JPH0551295B2 - - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention provides an X-ray CT apparatus that reconstructs images by converting fan beam data obtained by an X-ray fan beam into parallel beam data. Regarding.

(Prior Art) As a conventional third generation X-ray CT apparatus, the apparatus shown in FIG. 9 is known.

An X-ray detection element array 2 consisting of an X-ray tube 1 and a large number of X-ray detection elements 2a arranged on an arc centered on an X-ray source 1a at equal intervals along this arc.
are placed facing each other. This apparatus rotates the X-ray tube 1 and the X-ray detection element array 2 around the subject 3 around the body axis of the subject 3 at the same time, and rotates the X-ray at every predetermined rotation angle. An X-ray fan beam 1b is irradiated from the tube 1, and the
The X
It has a configuration in which X-ray detection data of a large number of channels obtained during one rotation of the ray tube 1 is processed by a data acquisition device 4.

The image reconstruction method used in conventional X-ray CT equipment configured in this way is mainly a parallel beam algorithm that uses a parallel beam algorithm after converting fan beam data to parallel beam data.
There is a View method and a direct fan reconstruction method that directly reconstructs fan beam data.

The Parallel View method will be explained with reference to FIG. 10. Figure a shows a diagram in which the X-ray source 1a is located at point F ₀ (directly above). Next, as shown in Figure b, the X-ray source 1a moves from point F ₀ to point F ₁
When the X-ray beam 1c is rotated nΔβ, the X-ray detection element _D1 detects the X-ray beam 1c. Next, as shown in Figure c,
When the X-ray source 1a further rotates and moves (n+1)Δβ from the point F ₀ to the point F ₂ , the X-ray detection element D ₂ detects the X-ray beam 1c. Similarly, as shown in figure d, the X-ray source 1a moves from point F ₀ to point F ₃ (n+2)Δβ
After rotation, a detection element D ₃ (not shown) next to the X-ray detection element D ₂ detects the X-ray beam 1c. In the X-ray beams 1c thus obtained, the intervals L ₁₂ and L ₂₃ between the X-ray beams 1a are not constant, as shown in FIG. 10d. Therefore, even if the fan beam data is converted into parallel beam data, the beam data is non-uniformly spaced, so interpolation processing is required to interpolate this beam data into data that is equally spaced.
This increases the computation time, and furthermore, there is a problem that the interpolation causes blurring of the image.

Further, although the direct fan reconstruction method requires a short calculation time, it has the problem of blurring around the image.

In addition, as another conventional example, the present applicant has proposed an X-ray detector having X-ray elements arranged at unequal intervals and the
An X-ray tube that faces the radiation detector and is rotatably arranged around the subject and emits fan beam X-rays, and detects data of the exposed X-rays at different timings for each X-ray detection element. We are proposing an X-ray CT system equipped with a data acquisition circuit that performs sampling.

This device uses an ionization chamber-type X-ray detection element that has an electrode plate and a gas that ionizes upon receiving X-rays in a closed container. Since the electrode plates are spaced unevenly, the characteristics of the X-ray detection elements become uneven, resulting in a problem that uniform data cannot be obtained.

Further, if the electrode plates constituting the X-ray detection element are arranged at equal intervals, there is a problem that gaps are created between the X-ray detection elements, resulting in poor X-ray utilization efficiency.

(Problems to be Solved by the Invention) Conventional X-ray CT apparatuses required interpolation processing to convert fan beam data to equidistant parallel beam data, resulting in a long reconstruction time. Further, in a method using an X-ray detection element with electrode plates arranged at irregular intervals, which does not require this interpolation process, uniform data cannot be obtained.

SUMMARY OF THE INVENTION An object of the present invention is to provide an X-ray CT apparatus that can shorten image reconstruction time and obtain uniform data.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides an X-ray source that irradiates an X-ray fan beam, and detects the X-ray beam from the A large number of X-ray detecting elements are placed facing each other across the center of rotation, and
In an X-ray CT apparatus in which a radiation source and an X-ray detection element are rotated while maintaining their relative positions, and an X-ray beam is taken in at every predetermined rotational position of the X-ray source by an intake control means, the X-ray detection element is The center of the tip of the X-ray detection element on the X-ray source side and the X-ray source are arranged on a common circumference, and from a fixed point located point-symmetrical to the X-ray source with respect to the center of this common circumference. The present invention is characterized in that the adjacent X-ray detection elements are arranged so that the difference in distance to the center of the tip is constant.

(Function) When a subject is placed between the X-ray source and the X-ray detection element, the X-ray source and the X-ray detection element then move around the subject. Rotates at the same time while irradiating the fan beam. Then, the capture control means captures the X-ray beam at each rotational angular position of the X-ray source where the X-ray beams incident on adjacent X-ray detection elements become parallel. Due to the geometrical arrangement of the X-ray source and the X-ray detection element, the captured X-ray beams provide equally spaced and parallel data. Therefore, interpolation processing becomes unnecessary, and the time for image reconstruction can be shortened.

