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anti scatter grids
anti-scatter grids
| Question | Answer |
|---|---|
| what controls scatter? | kvp |
| (compton) scattered radiation | incident x-ray photon changes direction with less energy following interaction within patient |
| what is the highest quantity of scatter generated from patient? | 180 degrees |
| what is the highest intensity from patient? | 90 degrees |
| the amount of scatter is equal to the same as what? | primary beam |
| energy is how much of primary beam photon energy? | .1% or 1/1000 |
| what is the relationship between kvp and absorption? | as energy increases, the percentage of scatter v. absorption increases |
| patient factors that can cause scatter | pathology such as edema, size of part, improper collimation, atomic # & mass of tissue and barium |
| atomic number | number of protons in the nucleus |
| atomic mass of tissue | protons and neutrons in the nucleus |
| barium | has water in it |
| when would you use a grid? | for a part larger than 10 cm. if you use more than 60 kvp |
| scatter and useful density on chest/abdomen | 50% chest 90% abdomen |
| what helps control scatter? | appropriate collimation helps control scatter |
| too much scatter does what? | fogs image, reduces contrast, and reduces visibility of detail/ spatial resolution |
| gustav bucky | in 1913 developed grid that had alternating grid lines |
| hollis potter | in 1920 developed a mechanism which moved the grid during exposure |
| potter/bucky diaphragm means | moving grid |
| what is the purpose of a moving grid? | to blur out lines and clean up scatter. improves contrast by reducing the amount of scatter reaching the IR |
| DR | has no moving grid |
| CR | has a moving grid oscillating and reciprocating |
| oscillating | circular motion |
| reciprocating | moving back and forth during exposure to improve image quality |
| the grid is placed where? | between the patient and IR |
| construction of grid | we have carbon fiber thin pb which has alternating radio-opaque pb which absorbs scatter and radiolucent interspace which absorbs x-ray beam light for latent image, plastic for low kvp |
| grid cut off | loss of density/ brightness due to absorption of imaging, forms pb strips |
| types of linear grids | parallel, focused, and crisscrossed |
| parallel type | lines are parallel |
| focused type | majority of grids are focused. lines are angled/ tilted inwards to meet divergent of the beam @ a specific focusing difference |
| crisscross type | most effective grid, two grids one atop another |
| latitude | range of SID you can use |
| non- focused types | increase positioning latitude and they are linear parallel and cross (hatch) |
| linear parallel | pb lines run in one direction parallel to each other |
| cross (hatch) | pb lines run in crisscross formation which improves clean up |
| construction of non- focused grids | problem is cut off (loss of density due to absorption of exit beam by pb strips) seen in periphery of image. |
| advantage of non- focused grids | increases SID latitude |
| focused parallel or cross grids | manufactured so peripheral grid lines are tilted to meet beam divergence to eliminate peripheral grid cut off. less latitude/ focusing range (grid radius) |
| grid ratio | determines grid efficiency along with amount of pb in grid |
| r= h/d | h= pb strip (grid) height D= interspace width |
| grid ratio | 2:1-16:1 |
| use of low v. high ratio | kVp & amount of s/s (scatter) |
| grid ratio formula | grid ratio = height of pb lines distance between grid lines thickness of innerspace |
| grid frequency | grid line in cm |
| thicker pb strips clean up more scatter t or f? | true, you want higher ratio and lower frequency. |
| clamp on grids are used for what? | mobile imaging |
| grid cassettes have | grid lines |
| we do not need moving grids because of what? | we use high frequency thin pb lines |
| moving grids | potter bucky diaphragm in table bucky. blur out grid lines and motion starts before exposure and stops afterwards. |
| what are the two types of movement for grids? | reciprocating and oscillating |
| patient dose w/grid vs. patient dose w/out grid | pt. dose w/grid is always higher incr. pb = increased sensitivity |
| k = | contrast w/grid/contrast w/out grid increased contrast improvement ability = increased contrast |
| grid position errors | upside down, off level, off center, and off focus |
| upside down | least common, severe peripheral cut-off |
| off level | more common (to light) angled grid or tube, cut-off entire image due to oblique nature, CR cuts across or perpendicular to grid lines |
| off center or | lateral decentering, cut off entire image with more on one side |
| off focus | cut off periphery of image, SID is not properly used |
| stroboscopic effect | captures grid lines when grid is not moving. is seen when reciprocating mechanism is broken and when using short exp time (shorter than movement of grid) |
| alternate method if there is no grid | film screen technology in which you would use the cassette backwards |
| air gap technique | increase distance between patient/part and IR (OID) incr. SID compensates and prevents magnification distortion |
| to prevent OID you would increase what? what is the rule? | SID. for every 1" OID, increase 1' SID. EX. 6" OID and 6' (72") SID |
| when using a grid what must be taken into account | pathology, part size greater than 10-12 cm, and 60>kVp |
Created by:
eckoultd1972
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