And just to make sure this is clear: converting DICOM to nrrd is not a simple well defined operation in general. DICOM is a very expressive and flexible standard and there are several possibilities for any one collection of DICOM files:
- it may correspond to many nrrd files (e.g. different series in a study)
- there may be more than one valid conversion and only the user knows which is preferred
- it may not be possible for nrrd to represent the DICOM data (such as a DICOM structured report or something)
That said, if you are dealing with a fairly standard case like converting one CT series to a nrrd scalar volume there are many good options as @fedorov points out. It all depends on the data and what you hope to use it for. Luckily there are lots of examples to learn from and by writing a script you can control the process to get what you need.