medical physics Flashcards
how are x-rays produced
- rapidly accelerating or decelerating charged particles
- their kinetic energy is transferred into high energy photons
how can you differentiate between x-rays and gamma rays
- X-rays and gamma rays have frequencies that overlap so cannot distinguish by wavelength
- use their method of production
- gamma rays come from radioactive decay or particle collisions with a mass defect
- X-rays produced by accelerating charged particles
why are X-rays used in medical imaging
have energy that is lower than gamma rays
general structure of an x ray tube
heat filament (cathode) and tungsten anode with a potential difference between them and sealed in a vacuum tube
how does an x-ray tube tube work
- electrons are emitted from the heated filament via thermionic emission and drain towards the anode
- collide with the anode and some of. their kinetic energy is released as x-rays in all directions
- the rest is transferred to heat energy within the anode
why does the x-ray tube need a vacuum
prevents electrons from colliding with molecules of air before they gain enough energy to release x-rays
how is the anode prevented from overheating
- rotating so new section is in contact with the x-rays
- using water as a coolant, circulating it through the anode
how are x-rays focused into one beam
- vacuum tube is encased in a material that is thinner in one area, so only x-rays that pass through that section are released from the tube
- they pass through a collimator - a series of straight parallel tubes that absorb any rays that are not travelling parallel to the axis of the tubes
why is it better for x-rays ti be in a beam rather than emitted in all directions
allows them to be directed at specific areas (like a broken bone) and minimises the patients exposure to them
what is x-ray attenuation
when a material absorbs x-rays decreasing the intensity exponentially
how can you calculate the intensity of x-rays leaving a material
I = I0e^-Px
I0= initial intensity
P = attenuation
x = thickness
explain the process of taking an x-ray of a patient
- x-rays directed at area of patients body an pass through bine and soft tissue
- bone higher attenuation coefficient it absorbs more x-rays then soft tissue
- if photographic film is placed behind the patient the areas where bine is will not blacken as much as areas of soft tissue creating an image
- nowadays digital detectors are used in place if photographic film
the greater the attenuation (absorption) coefficient…
the more the material will absorb incident x - rays
simple scattering
- x-rays of energy between 1 and 20KeV are directed at a material
- x-rays will reflect off layers of atoms or molecules in the material because they have insufficient energy to undergo more complex processes (like photoelectric effect)
photoelectric effect
- x0rays of energy less than 100KeV are directed at a material
- x-rays can be absorbed by electrons in the material if they have the same energy as the ionisation energy of the atoms
- x-ray is absorbed, a photoelectron is released and another electron may de-excite, releasing another photo in the process
Compton effect
- x-ray of energy between 0.5 and 5MeV are directed at a material
- x-rays will lose a small amount of their energy to electrons in the absorbing materials due to an inelastic collision between the photon and electron
- scattered x-ray photon will have less energy than before ( greater wavelength)
- the Compton electron will be scattered in a different direction as momentum must be conserved
pair production process
- an x-ray of energy greater than 1.02MeV passes through the electric field of an atom
- electron-position pair is produced
- positron will annihilate with another electron and produce two photons
- process not very important in medical x-rays as the photon energies are usually not high enough to cause pair production
contrast media and give two examples
- high attenuation coefficient materials that have heavy atoms with a large proton number and therefore a large number of electrons
- easily identified in x-ray images as they absorb a lot of x-ryas
- examples (barium (Z=56) or iodine (Z=53) compared to soft tissue (Z=7)
relationship between attenuation coefficient and proton number
P = Z^3
what does the CAT stand for in CAT scan
computerised axial tomography
what is a CAT scan and how do they work
- 3D x-ray image of a patients body made up of lots of 2d images
- x-ray tube generates a fan - shaped beam that is directed onto a patent that is laying down
- ring of detectors behind the patient to detect the beam intensity
- tube and detectors rotate around the patient and up and down their body to create 3d image
compare CAT scans to conventional x-ray images
- CAT scans Gove a better resolution image and having a 3d image makes it easier to assess the injury
- however CAT scans take significantly longer than conventional x-rays so the patient is exposed for longer
medical tracer
- compound made of radioactive isotope and specific elements that collects in a particular location in the body
- medical tracers can be used to locate things such as cancerous tumours in the body
how are tracers used in non-invasive diagnosis and which type if radiation is best-suited for it
- tracer is administered to the patient and its radioactive emissions are detected from the outside
- gamma emitters are the best since they are the least ionising and most penetrative
what are the characteristics of radioisotopes used in medicine and why are they important
- usually have high activities and short half-lives
- this is so image can be obtained quickly
- patient not exposed to harmful radiation for longer than necessary and only small amount of the radioactive substance is required
why are many of the radioactive sources needed for medical tracers produced on-site
- because they have such short half-lives meaning they need to be used almost immediately
what is technetium - 99m
- a gamma only emitter
- it is in a metastable state as shown by the m in the name
- meaning it remains in an excited state for a prolonged period of time
