6.5 Medical imaging Flashcards
(198 cards)
1
Q
How did Wilhelm discover X-rays?
A
He was carrying out experiments passing current through an evacuated glass tube with high voltage between the cathose and the anode. He noticed that an unknown type of radiation could travel through the glass and the dark paper around it, and it cause a flourecent plate near his apparatus to glow.
2
Q
What did Wilhen Rontgen see when he placed his hand between the tube and the flouescent plate in his experiment?
A
He saw flourescet even in the shadow of his hand, showing the radiation had passed through his hand.
3
Q
What properties do X-rays have?
A
Wave and particle properties (wave-particle duality).
4
Q
What did Max van Laue reason if X-rays are elecromagnetic radiation?
A
The X-rays are electromagnetic radiation ofm short wavelength, they should cause diffraction if a grating with a small enoigh grating spacing could be found.
5
Q
What did Max van Laue use to diffract X-rays? What did this tell us?
A
He used the regular array of atoms in a crystal as a diffraction grating and found a diffraction pattern with wavelengths between 10^-12 to 10^19m.
6
Q
How are X-rays produced when a fast-moving electron decelerates rapidly by smashing into a metal target?
A
As the electron interacts with the elctric field around the nucleus, an X-ray photon is emitted and by the principle of conservation of energy the kinetic energy lost by the electron is equal to the energy of the photon emitted.
7
Q
How are X-rays produced when a fast-moving electron ejects an inner electron of the target metal?
A
Another electron from a higher energy level mmoves to occupy the vacancy created, and this transition to a lower energy level releases energy as an X-ray photon of a specific wavelength coresponding to the difference in energy levels.
8
Q
What are the 2 ways X-rays are produced?
A
- A fast moving electron decelerates rapidly by smashing into a metal target.
- When a fast moving electron ejects an inner electron of the target metal.
9
Q
What does the production of X-rays require?
A
- High voltage
- Vacuum
- Anode
- Cathode
10
Q
How does a modern X-ray tube produce X-rays?
A
Electrons are emitted from a filament, with is heated by an electric current . The high voltage accelerates electrons between the filament (cathode) and the positive anode. The electrons have enough energy sp when they hot the anode, some X-rays will be emitted.
11
Q
In the modern X-ray tube, what property must the positive anode have?
A
Must be a metal with a high melting point such as tungsten.
12
Q
Why must there be a vacuum in a X-ray tube?
A
The electrons will lose energy by colliding with any gas molecule on their path to the positive anode, so the tube is evacuated.
13
Q
In an X-ray tube, why is the anode end shaped at an angle?
A
The anode is shaped so that the X-rays come off at the desired direction through a window, and the tube is surrounded by lead sheilding so that the radiographers themselves are not subject to radiation.
14
Q
In an X-ray tube, why must the anode by made of a metal with a high melting point? How is it kept cool?
A
Most of the kinetic energy of the electrons heat up the anode, so in many X-ray tubes the anode has to be cooled by oil flowing through it.
15
Q
In an X-ray tube, what is the energy output of X-rays compared to the energy input?
A
The energy output of X-rays is about 1% of the energy input.
16
Q
What is the kinetic energy equal to when an electron with energy e, is accelerated through a potential difference?
A
Kinetic energy= eV
17
Q
What is the maximum energy on a X-ray photon?
A
eV
18
Q
When will an X-ray have a minium wavelength?
A
when all the electrons kinetic energy is, transffered to producing an X-ray photon.
19
Q
Why is the wavelength of X-rays a spectrum?
A
Not all the electons lose all of their kinatic energy to produce X-rays with this wavelength, so there will be a specrum of wavelengths.
20
Q
What is the number of X-rays emitted diectionally proportional to?
A
It is directly proportional to the number of electrons emitted per second from the filament.
21
Q
What does the graph of intensity against wavelength for X-rays look like?
A
22
Q
What does the graph of intensity against wavelength for X-rays show?
A
- It shows no intensity of X-rays below a minumum value of wavelengths.
- A falling away of X-ray intensity at larger wavelengths.
- Peaks showing particularly strong emission at certain wavelengths.
23
Q
What do the peaks in the graph show?
