Patent Number: 050671448
Section: summary

BACKGROUND OF THE INVENTION The invention relates to radiography and particularly to equalized radiography which improves diagnostic capabilities by selectively subjecting denser parts of the examined object to higher intensity radiation to render them more fully exposed and image them with greater contrast and detail. In such radiography, a fan of penetrating radiation sweeps the object while being locally modulated to vary the radiation intensity both as between different sectors of the fan and in the scanning direction in order to selectively equalize the radiation delivered to the image plane. In conventional radiography, image quality and diagnostic value can be compromised when the object density differs too much as between different parts of the object. For example, in conventional chest x-rays the mediastinum and retrocardiac area could be underexposed, detracting from the diagnostic value of the image. Equalization radiography, which is capable of varying the local x-ray exposure to areas in the image to compensate for the local patient attenuation, particularly in the case of chest x-rays, can improve image quality and diagnostic value. Typical examples of such feedback-controlled equalization radiography are discussed in the commonly assigned Wang European Patent Application No. 86308224.4 (based on a U.S. application leading to U.S. Pat. No. 4,953,189; see, in particular, FIG. 8 showing wedge-shaped attenuator elements) as well as in Plewes U.S. Pat. No. 4,773,087 and U.S. Pat. application Ser. No. 07/242,644 filed Sep. 13, 1988 (see, in particular, FIGS. 3 and 5 showing overlapping shutter pins in two rows). Further examples are discussed in U.S. Pat. Nos. 4,675,893, 4,715,056, 4,677,652, and 4,741,012. All of the prior documents cited in this paragraph are hereby incorporated by reference in this specification. Further background material concerning the subject can be found in the documents made of record in said Plewes application. In a typical prior art example, an x-ray fan beam scans the patient and a modulator unit locally controls the x-rays before they reach the patient in order to modulate the radiation differently as between different sectors of the fan and as between different stages of the scanning movement. The degree and kind of local modulation are under the control of a feedback circuit which locally measures the x-rays in the fan exiting the object. The goal of this local, time varying modulation is to equalize the image, i.e., to reduce the difference in exposure as between different areas of the image. The modulator unit can use a row of modulators or shutters which are individually and selectively movable into the fan to modulate it locally, e.g., by varying the local attenuation, the local beam cross-section, and/or the local exposure time of the x-rays impinging on the object being examined. While such prior art systems could provide significant improvement, they also could introduce certain types of image artifacts. One type is in the direction of scan and could appear as streaks in the image. The inventors believe that this type of an artifact could be caused by ineffective smoothing between adjacent elements of the modulator unit, especially when at any one time during the scan the settings or positions of these adjacent elements are very different, and that artifacts could also be caused by limitations in parameters such as the sizes of the focal spot, the attenuator elements and the collimator slit at the modulator and the geometry of the modulation arrangement. Such artifacts are also noted in Vlasbloem, et al., RADIOLOGY, Vol. 169, pages 29-34 (Oct. 1988). See, also, Plewes, D.B. and Vogelstein, E., Exposure Artifacts in Raster Scanned Equalization Radiography, Med. Phys. Vol. 11. pp. 158-165 (1984). Other artifacts could be in the direction normal to the scan direction and others could be in other directions or positions. Of course, it is desirable to minimize any deleterious effects of such artifacts on the diagnostic value of the image. SUMMARY OF THE INVENTION One object of the invention is to improve equalization radiography by reducing image artifacts. Another is to use a modulator which provides smoother variations in modulation from one fan beam sector to another. Yet another is to smooth the overlap area between adjacent modulator elements, especially when the settings or positions of adjacent modulator elements differ significantly. Still another is to derive benefits from factors which may have been considered limitations in the prior art, such as the finite size of the focal spot. In an exemplary embodiment of the invention, an x-ray source/modulator assembly generates a fan beam which is thin in the horizontal direction and tall in the vertical direction. The assembly sweeps the beam horizontally across the object being examined while selectively and individually modulating sectors of the beam, by special modulator elements described below, to vary the intensity of the