1. Field:
This invention relates to radiography, including medical chest radiography. More particularly, this invention discloses an optical masking technique to improve the observed contrast on a radiograph.
2. State of the Art:
Since the discovery of the x-ray in the late 19th century, x-ray techniques or radiographing techniques have become widely used in a wide range of activities in both the medical and non-medical fields. For example, in the medical field, x-ray techniques are a major diagnosing tool. In non-medical areas, the x-ray has developed into a tool for use in, for example, quality control of structural steel welds.
In the medical area, the chest x-ray has become a widely used, almost common, diagnosing tool for discovering a wide variety of afflictions. A basic problem in chest radiography is the limited latitude of x-ray film. Even with so-called "latitude film", some areas of the radiographic chest image, such as the central mediastinum and the peripheral areas of the lung may be underexposed and suffer from poor contrast. Even relatively dense structures, such as vertebral bodies, may be poorly visualized as a result.
High voltage techniques (220-350 kilo/volt/peak) have been suggested as a means for obtaining improved penetration. Improved penetration, and in turn improved image contrast, in these areas is the objective. W. P. Dyke, J. P. Barbour: Depth Resolution: A Mechanism By Which High Kilovoltage Improves Visibility In Chest Films; Radiology 117:159-164, October 1975. The high kilovoltage technique requires special generators and tubes for the x-ray apparatus or machine. Such equipment is expensive. Further, images or radiographs produced suffer decreased contrast in other areas of the chest image, e.g., in the lung fields, because of decreased attenuation coefficients for both soft tissue and bone, and because of increased scatter radiation. "Trough" filters have also been employed to obtain a more balanced film density between the lungs and mediastinum. These filters are difficult to use because a single filter cannot be constructed to simulate accurately the wide variations of patient anatomy that are encountered. Frequently, the trough filter generates a "shadow" which is seen clearly and becomes an annoying superimposed structure on the chest film (radiograph).
The particular problem encountered is specifically associated with the chest and mediastinum areas. That is, the chest provides a wide range of different densities as to the x-ray radiation so that the x-ray film in turn is overexposed and underexposed. In particular, the lung areas contain air (low density) so that the film in that area is rather well exposed. The x-ray radiation penetrates thoroughly and generates an exposed portion which is sufficiently dark for visualization purposes when the film is visually examined. In other areas, such as the mediastinum and heart areas, the film is underexposed. That is, the body is quite thick and relatively solid (higher density) resulting in substantial absorption of x-ray radiation in those areas. The film, in turn, is underexposed (light or white) and suffers from a substantial lack of visual contrast. This inhibits, and may even preclude, visualization and interpretation in these areas.
Similar problems are experienced for other radiography applications. That is, any time an object to be radiographed has varying or differing densities as to the x-ray radiation, the x-ray film may have underexposed areas with low optical or visual contrast. An improved method or technique to improve the exposure of the underexposed areas and in turn the optical contrast or visually observable contrast of radiographs, and particularly medical radiographs, is desirable.
Some masking techniques have been suggested for improving images on film or photographs. See for example: W. F. Schreiber, Image Processing For Quality Improvement, Proceedings of the IEEE (Vol. 66, No. 12, December 1978, pp. 1640-1651); K. R. Kattan, M. C. Caplan, B. Felson, A Method For Improving Roentgenographic Illustrations For Publication--Blurred Undersubtraction (BUS), Medical Radiography and Photography (Vol. 49, No. 1, 1973, pp. 21-22); D. F. Malin, W. J. Zealey, Astrophotography With Unsharp Masking, Sky and Telescope (April 1979). Such techniques are, in effect, laboratory procedures which are not practical for daily use. These references do not disclose a simple technique and apparatus for the production of improved readable radiographs, and in particular medical radiographs, and more particularly chest radiographs.