A
The particurly strong emissions at certain wavelengths show the characteristics of the element in the anode.
24
Q
What is compton scattering?
A
Compton scattering is the effect whereby an X-ray deflected by by interaction with an orbital electron has a longer wavelength than its initial wavelength. The electron is ejected from the atom at hight speed.
25
What is attenuation?
Attenuation is a gradual decrease in intensity.
26
What is the attenuation (or absorption) coefficient?
It is a constant used to calculate how the intesity of X-rays decreases as they pass through a material. (m^-1)
27
How many ways can X-rays interact with matter?
4 ways
28
How can X-rays interact with materials?
* Transmitted
* Absorbed
* Reflected
29
What do the dark areas on an X-ray show?
The dark areas show where X-rays are transmitted with no reflection or absorption.
30
What are the 4 ways X-rays can interact with matter?
* Simple scattering
* The photoelectric effect
* Pair production
* The compton effect
31
What happens in all pair production, the phtotoelectric effect and the Compton effect?
Atoms or molecules in the material are ionised, and the energy of X-rays is reduced or attunated.
32
What happens in both the photoelectric effect and pair production?
All of the photon energy is absorbed.
33
What is simple scattering?
When low-energy X-rays encounter the electrons in an atom the energy of the X-ray photon is not sufficient to cause ionisation. The photon is scattered (deflected so that the direction is changed).
34
Is there any energy change of the photon in simple scattering?
No
35
Is the X-ray photon absorbed in simple scattering?
No
36
What causes noise in X-ray imaging?
Simple scattering causes 'noise' in the image due to the arrival at he detector of scattered X-rays from several X-rays from several angles as well well as from the main beam.
37
What happens in the photoelectric effect?
X-rays cause the emission of free electrons from atoms.
38
What is the binding energy called of a photoelectron?
The work function.
39
What is the maximum kinetic energy equal to in the photoelectric effect with X-rays?
The work function or the binding energy of the photoelectron is so small ir can be ignored when X-rays interact with an atom by the photoelectric, and the emited photoelectrons have a **maximum kinatic energy equal to the photon energy of the X-rays.**
40
What is measuring the energy of the emitted electron from the photoelectric effect a methord for?
Measuring the energy of the amitted electron is a methord of detecting the energy of X-rays.
41
What is monochromatic X-rays?
X-rays with just one wavelength.
42
How did Arthur Compton obtain monochromatic X-rays?
He obtained these by filtering out all the wavelengths except those corresponding to the strongest sharp peaks of the X-ray spectrum.
43
What did Compton find when he measured the wavelength of scattered radiation from a carbon target? How did he exaplain this?
He found that some deflected X-rays had longer wavelengths that the inital wavelength. He explained it by using quantum theory- he regsrded the X-rays as particles colliding with orbital electrons in carbon atoms, causing an electron to be ejected with a small fraction of the kinetic energy lost by the X-ray photon and a photon of the remaining emergy is emitted. Mass-energy is conserved.
44
In compton scattering, which way is the X-ray photon and the electron scattered?
They are scattered in different directions, due to the conservation of momentum.
45
What is compton scattering?
Regarding X-rays as particles/ photons, they collide with orbital electrons of atoms which causes the electron to be ejected with a small fraction of the kinetic energy lost by the photon and a photon with the remaining energy is emitted- mass-energy is conserved.
46
In compton scattering, how does the angle the photon is deflected through effect the energy and the wavelength of the X-ray?
* A photon deflected through a large angle will have lost more energy and so will have a longer wavelength.
* A photon through a small angle will have lost less energy, so its wavelength wil show little increase.
47
What is pair production?
When a X-ray interats with the nucleus of an atom, causing the photon the vanish, spontaneously producing a positron and an electron.
48
What is the equation for pair production?
49
What must be known to asses the does of an X-ray for a patient?
* Intensity of radiation
* Amount of absorption
* Time of exposure
50
What causes inoisations with X-rays?
Absorption of X-ray photons and the electrons released by the photoelectric effect and the compton scattering.
51
What varies the absorption of X-rays by done and soft tissues?
Frenquency of the X-rays.
52
How are low frequency X-rays absorbed by?
They are mostly absorbed by causing the emission of photoelectrons.
