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RAD 121 - Formulas
Question | Answer |
---|---|
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 |