy ensure?
that whole tumours can be irradicated reasonably uniformly
100
how come whole tumours can be irradicated reasonably uniformly during radiotherapy?
higher energies of the x-rays used
101
what are easier to kill - cancer cells or healthy cells?
cancer cells
102
what's the problem with the high energy x-ray doses used in radiotherpy?
they can cause secondary cancers (the risk of secondary cancers is approximately 10% after 25 years have passed)
103
absorbed dose symbol and equation
D = total energy of radioation absorbed/mass
104
absorbed dose (D) unit
Gy (gray) or Jkg-1
105
what does equivalent dose also take into account?
the relative danger of the radiation itself
106
equivalent dose symbol and equation (+explain)
equivalent dose is very similar to the absorbed dose however it also takes into account the relative danger of the radiation itself
H = DWR
WR = radiation weighting factor
107
when is Wr usually 1?
for x-rays and gamma rays and for beta radiation
108
WR for alpha radiation
20
109
how do we know that alpha radiation is 20x more dangerous than other radiations?
noramally WR = 1 for x-rays and gamma rays and beta radiation
WR = 20 for alpha radiation
110
effective dose equation + explanation
effective dose is also very similar to the absorbed dose however it takes into account both the relative danger of the radiation itself (WR) and the realtive dange to the tissue (WT)
WT = tissue weighting factor
E = HWT
combining with H equation for equivalent dose:
E = DWRWT
111
effective dose unit
Sv
112
what does WT vary from and for what?
varies from 0.01 for skin, brain and bone surface to 0.12 for lungs, colon and bone marrow
so, irradiating the lungs is 12x more dangerous than irradiating the skin
113
how do we know that irradiating the lungs is 12x more dangerous than irradiating the skin?
WT is 0.12 for the lungs and 0.01 for the skin
114
which effect do we rely on to produce and detect ultrasound?
the piozoelectric affect
115
what does the piozoelectric effect allow us to do?
produce and detect ultrasound
116
piozoelectric effect
physical deformation causes a voltage and vice versa
117
what happens to the piozoelectric crystal when a pd is applied?
it deforms
118
when does a piozoelectric crystal deform?
when a pd is applied
119
explain how ultrasound pulses are produced in a probe
a high frequency (2-15MHz) alternating pd is sent to the electrodes.
the reverse piozoelectric effect means that the crystal will deform at the same frequency producing ultrasound waves of the required frequency (2-15MHz)
when the ultrasound pulses are reflected from whatever is being analysed, they arrive back at the piozoelectric crystal. this time, the sound waves will deform the piozoelectric crystal and the piozoelectric effect mmeans that an alternating pd will be defelected across the electrodes
the absorber block and acoustic insulation help improve the quality and direction of the pulses by stopping reflections and absorbing waves going in the wrong direction
the plastic nose is designed to have the correct acoustic impendance for maximum trasmission and might even incorporate a lens to focus the ultrasound beam
120
what does an absorber block and acoustic insulation in a probe that produces and detects ultrasound do?
helps improve the quality and direction of the pulses by stopping reflections and absorbing waves going in the wrong direction
121
how is the plastic nose in a probe that produces and detects ultrasound designed?
to have the correct acoustic impendance for maximum transmission and might even incorporate a lens to focus the ultrasound beam
122
how many detectors do A scans have?
only one
123
what happens in an A scan?
an ultrasound pulse is sent and a series of reflections is received
124
what can be done with an A scan and how?
in an A scan, an ultrasound pulse is sent and a series of reflections is received.
various time-delays are then used to measure various distances
125
common use of an A-scan
to scan the eye for a detached retina or to check the thickness of the lens before an operation (the scan can be done with the eye closed)
126
how do we calculate time delays in A scans?
remember to double the thickness of a lens for example when the pulse is reflected back and passes through a second time
use the general std triangle
127
describe the probe used for B scans
the prove has multiple ultrasound emitters (each of which is a detector when the pulse is reflected back)
128
how many emitters is it common for ultrasound probes to have and what can these then do?
512
these can produce a line image that is 512 dots (or pixels) wide
128
how do you obtain a 2D image from a B scan?
the emitters produce a line image and when the probe is rotated, a 2D image is produced by joining many of these line images together
129
common use of B scans
imaging foetuses
130
acoustic impendance equation + define symbols
Z = cp
c = speed of sound in the material
p = density of the material
131
what is it that gives us reflections of ultrasound?
diffeences in acoustic impendance
132
what do differences in acoustic impendance give us for ultrasound?
reflections
133
what is the fraction of ultrasound reflected at a boundary between two materials (R) with acoustic impendance Z1 and Z2 given by?