53
How are high-frequency X-rays mostly absorbed by?
Mostly absorbed by compton scattering.
54
How are very high frequency X-rays absorbed?
By pair production
55
Why does pair production not cause attenuation in X-ray tubes in medial imaging?
X-ray tubes used in medical imaging do not poerate at such high energies, so pair production does not cause attenuation.
56
What is the definition of the intensity of X-rays?
Intensity- power transmitted per unit cross-sectional area.
I=P/A
57
What does the intesity equal you move away from it's source?
An inverse square law applies, therefore, the intensity at a distance 3x compared with the intensity of the distance x will be 1/9th of its value at x.
58
What does it mean if an X-ray is collimated?
The X-ray provides a parallel beam of X-rays, so the intensity hardly changes with distance.
59
What is the equation for a collimated beam of X-rays intensity with regards to the distance from it's source?
I=I0e^ux
60
What does each value stand for in the equation?
* I= Intensity
* I= Original intensity
* u= Attenuation coefficient of ghe materialX-rays are passing through
* x= Distance the X-rays have passed
61
What is the attenuation coefficient for a vacuum?
0
62
What is the attenuation coefficient for flesh?
100m^-1
63
What is the attenuation coefficient for bone?
300m^1
64
What is the attenuation coefficient for lead?
600m^1
65
Describe the role the distance has in the equation?
As with other exponential decays, the equation determines there is a constant distace which the intensity falls tp half the inital value. Twice this value will result the intensity being one quatre of it's inital value.
66
What is the distance for halving the intensity for a X-ray called?
The half-value thickness.
67
Does bones or soft tissues absorb X-rays better?
Bones absorb them better
68
Using X-rays, how can you see detail of different types of soft tissues which have similar attenuation coeefficients?
Using a contast material such as barium or iodine, that has a larger attenuation coeiffient.
69
How can you get a clear X-ray for someones intestines?
A patient can swallow a liquid containing barium sulfate (a barium material) before having an X-ray taken. The barium the coats the wall of the intestines absorb more X-rays than the surrounding tissues, so the image of the intestine is more enhanced in contrast with the abdoninal structures.
70
What causes a large attenuation coefficient?
A large attenuation coefficent is due to the large atomic number of certain elements. These atoms have electrons bound by an energy equivalent to the of X-ray photons and so absorb X-rays more often than elements with low atomic numbers.
71
What is computerised axial tomography?
A CAT scan is a process using multiple X-ray scans to produce images of 'slices' through the body on one plane, in order to ptoduce a 3D image.
72
What does CATT scan premitt us to do?
It allows image reconstruction in 3D of multiple cross-sections through the body.
73
How are CAT scan images produced?
* To build a 3D image, many X-ray scans must be taken from different angles.
* A thin-fan shaped X-ray beam iis produced- whch allows it to irradiate only a very thin slice of the patient at a time.
* Once passed through, the X-rays are detected by a stationary ring of detectors.
* The X-ray source is rotated around the patient.
* Once it's moved one revolution, both it and the detectors have moved up about a centimeter, so in the next revolution, it will look at the nect slice.
* A computer performs didgital geometry processing of the X-ray intensities, detected at different positions to constuct a 3D imagr of an organ.
74
In a CAT scan, what shape is the X-ray beam? Why?
The X-ray beam is a thin-fan shape, with little thickness so the X-rays irradiate only a very thin slice of the patient at one time.
75
In a CAT scan, what happens to the X-rays after they have passed through the patient?
The X-rays are detected by a stationary ring of detectors.
76
In a CAT scan, what is different once the X-ray source has completed one revolution of the patient?
One completing one revolution, both it and the detectors have moved up about a centimeter, so that on the next revolution it looks at the necct slice of the body.
77
In a CAT scan, what does a computer do to produce a 3D image of an organ on the screen?
The computer performs **digital geometry processing** of the **X-ray intensities** detected at different positions, constructing a 3D image on the computer screen.
78
In a CAT scan, what can a doctor do with the 3D image produced?
A doctor can rotate the image in orger to view it from any desired angle and zoom in on it.
79
What are the advantages of a CAT scan over an X-ray image?
