how to know if an image is int16 or uint16 in dicom file

Hi Comunity,

i am reading dcm files from scratch and i ve manage to get the header and the image matrix using the fread function.

dcm_pixel_data = fread(dcm_file_id, [pixel_data_Width pixel_data_Height], 'uint16')';

whene i compare my result with the output of the matlab function 'dicomread()' using 'ssim()' some time i get 1, but some times i don't get 1!!

whene i searched & debuged i found that the images are not always in 'uint16' bits but can be 'int16' (i think the negative pixels are noise but plz to correct me if i am wrong!) what parameter in the header indicates the whether to use 'int16' or 'uint16'!