JP2013040859A - X-ray detector and x-ray ct apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an X-ray detector and an X-ray CT apparatus capable of suppressing X-ray detector output reduction, eliminating causes of photoelectric transducer degradation and abnormal output, and preventing generation of artifacts.SOLUTION: An X-ray detector for X-ray CT apparatus according to an embodiment includes a scintillator array, collimators, and masks. The scintillator array includes scintillator elements arranged along a circumferential direction around a subject's body axis, and partition walls for partitioning adjacent scintillator elements. The collimators comprise a plurality of collimator plates, each of which being located at a position corresponding to each of the partition walls, with one end of each collimator plate positioned in the proximity of each partition wall and the other end of each collimator plate extending in the height direction thereof from the partition wall toward an X-ray source, and regulate incident direction of X-rays entering the scintillator elements. The masks, which absorb X-rays, are located between the partition walls and one ends of the collimator plates such that at least the partition walls are masked thereby in the direction of incident X-rays.

Description

  Embodiments described herein relate generally to an X-ray detector and an X-ray CT apparatus.

  A conventional X-ray CT apparatus has an X-ray source and an X-ray detector. The X-ray source and the X-ray detector are opposed to each other about the body axis of the subject, and are rotated in a direction around the body axis. The X-ray source generates X-rays by causing thermal electrons to collide with a focal point on a target (anode).

  The X-ray detector is arranged in correspondence with the X-ray detection element, an X-ray detection element arranged in a channel direction that is a direction orthogonal to the X-ray incident direction and the body axis direction (slice direction), It has a scintillator element that converts light, and a plurality of collimator plates. A partition wall which is a dead zone is provided between adjacent scintillator elements. Each collimator plate is disposed at a position corresponding to the partition wall. X-rays are emitted from an X-ray source, pass through the subject, are regulated in direction by a collimator plate, and enter the scintillator element.

  If the shadow of the collimator plate by X-rays is applied to the scintillator element, it becomes a factor that causes artifacts. Therefore, the partition wall is thicker than the collimator plate.

JP 2000-193750 A

  However, it is necessary to set the effective area of the scintillator element to which X-rays are incident as much as the partition wall is thick, and the geometric efficiency is lowered. Here, the geometric efficiency is represented by an area ratio. For example, when the area of the effective area is changed from A0 to A1, the geometric efficiency is A1 / A0. The geometric efficiency is represented by a value of 1 (= 100%) or less even when the area of the effective region is not reduced (A1 ≧ A0).

  In addition, it is necessary to set the effective area of the photoelectric conversion element to be smaller than the effective area of the scintillator element in consideration of the relative displacement between the scintillator element and the photoelectric conversion element that occurs during assembly. For example, if the area of the effective region of the photoelectric conversion element is A2, the geometric efficiency is A2 / A1 because the area of the effective region of the scintillator element is A1. That is, in an X-ray detector in which the effective area of the photoelectric conversion element is set smaller than the effective area of the scintillator element, the geometric efficiency is two steps of A2 / A0 (= A1 / A0 * A2 / A1). Since the output of the X-ray detector is proportional to the geometric efficiency, it decreases in two steps.

  Further, in the X-ray CT apparatus in which the focus on the target is moved and adjusted along the channel direction by the movement of the target, the angle in the X-ray incident direction with respect to the scintillator element changes due to the movement of the focus, and this angle increases. Accordingly, the shadow of the collimator plate by X-rays moves from the partition side toward the scintillator element. It is necessary to increase the thickness of the partition wall according to the amount of movement of the focal point so that the shadow does not reach the scintillator element. As a result, there is a problem that the output of the X-ray detector further decreases.

  Further, when the partition wall is thickened, there is a problem that X-rays enter the partition wall, pass through the partition wall, and cause deterioration of the photoelectric conversion element or generation of abnormal output due to scattered X-rays.

  Further, the end face of the scintillator element (surface in contact with the partition wall) is a processed surface, and is a portion where the sensitivity of X-rays is lowered. Therefore, when the end face of the scintillator element is included in the effective region, there is a problem that an artifact is generated.

