+We have to deal with 4 *very* different cases :
+-1) user created ex nihilo his own image and wants to write it as a Dicom image.
+-2) user modified the pixels of an existing image.
+-3) user created a new image, using existing images (eg MIP, MPR, cartography image)
+-4) user anonymized an image without processing the pixels.
+
+gdcm::FileHelper::CheckMandatoryElements() deals automatically with these cases.
+
+1)2)3)4)
+0008 0012 Instance Creation Date
+0008 0013 Instance Creation Time
+0008 0018 SOP Instance UID
+are *always* created with the current values; user has *no* possible intervention on
+them.
+
+'Serie Instance UID'(0x0020,0x000e)
+'Study Instance UID'(0x0020,0x000d) are kept as is if already exist,
+ created if it doesn't.
+ The user is allowed to create his own Series/Studies,
+ keeping the same 'Serie Instance UID' / 'Study Instance UID' for various images
+ Warning :
+ The user shouldn't add any image to a 'Manufacturer Serie'
+ but there is no way no to allowed him to do that
+
+ None of the 'shadow elements' are droped out.
+
+
+1)
+'Modality' (0x0008,0x0060) is defaulted to "OT" (other) if missing.
+'Conversion Type (0x0008,0x0064) is forced to 'SYN' (Synthetic Image).
+'Study Date', 'Study Time' are defaulted to current Date and Time.
+
+1)2)3)
+'Media Storage SOP Class UID' (0x0002,0x0002)
+'SOP Class UID' (0x0008,0x0016) are set to
+ [Secondary Capture Image Storage]
+'Image Type' (0x0008,0x0008) is forced to "DERIVED\PRIMARY"
+Conversion Type (0x0008,0x0064) is forced to 'SYN' (Synthetic Image)
+
+2)4)
+If 'SOP Class UID' exists in the native image ('true DICOM' image)
+ we create the 'Source Image Sequence' SeqEntry (0x0008, 0x2112)
+ --> 'Referenced SOP Class UID' (0x0008, 0x1150)
+ whose value is the original 'SOP Class UID'
+ --> 'Referenced SOP Instance UID' (0x0008, 0x1155)
+ whose value is the original 'SOP Class UID'
+
+3) TODO : find a trick to allow user to pass to the writter the list of the Dicom images
+ or the Series, (or the Study ?) he used to created his image
+ (MIP, MPR, cartography image, ...)
+ These info should be stored (?)
+ 0008 1110 SQ 1 Referenced Study Sequence
+ 0008 1115 SQ 1 Referenced Series Sequence
+ 0008 1140 SQ 1 Referenced Image Sequence
+
+4) When user *knows* he didn't modified the pixels, he may ask the writer to keep some
+informations unchanged :
+'Media Storage SOP Class UID' (0x0002,0x0002)
+'SOP Class UID' (0x0008,0x0016)
+'Image Type' (0x0008,0x0008)
+'Conversion Type' (0x0008,0x0064)
+He has to use gdcm::FileHelper::SetKeepMediaStorageSOPClassUID(true)
+(probabely name has to be changed)
+
+
+Bellow follows the full description (hope so !) of the consistency checks performed
+by gdcm::FileHelper::CheckMandatoryElements()