Hospital, Manchester and tDepartment of Radiology, Withington Hospital, West Didsbury, Manchester. (Received August 1986 and in revised form October 1986).
1987, The British Journal of Radiology, 60, 515-516
MAY
1987
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Division from different radiology departments in Kuwait, 1980-1984. Radiation Protection Division, Ministry of Public Health, Kuwait. UNSCEAR, 1977. Sources and Effects of Ionizing Radiation. (United Nations, New York).
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Radiation Doses in Nuclear Medicine and Diagnostic X-ray (CRC Press, Florida). MCKINLAY, A. & MCCAULEY, B., 1977. Spoilt films in X-ray departments. British Journal of Radiology, 50, 233-234. RPD, 1984. Statistics provided to the Radiation Protection
Digital ionography: preliminary results By R. V. Booler, *B. M. Moores, T. Dovas and t D . L. Asbury Department of Medical Biophysics, University of Manchester, 'Department of Medical Physics, Christie Hospital, Manchester and tDepartment of Radiology, Withington Hospital, West Didsbury, Manchester
(Received August 1986 and in revised form October 1986)
In order to ensure the widespread application of information technology to departments of medical imaging, there is a real need to develop a digitally compatible, film-replacement, radiographic imaging system. The feasibility of employing digital ionography in this capacity has been demonstrated (Moores et al, 1984, 1985) using a laboratory prototype system. We wish to present preliminary results produced by the clinical prototype digital ionography system in order to demonstrate its imaging capability as well as indicate the overall potential of this technology.
FIG. 1. Digital chest ionogram displayed at 1600x1200 pixels. Exposure factors are 65 kVp, 50 mAs and entrance skin dose 250 /iGy (25 mrad).
MATERIALS
A large-area ionography chamber, which has been outlined previously (Booler et al, 1985), is used to produce latent electrostatic images. These resultant images are read directly by means of scanning electrometers, and the resultant signals are amplified, sampled and stored in a display memory. The digital imaging data are displayed on a high-resolution 1600line television system, corresponding to a 1600x1200 pixel image format with 12 bits of grey-scale per pixel.
FIG. 2. Digital ionogram displaying over 3 line pairs per mm resolution.
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FIG. 4. Digital chest ionogram employing 72 kVp and 18 mAs exposure factors and corresponding to an entrance skin dose of 170 /iGy (17 mrad).
FIG. 3. Electronic enlargement by a factor of two of an area of image shown in Fig. 2.
height functions were employed in order to present what was deemed to be a suitable image, which was then photographed from the visual display unit. Clinical evaluation and further development of the system is under way. Part of this evaluation will involve a comparison with a rare-earth screen-film combination. That the system as presently designed can compete with rare-earth screen-film combinations in terms of speed is indicated in Fig. 4. This image employed exposure factors 72 kVp and 18 mAs on a three-phase unit and corresponds to an entrance skin dose of 170 /iGy (17 mrad), similar to that employed on the screen-film combination. Obviously, the results presented are preliminary but, hopefully, do demonstrate the potential of this technique. There is scope for major development of the existing system to improve greatly both the sensitivity and resolution, as well as to enhance greatly its operation with patients. The technology is also simple and cheap—the cost per image produced can be a fraction of the equivalent film costs.
RESULTS AND DISCUSSION
A digital ionography chest image is shown in Fig. 1. This image was produced with exposure factors 65 kVp and 50 mAs on a single-phase X-ray unit at a focus-film distance of 2 m and corresponds to an entrance skin dose of approximately 250 /iGy (25 mrad). The overall noise level in this picture is dictated by quantum statistics and the resolution limit of approximately 2 line pairs per mm is mainly governed by the display matrix size, i.e. the 40 cm x 30 cm field size sampled at 1600 x 1200 pixels corresponds to four data points per mm. The apertures of the electrometers used to read the electrostatic images are roughly 0.15 mm in diameter and correspond to a resolution limit in excess of 4 line pairs per mm at the electrometer-to-foil spacing employed. However, these samples are displayed onto a pixel size of 0.25 mm. This means that it should be possible either to increase the aperture of the electrometers, and thus increase the signal-to-noise ratio, for the same overall resolution, or the resolution can be increased for the same signal-to-noise ratio. The latter is demonstrated in Fig. 2, where a smaller input field size (roughly 17.5 cm x 17.5 cm) has been sampled on the 1200x 1200 pixel matrix. The resolution limit in this image is now approximately 3.4 line pairs per mm, which has been measured by means of a lead-bar pattern image. This level of resolution and sensitivity is now comparable with that of screen-film systems. However, given the technical developments possible with ionography systems the imaging capability shown by this prototype unit can be greatly increased. One of the potential advantages of digital imaging is highlighted in Fig. 3, where a portion of the image presented in Fig. 2 has been electronically magnified by a factor of two and the visualisation of fine detail is clearly demonstrated. It must be noted that, because the ionography process is linear, all the grey-scale detail present in an image cannot be viewed simultaneously. In all the images presented, window width and
ACKNOWLEDGMENTS
We wish to thank Messrs J. Norcott, A. Porter and A. Platt for their technical assistance. This work was undertaken with funds provided by the Medical Research Council.
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MOORES, B. M. & ASBURY, D. L.,
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field ionography—preliminary results. British Journal of Radiology, 58, 1141-1143. MOORES, B. M.,
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1984. Production and processing of digital ionographic Xray images. British Journal of Radiology, 57, 1157-1160. MOORES, B. M., DOVAS, T., PULLAN, B. & BOOLER, R., 1985.
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