Mask subtraction is very different in the COM and .NET versions of DicomObjects
DicomObjects.COM In the COM version of DicomObjects, two DicomImage properties associated with the Mask Subtraction operations are sometimes found a bit confusing to developers: Mask & MaskFrame.
Mask This functions as a switch to turn on/off the mask subtraction operation. If Mask Subtraction Sequence (0028, 6100) exists then DicomObjects will sets the default value of Mask to 1, otherwise 0.
Pixel shifting is a process used in conjunction with Mask Subtraction to correct for movement of the patient between exposures, so as to give the best alignment and therefore the best masking of fixed structures possible. In general, there needs to be a different shift factor for each frame in an image (if there has been movement between the mask and the image frames, then there is also likely to be movement between the image frames!