Patent Publication Number: US-2013235972-A1

Title: Method for manufacturing collimator, collimator and x-ray ct apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No.2012-052341, filed on Mar. 8, 2012; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a method for manufacturing a collimator, a collimator, and an X-ray CT apparatus. 
     BACKGROUND 
     In an X-ray CT (Computer Tomography) apparatus, in order to increase the number of detection points to increase spatial resolution, an X-ray detector using a scintillator has been used. 
     Upon request to take a photograph of a wide range at high speed and high definition, the X-ray detector including a plurality of photoelectric conversion elements both in a channel direction and a slice direction has been used. In such X-ray detector, when the number of the photoelectric conversion elements in the slice direction increases, it is needed to remove scattered X-rays in the channel direction as well as the slice direction. 
     For this reason, there is proposed a collimator formed by stacking a plurality of elements in which a flat plate-like bottom part and a plurality of wall parts protruding from the bottom part are integrally molded. 
     However, when the bottom part and the wall parts are integrally molded, a corner of each of intersections of the bottom part and the wall parts is rounded, thereby lowering aperture ratio. 
     In this case, the geometric efficiency of the X-ray detector is a ratio of an effective area of a detecting part to a total area of the X-ray detector. Thus, when the aperture ratio lowers, the geometric efficiency also lowers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram for illustrating schematic configuration of the X-ray CT apparatus. 
         FIG. 2  is a schematic perspective view illustrating the radiation detector. 
         FIG. 3  is a schematic sectional view showing an A-A cross section in  FIG. 2 . 
         FIGS. 4A and 4B  are schematic perspective views illustrating the collimator. 
         FIGS. 5A and 5B  are schematic views illustrating the plate-like parts constituting the collimator. 
         FIG. 6  is a schematic perspective view illustrating the section. 
         FIGS. 7A and 7B  are schematic perspective views illustrating the lattice structure part of modular unit. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a method is disclosed for manufacturing a collimator. The method can include forming a first plate-like part having a plurality of first slits inclined at a predetermined angle corresponding to a focal position of a radiation source. The method can include forming a second plate-like part having a plurality of second slits inclined at a predetermined angle corresponding to the focal position. The method can include causing the first slits and the second slits to face each other and assembling a plurality of the first plate-like parts and a plurality of the second plate-like parts so as to intersect each other. 
     The causing the first slits and the second slits to face each other and assembling a plurality of the first plate-like parts and a plurality of the second plate-like parts so as to intersect each other includes the followings. Portions of the second plate-like parts where the second slits are not provided are held on an opening side of the first slits. The second plate-like parts are inclined so as to follow an inclination of the first slits. The inclined second plate-like parts are moved toward a bottom of the first slits. 
     Embodiments of the invention will now be described below with reference to the drawings. The same constituents are given the same numerals throughout the figures and detailed description thereof is omitted as appropriate. 
     In following description, although a case where a radiation is an X-ray is used as an example, the invention can be also applied to other radiation such as a γ-ray. 
     Thus, for example, when an exemplified X-ray detector is applied to other radiation, “X-ray” may be replaced with “other radiation (for example, γ-ray)”. 
     First Embodiment  
     First, a collimator  1  and an X-ray CT apparatus  100  in accordance with the embodiment will be described. 
       FIG. 1  is a schematic block diagram for illustrating schematic configuration of the X-ray CT apparatus. 
     As shown in  FIG. 1 , the X-ray CT apparatus  100  includes an X-ray tube  101 , a rotational ring  102 , a two-dimensional detecting part  103 , a data acquisition circuit (DAS)  104 , a non-contact data transmission device  105 , a platform driving part  107 , a slip ring  108  and a processing part  106 . 
     The X-ray tube  101  as an X-ray source emitting an X-ray is a vacuum tube generating the X-ray and is supported by the rotational ring  102 . Electric power (tube current, tube voltage) necessary for exposure of the X-ray is supplied from an unillustrated high-voltage generator to the X-ray tube  101  via the slip ring  108 . The X-ray tube  101  causes an electron accelerated by a supplied high voltage to hit a target, thereby exposing the X-ray toward an object to be tested in an effective field of view FOV. 
     An X-ray tube-side collimator not shown for shaping the shape of an X-ray beam exposed from the X-ray tube  101  into a cone shape, quadrangular pyramid shape or fan beam shape is provided between the X-ray tube  101  and the object to be tested. 
