Dicom Image analysis; why converting unit8 is different values of matrix?

Sir,

I have 512x512 size Dicom images.

I convert these images into unit8 using im2unit8.

Some image values are 128 (all 512x512 Matrix). But some image values are 0 (all 512x512 Matrix) ie., Black Image.

Why these variations.

I want all image values are 128 after converting unit8.

How to do this?

Thank you.

[Image Analyst]

There is no unit8 data type and are no unit8 or im2unit8 functions in MATLAB. And you forgot to attach an image.