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Physics > Medical Imaging > Flashcards

Medical Imaging Flashcards

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1
Q

what is the process of x-ray production

A
  • external power supply is used to create a p.d between electrodes
  • cathode (filament) is heated and produces electrons through thermionic emission
  • the electrons are accelerated towards the anode
  • x-ray photons are produced when the electrons are decelerated by hitting the anode
  • kinetic energy of electrons is transferred into x-ray photons
  • the energy output is less than 1% of the kinetic energy of the incident electrons, the rest heats up the anode
  • the anode stays cool either by spinning or by having a coolant such as oil behind it
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2
Q

why is the x-ray tube a vacuum

A

so electrons do not lose energy through interacting with gas particles

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3
Q

what is the anode in an x ray tube made of and what is its main property

A

this is the target metal, such as tungsten, which has a large melting point

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4
Q

what does attenuation mean

A

attenuation is used to describe the decrease in the intensity of EM radiation as it passes through matter

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5
Q

what are the four attenuation mechanisms

A
  • simple scatter
  • photoelectric effect
  • compton scattering
  • pair production
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6
Q

what is simple scattering (x ray interaction with matter)

A
  • x ray photon is scattered elastically by an electron
  • x ray photon interacts with an electron in the atom, but has less energy than the energy required to remove the electron
  • occurs for x ray photons between 1 - 20 keV
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7
Q

what is the photoelectric effect (x ray interaction with matter)

A
  • x ray photon is absorbed by one of the electrons in the atom
  • the electron uses this energy to escape from the atom
  • x ray photons with less than 100keV
    common in x ray imaging
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8
Q

what is compton scattering

A
  • incoming x ray photon interacts with an electron within an atom
  • electron is ejected from atom, but the x ray photon does not disappear completely, it is just scattered with less energy
  • x ray photons with energy of 0.5 - 5MeV
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9
Q

what is pair production (x ray interaction with matter)

A
  • x ray photon interacts with the nucleus of the atom
  • it disappears and the EM energy of the photon is used to create an electron-positron pair
  • only occurs when x ray photon has energy equal to or greater then 1.02MeV
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10
Q

what does a higher attenuation coefficient mean

A

it absorbs x ray photons more than a material with a lower attenuation coefficient

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11
Q

what is a contrast medium

A

used to improve visibility of internal structures in x ray images

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12
Q

what are the two most common contrast mediums and what are each used for

A
  • iodine
  • injected into blood vessels to view certain liquids such as blood flow
  • barium sulfate
  • ingested to view the digestive system
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13
Q

what is the attenuation coefficient proportional to

A

the cube of the atomic number

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14
Q

what is the process of a CAT scan

A
  • patient lying on back slides into gantry
  • x ray tube on one side and x ray detectors on the other
  • x ray tube produces a fan shaped beam of electrons which irradiates a thin slice of the patient
  • x rays are attenuated by different amounts by different tissues
  • intensity of x rays is recorded by detectors
  • sends electrical signals to a computer
  • x ray tube and detectors rotate 360 degrees around patient creating 2D image of thin slice of patient
  • table moves forward, new image is created
  • images are manipulated to create a £D image of patient
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15
Q

what are the advantages of a CAT scan

A
  • shape, size and position of tumours can be determined
  • can distinguish between soft tissues of similar attenuation coefficients
  • non-invasive
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16
Q

what are the disadvantages of a CAT scan

A
  • slower and more expensive than a single x ray scan
  • subject to ionising radiation
  • patients must remain very still for a long period of time
17
Q

why are gamma emitting sources ideal for medical tracers

A
  • least ionising
  • can penetrate through the patient to be detected externally
18
Q

why must radioisotopes used in medical imaging have a short half life

A
  • to ensure high activity from the source so that only a small amount is required to form the image
  • so that patient is subjected to the radiation for a long period of time
19
Q

describe the gamma camera

A
  • gamma photons travel towards the collimator, a honeycomb of long, thin tubes made from lead
  • any photons arriving at an angle to the axis of the tubes are absorbed by the tubes, so only those travelling parallel can reach the scintillator
  • gamma photons that reach the scintillator material produces thousands of visible light photons
  • the chance of a gamma photon interacting with the scintillator is only 1 in 10
  • the photons of visible light travel to the photomultiplier tubes
  • photons of light are converted into voltage
  • the outputs are connected to a camera
  • these electrical signals are processed to locate the impacts of the gamma photons on the scintillator
  • these impact positions are used to construct a high-quality image that shows the concentrations of the medical tracer within the body
20
Q

why must gamma photons be travelling parallel to the collimator tubes

A

to produce a high-quality image that isnt blurred

21
Q

describe the process of a PET scan

A
  • patient lies on table and is surrounded by a ring of gamma detectors
  • FDG (fluorodeoxyglucose) is injected
  • decays, giving off a positron
  • positron annihilates and produces two gamma photons travelling in opposite directions
  • photons picked up by gamma camera
  • computer can determine the point of annihilation from the time difference in the arrival of the two gamma photons
22
Q

what are the advantages of a PET scan

A
  • non-invasive
  • diagnoses different types of cancers
  • can observe the function of the brain
23
Q

what are the disadvantages of PET scans

A
  • very expensive
  • only found at larger hospitals
  • not very common so only patients with complex health problems are recommended
24
Q

what is an ultrasound transducer

A
  • a device that changes electrical signals into sound and sound into electrical signals
  • generates and receives ultrasound
  • uses the piezoelectric effect
25
Q

how does the piezoelectric effect produce ultrasound

A
  • high frequency alternating p.d is applied across opposite faces of a crystal
  • this repeatedly compresses and expands the crystal
  • the frequency chosen to be the same as the natural frequency of the crystal, so it resonates and produces an intense ultrasound signal
26
Q

how does a piezoelectric crystal detect ultrasound

A
  • any ultrasound incident on the crystal will make it vibrate, soo the crystal is compressed and expanded by tiny amounts
  • this vibration generates an alternating p.d across the ends of the crystal, which can be detected by electronic signals
27
Q

what is an a-scan

A
  • single transducer used to record a straight line through the patient
  • usually used to determine the distance between the lens and the retina
28
Q

explain how a b-scan works

A
  • multiple a-scans
  • transducer moved over patient’s skin
  • output connected to a high-speed computer
  • each position, computer produces a row of dots corresponding to the boundary between two media
  • brightness of dot dictates the intensity
  • collection of dots produces a 2D image
29
Q

what is acoustic impedance

A
  • defined as the product of the density of the substance and the speed of the ultrasound in that substance
  • dictates the proportion of ultrasound that passes through a media
30
Q

what is coupling gel

A

a gel which matches acoustic impedance to that of your skin so less ultrasound is reflected by the skin

31
Q

how can you determine the speed of blood

A
  • transducer placed at angle to the skin
  • ultrasound reflects off moving blood cells with a changed frequency (increased when blood is moving towards the transducer)
  • transducer receives ultrasound pulses and is connected to an oscilloscope
  • speed of blood determined using the formula

Physics (13 decks)

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