Segmenting only within a ROI

Hi team, I am a novice user of 3D Slicer and trying to upskill. I have MRI images of the upper airway and I am trying to segment out only the airways (nasal and oral cavities). I have been using the threshold masking to some success, however the threshold of the air I am segmenting is the same as the image background and other areas of the head I do not wish to segment. I want to use a different threshold for the nasal airways than the oral airway. I thought it might be handy to create a ROI around the nasal airways and restrict my segmentation to that but I haven’t figured out how to segment only within the ROI i have made. Any help greatly appreciated