* It provides the doctor with a very accurate picture of the position of internal organs without being obscured by other structures in the body.
* CAT scans are very sensitive to changes in density.
* The digital technology allows the doctor to rotate the image and the view of the patient's body without having to take more images.
* The computer technology allows the doctor to remove areas with the densooty of bone or air by making them transparent.
80
What is the main advantage of a CAT scan?
It provides a very accurate picture of the position on the internal organs without being obscured by other structures in the body.
81
Why is it an adavantage that CAT scans are particularly sensitive to changes in density?
They give much better contast for different soft tissues than an X-ray could, e.g. cancerous tumours.
82
Why is it an advantage of a CAT scan that the digital technology allows the doctor to rotate the image?
So the doctor can view the body from a number of angles without having to take any more iimages of the patient which could expose them to more potential harmful X-rays.
83
Why is it an advantage of a CAT scan that the computer technology allows them to remove areas of bone or air?
BY making these areas transparent, the doctors can see parts of the image they may no have been able to see before. e.g. the ribs could be removed from an image if the doctor wanted to view the patients lungs.
84
What is a tracer?
A radioactive substance either injected by, or injected into a patient. It emits gammam photons to be detected by a gamma camera.
85
What is a gamma camera?
A gamma camera detects gamma photons from a pateint given am radioactive tracer. This is used to produce a real-time image on the path of the tracer through the body.
86
What is a collimator?
A collimater is a devise for producing a parallel-sided (collimated) beam of elecromagnetic radiation.
87
What is a scintillator?
A scintillator is a material that produces many photons of visable light when struct by a high energy photon.
88
What is a photomultiplier tube?
A photomultiplier tube is a devise used too give a pulse of electrons for each incident photon.
89
Why might radioactive material be put into a patients body?
To diagnose or treat an illness.
90
What must be considered when putting radioactive materials into a ptients body when either diagnosing or treating an illness?
* Gamma sources should be used
* The patient will remain slightly radioactive for some time after the tracer is injected.
* The half-life
* Source must not be toxic to humans
* How is the radioactive material going to get to where it's needed in the body?
* How is the tracer going to be monitored?
91
When either diagnosing or treating an illness with radioactive materials is a patient's body, why iis it important to consider what source must be used?
A gamma source must be used, because alpha and beta patricles would be absorbed by the body rather than pass through to aa detector, and would damage the body due to their greater ionising power.
92
When either diagnosing or treating an illness with radioactive materials is a patient's body, why iis it important to consider the pateint remaining radioactive for some time after the tracer being injected?
Due to the patient remaining radioactive, the patients family and madical staff will be exposed to some radioactivity of they are close to the patient for large amounts of time.
93
When either diagnosing or treating an illness with radioactive materials is a patient's body, why iis it important to consider the half life of the radioactive material?
The half life must be long enough to carry out the experiment, but not long enoigh for it to cause damage to cells.
Time must be long enough for the source to be bought from the manufacturing site to the hospital.
94
What radioactive source can be made in hospitals to be used as a tracer? Why?
Technetium-99m is a gamma source with a half life of 6 hours, so can be oved from production site to pateint very quickly without losing much activity.
95
When either diagnosing or treating an illness with radioactive materials is a patient's body, why iis it important to consider if the source is source is toxic to humans?
If a toxic gamma source is injected into the body, it cause cause the patient hasrm or even death.
96
When either diagnosing or treating an illness with radioactive materials is a patient's body, why iis it important to consider how the radioactive material is going to get to the part of the boody where it is needed?
Getting a source to where it is needed if often done by injection and attatching of the gamma emitter too a chamical that will go to a certain part of the body.
97
Using a tracer, how can we investigate the functioning of the thyroid galnd?
We use iodine-131, since this will be absorbed by the thyroid gland and it is a gamma emitter.
98
When either diagnosing or treating an illness with radioactive materials is a patient's body, why iis it important to consider how a doctir will monitor a tracer in the body?
For the tracking of a gamma-emitting source arounf the body, a **gamma camera** is often used.
99
What does a gamma camera detect?
It detects gamma photons that have been emitted by a source inside a patients body.
100
Regarding a gamma camera, how is the image of the tracer sharpened?
