6.5 - Medical Imaging Flashcards
How can you differentiate between X-rays and Gamma rays?
X-rays and gamma rays have frequencies that overlap, so you cannot distinguish them by their wavelengths. Instead you have to use their method of production – gamma rays come from radioactive decay or particle collisions with a mass defect, whereas X-rays are produced by accelerating charged particles.
How are X-rays produced?
By rapidly accelerating or decelerating charged particles – their kinetic energy is transferred into high-energy photons
Why are X-rays used in medical imaging often referred to as ‘soft X-rays’?
Because they have energies that are lower than gamma rays.
Describe the general structure of an X-ray tube.
Heated filament (cathode) and tungsten anode with a potential difference between them of up to 200kV and sealed in a vacuum tube.
How does an X-ray tube work?
Electrons are emitted from the heated filament via thermionic emission and drawn towards the anode. They collide with the anode and some of their Eₖ 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?
To prevent electrons from colliding with molecules of air before they gain enough energy to release X-rays.
How is the anode prevented from overheating?
By either rotating it so that a new section of it is in contact with the X-rays all the time, or by using water as a coolant, circulating it through the anode.
How are the X-rays focused into one beam?
The 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 then 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 to be in a beam rather than emitted in all directions?
Because it allows them to be directed at specific areas (like a broken bone) and minimises the patient’s 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?
I₀= initial intensity (W m⁻²)
μ = attenuation (absorption) coefficient (m⁻¹)
𝒙 = thickness of the material (m)
Explain the process of taking an X-ray of a patient.
X-rays are directed at an area of a patient’s body and pass through the bone and soft tissue. Since bone has a higher attenuation coefficient, it absorbs more X-rays than soft tissue does. If photographic film is placed behind the patient, the areas where the bone is will not blacken as much as the areas of soft tissue, creating an image of the inside of the patient’s body. However, nowadays, digital detectors are used in place of photographic film
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.