- In tis state it has a half life of 6 hours
- after this it will decay into technetium-99, which is a state isotope with a half life of 210000 years
what is a gamma camera used for
detecting gamma photons emitted by medical tracers inside the body
general structure of a gamma camera
- collimator which only allows photons travelling a certain direction through
- scintillation crystal which emits many visible light photons for every incident high energy photon
- photocathode which produces an electron for every incident visible photon
- photomultiplier tube which amplifies the signal
- computer which detects the signal and displays the image on a screen
explain the process that occurs in a gamma camera when gamma photons are incident on it
- photons are collimated and then incident on a scintillation crystal which absorbs the gamma photons and releases thousands of visible light photons
- these are then directed to a photocathode where an electron is produced for every incident visible photon
- the electrons are passed into a photomultiplier tube which releases more electrons
- the position of impact on the scintillator is used ti locate the emission site of the original gamma photon the signal is detected by the computer and the image is displayed
structure of a PET scanner
ring of gamma cameras placed around a patient in order to create a 3d image
Explain the process of PET scan
- positron emitter is administered to the patient
- the positrons travel only a few millimetres before annihilating with an electron and releasing two gamma photons which are detected at two diametrically opposed detectors in the camera ring
- the arrival times are recorded and speed is known so the location of the annihilation can b e calculated
- since annihilation is very close to the initial positron emission the location of the tracer can be estimated
- this is repeated until a 3d image can be produced
example of a radioisotope commonly used in PET scans
Fluorine-18
has a half life of approximately 110 minutes
what is flurodeoxyglucose and how is it used
gluscoes substituted with fluorine-18 it is used in PET scans to locate areas in the body with high respiration rates such as cancerous tumours or active areas of the brain
evaluate the pros and cons of PET scans
Pros: non- invasive, accurately demonstrate organ function , can be used to observe the effects of various medications
Cons: expensive, require tracers to be made on-site
ultrasound
longitudinal sound waves with frequency greater than the range of human hearing > 20KHz
what are the pros of using ultrasound
non - ionising, non- invasive, quick and affordable
what is ultrasound used for
finding the boundary between two media
explain the piezoelectric effect
An effect shown by crystals like quartz, when a potential difference is applied the crystal will mechanically deform. Likewise when the crystal is deformed a potential difference is produced
how do piezoelectric crystals work
- applying pd to piezoelectric crystal will cause it to produce ultrasound waves and a piezoelectric crystal absorbing ultrasound waves will produce an alternating pd they tend to be made from quartz, polymeric or ceramic materials
how does an ultrasound transducer work
- has alternating pd that causes a piezoelectric crystal to contract nd expand at a resonant frequency of the crystal to Maximus intensity
- once ultrasound has been created the pd is turned off and the reflected signal is detected by the transducer
ultra sound A-scan
- uses a single transducer to emit a signal and then detect the reflected signal. it is used to determine the distance from the device to the point of refection (usually the boundary between media) by using the time and the speed of sound in the medium
ultra sound B-scan
- series of A-scans that are stitched together to create a 2D image
- transducer is moved across the patients skin and uses the time and speed to calculate distance to boundary at each point
why are ultrasounds pulsed
allows time for the reflected signal to be received by the transducer
why do smaller wavelengths give more detailed images
allow the sound waves to diffract around smaller points a=of detail on the object that is being scanned
acoustic impedance
product of a sound waves density and the speed of sound in the medium
Z = Pc
Kgm^-2s^-1
explain what happens when an ultrasound hits a boundary between two media
- fraction of the waves energy/intensity is reflected and the rest is transmitted
- the amount is dependent on the acoustic impedance of each medium
reflection coefficient
ratio of the reflected intensity to the original intensity Ir/I0
how can you calc the reflection coefficient using the impedances of two media
Ir/I0 = (Z2-Z1)^2/(Z2+Z1)^2
where Z1 is the impedance of the first medium and Z2 is the impedance of the incident medium
- this formula only applies when the ultrasound is incident on the boundary at 90
When z1 and z2 are very close, what happens
most of the energy/intensity is transmitted
when Z1 and z2 are very different, what happens
Most of the energy/intensity is reflected
how can reflection be minimised when using a transducer against a patients skin
- since impedances if air and skin are very different
- most intensity would be reflected
- however using an impedance matching gel between the transducer and the skin (that has a Z similar to skin) minimises reflection
define the doppler effect
when there his a change in frequency of a wave when it is reflected or emitted by a moving source
doppler imaging
non-invasive technique to measure blood flow
explain the process of doppler imaging
- ultrasound waves are sent into a blood vessel
- the iron in the blood cells reflects the waves back to the transducer and the frequency is shifted depending on the direction and how fast the cells are moving
formula for the change in frequency of the ultrasound waves during doppler imaging
change f = (2fvcos(theta))/c
f - original freq
v - speed of blood flow
theta - angle between probe and the direction of blood flow
c - speed of ultrasound in blood
