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Appendix 2: Equalisation Techniques in Chest Radiography

As discussed throughout the report, the most difficult technical challenge in chest radiography is the very large range of X-ray radiation transmitted by the human thorax. This dynamic range can easily exceed 50, greater than the recording latitude of conventional radiographic film. Radiographic technique factors are typically selected so that the lung is imaged in the optimum linear portion of the H & D curve, giving good contrast in the lung region. However, this forces the mediastinum and retrodiaphragmatic regions into the toe of the H & D curve (Figure 4.2), resulting in the familiar under-exposed appearance in those areas. Film manufacturers have addressed this problem by producing special wide-latitude films for use in chest radiography, but this solution has the drawback of improving contrast in the mediastinum at the expense of reduced contrast in the lungs. The wide range of transmitted X-ray intensities also results in a wide range of signal-to-noise levels over the chest radiographic image. The signal-tonoise ratio in the dense mediastinum and obscured lung can be many times worse than in the unobscured lung because of poor X-ray statistics. This makes it more difficult for observers to detect subtle abnormalities in the poorly penetrated regions, a difficulty compounded by poor radiographic contrast in these areas.