I tend to prefer the more modern dcm2niix (which comes with MRIcroGL) over the mature but legacy dcm2nii (which comes with MRIcron). For Siemens, the dwell time into the "description" of the header - you can view this with any tool that reports the NIfTI header, for example fslhd:

descrip TE=30;Time=140000.990;phase=1;dwell=0.280

For Siemens, the information required to compute dwell time is not in the standard DICOM tags, but is hidden in the proprietary CSA header. For details see the dcm2niix source code, in particular  readCSAImageHeader
  https://github.com/neurolabusc/dcm2niix/...