A block of lead with tens of thousands of vertical holes is close to the pateint. These parallel tubes colimate the beam so only photons trvelling along the axiis of this colimtor can pass through to the detector. Photons moving in any other direction will be absorbed by the kead, which improves the shaprness of the image as **scattered photons are exluded**.
101
Ragarding a gamma camera, what happens to the photons after being collimated?
The gamma photons strike the scintillator which scintillates when it absorbs a gamma photon (emits many phhotons of visable light).
102
What is a sccintallator in a gamma camera?
It is a large crystal of **sodium iodide**- typically 400mm in diameter and 10mm thick- sodium iodide is a fluoresnt material and scintillates when it absorbs a gamma photon.
103
Regarding a gamma camera, what is behind the scintillator?
An array of photomultiplier tubes arranged in a hexagonal pattern.
104
In a gamma camera, what happens after the scintillator emits many photons of visable light?
The photons of visable light reach the photomultiplier tubes which initially emilt one electron for each photon by the photoelectric effect- they amplify the effct to release maany more electrons, giving an electrical pulse output for every incident photon of light.
105
What does a gamma camera look like? (Cross sectional area)
106
What is the photomultiplier tubes in a gamma camera connected to, what does this do?
The output of the photomultiplier tubes are connected to a computer, and the computer will biuld up an image based on the electrical siganls from all of the tubes.
107
What parts of the body are gamma scans used is diagones disease in?
Disease of the:
* Thyroid
* Liver
* Brain
* Kidneys
* Lungs
* Spleen
* Heart
* Circulatory system
108
For many organs in the body, why is the radioactive nuclide technetium-99 used for gamma scans?
* It decays in 6.0 hours.
* It's chemical properties enable a small quantity to be incorperated into many kinds of molecules so it can be directed at a large numebr of target organs.
109
How can technetium be used to test tye function of the thyroid gland?
An iodine compound containing some technitium-99 can be injected to test the function of the thyroid gland. **If the tyroid galnd does not absorb the compound**, **little radioactivity will show** on the camera, indication **poor thyroid function**.
110
Why do some modern gamma cameras have more than one scintillator?
With 2 crystals at right angles to one another, around the patient, it is possible to obtain a 3D image of an organ.
111
What mathords are being developed to administer radioactive material with a longer half-life to people with certain cancers?
The idea is to attatch the radioactive material to cancerous cells in the tumor, thereby destorying them without dangerous doses of radiation to healthy tissue.
112
Define positron emission tomography?
(PET) is the use of gamma photons produced when positrons annihilate with electrons inside the body to map out biologically active areas within the body.
113
What is the structure of a PET scanner? What does this allow us to do?
Sensors are positioned around the patient, and mathematical techniques of computerised tomography are used to reconstruct a 3D image of parts of the body that the gamma rays gave passed through.
114
What is PET scanning based on?
Its based on the gamma rays produced by positron-electron annihilation following the radioactive decay of a positron-emitting radioisotope, or tracer.
115
In a PET scanner, why are detectors placed on both sides of the body?
Pairs of gamma rays are produced at 180° to each other, detectors are placed on either side of the body to allow accurate tracing back to the source.
116
What does the computerised 3D image show in a PET scan?
It shows the concentration of the tracer throughout the part of the body that is being analysed.
117
What tracer is commonly used for a PET scan? How long does the doctor wait between the injection and the PET scan.
A sugar called flurodeocyglucose (FDG), which is tagged with a radioactive flourine-18 (a beta-plus emitter).
Doctors will need to wait an hour before FDG becomes concentrated in certain areas of the body.
118
How is the tracer FDG tagged with flouride-18 put into a patients body?
It is injected
119
Why are PET scans used to look for cancers?
Cancer cells are more active than other cells. They will uses more energy, so require more glucose than non-cancerous cells- so the radioactive glucose becomes more concentrated in these cells and will appear brighter on the scan.
120
During a PET scan, what detects the radiation emitted from the tracer?
The ring of detectors around the body.
121
How will the tracer produce gamma rays in the patient which will be detected in a PET scan?
