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RAD 121 - Formulas

QuestionAnswer
mAs, mA, or Time when 2 of 3 are known. mAs = mA x Time; mAs/Time = mA mAs/mA = Time
Finding percentage difference in mAs or kVp. High value - lower value; Answer divided by original value; multiply by 100
Application of 15% kVp rule: a) Double receptor exposure. AOFC b) Halve receptor exposure. AOFC c) Increase contrast and maintain receptor exposure. d) Decrease contrast and maintain receptor exposure. a) Double receptor exp AOFC orig kVp x 1.15 = new b) Halve receptor exp AOFC orig kVp x .85 = new c) Increase contrast, maintain receptor exp orig kVp x .85 and double mAs d) Decrease contrast, maintain receptor exp orig kVp x 1.15 and half mAs
Maintain receptor exposure with mAs for changes in SID. (mAs distance compensation formula) mAs 1 / mAs 2 = SID 1 squared / SID 2 squared
Maintaining receptor exposure with change in Grid Ratio. mAs 1 / mAs 2 = GR 1 / GR 2
Formula to calculate geometric unsharpness. P = FSS - OID / SOD
Magnification factor based on distances: MF = SID / SOD
Magnification factor based on object and image: MF = IW / OW (which also equals SID / SOD)
Percent of magnification based on distance: %M = SID - SOD / SOD x 100
Percent of magnification based on object and image: %M = IW - OW / OW x 100
Exposure rate with changes in distance from source: Inverse Square Law I 1 / I 2 = SID 2 squared / SID 1 squared
Grid Ratio formula GR = Height of lead strips/ Distance between lead strips
Double receptor exposure with mAs OLD mAs x 2 = NEW mAs
Double receptor exposure with kVp OLD kVp x 1.15 = NEW kVp
Created by: Larobbins