Chapter 27 - Medical Imaging Flashcards
define X-rays
- EM waves with wavelength 10^-8 —> 10^-13 m
what are some key features of X-rays
- travel at speed of light
- can be polarised
- can be diffracted by atoms in crystals
- have 10-10000x more energy (per photon) than visible light
- can be harmful to cells
describe the general way that X-rays are produced and what we can say about energy transfer
- X-rays are produced when fast-moving electrons are decelerated by interactions with atoms of a metal such as tungsten
Ek of electrons = Energy of photons (if a photon is released)
electrons and photons interact one-to-one
explain the structure of an X-ray tube
- consists of an evacuated tube (so that electrons don’t interact with gas particle) containing two electrodes
- external power supply produces large accelerating P.D. between the electrodes
- cathode (negative) also has a low voltage supply over it, the cathode is a filament from which electrons are produced by thermionic emission as it is heated
- the electrons accelerate to the positive anode, this is a piece of high bpt metal such as tungsten
- x-ray photons are produced when the electrons are decelerated when hitting anode
what is the general efficiency of an X-ray tube and what often happens at the anode because of this
- generally only about 1% of the electrons’ energy is converted to X-rays, the rest is dissipated as thermal energy
- so often the anode needs to be cooled or rotated
how can we calculate the wavelength of the X-rays from the accelerating P.D.
W = VQ E = eV hf = eV hc/lambda = eV lambda = hc/eV
wavelength = hc/eV
what is the effect of increasing the number of electrons but with a constant energy
- intensity of X-rays increases because more photons are produced but their wavelength remains cosntant
what generally occurs when X-rays interact with matter
- X-ray photons interact with the atoms of the matter they pass through, they can be scattered or absorbed, decreasing intensity
what is attenuation
Attenuation is the decrease in intensity of Electromagnetic radiation as it passes through matter
name the 4 attenuation mechanisms, which one does not involve the electron inside the atom
- Simple scatter
- Photoelectric effect
- Compton scattering
- pair production - this one does not involve electrons inside the atom
explain what occurs in simple scattering and for which energies of photons it’s most prominant
- where Ephoton = 1 - 20 keV
- X-ray photon interacts with electron in the atom but has less energy than is required to remove it (E < work function)
- the photon ‘bounces off’/is scattered in an elastic collision
explain what occurs in the photoelectric effect (for X-rays and matter) and for which energies of photons it’s most prominant
- where Ephoton < 100 keV
- X-ray photon is absorbed by an electron, this gives the electron sufficient energy to escape from the atom
- this is the dominant mechanism in X-rays (medical) because V = 30-100 kV
explain what occurs in Compton scattering and for which energies of photons it’s most prominant
- Where Ephoton = 0.5 - 5 MeV
- incident X-ray photon interacts with an electron, electron is ejected from the atom, photon is scattered with reduced energy
- both energy and momentum are conserved
explain what occurs in pair production and for which energies of photons it’s most prominant
- onle occurs where Ephoton > 1.02 MeV
- X-ray photons interacts with the nucleus of an atom, it disappears and its energy is used to produce an electron and a positron
what is the amount of attenuation dependent on
- for a given energy of photon, attenuation is dependent on thickness of material and type of material
what is the equation for calculating the intensity of X-rays after having passed through a medium
I = Io (e^-(mew)(x)) I = final intensity Io = initial intensity Mew = attenuation coefficient x = thickness of material
what make better contrast media in medical x rays and why
photoelectric effect is the dominant attenuation mechanism
- in the photoelectric effect, mew is direct prop to Z^3
- therefore higher mass number elements are better for this
why do we sometimes have to use contrast media, what are some common contrast media
- soft tissues have low absorption/attenuation coefficients, so contrast mediums with high attenuation coefficients are used to make internal structures more clear
- they allow you to view the outline of the internal structures/soft tissues
- Barium and Iodine are common elements that are used because in X-ray imaging
where are barium and iodine generally used as a contrast media
- iodine in liquids and the blood
- barium in digestive systems
how can x-rays be used for therapeutic purposes
- high energy X-ray photons can be used in cancer therapy
- they can kill cancer cells through Compton scattering and pair production
what are the advantages and disadvantages of X-ray scans
Adv:
- cheap, easy
- shows internal structures
Disadv:
- the image produced is only 2D so overlapping structures can’t be differentiated
- not good for imaging soft tissue
briefly explain what a CAT scan is
CAT scanners take a large number of X-rays from different angles and compile them into one large 3D image using software
what is the structure of CAT scanner
- patient lies on an examination table that is able to move/slide back and forth into the scanner
- the scanner is a tube/gantry that contains an X-ray tube on one side and some X-ray detectors opposite
- these scanners/detectors can rotate within the gantry/tube
explain the process of taking a CAT scan, what moves etc.