  This embodiment solves the above-described problem, suppresses a decrease in the output of the X-ray detector, eliminates a factor that deteriorates the photoelectric conversion element or generates an abnormal output, and further generates an artifact. An object of the present invention is to provide an X-ray detector and an X-ray CT apparatus capable of preventing the above-described problems.

  In order to solve the above-described problems, an X-ray detector for an X-ray CT apparatus according to an embodiment is disposed so as to face an X-ray source that generates X-rays around the body axis of the subject, and around the body axis. The X-ray is detected by rotating in the direction of, and has a scintillator array, a collimator, and a mask. The scintillator array has scintillator elements arranged along the direction around the body axis, and partition walls that partition adjacent scintillator elements. The collimator has a plurality of collimator plates provided corresponding to the partition walls. One end portion of each collimator plate is close to the partition wall, and the other end portion of each collimator plate is in the height direction from the partition wall to the X-ray source. The X-ray incident direction that is extended and incident on the scintillator element is regulated. The mask is disposed between the partition wall and one end of the collimator plate so as to cover at least the partition wall from the incident direction of the X-ray, and absorbs X-rays.

1 is a schematic diagram of an X-ray CT apparatus according to an embodiment. The schematic diagram of an X-ray detector. The partial perspective view of a X-ray detector. The fragmentary sectional view of a X-ray detector. The figure which represented angle (theta) of the X-ray incident direction in a part of X-ray detector when it sees from a Y direction. The fragmentary sectional view of the X-ray detector which concerns on a comparative example.

  An embodiment of the X-ray CT apparatus will be described with reference to each drawing.

  FIG. 1 is a schematic diagram of an internal structure in a gantry of an X-ray CT apparatus. The gantry is rotatably provided with a rotating ring 1. In the rotating ring 1, an X-ray source 2 and a two-dimensional array type X-ray detector 3 are disposed so as to face each other with the subject P as a center. X-rays 5 radiated in a fan shape from the focal point 4 of the X-ray source 2 pass through the subject P and are detected by the X-ray detector 3. Projection data detected by the X-ray detector 3 is stored in a data acquisition unit (not shown), and the computer 6 reconstructs tomographic image data relating to at least one slice based on the projection data. The reconstructed tomographic image data is stored in a data storage unit (not shown). The display unit 7 displays a tomographic image according to the tomographic image data.

[X-ray source]
The X-ray source 2 has focal point moving means (not shown) that moves the focal point position along the channel direction and the body axis direction (slice direction). The channel direction and the body axis direction (slice direction) may be referred to as the X direction and the Z direction. In the following, the case where the focal point is moved along the channel direction (X direction) will be described. However, the configuration of this embodiment is also applied to the case where the focal point is moved along the body axis direction (Z direction). It is possible.

[X-ray detector]
FIG. 2 is a schematic diagram of an X-ray detector. FIG. 2 shows the X-ray detector as viewed from the Z direction.

  As shown in FIG. 2, the X-ray detector 3 includes a scintillator array 31, a photoelectric conversion element array (PDA: Photo Diode Array) 32, a collimator 33, and a mask 8.

(Scintillator array)
FIG. 3 is a partial perspective view of the X-ray detector. FIG. 3 shows a portion of the X-ray detector when the XZ plane is viewed obliquely from above.
As shown in FIGS. 2 and 3, the scintillator array 31 includes scintillator elements 34 that are two-dimensionally arranged in the X direction and the Z direction, and a partition wall 35 that partitions adjacent scintillator elements 34. As the scintillator element 34, for example, a ceramic scintillator made of a sintered body of rare earth oxysulfide can be used. However, the present invention is not limited to this.

  As shown in FIG. 3, N modules each having M scintillator elements 34 arranged in a line along the Z direction are arranged in an arc shape along the X direction. The scintillator element 34 receives X-rays and outputs fluorescence. In FIG. 3, four modules in which six scintillator elements 34 are arranged in a line are arranged along the X direction, and the other modules are indicated by two-dot chain lines. The number of partition walls 35 arranged in the Z direction is M + 1, which is one more than that of the scintillator element 34. The number of partition walls 35 arranged in the X direction is N + 1, which is one more than that of the scintillator element 34.

