JPS6251622B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention comprises an X-ray source for generating a flat fan-shaped X-ray beam and an X-ray detector consisting of a large number of detection elements arranged along an arc, The present invention relates to an apparatus for computer tomography in which a radiation source and an X-ray detector are rotatable about a common axis of rotation perpendicular to the plane of the fan-shaped X-ray beam.

Such a device is particularly suitable for X-ray diagnostics. During such a diagnosis, a certain part of the patient's body is illuminated from various directions by means of a flat fan beam.
The locally transmitted radiation is measured, and the measurement data thus obtained is used to calculate the concentration distribution of the patient's body part in the irradiation plane using a computer, and the results are displayed on, for example, a television monitor. .

A device of the above-mentioned type is known from the already published Dutch patent application no. It uses a line beam. In order to obtain a suitable number of measurement data, the X-ray source and X-ray detector are rotated together around the patient, which is placed near their common axis of rotation. The device comprises means for compensating for the effect on the measurement signal of differences in sensitivity of the various detection elements.

The X-ray source and X-ray detector of conventional devices rotate together about a common axis of rotation at a uniform speed during diagnosis. The output signal of the detection element is integrated over a short period of time in order to obtain a reliable measurement value, during which the X-ray source and the X-ray detector are rotated by a small angle, for example 1 DEG. Then, the logarithm value of the radiation intensity measurement is determined. Therefore, during diagnosis, each detection element detects the X that passes through the patient's body from various directions.
- provides a set of logarithmic values for the line intensity; During diagnosis, these data are stored in a first electronic memory;
After completion of the diagnosis, the data arriving from the multiple detection elements are classified into multiple sets of logarithmic values relating to the intensity of the transmitted X-rays along sets of parallel paths oriented in corresponding directions. The logarithmic values of each of these sorted sets are stored in a second electronic memory. Then, the local density reproduced as pixels on the irradiated surface of the diagnosed human body part is calculated by a reconstruction method. This means that the measurement data for all measurement paths are added from the logarithmic values of each of the classified sets for the corresponding reproduction pixel.

With conventional equipment, when displaying the calculated concentration distribution,
The occurrence of a troublesome annular interference pattern called an annular virtual image pattern that does not actually exist is suppressed by correcting the difference in sensitivity of the detection elements. This makes the fast motion of the X-ray source
- separately superimposed on an equal common rotational movement of the radiation source and the X-ray detector, so that each measurement by each detection element is repeated by the detection element adjacent to each element during diagnosis; The detection elements are compared with each other based on the measured values obtained in this manner, and differences in sensitivity of the detection elements are corrected. Additional electronic processing circuitry is therefore provided for this correction purpose.

It is an object of the present invention to prevent the occurrence of annular virtual image patterns from becoming apparent due to differences in sensitivity of the detection elements, and to provide a set of images transmitted through the patient in parallel directions by each detection element during diagnosis. It is an object of the present invention to provide an apparatus for computer tomography, which generates measured values regarding the X-ray intensity of X-rays, thereby making it possible to omit classification of the measured values after the diagnosis is completed.

The present invention includes an x-ray source for generating a flat fan-shaped x-ray beam, the x-ray source having a relatively small x-ray emitting area that is substantially fixed relative to the body of the x-ray source during operation. and further comprising an X-ray detector consisting of a number of detection elements arranged along an arc to receive X-ray radiation from the fan beam after passing through the object under diagnosis. , the X-ray detection area of each of the detection elements is made relatively small, the first and second rotatable support members are rotatably supported by fixed support means, and the first and second rotatable support members are rotatably supported by fixed support means.
a rotatable support member is rotatable about a common axis oriented perpendicular to the plane of the fan beam, and the X-ray source is oriented such that the fan beam is oriented generally toward the common axis. is fixed to the first rotating support member, and the distance between the X-ray detection surface of each detection element and the common axis is the same as the distance between the X-ray emission area of the X-ray source and the common axis. the X-ray detector is fixed to the second rotary support member so that the distances are substantially equal to each other; An apparatus for computer tomography is characterized in that it is configured to include means for rotating in the rotational direction of the computer tomography apparatus.

To obtain adequate measurement accuracy during diagnosis, the output signal of the detection element is integrated over a short period of time, during which the X-ray source and the X-ray detector are rotated through a small angle, e.g. 1°. . The X-ray source and X-ray detector move in opposite directions, so that the body part to be diagnosed is exposed to the X-ray in a continuous, generally parallel path.
- scanned by a radiation source and a defined detection element;
This means that each calculated value regarding the density reproduced as each pixel of the irradiated surface includes the measured value by each detection element. Therefore, when displaying the calculated concentration distribution, interference patterns caused by differences in sensitivity of the detection elements can be substantially reduced. Furthermore, the annular virtual image pattern can also be reduced.

