8. Beam Geometry Flashcards
What is magnification caused by?
The diverging beam, the outer part of the beam makes the object appear larger. Also OID
What is the magnification factor?
How much larger the object appears in the image
What are two ways to reduce magnification?
- Increase SID
- Decrease OID (has a greater effect)
What is distortion affected by?
- SID
- OID
- Object position
- Thickness of object
What are the 2 types of shape distortion?
- Foreshortening
- Elongation
When does elongation happen?
When object is parallel to IR but central ray is angled and when object is parallel to CR but IR is angled
When does foreshortening happen?
When object is angled but IR is flat
What are two ways that we can use angles to our advantage?
- Bisecting angle method
- Superimposition
How does the bisecting angle method reduce distortion?
The angle of the CR to the IR is 1/2 angle of object to the CR
What is focal spot blur caused by?
multiple point sources on the larger fs
What is another word for geometric unsharpness?
penumbra
What is the inverse square law caused by?
The diverging beam: as you get farther from the source, more and more of the diverging beam misses the object
What are the pros for the LARGE focal spot?
- More area to be hit by x ray beam
- Less pitting of target surface
- Higher ratings
What are the cons for the LARGE focal spot?
- More point sources
- More blur
What are the pros for the SMALL focal spot?
- Sharper image
- Less blurring
What are the cons for the SMALL focal spot?
- Lower ratings
- More easily damaged
What is the line focus principle?
Uses angles to achieve large actual focal spot and effective small focal spot
What is the effective focal spot?
-Focal spot as seen from the POV of the IR
-Square shape
-Point sources are closer together which creates better spatial resolution
What does it mean if the anode angle from the vertical is 45 degrees?
Effective size is equal to the incident electron beam size
What does it mean if the anode angle from the vertical is less than 45 degrees?
The effective size is smaller than incident electron beam size
What is the usual anode angle from the vertical
12-17 degrees
What is the anode heel affect?
Caused by the angled target, some photons travel through the target causing attenuation. It happens with steeper angles
What are the benefits of the anode heel affect?
- Can act like a filter (Align anode to thinner portion of the body)
- Used for T spine and abdomen
How can you reduce the anode heel effect?
Longer SID and smaller field sizes
