NEED HELP WITH PHOTOSHOP

"Any measurement scale present in a DICOM file is automatically imported with the file. If no scale is present, the default scale of 1 pixel = 1 mm is added as a custom measurement scale."

I am using DICOM file to measure an area. However, the scale is off when I use default measurement. The above is what the website says. Do you know I can ensure the DICOM file keeps its scale?

Please help! Thanks!