(Photoelectric conversion element array)
As shown in FIGS. 2 and 3, the photoelectric conversion element array 32 includes photoelectric conversion elements 36 arranged corresponding to the scintillator elements 34. The photoelectric conversion element 36 receives the fluorescence output from the scintillator element 34 and outputs it as an electrical signal. As the photoelectric conversion element 36, for example, a PD (photodiode) can be used. However, it is not limited to this. Although only four photoelectric conversion elements 36 are shown in FIG. 3, in actuality, like the scintillator elements 34, they are arranged in two two dimensions in the X direction and the X direction.

(Collimator)
As shown in FIGS. 2 and 3, the collimator 33 has a plurality of collimator plates 37. Each collimator plate 37 is arranged corresponding to the partition wall 35. The collimator plate 37 regulates the incident direction of X-rays incident on the scintillator element 34. The collimator plate 37 is made of, for example, W (tungsten), Mo (molybdenum), Ta (tantalum), Pb (lead), or an alloy containing one of these heavy metals.

  The partition walls 35 are formed in a lattice shape in which the X direction and the Z direction are vertical and horizontal. That is, the number of collimator plates 37 arranged in the Z direction is M + 1. Further, the number of collimator plates 37 arranged in the X direction is N + 1.

  FIG. 4 is a partially enlarged sectional view of the X-ray detector 3. FIG. 4 shows a part of the X-ray detector 3 when viewed from the Z direction. The X-ray incident direction with respect to the scintillator element 34 is indicated by R1, and the opposite direction is indicated by R2. As shown in FIG. 4, one end portion 37a of the collimator plate 37 is brought close to the partition wall 35, and the other end portion 37b of the collimator plate 37 extends from the partition wall 35 in a direction R2 opposite to the X-ray incident direction R1. . 4 is the X-ray incident direction when the angle of the X-ray incident direction with respect to the scintillator element 34 is 90 °.

(mask)
As shown in FIG. 4, the mask 8 is disposed between the one end 37 a of the collimator plate 37 and the partition wall 35 so as to cover the partition wall 35 from the X-ray incident direction R <b> 1 and has an opening 81 facing the scintillator element 34. ing. Examples of the X-ray absorbing material (material that hardly or hardly transmits X-rays) used for the mask 8 include, for example, W (tungsten), Mo (molybdenum), Ta (tantalum), Pb (lead), or these heavy metals. An alloy including one is used.

  The mask 8 is formed in a lattice shape corresponding to the partition wall 35, and an opening 81 is a portion surrounded by the lattice.

  The mask 8 has an overhang portion 82 that projects from the position of the end face 38 of the scintillator element 34 in contact with the partition wall 35 toward the position of the central portion 39 of the scintillator element 34. The ends 83 of the overhanging portion 82 face each other through the opening 81.

  The overhang portion 82 is provided according to the position of the other end portion 37b of the collimator plate 37 in the X direction and the X-ray incident direction, and the angle in the X-ray incident direction. As described above, since the angle in the X-ray incident direction changes as the focal point moves along the X direction, the overhanging portion 82 takes into account the change in the angle in the X-ray incident direction. .

  Next, the interrelation between the position of the overhang portion 82, the position of the other end portion 37b of the collimator plate 37, and the angle in the X-ray incident direction incident on the scintillator element 34 will be described with reference to FIG.

  FIG. 5 shows a part of the X-ray detector as viewed from the Z direction, where the X-ray incident direction is indicated by R and the angle is indicated by θ.

5, the length from the position of the other end 37b of the collimator plate 37 in the X direction to the position of the tip 83 of the overhang 82 is L, and the position of the other end 37b of the collimator plate 37 in the height direction. The length from the position of the overhanging portion 82 (the position of the mask 8) to H is indicated by H, and the angle of the X-ray incident direction R regulated by the collimator plate 37 is indicated by θ. L, H, and θ expressed in this way have the relationship shown in the following equation.
L = H / tan θ (1)
Here, the height direction refers to the incident direction when θ = 90 °. In FIG. 5, the height direction is indicated by Y.

