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, such 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 the scale which is a part of 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 in the dialog shown and select OK

5. The calibration line is shown in a physical unit now, and all next measurement that will be drawn will be shown in a physical unit.

   The measurement calibration can only be done for images individually, so you should calibrate multiple uncalibrated images separately if you have more than one uncalibrated image.