     The two-dimensional detecting part  103  is a detector system for detecting the X-ray passing through the object to be tested and is supported by the rotational ring  102  so as to face the X-ray tube  101 . A radiation detector  10  is attached to an outer circumferential side of the two-dimensional detecting part  103  (opposite side of the object to be tested). That is, the radiation detector  10  including the collimator  1  described later, a scintillator  4  for receiving the X-ray to emit fluorescence and a photoelectric converting part  12  for converting the fluorescence into an electric signal is attached to the outer circumferential side of the two-dimensional detecting part  103 . 
     Details of the collimator  1  and so on will be described later. 
     The X-ray tube  101  and the two-dimensional detecting part  103  are supported by the rotational ring  102 . The rotational ring  102  is driven by the platform driving part  107  and rotates about the object to be tested. 
     The data acquisition circuit (DAS)  104  has a plurality of data acquisition element rows in which DAS chips are arranged, and receives an input of data detected by the two-dimensional detecting part  103  (hereinafter referred to as raw data). Then, the input raw data is amplified and A/D converted and then, transmitted to the processing part  106  via a data transmitter  105 . 
     The platform driving part  107  performs driving and its control, for example, integrally rotates the X-ray tube  101  and the two-dimensional detecting part  103  about a central axis that is parallel to a body-axis direction of the object to be tested inserted into a diagnostic opening. 
     The processing part  106  creates “projection data” by performing correction of the sensitivity of the raw data and correction of the intensity of the X-ray. Then, reconstructed image data of predetermined slices is created by reconstructing the projection data on the basis of predetermined reconstruction parameters (reconstruction region size, reconstruction matrix size, threshold value for extracting concerned region and so on). The reconstructed image data is subjected to image processing for display, such as window conversion and RGB processing, and is outputted as an image to a display device not shown. 
     That is, the processing part  106  reconstructs a tomographic image of the object to be tested on the basis of the intensity of the X-ray detected by the radiation detector  10 . 
       FIG. 2  is a schematic perspective view illustrating the radiation detector. 
       FIG. 3  is a schematic sectional view showing an A-A cross section in  FIG. 2 . 
     As shown in  FIG. 2 , the radiation detector  10  includes a detecting part  2  and the collimator  1 . A holding part  6  is a member provided at the two-dimensional detecting part  103  for holding the radiation detector  10 . 
     As shown in  FIG. 2 , the collimator  1  has a lattice structure formed of an X-ray shielding plate (plate-like parts  11 ,  21  described later) for shielding the X-ray, and each section of the lattice structure corresponds to each section of the scintillator  4 . In this case, when the collimator  1  is provided at a predetermined position in the X-ray CT apparatus  100  shown in  FIG. 1 , each section of the lattice structure of the collimator  1  faces the focus of the X-ray tube  101  (X-ray source). For example, as shown in  FIG. 2 , each rectangular section can be configured so as to be shaped like a quadrangular pyramid in a plan view. Such lattice structure can be formed by inclining each X-ray shielding plate constituting each section at a predetermined angle in both the channel direction and the slice direction of the collimator  1  so as to face the focus of the X-ray tube  101  when the collimator  1  is provided at the predetermined position in the X-ray CT apparatus  100  shown in  FIG. 1 . Details of the collimator  1  will be described later. 
     As shown in  FIG. 3 , the detecting part  2  is provided with the scintillator  4 , a light reflecting part  17 , an adhesive layer  3 , the photoelectric converting part  12 , a circuit board  18  and a bottom part  7 . 
     The scintillator  4  is divided into sections corresponding to detection sections of the photoelectric conversion elements  12   a  provided in the photoelectric converting part  12 , and a groove  16  is formed between the respective detection sections. That is, each scintillator  4  is divided by the groove  16 . The scintillator  4  is bonded to the photoelectric converting part  12  so that their sections correspond to each other. 
     The scintillator  4  is provided facing the collimator  1 , receives radiation such as the X-ray and emits the fluorescence. The fluorescence is, for example, light such as a visible light ray. Since maximum luminous wavelength, attenuation time, reflection coefficient, density, light output ratio and temperature dependency on the fluorescence efficiency of the scintillator  4  vary depending on the material for the scintillator  4 , the material can be selected according to usage. For example, a ceramic scintillator formed of a sintered body of rare-earth oxysulfide is used for the X-ray CT apparatus. However, the material is not limited to this, and may be appropriately changed. 
     The light reflecting part  17  formed by inserting and bonding a body having a function of reflecting light of wavelength in the vicinity of luminous wavelength of the scintillator  4  (for example, a while plate-like body) is provided in the groove  16  between the scintillators  4 . 
     The light reflecting part  17  that sections the scintillator  4  for each photoelectric conversion element  12   a  serves to perform optical separation between the sections of each scintillator  4  and reflection, thereby suppressing optical crosstalk between the respective sections. 
