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Calibrating radiographs

Calibration tells the app the real-world size of one pixel. Every length in millimetres and every implant template depends on this. Without it, Line, Angle, Circle, TPA, the osteotomy and Reposition tools and the implant library are locked; CORA, Norberg and annotation still work.

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 → General → Planning Defaults → Your Calibration Markers; they are kept for your account on that computer, so a second machine in theatre needs setting too, and so does a colleague who signs in on yours.

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. Without a scale the app will not place an implant at all. Calibrate off the wrong thing, and a 65 mm implant lands 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, and for a ruler when there is no ball. The size it assumes is the one set for your account, on whichever computer you set it, under Settings → General → Planning Defaults → Your Calibration Markers — 25 mm for a ball and 50 mm for a ruler unless you change them.

• Confident match: a Read from this film panel opens with the ball selected and your ball size in the box, which you can change for this film. Choose Use this marker to set the scale, or No scale from this film — set it later. There is nothing to click on the image, and nothing changes until you choose.
• 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 searches the whole image for your ball and measures it. With no ball, it looks for your ruler instead and lays a line along it, its length set to your ruler size. Check either one, then choose Set Scale. If it finds neither, or more than one ball, click once inside the ball and it measures it for you.
• Circle — place a circle over any round object of known diameter, drag its three handles onto the object’s edge, 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 if the fit isn’t clean?

A fit that is worth a second look says so in one line under Set Scale — for example Something crosses the ball; 40 degrees of the rim was ignored, or that the ball’s edge is faint, or that the measurement moved a little between passes. Look at the image, not just the number, before you accept it.

A fit too poor to use is never offered. The app says No marker under that click — click inside the ball, and after a second miss it hands you Circle to fit the ball by hand.

• 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, which is what the gap warning is about.
• 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 own marker size, and says which figure it used. The size is still editable film by film — enter the ball’s true diameter before accepting. A size typed on the upload panel applies to that film only. A size you confirm with Set Scale in Calibration is remembered as that shape’s size for the next film: 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 offers it — as a row on the panel shown when the film is imported, and as Use DICOM scale in Calibration while the film has no scale — but never applies it unless you choose it. 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.

What does “Estimated” beside the scale mean?

That the scale came from the DICOM’s own pixel size, not from a marker on the film. The amber Estimated badge sits beside the px/mm figure in the sidebar for as long as that is the film’s scale. Everything works, but lengths, circles and implant sizes are only as good as that figure — see the caution above. Angles are not affected: an angle is the same at any scale. To replace an estimated scale, calibrate off a ball or a known length; the new scale takes over and the badge goes.

How do I check the calibration is right?

• The ticks and the caution line. Before you press Set Scale, check the green ticks run round the whole rim of the ball, and that there is no caution line under the button. If either looks wrong, something in the image is confusing the fit — use Circle to fit it by hand 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 take a DICOM’s own pixel spacing (read the caution above), or lay Line over anything in the picture whose true length you know. Until a scale is set, Line, Angle, Circle, TPA, the osteotomy and Reposition tools and the implant templates stay locked; CORA and Norberg still work.

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. Implants and TPLO saw blades are resized to keep the millimetres they were chosen at; measurements and cuts stay exactly where you drew them and read against the new scale.

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.