RAD BOARDS 2
Terms
undefined, object
copy deck
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Film-screen combo summary
* faster-speed system -
*higher contrast (blk/wht)
*narrow latitude
*less recorded detail
*increased density -
speed controlled by...
contrast controlled by... -
A. screen
B. film -
Film-screen combo summary
* slower-speed system -
*lower contrast (gray-scale)
*wider latitude
*greater recorded detail
*decreased density -
Film-screen combo summary
* single emulsion films -
*one intensifying screen
*slower
*lower contrast (grays)
*wider latitude
*better recorded detail
*decreased density -
Film-screen combo summary
* double emulsion film -
*2 intensifying screens
*faster
*higher contrast (blk/wht)
*narrower latitude
*pooper recorded detail - Identified by relative speed #'s - 100sp is base
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*based on IF
exp w/o screens
IF = ---------------
exp w/ screens - thicker phosphor layer means
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*increased screen speed
*decreased recorded detail
*decrease patient dose - lrg phosphor crystal sz means
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*increased screen speed
*decreased recorded detail
*decreased patient dose - reflectv layer
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*increased screen speed
*decreased recorded detail
*decreased patient dose - absorbing layer
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*decreased screen speed
*increased recorded detail
*increased patient dose - dye in phosphor layer means..
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*decreased screen speed
*increased recorded detail
*increased patient dose - 200 screen sp system is twice as fast, so...
- 1/2 the mAs is required to produce same density
-
slower systems used for...
faster systems used for... -
A. extremity radiography
B. spine, abd, trauma, peds - Spectral Matching
- *wavelength of light emitted by screens must match w/ wavelengths to which film is most sensitive
- film-screen contact
-
*must be perfect
*poor contact = loss of recorded detail - Film-screen contact tested by...
- radiographing a wire/mesh screen
- What are grids used for?
- * to reduce the amt of scatter radiation reaching the film
- Scatter travels in divergent paths and is...
- * more likely absorbed in the grid
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Grid Construction
lead strips separated by... -
* aluminum interspacers
(increase technique) - Grid Ratio is
-
height of the lead strips divided by the distance between the lead strips
H/D - Ratios range frm 4:1 to 16:1
-
no grid - x1
4:1 - x2
6:1 - x3
8:1 - x4
12:1 - x5
16:1 - x6 - Grid Frequency is...
- * Number of lead strips per inch (centimeter)
- what do grids do?
- * reduce the amt of scatter reaching film
-
scatter travels...
image producing rays... -
A. in divergent paths
B. perpindicular path - scatter is...
- likely absorbed in the grid
- grids are used when...
- * parts are more than 10 cm thick
- grid ratio
-
* height of the lead strips divided by the distance between the lead strips
H/D - grid frequency
-
* number of strips per inch
* as grid frequency increases, lead strip thickness decreases and becomes less visible - grid frequency ranges frm???
- * 60 to 150 lines per inch
-
grid types
* linear grids -
* lead strips are parallel
* tube may be angled along the length of the grid w/o cutoff
* grid cutoff-decreased density along the periphery of the image caused by absorption of image-forming rays
* used w/ lrg sid or sm field -
grid types
* focused grids -
* lead strips are angled to coincide w/ divergence of xray beam
* used specific ranges of SID
* grid radius: distance at which focused grid may be used
* focal range is wide for low-ratio grids
* focal range is narrow for high-ratio grids
* focal range is stated on the front of the grid -
grid types
* crossed grids -
* also known as crosshatch
* consist of 2 linear grids placed perpindicular to each other
* superior clean-up
* no angulation of xray beam
* requires perfect positioning & centering
* primary use is biplane cerebral angiography -
grid characteristics
contrast improvement factor?? -
* measure of a grids ability to increase contrast
* expressed as ratio of contrast w/ grid to w/o grid - grid selectivity
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* espressed as ratio of primary rad transmitted thru grid to 2nd rad transmitted thru the grid
* higher grid frequency & grid ratio, the more selectv it will be
* high selectv, high efficiency of scatter cleanup - grid conversion factor (known as bucky factor)
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* amt of increased tech to compensate for absorption of scatter and image forming ray
* used to indicate increase in mAs when converting frm grid to nongrid
* grid conversion factors increase w/ higher kVp due to increased Comptons -
grid motion
stationary grids -
* do not mv during exposure
* grid lines may be seen -
grid motion
moving grids -
* mv back and forth during exposure
* eliminate visibility of grid lines