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Electronic Grids for Electrostatic Imaging

Systems 1

Radiation Physics

E. Phillip Muntz, Ph.D., George Jacobson, M.D., Emil M. Kaegl, M.Sc., and David J. Klein, Ph.D. Electronic grids have been substituted for conventional grids in an electrostatic imaging system and have been useful in chest radiography. Electronic grids also appear to be useful for clean-up of small amounts of scattered radiation in radiographs of the extremities. More clinical work is needed before a statement about the use of electronic grids for other examinations can be made. INDEX TERMS: Rad"iographs, enhancement. Radiography, electrostatic. Chest, miscellaneous special technique, 6 [0] . Radiology 121:197-204, October 1976

• HE SCATIERED RADIATION field generated by the subject of a radiological examination has angular and spatial distributions that are different from the attenuated primary beam. A conventional fixed or moving grid uses the difference in angular distribution to filter the scattered radiation. In air gap techniques (1,2) it is again the difference in angular distribution between the primary and scattered radiation that causes the scattered radiation fluence2 at the image plane to be diminished relative to the primary radiation. The air gap required to reduce the scattered fluence to acceptable levels is at least 15 cm (1, 2) but preferably larger, even for such a relatively weak scatterer as a chest (3). As a result, use of the air gap technique is generally precluded except in the special case of chest roentgenography and when 300-cm (or greater) film focal distances are available (1, 2). While both conventional scatter suppression techniques depend on the angular spread of the scattered radiation field, it is possible to use another characteristic of the scattered field, namely, the spatial distribution of scattered radiation fluence in the image plane, to suppress the scattered radiation effects on the visualized image. The scattered radiation has a tendency to have a uniformly distributed average value (averaged over local spatial variations) in the plane of the radiographic image, as well as to have no significant components with high spatial frequencies. This is particularly the case for typical separations (~3-5 ern) between film and subject that exist in some radiographic examinations. If the scattered radiation were precisely uniform over the image plane, and if a convenient way of removing or subtracting all or any arbitrary portion of this uniform contribution to the image were available, a perfect grid would have been achieved. This perfect grid would have no focus requirements and thus no positioning tolerances. There would be no attenuation of primary radiation, the

effective grid ratio could be varied at will, there would be no requirement for grid alignment or motion mechanisms, there would be no grid to be damaged, and no grid lines could appear in the image. Such a subtraction of scattered radiation can be accomplished in electrostatic imaging systems by applying a uniform bias potential or voltage to the image during development. Scatter suppression can also be accomplished to some extent by the selective spatial frequency filtering (edge or high-frequency enhancement) that is one of the characteristics of electrostatic systems (for a discussion of electrostatic image processing in diagnostic radiography see Ref. 4). The realization of such an electronic grid is described in the following sections of this paper. The manipulations that are required to make the scattered radiation field suitable for the application of electronic grids are also outlined. Video display images are similarly subject to manipulation for purposes of suppressing the effects of scattered radiation as discussed recently by Mika and Reiss (5). To clarify the scatter removal operation of an electronic grid of the simple subtraction type, examine the sketches in Figure 1. Imagine a latent electrostatic radiographic image. Consider an arbitrary line across the image along which the image potential (voltage) is traced as a function of distance, with the result as illustrated in Figure 1, A by the curve labeled "total signal." The total signal is made "up of contributions from scattered and primary x rays. For illustrative purposes a typical scattered contribution to the total signal is shown in Figure 1, A by the diagonally hatched area. In electrostatic image processing it is easy to arrange for a portion of the image potential between zero and a bias potential value Vb to be excluded from the development of a visible image. The contribution of scatter radiation can be excluded by the use of such a bias potential at the level

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1From the Department of Radiology (E. P. M., G. J., D. J. K.), University of Southern California, Los Angeles, Calif.; and from Xonics Incorporated (E. M. K.), Van Nuys, Calif. Accepted for publication in M~rch 1976. 2National Bureau of Standards Handbook #89, P 31. elk

197

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PHILLIP MUNTZ AND OTHERS

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October 1976

indicated in Figure 1, A. The resulting image potential distribution that is made visible by the electrostatic development process is shown in Figure 1, B with only a small amount of the contribution from scattered radiation remaining. It is of course possible to raise the bias potential and obtain an image of even greater contrast, although, if the level is raised too high, information will be lost (Fig.

