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24 Cards in this Set
- Front
- Back
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Inverse Square Law
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As distance increases, rad dose decreases.
Old Intensity New Distance^2 __________ = _____________ New Intensity Old Distance^2 If the distance from the x-ray doubles, the rad dose is 1/4 of original dose. If the distance from the x-ray tube gets cut by 1/2, the rad dose is 4 times more. |
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Inverse Square Ex:
At 36" SID, the rad dose in air is 100 R. What would be the dose in air if the SID was increased to 72' SID?? |
100R 72^2
____ = _____ x 36^2 (Cross Multiply) X = 25R |
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mAs Distance Conversion
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Old mAs Old SID^2
_______ = ________ New mAs New SID^2 If distance increases, mAs must increase to compensate for the loss of radiation. |
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mAs and Density
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mAs controls film density
(blackness on radiograph) As mAs increases, density increases. As mAs decreases, density decreases. |
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Contrast Scale Changes
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A 5% increase or decrease in kV is necessary to see a visible change in contrast so using the 15% kVp Rule is recommended
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To LENGTHEN the scale of contrast
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To lengthen the scale, you increase the kVp by 15% and decrease the mAs by 50% (or 1/2)
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Formula to SHORTEN the scale of contrast
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To shorten the scale, you decrease the kVp by 15% and increase the mAs by 100% ( or x2)
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Density Changes & Relationships
30% mAs Rule (Use when the density is too dark or too light.) |
In order to see a visible change in radiographic density, the 30% mAs rule should be used.
Add or subtract 30% of the original mAs |
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kVp changes using 15% Rule
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Decrease kVp 15%: mAs x 2
Increase kVp 15%: mAs / 2 |
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SID changes using mAs-distance formula
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mAs1 D1^2
____ = ____ mAs2 D2^2 |
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Grid changes using mAs-grid formula
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mAs2 = mAs1 x (G2)
(G1) |
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Screen changes using mAs-screen formula
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Old Screen Sp #
mAs2 = mAs1 x ______________ New Screen Sp # |
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Intensification Factor
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Exposure required w/o Screen
IF = _________________________ Exposure required w/ screen |
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Systematic Method for Screen Speed Conversions:
As screen speed /\ , Contrast /\ , density /\ , recorded detail \/ , rad dose \/ |
Slow to Par = mas/2
Par to High = mAs/2 Slow to High = mas/4 Par to Rare Earth = mas/4 Par to Slow = mAs x 2 High to Par = mAs x 2 High to Slow = mAs x 4 Rare Earth to Par = mAs x 4 |
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Numerical Method for Screen Speed Conversions
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Relative Speeds
No screen 1 Slow (Detail) 50 Medium (Par) 100 High (Fast) 200 Rare Earth 200, 400, 800, 1200 |
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Formula for Screen Speed Conversion
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Old Scr Sp #
New mAs = Old mAs x ___________ New Sr Sp # |
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Formula for Grid Ratio
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Height of Lead Strip
Grid Ratio = ________________ Distance b/w strips |
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Grid Conversions: Numerical Factor
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No grid = 1
5:1 = 2 6:1 = 3 8:1 = 4 10:1 & 12:1 = 5 16:1 = 6 |
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Grid Conversion Formula
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New Grid #
New mAs = Old mAs X _________ Old Grid # mAs2 = mAs1 x G2 G1 G2 = g1 x mAs2 _________ mAs1 |
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kVp changes from one grid ratio to another
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From non-grid to ___ increase kVp by ___
5:1 +8 6:1 +12 7:1 +13 8:1 +15 10:1 +20 12:1 +23 16:1 +25 |
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Unsharpness Formula
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FSS x OID
__________ SOD SID - OID = SOD |
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Magnification Formula
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OW SOD
___ = ____ IW SID |
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Percent Magnification Formula
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OID
% Mag. = ____ x 100 SOD ~~~~~~~~~~~~~~~~~~~~~~~~~~ IW - OW %Mag = ________ x 100 OW |
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Beam Width/Length/Area
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BW1 D1
____ = ____ BW2 D2 |
