Medical physics Flashcards
How are X-rays produced?
By rapidly accelerating or decelerating charged particles so that kinetic energy is transferred into high energy photons
How can you differentiate between x-rays and Gamma rays?
X-rays and gamma rays have frequencies that overlap so you cannot distinguish between 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 charge particles.
Why are x-rays 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 kinetic energy is released as x-rays in all directions (the rest is transferred to heat energy with 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 the 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 (window), so only x-rays 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 traveling parallel to the axis of the tubes.
But why is it better for X-rays to be in a beam rather than emitted in all directions?
It allows them to be directed at specific areas (like a broken bone) and minimises a 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?
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 area 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 100KeV are directed at a material.
The x-rays can be absorbed by electrons in the material if they have the same energy as 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 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?
Define contrast media and give two examples.
Define the relationship between attenuation coefficient and proton number.
What does CAT stand for in CAT scan?
Computerised Axial Tomography
What is a CAT scan and how do they work?
Compare CAT scans to conventional X-ray images.
Define medical tracer.
How are tracers used in a non-invasive diagnosis and which type of radiation is it best suited for?
What are the characteristics of radioisotopes used in medicine and why are they important?
Why are many of the radioactive sources needed for medical tracers produced on-site?
What is technetium-99m?
What is a gamma camera used for?
Describe the general structure of a gamma camera.
Explain briefly the process that occurs in a gamma camera when gamma photons are incident on it.
Explain the process of a PET scan.
Give an example of a radioisotope commonly used in PET scans.
What is fluorodeoxyglucose and how is it used?
Evaluate the pros and cons of PET scans.
Define ultrasound.
Explain the Piezoelectric effect.
How do piezoelectric crystals work?
How does an ultrasound transducer work?
What is an ultrasound A-scan?
What is an ultrasound B-scan?
Why are ultrasounds pulsed?
Why do smaller wavelengths give more detailed images?
What is an acoustic impedance?
Explain what happens when an ultrasound hits a boundary between two media?
What is the reflection coefficient?
How can you calculate the reflection coefficient using the impedances of two media?
Most of the energy/intensity is transmitted.
Most of the energy/ intensity is reflected.
How can reflection be minimised when using a transducer against a patient’s skin?
Define the Doppler Effect,
Explain the process of Doppler imaging.
Give the formula for the change in frequency of the ultrasound waves during the Doppler imaging.
A patient is having an x-ray scan. The x-rays interact with the atoms in the patient. Name and describe the 3 methods by which x-rays interact with matter. [4]
[3]
