Medical Physics Flashcards
1
Q
Types of Scans
A
X-rays, CAT scans, PET scans, Gamma camera, Ultrasound.
2
Q
Simple Scatter
A
- X-rays of energy 1-20 keV will reflect off layers of atoms or
molecules in the material as they do not have enough energy to undergo more
complex processes
3
Q
Photoelectric effect
A
- X-rays of energy less than 100 keV can be absorbed by
electrons. - When an X-ray is absorbed by an atom, a photoelectron is released and another electron may transition down to the lower energy level emitting a
scattered photon in the process.
4
Q
Compton Effect
A
- X-rays of 0.5 to 5 MeV lose only a fraction of their energy to electrons in the absorbing materials.
- The scattered X-ray photon will have less energy than before, and so its wavelength will be greater.
- The Compton electron will be scattered in a different direction as momentum must be conserved.
5
Q
Pair Production
A
- When X-ray energy is greater than 1.02 MeV passes through the
electric field of an atom it will spontaneously produce an electron-positron pair
via the mass-energy relation. - This process is not very important in
medical X-rays as the photon energies are usually not high enough to produce an
electron-positron pair.
6
Q
Attenuation
A
Gradual decrease in energy or intensity of the x-ray beam
7
Q
X-ray Attenuation Mechanisms
A
- Different materials attenuate X-rays to a different extent
- Tissues can be contrasted by measuring the intensity of the attenuated beam once it has passed through the patient.
- For example, bone attenuates X-rays to a greater extent than flesh or other soft tissues.
- This is clearly seen as a white outline of the patient’s skeleton.
8
Q
What does CAT stand for?
A
Computerised Axial Tomography
9
Q
What does PET stand for?
A
Positron Emission Tomography
10
Q
What does Z stand for?
A
Proton Number - smallest number on the element
11
Q
u is proportional to
A
Z^3
12
Q
Two contrast mediums
A
Iodine and Barium
13
Q
Why is contrast medium used?
A
To highlight internal structures when conducting a scan
14
Q
How does a CAT scan work?
A
Records a large number of 2D X-ray images then assembles them into a 3D image with
the help of computer software.
15
Q
Advantages of CAT scans
A
Produces a 3D image
