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grids and scatter
| Question | Answer |
|---|---|
| What is secondary radiation? | A: Any radiation that comes from the patient. |
| What is the relationship between scatter and secondary radiation? | A: All scatter is secondary, but not all secondary radiation is scatter. |
| What happens during Compton scatter? | A: A photon hits an outer-shell electron, ejects it, and the photon changes direction and loses energy. |
| What does kVp control? | A: Beam quality, penetrability, and image contrast. |
| photoelectric interactions ___ when kVp increases | decreases, but scatter increases |
| the kVp is ___ in short scale contrast | low |
| the kvp is ___ in long scale contrast | high |
| How does kVp affect patient dose? | A: Higher kVp decreases patient dose; lower kVp increases patient dose. |
| What is contrast resolution? | A: Ability to distinguish subtle differences in density. |
| What increases scatter production? | A: Increased kVp, patient thickness, and field size. |
| When are grids used? | : Body parts >10 cm or >60 kVp. |
| What is grid ratio? | A: Height of lead strips divided by distance between them. |
| What does a higher grid ratio do? | A: Removes more scatter and improves contrast. |
| What is grid frequency? | A: Number of lead strips per inch or cm. |
| What technique must be adjusted when using grids? | A: mAs (not kVp). |
| Advantage and disadvantage of linear grid | allows angulation but only along the direction of the strips |
| advantage and disadvantage of cross hatched grid | excellent at reducing scatter, beam must be lined up exactly |
| What is the relationship between scatter and part thickness | thicker tissue creates more scatter |
| collimators are also known as | variable light aperture |
| collimators are composed of | lead shutters, light, and a mirror |
| what are the possible 6 grid ratios? | 5:1, 6:1, 8:1, 10:1, 12:1, 16:1 |
| simplest type of grid that is used in mobiles and in fluoroscopy | parallel grid |
| grid where the lead strips match the divergent quality of the beam | focus grid |
| advantage and disadvantage of a focus grid | lead strips match the angle of the beam, has a very specific SID. |
| most grid cut off occurs on the ____ dimension | short; along the short axis of the gri |
| off-level grid cut off is caused by ___ and the image will appear | tilt of the tube or grid, uniformly light |
| off center grid cut off is worse with ___ grids and will appear | cross hatched, uniformly light |
| off focus grids are caused by __ and will appear | using incorrect sid with focus grids, light edegs, center ok |
| upside down grid cut off will appear | dark center, edges extremely light |
| lateral decentering is an issue of the __ and will appear | tube shifted laterally, one side light, one side ok |
| when using the air-gap technique, the ______ to be effective | IR must be at least 6 inches from body part, mAs is increased 10% for every cm to compensate |
| when using the air gap technique, the pt dose is ___ to using a grid | less or same |
| advantges and disadvantages of the air gap technique | reduces scatter, increase contrast dis- increases magnification, contrast resolution decreases |
| moire effect | when the grid frequency conflicts with the computed laser scan frequency and it creates a zebra pattern on the image. |
| With what grid does the moire effect occur? | stationary grids. recommend to take out the grid and use air gap technique |
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user-1989152