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# Radiography Formulas

### Review

Question | Answer |
---|---|

For every 1" of OID, how much compensation in SID is required? | For every 1" OID, an increase of 7" in SID is required. |

What is the formula for Density (Incident light & transmitted light)? | Density = log x incident light intensity/transmitted light intensity |

What are the GCF for the following Grid ratios........No grid, 5:1 grid, 6:1 grid, 8:1 grid, 12:1 grid, 16:1 grid? | No grid = 1x the original mAs, 5:1 grid = 2x original mAs, 6:1 grid = 3x original mAs, 8:1 grid = 4x original mAs, 10:1 or 12:1 grid = 5x original mAs, 16:1 grid = 6x original mAs |

What formula is used when converting from one grid ratio to another? | mAs original/mAs new = GCF original/GCF new |

In digital imaging, how is pixel size determined? | By dividing the field of view (FOV) by the matrix. Pixel size = FV/Matrix |

How can the size of a particular structure or image can be calculated? | Image size/Object size = SID/SOD |

What is SOD the difference of? | SOD = SID - OID |

What is the formula for the Magnification Factor? | MF = Image Size/Object Size |

What is the formula for Heat Units (HU) for single phase? What are the correction factors used for 3 phase 6 pulse & 3 phase 12 pulse? What % increase is the intensity of the beam? | HU = mA x Sec x kV. 3 phase 6 pulse 1.35 (35%) & 3 phase 12 pulse 1.41 (41%). |

What is the transformer law? What formula is used to determine the voltage & current? | Vs/Vp = Ns/Np (V=voltage & N=number of turns). to determine current...Ns/Np=Ip/Is (N=number of turns & I=current). |

What is the Grid Ratio? | GR = H/D (GR = Grid Ratio, H = Height of Pb stip, D = Distance of the interspace material between the Pb strips). |

What is the Grid Frequency? | The # of Pb (lead) strips per inch. |

What are the mAs correction factors when changing from single phase to 3 phase 6 pulse & 3 phase 12 pulse? | 2/3 X for 3 phase 6 pulse & 1/2 X for 3 phase 12 pulse (X = single phase mass). |

what are the screen speed conversion factors for the following screen speeds mAs conversion factors, 50, 100, 200, 400, 800? | screen spped 50=4, 100=2, 200=1, 400=0.5 & 800=0.25. |

How much kVp increase in technical factors for each increase of 1cm tissue thickness? | For each 1cm increase of tissue thickness an increase of 2 kVp to compensate. |

What is the 15% rule? How will I apply it, if repeating & wanting to keep the same the density? | A 15% increase of kV is equivalent to increasing mAs by 50%. A 15% decrease in kVp is equivalent to a 50% decrease in mAs. If repeating & wanting the same density, increase my kVp 15% & decrease my mAs by 50%. |

What is the formula used to determine the amount of image magnification? | Image size/Object Size = SID/OID |

How can the SID be determined with the OID & SOD? How can the SOD be determined with the SID & OID? How can the OID be determined with the SID & SOD? | SID = OID + SOD. SOD = SID - OID. OID = SID - SOD. |

What is the formula to calculate Equivalent Dose? | EqD = D x Wr. (D) dose, (Wr) Radiation weghting factor. |

What is the Cumulative Effective Dose? What is the radiation unit? How would it be determined? | The Collective Effective Dose describes low doses of ionizing radiation exposure of a population or a group. The person-seivert. The # of people x the average effective dose. |

What are the annual occupation limits for Effective Dose? How many mSv are in each rem? What are the limits in mSv? What is the occupational yearly dose limit for students under 18 years of age? | 5 rem for the whole body, 50 rem for the skin & extremities, 15 rem for the lens of the eye. 1 rem = 10 mSv. 50 mSv for whole body. 500 mSv for skin/extrmities & 150 mSv for lens of the eye. 0.1 rem (100 mrem or 1 mSv) in given year for students under 18. |

What is the gestational dose equivalent limit for the embryo/fetus of a pregnant radiographer? What is the limit per month? | 5 mSv (.5 rem) for the gestational period & not to exceed .5 mSv per month (.005 rem, or 5 mrem). |

Can excessive radiographic density be attributed to over-development? Why? | Yes. Overdevelopment can be caused by the developer temp being too high, which results in chemical fog. |

Which of the following is likely to result from using single emulsion film in an IR that has 2 intensifying screens.....Double exposure, Decreased density, Increased recorded detail, Greater latitude? Why is this? | Decreased density. If single emulsion film is in dual screen cassette, the film will only receive half the exposure. |

How does an decrease in kV affect exposure latitude? What does this mean? | An decrease in kV also decreases exposure latitude. There is less margin for error in lower kV ranges. |

What is the qualitative factor (weighting factor) of the following radiations.....fast neutrons, alpha particles, protons, Dx x-rays & gamma rays? What is the ranking of Relative Biological Effectiveness (RBE) from least damaging to most? | Dx x-ray & gamma rays 1, protons 5, fast neutrons 10, alpha particles 20. |

What is the inverse square law? | Old Intensity/New Intensity = New distance squared/Old Distance squared. |

What is the density maintenance formula? | Old Intensity/New Intensity = Old Distance squared/New Distance squared |

What is the formula to calculate Effective Dose? | EfD = D x Wr x Wt. (D) dose,(Wr) radiation weighting factor, (Wt) tissue weighting factor |