1. A method of evaluating the amount of a particular substance within an object of interest in a subject body, wherein the substance in said object of interest is at least partially obscured by an additional quantity of such substance, said method utilizing a digital imaging system having source and detector means for employing penetrative radiation and for generating and storing pixel values including signals corresponding to detected radiation emergent from the subject, said method comprising the steps of: (a) actuating the source for causing penetrative radiation to pass through both the object of interest and the obscuring quantity of the substance to emerge in a pattern; (b) detecting the emergent pattern; (c) generating signals defining a set of pixel values related to the emergent pattern; (d) identifying first and second portions of said pattern, whereby a first set of pixel values corresponding to said first pattern portion and a second set of pixel values corresponding to said second pattern portion are identified; (e) combining said first set of pixel values; (f) combining said second set of pixel values; and (g) producing a scalar representation of the difference between the resultants produced by combining said first set of pixel values and said second set of pixel values.