How the problems work
Pick topics and a number of problems. Each set gets a code, and a class link carries the code and every setting, so each student who opens it gets the same numbers. Every answer is worked exactly and rounded once, at the end. Within 1% of the key counts as close.
Known slips get a hint: the inverse square law upside down, distances not squared, the 15% applied to the mAs, a grid ratio used as its factor, or OID used where SOD belongs.
One combined change, worked through
A good image was made at 72 in SID with no grid, 70 kVp and 10 mAs. The next image is at 40 in with an 8:1 grid (factor 4), and the kVp is raised by 15%.
- SID: × 40² ÷ 72² = × 1,600 ÷ 5,184.
- Grid: × 4 ÷ 1.
- kVp up 15%: ÷ 2.
- 10 × 1,600 ÷ 5,184 × 4 ÷ 2 ≈ 6.17 mAs.
Try ten combined-change problems
Formulas and where they come from
| What | Formula | Source |
|---|---|---|
| mAs | mA × time in seconds; same mAs, same exposure | PMI RA110 problems; Reading Hospital MI 263 (2022) |
| Inverse square | I₂ = I₁ × D₁² ÷ D₂² | Al-Mustaqbal University lectures (2022, 2024) |
| mAs-distance | mAs₂ = mAs₁ × SID₂² ÷ SID₁² | Reading MI 263; Al-Mustaqbal |
| 15% rule | kVp × 1.15 → mAs ÷ 2; kVp × 0.85 → mAs × 2 | How Radiology Works; Reading MI 263 |
| Grids, common | none 1, 5:1 2, 6:1 3, 8:1 4, 10:1 5, 12:1 5, 16:1 6 | Reading MI 263; Saia's Radiography PREP (citing Bushong); How Radiology Works |
| Magnification | MF = SID ÷ SOD = image ÷ object; SOD = SID − OID | Al-Mustaqbal (2022); How Radiology Works |
| Focal spot blur | focal spot × OID ÷ SOD | How Radiology Works |
| Heat units | kVp × mA × s × generator factor | Radiopaedia (1.35); Al-Mustaqbal (1.41) |
Sources checked October 24, 2026. ARRT's 2027 radiography outline lists grid conversion factors and the inverse square law, and says it avoids content where references conflict.
Questions
How do you work an inverse square law problem?
Multiply the old intensity by the old distance squared, then divide by the new distance squared: I₂ = I₁ × D₁² ÷ D₂². At twice the distance the intensity is a quarter. From 400 mR at 40 in to 72 in: 400 × 1,600 ÷ 5,184 = 123.5 mR.
What is the difference between the inverse square law and the mAs-distance formula?
The inverse square law says how much the intensity drops as you move away. The mAs-distance formula (also called the direct square law or exposure maintenance formula) says how much mAs makes up for it: mAs₂ = mAs₁ × SID₂² ÷ SID₁². Same squares, opposite way up.
How does the 15% rule work?
Raising the kVp by 15% roughly doubles the exposure at the image receptor, so halve the mAs to keep it the same. Lowering the kVp by 15% (× 0.85) roughly halves it, so double the mAs. The 15% always applies to the kVp, never to the mAs.
Which grid conversion factors should I use?
The common table gives no grid 1, 5:1 2, 6:1 3, 8:1 4, 10:1 5, 12:1 5 and 16:1 6 (Reading Hospital's handout, Saia's Radiography PREP, How Radiology Works). If your course uses other factors, choose Your own factors under Grid factors, units and rounding and type them in.
How do I find the magnification factor?
MF = SID ÷ SOD, where SOD = SID − OID. At 40 in SID and 4 in OID, MF = 40 ÷ 36 = 1.11, about 11% magnification. To get the true size of a structure, divide its size on the image by the MF.
Are heat units on the registry?
Neither ARRT's current radiography outline (2022) nor the 2027 one names heat units; both list tube loading. Heat units and focal spot blur are here for courses that teach them. Sources give different generator factors, so each heat unit problem states the factor to use.
Can I use these numbers on a patient?
No. They are practice values for exam arithmetic, not a technique chart. Real techniques come from your department's charts, the equipment and your clinical instructors. This site is not affiliated with ARRT or any school.
Sources: ARRT Radiography Examination Content Specifications, 2027 (PDF); Reading Hospital School of Health Sciences, MI 263 Technical Factor Review, 2022 (PDF); PMI Radiography, RA110 Grid Ratio and Grid Conversion Problems; Al-Mustaqbal University, Grid Usage lecture, 2024-25 (PDF); How Radiology Works, exposure relationships and grid table; How Radiology Works, magnification and blur; Radiopaedia, Tube rating.