How to get to negative value in threshold (discom format)



I’m new to image J. I want to measure the visceral fat area in CT imaging. I get image from CT scan in discom format. I want to adjust the hounsfield unit between (-190~-30). However, the image I open in imageJ can only adjust the threshold between 0~255. The CT image is 8 bit. How can I get the 16bit image?
Thank you in advance!