Physics: Rapid Review Flashcards
What is the approximate average energy of an xray beam?
1/3 kVp
Note: The beam is mostly (80%) due to Bremsstrahlung interactions creating xrays.
Which electron is ejected from an atom during characteristic xray production?
An inner shell electron
What is the binding energy for Tungsten that creates a characteristic xray peak?
70 KeV
What is the purpose of the glass enclosure/envelop of an xray tube?
- Maintain a vacuum
- Allow the amount and speed of the electrons to be controlled independently
What does the xray tube cathode do?
It is the filament that emits electrons to be accelerated towards the anode
What does the xray tube focusing cup do?
Help the electron beam strike an appropriately sized focus on the anode
What does the anode do?
Provide the target material (e.g. tungston) to be hit by the electron beam to make xrays
What happens if you increase the target anode atomic number (Z)?
You increase the quantity and quality of xrays produced
What happens if you increase the xray tube kVp?
You increase the quality and quantity of xrays produced
What happens if you increase the xray tube mAs?
You increase the quantity of xrays produced (but not their average energy)
What happens if you increase the voltage ripple in an xray tube?
You decrease the quantity and quality of xrays produced
What happens if you add filtration to an xray tube?
You increase the quality but decrease the quantity of xrays produced
On which side should you place thicker parts to minimize heel effect?
The cathode side
How should you change the xray tube angle to minimize heel effect?
Increase angle
Note: Smaller angles are associated with worse heel effect.
How does the focus to film distance (FFD) affect the heel effect?
Smaller FFD means less heel effect
How does the field of view affect the heel effect?
Smaller field of view (film) means less heel effect
For mammography, you should put the chest side on the…
Cathode side (to minimize heel effect)
How does increasing mAs affect the average energy of xrays produced?
It doesn’t change the average energy (it only results in a higher quantity of xrays produced)
How can you determine the kVp for a given target curve?
It is the maximum energy of xray photon produced (where the curve falls back to 0 on the x axis)
You lower the kVp and now no longer see a characteristic peak on the target curve…
You lowered the kVp below the binding energy for the target (no characteristic xrays can be produced)
What happened if the target curve now has a different characteristic xray peak?
You changed target material
Note: Tungston will always have a characteristic peak at ~70 KeV (as long as the kVp is set to 70 or above).
How does k shell binding energy change with atomic number?
Higher atomic number means higher k shell binding energy (need a higher kVp to produce characteristic xrays)
Compton interactions are a main contributor to…
Scatter/noise
Note: Compton scatter involves the outer shell electrons, whereas the photoelectric effect (which contributes to desired contrast) involves inner shell electrons.
Magnification is proportional to
SID/SOD
Note: Increased source-image distance and decreased source-object distance both increase magnification.