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Measurement Calibration

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Most DICOM images come calibrated.  That is, the size of each pixel in millimetres is encoded in the image.  However, there are some – most commonly ultrasound – which are not.  Such images are easily identified--measurements are reported in pixels instead of a physical unit like millimetres, and the image scale lines are not visible.  Uncalibrated images may be manually calibrated by using the Calibration Tool:

1.Using the Ruler Tool, draw a line that corresponds to a known physical length.  The longer the line is, the more accurate the result will be.  On ultrasounds, a scale of some kind is often available on the image itself that can be used to calibrate measurements.
2.Right-click on the measurement and select Calibrate.

3.In the dialog that appears, enter the physical length of the line.

4.Click OK.
5.The measurement should now appear in physical units and the image scale should become visible.

Technical Notes

Manual calibration applies only to one image at a time.  If you have a number of uncalibrated images in the display set, you need to calibrate them individually.  This is because it is not always reasonable to assume that the calibration is the same throughout the image stack.
You are allowed to manually recalibrate images that are already calibrated.  Be very careful when you do this as you may inadvertently make measurements less accurate. We allow this because sometimes fiducials are used during the scan to provide more accurate measurements, especially in projection based modalities (i.e. DX, CR, MG).
If you calibrate an image when the Calibration Method text overlay item is visible, the method will change to User.

 

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