Accessing elements of an image using C++

Hi all,

I am new member to this group. Actually, I have read a data file of an
image from its sub header stored the data in a buffer.*imdata is a pointer
to the buffer for data. The data size is 100x100x81 and no. of bytes per
pixel is 4 bytes and the format type is float. Now I have to take the
center slice of this 3d image and locate ROI (region of interest) and find
mean within this region.
My question is how to access the center slice (40th slice) through this
pointer imdata. Thanks in advance.