RADIOGRAPHIC SUBTRACTION BY COLOR ADDITION.

Abstract

The radiographic subtraction technic is frequently used to accentuate the vascular tree in angiographic studies. The photographic method of doing this is rather time-consuming and requires the services of a skilled photographer. The electronic method which uses two television cameras, one yielding a positive image, is quite expensive. The procedure herein described uses a very old principle of combination of colors. When three primary colors, or a color and its complement, are added by being projected onto a screen or with beam-splitting mirrors, they give the appearance of white. By placing appropriate black-and-white color separation transparencies in the beams, full color pictures can be displayed. A description of this method, which has been employed for many years, may be found in any basic physics text. We have used radiographs from the arterial and venous phases of cerebral angiograms, projected respectively through red and green filters. The original radiographs, or slides made from them, may be projected with two identical overhead projectors. The filters to be used will vary with the color temperature of the light source. With conventional tungsten projection lamps, Wratten filters numbers 25 and 40 have given a satisfactory white in the areas of addition. Two radiographs in identical projection and good registration will show the arteries in red, the veins (or ventricles, if a pneumoencephalogram is used instead of the venous phase of the arteriogram) in green, and the bones, which appear in both radiographs, in white. The spectacle is, if nothing else, quite dramatic. If a permanent record is desired, a color photograph can be taken of the screen. We are presently designing a beam-splitting device to achieve the same effect without the necessity for projection. With this apparatus we will be able to place the radiographs on an illuminated table and view them directly, much as with a conventional stereoscope. In this connection, it is of interest that the effect can be obtained if the different colored images are presented to different eyes, but an unpleasant flickering sensation renders viewing difficult. In conclusion, the color addition technic for radiographic subtraction promises the advantages of (a) simplicity, (b) cheapness, (c) speed, (d) simultaneous viewing of the relationships of arteries, veins, and bony landmarks, and (e) a dramatic graphic presentation for teaching purposes. Whether its clinical usefulness will surpass or even equal that of conventional subtraction technics remains to be determined.