Dear developer of RADIANT SOFT WARE,
I am a fans of RADIANT. I found there is some error in measuring DSA picture. The result of measurenent is larger than the actual size.The measurenent result in RADIANT is the vascular projection on the detetor of DSA. So it is enlarged. The result of measurenent should be corrected by a calibrating ruler. There are some DICOM TAGs including these calibrating infomation. These TAGs are "(0018,1110)Distance Source to Detector" and "(0018,1111)Distance Source to Patient". The corrected measure result is "Not calibrated result"*"Distance Source to Patient"/"Distance Source to Detector".

Thank you very much.

Wang kz

Thank you for your comments.
Please note that depending on the source of calibration data the length label is displayed in different color: https://www.radiantviewer.com/dicom-viewer-manual/length.html

The only way to obtain exact measurements in the DSA picture is to calibrate length measurements to a known object, e.g., a catheter inside a blood vessel.
The formula that you provided does not account for the thickness of the examined object. There would still be an error between two measurements located on the opposite sides of the examined object.