Medical Physics Flashcards
(40 cards)
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
