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OCR Physics A2 > Medical Imaging (DONE) > Flashcards

Medical Imaging (DONE) Flashcards

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1
Q

who discovered x - rays and when were they discovered?

A
  • 1895

- Wilhelm Roentgen

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2
Q

How were x - rays discovered?

A
  • Roentgen was carrying out experiments passing current through an evacuated tube with a high voltage.
  • He noticed radiation could travel through the glass and the dark paper around it.
  • a plate within the room started to fluoresce.
  • He put his hand between the tube and fluorescent plate and found the radiation passed through his hand
  • He put a photographic plate infant of the plate and got his wife to put her hand in front of the plate.
  • He took the photo.
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3
Q

name the components of an x - ray machine

A
  • vacuum tube
  • power source
  • cathode
  • anode
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4
Q

What P.Ds are used in an x - ray machine?

A
  • 80 to 120kw for looking inside body

- 200+ kw for fighting cancer

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5
Q

what is a cathode?

A
  • the negatively charged electrode by which electrons are emitted
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6
Q

What is an anode?

A
  • the positively charged electrode by which the electrons are accepted
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7
Q

How are x - rays produced in an x - ray machine?

A
  • A filament (cathode) is heated by an electrical current.
  • electrons are emitted from the filament.
  • the high P.D. accelerates electrons between the filament and the anode which is normally a metal with a high melting point e.g. Tungsten.
  • the tube is evacuated as electrons would lose energy on their path to the anode through colliding with gas particles otherwise.
  • if the electrons have enough energy they will hit the anode and x - rays will be emitted.
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8
Q

What safety precautions are taken with an x - ray machine?

A
  • the vacuum tube is surrounded by lead shielding to prevent radiographers being exposed to radiation.
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9
Q

What safety precautions are taken with an x - ray machine?

A
  • the vacuum tube is surrounded by lead shielding to prevent radiographers being exposed to radiation.
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10
Q

How can you distinguish an x - ray from a gamma ray of the same frequency?

A
  • x - rays form when electron is decelerated

- gamma ray is formed when an electron is emitted from the nucleus.

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11
Q

what is the equation for the gain in kinetic energy of an x - ray photon?

A

Ek = eV

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12
Q

how do you do derive an equation for frequency from E = hf

A

E = hf = eV
- therefore:
f = (eV)/h

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13
Q

How do you derive an equation for wavelength of an x - ray photon?

A

E = hc/wavelength
- therefore
wavelength = hc/eV

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14
Q

what happens to frequency and wavelength when voltage increases in an x - ray machine?

A

when voltage increases:

  • frequency increases
  • wavelength decreases
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15
Q

when do you achieve minimum wavelength of an x - ray photon?

A
  • when all of the electrons kinetic energy is transferred to x - ray photons.
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16
Q

why does the graph of intensity against wavelength vary as wavelength increases?

A
  • because not all electrons transfer 100% of their kinetic energy to x - ray photons
  • some energy is lost through heat.
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17
Q

what is the relationship between x - rays emitted and electrons emitted from the filament per second?

A
  • x - rays emitted is directly proportional to the electrons emitted from the filament per second.
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18
Q

how does accelerating p.d. change minimum wavelength of an x - ray photon?

A
  • high accelerating voltage = lower min wavelength

- low accelerating voltage = higher min wavelength

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19
Q

Describe the pattern of the intensity against wavelength graph for x - ray photons

A
  • where 100% of the kinetic energy is transferred to x - ray photons, the minimum wavelength is produced.
  • intensity increases as wavelength increases up until a large wavelength.
  • the x - ray intensity then falls away at a large wavelength.
  • this continuous line continues to decrease as wavelength increases.
  • sharp peaks throughout the continuous data show characteristics of the anode material.
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20
Q

why are there characteristic changes in the graph of x - ray intensity against wavelength?

A
  • as electrons collide with the anode, the electrons within the anode rearrange and change shells.
  • when they return to their original state photons with specific frequencies are released meaning there are sharp peaks on the graph.
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21
Q

what is attenuation?

A
  • a gradual decrease in intensity.
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22
Q

describe the simple scatter interaction of x - ray photons

A
  • occurs only with low energy x - ray photons.
  • where the energy of the photons are not sufficient to cause ionisation.
  • the photon collides with an electron and is scattered (deflected) however there is no change in energy.
  • the scattering causes ‘noise’ in the image due to the arrival at the detector of scattered rays.
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23
Q

describe the interaction of x - ray photons via the photoelectric effect

A
  • x - ray photons can cause the emission of electrons from their energy shells within an atom.
  • provided the photon has enough kinetic energy the electron will escape from the atom.
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24
Q

what is the work function?