     The photoelectric converting part  12  has the photoelectric conversion elements  12   a  for converting the fluorescence from the scintillator  4  into an electric signal. The photoelectric conversion elements  12   a  are, for example, silicon photo diodes with pin structure. 
     The adhesive layer  3  is made of, for example, a transparent adhesive, and bonds the scintillator  4  to the photoelectric converting part  12  while improving transmission of light between them. 
     The circuit board  18  is provided on the face opposite to the face on the side of the photoelectric converting part  12  to be bonded to the scintillator  4 . The circuit board  18  is also sectioned so as to correspond to the sections of the scintillator  4  and is configured to allow take-in of the electric signal of each section. 
     The bottom part  7  is shaped like a flat plate, and on a main surface thereof, the circuit board  18 , the photoelectric converting part  12 , the adhesive layer  3  and the scintillator  4  provided with the light reflecting part  17  are provided in a stacked manner. The bottom part  7  can be attached to the holding part  6  by use of a fastening means such as a screw not shown. By attaching the bottom part  7  to the holding part  6 , the scintillator  4  and the like provided in a stacked manner are held by the holding part  6 . 
     The holding part  6  provided in the two-dimensional detecting part  103  to hold the radiation detector  10  can be shaped like a circular arc so that each scintillator  4  faces the focus of the X-ray source (X-ray tube  101 ). The pair of holding parts  6  is spaced at a predetermined interval so as to face each other, and holds the collimator  1  therebetween. In this case, for example, by bonding the collimator  1  between the holding parts  6  by use of an adhesive, the collimator  1  can be held by the holding parts  6 . However, the holding method of the collimator  1  is not limited to bonding using the adhesive and may be appropriately changed. For example, by fitting the collimator  1  into a groove not shown provided in the holding part  6 , the collimator  1  can be held by the holding parts  6 . 
     The bottom part  7  provided at the detecting part  2  is held on an outer circumferential side (convex side of the circular arc) of the pair of holding parts  6 . A plurality of bottom parts  7  are provided along the circumferential faces of the holding parts  6  so as to conform to the outer circumferential shape of the holding parts  6 . 
     Next, the collimator  1  will be further described. 
     As shown in  FIG. 2 , the collimator  1  has the lattice structure on the cross section intersecting the passage direction of the X-ray emitted from the X-ray tube  101 . The rectangular sections are formed in the lattice structure so that the area of the cross section of the structure becomes larger as the distance thereof increases from the X-ray tube  101 . Here, the lattice structure can be provided in a configuration, for example as shown in  FIG. 2 , so that each rectangular section is shaped like a quadrangular pyramid. As shown in  FIG. 3 , the collimator  1  controls the X-ray incident to each scintillator  4 , and absorbs the scattered X-rays to reduce crosstalk due to the scattered X-rays. 
     Examples of the material for the collimator  1  include W (tungsten), Mo (molybdenum), Ta (tantalum), Pb (lead) and an alloy containing at least one of these heavy metals. However, the material is not limited to these and a material having an excellent X-ray shielding characteristic can be appropriately selected. 
     As described later, the lattice structure of the collimator  1  can be also configured by preparing a plurality of lattice structures of modular units (or referred to as block units) and combining the lattice structures of modular units. In this case, the lattice structures of modular units are attached in line with the holding parts  6  (support members) while positioning the lattice structures so that each section faces the focus of the X-ray tube  101  (X-ray source). 
     The lattice structures of modular units may be detachable with respect to the holding parts  6 . 
     Here, the collimator may be integrally molded by using a member obtained by bending a thin plate so that the rectangular sections are formed on the above-mentioned cross section (that is, the cross section intersecting the passage direction of the X-ray), or sections having rectangular cross section are formed by stacking the plurality of integrally molded elements. However, in doing so, any of four corners of the rectangular cross section is rounded and thus, the lattice shape becomes non-uniform, resulting in a decrease in the aperture ratio. 
     In such case, in terms of an image taken from the object to be tested, since the geometric efficiency of the radiation detector  10  is a ratio of the effective area of the detecting part  2  to the total area of the radiation detector  10 , when the aperture ratio decreases, the geometric efficiency also decreases. In the case where the collimator with the decreased geometric efficiency is used, in the X-ray CT apparatus, the quality of the taken image of the object to be tested deteriorates. 
     In recent years, to increase the resolution of the X-ray CT apparatus, higher definition of acquired data such as an image has been achieved through multi-row detectors including collimators and therefore, the size of the section tends to be small. For this reason, when any of four corners of the rectangular cross section of the section is rounded, the effect can be great. 
