JP3711166B2 - X-ray CT system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray CT apparatus. In computed tomography currently used in medical technology, a thin fan beam consisting of the X-ray beam of an X-ray emitter is emitted, and the beam transmitted through the object to be detected is detected using individual detector rows. Thus, photographing data necessary for the cross-sectional image is obtained. At this time, the length of each detection element in the z direction (the direction of the system axis) is such that the beam can be received when each detection element has the largest adjustable slice thickness (usually a 10 mm slice). Determined.
[0002]
[Prior art]
The thickness of the various slices is formed by adjusting the diaphragm near the X-ray tube and the diaphragm on the detector side accordingly. Since such an apparatus can only capture data for one slice each time, the X-ray beam emitted by the X-ray emitter is used very inefficiently. Therefore, in general in three-dimensional imaging techniques, the detected volume is limited by the effective continuous output of the X-ray emitter. The required imaging and inspection times are correspondingly longer.
[0003]
With a surface detector according to US Pat. No. 5,291,402 such limitations of the measurement system are well overcome. The surface detector is a two-dimensional array of detector elements (mosaic). That is, the surface detector is constituted by a plurality of parallel detector rows. Therefore, an X-ray beam bundle extending in the z direction can be used for image construction instead of a thin fan beam. In contrast to a conventional single row detector, a surface detector consists of detector elements that are individually arranged in the z direction. During the rotation of the measurement system, a large number of slices can be taken simultaneously according to the spread of the surface detector in the z direction. At this time, adjacent rows of the surface detector detect adjacent slices. Therefore, the length of the detector elements in the z direction is selected to detect the desired slice (usually a 1 mm slice) with the smallest detector row.
[0004]
That is, the surface detector has an array of N rows in the z direction, and M detector elements are arranged in one row, for example, in a direction perpendicular to the z axis. Thus, the surface detector consists of N × M detector elements. The configuration of the detector elements perpendicular to the z-axis is advantageously designed in the shape of an arch, with the X-ray focus being formed in the center of this arch.
[0005]
The surface detector consists of a mosaic of N × M detector elements and has the great advantage of using the X-ray output very efficiently by imaging a volume or partial volume simultaneously.
[0006]
On the other hand, the surface detector has the following drawbacks.
[0007]
a) When reading the individual detector elements, a correspondingly large number of N × M detector channels are required.
[0008]
b) Compared to a CT device with a single row detector, the amount of data detected increases in proportion to the number N of rows.
[0009]
c) Reconstruction using filter backprojection, known as planar tomography, is inaccurate for slicing the edge region of the X-ray bundle and at least requires a very expensive computational process is necessary. As an alternative method, a true three-dimensional method can be used for image reconstruction (European Patent Application Publication No. 0526157). However, in this case, the calculation cost is significantly increased as compared with the back projection or Fourier reconstruction by the conventional filter. Correspondingly, the required computer output increases.
[0010]
[Problems to be solved by the invention]
The problem of the present invention is that in an X-ray CT apparatus having a surface detector, with respect to readout, data volume and reconstruction, the significant complexity of the surface detector is sufficiently avoided and reduced to a practical level, and at the same time important Is to keep the benefits.
[0011]
[Means for Solving the Problems]
The above problem is solved by the present invention.
The detector consists of a plurality of parallel detector rows, each of the detector rows being formed by a single column of detector elements,
The X-ray beam bundle is rotatable around the system axis, and a detector signal formed during various radiographs is supplied to the computer, which calculates an image of the object to be examined from the detector signal,
An adjustable aperture is provided to select the number of detector rows that the x-ray beam strikes, and a multiplexer connects a predetermined number of output channels to the selected detector row. If the number of detector rows selected using the aperture is greater than the number of output channels, the multiplexer solves by grouping the output signals of detector rows adjacent to each other. The
[0012]
In this regard, the basic idea is as follows.
