Analysing visceral, retroperitoneal and subcutaneous fat by using Macro in image J in CT- imagej

Dear,

I am trying to image J for calculating the percentage of fat in a CT- image. The file is NIFTII. I have 16 bit images and I need to find out the fat from specific slice. I have used the technique of histogram and pixel values in 8 bit image.
But, I require a more accurate technique if possible.