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!)

Due to this complexity, Pixel shifting is not directly supported in the COM_version of DicomObjects through simple properties (like Mask or Maskframe for unshifted subtraction) but is is supported through use of the DICOM "Mask Subtraction Sequence" (Mask Module) which may be present in either the image itself, or more commonly in a DICOM Presentation_state object. This sequence allows a great deal of complexity and is used internally by DicomObjects, and the following VB code gives an idea how to use it.

Note: This example uses modification of the image as stored in memory, and developers should be careful as to whether or not they wish to persist any such modificiation in images sent or stored elsewhere. In general, modification of existing images is not recommended!

This example uses two items in the sequence to indicate that:

• Frames 1-5 should not be subtracted (they are not listed in either item)
• Frames 6 to 10 should have frame 4 subtracted, with a shift of 1 pixel vertically and -2 horizontally
• Frames 11 to 15 should have frame 5 subtracted, with a shift of 0 pixels vertically and 3 horizontally

More complex examples are also supported using "Time Interrval Differencing" or averaging of multiple mask frames, but these are beyond the scope of this article - see DICOM Part 3 section C.7.6.10 for more information.

Dim shift(1 To 2) As Single                             ' need an array to hold two shift values 
Dim range(1 To 2) As Single                             ' need an array to hold ends of frame range 
Dim MaskSequence As New DicomDataSets 

The first range:

Dim MaskSequenceItem1 As New DicomDataSet
MaskSequenceItem1.Attributes.Add &H28, &H6101, "AVG_SUB" ' fixed value to indicate normal subtraction

range(1) = 6
range(2) = 10
MaskSequenceItem1.Attributes.Add &H28, &H6102, range

MaskSequenceItem1.Attributes.Add &H28, &H6110, 4         ' the mask frame

shift(1) = 1                                             ' note that Y comes first!
shift(2) = -2
MaskSequenceItem1.Attributes.Add &H28, &H6114, shift

MaskSequence.Add MaskSequenceItem1

The second range:

Dim MaskSequenceItem2 As New DicomDataSet
MaskSequenceItem2.Attributes.Add &H28, &H6101, "AVG_SUB" ' fixed value to indicate normal subtraction

range(1) = 11
range(2) = 15
MaskSequenceItem2.Attributes.Add &H28, &H6102, range

MaskSequenceItem2.Attributes.Add &H28, &H6110, 5         ' the mask frame

shift(1) = 0                                             ' note that Y comes first!
shift(2) = 3
MaskSequenceItem2.Attributes.Add &H28, &H6114, shift

MaskSequence.Add MaskSequenceItem2

Add to the image, and suggest use of subtraction mode for viewing

im.Attributes.Add &H28, &H6100, MaskSequence
im.Attributes.Add &H28, &H1090, "SUB"     ' indicate to display in subtraction mode

NOTE: Due to a bug in earlier versions of DicomObjects, only the first such sequence will actually be considered in versions before 4.3.54.x