Calibrating radiographs
Calibration tells the app the real-world size of one pixel. Every measurement and every implant template depends on this — without it, the rest of the app’s tools are locked.
What do I need in the film to calibrate it?
A radiopaque reference of known dimension, sitting in the same plane as the bone of interest — typically a 25 mm sphere taped to the limb. A linear marker (a ruler) works too, and so does any round object whose true diameter you know. Set your own sizes under Settings → Planning Defaults → Your Calibration Markers; they are kept on that computer, so a second machine in theatre needs setting too.
Why does calibration matter?
Radiographic magnification varies between exposures. The same dog’s tibia may project as 130 mm on one X-ray and 142 mm on another. If you skip calibration and try to overlay a 65 mm implant, it’ll land at the wrong size — and you’ll plan with the wrong dimensions.
Every measurement and overlay in the app uses the calibration you set. Get this step right.
How do I calibrate a radiograph?
The app checks every radiograph for a calibration ball as it is imported. The size it assumes is the one set on this computer, under Settings → Planning Defaults → Your Calibration Markers — 25 mm for a ball and 50 mm for a ruler unless you change them.
- Confident match: a toast shows the measured scale. Choose Yes, use it to confirm; there is nothing to click on the image.
- Uncertain or no match: nothing is selected and no tool is armed. Calibration stays blank until you choose to set it yourself.
To calibrate an image by hand, click Calibration in the sidebar. It has three sub-tools and a Clear scale button:
- Auto — the app tries the whole image once more. If it is still unsure, click once inside the ball and it measures it for you.
- Circle — place a circle over any round object of known diameter and set the scale from that.
- Line — for a standard linear marker. Place the line over the marker and adjust its two ends. The confirmation starts at the length shown — 50 mm by default. Change that number if your marker is a different length, then choose Set Scale. Until then, the scale is unchanged.
Where the film is a DICOM carrying its own pixel spacing, a fourth button offers that figure. Read the warning below before taking it.
If a click does not land a confident result, try again nearer the centre of the ball, or use Circle to fit it by hand. The app does not guess when the image is ambiguous.
What do GOOD, CHECK and REJECT say about the fit?
- GOOD — a clean, confident fit round the whole ball. Safe to accept.
- CHECK — a fit was found, but something about it is worth a second look before you trust it (partial coverage, a noisy edge). Look at the image, not just the number.
- REJECT — not confident enough to use. Click again, or switch to the manual circle tool.
Two more things the card tells you:
- Coverage and gaps. The fit is built from points sampled all round the ball’s rim. Anything crossing the ball — bone, soft tissue, another marker — blocks that stretch of rim from being used. A gap warning means a meaningful chunk of the edge was skipped, not measured.
- Green and red ticks on the ball. Green ticks are the edge points the fit used. Red ticks are edge points it found but threw out because they didn’t agree with the rest. A lot of red, or red bunched on one side, is worth a second look.
Can I change the marker size?
Upload recognition offers your practice’s marker size, and says which figure it used. The size is still editable film by film — enter the ball’s true diameter or the ruler’s true length before accepting. Whatever you accept is remembered as that shape’s size: a ball’s diameter never overwrites a ruler’s length, or the other way round.
Should I trust a DICOM’s own scale?
Some DICOM files carry a pixel spacing tag, and the app will use it as a starting scale. That value describes the X-ray detector’s pixel size, not the size of anything at the bone — the bone sits above the plate and is projected onto it larger than life. Left as-is, lengths read off that spacing can come out 10–25% too large, which makes implant templates size out relatively too small and nudges you toward an oversized plate.
Whenever a calibration ball is in the field, calibrate off it. The ball-based scale reflects the plane the bone is actually in; the detector’s own spacing doesn’t.
How do I check the calibration is right?
- Repeatability. Click five different spots inside the same ball. Every one should give the same diameter and the same scale. If they don’t agree, something in the image is confusing the fit — use the manual circle tool instead.
- Self-measurement. After calibrating, measure straight across the ball with the Line tool. It should read 25.0 mm (or whatever diameter you set). If it doesn’t, the calibration hasn’t taken — recalibrate before planning off it.
Why does calibration go wrong?
- Chained or stacked markers. Two or more spheres touching can look like one blob. Auto-detection can be fooled by this — verify the ball it picked is the correct individual marker.
- Marker off-plane. A ball taped next to the bone but sitting at a different distance from the X-ray plate has different magnification to the bone itself. Calibration will look right but the rest of your planning will be subtly off.
- No marker visible. You can’t calibrate without one. The app won’t let you measure or place templates until a scale is set.
When should I re-calibrate?
- New radiograph in the same case — every image needs its own calibration.
- Suspicious measurements (a 6 kg dog’s tibia measuring 180 mm) — recalibrate before trusting the rest.
You do not have to clear the old scale first. Setting a new one replaces it, and anything already placed on the film is resized to keep the millimetres it was chosen at.
How do I clear a scale and start again?
Calibration → Clear scale takes the image back to uncalibrated: the px/mm figure goes, and measuring, osteotomy planning and implant templates lock again until you set a new one. The marker you calibrated off is removed with it.
Nothing you have measured is moved or deleted — lengths, angles and cuts stay exactly where you drew them, and simply stop reading in millimetres until there is a scale again. Undo on the confirmation puts everything back.
Use it when the scale itself is wrong rather than merely out — the wrong ball was measured, or the marker turned out to be off-plane — and you would rather have no number than a wrong one while you sort the film out.