Measurement Calibration

Precision is key when evaluating x-rays, and sometimes you need to be able to accurately measure an exact length on the screen. This is obviously tricky, as pixels (the little dots that make up every image on your computer screen) are not a practical real-life measurement.

Thankfully, we have a Measuring Tool in Digital Radiography that allows you to measure real-life lengths of a scanned image. The problem is that this isn't always perfectly accurate, depending on whether we have calibrated the measurement properly.

We can control the Pixels Per Unit in the Preferences > Digital Radiography > Image Tab > Measurement Calibration table, but in order to accurately gauge this, we need to determine the actual length of a scanned item, and use that to determine what our Pixels Per Unit should be. Pixels Per Unit calibrate an output of measurement to millimeters.

Note: For a quick and dirty guideline, we find that 30 pixels per unit works well for measuring at 1mm.

Measuring Pixels Per Unit

If we need to be more precise, you will need to calibrate for a more accurate measurement. In order to do this, we need to start off with taking an X-ray of an image we know is exactly a certain distance. We'll want to keep the sensor taking the x-ray at this same distance for optimal accuracy.

Once you have an image open in the Digital Radiography ability, use the Ruler tool and measure between two points.

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Once you have a measured line, it will display the length. Double click the line.

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Double click the line and enter in the actual distance. Press the tab key. Once you press tab, you will see a new calculation for the "Pixels Per Unit", as shown in the above image.

With your "true" Pixels Per Unit determined, you'll take that number and enter it in Preferences > Digital Radiography > Image > Measurement Calibration under the specific integrated device you're using.

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If you have a generic device, you'll use "Other Device" under the Device column.

These Preferences are local, meaning that for each MacPractice client with a device that requires calibration will need to be done individually on each machine.