       FIGS. 4A and 4B  are schematic perspective views illustrating the collimator. 
       FIG. 4A  is a schematic perspective view illustrating outer appearance of the collimator, and  FIG. 4B  is a schematic exploded view of the collimator. 
     To avoid complexity, the plate-like parts are thinned out. 
       FIGS. 5A and 5B  are schematic views illustrating the plate-like parts constituting the collimator. 
     As shown in  FIGS. 4A and 4B ,  FIGS. 5A and 5B , the collimator  1  includes the plurality of plate-like parts  11  arranged spaced apart from each other (corresponding to an example of first plate-like part) and the plurality of plate-like parts  21  arranged spaced apart from each other in a direction intersecting the plate-like parts  11  (corresponding to an example of second plate-like part). 
     A plurality of slits  11   a  (corresponding to an example of first slits) are formed spaced apart from each other in the plate-like part  11 . The number of the slits  11   a  can be set to the number of the fitted plate-like parts  21 . A width W 1  of the plate-like part  11  can be made equal to a width W 2  of the plate-like part  21 . 
     The width W 1   a  of the slit  11   a  is slightly larger than a thickness of the plate-like part  21 . A length L 1  of the slits  11   a  can be set to, for example, about a half of the width W 1  of the plate-like part  11 . 
     The slits  11   a  are formed to be inclined at a predetermined angle corresponding to the focal position of the X-ray source (X-ray tube  101 ). For this reason, by fitting the plate-like parts  21  into the slits  11   a , the plate-like parts  21  can be inclined at the predetermined angle corresponding to the focal position of the X-ray source. 
     A plurality of slits  21   a  (corresponding to an example of second slits) are formed spaced apart from each other in the plate-like part  21 . The number of slits  21   a  can be set to the number of the fitted plate-like parts  11 . 
     The width W 2   a  of the slits  21   a  is slightly larger than a thickness of the plate-like part  11 . A length L 2  of the slits  21   a  is set to, for example, about a half of the width W 2  of the plate-like part  21 . 
     The slits  21   a  are formed to be inclined at a predetermined angle corresponding to the focal position of the X-ray source. For this reason, by fitting the plate-like parts  11  into the slits  21   a , the plate-like part  11  can be inclined at the predetermined angle corresponding to the focal position of the X-ray source. 
     In this case, at a position where the plate-like parts  11  intersect the plate-like parts  21 , the slits  11   a  and the slits  21   a  face each other. 
     That is, portions of the plate-like parts  21  where the slits  21   a  are not provided are fitted into the slits  11   a , and portions of the plate-like parts  11  where the slits  11   a  are not provided are fitted into the slits  21   a , resulting in that the plate-like parts  11  intersect the plate-like parts  21 . 
     When the plate-like parts  11  and the plate-like parts  21  are assembled to each other to form the collimator  1 , as shown in  FIG. 4B , the slits  1   a  of the plate-like parts  11  are caused to face the slits  21   a  of the plate-like parts  21  and the portions of the plate-like parts  21  where the slits  21   a  are not provided are fitted into the slits  11   a . At this time, the portions of the plate-like parts  11  where the slits  11   a  are not provided are fitted into the slits  21   a.    
       FIG. 6  is a schematic perspective view illustrating the section. 
     As described above, by assembling the plate-like parts  11  and the plate-like parts  21  to each other, the plate-like parts  11  and the plate-like parts  21  are inclined at the predetermined angle corresponding to the focal position of the X-ray source. 
     For this reason, an outer shape of a section la formed by being defined by the plate-like parts  11  and the plate-like parts  21  is a quadrangular pyramid as shown in  FIG. 6 . 
     In this case, since the section  1   a  is formed by fitting the plate-like slits to the corresponding plate-like parts, the four corners of the rectangular cross section of the section  1   a  are hardly rounded. For this reason, the decrease in the aperture ratio can be prevented, thereby improving the geometric efficiency. Accordingly, in the detector including the collimator, multi-row in the channel direction and the slice direction can be addressed. By using such collimator with improved geometric efficiency, in the X-ray CT apparatus, the spatial resolution and an image quality of the taken image of the object to be tested are improved, enabling acquisition of high-definition data. 
     It should be noted that the plate-like part  11  and the plate-like part  21  are not necessarily fixed to each other. 
     However, by fixing the plate-like parts  11  and the plate-like parts  21  to each other, the effect such as vibration is hard to occur. 
     In this case, the plate-like parts  11  and the plate-like parts  21  can be fixed to each other by use of an adhesive. Details of fixation using the adhesive will be described later. 
     Second Embodiment 
     Next, a method for manufacturing the collimator in accordance with the embodiment will be described. 
