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# PRI Formulas

### Formulas for PRI

Calculating mAs mAs = mA x seconds
Calculating Heat Units HU = mA x time x kVp x generator factor
Adjusting Milliamperage or Exposure Time 100 mA @ 0.10s = 10mAs. To increase the mAs to 20, you could use: 200 mA @ 0.10 = 20mA; 100mA @ 0.20s = 20mAs
Adjusting Milliamperage and Exposure Time to Maintain mAs 100mA @ 100 ms(0.10s) = 10mAs. To maintain the mAs, you could use: 200mA @ 50 ms(0.5s) = 10mAs; 50mA @ 200 ms(0.20s) = 10mAs
Using the 15% Rule: To increase density Multiply the kVp by 1.15 (original kVp + 15%0
Using the 15% Rule: To decrease density Multiply the kVp by 0.85 (original kVp - 15%)
Using the 15% Rule: To maintain density When increasing kVp by 15% (kVp x 1.15), divide the original mAs by 2. When decreasing the kVp by 15% (kVp x 0.85), multiply the mAs by 2
Inverse Square Law Formula I1/I2 = (D1)^2/(D1)^2
Density Maintenance Formula mAs1/mAs2 = (SID1)^2/(SID2)^2
Adjusting mAs for Changes in Grid mAs1/mAs2 = Grid conversion factor1/Grid conversion factor2
Adjusting mAs for Changes in Film-Screen System Speed mAs1/mAs2 = RS1/RS2
Adjustiong mAs for Changes in Part Thickness An optimal radiograph was obtained using 40 mAs on an naatomic part that measured 18cm.The same anatomic part is radiographed in another patient and measures 22cm.What ne mAs is needed to maintain density?Because
the part thickness was increased by 4cm the original mAs is multiplied by 2 yielding 80 mAs.If another patient for the same part measures 26 cm what new mAs is needed to maintain density?Because the part thickness increased by another 4cm the mAs is
by another 4cm the mAs is multiplied by 2 yielding 160mAs.This mAs is 4 times greater that for the original patient who measured 8cm less.
Calculating Geometric Unsharpness Geometric unsharpness = Focal spot size x OID/SOD where Focal spot size = Dimensions of the nominal focal spot in millimeters(mm)
OID=the distance between the object(area of interest) and the image receptor;SOD=the distance between the focal spot(source) and object (area of interest); SOD=SID - OID. When calculating the amount of geometric unsharpness the radiographer must keep the
radiographer must keep the distance measurement in the same units(i.e. inches or centimeters)
The Magnification Factor SOD=SID - OID; MF=SID/SOD
Determining Object Siz Object size = Image size/MF
Created by: dawn2000