In a preferred implementation of the computer tomography apparatus according to the invention, the X-ray detector is constructed with a large number of detection elements arranged in a circular arrangement. By doing this, the X transmitted in the parallel direction
- Each set of measurements regarding the intensity of the line can be determined for all directions of the diagnostic plane.

The invention will be explained with reference to the drawings.

1 and 2 show a computer system according to the present invention.
FIG. 1 is a longitudinal and transverse cross-sectional view showing an example of a tomography device. A patient 1 placed on a patient table 2 is irradiated with a flat fan-shaped X-ray beam 3. This X-ray beam 3 has an aperture angle (hereinafter referred to as sector angle α) of, for example, 30° in the plane of the drawing of FIG. 2, which is relatively flat with respect to the direction perpendicular to said plane. Yes, its thickness is approximately 10mm
It is. The fan angle α is large enough so that the beam 3 straddles the entire patient in its fan direction. X
- The ray beam 3 is generated by an X-ray tube 4 with a rotating anode (not shown). The emitting surface of the rotating anode (the actual X-ray source) is relatively small, with a length and width of approximately 2 mm, so that the X-ray source can be considered as point-shaped in practice. The radiation transmitted through the patient is measured by an X-ray detector 5. The detector 5 includes a series of, for example, 400 detection elements 6 arranged in a circular manner. As will be clear from the description below, the dimensions of the detection elements 6 are relatively small, and these elements can also be considered as point-like elements in practice, and these elements may be composed of, for example, a scintillation crystal and a photodetector. do. The X-ray tube 4 is mounted on a ring 7, which constitutes a rotatable support member, which is supported by a wheel 10, which is mounted on a sling 9, which constitutes a fixed support means. This ring 7 can be rotated by a drive motor 12 about an axis 14 extending at right angles to the plane of the fan beam 3. The actual X-ray source, ie the X-ray emitting surface at the surface of the rotating anode, is rotated about the axis 14 so that it follows a circular path 15. X
- connecting the line detector 5 to a ring 8 forming a further rotatable support member; This ring 8 is supported by a wheel 11 connected to a sling 9 so that it can be rotated about an axis 14 by a drive motor 13. Rings 7 and 8 are rotated in opposite directions during diagnosis. The detection element 6 rotates in a circular path, the radius of which is approximately equal to the radius of the circular path in which the actual X-ray source rotates, the radius of each of these circles being e.g.
cm and 85cm. Next, processing of measurement data will be explained in detail with reference to FIG.

FIG. 3 shows the data processing circuitry for a device of the type described above, having a patient 20, an X-ray source 21 and an X-ray detector 22 with a series of detection elements 23 arranged in a circular manner. X-ray source 21 and detection element 23 are rotated about a central axis (14 in FIG. 1) in opposite directions so that they follow substantially the same circular path 24 during diagnosis. All detection elements are connected to a corresponding integrating circuit 25 (only three detection elements are shown in the drawing), and this circuit integrates the measurement signal of the detection element 23 over a short period of time. The integration time is, for example, 10 milliseconds to obtain adequate measurement accuracy. Since the X-ray source 21 and the X-ray detector 22 move in opposite directions,
The region of the human body 20 to be diagnosed is scanned in a relatively generally parallel continuous path as shown in FIG. FIG. 4 shows the positional relationship between the X-ray source 21 and one detection element 23 for successive instants t _a , t _b , t _c . . . _th . Since the X-ray source 21 and the X-ray detector 23 follow approximately the same circular path 24 during the diagnosis, the distance between the X-ray source and the detection element changes during the diagnosis and is therefore X- to be measured
The intensity of the radiation varies independently of the absorption of the X-rays in the local area of the patient during diagnosis. These changes are corrected by circuit 26. After this correction, a logarithmic amplifier 27 forms a logarithmic value of the signal measured by the detection element, and the signal thus obtained is stored (stored) in a memory 28. During diagnosis, the detection element 23 thus generates a set of logarithmic values for the intensity of the X-rays transmitted through the patient in approximately parallel directions. After the diagnosis is completed, the computer 29 calculates the local concentration distribution in the irradiated portion of the human body 20 using a reconstruction method, and displays this on, for example, a television monitor 30.

FIG. 5 is a perspective view showing an example of an X-ray detector 31 particularly suitable for use as a detection element in a computer tomography apparatus such as that described above, and this detector 31 has a diameter of, for example, 5 mm. It includes a cylindrical scintillator 32 with a length of 20 mm and a photodetector 33 connected in the axial direction of the scintillator.
X-ray beams incident on the detector from different directions (only two X-ray beams 34 and 35 are shown) are detected in the same way due to the cylindrical shape and symmetry of this detector. These characteristics of the detector are particularly suitable for the above-mentioned applications. The reason for this is that the direction of the X-rays incident on the detection element constantly changes during diagnosis. Since this change in the direction of incidence of the X-rays is independent of the degree of absorption of the X-rays by local areas of the patient, such changes have no effect on the measurement signal.

