6.5 Medical Imaging Flashcards
What are some properties of X-rays
Travel in straight lines
Travel at the speed of light
They penetrate matter. Penetration is least in dense materials with high atomic
number. All but the most penetrating are stopped by about 1mm of lead.
They are not deflected by magnetic or electric fields.
They affect photographic film.
They eject electrons from matter by the photoelectric effect and other mechanisms.
They ionise a gas permitting it to allow discharge of electrically charged bodies.
They make certain materials fluoresce eg rock salt, calcium compounds barium platinocyanide.
What is the frequency of X-rays
The frequency range of x-rays is approximately 1017 – 1020 Hz
How are x-rays produced
Electrons are accelerated across the evacuated tube and strike a target material The majority of the kinetic energy is converted to heat As the electrons decelerate they can produce Bremsstrahlung radiation in the X-ray part of the spectrum Some electrons in atoms move into excited energy levels after a collision with an accelerated electron. When they drop down levels they produce characteristic line spectra
The maximum energy of an x ray photon will occur when all the Ek of the incident
electron is converted into a single photon
What are the energy changes in an X-ray tube
- Electrical (filament current)
- Heat (thermionic emission in filament)
- Electrical (tube voltage accelerates electrons
- Kinetic Energy (of electrons)
- Heat (target material) and EM Waves (X-rays)
What is X-ray spectra
X rays are emitted with a range of wavelengths corresponding to the range of different photon energies produced by the x-ray tube. A spectrometer can analyse these photons to produce a graph of intensity versus wavelength. The graph has two main parts. A continuous spectrum which has a definite lower limit, increases to a maximum intensity at a certain wavelength and then decrease gradually to longer wavelengths. A line spectrum superimposed on top of this. The series of peaks produced are denoted by the letters K,L,M etc and the individual peaks by the alpha, beta, gamma etc. The wavelengths of the peaks are characterised by the target material.
What are the 4 types of X-ray attenuation
- Simple scatter 2. Photoelectric effect 3. Compton scatter 4. Pair Production
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-μx I0= initial intensity (W m-2) μ = attenuation (absorption) coefficient (m-1) x = thickness of the material (m)
The greater the attenuation (absorption) coefficient…
…The more the material will absorb incident X-rays.
Explain the process of simple scattering.
● X-rays of energy between 1 and 20keV are directed at a material
● The X-rays will reflect off layers of atoms or molecules in the material because they have insufficient energy to undergo more complex processes (like the photoelectric effect)
Explain the process of the photoelectric effect.
● X-rays of energy less than 100keV are directed at a material
● The X-rays can be absorbed by electrons in the material if they have the same energy as the ionisation energy of the atoms
● When an X-ray is absorbed, a photoelectron is released and another electron may de-excite, releasing another photon in the process
Explain the process of the Compton Effect.
● X-rays of energy between 0.5 and 5MeV are directed at a material
● The 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.
● The 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.
Explain the process of pair production.
● An X-ray of energy greater than 1.02MeV passes through the electric field of an atom
● An electron-positron pair is produced
● The positron will annihilate with another electron and produce two photons
● This process is not very important in medical X-rays as the photon energies are usually not high enough to cause pair production.
Define 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). They are easily identified in X-ray images as they absorb a lot of X-rays. Examples of contrast media include Barium (Z=56) or Iodine (Z=53) – compared to soft tissue (Z≈7).
Define the relationship between attenuation coefficient and proton number.
μ ∝ Z3
What does the CAT stand for in CAT scan?
Computerised Axial Tomography
What is a CAT scan and how do they work?
A CAT scan is a 3D X-ray image of a patient’s body made up of lots of 2D images. An X-ray tube generates a fan-shaped beam that is directed onto a patient that is laying down. There is a ring of detectors behind the patient to detect the beam intensity. The tube and the detectors rotate around the patient and up and down their body to create a 3D image of their whole body.
Compare CAT scans to conventional X-ray images.
CAT scans give 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.
Define medical tracer.
A compound made of a 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 a non-invasive diagnosis and which type of radiation is best-suited for it?
The tracer is administered to the patient and its radioactive emissions are detected from the outside. Gamma emitters are the best for this since they are the least ionising and most penetrative.
