Patient position descriptor relative to the equipment. Required for images where Patient Orientation Code Sequence (0054,0410) is not present and whose SOP Class is one of the following: CT ("1.2.840.10008.5.1.4.1.1.2") or MR ("1.2.840.10008.5.1.4.1.1.4") or Enhanced CT ("1.2.840.10008.5.1.4.1.1.2.1") or Enhanced MR Image ("1.2.840.10008.5.1.4.1.1.4.1") or Enhanced Color MR Image ("1.2.840.10008.5.1.4.1.1.4.3") or MR Spectroscopy ("1.2.840.10008.5.1.4.1.1.4.2") Storage SOP Classes.
May be present for other SOP Classes if Patient Orientation Code Sequence (0054,0410) is not present.
When validating my registration object, this scenario is throwing an error because the (0018,5100) Patient Position tag is not defined.
As far as I can tell from the DICOM standard, this tag should only be required for CT and MR images.
From http://medical.nema.org/dicom/2013/output/chtml/part03/sect_C.7.html :
Patient position descriptor relative to the equipment. Required for images where Patient Orientation Code Sequence (0054,0410) is not present and whose SOP Class is one of the following: CT ("1.2.840.10008.5.1.4.1.1.2") or MR ("1.2.840.10008.5.1.4.1.1.4") or Enhanced CT ("1.2.840.10008.5.1.4.1.1.2.1") or Enhanced MR Image ("1.2.840.10008.5.1.4.1.1.4.1") or Enhanced Color MR Image ("1.2.840.10008.5.1.4.1.1.4.3") or MR Spectroscopy ("1.2.840.10008.5.1.4.1.1.4.2") Storage SOP Classes.
May be present for other SOP Classes if Patient Orientation Code Sequence (0054,0410) is not present.