X-ray or gamma ray systems or devices – Beam control – Antiscatter grid
Reexamination Certificate
2000-05-03
2001-11-27
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Antiscatter grid
Reexamination Certificate
active
06324259
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scattered-ray grid, particularly for a medical X-ray device, of the type having a carrier with absorption elements, particularly in the form of lead elements, which are arranged in spaced rows, as well as to a method for determining the position of the absorption elements in a scattered-ray grid.
2. Description of the Prior Art
In radiographic technology, particularly medical diagnostics, scattered-ray grids are frequently utilized to attenuate the scattered radiation which is always present with the primary radiation. The grids that are currently used most are composed of a sequence of line-like radiation absorption elements in the form of lead lamellae, which, alternately, are coated with lamellae made of a base material. X-rays that are incident in the plane of the lamellae are only insignificantly attenuated by the base material. By contrast, the lead lamellae highly absorb radiation that is obliquely incident. Since such lead lamellae generate unavoidable lines on the radiograph and since the number of lines per centimeter is limited due to manufacturing reasons, it has been suggested to use pins made of lead or another absorption material arranged in rows, the pins being spaced apart, instead of using lead lamellae in a silicon base material. Such scattered-ray grids are known from the German OS 197 26 846 and OS 197 29 596, for example.
German OS 197 26 846 describes a configuration concerning the arrangement of the pins (and thus of the rows), wherein the rows extend to the center of the grid in a spoke-like manner. In this arrangement, many rows start at the same radius. The density of the absorption elements (seen in the radial direction) considerably varies as a result and grey tone discontinuities also occur, and the row spaces (seen in the tangential direction) significantly vary as well. This has a disadvantageous effect on the entire absorption behavior of the grid and therefore on the image quality.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a scattered-ray grid, which is improved compared to known scattered-ray grids with respect to the arrangement of the absorption element rows.
This object is achieved in accordance with the invention in a scattered-ray grid of the type described above wherein, inventively, the rows extend essentially radially from a center region and wherein, apart from one or more rows starting in the center point, the individual, identically structured rows of the scattered-ray grid or of the grid sectors of the scattered-ray grid proceed from starting points at respectively different radii, and wherein the origin (starting point) of each row is situated in an angle section, the angle section being determined by two points lying on a circle (or arc thereof) with a predetermined radius and which is divided in a predefined ratio for determining the position of the origin. The predetermined radius is incremented in a stepped manner to define the respective origins for all of the rows.
In the inventive scattered-ray grid, the origins of the lines, which all extend in the direction of the grid center, therefore produce an asterisk-shaped configuration, the lines respectively starting at different radii with respect to the grid center, so that a scattered-ray grid results which is significantly more homogenous and radial. In one embodiment, only one row in the entire grid can have its origin at any given radius. Alternatively, the grid configuration can be sector dependent i.e. the entire grid surface is composed of a number of angle sectors, for example, four sectors each of 90°, with the row configuration being essentially identical in every sector, i.e. it annularly periodically repeats. In this case, a number of rows would therefore have their origins at every selected radius, but only a single row has its origin at a given radius in every sector. The position of the origin is determined on the basis of an angle section, which is determined on the basis of specific points situated at a predetermined radius. This angle section is divided in a predefined ratio p:q, is preferably with p≠q.
As described, an inventive scattered-ray grid can be configured by determining the row arrangement such that only one row has its origin at a circle with a fixed radius. In this case, a first row that starts at the grid center is taken as a starting basis, which represents the iteration basis. Then, the first angle section, which is to be divided in the ratio p:q, is determined on the basis of this row. In this case, the two points are defined by the first row itself, the angle section is full 360° in this case. Now, this angle section is divided according to the predefined ratio for determining the position of the origin of the second row, so that the angle position of the origin of the second row is fixed at the relevant radius. Then, the position of the third row and of every further row is determined on the basis of the largest angle section, which exists between two rows that intersect the relevant circle or circle arc. The new origin is always placed in the largest angle section.
Alternatively, for a sector-by-sector row configuration, at least two rows that start in the center are used as a starting basis, by means of which rows the grid is divided into the sectors, and the origin of every further row is placed between two rows that start closer to the center, these rows defining the aforementioned points on the predetermined circle and therefore the angle section and exhibit the largest possible angular spacing of all row pairs. Therefore, the origin of a new row always lies between two rows which originate closer to the center than the origin of the new row, these two rows being the rows which are spaced furthest from one another.
Spiral-shaped density variations, even though slight may arise given the determination criterion of the angle section which is to be divided on the basis of the intervening angular space of the respective row pairs. This can be avoided in an embodiment of the invention wherein the row origins are determined starting with two rows that originate at the grid center, and the origin of the next row is placed between the row pair which has a maximum sum of the angular spacing between the rows and an additional angle value, this sum being allocated to the angle section. In this embodiment, the angle section to be divided in the ratio p:q is therefore not only determined on the basis of the actual angular spacing between the row pair, but is determined on the basis of the sum of the actual angular spacing between the row pair and an additional angle value. This additional angle value is determined by weighting, with a defined weighting factor, the respective angular spacings, on opposite sides of the adjacent row pair, between the adjacent row pair and the closest row thereto. For example, the sum of these two angular spacings (one from each side of the adjacent row pair) can be multiplied with the predefined factor, which can be <1 according to the invention, and this value can then be added with the angular spacing between the two adjacent rows. Then, the angle section, at which the largest of said sums is allocated, is divided corresponding to the predefined ratio. An even more homogenous distribution of rows results.
Apart from the scattered-ray grid itself, the invention also relates to a method for determining the position of the absorption elements of a scattered-ray grid, which are arranged in rows and which extend essentially radially relative to a center. Apart from one or more rows that start at the grid center, the origin of each row of the scattered-ray grid, or of a grid sector of the scattered-ray grid, is placed in an angle section, which is determined on the basis of two points lying on the circle or arc of circle with the origin radius r
0
+n&Dgr;r and which is divided for purposes of determining the position of the origin in a predefined ratio.
Beginning with a single row that origi
Keller Wolfgang
Lehmann Volker
Ho Allen C.
Kim Robert H.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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