A
  • the amount of energy needed for an electron to escape the atom.
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25
Q

what is the equation used to calculate the energy of the incoming photon during the photoelectric effect?

A

energy of photon = work function + max kinetic energy of photoelectron

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26
Q

what is a photoelectron?

A
  • an electron emitted from an atom through interaction with a photon.
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27
Q

what is ionisation?

A
  • the addition or removal of an electron to create an ion.
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28
Q

describe how x - ray photons interact through the Compton effect/scattering

A
  • x - ray photons fired at an atom collide with electrons.
  • the electron is ejected from the atom with a small amount of kinetic energy lost from the photon.
  • a photon of the remaining energy is emitted.
  • the photon and electron are scattered in different directions.
  • a photon deflected through a larger angle will have lost more energy and so will have a longer wavelength.
  • this is what Compton found and it was regarded as strong evidence for the quantum theory.
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29
Q

what are x - rays of just one fixed wavelength called and how are they obtained?

A
  • monochromatic x - rays
  • obtained by filtering out all of the wavelengths except those that correspond to the strongest sharp peaks of the x - ray spectrum.
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30
Q

how was Compton scattering discovered?

A
  • Arthur Compton
  • conducted experiments on how materials scattered radiation.
  • he obtained monochromatic x - rays and used an x - ray spectrometer to measure the wavelength of the scattered radiation from a carbon target.
  • he found that some deflected rays had larger wavelengths than the initial wavelength.
  • he explained this using quantum theory.
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31
Q

what happens during Compton scattering if the angle at which the photon is deflected increases?

A
  • as the angle at which the photon is deflected increases, the energy lost by the photon will also increase.
  • therefore the photon will have a longer wavelength.
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32
Q

describe x - ray interaction through pair production?

A
  • where a beam of high frequency x - rays are fired at the nucleus of an atom
  • the x - ray photon interacts with the nucleus and the photon vanishes, producing a positron and an electron.
    (2 bits of matter created out of energy)
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33
Q

when is the interaction of x - rays through pair production likely to happen?

A
  • when a voltage of over 1,000,000V is used to accelerate electrons from the cathode to the anode.
34
Q

what is a collimated x - ray beam?

A
  • a beam where all rays are parallel and do not disperse over time
35
Q

what happens to a collimated beam as it passes through a substance?

A
  • the intensity decreases with distance.
36
Q

what does the graph of intensity against distance show?

A

y = e^-x

37
Q

what happens to the intensity of x - rays if the thickness of the material is doubled?

A
  • the intensity becomes a 1/4 of the original value.
38
Q

what is the attenuation coefficient?

A
  • tells us the amount of rays absorbed per metre of the substance.
39
Q

what is the formula for the intensity of of collimated rays passing through a substance?

A

I = I0 x e^(Mu)x

- where:
I = intensity
I0 = original intensity
e = elementary charge
Mu = attenuation coefficient
x = distance through medium
40
Q

what is z in terms of attenuation and x - rays?

A
  • the number of protons within the nuclei of a substance
41
Q

what is the relationship between the attenuation coefficient and z?

A
  • the attenuation coefficient is proportional to z^3
42
Q

what do attenuation coefficients depend on?

A
  • vary with the energy of the x -rays used, therefore the accelerating p.d. impacts.
  • also the value of z
43
Q

what units does the attenuation coefficient have?

A

m^-1

44
Q

what is the distance for halving intensity often called? (x - rays)

A
  • the half value thickness
45
Q

what are contrast materials used for in x - rays?

A
  • to see different types of soft tissues with similar attenuation coefficients.
46
Q

what contrast materials are often used for x - rays?

A
  • iodine

- barium

47
Q

how is barium sulphate (barium meal) useful for x - rays?

A
  • patient may swallow barium meal before x -ray of digestive tract
  • the barium coats the intestine and absorbs more x -rays than flesh.
  • ## there image is enhanced in contrast with other abdominal structures
48
Q

how can iodine be useful for x - rays?

A
  • can be injected in blood
  • absorbs more x - rays than flesh
  • shows contrast against bones
  • shows where blood is flowing.
49
Q

what characteristic makes barium sulphate and iodine ideal for contrast on x - ray images?

