X-ray or gamma ray systems or devices – Beam control – Antiscatter grid
Reexamination Certificate
2000-08-11
2001-12-04
Church, Craig E. (Department: 2876)
X-ray or gamma ray systems or devices
Beam control
Antiscatter grid
Reexamination Certificate
active
06327341
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scattered-ray grid, particularly for medical radiography systems having an X-ray source that generates an X-ray beam with a center ray, the scattered-ray grid being of the type having a carrier with absorption elements, particularly in the form of lead elements, which are formed as spaced pins in rows which are spaced apart from one another.
2. Description of the Prior Art
In radiography, particularly in medical diagnostics, scattered-ray grids are frequently employed to attenuate the scattered radiation emanating from the examination subject relative to the effective radiation. The grids commonly used today consist of a sequence of lead lamellas which are layered in alternation with lamellas made of a carrier material. X-rays that strike in the layers of the lamellas are attenuated only by the carrier material. Oblique radiation, on the other hand, is more or less absorbed by the lead lamellas.
Since such lead lamellas generate unavoidable lines on the X-ray image, and furthermore the number of lines per centimeter is limited for technical reasons having to do with production, German OS 197 29 596, corresponding to U.S. Pat. No. 6,047,044, discloses using spaced pins made of lead or some other absorptive material instead of the lead lamellas.
Owing to the conical beam geometry of the X-radiation which is common in projection radiography, the lead lamellas—and this applies accordingly to the parallel pins that serve in their stead—must not be oriented in parallel fashion. Rather, they must be directed in such a way that they are all oriented to the focus of the X-ray tube. This requirement means a significant outlay for production. Furthermore, the focusing of the scattered-ray grid is calculated only for a specific interval between focus and grid. When this interval is changed, the orientation conditions are no longer correct for the peripheral radiation, and clearly visible shadowing occurs at the image margins. Previously, the expense for producing focused grids was accepted as unavoidable, and beyond this several grids were employed, which had to be exchanged according to the selected focus-grid interval. But this was a significant disadvantage in terms of both production and handling, which was associated with significant added costs.
European Application 0 333 276 describes a scattered-ray grid for compensating vignetting which is provided with perforations that are arranged circularly, the density of which is not constant over the area.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a scattered-ray grid of the above mentioned type such that it can be produced more easily and can be employed independent of the focus-grid interval.
This object is achieved in accordance with the principles of the present invention in a scattered-ray grid having rows of pins which are respectively oriented so as to proceed toward the point of intersection of the center ray of the X-ray beam with the scattered-ray grid.
A significant advantage of the inventive arrangement, in which the pins or the lamellas made of absorptive material are no longer arranged in parallel rows but instead are arranged radially symmetrically with respect to the center axis of radiation, is the avoidance of the aforementioned focusing problem. In principle lamellas could be used instead of the pin rows, though problems arise in the region of the midpoint in the lamellar arrangement, since the absorptive material there would completely gate out a disk-shaped region on the center axis of the radiation, and the production of radially arranged lamellas in alternation with carrier material would be extremely complicated. Because of the radially symmetrical arrangement of the pin rows, the size of the focus-grid interval is of no significance whatsoever to the permeability of the inventive scattered-ray grid to the actual effective radiation, since in any case the effective radiation can pass unobstructed between two radial absorption rows which are spaced apart from one another in the perimeter.
In order still to obtain a sufficient scattered-ray absorption at a great distance from the center point although the radial rows of radiation-absorbent separated pins are already arranged at very large intervals, additional intermediate rows of pins can be arranged between the rows of pins that extend continuously into the vicinity of the center point, which additional rows commence radially only at a distance from the center point.
What is important is to guarantee that the mean surface coverage of the absorptive pins is as uniform as possible over the entire surface of the scattered-ray grid. This ensures an optimally homogenous transparency for the effective radiation.
Such an arrangement nevertheless has the disadvantage of a symmetry which may become visible later in the imaging process. For this reason, in a more advantageous embodiment of the invention, the intermediate rows of pins are arranged at least partly phase-shifted, whereby the pins of the rows along a radius can be phase-shifted section by section, i.e. approximately offset somewhat to the left or right relative to a ray emanating from the radius. In this way it is possible to prevent disturbance to the imaging due to periodic structures.
A silicon disk, particularly a monocrystalline disk, can serve as the carrier.
To produce an inventive scattered-ray grid, holes can be etched into the carrier using a directionally selective etching method, into which holes the absorptive material is introduced in a liquid or semiliquid state and then cooled, and excess absorptive material is removed following cooling, for instance by polishing (“directionally selective” means a higher etching rate depthwise than laterally). An electrochemical etching method or a plasma etching method can be used, whereby a lithographic etching mask that corresponds to the pin pattern to be generated is placed onto the surface of the carrier prior to the etching and removed again following etching.
REFERENCES:
patent: 4969176 (1990-11-01), Marinus
patent: 5416821 (1995-05-01), Frazier et al.
patent: 5418833 (1995-05-01), Logan
patent: 6047044 (2000-04-01), Lehmann et al.
patent: 0 333 276 (1989-09-01), None
patent: 0 603 043 (1994-06-01), None
Patent Abstracts of Japan Publication No. 03267049, for Japanese Application 02064104.
Church Craig E.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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