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Patent Issued for Diffraction Grating and Alignment Method Thereof, and Radiation Imaging System

July 2, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventors Kaneko, Yasuhisa (Kanagawa, JP); Ishii, Hiroyasu (Kanagawa, JP), filed on January 29, 2011, was published online on June 17, 2014.

The assignee for this patent, patent number 8755487, is Fujifilm Corporation (Tokyo, JP).

Reporters obtained the following quote from the background information supplied by the inventors: "The present invention relates to an alignment method of plural diffraction gratings, and the diffraction gratings that are easy to align, in a radiation imaging system for capturing a phase contrast image of an object with use of the diffraction gratings.

"X-rays are used as a probe for imaging inside of an object without incision, due to the characteristic that attenuation of the X-rays depends on the atomic number of an element constituting the object and the density and thickness of the object. Radiography using the X-rays is widely available in fields of medical diagnosis, nondestructive inspection, and the like.

"In a conventional X-ray imaging system for capturing a radiographic image of the object, the object to be examined is disposed between an X-ray source for emitting the X-rays and an X-ray image detector for detecting the X-rays. The X-rays emitted from the X-ray source are attenuated (absorbed) in accordance with the characteristics (atomic number, density, and thickness) of material of the object present in an X-ray path, and are then incident upon pixels of the X-ray image detector. Thus, the X-ray image detector detects an X-ray absorption image of the object. There are some types of X-ray image detectors in widespread use, such as a combination of an X-ray intensifying screen and a film, an imaging plate containing photostimulated phosphor, and a flat panel detector (FPD) that takes advantage of semiconductor circuits.

"The smaller the atomic number of the element constituted of the material, the lower X-ray absorptivity the material has. Thus, the X-ray absorption image of living soft tissue, soft material, or the like cannot have sufficient contrast. Taking a case of an arthrosis of a human body as an example, both of cartilage and joint fluid surrounding the cartilage have water as a predominant ingredient, and little difference in the X-ray absorptivity therebetween. Thus, the X-ray absorption image of the arthrosis hardly has sufficient contrast.

"With this problem as a backdrop, X-ray phase imaging is actively researched in recent years. In the X-ray phase imaging, an image (hereinafter called phase contrast image) is obtained based on phase shifts (shifts in angle) of the X-rays that have passed through the object, instead of intensity distribution of the X-rays having passed therethrough. It is generally known that when the X-rays are incident upon the object, the phases of the X-rays interact with the material more closely than the intensity of the X-rays. Accordingly, the X-ray phase imaging, which takes advantage of phase difference, allows obtainment of the image with high contrast, even in capturing the image of the object constituted of the materials that have little difference in the X-ray absorptivity. As a type of such X-ray phase imaging, is proposed an X-ray imaging system using an X-ray Talbot interferometer, which is constituted of two transmission diffraction gratings and the X-ray image detector (refer to Japanese Patent Laid-Open Publication No. 2008-200359 and Applied Physics Letters, Vol. 81, No. 17, page 3287, written on October 2002 by C. David et al., for example).

"The X-ray Talbot interferometer is constituted of the X-ray source, the X-ray image detector, and first and second diffraction gratings disposed between the X-ray source and the X-ray image detector. The first diffraction grating is disposed behind the object. The second diffraction grating is disposed downstream from the first diffraction grating by a specific distance (Talbot distance), which is determined from a grating pitch of the first diffraction grating and the wavelength of the X-rays. The Talbot distance is a distance at which the X-rays that have passed through the first diffraction grating form a self image by the Talbot effect. If the object is disposed between the X-ray source and the first diffraction grating or between the first diffraction grating and a self image observation position (in this case, the Talbot distance), this self image is spatially modulated according to the interaction (phase shifts) between the X-rays and the object.

"In the X-ray Talbot interferometer, the second diffraction grating is overlaid on the self image of the first diffraction grating to obtain a fringe image subjected to intensity modulation. The fringe image is detected by a fringe scanning technique. From variation (phase shift) of the fringe image due to the presence of the object, an intensity contrast image of the object is obtained. In the fringe scanning technique, a plurality of images are captured, while the second diffraction grating is slid relatively against the first diffraction grating in a direction substantially parallel to a surface of the first diffraction grating and substantially orthogonal to a grating direction of the first diffraction grating at a scan pitch that corresponds with an equally divided part of a grating pitch. By this scanning operation, is obtained series data (hereinafter called intensity modulation signal) composed of pixel data the intensity of which is periodically changed on a pixel basis of the X-ray image detector. From a phase shift amount (a phase shift amount between the presence and the absence of the object) of this intensity modulation signal, a differential phase image (corresponding to angular distribution of the X-rays refracted by the object) is obtained. Furthermore, integration of the differential phase image along a fringe scanning direction allows obtainment of the phase contrast image. This fringe scanning technique is also adopted in an imaging system using laser light (refer to Applied Optics, Vol. 37, No. 26, page 6227, written on September 1998 by Hector Canabal et al.).

"If the focus size of the X-ray source is large, the self image of the first diffraction grating is blurred, and the blur causes degradation in image quality of the phase contrast image. To prevent this problem, is known an X-ray imaging system having a third diffraction grating (source grating) disposed just behind the X-ray source. The third diffraction grating partly blocks the X-rays emitted from the X-ray source to reduce the effective focus size, and forms a number of narrow point sources (distributed light sources), in order to prevent the blur of the self image.