A
  • both have a high z number
50
Q

why do atoms with high z numbers absorb more x - rays than atoms with low z numbers?

A
  • the high z atoms have electrons bound by an energy equivalent to that of the x - rays photons.
  • therefore they absorb more than low z atoms.
51
Q

name 3 examples of low z atoms

A
  • hydrogen
  • carbon
  • oxygen
    (found in soft tissue)
52
Q

How are x-ray images formed inside an x-ray machine?

A
  • to find out what is inside a person we use a clear plastic inside the machine which is coated in silver halide like normal photographic film.
  • however the x rays go straight through this plastic so we take the plastic and sandwich it in between 2 sheets which are coated in phosphor.
  • this is because the x rays do not interact with the plastic so we get the x rays to interact with the phosphor coating instead.
  • When the x rays reach the phosphor coated sheets they cause a small amount of light.
  • the flash of light is picked up by the silver halide ions on the plastic which become silver ions and we get an image recorded.
53
Q

What can be used as an image intensifier for x-ray images and how does it work?

A
  • x - ray photons are fired at the patient and reach the input phosphor screen.
  • the input phosphor emits light where the x - rays hit and the light then hits the photo cathode.
  • the photo cathode turns the light into a beam of electrons.
  • the electrons are focused and directed by electrodes (cathodes) toward the anode which accelerates the electrons to the output phosphor screen.
  • the output phosphor screen turns the electron beam into light.
  • a camera takes a picture of the light and it is processed by a computer.
54
Q

Why do we use x-ray image intensifiers?

A
  • in order to use as few x-rays as possible.

- and also to have them as weak as possible as they are ionising.

55
Q

What is a CAT scan?

A
  • computerised axial tomography.

- used to create 3D images of x-rays.

56
Q

What is inside a CAT scanner and how does it work?

A
  • the x-ray machine rotates 360 degrees around the patient very quickly while it sends out a narrow fan shaped beam of x-rays.
  • there is often a full ring of x-ray detectors around the patient, sometimes the detectors move with the x-ray machine.
  • as the x-ray machine rotates around 360 degrees it starts to take various x-ray images.
  • it will also move lengthways across the patient.
57
Q

What are the benefits and limitations of CAT scanners?

A

+ they provide very detailed 3D images.
+ also allow us to see soft tissue detailed.
- Expensive cost
- the process can take a few minutes so the patient can be exposed to a lot of radiation.

58
Q

What are the 2 main uses of radioisotopes in medical physics?

A
  • diagnosis (can be injected into the body to find the problem).
  • therapy (if we know the problem we can use it when fighting cancer).
59
Q

What types of radiation is used in diagnosis and why?

A
  • beta + and gamma
  • alpha and beta minus are highly ionising and weakly penetrating so are unable to leave the body and therefore are not used.
60
Q

How can we use Beta + emitters for diagnosis in medical physics?

A
  • we use fluorine-18 because it has a short half life of 110 minutes and when it decays turns into oxygen + Beta plus + neutrino + gamma radiation.
  • the positron emitted from the decay of fluorine-18 is used in PET scanners where electron and positron annihilate emitting gamma radiation.
61
Q

How can we use gamma radiation for diagnosis in medical physics?

A
  • gamma radiation can pass through the body meaning we can detect it using a gamma camera.
  • the element used for this is technetium-99m which when combined with other radio pharmaceuticals tells us how major organs in the body work.
  • the element releases a gamma photon to become more stable, allowing a gamma camera to detect it.
62
Q

How is technetium-99m formed and why is it used in medical physics?

A
  • in order to obtain it we start with molybdenum-99 which decays into technetium-99m/.
  • the m stands for something which is metastable which is a highly energised state of the element but still with the same amount of protons.
  • this means the element is not as stable as it could be so in order to become more stable it becomes technetium-99 and releases a gamma photon.
  • therefore the m in technetium-99m shows we have a radioisotope.
  • the half life is 67 hours meaning it is not too short or long.
63
Q

What can you do with fluorine-18 and technetium-99m in medical physics?

A
  • you can combine fluorine-18 and technetium-99m to make a medical tracer (radio pharmaceutical).
  • we can detect where elements are within the body using gamma cameras and PET scanners to see which parts of the body are working well.
64
Q

What is a gamma camera used for?

