ENVI works with raster image files.  Rasters always must always be rectangular with a certain number of samples and lines (rows and columns) defining it.  If your area of interest is not rectangular or irregular shaped, all areas outside of the area of interest will be background set to whatever value you choose, but typically they are set to 0.  The best you can do is subset to the smallest bounds around your area of interest to exclude as much background as possible, but again, if your area is not rectangular, then you will still have some background. ArcGIS works in a similar way with rasters.  Rasters are still a certain number of samples and lines.  However, instead of assigning a background value to areas outside your area of interest, they assign a 'no data' value which can be made transparent.  The data are still rectangular but this area is not viewable.