Double exposure technique for demonstration of osseous and pulmonary structures on a single CT film

Abstract

Marked differences in linear attenuation coefficients among lungs, soft tissue, and bony thorax necessitate viewing CT scans of the thorax at relatively wide window settings and variable window levels. Bony and soft tissue structures may best be seen at window levels near water density (0 window level), while pulmonary panenchyma and vasculanity are best appreciated at much lower window levels (-300 window level). Hard copy on Polaroid or multiformat camera has required two pictures for each CT section to adequately demonstrate all tissue structures [1]. We have developed a double exposure, single image technique clearly delineating lung, soft tissues, and bony structures on one film. In addition, this technique has an added benefit of edge enhancement not appreciated on routine filming at either low or high window levels. Double exposure images may be obtained from any cathode nay tube unit as long as a reverse mode is available. The technique involves the following process: (1) first exposure at 400 window width, 0 window level, normal image mode; and (2) second exposure at 400 window width, -300 window level, reverse image mode on the same film as in (1). These exposures may be taken in any order as long as the negative window level is exposed in reverse mode. A Polaroid reproduction of the thorax at settings to demonstrate osseous structure is shown in figure 1A, and figure lB is a film of the same CT image at window settings to show pulmonary structure. Figure 1C combines the two images as a double exposure on a single Polaroid film. Note that osseous structure and pulmonary metastases are well delineated on this double ex-