Limitation in display of CT numbers The human eye can distinguish only about 20 levels of gray in the gray scale and cannot appreciate the large range of information contained in CT scan The human eye can distinguish only about 20 levels of gray in the gray scale and cannot appreciate the large range of information contained in CT scan This is solved by using the 'window width' & 'window level' This is solved by using the 'window width' & 'window level'