The radioisotope that is attached to the FDG emits positrons- each positron will travel a short distance before interacting with an electron in one of the persons atoms, causing electron-positron annihilation. 2 gamma rays are produced which travel in opposite directions (conserving energy) to be detected by the scintillators in the gamma camera.
122
What is the energy of a gamma ray emitted by electron-positron annihilation?
511keV
123
How does the gamma cameras detects where the gamma rays are emitted in a PET scan?
If detected at opposite sides of the detection equipment within a fraction of a nanosecond of each other, the computer software uses the difference in arrival time between 2 gamma rays to calculate the point at which the rays originated.
124
Why are some photons emitted in a PET scan ignored?
If photons are detected that are not within a nanosecond of each other are detected, they are ignored since they cannot have come from the same point.
125
What can a PET scan show in someones brain?
It can show how a persons brain activity varies depending on wether they are seeing, hearing or speaking.
126
Why is the half-life of the tracer used for a PET scan significant?
They need to have short half lives, so they need to be produced and incorporated into the biologically active molecule very quickly, otherwise the activity of the tracer will be too low for gamma rays to be detected.
127
How are the tracers used for PET scans made?
The tracers are made in machines called cyclotrons before being chemically prepared and then injected into the patient.
128
What diagnostic purposes are PET scans used for?
To study blood and metabolism
To detect problems with the nervous system including diseases such as Parkinson disease, multiple sclerosis, Huntington and stroke
Find changes in the brain that may lead to epilepsy
Study and analyse certain cancers
Determine how advanced a caner is and to choose the best treatment.
129
Compare PET scans and CAT scans.
Pet scans are more expensive than CAT scans.
Pet scans take longer than CAT scans.
The injection of the tracer for a PET scan is less radiation than a patient will receive in a CAT scan.
Cat scans show tissue and bone structure while PET scans show the biological function of a part of the body.
130
How is the time taken for a PET scan and a CAT scan different?
A PET scan take 2-4 hours while a CAT scan takes 30mins
131
How is the radiation experiences by the patient different in a PET scan and a CAT scan different?
The injection for a PET scan is equivalent to an exposure of a standard x-ray while CAT scans are much higher- about the same as 5 years of exposure to background radiation- which has been linked to cancers including leukaemia in individuals.
132
Define ultrasound.
Waves are longitudinal waves above the upper limit of the audible range, with frequencies greater than 20,000Hz.
133
What is a PET scans full name?
Positron emission tomography
134
Define a transducer.
A transducer is a devise, such as a microphone, which converts a non-electrical signal (e.g.sound) into an electrical signal.
135
Define the piezoelectric effect.
The piezoelectric effect is the change in volume of a material when a p.d. is applied across its opposite faces. Alternatively, it is the production of an induced e.m.f. when certain crystals are placed under stress.
136
Why is ultrasound scanning to examine a fetus?
It does not expose the patient to large amounts of ionising radiation.
137
What are waves that have a frequency above the audible range called?
Ultrasound
138
What is the range of frequencies for audible sound?
20Hz - 20,000Hz
139
What are the range of frequencies used in ultrasounds for medical purposes?
1 - 15MHz
140
How many ultrasound scans are pregnant women usually offered during their pregnancy?
2
141
What is the 1st ultrasound scan used for when examining a fetus?
Around 8-14 weeks, it can determine, from measurements of the dimensions of the fetus, when the baby is due to be born.
142
What is the 2nd ultrasound scan used for when examining a fetus?
Around 18-21 weeks, checks for structural abnormalities, particularly in the babys head or spine.
143
What is the 2nd ultrasound scan used for when examining a fetus?
Around 18-21 weeks, checks for structural abnormalities, particularly in the babys head or spine.
144
Benefices examining fetuses, what other structural information can be obtained using an ultrasound?
Can help detect kidney stones or gall stones in the gall bladder.
145
Compare ultrasounds with other medical techniques.
There are no known dangers associated with ultrasounds.
You can obtain real-time images of soft tissue such as the heart (echocardiogram)
Ultrasound machines are inexpensive and portable
145
Compare ultrasounds with other medical techniques.
There are no known dangers associated with ultrasounds.
You can obtain real-time images of soft tissue such as the heart (echocardiogram)
Ultrasound machines are inexpensive and portable
146
Why are there no known dangers associated with ultrasounds?
