Certain DTI image files (e.g., V1, the vector image showing the principal diffusion tensor direction) are multi-volume (4D), like FMRI image files. To view such images in DTI Mode, select the image in the Image List and press the Image Info button . This will open the Image Info dialog. Also, when DTI Display is set to RGB the image intensity can be modulated, e.g. with the fractional anisotropy image. To do this, load the FA image into FSLView and then select this in the Modulation menu on the Image Info dialog. The DTI image will now be modulated by the anisotropy image. Where anisotropy values are very low the DTI image will appear more transparent. Where the anisotropy are very high the DTI image will appear more opaque. When the DTI image is highlighted in the Image List the transparency slider will split into 2 parts. The left hand slider controls the overall transparency. The right hand slider controls the amount of transparency change caused by the modulation. If the right hand slider is set to the furthest point on the right then the modulation will have no effect at all. Source.