7. Beam Restriction Flashcards
What are the 2 purposes of beam restriction?
- Protects patient from radiation
2.Improves image quality
How does beam restriction protect the patient?
-↓Field size = ↓tissue irradiatedd
-Less energy absorbed by patient
-Lowers dose
How does beam restriction improve image quality?
-Reduces the scatter radiation
What is an aperture diaphram?
-Lead plates with holes cut in them
-Designed for specific film sizes
-Constricted the size of the beam
What did cones and cylinders reduce?
-Reduce more of the beam because of increased length
What are the pros and cons of cones and cylinders?
Pros:
-Very little blur
-Better image quality than just using diaphragm alone
Cons: Made the x ray field circular
What is a light beam collimator/variable aperture collimator?
-Multiple levels of lead shutters to restrict beam more effectively
-2-3 shutters
-Light bulb and mirror to represent x-ray field
What is the functions of the first and second shutters?
- Reduces off-focus radiation, less blur
- Allows field size/shape selection and collimate to include anatomy of interest
How is the bulb and mirror set up in the tube and what is its purpose?
-Light field shows where x-ray beam will be
-Allows us to be precise
-Mirror must be at 45⁰
What happens when the bulb is too close to the mirror?
Light field is bigger than x ray field
How much can the beam be misaligned?
<2% of SID
What is the purpose of positive beam limitation?
-Limit the size of the collimation to the size of the IR
-Makes sure you cant open more than the detector
When is the collimation scale used?
- For when PBL is not available and you can’t see the IR