A
  • used to look inside the body to find medical tracers/gamma emitters such as technetium-99m.
  • you can inject radioisotopes into a patients body but you then need to find where the isotope which is why you use a gamma camera.
65
Q

How do gamma cameras detect radioactive material in the body?

A
  • usually there are 2 cameras at 90 degrees to each other to pinpoint where the material is.
  • firstly you have the collimator which is a piece of lead with a huge amount of holes drilled into it meaning it absorbs any gamma photons that don’t pass through the holes.
  • then we have the scintillator, it is a sodium iodide crystal that when hit by a gamma photon gives a small flash of visible light.
  • the light guide detects the light and guides it towards the photomultiplier.
  • the photomultiplier takes the light and converts it to electrons which are multiplied creating a cascade of electrons.
  • we then get a measurable current and therefore voltage which is sent down wires to a computer.
  • using a computer we can build an image of where gamma photons are coming from in the patient showing which parts of the body are working well.
66
Q

What is a pet scanner?

A
  • positron emission tomography.

- a gamma imaging test which detects medical tracers in your body in order to diagnose disease.

67
Q

How do PET scanners work?

A
  • when we have the annihilation of a beta plus and minus particle, 2 gamma photons are given out.
  • this is because matter and antimatter annihilate to make pure energy.
  • This energy is in the form of 2 gamma photons.
  • The energy of these is around 0.51Mev
  • the PET scanner has a ring of detectors that detect where gamma rays are given out.
  • each detector is a collimator connected to a scintillator and a photomultiplier.
  • the 2 gamma rays are given out at 180 degrees to each other due to conservation of momentum, this means the source is on a point somewhere between the where 2 gamma rays were detected.
  • If the gamma is detected by one detector quicker than the other, it must mean the source is closer to that detector.
  • They can use the time difference to calculate the distance therefore the location of the source.
68
Q

How is a beta plus particle produced in PET scanners?

A
  • fluorine-18 is used as it emits positrons when it decays, it also replaces oxygen in glucose.
  • another benefit of fluorine is that it has a short half life.
  • what we then make is FDG which stands for fluoridedeoxy glucose.
  • this can be injected into the body and is used most by cells which are functioning the most, for example cancerous cells which need glucose to multiply.
  • We can also use carbon-11 as a substitute to fluorine, as it can be used in carbon monoxide which is taken in the blood and is moved around the body
69
Q

Advantages and disadvantages of PET scanners?

A

+ non invasive
+ good at looking how the brain functions and detecting cancer.
- expensive
- making fluorine-18 is difficult and large resources are needed.

70
Q

What is ultrasound?

A
  • any kind of longitudinal mechanical wave that has a frequency greater than 20 kilohertz.
71
Q

why do we use ultrasound to see what is inside a person?

A
  • We can send waves through bodies and it doesn’t do the body any harm.
  • We tend to use frequencies of around 5 megahertz MHz.
  • This means if you use a frequency like this the wavelengths produced will be smaller than 1mm.
  • In order to produce and then detect the ultrasound we need to use the Piezoelectric effect.
72
Q

What is the piezoelectric effect?

A
  • Piezoelectric Effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress.
  • If you have certain crystal, we can do 2 things, we can cause it to move ourselves by compressing it or we could apply a potential difference across it.
  • if you apply a potential difference across it, the shape of the crystal will change resulting in movement.
  • The reverse of this is also true, if you take a crystal and compress it causing it to move what we can then take is that movement induces an emf across the crystal.
  • this means that crystals can be used to detect very small amounts of movement.
  • this type of crystal and effect is what we find in a transducer.
73
Q

How are you able to give out and detect ultrasound using a transducer?

A
  • A transducer is connected to a computer and inside it we have a crystal surrounded by a damper.
  • The transducer is placed next to your skin and sends a small pulse of ultrasound through the body, and you wait for it to reflect back.
  • The reflected echo of the ultrasound causes the piezoelectric crystal to move, creating an emf which we can then detect using a computer.
  • There are 2 main types of ultrasound scan, the A-scan and the B-scan.
74
Q

What is an A-scan and how does it work?

A
  • The A-scan is often seen on an oscilloscope.
  • If you sent an ultrasound through material 1 and material 2 which are joined together, you would find where the signal enters material 2, we have is a partial reflection.
  • this means that some of the ultrasound is reflected back to where it came from and the rest moves through the medium, this process will repeat for each new medium.
  • this is why on a graph of intensity against time, we have an initially high peak where ultrasound is given out and then get a few further return peaks which are of gradual lower intensities.
  • this is a useful way of measuring the thickness of things such as an eye lense.
75
Q

What can you calculate using an A-scan?

