WAR MACHINE- Testable trivia Flashcards
What is the primary purpose of the envelope?
to maintain a vacuum
Why is a vacuum needed?
To allow the amount and speed of the electrons to be controlled independently
What is the purpose of a focusing cup?
to help the electron beam strike the target anode in an acceptable size
What is the purpose of a rotating anode?
to spread the heat produced over a larger surface area to prevent melting
What focal spot size do general x-ray machines use?
0.6 and 1.2 mm
What causes differential x-ray attenuation?
steeper (smaller) anode angle
What are ways to reduce heel effect?
- large anode angle
- put thicker tissues toward cathode side
- smaller plate
- larger focal spot to film distance (FFD)
What is the minimum aluminim filter thickness?
2.5 mm
In Bremsstrahlung, what dictates the highest achievable energy x-ray?
Dictated by the original energy of the electron
Max energy (max keV)= Max kVp
Is Bremsstrahling more likely with higher or lower Z?
higher - more protons to make the electrons break
In nuc med, why are low Z materials used to shield Beta emitting particles?
to minimize Bremsstrahlung production - lead makes this worse
what is the K-shell binding energy of tungsten?
-69.5 keV
How is the energy of characteristic x-rays determined?
determined by the specific/characteristic binding energies of the element of the target atom (not determined by voltage)
80% of x-rays produced are what type?
Brensstrahlung
How to kVp and mAs effect quantity and quality?
- mAs only affects QUANTITY
- kVp influences both QUANTITY and QUALITY
What is the 15% rule?
If you increase kVp by 15%, youn decrease mAs by 1/2 to maintain the same x-ray density on the film
How do single and triple phase generators compare as far as voltage ripple and quality and quantity?
Single phase: more ripple = less quality and quantity
Triple phase: less ripple = more quantity and quality
What is HVL (half value layer)?
amoutn of material required to attenuate an x-ray photon to 1/2 the original output
What does increasing kVp do to the spectrum/spectrum diagram?
- incr total quantity of x-rays
- incr average energy
- incr max energy
What are a few important characterisitc of Coherent/Classic scatter?
- Do NOT result in ionization
- Do NOT result in net transfer of energy - changes direction without loss of energy
- Does NOT contribute to image
- Adds tiny dose to patient
- Seen at LOW energies (less than 10keV)
What occurs with Compton interactions?
- high energy incident x-ray strikes and frees outter shell electron - ejects as “compton” electron
- thus ionizes the atom
- deflects the incident x-ray as “scattered photon”
What are some negative characteristics about Compton scatter?
- dominant force contributing to scatter/image fog
- major source of occupational exposure
- materials rich in hydrogen have higher risk of Compton scatter - e/g animals
- does not depend on z of atom
Describe photoelectric interactions
- Occurs throughout the diagnositc ranges but dominates at lower energy
- x-ray strikes inner chell electron and transfers all its energy (is absorbed)
- all or nothing - either rleases the k-shell electron or it doesn’t
- atom is ionized - as electrons shift down, either release characeristic x-rasy or Auger electrons
- Auger dominates in biologic tissues
Describe the probability of photoelectric effect in relation to incident photon (IP) energy and atomic number
- inversely proportional to the IP energy cubed: 1÷E3
- directly proportional to Z cubed: Z3