It is not a type of ionising radiation in the way that x-rays and gamma rays are. Nevertheless, care must be taken to keep the intensity of the ultrasound as low as possible, as the energy of the waves is absorbed by the tissue. Too intense and it can be destructive.
147
Why must the intensity of the ultrasound must be kept as low as possible?
Because the energy of the waves is absorbed by tissue- if its too intense, it can be destructive.
148
Why is a gel used in ultrasounds?
To stop the ultrasound just reflecting from the skin.
149
What are the 2 facts about ultrasounds that make it very different from other types of scanning?
Ultrasound is reflected from surfaces rather than going straight through the body.
Ultrasound sent into the body must be pulsed.
150
How are ultrasounds reflected from surfaces in the body?
Echoes are used. A boundary between tissue and liquid, or tissue and bone, or air and skin, ect., reflects the waves.
151
Ultrasounds sent into the body are pulsed, what happens after this?
After the pulse of ultrasound is sent into the body from the transducer, there is a pause while reflected echoes come back and are picked up by the transducer.
152
What is the pulse repetition frequency?
The pulse repetition frequency is the number of ultrasound pulses emitted by the transducer over a designated period of time.
153
What must the minimum time for the reflected echo to be received be like compared to the boundaries near the transducer, in an ultrasound?
The minimum time for a reflected echo to be received must be much smaller than this for boundaries near the transducer, so must of the time between pulses needs to be spent receiving.
154
What must the start of the pulse be like in an ultrasound? Why?
Its important that the pulse has a sharp start. If it does not, then the timing becomes difficult for its reflected echo.
155
How many ultrasound waves will a pulse of the transducer contain?
A few.
156
What will the reflected echo be like in an ultrasound?
A reflected echo will have many different pulses of different amplitudes from surfaces at different distances from the transducer.
157
What does a transducer contain?
Piezoelectric crystals that acts are transmitters and receivers.
158
What physical effect is used to produce ultrasounds?
The piezoelectric effect.
159
How does the piezoelectric effect work in a transducer to produce ultrasound waves?
When crystals have a pd applied to them, they contract a little. When a high frequency alternating pd (called a signal) is applied, the crystals oscillate at the frequency of the signal and emit ultrasound waves.
160
What is an example of a crystal used in a transducer to produce ultrasound waves?
Zicronate titanate
161
What is the resonance effect in transducers when receiving ultrasound waves?
Used to increases the vibration of the crystal, the crystal is cut so that its thickness is a whole number of wavelengths.
162
How are ultrasound waves received by a transducer?
If a compression from an ultrasound wave arrives at the crystal, a pd is created across it. This can be amplified electronically.
163
Define acoustic impedance.
Z, is defined by the equation Z=pc. The units are kgm^-2s^-1.
164
Define acoustic impedance.
Z, is defined by the equation Z=pc. The units are kgm^-2s^-1.
165
Define impendance matching.
The **reduction in intenisity** of reflected ultrasound at a bounary between 2 substances, acheieved when the **2 substances have similar or identical acoustic impedances**.
166
What is an A-scan called?
An amplitude scan.
167
How is an A-scan conducted?
A short pulse of ultrasound wave is sent into the body at the same time that an electron beam travels across the screen of a cathode ray oscilliscope (CRO). The transducer recieves reflected pulses whuch cause vertucal spiked on the CRO screen.
168
During an A scan, what diaplays the relected ultrasound waves?
A cathode ray oscilliscope (CRO).
169
During an A scan, what is displayed of the CRO?
Vertical spikes ahowing the refleted ultrasound waves.
170
What are the different axis on the CRO in an A scan?
* X axis = time echo took to be detected by transducer
* Y= signal strength
171
What can be worked out from the CRO from an A scan?
Can be used to fine the depths or thicknesses of reflecting tissues in the body.
172
What is a B scan?
A **real-time 2D or 3D** image of the area is built up from **many returning echoes recorded** from **several transducers** in an array, or a transducer is moved to different posistions or angles arounf the patient.
173
What is a B scans full name?
A brightness scan.
174
What are the 2 types of utlrasound scan?