     First, the plate-like part  11  and the plate-like part  21  are formed. 
     That is, the plate-like part  11  having the plurality of slits  11   a  inclined at a predetermined angle corresponding to the focal position of the X-ray source is formed. The plate-like part  21  having the plurality of slits  21   a  inclined at a predetermined angle corresponding to the focal position of the X-ray source is formed. 
     Blanks of the plate-like part  11  and the plate-like part  21  are cut out from a flat plate material using a material excellent in X-ray shielding characteristic. 
     Then, the slits  11   a  having predetermined shape and dimension are formed in the blank of the plate-like part  11  and the slits  21   a  having predetermined shape and dimension are formed in the blank of the plate-like part  21 . 
     The lattice structure formed of the plate-like parts  11 ,  21  is formed in the collimator. Here, when the lattice structure is provided at a predetermined position in the X-ray CT apparatus, the sections of the lattice structure needs to be configured so as to face the focus of the X-ray tube  101  (X-ray source). 
     Accordingly, the slits  11   a  of the plate-like part  11  and the slits  21   a  of the plate-like part  21  need to have the predetermined shape and dimension so as to achieve the collimator with such configuration. 
     In this case, examples of a material excellent in X-ray shielding characteristic include W (tungsten), Mo (molybdenum), Ta (tantalum), Pb (lead) and an alloy containing at least one of these heavy metals. However, the material is not limited to these and the material excellent in X-ray shielding characteristic can be appropriately selected. 
     The slits  11   a  and the slits  21   a  can be formed, for example, by etching. 
     Next, the plate-like part  11  and the plate-like part  21  are assembled so as to intersect each other. 
     Here, the collimator  1  can be manufactured by sequentially assembling the plate-like part  11  or the plate-like part  21  one by one. 
     Such lattice structure can be formed by inclining each X-ray shielding plate constituting each section in two directions: the channel direction and the slice direction of the collimator  1  at a predetermined angle so as to face the focus of the X-ray tube  101  when the collimator  1  is provided at the predetermined position in the X-ray CT apparatus  100  shown in  FIG. 1 . 
     The lattice structure of the collimator  1  can be also configured by preparing a plurality of lattice structure parts of modular units and combining these lattice structure parts of modular units. 
       FIGS. 7A and 7B  are schematic perspective views illustrating the lattice structure part of modular unit. 
       FIG. 7A  is a schematic perspective view illustrating an outer appearance of the lattice structure part of modular unit, and  FIG. 7B  is a schematic exploded view of the lattice structure part of modular unit. 
     To avoid complexity, the plate-like parts are thinned out. 
     As shown in  FIGS. 7A and 7B , the plate-like parts  11 , the plate-like parts  21 , connecting parts  31  and a covering part  32  are provided in the lattice structure part  13 . 
     The connecting part  31  is made of a material having a high rigidity, such as metal, and can be bonded to the ends of the plate-like parts  11  by use of an adhesive or the like. 
     The covering part  32  is shaped like a flat plate and covers an incident face of the X-ray. 
     The covering part  32  may have grooves not shown for fitting the ends of the plate-like parts  11  and the plate-like parts  21 . 
     The covering part  32  is made of a material having a high transmittance of the X-ray and a high rigidity. The covering part  32  can be made of, for example, carbon fiber reinforced plastics (CFRP). 
     The covering part  32  can be bonded to the plate-like parts  11  and the plate-like parts  21  by use of an adhesive or the like. Further, the covering part  32  can be also bonded to the connecting parts  31  by use of an adhesive or the like. 
     In this case, the collimator  1  is configured by attaching the lattice structure part  13  of modular units in line with the bow-like holding parts  6  via the connecting parts  31  while positioning the lattice structure part  13  so that each section thereof faces the focus of the X-ray tube  101  (X-ray source). Here, the bow-like holding parts  6  are formed so that each point thereof draws a circular arc with a predetermined curvature so as to face the focus of the X-ray tube  101  (X-ray source) when the collimator  1  is provided at the predetermined position in the X-ray CT apparatus  100  shown in  FIG. 1 . 
     The lattice structure part  13  of modular units may be detachable with respect to the holding parts  6 . 
     According to the above-exemplified embodiments, the manufacturing method of collimator, the collimator and the X-ray CT apparatus that can improve the geometric efficiency can be realized. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention. Moreover, above-mentioned embodiments can be combined mutually and can be carried out. 
     For example, shape, size, material, arrangement and the number of each constituent included in the collimator  1  and the X-ray CT apparatus  100  are not limited to those exemplified and may be appropriately changed.