Medical Physics Flashcards
Larmor frequency
The frequency of precession of nuclei in an external magnetic field
Define acoustic impedance
The property of a material that determines the intensity of ultrasound refracted at a boundary with another material
Z = pc (measured in kg m^-2 s^-1)
How are X-rays produced?
Bombarding tungsten with high energy particles
What is the wavelength of X-rays?
10^-13 to 10^-8
What is the typical frequency of ultrasound used in medicine?
1-15MHz
What is the name of the frequency at which the protons precess?
Larmor Frequence
After the pulses of radio waves has ceased the nuclei relax and emmit what kind of wave?
Radio
Gamma Cameras are used with what?
Radioactive tracers
Describe X-ray production
A tungsten cathode is heated so that it releases electrons
These electrons are fired across a vacuum by a voltage generated by the mains
These electrons hit the negative anode and some of their energy (about 1%) is converted into X-ray photons
These X-ray photons can only exit through a window in the casing and there may be sheets of metal either side of the window to absorb X-rays not travelling straight, forming a collimated beam
The rest of the energy of the electron is gained by the anode as heat, so it rotates very fast to get rid of excess heat
The energy of one X-ray photon is virtually the same as the K.E. of the electrons, as the work function is negligible
The energy of an electron is eV and the energy of an X-ray photon is hf. Therefore…
hf = eV
And as λ = v/f, λ = c/ (eV/h), λ = hc/ eV
Describe X-ray absorption, with reference to the photoelectric effect
As with light rays, X-rays are also capable of producing photoelectrons
Energy of incoming photon = work function + KE of photoelectron
hf = ϕ + 1/2 mv^2
The work function is the energy required to break the bonds holding an electron in place
Because the energy of X-rays is so high, the work function is considered negligible
Therefore the photoelectron s produced have virtually the same energy as the X-ray photons that caused them
This is the main way that low energy X-rays are absorbed
Describe Compton Scattering
Occurs for higher energy X-rays (0.5-5MeV)
Instead of giving all their energy to an electron, the X-ray photon only loses a small amount of energy when it hits one
The ‘Compton Electron’ is knocked off its orbit and moves of in a different direction, ionising the atom
The X-ray photon now has slightly less energy, so a smaller frequency (E=hf) and hence a longer wavelength (λ = v/f) and is also deflected
Describe pair production with X-rays
Pair production occurs when an X-ray photon with energy above 1.02 MeV spontaneously splits into a positron and an electron when entering the electric field around the nucleus of an atom
The energy of the X-ray photon is converted into the mass of the electron and positron as E = mc^2 applies
The positron is soon annihilated when it collides with an electron, producing two gamma-ray photons that move off in opposite directions to each other
Describe film intensifiers
To intensify the image produced from a traditional film X-ray, intensifier sheets are used
These sheets are of material that contain a phosphor – a substance that emits visible light photons when an X-ray photon hits it
The film is placed behind an intensifier screen and many light photons produced by the screen blacken the film
This dramatically reduces the energy of the X-rays that needs to be used, so reducing the risk to the patient
Describe digital intensifiers
Incoming X-rays strike a phosphor screen which releases thousands of visible light photons for each X-ray photon that hits it
These then hit a photocathode, which releases an electron for each light photon, via the photoelectric effect
These electrons are then focused onto a screen which is another phosphor that gives out visible light
Image intensifies are used to reduce the length of time the patient has to be exposed to X-rays, and so that a lower intensity can be used
Describe contrast media
Used to show up a certain tissue that has a similar attenuation coefficient to other tissues in the body, which would normally not show up well on an ordinary X-ray scan
The medium used has to be a good absorber of X-rays, so has to have lots of electrons and hence a high atomic number (Z)
The medium, often barium, is injected into the tissue of interest, causing the tissue to become a better absorber of X-rays, so it’s edges are more clearly defined on the final image
Describe X-ray attenuation
