Medical Imaging (DONE) Flashcards
who discovered x - rays and when were they discovered?
- 1895
- Wilhelm Roentgen
How were x - rays discovered?
- 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.
name the components of an x - ray machine
- vacuum tube
- power source
- cathode
- anode
What P.Ds are used in an x - ray machine?
- 80 to 120kw for looking inside body
- 200+ kw for fighting cancer
what is a cathode?
- the negatively charged electrode by which electrons are emitted
What is an anode?
- the positively charged electrode by which the electrons are accepted
How are x - rays produced in an x - ray machine?
- 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.
What safety precautions are taken with an x - ray machine?
- the vacuum tube is surrounded by lead shielding to prevent radiographers being exposed to radiation.
What safety precautions are taken with an x - ray machine?
- the vacuum tube is surrounded by lead shielding to prevent radiographers being exposed to radiation.
How can you distinguish an x - ray from a gamma ray of the same frequency?
- x - rays form when electron is decelerated
- gamma ray is formed when an electron is emitted from the nucleus.
what is the equation for the gain in kinetic energy of an x - ray photon?
Ek = eV
how do you do derive an equation for frequency from E = hf
E = hf = eV
- therefore:
f = (eV)/h
How do you derive an equation for wavelength of an x - ray photon?
E = hc/wavelength
- therefore
wavelength = hc/eV
what happens to frequency and wavelength when voltage increases in an x - ray machine?
when voltage increases:
- frequency increases
- wavelength decreases
when do you achieve minimum wavelength of an x - ray photon?
- when all of the electrons kinetic energy is transferred to x - ray photons.
why does the graph of intensity against wavelength vary as wavelength increases?
- because not all electrons transfer 100% of their kinetic energy to x - ray photons
- some energy is lost through heat.
what is the relationship between x - rays emitted and electrons emitted from the filament per second?
- x - rays emitted is directly proportional to the electrons emitted from the filament per second.
how does accelerating p.d. change minimum wavelength of an x - ray photon?
- high accelerating voltage = lower min wavelength
- low accelerating voltage = higher min wavelength
Describe the pattern of the intensity against wavelength graph for x - ray photons
- 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.
why are there characteristic changes in the graph of x - ray intensity against wavelength?
- 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.
what is attenuation?
- a gradual decrease in intensity.
describe the simple scatter interaction of x - ray photons
- 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.
describe the interaction of x - ray photons via the photoelectric effect
- 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.
what is the work function?
- the amount of energy needed for an electron to escape the atom.
what is the equation used to calculate the energy of the incoming photon during the photoelectric effect?
energy of photon = work function + max kinetic energy of photoelectron
what is a photoelectron?
- an electron emitted from an atom through interaction with a photon.
what is ionisation?
- the addition or removal of an electron to create an ion.
describe how x - ray photons interact through the Compton effect/scattering
- 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.
what are x - rays of just one fixed wavelength called and how are they obtained?
- 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.
how was Compton scattering discovered?
- 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.
what happens during Compton scattering if the angle at which the photon is deflected increases?
- 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.
describe x - ray interaction through pair production?
- 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)
when is the interaction of x - rays through pair production likely to happen?
- when a voltage of over 1,000,000V is used to accelerate electrons from the cathode to the anode.
what is a collimated x - ray beam?
- a beam where all rays are parallel and do not disperse over time
what happens to a collimated beam as it passes through a substance?
- the intensity decreases with distance.
what does the graph of intensity against distance show?
y = e^-x
what happens to the intensity of x - rays if the thickness of the material is doubled?
- the intensity becomes a 1/4 of the original value.
what is the attenuation coefficient?
- tells us the amount of rays absorbed per metre of the substance.
what is the formula for the intensity of of collimated rays passing through a substance?
I = I0 x e^(Mu)x
- where: I = intensity I0 = original intensity e = elementary charge Mu = attenuation coefficient x = distance through medium
what is z in terms of attenuation and x - rays?
- the number of protons within the nuclei of a substance
what is the relationship between the attenuation coefficient and z?
- the attenuation coefficient is proportional to z^3
what do attenuation coefficients depend on?
- vary with the energy of the x -rays used, therefore the accelerating p.d. impacts.
- also the value of z
what units does the attenuation coefficient have?
m^-1
what is the distance for halving intensity often called? (x - rays)
- the half value thickness
what are contrast materials used for in x - rays?
- to see different types of soft tissues with similar attenuation coefficients.
what contrast materials are often used for x - rays?
- iodine
- barium
how is barium sulphate (barium meal) useful for x - rays?
- 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
how can iodine be useful for x - rays?
- can be injected in blood
- absorbs more x - rays than flesh
- shows contrast against bones
- shows where blood is flowing.
what characteristic makes barium sulphate and iodine ideal for contrast on x - ray images?
- both have a high z number
why do atoms with high z numbers absorb more x - rays than atoms with low z numbers?
- 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.
name 3 examples of low z atoms
- hydrogen
- carbon
- oxygen
(found in soft tissue)
How are x-ray images formed inside an x-ray machine?
- 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.
What can be used as an image intensifier for x-ray images and how does it work?
- 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.
Why do we use x-ray image intensifiers?
- in order to use as few x-rays as possible.
- and also to have them as weak as possible as they are ionising.
What is a CAT scan?
- computerised axial tomography.
- used to create 3D images of x-rays.
What is inside a CAT scanner and how does it work?
- 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.
What are the benefits and limitations of CAT scanners?
+ 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.
What are the 2 main uses of radioisotopes in medical physics?
- diagnosis (can be injected into the body to find the problem).
- therapy (if we know the problem we can use it when fighting cancer).
What types of radiation is used in diagnosis and why?
- 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.
How can we use Beta + emitters for diagnosis in medical physics?
- 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.
How can we use gamma radiation for diagnosis in medical physics?
- 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.
How is technetium-99m formed and why is it used in medical physics?
- 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.
What can you do with fluorine-18 and technetium-99m in medical physics?
- 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.
What is a gamma camera used for?
- 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.
How do gamma cameras detect radioactive material in the body?
- 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.
What is a pet scanner?
- positron emission tomography.
- a gamma imaging test which detects medical tracers in your body in order to diagnose disease.
How do PET scanners work?
- 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.
How is a beta plus particle produced in PET scanners?
- 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
Advantages and disadvantages of PET scanners?
+ non invasive
+ good at looking how the brain functions and detecting cancer.
- expensive
- making fluorine-18 is difficult and large resources are needed.
What is ultrasound?
- any kind of longitudinal mechanical wave that has a frequency greater than 20 kilohertz.
why do we use ultrasound to see what is inside a person?
- 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.
What is the piezoelectric effect?
- 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.
How are you able to give out and detect ultrasound using a transducer?
- 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.
What is an A-scan and how does it work?
- 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.
What can you calculate using an A-scan?
- 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.
How do B-scans work and why do we use them?
- 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.
What is acoustic impedance?
- 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
What can the acoustic impedance be used to calculate?
- 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.
How can acoustic impedance be used on a persons body?
- 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.
What is coupling gel in acoustic impedance?
- 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.
How can we use the Doppler effect to look at blood in the body?
- 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.
What can you calculate using the doppler effect and a transducer?
- 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.