[0013]
An N-row surface detector with M detector elements per row is fabricated for imaging N slices with the smallest slice thickness w _o (eg, w _o = 1 mm). Depending on the actual imaging mode, eg slice thickness w> _wo , the output signals of one row of detector elements are grouped appropriately. Therefore, the number n of necessary read channels is n <N. At this time, of course, it is desirable that n can be kept constant. Thus, if not all N rows of the surface detector are used, the corresponding X-ray tomogram can be formed with an adjustable aperture.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be described in detail based on embodiments with reference to the drawings.
[0015]
FIG. 1 shows a main part of an X-ray CT apparatus according to the present invention. FIG. 1 shows a focal point 1 of an X-ray emitter, from which a fan-shaped X-ray beam bundle 2 formed by a diaphragm not shown is emitted. The X-ray beam bundle 2 passes through the test object 3 and hits the detector 4. The detector 4 consists of a number of parallel detector rows, each of which is formed by a single column of detector elements. The measurement systems 1 and 4 can rotate around the system axis 6, so that the test object 3 is transmitted and irradiated under various imaging. From the detector signal formed at this time, the computer 7 calculates an image of the subject 3 to be reproduced on the monitor 8. Acquisition of the detector signal is performed by a multiplexer 9 which will be described in detail with reference to FIGS.
[0016]
The detector 4 is only shown schematically in FIG. 2 to 5 show that the detector 4 consists of 16 parallel detector rows 4a to 4p in the embodiment based on these figures. A diaphragm 10 that can be adjusted in the z direction is provided in the beam direction seen in front of the detector 4. The number of detector rows 4a to 4p that are hit by the X-ray beam bundle is selected by the diaphragm 10. The multiplexer 9 connects a predetermined number of output channels, in this embodiment four output channels, to the selected detector rows 4a-4p. If the number of selected detector rows 4a-4p is greater than the number of output channels, the multiplexer 9 groups the output signals of adjacent detector rows into a group.
[0017]
In the aperture position of FIG. 2, four detector rows 4g-4j are connected to four output channels 11-14. In the aperture position of FIG. 3, detector rows 4e-4l are connected to four output channels 11-14. Corresponding to detecting the output signals of the eight detector rows, the output signals of two adjacent detector rows, eg 4e and 4f, are grouped together. 4, the output signals of the detector rows 4c to 4n are detected, and the output signals of three mutually adjacent detector rows are grouped into one group correspondingly. Further, in FIG. 5, since the diaphragm 10 is fully opened, the output signals of four mutually adjacent detector rows are combined into one group.
[0018]
In this embodiment, the detector 4 includes N = 16 detector rows 4a to 4p, and is suitable for imaging 16 slices with a layer thickness of 1 mm. However, for reading, only n = 4 channels need be used for each column. Therefore, the total number of channels is 4 × M.
[0019]
By squeezing accordingly, the innermost four detector rows 4g-4j are used in a 1 mm slice. The outputs of the detector rows 4g-4j are switched to the effective channels 11-14, respectively (FIG. 2).
[0020]
When operating with a slice thickness of 2 mm, the innermost eight detector rows 4e-4l are used and the output signals of the detector rows 4e-4l are grouped by two each. The four output signals of each pair generated in this way are subsequently switched to channels 11 to 14 (FIG. 3).
[0021]
Similarly, 4 slices with a layer thickness of 3 mm are each formed by combining the signals of three adjacent detector elements (FIG. 4), or 4 slices with a layer thickness of 4 mm each. It is formed by combining the signals of adjacent detector elements.
[0022]
From such a reference mode having a clinically important slice thickness of 1 mm to 4 mm, a larger slice thickness, for example 6 mm or 8 mm, can be formed by a separate addition step. In such a case, the detector 4 operates as an effective two-row detector.
[0023]
In order to actually realize, an adder element having at least four input sides is required based on FIGS. Depending on the thickness of the selected slice, up to four output signals of the detector element are connected to the input side of the summing element. The output side of the adding element is fixedly connected to the four channels 11 to 14 of the readout electronic device.
[0024]
It is self-evident that the selected numerical examples are merely exemplary in nature and can be arbitrarily expanded (eg, N = 32 detector rows and n = 8 readout channels).
