Calculate entropy of an MR image

I have to calculate entropy of mr image. I used both methods but it gives different ans... Can anyone help??

% Find entropy

E = entropy(realimg(:));

% lg = log2(sum(realimg(:)));

en = -sum(realimg(:).*log2(realimg(:)));

Answer 1

[Image Analyst]

Simply do

>> edit entropy.m

to discover what their formula is and how it differs from yours. It will be obvious.

[Zee Idrees]

Thank u :)