(Example) An example of the present invention will be described with reference to the drawings.

FIG. 1 is a configuration diagram of an X-ray CT apparatus according to an embodiment of the present invention, in which 1 is an X-ray tube having an X-ray source 1a, 20 is an X-ray detection element array consisting of a large number of X-ray detection elements 20a, 3 is the subject, 40 is the data collection device, 5
is an image processing device.

The X-ray detection element 20a is an ionization box type X-ray detection element that has an X-ray incidence space 20c on the side of the X-ray source 1a and is sandwiched between electrode plates 20b having a predetermined length d in the X-ray incidence direction. X-ray tube 1 and X-ray detection element array 20
and are located opposite each other with the subject 3 in between. The end 20d of the electrode plate 20b on the X-ray source 1a side is arranged on the common circumference C1 with the X-ray source 1a so that the distance P between the electrode plates 20b is equal, and the electrode plate 20b is , are arranged along the incident direction of the X-rays. The data collecting device 40 is a data collecting device 4 corresponding to the X-ray detection element 20a which is a capturing means.
It consists of 0a.

The X-ray tube 1 and the X-ray detection element array 20 maintain their opposing positions and rotate around the subject 3 while irradiating the subject 3 from the X-ray source 1a. The X-rays transmitted through the object 3 are transmitted to the X-ray detection element array 2
0, and this detected amount is detected by the X-ray detection element 20a.
After the data collection device 40 collects data at different timings, the image processing device 5 reconstructs the image and displays the image.

FIG. 2 is a diagram showing in detail the positional relationship between the X-ray source 1a and the X-ray detection element 20a.

Point F ₀ indicates the position of the X-ray source 1a, and 1c indicates the X-ray beam incident on the X-ray detection element 20a. The X-ray source 1a and the X-ray focal point 1a of the X-ray detection element 20a
Let G be the point where the circumference of the common circle C1 in which the side tip center E is arranged intersects with a line passing through the center M of the circle C1 from point _F0 . Point G is the X-ray detection element 20
This is a reference position that serves as a reference for determining the position of a. The center of rotation is O, the midpoint of line segment ₀ is N, and the circle centered at this N is C2. n-th and n+1-th X-ray detection elements 2 counterclockwise from point G
The center of the tip on the point F ₀ side of 0a is set as E _o and E _o+1 , respectively, on the circumference of the circle C1. Then, if the lengths of line segment _o and line segment _o+1 are h _o and h _o+1 , respectively, then Δh
(=ho _{+1 −ho} ) remains constant regardless of n. Therefore, the relationship is _o+1 = h _o+1 = (n+1)Δh.

Also, the tip of the electrode plate 20b on the point _F0 side is also connected to the circle C
The X-ray beam 1c is located on the circumference of the X-ray beam 1c, is located midway between the tip centers E and E, and has a length d along the X-ray beam 1c.

Next, let the points where line segment F ₀ E _o and line segment _{0 o+1} intersect with circle C1 be P _o and P _o+1 , respectively, and the length of line segment _o is r _o and the length of line segment _o+1 is Let the value be r _o+1 . Since the point E _o is on the circumference of the circle C1 and the point P _o is on the circumference of the circle C2, ΔF ₀ GE _o and ΔF ₀ OP _o form a right triangle.
Furthermore, since ∠GE ₀ E _o =∠OF ₀ P _o , ΔF ₀ GE _o and ΔF ₀ OP _o are similar. This similarity relationship means that the X-ray source 1a (point F ₀ ) and the tip center E on the point F ₀ side of the X-ray detection element 20a are located in a common circle C.
It is because it is on the circumference of 1 that it is guided.

Therefore, r _o / _ho = ₀ / ₀ , and if this _equation is transformed using the above relational expression, r _o = _ho・( _0/0 ) =n・Δh・(F ₀ O/F ₀ G) Here, if Δr=r _o+1 −r _o , Δr=(n+ ₁ )・Δh・( _0/0 ) −n・Δh・( _0/0 )= _{(0/0 ) ・}Δh, and Δr is constant regardless of n.

FIG. 3 shows how parallel and equally spaced projection data are obtained as the X-ray source 1a and the detector array 20 rotate.