[0025]
There is little time requirement for the process of switching from the detector signal to the summing element. This is because such switching is fixed at the beginning of the shooting mode and can be maintained without change during the shooting period. However, in the spiral technique, it is also possible to switch the slice thickness during the volume imaging period.
[0026]
【The invention's effect】
The detector arrangement is simplified as already described with respect to readout electronics, data volume and image reconstruction compared to a fully readable surface detector. Compared with the single row detector of the conventional CT apparatus, the present detector apparatus has the following important advantages.
[0027]
a) Depending on the choice of n and N, n slices with the maximum layer thickness determined by N / n can be taken simultaneously.
[0028]
b) In the device corresponding to the addition circuit selected for photographing the volume in the spiral technique, n effective single-row detectors are used for the width w of the slice to detect the data. At this time, the pitch P = (horizontal forward feed (unit, mm) every 360 ° rotation of the measurement system) / (slice thickness (unit, mm)) can be largely selected. For example, if P = n or P = n + 1, the volume can be scanned faster by a factor n or n + 1 compared to a single row detector. At this time, P = n (or P = n + 1) corresponds to the effective pitch p ₁ with reference to an effective detector row of p ₁ = 1 (or p ₁ = 1 + 1 / n). Thus, the same scan density is actually obtained in the z direction despite the faster volume scan.
[0029]
c) The choice of P is based on actual demand. In order to avoid many scans in the spiral mode, it is generally preferable to select an odd number P (for example, P = n + 1) for an even number n. If n is an odd number, P = n can also be selected.
[0030]
d) In particular, scanning in the z direction is desirable to photograph thin slices. This can be done by selecting 1 <P <n. For a single row detector with p ₁ <1, the time factor related to the imaging time remains P / p ₁ >> 1.
[0031]
e) Multiplex switching of the detector elements of the present invention can directly use methods that reduce non-linear partial volume effects. That is, by operating the detector in the 4 × 1 mm mode and adding signals (digital) of adjacent 1 mm slices, a slice of about 2 mm can be formed. At the same time, the partial volume artifacts are reduced and a corresponding correction signal / noise ratio of the image is obtained. This is also true for various slice thicknesses.
[Brief description of the drawings]
FIG. 1 shows a main part of an X-ray CT apparatus according to the present invention.
FIG. 2 shows the detector of the CT apparatus of FIG. 1 with a multiplexer corresponding to a slice thickness.
FIG. 3 shows the detector of the CT apparatus of FIG. 1 with a multiplexer corresponding to the thickness of a slice.
FIG. 4 shows the detector of the CT apparatus of FIG. 1 with a multiplexer corresponding to a slice thickness.
FIG. 5 shows the detector of the CT apparatus of FIG. 1 with a multiplexer corresponding to a slice thickness.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Focus 2 X-ray beam bundle 3 Test object 4 Detector 4a-4p Detector row 6 System axis line 7 Computer 8 Monitor 9 Multiplexer 10 Diaphragm 11-14 Output channel

Claims (1)

In an X-ray CT apparatus having an X-ray emitter (1) that emits a fan-shaped X-ray beam bundle (2) and a detector (4) that the X-ray beam bundle (2) hits,
The detector (4) consists of a plurality of parallel detector rows (4a-4p), each of the detector rows (4a-4p) being formed by a single column of detector elements,
The X-ray beam bundle (2) can be rotated around the system axis (6), and detector signals formed during various radiographs are supplied to the computer (7), and the computer (7) Calculate the image of the test object (3) from the instrument signal,
An adjustable aperture (10) is provided to select the number of detector rows (4a-4p) that the X-ray beam bundle (2) will hit,
Multiplexer (9) is provided for connection to pre-determined plurality of output channels (11-14) selected detector rows number (4a-4p),
If the number of detector rows (4a to 4p) selected using the diaphragm (10) is greater than the number of output channels (11 to 14), the multiplexer (9) is adjacent to each other of the detector rows (4a An X-ray CT apparatus characterized by grouping output signals of ˜4p) into groups.

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