Attenuation is the decrease in the intensity of X-rays as they travel through matter
Ordinary X-rays decrease in intensity according to the inverse square law
For a collimated (does not spread out – parallel beams) beam of X-rays, the intensity varies
This is a form of exponential decay
Bone is a better absorber of X-rays than flesh, so it has a higher attenuation coefficient
Describe a CAT scan
CAT stands for computerised axial tomography
The patient lies inside a ring of X-rays detectors whilst an X-ray tube rotates around them
In modern CAT scanners the patient is moved through the ring of detectors so that a picture of the whole body can be built up
The X-ray tube exposes the patient to a fan shape beam of X-rays, and the intensities of the X-rays after passing through the body are determined by the detectors opposite the X-ray tube
This information is sent to a computer which builds up a 3D picture of the inside of the patient
Slices or cross-sections through the patient can then be viewed
What are the advantages and disadvantages of CAT scans
Advantages
You can see 3D images compared to 2D images produced by conventional X-rays
Tumours can be located accurately
They are better at distinguishing between tissues with similar attenuation
Disadvantage
Exposes the patient to several years worth of background ionising radiation
Describe the gamma camera
Used to detect gamma ray photons
The photons pass through a collimator (a series of small lead tubes that ensures only photons travelling straight that are passed through it) The rest are absorbed by the lead
The gamma ray photons hit the crystal, which is a scintillator as it produces flashes of visible light (light photons) when hit by a gamma ray photons
A visible light photon is used by a photocathode to release an electron via e photoelectric effect
The electron enters a photomultiplier tube and is accelerated to a series of dynodes at increasing voltages. Each time it collides with a dynode it releases two or three secondary electrons which are also accelerated to the next dynode. By the time it gets to the final dynode there are thousands of electrons hitting it
This forms an electrical pulse which is processed by a computer to display a dot on the screen
Each pixel on the screen corresponding to one photomultiplier tube
How is the gamma camera used?
The image can be coloured to show areas where lots of gamma ray photons were emitted compared to areas that only emitted a few
It is often used to check blood flow through organs, especially kidneys, as a damaged kidney will not show up as well as a normally functioning kidney as it has reduced blood flow through it
To produce the image a person has to be given a beta positive or gamma ray emitting substance
What is a PET scan?
PET stands for positron emission tomography
It is another technique that utilises gamma ray photons
The radiopharmaceuticals (radioactive substances that can be taken into the body) used to emit positrons (beta-positive radiation)
These positrons soon collide with electrons and the two annihilate each other producing two gamma photons in the process
These gamma ray photons travel in completely opposite directions
Each pair of gamma photons produced is picked up by opposite detectors, and by measuring the tiny time difference in the detection of the photons, the position of their emission can be calculated
Gradually a 3D image of the distribution of the tracer can be built up by the computer
This is often used to show up tumours, but can also be used to monitor brain activity
The tracer used is usually glucose tagged with Fluorine-18, so it will be taken up more in tissues with a high rate of respiration
Describe ultrasound
Ultrasound is a sound wave with a frequency above the limit of human hearing
Ultrasound is produced by applying a voltage across a piezoelectric crystal
A piezoelectric crystal contracts by around 1% when a pd is applied across it, and this creates an ultrasonic wave
Ultrasound waves incident on the crystal also cause it to contract, and this produces an emf across it
This effect is used to detect reflected ultrasound waves
What is a transducer?
A device that converts a non-electrical signal into an electrical signal
Why is ultrasound used in medical imaging?
Because the ultrasound waves are partially reflected at the boundary between two different materials (e.g. Bone and tissue)
The fraction of ultrasound wave reflected can be calculated if we know the acoustic impedance
The fraction of the original intensity can be worked out.
Why is gel used with ultrasounds?