The electrode plate 20b is arranged on a line connecting the X-ray source 1a and the rotation center O, and when the X-ray source 1a rotates from point _F0 by a predetermined angle _β1 to point _F1 ,
X-ray beam 1 is placed at the center E ₁ of the tip of the ray detection element 20a.
c is incident, and when the X-ray source 1a is further rotated by a predetermined angle β ₂ from point F ₀ to point F ₂ , the X-ray beam 1c is incident on the tip center E ₂ of the X-ray detection element 20a, and the When the X-ray beam 1a rotates to the position _F3 , the X-ray beam 1c is incident on the center _E3 of the tip of the X-ray detection element 20a. In this way, the interval between the X-ray beams 1c incident on the X-ray detection element 20a becomes constant Δr.

In the device of this embodiment, the pair of electrode plates 20b and the space sandwiched between the electrode plates 20b serve as the X-ray detection element 20.
a, and an electrode plate 2 is placed at a position facing the point _F0 .
Since 0b is placed, Δr/2 from the center O
The space at the shifted location is detected by the X-ray detection elements 20a on both sides with the electrode plate 20b directly below in between.

Next, the timing of collecting equidistant parallel beams will be explained using FIGS. 4 and 5.

Among the X-ray detecting elements 20a with the electrode plate 20b directly below facing the point _F0 shown in _FIG . Let it be D _o . Then, as shown in Fig. 4, the X-ray detection element DO detects the X-ray beam XO that is Δr/2 away from the center 0, and detects the X-ray beam _XO that is parallel to the X-ray _beam The X-ray detection element D _o detects it, and in this way, equally spaced parallel beam data is obtained.

In Figure 4, the position of the X-ray source 1a that emits the X-ray beam XO shifted by Δr/2 is F ₁ , and the vertical line ₀ and the line segment
Let the angle between it and OF ₁ be θO. For example, as shown in Figure 5, U is tilted by an angle θ from the vertical line _0.
When the X-ray detection element D _o detects parallel data at equal intervals parallel to the direction, the angle between the vertical line ₀ and the line segment _o is β, and the angle between the line segment _o and _{the line segment o is} φ _o . Then, β is expressed as β=θ+φ _o . Image reconstruction is facilitated by collecting data at an angle θ between the projection direction U and the vertical line ₀ that is an integral multiple of a constant pitch angle Δθ. θ is expressed as θ=θO+kΔθ (k=0, 1, 2,...).

Also, when the X-ray source comes to the position F _o where β = β _k = θ _k + φ _o , the data acquisition device samples the output of the X-ray detection element D _o , so β = kΔθ + (θ o + φ _o ) You can sample when . In other words, since φ _o is constant regardless of θ corresponding to the X-ray detection element D _o , any X-ray detection element 20a has Δθ
The sampling system is simple because it is sufficient to repeat sampling at angular positions that are integral multiples of .

Furthermore, if Δθ is determined so that 2π/Δθ is an integer, as in the conventional technology, if the point F rotates at least once around the center O, the necessary data θ=θO+kΔθ(k=0,1 ,...,2π/Δθ−
1) All of r= _ro (n=-N to N) are present.

Next, the data importing operation will be explained with reference to the drawings. FIG. 6 shows the data collector 40 of the device.
This is a detailed block diagram of ai, and FIG. 7 is a time chart for explaining its operation.

The X-ray intensity detected by the X-ray detection element 2ai is converted into a current corresponding to this X-ray intensity, which is input to an integrator 40b in the data collector 40ai and integrated. When the signal SO from the timing generator 6 enters the timing delay circuit 40d, after a delay time t ₁ (see FIG. 7) previously set in the ROM 40c for each X-ray detection element 2ai, the timing delay circuit 40d /H (sample hold circuit)
A signal _S1 is output to 40e. According to this signal _S1 , the S/H (sample and hold circuit) 40e samples the output integrated by the integrator 40b, and the analog data sampled by the S/H 40e is output to the ADC 40f and converted into digital data. Sent to an image processing device. Immediately after the signal S ₁ is output (after a predetermined time t ₂ has elapsed), the timing delay circuit 40d outputs the signal S ₂ to the integrator 40b to reset the data in the integrator 40b.

In this way, the X-ray source F ₀ and the tip center E _o of the X-ray detection element D _o are arranged on a common circle C1, and the difference in distance from the reference position G to the tip center E is a constant Δr. Therefore, since data can be collected at a predetermined timing for each X-ray detection element, equally spaced parallel converted data can be obtained without interpolation processing.

Further, the electrode plates 20b are arranged at a constant distance P for any X-ray detection element 20a, and all the electrode plates 20b face the X-ray source 1a, so that each Since the sensitivity characteristics of the radiation detection element 20a are uniform, the X-ray intensity after passing through the object can be detected faithfully.

Furthermore, in the conventional image reconstruction method, direct fan reconstruction, the area around the scan area would be blurred due to errors, but in this example, it is changed to equally spaced parallel beams, so an accurate reconstructed image can be obtained. is obtained.

