With MRIcron 
 1.) Open the image where the ROI was drawn
 2.) Draw/Convert/ROI->VOI

The old ROI format does not have its own header, so it depends on being paired with a NIfTI/Analyze format header. The newer voi format is actually a fully fledged NIfTI image (the only difference between filename.voi and filename.nii.gz is the file name's extension, this helps the Draw/Open menu only show valid drawings while making it easy to exchange images with other tools).