Calibration corrects for missing or incorrect pixel-spacing metadata. It is most often needed on legacy radiographs or scanned films, where the DICOM header lacks real-world dimensions. Without calibration, length measurements are not in true mm.
Step 1: Find a known reference
Identify a structure of known size in the image. A calibration marker ball, a coin placed on the plate, or a known implant length all work. For clinical calibration, use a ruler or marker placed at acquisition.
Step 2: Apply calibration
Click Calibration Line in the Measurements toolbar. Click the two endpoints of your known reference, then enter its true length in mm. All subsequent length measurements on that image use the new calibration.
Step 3: Save or clear calibration
Calibration persists for the current session. To save, include it in a Presentation State. To clear, use Reset on the viewport.
Tips
- Calibration is image-specific, not series-specific. If pixel spacing is uniformly missing across a series, you must repeat calibration on every instance. Flag this to your modality vendor, it is usually a configuration error.
- Always document when measurements come from calibrated vs header-based pixel spacing. In a report, a line like "measurements performed after manual calibration against a 25 mm reference" prevents future confusion.
Related articles
- Basic measurements: length, angle, ROI, area and volume
- Radiology-specific measurements
- Viewing DICOM tags