In addition, the X-ray source F and the tip 20d of the electrode plate 20b
Since these are arranged on the circumference of a common circle C1, the scanner can be made smaller. This can be seen in Figure 8a.
A conventional scanner is shown in FIG. 1, and a scanner according to an embodiment of the present invention is shown in FIG. Point F ₀ of X-ray source 1a
The distance F ₀ OD between the rotation center 0 and the diameter SED of the image reconstruction area are the same, and the two are compared. Minimum clearance between X-ray tube 1 and cover COVa in figure a
G3 has X-ray detection element array 2 and cover COVa
Under the condition that the minimum gap G2 is larger than the appropriate minimum gap G2, the scanner can be downsized with the device of this embodiment. In other words, the X-ray tube 1 and cover COVb in Figure b
The minimum gap G1 between the X-ray detection element array 20 and
The minimum gap G2 between the cover COVb and the cover COVb can be made as small as the minimum gap G2, and the scanner dimension Hb of the apparatus of this embodiment can be made smaller than the conventional scanner dimension Ha.

Although one embodiment has been described above, the present invention is not limited to this, and various modifications can be made without changing the gist thereof.

For example, although the reference position G is set to be a position on the circle _C1 facing the point _F0 , it may be set to a position on the circle C1 that is deviated from this point.

Further, an electrode plate is provided on the extension of the line passing from the X-ray source 1a through the rotation center 0, for example, Δh/
Each X-ray detection element may be arranged so that the X-ray detection element is provided on the extension with a shift of 2.

Further, although the X-ray detection element is an ion chamber type X-ray detection element, similar effects can be obtained by using a solid-state detection element using a semiconductor or other X-ray intensity detection means.

[Effects of the Invention] Since the present invention is configured as described above, it produces the following effects.

In the X-ray CT apparatus according to claim 1, the geometric arrangement of the X-ray source and the X-ray detection element enables fan beam data to be obtained without the need for interpolation processing to make irregularly spaced parallel data into equal spaces. Can be converted to equally spaced parallel beams. Therefore, it is possible to provide an X-ray CT apparatus that can shorten image reconstruction time and obtain uniform data.

Furthermore, in the X-ray CT apparatus according to claim 2,
In addition to the effect described in claim 1, since the X-ray detection elements in which the electrode plates are equally spaced are used, the sensitivity characteristics between the X-ray detection elements are uniform, and the X-ray intensity after passing through the object is faithfully detected. can.

In the X-ray CT apparatus of claim 3, claim 1
In addition to the effects described, the use of a solid-state detector that is resistant to vibrations provides a more stable signal.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an X-ray CT apparatus according to an embodiment of the present invention, FIG. 2 is a diagram showing the positional relationship between an X-ray source and an X-ray detection element, FIGS. 3 a to d, FIGS. 4 and 5 6 is a block diagram showing the configuration of a specific X-ray detection element and data processor, FIG. 7 is a time chart showing its operation, and FIG. 8a is a conventional FIG. 8b is a front view of a scanner according to an embodiment of the present invention, FIG. 9 is a configuration diagram of a conventional X-ray CT device, and FIGS. 10a to 10d are diagrams of the X-ray beam of the conventional device It is an explanatory diagram of import. 1...X-ray tube, 1a...X-ray source, F...X-ray source position, 1b...X-ray fan beam, 1c...X
Ray beam, 20...X-ray detection element array, 20a
...X-ray detection element, 20b...electrode plate, 3...subject, 40...data acquisition device (intake means),
C1...Circle (common circle), E...Tip center, Δh...
...difference in distance.

Claims (1)

[Scope of Claims] 1. An X-ray source that emits an X-ray fan beam and a number of X-ray detection elements that detect the The X-ray source is rotated while maintaining the relative position of the X-ray source and the X-ray detection element, and an X-ray beam is captured at each predetermined rotational position of the X-ray source by a capture control means.
In the CT apparatus, the X-ray detection element is
The center of the tip of the X-ray source side of the radiation detection element and the X-ray source are arranged on a common circumference, and the X-ray source and the center of the tip of the radiation detection element are arranged on a common circumference. An X-ray CT apparatus characterized in that the adjacent X-ray detection elements are arranged so that the difference in distance to the center of the tip is constant. 2 The X-ray detection element is an ionization box type X having an electrode plate and a gas that ionizes upon receiving X-rays in a closed container
The X-ray CT according to claim 1, wherein the X-ray CT is a radiation detection element, and the electrode plate is arranged along the incident direction of the X-ray beam.
Device. 3. The X-ray CT apparatus according to claim 1, wherein the X-ray detection element is a solid-state detection element having a scintillator that emits light upon receiving X-rays, and a photodetector that converts the light from the scintillator into an electrical signal.