Exposure factors are the settings that control how an X-ray image is produced. Understanding them is essential for the ARRT radiography exam and for producing good images at a reasonable dose. This guide explains the main factors in plain language, gives you a free calculator for the core formulas and finishes with practice problems with worked answers.
[TU EXPERIENCIA: 2–3 sentences with a real example of how you adjust technique in the room, such as for a cast, a child or a larger patient, noting that you practice in Spain.]
Key takeaways
- kVp controls the energy and penetrating power of the x-ray beam and affects contrast.
- mAs (mA x time) controls the quantity of x-rays and is directly proportional to receptor exposure.
- The 15% rule: raising kVp by 15% has about the same effect on receptor exposure as doubling mAs.
- Changing SID or adding a grid means adjusting mAs to keep the same exposure to the image receptor.
- In digital radiography, image brightness is adjusted by the computer, so the exposure indicator is how you check your technique.
The main exposure factors
| Factor | What it controls | Main effect on the image |
|---|---|---|
| kVp (kilovoltage peak) | Beam energy (quality) and penetration | Higher kVp gives lower subject contrast and a longer gray scale. It also increases beam quantity. |
| mA (tube current) | Number of electrons crossing the tube | Directly affects x-ray quantity |
| Exposure time (s) | How long the beam is on | Directly affects quantity; shorter times reduce motion blur |
| mAs (mA x s) | Total x-ray quantity | Directly proportional to receptor exposure; no effect on contrast in digital imaging |
| SID (source-to-image distance) | Distance from tube to receptor | Longer SID reduces exposure (inverse square law) and magnification |
| Grid | Removes scatter radiation | Improves contrast but needs more mAs |
| Focal spot size | Size of the x-ray source | A small focal spot improves spatial resolution |
Key formulas
mAs: mAs = mA x exposure time (seconds)
15% rule: kVp x 1.15 with mAs / 2 gives about the same receptor exposure. kVp x 0.85 with mAs x 2 gives about the same receptor exposure.
Exposure maintenance (changing SID): new mAs = old mAs x (new SID / old SID)²
Inverse square law (intensity): new intensity = old intensity x (old distance / new distance)²
Grid conversion: new mAs = old mAs x (new grid factor / old grid factor)
Common textbook grid conversion factors:
| Grid | Conversion factor |
|---|---|
| No grid | 1 |
| 5:1 | 2 |
| 6:1 | 3 |
| 8:1 | 4 |
| 12:1 | 5 |
| 16:1 | 6 |
Exposure factors calculator
Use the calculator to check your work on any of the formulas above.
Practice problems
Try each problem first, then check the answer.
1. mAs. What is the mAs at 300 mA and 0.05 seconds? Answer: 300 x 0.05 = 15 mAs.
2. Exposure time. You need 12 mAs at 400 mA. What exposure time should you use? Answer: 12 / 400 = 0.03 s (30 ms).
3. The 15% rule. A technique is 75 kVp at 16 mAs. You want to increase kVp by 15% to reduce dose. What’s the new technique? Answer: 75 x 1.15 = about 86 kVp, and 16 / 2 = 8 mAs.
4. Increasing SID. A technique uses 10 mAs at 40 inches SID. What mAs is needed at 72 inches to keep the same receptor exposure? Answer: 10 x (72 / 40)² = 10 x 3.24 = 32.4 mAs.
5. Decreasing SID. A technique uses 20 mAs at 72 inches SID. What mAs is needed at 40 inches? Answer: 20 x (40 / 72)² = 20 x 0.309 = about 6.2 mAs.
6. Inverse square law. The exposure rate is 0.4 mGy per minute at 2 meters from the source. What is it at 4 meters? Answer: 0.4 x (2 / 4)² = 0.4 x 0.25 = 0.1 mGy per minute. Doubling the distance cuts intensity to one-quarter.
7. Adding a grid. A non-grid technique uses 8 mAs. What mAs is needed with a 12:1 grid? Answer: 8 x (5 / 1) = 40 mAs.
8. Changing grids. A technique uses 25 mAs with a 12:1 grid. What mAs is needed with a 6:1 grid? Answer: 25 x (3 / 5) = 15 mAs.
Exposure factors in digital radiography
In film radiography, too much or too little mAs made the image too dark or too light. In digital radiography, the computer adjusts brightness automatically, so a poor technique can still look acceptable on screen. That makes it easy to overexpose patients without noticing, sometimes called «dose creep.»
That’s why technologists check the exposure indicator (EI) and the deviation index (DI):
- The EI shows how much radiation reached the image receptor.
- The DI compares that EI with the target for the exam. A DI near 0 means the exposure was on target.
- Underexposed images look noisy (grainy). Overexposed images may look fine but deliver unnecessary dose.
Good practice is to use the lowest exposure that gives a diagnostic image, following the ALARA principle (as low as reasonably achievable).
Frequently asked questions
Does kVp or mAs control contrast? kVp is the main technical factor affecting subject contrast. In digital imaging, displayed contrast is also shaped by processing. mAs controls quantity, not contrast.
Why use the 15% rule? Raising kVp and halving mAs usually lowers patient dose while keeping a similar exposure to the receptor. The trade-off is lower subject contrast and potentially more scatter.
Do I need to memorize grid conversion factors for the ARRT exam? It’s useful to know them and how to apply the formula. Check your program’s materials for the values they use.
Is the inverse square law the same as the exposure maintenance formula? They’re related. The inverse square law describes how intensity changes with distance. The exposure maintenance formula tells you how to change mAs to compensate.
Sources
- American Registry of Radiologic Technologists (ARRT): Radiography Examination Content Specifications.
- Standard radiographic physics texts, such as Bushong, Radiologic Science for Technologists, and Carlton and Adler, Principles of Radiographic Imaging.
- American Association of Physicists in Medicine (AAPM) Report 116: exposure indicators for digital radiography.
For education and exam practice only. In clinical work, follow your department’s technique charts, protocols and radiation safety rules.