R = (Z2 -Z1)^2/(Z2 + Z1)^2
134
if our reflection coefficient (R) is 1, what does this mean?
nothing is transmitted (All reflected)
135
when do we know that nothing is transmitted and that all of the ultrasound is reflected in terms of the reflection coefficient?
if its 1, nothing is tranmitted
136
what do we need to consider when we use the doppler shift to measure the speed of blood flow?
reflection and angles
137
how do we measure the speed of blood flow?
using the doppler shift
138
how do we consider the velocity of flow when working out the speed of blood flow?
as a component in the direction of the ultrasound waves
139
what 3 things do we need to consider when measuring the speed of bloof flow?
consider the velocity of flow as a component in the direction of the ultrasound waves
consider that the waves encountered by red blood cells will be squashed
the red blood cells, when they reflect, are also moving sources
140
equation for the doppler shift to work out the speed of blood flow
deltaf/fo = 2v/c costheta
or
-deltalambda/lambdao = 2v/c x costheta
141
MRI
magnetic resonance imaging
142
what is magnetic resonance imaging based on?
the spin of hydrogen nuclei (protons)
143
how come there are plenty of hydrogen nuclei in our bodies for an MRI scan?
our bodies contain 75% water (H2O) and so there are plenty of hydrogen nuclei in our bodies to assist the MRI scan
144
what do all spinning charges have?
magnetic fields
145
how come a hydrogen nucleus is a tiny magnet?
all spinning charges have magnetic fields
146
what happens when we apply a B field to a hydrogen nucleus?
the hydrogen nuclei tend to align in the same direction as the applied B -field (As any magnet would)
147
what do hydrogen nuclei do in addition to aligning in the same direction as the applied B-field?
precess around the direction of the applied B-field
148
what can happen to a hydrogen atom if its gains enough energy?
it can flip to a higher energy state where its own magnetic field is in the opposite direction to the applied field (but it still precesses)
149
what does the frequency at which the hydrogen nuclei precess depend on?
the applied B field
150
equation for the frequency at which the hydrogen nuclei precess + name
f = 42.6x10^6B
larmor frequency
151
larmor frequency
the frequency at which the hydrogen nuclei precess
f = 42.6x10^6B
152
what would happen if we sent radio waves at the hydrogen nuclei at the larmor frequency? how about when we remove the radiowaves?
resonance will occur and many hydrogen nuclei will flip the the higher energy state (opposite B-field orientation)
removed --> the nuclei gradually will flip back to align with the B-field
153
what do hydrogen nuclei do when they flip back to align with the B field after removing the applied radiowaves?
the hydrogen nuclei emit radiowaves at the same frequency when they flip back. these radio waves are detected to produce the MRI image
154
what's different in terms of different tissues for an MRI scan?
different tissues take a different characteristic time to flip back and this is the main reason that MRI gives good contrast for different soft tissue
155
why does an MRI scan give good contrast for different soft tissue?
because different tissues take a different characteristic time to flip back
156
what is the "flipping back" time of protons usually called?
the "relaxation time"
157
how can we produce multiple MRI images to build a 3D scan?
we can image one slice at a time
158
how do we image one slice at a time during an MRI scan to build a 3D scan?
by having a B-field that varies from head to toe
159
explain how having a B-field that varies from head to toe helps to build a 3D MRI scan
as the B-field varies from head to toe, we can "tune in" to different slices of the body by varying the frequency of the radiowaves. only the slice with the correct resosance frequent will respnod to the radio waves and only that slice will be imaged.
160
what does PET stand for?
positron emission tomography
161
what is a PET scan based on?
position-electron annihilation
162
what happens when a position annihilates an electron?
two gamma ray photons of energy 0.51MeV are produced
163
how can we check how much energy is released when a position annihilates an electron?
using E = mc^2
164
what is the mass lost in the reaction of when a position annihilates an electron and why?
the mass of an electron plus the pass of a position (since photons have no mass)
165
compare the mass of an electron to the mass of a position
exactly the same
166
why is the mass lost in the annihilation of an electron by a position equal to 2 electron masses?
since a position has the exact same mass as an electron
167
J to eV
divide
168
eV to J
multiply
169
why is the energy released during positron-electron annihilation always divided equally between the 2 photons?
because of the conservation of momentum - the initial momentym of the position and electron is always negligible so that the momentum of the two photons must cancel
170
what do we do when performing a PET scan?
enclose the area being scanned by a ring of gamma detectors
171
when do we know that position-electron annihilation has taken place in a PET scan?
when two photons are detected almost simultaneously
172
how do we work out exactly where positron-electron annihilation took place during a PET scan?