A
  • we can measure the time it takes to go from medium 1 to medium 2 and reflect back to medium 1 again, t1.
  • If we know the time t1 and the speed of the ultrasound c, then we can then work out the distance travelled.
  • This is a useful way of measuring the thickness of things such as an eye lense.
76
Q

How do B-scans work and why do we use them?

A
  • A-scans do not give any indication of what is inside a body so this is why we use a B-scan.
  • A B-scan is many A-scans which have all been superimposed on each other
  • the computer works out what the image is inside which gives us a lot more detail.
  • B-scans are usually 2D but are also 3D and 4D.
77
Q

What is acoustic impedance?

A
  • Acoustic impedance is a measure of how well a sound moves through a material.
  • It has the symbol Z and = density x speed of sound within the substance
  • Where density has the symbol rho, and the speed of sound has the symbol c.
  • Due to density having units kgm^-3 and speed having units ms^-1, the units of acoustic impedance Z is kgm^-2s^-1
78
Q

What can the acoustic impedance be used to calculate?

A
  • using the acoustic impedance Z1 and Z2 of the mediums, you can calculate how much energy will be reflected at the boundary.

(Ir/Io) = [(Z2-Z1)/(Z2+Z1)]^2

  • where Ir = the intensity of the reflected wave.
  • and Io = the intensity of the original wave.
  • This means that if Z1 is approximately = to Z2 (e.g. going from tissue to blood) then what we will find is that Ir/Io is small and therefore only a small amount of ultrasound is reflected.
  • However if Z1 was a lot smaller than Z2, the Ir/Io would be equal to a large value and therefore a lot more of the ultrasound wave is reflected at the boundary.
79
Q

How can acoustic impedance be used on a persons body?

A
  • if we have a transducer next to a persons skin, if the transducer sends an ultrasound signal through the air, and then into the body we find that the value of Z1 (air) is a lot smaller than Z2 (body) meaning that most of the ultrasound wave gets reflected back.
  • in order to prevent most of the ultrasound being reflected we need to try and match the acoustic impedance for the transducer, the air and the body.
  • We do this through using a coupling gel which gets rid of the air between the body and what we find is that Z1 is approximately equal to Z2 and little ultrasound is reflected.
80
Q

What is coupling gel in acoustic impedance?

A
  • coupling gel gets rid of the air between the body and transducer so that Z1 is approximately equal to Z2.
  • the coupling gel provides acoustic/ impedance matching.
  • this is why in a hospital when looking at a baby inside the mothers womb, they put a gel on the belly.
81
Q

How can we use the Doppler effect to look at blood in the body?

A
  • the Doppler effect is why racing cars change sound when they pass you at a high speed and it is due to the change in wavelength or frequency as the signal is moving towards/away from you.
  • We can apply this to blood to tell us when the red blood cells in veins/arteries are moving towards or away from an ultrasound signal.
  • so if we have an ultrasound signal that is moving towards a red blood cell also heading for the signal, the red blood cell reflects the ultrasound.
  • because it is moving towards it the frequency increases and the wavelength decreases.
  • the reverse is also true, if the red blood cell is moving away as it reflects the ultrasound it increases the wavelength and decreases the frequency.
  • we can then pick up the change in frequency using the transducer and determine whether the blood is moving away from or towards the transducer.
82
Q

What can you calculate using the doppler effect and a transducer?

A
  • using a blood vessel with blood moving inside it we can put the transducer at a certain angle to measure which way the blood is moving.
  • the ultrasound waves will therefore enter the body at an angle theta to the blood vessel.
  • we know how fast the ultrasound travels in blood c, and we know that the red blood cells are moving with a velocity v.
  • we can then work out if the red blood cells move towards the transducer through the change in frequency.
  • we can use the equation that the change in f = [2fv x cos(theta)]/c
  • Where f = frequency of the ultrasound,
  • v = speed of the red blood cells,
  • c = speed of the ultrasound in the medium
  • theta = the angle at which the transducer is placed
  • the transducer has to be at an angle because if the transducer was at 90 degrees to the vessel, we would have cos(90) which is equal to 0.
  • from this you can then tell if the body is working as it should.

OCR Physics A2 (15 decks)

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