| Question | Answer |
| what is the energy of the scattered xrays 1 meter from the pt. | .1% or 1/1000 of primary beam |
| what are the patient factors that affect scatter radiation | -tissue density
-size of part
-atomic # of tissue
-atomic mass of tissue
-beam restriction
*barium causes more scatter |
| what percentage of scatter makes up the useful density on a chest xray | 50% |
| what percentage of scatter makes up the useful density on a abdomen xray | 90% |
| what happens if you have too much scatter | -fogs image
-reduces contrast
-reduces visibility of detail |
| what is the purpose of an xray grid | improves contrast by reducing the amount of scatter reaching the IR |
| where is the grid located | between the patient and the IR |
| when should we use a grid | -kVp higer than 60
-part thicker than 10-12 cm
-pathology such as edema or ascites |
| how are grids constructed | alternating radio-opaque lead and radiolucent interspace(aluminum,carbon fiber,or plastic fiber) |
| what causes grid cutoff | loss of density due to absorbtion of unatenuated beam from pt |
| _________ grids cause grid cutoff on the periphery, but you can use any SID | parallel grid |
| __________ are the most commonly used grid. eliminates grid cutoff on periphery, but has limited SID(SID must be in focussing range to avoid cutoff) | focussed grids |
| cross type grid | can only be used in stationary bucky and provides no positioning lattitude |
| grid ratio | -determines grid efficiency along with amont lead in grid
-R=H/D (H=heigt of strips, D=interspace distance)
-ratio range=2:1-16:1 |
| grid frequency | -number of grid lines per unit of measurement
-60-196 per inch or 152-498 per cm
-80-103 lines/in or 203-261 lines/cm is common |
| what kind of grids are used for mobile imaging | -stationary
-clip on
-grid cassettes |
| potter bucky diaphragm | -grid is held inside bucky
-recipricating or oscilating
-motion statrts before exposure and ends after and is used to blur out grid lines |
| bucky factor | -measures grid quality by analyzing pt dose with a grid vs. dose w/o a grid
-exit radiation vs radiation that actually forms image |
| when using a grid what must we do | increase mAs or kVp |
| using a grid does what to our gray scale | shortens it |
| contrast improvement ability | -measures grid quality by analyzing contrast with a grid vs. contrast w/o a grid
-increased contrast improvement ability=increased contrast |
| what are the types of grid positioning errors | -upside down
-off level
-off center
-off focus |
| causes severe peripheral cut-off and is the least common of grid errors | upside down grid |
| an angled grid or tube, causes cut-off over entire image due to oblique nature, CR cuts across or perpendicular to grid lines and is the most common of all grid errors | off level grid |
| causes cut-off of entire image with more on one side | off center(lateral decentering)grid |
| causes cut-off on periphery of image | off focus grid |
| stroboscopic effect | -in moving grids
-motion of the grid is frozen
-when using exposure time shorter than grid movement
-reciprocating mechanism may be broken |
| what are some alternate methods of reducing scatter | -in film screen, you can use a backwards cassette
-air gap technique |
| what is the rule for distance when using the air gap tecnique | for every inch of OID you must increase SID by a foot |
| focusing distance | specific range of SID's that can be used for focused grids |