when two photons are detected almost simultaneously, we know that a position-electron annihilation has taken place
we also know that the annihilation took place somewhere on the line between the two detectors
the gamma detectors can measure time delays of around 1ps and this means they know where along the line annihilation took place
173
what can gamma detectors do during a PET scan?
can measure time delays of around 1ps and this means they know where along the line annihilation took place
174
what can be done after completing a PET scan?
a 3D image of annihation hotspots can be build by using a computer and averaging over time
175
how would we do a calculation to work out where during a PET scan annihilation took place?
distance = velocity x time
use 3x10^8 as velocity and the time given
176
how do we get the positions for a PET scan inside the patient in the first place?
use fluorine-18 as the beta+ emitter
177
why don't we need to supply annihilating electrons for a PET scan?
because there are plenty of those in the body already
178
show how fluorine-18 is a beta+ emitter
18F9 --> 18O8 + +1B + Ve
179
half life of fluorine-18?
110 minutes
180
how must fluorine-18 be produced?
using a cyclotron
181
why are PET scans not available in small hospitals?
require fluorine-18 for beta+ to be emitted, and fluorine-18 must be produced using a cyclotron
182
what must you do once fluorine-18 has been produced?
must react it (chemically) quickly to produce a glucose (called Fluorodeoxyglucose, FCG)
183
what is done to the glucose formed from fluorine-18?
it's injected into a vein where it travels to the parts of the body that use it
184
how come hotspots appear in a PET scan?
the glucose from the beta+ emittiing fluorine-18 is used by the cancers since cancers require a lot of energy and glucose
185
186
How is blood volume calculated? describe the method
By injecting a radioactive chemical into the patients blood
You then wait 10 to 20 minutes for the tracer to mix uniformly throughout the patient’s blood.
Then, a small amount of blood is obtained from the patent (around 10cm^3)
The activity of this 10cm^3 sample can be used to calculate the total volume of the patient’s blood
187
What do we assume when calculating blood volume?
That the activity is the same throughout the time period
188
Total volume of blood equation
Activity of tracer x sample of blood taken/activity of sample
189
Radioactive tracers
Radioactive chemicals which are put inside a patient (either injected or ingested)
190
What must radioactive tracers for imaging do?
Emit body-penetrating gamma rays that can be viewed by a gamma camera outside the body
191
What should the half life of a radioactive tracer be and why?
A few hours
Long enough to take an image but short enough not to give a large radioactive dose to the patient
192
Most common radioactive tracer
Technetium-99m
193
Explain why technetium-99m is a commonly used radioactive tracer
It has a half life of 6 hours and emits gamma rays
194
Why is technetium-99m only produced in specialised hospitals?
Requires a cyclotron
195
Collimator
A collection of hollow lead tubes
196
A collection of hollow lead tubes
Collimator
197
What will Collimator do?
Only collect rays moving vertically upward
198
What does a scintillator counter count?
Electric pulses
199
Explain how images are produced using technetium-99m and a gamma camera
The patient will be emitting gamma rays in all directions, so we need a Collimator to collect only the rays moving vertically upward
The gamma rays are absorbed by the scintillator which produces approximately 4000 light photons for each incident gamma ray - a large 2D network of photomultiplier tubes provide an electric pulse each time light is detected in front of them
The scintillator counter counts these pulses and passes the information to a computer which displays a 2D image on the screen
200
Does a computer have to do anything clever to produce an image using technetium-99m and a gamma camera? Explain
Unlike CT, MRI and PET, no.
It merely reproduced the faint image that appears on the scintillator
201
What type of cameras can be set up to view the image on a scintillator?
Sensitive CCD cameras (similar to the set up for fluoroscopy)
202
How do the images using technetium-99m and a gamma camera tend to come out and why?
Low resolution
The Collimator tubes have to be quite thick to absorb the misdirected gamma rays
203
For the imaging technique ultrasound, explain its…
Radiation exposure
Application
Biological effects
Cost
Conditions
3D imaging
Definition
When not to use
No ionising radiation
Generally soft tissue including foetus, skeletal joints
No known hazards in imaging
Low cost
Short time, usually painless
Usually 2D images
Not high - depends on the skill of the practitioner
Always safe. Not usually suitable for lung imaging but used for collapsed lung
204
For the imaging technique standard x-ray explain its…
Radiation exposure
Application
Biological effects
Cost
Conditions
3D imaging
Definition
When not to use
Varying but usually low exposure to ionising radiation
Mainly bone breakages, with contrast agent can also be used for soft tissue
Carcinogenic effects and developmental defects in embryos
Low cost
Very short time
Usually 2D images
High definition of bony structures
Pregnancy
205
For the imaging technique CT scan explain its…
Radiation exposure
Application
Biological effects
Cost
Conditions
3D imaging
Definition
When not to use
Higher exposure to ionising radiation (0.02 - 10m5v) up to 5 years background
Bone injuries, lung and chest imaging, cancer detection, A&E investigations
Carcinogenic effects and developmental defects in embryos
About 1/2 the cost of MRI
Quite a short time (5 min), ideally no movement but less of a problem than MRI
Yes, using a helical scan
High definition of bony structures, moderate definition of soft structure
Pregnancy. Weight limit of about 200kg because of space and strength of moving table
206
For the imaging technique MRI, explain its…
Radiation exposure
Application
Biological effects
Cost
Conditions
3D imaging
Definition
When not to use
No ionising radiation
All kinds of soft tissue imaging e.g brain, injuries, tumours
No known hazards
High cos
Long time, uncomfortable (no movement allowed), noisy, claustrophobic
Yes
High definition (but requires a stationary subject)
Some mental implants, heart pacemakers. Weight limit about 150kg due to the space and strength of the table
207
What does the metal filter do in an x-ray machine?