The angle θ in the X-ray incident direction is changed by the movement of the focal point along the X direction. When the focal point moves along the X direction, when the angle in the X-ray incident direction changes in the range of −θ1 to θ1 (θ1> 0), L1 when θ = θ1 is the maximum length of the overhang portion 82. Become. This is expressed by the following equation.
L1 = H / tan θ1 (2)

  By setting the maximum length of the overhang portion 82 to L1, no matter how the focal point moves along the X direction, the shadow of the collimator plate 37 caused by X rays does not reach the scintillator element 34, and artifacts are generated. It becomes possible to prevent. Note that the maximum length of the overhanging portion 82 that faces the overhanging portion 82 via the opening 81 is L1 when θ = −θ1. The overhang portion 82 having the maximum length L1 is shown in FIG.

  Further, no matter how the focal point moves along the X direction, the X-rays are applied only to the overhanging portion 82 of the mask 8, so that the X-rays 5 do not enter the partition walls 35, pass through the partition walls 35, and are scattered. The X-rays do not deteriorate the photoelectric conversion element 36 or cause abnormal output.

  Next, the relationship between the thickness of the partition wall 35 and the thickness of the collimator plate 37 will be described with reference to FIG. FIG. 4 shows the thickness of the partition wall 35 and the thickness of the collimator plate as T1 and T2.

  As shown in FIG. 4, the thickness T1 of the partition wall 35 is substantially equal to the thickness T2 of the collimator plate 37 (T1≈T2). It is also possible to make the thickness T1 of the partition wall 35 equal to or less than the thickness T2 of the collimator plate 37 (T1 ≦ T2).

  Thus, the reason why the partition wall 35 can be made thin is that the shadow of the collimator plate 37 caused by X-rays does not reach the scintillator element 34 regardless of how the focal point moves along the X direction, thereby preventing the occurrence of artifacts. This is because the overhanging portion 82 of the mask 8 is provided.

  Next, the effective area of the scintillator element 34 and the effective area of the photoelectric conversion element 36 will be described with reference to FIG. In FIG. 4, the effective area of the photoelectric conversion element 36, the opening 81, and the width of the scintillator element 34 in the X direction are indicated by W1, W2, and W3. Note that the width of the opening 81 corresponds to the width of the effective region of the scintillator element 34.

Since a part of the scintillator element 34 is covered by the overhanging portion 82, the width W2 of the opening 81 is narrower than the width W3 of the scintillator element 34 (W2 <W3). Thereby, the effective area of the scintillator element 34 is reduced and the geometric efficiency is reduced. That is, if the area of the effective region before and after becoming small is A0 and A1, the geometric efficiency is A1 / A0.
However, the width W1 of the effective region of the photoelectric conversion element 36 is equal to or smaller than the width W3 of the scintillator element 34 and equal to or larger than the width W2 of the opening 81 (W2 ≦ W1 ≦ W3).

  Therefore, the effective area of the photoelectric conversion element 36 (the width is indicated by W1 in FIG. 4) does not need to be smaller than the effective area of the scintillator element 34 (opening 81: the width is indicated by W2 in FIG. 4). There is no reduction in geometric efficiency. That is, the geometric efficiency is 1.

  As a result, the overall geometric efficiency becomes A1 / A0 (= A1 / A0 * 1), which does not decrease in two stages, and prevents the output of the X-ray detector 3 from decreasing in two stages. It becomes possible.

(Comparative example)
Next, the X-ray detector 3 according to the comparative example will be described.

  FIG. 6 is a partially enlarged cross-sectional view of the X-ray detector 3 according to the comparative example. The comparative example is different from the configuration of the embodiment in that the mask 8 is removed.

  That is, as shown in FIG. 6, the X-ray detector 3 includes a photoelectric conversion element 36 arranged in the channel direction (X direction), a scintillator element 34 arranged corresponding to the photoelectric conversion element 36, and a collimator plate. 37. A partition wall 35 that is a dead zone is provided between adjacent scintillator elements 34.

  In the comparative example, the thickness T1 of the partition wall 35 is set so that the shadow of the collimator plate 37 by the X-ray 5 is not reflected on the scintillator element 34 even when the incident direction angle θ is the predetermined angle θ1. It is larger than the length T2 (T1> T2). Therefore, in the technique of the comparative example, no matter how the focal point moves along the X direction, the shadow of the collimator plate 37 due to the X-ray does not hit the scintillator element 34, and the advantage of preventing the occurrence of artifacts is obtained. Have.