radiation delivered to the object by the respective beam sectors. A detector/film assembly receives the fan exiting the object and, as the fan sweeps across the object, one part of this assembly measures the x-ray intensity distribution to generate feedback information while another part forms an x-ray image of the object. The feedback information, along with information related to the effect that the modulator has on individual detector elements, is used to estimate the effect of the object being examined and to control the modulator so as to increase or decrease the local amount of radiation delivered to the object. In a single scan mode, the information required to adjust the modulator is generated and used substantially in real time, to the extent permitted by inherent circuit delays. In a dual scan mode, the first scan is at reduced x-ray intensity and can be either equalized or non-equalized. The x-ray intensity can be reduced to a level that would not cause any significant exposure on the film, e.g., by controlling the x-ray tube filament current. The detected intensity levels coupled with the modulator element attenuation positions can be used to determine desired exposure settings and to calculate an equalization function used to perform "real-time" equalization during the second scan. If in a single scan mode the modulator is locked to a fixed position, the result would be similar to a conventional x-ray image. In order to reduce artifacts due to the modulation, special modulator elements are used. In a preferred but non-limiting example of the invention, they are in the form of modulator pins which slide back and forth in the horizontal direction into the fan of x-rays. This fan is defined by a vertically extending pre-patient collimator slit aperture. An object field collimator provides adjustments for film orientation and also for at least the lower edge of the object exposure field. The size of the slit aperture is adjustable horizontally to set the fan dimension in the horizontal plane (e.g., from about 0" to 0.5" at the slit aperture plane, which translates to about 0" to 3.5" at the image plane). In a non-limiting example, 35 modulator pins are used, made of an attenuating material such as aluminum and arranged in two rows which are along respective arcs that are centered at the focal spot and are in a vertical plane that includes the focal spot. The parts of the modulator pins that slide into the fan are in the shape of wedges which have generally triangular sections in a vertical plane. The bases of the triangular sections are along the arcs which are centered at the focal spot. The bases of the pins within a row are as close to each other as practical considerations would allow and, as viewed from the focal spot when all pins are fully into the fan, adjacent pins from different rows overlap so much that any ray from the focal spot is more likely than not to be intercepted by two pins, one from each row. The attenuation of the fan beam due to any one modulator pin is a function of how far into the fan the pin extends. For any one vertical plane in the fan, the attenuation due to a given modulator pin is a function of the area of the generally triangular section of the pin which is in that vertical plane. Stated more broadly, the modulator comprises portions of a radiation attenuating material which are adjacent each other along the larger angular extent of the fan of radiation and individually and selectively slide into the fan to: (1) vary the radiation along the larger angular extent of the fan smoothly while individually and selectively modulating the sectors of the scanning fan to reduce or eliminate objectionable artifacts at the image plane; and (2) make uniform to a selected degree the exposure which the fan exiting the object delivers to the image plane. While in the currently preferred example the attenuating portions are in the form of individual modulator pins, which could have rounded or truncated wedge ridges and tips, in an alternative embodiment the modulator uses a flexible diaphragm of a material such as leaded rubber of which portions are pushed into the fan by pins which also are arranged in one or more vertically extending rows and slide individually and selectively horizontally in or toward the plane of the fan. Further, while in the currently preferred embodiment which is described in detail below the fan sweeps the object generally horizontally, other scanning directions are possible. For example, in another preferred embodiment, the larger angular extent of the fan is generally horizontal and the fan sweeps the object in the vertical direction, in which case the modulator pins (which scan together with the fan) slide generally vertically into the plane of the fan. The detailed description below is for a fan scanning in the horizontal direction, but it should be understood that the invention is applicable to a vertically scanning fan as well and the same description applies with an appropriate change in the directional terms.