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Materials and Methods

The 70 mm technique can be adapted to any roentgen tube. It is necessary to use as small an anode focal spot as possible. A size of 0.3 mm provides a sharp image, but with thick tissue layers the load on the tube becomes too great. With a high speed rotating anode a focal spot of 0.6 mm should be considered feasible. The convex shape of the input screen causes distorsion of the image, especially at the periphery of the image. This distorsion is increased when the image from the input screen is focused onto the flat output screen. The dis­ torsion is increased further as the electrons released from the photocathode follow a curved path twisting about the linear axis of the image intensifier. The optics of the camera photographing the output screen gives an additional distorsion (Figs 1, 2 and 3). The thickness of the input screen decreases the detail definition of the image. Thus, the image of a point on the input screen is registered on the output screen as a blot with cloudy borders. Too small a radiation dose to the input screen will result in the so-called quantum mottle, which results in a granular picture.