Veterinary Radiographic Technique & Exposure Principles Reference
There is no single kVp/mAs chart that works across every practice - exposure factors depend on your specific x-ray generator, detector type, and grid use. This reference explains the underlying principles (kVp, mAs, the 15% rule, the inverse square law, and grid selection) so you can build and calibrate a technique chart for your own equipment.
There is no universal kVp/mAs chart
Exposure factors depend on your specific generator's output, detector type (film-screen, CR, or DR), grid use, and patient thickness. A technique chart copied from another practice or a generic published source will not be correctly calibrated to your equipment. This page explains the underlying principles so you can build and calibrate your own chart - it does not provide specific kVp/mAs settings to use directly.
kVp (kilovoltage peak)
Controls beam penetration and image contrast
| Effect |
|---|
| Set primarily by part thickness Thicker or denser parts need higher kVp to adequately penetrate the tissue. |
| Higher kVp = more penetration, less contrast More scatter reaches the detector, producing more shades of gray. |
| The "15% rule" Increasing kVp by 15% roughly doubles exposure reaching the detector - equivalent to doubling mAs - while reducing contrast. |
mAs (milliamperage-seconds)
mA x exposure time - controls beam quantity and image density
| Effect |
|---|
| Higher mAs = darker image Increases overall exposure/density with minimal effect on contrast. |
| Trade-off with exposure time Higher mA with shorter time reduces motion blur risk for the same mAs - useful for uncooperative or breathing patients. |
Focal-film distance (FFD)
Inverse square law
| Effect |
|---|
| Exposure changes with distance squared New mAs = Old mAs x (New distance / Old distance)^2, to keep image density equivalent if FFD changes. |
Grid use
Reduces scatter, improves contrast - but requires more exposure
| Guidance |
|---|
| Commonly used above ~10-12 cm part thickness E.g. thorax/abdomen in a medium-large dog. Thinner extremities generally don’t need a grid. |
| Grid technique needs more mAs Commonly 2-3x more mAs than a non-grid technique for the same body part - grid and non-grid settings are not interchangeable. |
Digital radiography (CR/DR) has much wider exposure latitude than film and can often correct a suboptimal exposure in post-processing - but overexposure still means unnecessary radiation dose to the patient and staff (the ALARA principle: As Low As Reasonably Achievable), so calibrated technique remains important for radiation safety even when the image still looks diagnostic.
Building a technique chart for your equipment
- Measure part thickness with calipers at the thickest point of the area being radiographed.
- Start from your machine manufacturer's suggested baseline settings (often provided with the unit or software presets by body part and species).
- Make a few calibration exposures, adjusting kVp and mAs based on the resulting image density and contrast.
- Record the finalized settings by body part and thickness range for your specific machine, so your whole team can use a consistent, reproducible chart.
Frequently asked questions
Why isn’t there a universal kVp/mAs chart for veterinary radiography?
Exposure factors depend on your specific machine’s output, detector type, grid use, and other equipment-specific variables, so a technique chart must be calibrated to your equipment rather than copied from a generic published source - always start from your machine manufacturer’s guidance and calibrate from there.
What does the "15% rule" mean in radiography?
Increasing kVp by 15% roughly doubles the exposure reaching the detector, equivalent to doubling mAs, while also reducing image contrast - useful for adjusting technique while managing patient motion or overall dose.
When should I use a grid?
Commonly recommended for body parts thicker than about 10-12 cm, such as the thorax or abdomen of a medium-to-large dog, since a grid reduces scatter radiation and improves image contrast - but it also requires increased exposure, commonly 2-3x more mAs, so grid and non-grid images aren’t interchangeable at the same settings.
Does correct exposure matter less with digital radiography since the image can be adjusted afterward?
Image density can often be corrected in post-processing on DR systems, so a suboptimal exposure may still look diagnostic - but overexposure still means the patient and staff received unnecessary radiation dose per the ALARA principle, so appropriate technique remains important for radiation safety, not just image quality.