Some DICOM images (usually ultrasound) might not be pre-calibrated so that the size of a pixel in the image is not known. For such images, measurements are displayed in pixels instead of a physical unit like centimeters, millimeters, etc. However, those images can be calibrated by Ruler tool since they have a scale as a part of the image.

1. Activate Ruler tool by Tools > Measurement > Ruler

2. Draw a line to calibrate on a scale which is found on the image.

   

   For more accurate results, it is recommended to use a calibration line as long as possible.

3. Right-click on the calibration line, and select Calibrate
4. Enter the length of the calibration line in a physical unit as indicated on the scale where the line is drawn, and select OK
   

5. The calibration line is shown in a physical unit now, and all other measurements are shown in a physical unit.

   

   Measurements can only be calibrated for images individually, so a calibration done for an uncalibrated image does not apply to other uncalibrated images.