Subtraction Radiography by Means of Additive Color
Gerhart S. Schwarz · Radiology · 1966
Color radiography has been employed in the past mainly to (a) extend the photographic range of the radiographic process, i.e., utilize more of the available perceptive power of the human eye, (b) substitute color for contrast, likewise a psychophysical method, (c) discriminate between a high- and low-kilovoltage radiograph of the same subject, and (d) hand-color lecture transparencies, i.e., selectively emphasize certain anatomic structures by artificial means. Below, a method of subtraction radiography will be described which generates its own color automatically. Black - and - white subtraction radiography, though first described in 1935 (1), has not gained acceptance to date for the following two reasons : a. It requires an added photographic process in which one of the two radiographic mates is being reversed by contact printing. Recently, a dual closed-circuit television viewer has been marketed which reverses the image of one of the mates electronically but at the expense of resolution. b. It requires extremely accurate super-imposition of two radiographic mates so that the undesirable parts of the visible anatomy may cancel each other. When it fails, i.e., when a slight parallax develops between the two mates, the intended cancellation turns into its opposite, namely into a relief-type of contour enhancement which defeats the purpose of subtraction radiography altogether. If the subtraction process is based on color, on the other hand, the registry of the two radiographs is not critical because the images which heretofore had to disappear may now remain visible since they assume a color hue which is distinct from that of the structure to be emphasized by x-rays. Method: In its simplest form, two conventional black-and-white radiographs are taken, one before and one after administration of a contrast medium. The films are placed in two matched projectors and cast upon a single screen. One of the projectors is provided with a red filter, the other with a blue or green filter (any other two colors can also be used ; they need not necessarily be complementary). The result is that those anatomic structures which are identical on both films will appear in a nondescript muddy ivory hue, but those areas occupied by contrast material images on one film but not on its mate will assume a brilliant primary color. In short, the contrast material paints its own color on the screen which makes it very distinct. An example is shown in Fig. 1, an Ektachrome photograph of the projection screen. No color photography is needed for viewing, however. Two black-and-white films instantaneously produce the color when projected. Only for permanent record-keeping or publication is a color photograph of the screen desirable. A Polaroid color-pack camera is now being experimented with for recording these images.