Query based on absence of attribute #230
Description
This issue came up with some GE DWI scans, where the DiffusionBValue was present when B was greater than zero, but was absent when B was equal to zero. So querying a DWI scan with "-k DiffusionBValue=0" returned nothing, making it impossible (or at least difficult) to extract the B0 images from the scan.
Likewise, there isn't any way to query whether an attribute is empty.