Due to limitations of the modality or IMPAX interpretation of the modality data, some images do not contain sufficient information to accurately translate dimensions from pixels to real-world measurements. Before measuring areas on such images, you must calibrate them. You can do this by drawing a line of known length on the image and entering its value. Future measurements are then based on this entered value.
 | Note: If the user name contains special characters due to language settings, images cannot be calibrated. Ensure that the system locale is changed to handle any special characters for a particular language. |
Calibrations are applied on a user basis, meaning that your calibrations do not affect measurements made by other individuals.
For calibrated images, the accuracy is limited by the calibration provided with the image, and by the position of the start and end points of the line drawn which may vary by one pixel at each end. Based on the calibration provided, pixels are converted into millimeters and rounded to the nearest millimeter. For non-calibrated images, measurements are displayed in pixels (px) and the accuracy of the measurement is limited by the pixel position at each end of the caliper measurement.
 | CAUTION! User calibration takes precedence over any other calibration that can apply to the image—including modality calibration, in the case of CR,DR, and DX studies. MR and CT studies with DICOM spacing cannot be user calibrated. The Image area status bar indicates the current calibration source. |
To calibrate images
From the Image area context or top toolbar, click User Image Calibration.
Or, if using the context toolbar text menu (Ctrl + right-click), select Markup > User Image Calibration.
The pointer shape changes. If the image cannot be calibrated (as with CT and MR images with pixel spacing), an icon is displayed beside the pointer.
Click the start point of the area you are calibrating to.
Move the pointer to the end point of the area and click again.
To adjust either of the end points, drag them to a new position.
To set the calibration line, right-click.
In the Calibrate Image dialog Values field, type the value of the line length (up to 10 digits).
From the Units list, select the unit of measurement.
Under Scope, select whether to apply this calibration to the Image, Series, or Study.
Under Calibration Source, leave the Source value at User.
Click Save.
Your calibration is now in effect for the current image, series, or study. The status bar indicates the user ID as the calibration source. New measurements use the selected units and are suffixed with (ui). The Zoom True Size image display is also affected.
| Note: The user calibration is retained only if you save the study with the calibration changes. IMPAX can configured to save calibration changes automatically. |