1, C). SCATTERED RADIATION CONTRIBUTION TO IMAGE

LATENT IMAGE ALONG A-A

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ELECTRONIC GRID ELIMINATING MOST OF THE EFFECT OF SCATTERED RADIATION ALONG A-A

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ELECTRONIC GRID SET TOO HIGH THUS DELETING INFORMATION ALONG A-A

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REGIONS OF INFORMATION LOSS

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This sketch illustrates the operation of an electronic grid.

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This is a simple model of chest scattering.

The simple scatter subtraction electronic grid is applied during the development or visualization of electrostatic radiographic images. The grid neither erases information nor does it attenuate the main beam; it only makes certain portions of the latent image unavailable for development or visualization. In principle, the image could be reprocessed to bring out further information if inspection indicated information loss (Fig. 1, C). In practice this is probably an unwieldy approach and would not be used. The operation of the other type of electronic grid, a spatial frequency filter, can also be appreciated from the sketch in Figure 1. If there is a great difference in the spatial frequency content of the scattered radiation (mostly very low frequency in Figure 1) and the primary radiation that forms the image, the scattered radiation can be removed by filtering the i.mage with a filter that selectively removes low-frequency components. No work was done on this approach in the present investigation, and although mentioned at certain places in the text, a detailed discussion is deferred to a future communication. It is important to emphasize that an electronic grid is applied in the processing or visualization step of electrostatic radiographic image generation. The latent electrostatic image contains the scatter, unlike a conventional radiograph made with a grid where scatter is controlled prior to reaching the film. The clinical demonstration of an electronic grid was limited to chest radiographs along with some examples of scatter clean-up in radiographs of extremities. The images were obtained using the electrostatic system called E'lectron Radiography by its inventors (6-8); other workers in the field prefer the term ionography or high-pressure ionography (9, 10). RELEVANT CHARACTERISTICS OF THE SCATTERED RADIATION FIELD

It is instructive to develop an idealized theoretical model of a scattered radiation field's characteristics. The notions obtained from the idealized modeling were substantiated using experimental results from phantoms. The information was subsequently used to suggest parameters for chest radiograms employing an electronic grid. The experimental work of Seemann and Splettstosser (11) and the results given by Ter-Pogossian (12) suggest that the scattered radiation has spectral characteristics similar to the primary radiation. This is to be expected for reasons listed by Ter-Pogossian (12). The assumption is made here that the spectral distributions of both the scattered and primary radiation are identical. Further, it is sufficient for present purposes to model.the scatter (e.g.,

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chest) as originating at a surface a distance (0 d) from the film plane where d is the air gap as normally measured, and 0 is the added displacement of the hypothetical emitting surface. Figure 2 shows that (0 d), the distance of the surface from which the modeled scattered radiation originates, is located such that 0 is about equal to the thickness of the chest. As will be shown later, 0 for the phantom chest used in this study was found to be about 35 cm. The purpose of this simple modeling is to examine the effect of an air gap on the average scattered radiation fluence compared to the smoothing of spatial fluctuations in the scattered' field.

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To do the modeling simply, some further assumptions are necessary. The angular distribution of scattered radiation in directions away from the normal to the emitting surface must be specified. From the theoretical expressions for the angular distribution in Compton scattering, as well as the experimental results of Seemann and Splettstosser, it is reasonable to assume that the scattering per unit surface area per unit solid angle drops off something like F(O) cos 3(O) or F(O) cos 2(O) where 0 is the angle between a given direction and the surface normal. As it bins out, whether the functional relationship is cos 20 or cas 3lJ does not affect the conclusions that can be drawn from the following calculations. First, the average scattered radiation fluence