  However, in the technique of the comparative example, when the thickness T1 of the partition wall 35 is larger than the thickness T2 of the collimator plate 37, the effective area of the scintillator element 34 correspondingly decreases, and the geometric efficiency decreases in proportion thereto. Further, since it is necessary to make the effective area (the width of which is indicated by W1 in FIG. 6) of the photoelectric conversion element 36 smaller than the effective area of the scintillator element 34 (the width of which is indicated by W2 in FIG. 6), The same problem as in the conventional technique arises in that the optical efficiency decreases in two stages and the output of the X-ray detector 3 decreases in two stages.

  Furthermore, there is a conventional problem that the photoelectric conversion element 36 is deteriorated or an abnormal output is generated by the X-ray 5 scattered through the partition wall 35. In FIG. 6, the X-ray 5 that has passed through the partition wall 35 is scattered and a part of the X-ray 5 that enters the photoelectric conversion element 36 is indicated by a broken line.

  The end face 38 of the scintillator element 34 (the face in contact with the partition wall 35) is a processed surface, and the sensitivity of X-rays is reduced. Then, since the end face 38 of the scintillator element 34 is covered with the overhanging portion 82 of the mask 8 and is not used as an effective area of the scintillator element 34, it is possible to prevent the occurrence of artifacts.

  In the embodiment, the mask 8 is provided separately from the partition wall 35, but may be formed integrally with the partition wall 35. At this time, the partition wall 35 may be formed of the same material as the mask 8. Further, the mask 8 may be formed integrally with the one end portion 37 a of the collimator plate 37.

  In the embodiment, the configuration in which the mask 8 is provided so that the scintillator element 34 is not shaded by the collimator plate 37 by X-rays when the focal point is moved along the channel direction (X direction) has been described. The configuration of this embodiment can be applied in the same manner when moving the body along the body axis direction (Z direction).

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, rewrites, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 1 Rotating ring 2 X-ray source 3 X-ray detector 31 Scintillator array 32 Photoelectric conversion element array 33 Collimator 34 Scintillator element 35 Bulkhead 36 Photoelectric conversion element 37 Collimator plate 37a One end part 37b Other end part 38 End face 39 of a scintillator element Center 4 Focus 5 X-ray 6 Computer 7 Display unit 8 Mask 81 Opening 82 Overhang 83 Tip

Claims (8)

In an X-ray detector that is disposed opposite to an X-ray source that generates X-rays around the body axis of the subject and detects X-rays by rotating in a direction around the body axis,
A scintillator array arranged along a direction around the body axis, and a scintillator array having a partition partitioning the adjacent scintillator elements;
A plurality of collimator plates provided corresponding to the partition walls; one end portion of each collimator plate is adjacent to the partition walls; and the other collimator plates in the height direction from the partition walls to the X-ray source A collimator having an end extending and restricting an incident direction of X-rays incident on the scintillator element;
A mask that absorbs the X-ray, and is disposed between the partition and the one end of the collimator plate so as to cover at least the partition from the incident direction of the X-ray;
An X-ray detector comprising:   The X-ray detector according to claim 1, wherein the mask is formed integrally with the partition wall of the scintillator.   The X-ray detector according to claim 1, wherein the mask is formed integrally with one end of the collimator plate.   2. The X-ray detector according to claim 1, wherein the mask has an overhanging portion that protrudes from a position of an end face of the scintillator element in contact with the partition wall toward a position of a central portion of the scintillator element.   The X-ray detector according to claim 4, wherein the projecting portion is provided according to a position of the other end portion of the collimator plate in the arrangement direction and the height direction, and the incident direction. The length from the position of the other end of the collimator plate in the arrangement direction to the position of the tip of the overhang is L1, and the overhang from the position of the other end of the collimator plate in the height direction. When the length to the position of the part is H and the maximum angle in the change range of the angle in the incident direction is θ1, the following relationship is established: L1 = H / tan θ1
The X-ray detector according to claim 5. The partition is formed in a lattice shape with the body axis direction and the direction around the body axis as vertical and horizontal directions,
The X-ray detector according to claim 1, wherein the mask is formed in a lattice shape corresponding to the partition wall.   An X-ray CT apparatus comprising the X-ray detector according to claim 1.

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