X-ray or gamma ray systems or devices – Beam control – Antiscatter grid
Reexamination Certificate
1999-11-24
2002-06-18
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Antiscatter grid
C378S149000
Reexamination Certificate
active
06408054
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to the field of x-ray imaging.
2. Description of Related Art
X-ray radiation is widely used for medical x-ray imaging and non-destructive evaluation. X-ray radiation easily penetrates many materials and allows images to be taken based on the shadows of dense materials that absorb x-rays. X-ray imaging is used for both thick and thin tissue procedures in medical imaging radiology and fluoroscopy. Exemplary applications of x-ray imaging in non-destructive evaluation include the testing of buildings, structural members, pressure vessels, welds and airplane fuselage constructions, and the like for the presence of defects and structural integrity.
SUMMARY OF THE INVENTION
The application of x-ray imaging presents difficult technical problems. One particular problem is that the absorption of x-rays by materials at higher energies (greater than 100 keV) competes with the Compton scattering process. Compton scattering deflects x-rays through a small angle from their original trajectories. For imaging dense and/or thick materials, Compton-scattered x-rays can obscure the image formed by the absorption of direct unscattered x-rays.
FIG. 1
shows a conventional x-ray imaging system
20
configuration for imaging objects. The x-ray imaging system
20
comprises an x-ray source
22
and an image contrast grid (antiscatter grid)
24
placed between the x-ray source
22
and a detector
26
. The x-ray source
22
emits x-rays
32
that impinge on an object
34
to be imaged. For example, the object
34
can be a human body. The transmitted x-rays
36
strike the surface
38
of the detector
26
.
As shown in
FIG. 2
, the detector
26
may include a film cassette with a film
30
sandwiched between phosphors
28
. As shown in
FIG. 3
, the detector
26
may alternatively include an electronic detector such as an a-Si detector
48
combined with a phosphor or photoconductor
28
as described in J. Rahn et al., “High Resolution, High Fill Factor a-Si:H Sensor Arrays for Optical Imaging,”
Materials Research Society Proc
. 557, April 1999, San Francisco, Calif.; and R.A. Street, “X-ray Imaging Using Lead Iodide as a Semiconductor Detector,”
Proc. SPIE
3659
, Physics of Medical Imaging
, Feb. 1999, San Diego, Calif., each incorporated herein by reference in its entirety.
As shown in
FIG. 4
, some of the non-normal x-rays
40
strike dense material
42
in the body, such as bone, and are absorbed by the dense material. However, other x-rays
44
are scattered and do not strike the dense material
42
and pass through the soft body tissue without being absorbed. These scattered x-rays are known as Compton-scattered x-rays.
The Compton-scattered x-rays
44
that do not strike dense material
42
in the object
34
adversely affect the formed image of the dense material. That is, the Compton-scattered x-rays
44
exit from the object
34
at positions that are laterally spaced from the positions at which they entered the object
34
. Based on their exit locations, the Compton-scattered x-rays
44
would appear to have passed through the region of the object
34
where the dense material
42
is located, but without having been absorbed by the dense material
42
.
As shown in
FIG. 5
, the image contrast grid
24
is provided in the x-ray imaging system
20
to absorb the Compton-scattered x-rays
44
that are not absorbed by dense material
42
in the object
34
. The Compton-scattered x-rays
44
affect the darkness (contrast) of the image of the dense material
42
that is formed by the actual absorption of the x-rays
40
by the dense material
42
. The image contrast grid
24
reduces the effects of the Compton-scattered x-rays
44
on the image formed by the absorption of direct x-rays by eliminating the Compton-scattered x-rays
44
that travel in a direction through the object
34
that does not point to the x-ray source
22
. By eliminating the Compton-scattered x-rays
44
, the image contrast is enhanced.
In general, image contrast grids are required for all “thick” tissue medical imaging procedures; i.e., procedures in which the screen is not located close (within about the thickness of the screen) to body tissue during medical imaging procedures.
Image contrast grids have been formed by laminating together foils of x-ray transparent material, such as aluminum, and x-ray absorbing material, such as lead, to form an extended sandwich structure.
FIG. 6
illustrates a known sandwich structure image contrast grid
124
including aluminum foils
126
and lead foils
128
forming an alternating, parallel arrangement.
Other methods of forming image contrast grids have been described, for example, in U.S. Pat. Nos. 5,581,592 and 5,557,650, incorporated herein by reference in their entirety.
However, known image contrast grids, such as the image contrast grid
124
, and the processes for forming the grids are unsatisfactory for at least several reasons. First, these processes are complicated and expensive to perform, leading to a high cost of the grids.
Second, known image contrast grids, such as the image contrast grid
124
, have a relatively coarse structure that produces grid lines in the formed images. For example, to reduce this problem, the grids can be moved slightly back and forth in a direction
46
approximately perpendicular to the normal (i.e., the direction of the x-rays
36
) to blur the image of the grid lines formed on the film. This movement of the grids is known as the “Bucky system.” However, the Bucky system requires the imaging system to include additional components and, thus, increases the cost and complexity of the system.
Third, known image contrast grids, such as the image contrast grid
124
, only remove the Compton-scattered, non-normal (off-z-axis) photons in one dimension (i.e., along either the x-axis or the y-axis). In order to provide two-dimensional photon removal using these grids, two grids, such as two of the image contrast grids
124
, have been stacked with their respective foils oriented orthogonal with respect to those of the other grid. Although the combined use of two grids may improve Compton-scattered photon removal in a second direction, the cost of the imaging system is also significantly increased by the added cost of the second grid. Thus, the value of improving the performance of the imaging system by using two image contrast grids may not justify the associated added cost to achieve the improved performance.
This invention provides improved image contrast grids that can overcome the above-described problems of the known image contrast grids and the processes used to form the known image contrast grids.
This invention separately provides image contrast grids that have improved x-ray transmission efficiencies, i.e., rejection ratios, that thus reduce the required dosage of source radiation that is needed to obtain an image of an object.
This invention separately provides image contrast grids that have increased open aperture ratios.
This invention separately provides image contrast grids that can be used to form images with improved contrast.
This invention separately provides image contrast grids that have fine structures that reduce or eliminate the need to use a Bucky system during imaging.
This invention separately provides image contrast grids that remove Compton-scattered x-rays in two, co-planar dimensions, e.g., the x and y dimensions, and thus eliminate the need to use two image contrast grids simultaneously.
This invention separately provides methods of making the image contrast grids that are economical, controllable and reproducible.
This invention separately provides methods of using the image contrast grids in imaging systems for imaging objects.
Various exemplary embodiments of the image contrast grids according to this invention comprises a body forming a continuous matrix and openings. The body comprises one of a first material that is at least substantially transparent to x-rays and a second material in the openings that absorbs the x-rays without substant
Apte Raj B.
Rahn Jeffrey T.
Oliff & Berridg,e PLC
Porta David P.
Xerox Corporation
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