- X-ray tube produces a fan-shaped beam of X-rays about 1-10mm thick
- this irradiates the patient as a normal X-ray would and the detectors opposite record the results and send it to a computer
- as the X-ray tube rotates the table moves, approx. 1cm per rotation
- then the process repeats giving X-ray images of ‘slices’ of the body
how long does a typical CAT scan last and what path does the X-ray tube follow
- approx. 10-30 mins
- a spiral path
how are the images from a CAT scan analysed
- radiographers can view each 2D ‘slice’ individually
- or they can be compiled by software into one big 3D image
what are the advantages and disadvantages to using CAT scans
Adv:
- can create a 3D image
- better for distinguishing between soft tissues with similar attenuation coefficients
Disadv:
- gives a higher radiation dose than normal X-rays
Briefly explain what a gamma camera is
The gamma camera is a detector of gamma photons emitted from radioactive nuclei injected into the patient
why is gamma radiation used in PET scanning
Gamma radiation is used because:
- it’s not very ionising so will not cause much harm to the patient’s cells
- it’s the most penetrative form of radiation so it can easily pass through the patient’s skin/flesh to the ‘camera’
what feature must the Gamma radioisotopes have that are used in a gamma camera
They must have a short half-life:
- this gives them a high initial activity so a ‘picture’ can be taken and only small amounts of the isotope are required to be ingested
- they also decay quickly, minimising radiation exposure time
where are gamma radioisotopes for gamma cameras generally produced and why
they tend to be produced on-site as they have short half lives so cannot be stored or transported for long periods of time
what are two examples of gamma radioisotopes used in the gamma camera
Tc-99m = half life of 6 hours
F-18
what are medical tracers why are they required
- medical tracers are chemicals that are bonded to the radioisotope that can target a particular area of the body or organ in order to make sure the isotope reaches the correct place
- the concentrations of this tracer can be detected to look for irragularities
what is the first step of detecting gamma photons, what is the collimator
- the collimator is a honeycomb structure of long thin tubes made of lead
- emitted gamma photons move towards the collimator
- only those that are perfectly parallel with it can pass through
- the rest are absorbed
what is the second step of detecting gamma photons, what is the scintillator
- the scintillator is a material such as sodium iodide that absorbs single gamma photons and releases thousands of visible light photons
- only those gamma photons which make it through the collimator interact with the scintillator, approx 1 in 10
what is the third step of detecting gamma photons, what is the photomultiplier tube
- the visible light photons that are emitted from the scintillator travel through the light guide into a photomultiplier tube
- the photomultiplier tubes are collections of tubes containing photocathodes and dynodes that convert single visible light photons into electrical voltage
how are the detected gamma photons analysed
- outputs of the photomultiplier tube are connected to a computer
- software calculates where and when the gamma photons hit the scintillator
- it constructs an image to show the concentrations of the medical tracer at different points in the body
how can the quality of image of a gamma camera be improved
using narrower/longer/thinner collimators
or
longer scanning time
what is a common radioisotope used in PET scans and what is its half life
fluorine-18 and 110 mins
what is the radiation from fluorine 18, what part do we detect
beta +
(18,9)F —> (18,8)O + (0,1)e+ + Ve + gamma
- we do not detect the gamma directly
- we detect the gamma released by the annihilation of the positron with electrons in the body
how can Fluorine-18 be made
- in particle accelerators near the site
- collide a proton with oxygen-18
(18,8)O + (1,1)p —> (18,9)F + (1,0)n
in what ways are CAT scans and PET scans similar and different
- both use EM radiation to produce 2D ‘slices’ through the body that can be formed into a 3D image using software
- CAT scans use X-rays, PET scans use gamma
what are two common medical tracers used in PET scans
Fluorodeoxyglucose (FDG)
- like glucose but an oxygen has been replaced with F-18
- treated by body like normal glucose
- shows where areas of high respiration are
Carbon Monoxide:
- uses C-11 instead of C-12, this is positron emitting
- used for blood stream
what is the structure of a PET scanner
- patient lies on a horizontal examination table
- they are surrounded by Gamma detectors/cameras, (sodium iodide scintillator and photomultiplier tube)
- they are injected with one of the medical tracers
what occurs in a PET scan and what is detected
- the medical tracer injected is usually positron emitting
- these positrons annihilate almost immediately with electrons, emitting two gamma photons that travel in opposite directions
- there are gamma detectors diametrically opposite which are connected to a computer
- it can calculate the points of annihilation (and therefore where the medical tracer is) by the difference in arrival times by the gamma photons
what are some advantages and disadvantages of PET scans
Advantages:
- non-invasive
- good for detecting cancers, and analysing the heart
- good for analysing the brain e.g. Alzheimer’s disease
Disadvantages:
- very expensive due to the cost of producing radioisotopes
define ultrasound
Ultrasound is longitudinal waves with a frequency greater than 20kHz
what are the benefits of ultrasound as a medical scanning technique
- non-invasive
- cheap
- non-ionising and therefore mostly harmless
what are some features of ultrasound as a wave
- it can be reflected, refracted and diffracted in the body just like other waves
what frequencies of ultrasound tend to be used in medical imaging
1 - 15 MHz
what is an ultrasound transducer
“an ultrasound transducer is a device to generate and receive ultrasound through the Piezoelectric effect”
what is the Piezoelectric effect
- when an emf is induced over the end of some crystals, they can stretch and squash
- the same occurs in reverse, when they are stretched or squashed they can produce an alternating P.D.