* A-scan
* B-scan
175
In a B scan, Why are different brightnesses seen on the scan?
The **greater the amplitude** of the reflected pulse, the **brighter** each dot will be, so a range of brightnesses will be shown in the scan where different bone, liquid and soft tissue **reflect different proportions of the transmitted ultrasound beam**.
176
What does an ultrasound scan depend on?
Depends on the ultasound being reflected at a boundary between 2 materials.
177
What is the key to being able to get multiple reflections from different boundaries during an ultrasound?
It depends on the **fraction of the intensity of reflected** ultrasound to the **fraction emitted.**
178
When is ultrasounds reflected?
When there is a change in the density of material.
179
What can ultrasounds not pass through, what limitation does this cause?
Ultrasounds cannot pass through air, meaning it cannot be used to look at organs behind the lungs.
180
Why can the fact ultrasound is reflected by denser materials be a limitation?
* We cannot use it for brain imaging.
* Cannot use it to detect fractures.
181
What is used to determine the fraction of the signal intensity that is reflected at a boundary between 2 materials of different acoustic impedances?
Acoustic impedance
182
What is acoustic impeance used for?
used to determine the **fraction of the signal intensity that is reflected at a boundary** between 2 materials of different acoustic impedances?
183
What does each bit stand for in the equation: Ir/Io=(Z2-Z1)^2/(Z2+Z1)^2
* Ir= intensity reflected
* Io= incident intensity
* Z1= ultrasound is at a boundary and leaving one material
* Z2= ultrasound entering new material
184
Why is gel used in an ultrasound?
They need to match up similar impedances to get good transmission/ reflection values- this is called impedance matching.
185
If you send a oulse of elecromagnetic radiation at a moving object, how will the object act?
It will act like a mirror.
186
What occurs when a seed gun is used to detect the speed of a moving car?
There is a double Doppler shift when the radiation is reflected back to the transmitter/ reciever from a moving source.
187
What is the Doppler equation that we obtain when finding the speed of a moving car to obtain a value of the new frequency?
f'= (c/(c-2v)) xf
188
What does each bit stand for in the doppler equation : f'= (c/(c-2v)) xf
* f'= New frequency
* c= Velocity of elecromagnetic radiation from the speed gun
* v= Speed of the moving object
* f= Frequency of transmitted new waves.
189
What is the use of the Doppler effect in medical physics?
To measure the speed of a moving reflector can be used with ultrasound to determine the speed of blood flow in the heart or the artery. The red blood cells reflect the ultrasound.
190
What is shown in the diagram?
It shows the **pieozoelectric transducer** that has been placed on the skin and aimed at an **angle of θ** towards a blood vessel. Within the vessle, the **blood is flowing at v ms^-1**. The transducer emits ultrasound waves of f0 Hz, and the **echoes generated** by the ultrasound from the reflection of the ultrasound will be reflective, iron-rich blood cells will be at **fr**.
191
What equation shows the difference between the 2 frequencies of the blood speed found with the Doppler effect?
* Δf = f0-fr= 2f0vcosθ/c
* Vf/f0 = 2vcosθ/c
192
What needs to be measured when finding the speed of the blood cell can be found when using ultrasound for the Doppler effect?
**Time** must be measured. The **new frequency of the reflected pulse** must be measured.
193
What is done so that a readable image is produced when using ultrasounds in the Doppler effect?
The frequency of the ultrasound waves is increased to about 8MHz.
194
What shows a greater speed in the blood when using ultrasounds to measure flow blood flow?
The greater trhe difference between the frequency of the transmitted and recieved signal, the greater the speed of the blood.
195
What can a doctor tell about the blood vellsle or heart from lookign at a scan?
They can establish weather or not the heart and blood vessels is functioning correctly.
196
How can a Doppler scan be colour coded?
The difference in frequencies on the snac can be colour coordinated.
197
Give an example of when a Doppler scan can be colour coded by its frequency?
If a vein and an artery are close to each other, then the blood will be flowing in opposite direction to each other. A flow of **blood towards the source of the ultrasound wave will increase in the frequency** of the recieved wave- this will be colour coded **red**. Blood flowing in the **opposite dirrection will result in a decrease in the frequency**- this is colour coded **blue**.