The fraction of the original intensity that is reflected at the air-skin boundary is around 99.95%, hence most ultrasound would be lost before even entering the body
Ultrasound gel with a similar acoustic impedance to skin is used between the transducer and the skin to minimise this effect
This is called impedance matching
Describe an A-scan
An A-scan is the simplest type of scan
A pulse of ultrasound is sent into the body and the reflected waves are displayed as pulses on a voltage-time graph
The depth of the tissues can be determined by the time taken for the pulse to be received
The type of reflecting tissue can be gathered from the amplitude of each pulse
The thickness of a certain material can also be calculated by working out the distance travelled by the ultrasound and dividing it by 2, as the reflected wave has to travel back through the material again
Describe a B-scan
A B-scan is a detailed image of the area of interest built up by moving the ultrasound transducer across the area
Each reflected pulse is analysed to determine the depth of the reflecting material (the time taken for the reflection to be detected) and the type of reflecting material, from the p.d. generated by the reflected wave
Surfaces that strongly reflect ultrasound waves appear brighter (bone) than areas that don’t (tissue)
This is the scan that is typically used to show a foetus in the womb
Describe a Doppler Ultrasound
This is used to check that blood is flowing smoothly (at the same speed) through a blood vessel, and that the heart is functioning properly
If the blood is flowing at different speeds in different area, it indicates that a blockage may be forming
The procedure uses the Doppler Effect - blood cells moving away from an incident ultrasonic wave reflect the waves at slightly longer wavelength (and consequently lower frequency), as each wave is reflected at a distance slightly further away from the source, so the string of reflected waves is slightly longer
The opposite applies if the blood cells are moving towards the source of the ultrasonic waves
Describe an MRI scan
When protons spin (which they do all the time) they behave as tiny magnets when placed in an external magnetic field (as they are charged, so when they spin they create a current, which creates a magnetic field)
Hence when a strong external magnetic field is applied, they align themselves with it
However they do not align directly with it, rather they process around it at an angle, like a gyroscope in a gravitational field
The angular frequency of the precession is called the Lamor frequency
When the protons are stimulated with a radio wave, they will resonate, absorb the energy and ‘flip’ up into a higher energy state where their magnetic fields are aligned antiparallel to the external magnetic field
When the radio frequency waves are switched off, the protons ‘relax’ back to their lower energy state, emitting their excess energy in the form of radio waves of the same frequency as the ones they absorbed
The rate of relaxation is different in different tissues
This allows the different tissues to be distinguished by the rate at which they relax.
A false colour image can then be produced by a computer, showing the different tissues
What do MRI scanners consist of
A large superconducting magnet to produce the strong external magnetic field. Kept around 4K by liquid helium (to reduce resistance to virtually 0)
One radio frequency coil to transmit and one to receive radio waves
A set of gradient coils, to produce small differences in the external magnetic field across the patient
What are the advantages of MRI?
Doesn’t use ionising radiation
Non-invasive
Gives better contrast between soft tissues than CAT scans
3D images and cross sections can be produced
Easy to locate tumours
What are the disadvantages of MRI?
Patients with pacemakers or surgical pins cannot undergo an MRI as the metal heats up
The scanners are expensive and take a long time to produce an image
They have to be shielded from any external radio waves
Explain how a collimator in a Gamma Camera works
Gamma ray photons travel along the axis of lead tubes, and this allows parallel gamma rays
Having narrow lead tubes makes the image sharper
Explain how a scintillator in a gamma camera works
Gamma ray photons hit the scintillator and produce thousands of visible light photons
Explain how photomultiplier tubes work in a gamma camera
A visible light photon is used by a photocathode to release an electron via the photoelectric effect
The electron enters the photomultiplier tube, where the electrons produce an electric pulse
Explain how a computer is use in a gamma camera
Signal from the photomultiplier tubes are used to produce an image
Each pixel on the screen corresponds to one photomultiplier tube
Describe how an ultrasound is used to determine the speed of the blood in an artery
Ultrasound is reflected by moving blood cells
The frequency / wavelength is changed
The change in frequency/wavelength is repeated to the speed of the blood
Describe the use of medical tracers to diagnose the condition of organs
A radioactive substance is injected/ingested by the patient
The tracer is absorbed by the organs and shows any blockages or damage to the organs
A beta detector or gamma camera is used to detect the radiation in the body
State the properties of X-rays
They are EM waves
They travel at the speed of light
They travel in a vacuum
They are transverse waves
Can cause ionisation
They are high energy photons
