3.10.5 X-Ray Imaging Flashcards
When does TIR occur in a glass rod?
If the light (or any EM wave) enters at incident angle > critical angle
What is the core of an optical fiber surrounded by?
Cladding
Which has a higher optical density and why?
Core or Cladding
Core
So that TIR can occur
(TIR only occurs when travelling from dense to less dense medium)
Other than allowing TIR to occur, what does the cladding do?
Protects core from damage
Prevents ‘crosstalk’
What is ‘crosstalk’?
When EM waves cross into adjacent fibers in the bundle (creating blurred images.)
What are the 2 types of bundles?
Coherent
Incoherent
What are coherent bundles?
The relative positions of fibers remain constant.
Good for imaging
(smaller diameter fibers have better resolution)
What are incoherent bundles?
The relative positions of fibers do not remain constant.
Images would be ‘jumbled’
Good for transmitting light.
(cheaper and easier to lay)
What are the main components of an x-ray tube?
Glass envelope
Cathode
Filament
Vacuum
Anode (with focal spot)
Rotor
Fixed shaft
Bearings
How does an x-ray tube work?
Electrons are released from filament, through thermionic emissions; a filament (of tungsten) emits electrons when heated.
They are attracted towards positive anode.
When electrons are brought to a sudden stop, their energy is dissipated in the form of x-rays.
What is continuous spectra of x-rays?
Due to the deceleration of electrons, KE = transferred to x-ray photons, whose max. energy = limited by the KE.
The electrons are decelerated by diff. amounts, - continuous spectra emitted.
What is line spectra of x-rays?
Depending on material being struck by the electrons, certain energies (frequencies) of x-rays appear with greater intensity.
Photons of specific energies are able to remove inner electrons of atoms - creates vacancy that higher electrons drop in to fill = release specific photon of radiation.
What does p.d control (in regards to x-rays) ?
Max. photon energy
Peak value of radiation activity
Amount of energy of peak radiation (moves position of peak, with respect to x-axis)
What does p.d not effect?
Position of line spectra for given target material.
What is the anode?
The anode is a rotating tungsten disc; spinning at 3000rmp, with a beveled edge
Why does the anode rotate?
The rotating spreads the heat over a larger area to prevent overheating and reduces risk of melting.
Allows higher power use, allowing higher x-ray production.
Why does the anode have a beveled edge?
Means larger surface area for electron beam.
-reduces heat build up, while still giving small focal spot for a sharper image.
What are x-rays used for?
To view internal structures of objects and materials.
What happens when x-rays go through a body?
X-rays usually pass though soft tissue and muscles.
Absorbed by bones.
(how much energy absorbed by a material depends on atomic number - tissue and bones have diff. atomic number, so will contrast in x-ray image)
What is required for clear x-ray image?
A clear x-ray image requires a large diff. in proton number or density between target area and surrounding tissue.
How would soft tissue be imaged (e.g. to see something stuck in esophagus) ?
A contrast medium is used to improve image quality.
Contrast medium = high proton number compound (‘barium meal’).
- barium passes through gastrointestinal tract, making it clearly visible against surrounding tissues on the x-ray.
What happens to a beam of monoenergetic x-rays as they pass through matter?
It’s intensity is reduced/ attenuated
What does the amount of absorption of x-rays through a material depend on?
Thickness of medium
Atomic number of medium
Density of medium
What is a photon of an x-ray absorbed by an atom through?
Photoelectric absorption
What happens to the electron when a photon strikes an inner shell electron?
Electron escapes its shell - excess energy contributes to the electron’s KE.
What happens to the x-ray photon when it strikes the atom (photoelectric absorption)?
Atom completely absorbs the photon, so it does not make it to the detector.
How does proton number relate to absorption?
Higher proton number atoms contain more electrons, which increases chance of absorption, because there are more interactions.
What is linear attenuation coefficient? [μ]
The fractional change in number of photons [N], for each unit of distance through the material.
How does density relate to linear attenuation coefficient?
Higher density = higher μ
(more atoms per unit area = more interactions between photons and electrons)
What is mass attenuation coefficient?
μ/ρ = constant
