FSLView can read Analyze and NIFTI files, either uncompressed or compressed, and either as a single file (e.g., image.nii.gz) or two files (e.g., image.hdr & image.img). To open an image, select the Open option from the File menu. A file opening dialog will appear. It will list all directories and all files with a suitable extension. Select the image file that you want to load and press the Open button.
The image will then be opened in Ortho view; coronal, sagittal and axial views are displayed simultaneously. If there is sufficient information present in the image header, L, R, A, P, S, I (Left, Right, Anterior=front, Posterior=back, Superior=top, Inferior=bottom) orientation markers will be displayed, making the orientation clear. The image name will be displayed in the Image List. Because this is the first image to be loaded it will be the "Main Image"; in the Image List it will be marked with a small set of arrows .
To load a second image (an "overlay"), select File -> Add. The image to be added must have the same dimensions as the Main Image, otherwise a message will appear saying Cannot load incompatible overlay.
When the second overlay is added, its name will appear in the Image List. Since it is not the Main Image it will not be marked with arrows. The Image List control can be used to change the transparency of images, change the order in which they are drawn, make them visible or invisible and lock or unlock them (for editing).