1. A diagnostic device comprising: an arrangement for generating raw data representing contents of a volume; a computer connected to said arrangement for calculating gray-scale values, representing a three-dimensional image of said volume contents, from said raw data; an image system connected to said computer for generating image signals, according to an algorithm employing a trapezoidal transfer function which assigns at least one optical property to the respective gray-scale values, from said gray-scale values; a monitor connected to said image system and supplied with said image signals for displaying said three-dimensional image; a user-operable input unit connected to said image system; and said image system generating a histogram distribution of said gray-scale values and displaying user interface graphics on said monitor including a histogram window in which said histogram distribution is displayed, said image system in said graphics also displaying a symbolic trapezoidal transfer function with a plurality of input fields respectively associated with different points of said symbolic trapezoidal transfer function, said input unit allowing entries into said input fields to select the trapezoidal transfer function employed in said algorithm, and said image system inserting a representation of the selected trapezoidal transfer function in said histogram window.