X-ray or gamma ray systems or devices – Beam control – Antiscatter grid
Reexamination Certificate
2002-08-20
2004-09-21
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Antiscatter grid
Reexamination Certificate
active
06795529
ABSTRACT:
FIELD
The present disclosure relates to X-ray imaging and, more particularly, to a dynamic X-ray tube output function combined with a high-ratio, high primary transmission grid system for use in general radiography.
BACKGROUND
Transmission X-ray imaging involves a point source (sometimes referred to as a focus or X-ray focus) of X-rays and a collimator to limit the X-rays to the region of interest. When the X-rays pass through the object, X-ray attenuation differences due to structures in the object give rise to differences in transmitted X-ray intensity. These intensity differences are in turn detected by an image receptor giving rise to the detected X-ray image. The detected X-ray image is comprised of two parts. The primary image consists of detected X-rays that have traveled on a straight-line path from the source to the image receptor. The secondary image consists of detected X-rays that have interacted with atoms and electrons in the object and were deflected or scattered from their original path (scattered X-rays). These scattered X-rays form a diffuse, out-of-focus image that is superimposed on the primary image. X-ray image contrast is reduced by scattered X-rays with the problem becoming more acute as the thickness and density of the object being imaged increases. In many applications of medical radiography the ratio of the scattered to primary X-rays can be quite large. For example, in abdominal radiography this ratio can exceed 10:1, causing a severe loss of subject contrast. Therefore, for maximum image contrast, it is important to reduce the contribution of scattered radiation to the detected X-ray image to a minimum.
In 1915 anti-scatter grids were introduced to improve image contrast in general radiography (G. Bucky, Method and Apparatus for Projecting Roentgen images, U.S. Pat. No. 1,164,987). As illustrated in
FIG. 1
, a conventional anti-scatter grid consists of an array of radiopaque foil strips or septa, interspersed with strips of radiolucent interspace material. The septa are composed of material that absorbs X-rays, such as, but not limited to lead, while the interspace material is composed of material that does not substantially absorb X-rays, such as, but not limited to, aluminum, polycarbonate fibers or paper. The width of the individual septa are indicated in
FIG. 1
as “d,” while the width of the interspace material is indicated as “D.” By combining the width of the septa (“d”) and the interspace material (“D”), the grid pitch, or grid period, can be determined (“d +D”).
The grid is positioned in the X-ray beam after the object so that the X-rays traveling in a straight-line path from the focus through the patient to the image receptor (primary X-rays) strike only the edges of the radiopaque septa. The septa are thus projected onto the image receptor as lines. To address this issue, during an exposure the grid is commonly moved orthogonal to the lines through a distance of at least 20 grid pitches to blur out the X-ray shadows of the lines. When these shadows are not eliminated, the result is an image flaw known as a “gridline artifact.” On the other hand, scattered X-rays do not travel in a straight line from the focus to the image receptor, but are deflected within the patient and approach the grid at an angle, and have a much greater probability of striking the sides of the septa and being absorbed. As a result, the contribution of scattered X-rays to the detected X-ray image is reduced and the image contrast is improved. Conventional general radiography grids typically absorb 30% to 50% of the primary X-rays, and 85% to 95% of the scattered X-rays.
A variety of techniques have been suggested for suppressing gridline artifacts. The simplest is to use a high line density grid, in which the number of septa per centimeter is sufficiently high that the image receptor is incapable of recording the image of the grid lines. Alternately, the grid can be moved during the exposure. The motion of the grid has the effect of blurring the grid line artifacts, reducing their visibility on the final image. Typically, the grid needs to be moved by more than 20 times the grid pitch in order to adequately suppress the grid line artifacts; however, given a constant tube output during the exposure the artifacts can be completely suppressed by moving the grid any integral number of grid pitches.
One problem with moving the grid is that the available distance is limited, and it is possible during long exposures for the grid to run out of travel space. The direction of motion can then be reversed, but this raises the possibility of a gridline artifact to develop while the grid is stationary. One technique that has been developed and is in clinical use is to use a variable velocity. The grid is moved quickly during the start of the exposure, guaranteeing that even for short exposures the grid moves at least 20 times the grid pitch. Then the grid is progressively slowed, to ensure that even for long exposures the grid does not reach the end of its travel space.
In 1931 Potter suggested a technique he described as “feathering”(H. E. Potter,
Am. J. Roent
., Vol XXV (May 1931), pp. 677-683). In this approach the tube output is gradually increased at the beginning of an exposure and gradually decreased at the end of the exposure. This has the effect of blurring the edges of the grid line artifacts. However, Potter did not describe how to accomplish the feathering, and did not implement this approach on a clinical x-ray machine.
In general radiography, the anti-scatter grid is typically integrated into the radiographic examination table. The X-rays pass successively through
a) the patient,
b) the table top,
c) the anti-scatter grid,
d) the automatic exposure control (AEC) sensor, and
e) the image receptor.
The AEC sensor is used to measure the amount of radiation (X-rays) that has reached the image receptor. This allows the X-ray generator to terminate the exposure when the desired exposure level has been reached.
The efficiency of an anti-scatter grid is reduced if it is not properly aligned. For a properly made grid, the primary planes of the septa all intersect along a line known as the focal axis (FIG.
2
A). The distance from the focal axis to the grid is the focal length of the grid. When the grid
5
is properly aligned (FIG.
2
A), the focal spot
4
lies on the focal axis
6
, and any x-rays traveling directly from the focal spot (primary x-rays) either pass between the septa or strike the septa edge and are absorbed. In this orientation, the projections of the septa on the image receptor are minimized. If the grid
5
is not aligned (FIG.
2
B), then the focal spot
4
does not lie on the focal axis
6
, a fraction of the primary x-rays strike the sides of the septa, and a higher fraction of the primary X-rays are attenuated than desired. This condition can be caused by poor initial positioning, but can also occur when the grid is moved to blur the septa shadows, when there are manufacturing defects in the grid, or when the distance from the focus to the grid changes from its ideal value.
Grid ratio is an important parameter in determining the effectiveness of an anti-scatter grid. Grid ratio is defined as the ratio of the height of the radiopaque septa (indicated as “h” in
FIG. 1
) to the interspace distance between septa (the width of the interspace material, indicated as “D” in FIG.
1
). The higher the grid ratio, the more efficient the grid is in controlling scattered X-rays. However, as the grid ratio increases, grid alignment difficulties are compounded. Also, if the grid ratio of a conventional grid is increased by increasing the height of the septa (“h”), attenuation of the primary X-rays by the interspace material increases. If the grid ratio is increased by decreasing the interspace distance (“D”) without a corresponding decrease in the thickness of the individual septa (“d”), attenuation of the primary X-rays by the septa increases. Typically, in general radiography the grid ratios range from 8:1to 16:1, and the strip densities range from 30 to
Barnes Gary T.
Gauntt David M.
Bradley Arant Rose & White LLP
Church Craig E.
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