Produced a beam of x-rays of suitable wavelength
Absorbs low energy x-rays so they’re not absorbed by the tissue, causing damage
208
How do we work out the force exerted by an electron beam on a target?
F = mv x no. of electrons per second
209
How do we work out the number of electrons per second?
Q/charge of an electron
210
Disadvantages of a PET scan
Expensive
Low resolution
211
Why would an ultrasound B scan not work for detecting a cancerous tumour in a person’s lung?
Too much air reflection
212
Another use of radioactive tracers
Blood flow through the brain check for blockages
213
Equation to work out the minimum wavelength on an x-ray spectrum
Minimum wavelength = hc/E
214
How to work out energy from voltage
E =Ve
215
Describe how the Doppler shift principle can be used to measure the speed of blood through an artery
Pulse sent by transducer at an angle to the blood vessel
Pulse reflected by moving blood cells
Pulse detected by transducer
Speed calculated using the Doppler shift of the ultrasound
216
Give all of the properties of technetium-99m that make it a good radioisotope in the effective diagnosis of medical problems
Emits gamma rays which penetrate through the skin and so are detectable outside of the body
Half life of 6 hours —> long enough to take an image but short enough not to give a large radioactive dose to the patient
Daughter products are stable
Low ionisation
217
Dose unit
Sv
218
How do we get larger photon energies (x rays)?
Greater pd
219
What does a greater pd mean in terms of photon energies?
Greater photon energies
220
When will we get an ultrasound echo in terms of acoustic impedance? Explain
Acoustic impedance is significantly different in both materials = will reflect = ultrasound echo
221
What happens when acoustic impedance is significantly different in both matters? Explain
Will reflect = ultrasound echo
222
What do we call the light photons produced by a scintillator?
Scintillations
223
What should be equal in a PET scan and why?
Mass-energy of electron should be equal to the photon energy because 2e —> 2 gamma ray photons
224
Would MRI image soft tissue?
Yes
225
Would MRI provide moving images?
No
226
What does a greater current do for an x ray?
Greater intensity
227
How do we get a greater intensity (x ray)?
Greater current
228
What does a greater pd do in an x ray?
Larger photon energies
229
How do we get larger photon energies in an x ray?
Greater pd
230
How come x-rays are absorbed by bones?
They are absorbed by dense matter or high atomic number elements (not penetrated)
231
What are x rays if they’re not absorbed by something?
Penetrated
232
Why do we need a vacuum between the heater and the tungsten target in an x-ray tube?
To avoid collisions on the way to the target
233
What do we need to remember to do for speed of blood flow questions?
Use c from the question - not from the data book!
234
Are contrast media used for ct scans?
Yes
235
Where do radioactive tracers go?
To hyperactive parts to locate problem areas
236
What do we need to be careful with in x-ray absorption questions?
Be careful when a question says reduced intensity *by* 60% for example - the new intensity is 40% not 60%
237
Key word for when describing piezoelectric crystals
The crystal *vibrates*
238
What kind of images can B-scans generate?
2D
Moving images
239
What do we need to do at the end of the calculation when working out the thickness of fat using acoustic impedance and why?
Distance divided by 2
Since the ultrasound pulse has travelled through the source and back again over this distance, so the thickness is half of this
240
What do we do when we have a question about efficiency and the rate of production of heat?
Use P= IV
241
Why can’t we have the wavelength of x-rays as zero?
Voltage/energy would have to be infinite
242
What type of voltage is needed for the piezoelectric effect to produce ultrasound?
Alternating, high frequency
243
What type of voltage is needed for the piezoelectric effect to produce ultrasound?
Alternating, high frequency
244
Which of these is in real time and which isn’t - CT scan, x-ray
CT scan = real time
x-ray = not real time
