16 bit Greyscale histogram changes to rgb

Hi there,
I’m quite new to imageJ and have some issues with 16bit greyscale data.

I have .IMA greyscale sequences (obtained from CT scans of a sandbox) that I need to convert to 16bit .raw files. This shouldn’t be too much of a problem as I simply can save the the sequences as .raw files. There’s, however, a Problem with the intensities. I’ve been told that the intensity range shouldn’t exceed 1600 per .raw file. If I check the histogram, however, its range goes from -1024 up to 1000 and something, which clearly exceeds the 1600 I’ve been told. Further, if i click on histogram it converts my 16 bit sequence to rgb. Is this a bug or simply a step I don’t understand? (I’m using the mac 64 bit 1.52 version).

Once the sequence is converted to a 16bit .raw file, I want to repeat this step for four more sequences (I have five different scan sequences from the same position). Having then five 16 bit .raw files I add them all together using the “image calculator plus” plugin. The problem is then that it converts to 32 bit as the intensity ranges exceed the 1600 intensity. The aim, however, would be to get a 16 bit .raw file of the superimposed five .raw files to cancel out noise of the scanned data.

Any help would be appreciated.
Timothy