- strain < 0.1%
how is ultrasound in medical scanners produced and why is this frequency chosen
- an alternating P.D. of about 5 MHz is applied across the crystal
- this makes it rapidly expand/compress, producing ultrasound
- frequency chosen = natural frequency of crystal
in what format is this ultrasound emitted from the ultrasound transducer
5000 pulses per second of 5 MHz sound
how is the ultrasound detected
- the transducer can also detect ultrasound, the ultrasound can cause the crystal to compress/stretch and this produces an alternating EMF, this can be detected by a circuit
- most ultrasound scanners use Lead Zirconate Titanate
what is an A-scan
- an A-scan is the simplest type of ultrasound scan
- a single transducer is use to record along a straight line through the body
how does an A-scan work
- A transducer sends ultrasound pulses into a patient’s body, each pulse is partly reflected and transmitted an any boundary between tissues
- a reflected ‘echo’ pulse is received at the transmitter
- this ‘echo’ pulse is at a lower energy than the original pulse, the intensity of this pulse is dependent on the acoustic impedance at tissue boundaries
how are the results from an A-scan analysed
- the pulsed voltage from the ‘echos’ is displayed on a voltage-time plot
- the average speed of ultrasound in the body and the time between echos can be used to calculate distances between things in the body
what is a B-scan and how does it work
- B-scans are ultrasound scans that can produce a 2D image on a screen
- a transducer is moved over the patient’s skin, it is attached to a high speed computer
- for each transducer position a computer produces rows of dots on a screen where each dot represents a tissue boundary
- the brightness of each dot is directly proportional to the intensity of the reflected ultrasound
- collections of dots combine to produce an image
what is a reason for using non-invasive medical diagnosis techniques
less chance of infection
what occurs to ultrasound when it is incident at a boundary, what is this dependent on
- when a beam of ultrasound is incident at a boundary, some of its intensity is reflected and some of its intensity is refracted
- the proportion of the intensity that is reflected depends on the two material’s acoustic impedances
define acoustic impedance and give the corresponding equation and units
“the acoustic impedance, Z, of a substance is defined as the product of the density, p, of the substance and the speed, c, of the ultrasound in the substance”
Z = pc
Z is measured in kgm^-2s^-1
give some typical acoustic impedance values
air = 0.0004 (x10^6) fat = 1.38 (x10^6) muscle = 1.69 (x10^6) bone = 7.6 (x10^6)
what is the equation for the intensity reflection coefficient, and what does this actually mean
Ir/Io = (Z2 - Z1)^2 / (Z2 + Z1)^2
this is the ratio of the reflected intensity of ultrasound to the incident intensity of ultrasound for a collimated beam of ultrasound that is normally incident at a boundary between two surfaces of acoustic impedance Z1 and Z2
what is the effect on reflection if the difference in acoustic impedance is greater
the proportion of the intensity that is reflected is greater
what is coupling gel and why is it needed
- it is a gel that has a similar acoustic impedance to skin
- if an ultrasound is done where this isn’t used, air bubbles between the transducer and the skin can cause a large amount of the ultrasound to be reflected
- because air and skin have very different acoustic impedance values
- the gel both has a similar acoustic impedance to skin and fills in air gaps
what is the principle of behind Doppler imaging
- the frequency of ultrasound changes when it is reflected off a moving object
- Doppler imaging uses the reflection of ultrasound off moving blood cells to assess blood flow
how does a colour Doppler scan work
- the transducer is pressed lightly on the skin over the blood vessel
- transducer sends pulses of ultrasound and receives reflected pulses
- ultrasound reflected off tissue returns with the same frequency
- ultrasound reflected off moving objects (blood cells) return with a changed frequency
- the transducer is connected to a computer which can analyse all of this and produce a colour coded image
how does the frequency of the returning ultrasound waves change in relation to the blood cells
If:
- the blood cells are moving towards the transducer, the frequency increases
- the blood cells are moving away from the transducer, the frequency decreases
change in frequency is directly proportional to the speed of the blood cells
what is the equation for the speed of red blood cells in a blood vessel
delta(f) = 2fvcos(theta)/c
delta(f) = change in frequency of ultrasound f = frequency of incident ultrasound v = speed of blood cells theta = angle of ultrasound (to the blood flow) c = speed of ultrasound in blood
how would you determine the speed of blood in an artery in the arm
- place ultrasound transducer at an angle to the arm
- record initial frequency of ultrasound
- the ultrasound is reflected by moving blood cells and thus its frequency changes
- record the frequency of the reflected ultrasound and calculate change in frequency
- use DeltaF = 2fvcos(theta)/c
how are X-ray images produced
- X-rays are detected by a film/ scintillation counter
- shadow forms on the film on exposure to X-rays, giving a negative image of the internal structures from different attenuation coefficients
- generally attenuation occurs due to the photoelectric effect as medical X-rays are generally E < 100KeV