"To capture the phase contrast image of high image quality, it is necessary to precisely align the X-ray source, the first to third diffraction gratings, and the X-ray image detector. Especially, it is expected that a relative rotation angle among the first to third diffraction gratings is difficult to adjust because slight deviation in the relative rotation angle severely affects the image quality of the phase contrast image. In the X-ray imaging system used in a laboratory, the precise alignment is carried out through use of a measuring instrument with high degree of accuracy. However, in the commercial X-ray imaging system set up in a hospital or the like, the use of the highly accurate measuring instrument is out of reality, and the establishment of any alignment technique is desired."

In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventors' summary information for this patent: "An object of the present invention is to provide a method for adjusting the position and angle of plural diffraction gratings without use of a measuring instrument having high degree of accuracy, and the diffraction grating convenient for the adjustment in a radiation imaging system for capturing a phase contrast image with use of the diffraction gratings.

"To achieve the above and other objects of the present invention, an alignment method of plural diffraction gratings according to the present invention includes the steps of disposing one of the plural diffraction gratings to be first aligned in a Z axis orthogonal to a detection surface of the radiation image detector; after the disposition of the diffraction grating to be first aligned, while the diffraction grating to be first aligned is rotated in a .theta.x direction about an X axis orthogonal to the Z axis and in a .theta.y direction about a Y axis orthogonal to the Z and X axes, measuring a dose of the radiation having passed through the diffraction grating to be first aligned, and locating the diffraction grating to be first aligned in such a position in the .theta.x and .theta.y directions that the dose of the radiation is maximized; after the location of the diffraction grating to be first aligned, disposing in the Z axis another one of the plural diffraction gratings to be aligned second or later, so that the radiation forms a moire pattern; after the disposition of the diffraction grating to be aligned second or later, while the diffraction grating to be aligned second or later is rotated in the .theta.x and .theta.y directions, detecting the moire pattern by the radiation image detector, and locating the diffraction grating to be aligned second or later in such a position in the .theta.x and .theta.y directions that a frequency of the moire pattern becomes uniform; and adjusting at least one of a relative position among the plural diffraction gratings along a direction of the Z axis and a relative position among the plural diffraction gratings in a .theta.z direction about the Z axis, so that the radiation image detector loses the detection of the moire pattern.

"In the adjustment step, at least one of the relative position among the plural diffraction gratings along the direction of the Z axis and the relative position among the plural diffraction gratings in the .theta.z direction may be adjusted, so that a period of the moire pattern becomes larger than a detection area of the radiation image detector.

"It is preferable that the plural diffraction gratings include a first diffraction grating for passing the radiation and producing a fringe image, and a second diffraction grating for applying intensity modulation to the fringe image in each of plural relative positions out of phase with one another with respect to a periodic pattern of the fringe image.

"The plural diffraction gratings may further include a third diffraction grating disposed between the radiation source and the first diffraction grating. The third diffraction grating partly blocks the radiation emitted from the radiation source and produces plural line sources. The third diffraction grating is to be aligned first or second.

"In the disposition step of the diffraction grating to be aligned second or later, the diffraction grating to be aligned second or later may be disposed so as to deviate from a target position in the direction of the Z axis or in the .theta.z direction.

"A diffraction grating according to the present invention includes plural radiation shield members arranged in one direction, and an alignment section. Each of the radiation shield members extends in a direction orthogonal to the arrangement direction. The plural radiation shield members form a grating pattern. The alignment section includes a grid pattern different from the grating pattern.

"The alignment section may have a grid pitch different from a grating pitch of the grating pattern. Otherwise, the grid pattern of the alignment section may be inclined relative to the grating pattern.

"The alignment section may include a first alignment pattern having the grid pitch smaller than the grating pitch, and a second alignment pattern having the grid pitch larger than the grating pitch. The alignment section may include a third alignment pattern rotated in a first direction about an axis orthogonal to a grating surface of the diffraction grating, and a fourth alignment pattern rotated in a second direction opposite to the first direction. Furthermore, the alignment section may include a fifth alignment pattern having the grid pattern same as the grating pattern.

"The alignment section may be disposed on at least one of four corners of the diffraction grating of a rectangular shape.

"A radiation imaging system according to the present invention includes a radiation source for emitting a radiation, a radiation image detector for detecting an image carried on the radiation, and plural diffraction gratings disposed between the radiation source and the radiation image detector. Each of the diffraction gratings includes plural radiation shield members arranged in one direction. Each of the radiation shield members extends in a direction orthogonal to the arrangement direction, and the plural radiation shield members form a grating pattern in each of the diffraction gratings. At least one of the diffraction gratings includes an alignment section including a grid pattern different from the grating pattern. The radiation image detector detects a moire pattern produced by passage of the radiation through the grating pattern of the diffraction grating without having the alignment section and through the grid pattern of the alignment section, and a relative position among the plural diffraction gratings is adjusted based on the moire pattern.

"According to the present invention, the position and angle of the plural diffraction gratings can be precisely adjusted based on the dose of the passing X-rays and the moire pattern, which are measurable without use of a measuring instrument with high degree of accuracy, and hence it is possible to obtain the phase contrast image of high image quality. Besides, by providing the alignment section in the diffraction grating to check a state and direction of positional deviation, the position and angle of the plural diffraction gratings are precisely adjustable."

For more information, see this patent: Kaneko, Yasuhisa; Ishii, Hiroyasu. Diffraction Grating and Alignment Method Thereof, and Radiation Imaging System. U.S. Patent Number 8755487, filed January 29, 2011, and published online on June 17, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8755487.PN.&OS=PN/8755487RS=PN/8755487

Keywords for this news article include: Fujifilm Corporation.

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Source: Journal of Engineering


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