One form of scanning operation that can be performed with the above-described apparatus will now be described in detail with reference to FIG. Here, a circle 41 indicates the path followed by an X-ray detector 43 comprising a point-like X-ray source 42 and a series of detection elements 44 . The X-ray source 42 is shown in position at three instants: at the beginning of the diagnosis, t=0, at the end of the diagnosis, t=T, and at moments in between. Detector 43 is shown in its position at time t=0. During diagnosis, the X-ray source 42 moves around a common central axis passing through the center M of the circle 41 with a uniform angular velocity ω, and the total angular displacement of the X-ray source and detector is ω·T. The fan angle α of the X-ray beam is determined by the diaphragm 45. This diaphragm 45 is moved together with the X-ray source so that the center of the X-ray beam is always directed towards the center M of the circle 41. During diagnosis, each detection element produces a set of measurements regarding the intensity of the X-rays transmitted through the patient in a relatively parallel direction (this direction is located between X-rays 46 and 47). The two X-rays 46 and 47 enclose the diagnostic angle φ. As is readily apparent from the triangle defined by the position of the X-ray source at the instants t=0 and t=T and the intersection of the X-rays 46 and 47, ω·T=α+φ. Since radiation is only measured in a relatively parallel direction located between the outermost X-rays 46 and 47, the X-ray beam other than X-ray 46 at instant t=0 and the instant t= X- other than X-ray 47 at time T
The ray beam is screened by another diaphragm 48 moving together with the X-ray source 42. This considerably reduces the patient's radiation exposure. diaphragm 4
Since 8 is only translated from left to right in FIG. 6, the opening angle β of the diaphragm is equal to φ, as is clear from this figure. During diagnosis, the detector 43 follows a circular path 41 in the direction shown at an angular velocity ω. Up to time T, the detection element measuring the X-ray 47 at instant t=T is displaced by an angle ωt from its initial position. In order to make diagnosis possible, the detection angle θ of the detector should be equal to ω・T+Δ, and for this purpose (6th
As is clear from the figure) Δ=360−ω・T−2
(180−α−Δ)=ω・T−2α, θ=2
It is necessary to ensure that (ω·T−α)=2φ.

As is clear from the above, sector angle α=60
In order to be able to perform a diagnostic scan over φ = 180° at
It is necessary to rotate it in the opposite direction over T=240° so that the detection angle θ of the detector is θ=360°. In this case, the opening angle β of the diaphragm 48 is preferably set to β=180°. It is clear that when the detection angle θ=360°, each detection element 43 is used twice and ωT is set to 420°, thereby increasing the diagnostic angle φ to φ=360°.

If the diagnostic angle φ is 180° or more, the first and second
Although the illustrated X-ray tube 4 and X-ray detector 5 cannot move in practically the same circular path, it is possible for them to move in circular paths that differ by a few centimeters in radius. In this case, during diagnosis, each detection element makes a set of measurements regarding the intensity of the radiation transmitted through the patient in a number of relatively not exactly parallel directions (X-ray source and detector moving with the same angular velocity). supply Such non-parallelism is caused by a sector angle of approximately
30° and the radius of the circular path is very small for a length of 100 cm, and the effect of this non-parallelism on the calculation of the local concentration at the X-ray irradiation plane is negligible. Such non-parallelism can be
This can be corrected by rotating the X-ray source moving in a smaller circular path slightly faster than the X-ray detector.

In the case of a sector α = 30°, the detection angle of the detection element 44 of FIG. 6 used at each instant is only 60°, so that for example three or five By arranging the X-ray source in the circular channel 41 in the form of an equilateral triangle or a regular pentagon, the diagnostic time can be accelerated.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view showing an example of a computer tomography apparatus according to the present invention, and FIG.
3 is a block diagram showing an example of a data processing circuit for the apparatus of the present invention, and FIG. 4 is an explanatory diagram showing the positional relationship of the X-ray source and the detection element with respect to each successive instant. , FIG. 5 is a perspective view showing an example of an X-ray detection element particularly suitable for use as a detection element, and FIG. 6 is an explanatory diagram showing a scanning method performed by the apparatus shown in FIGS. 1 and 2. be. 1... Subject (patient), 2... Table, 3...
...X-ray beam, 4...X-ray tube, 5...X-ray detector, 6...detection element, 7, 8... ring, 9
...Gairin, 10,11...Wheel, 12,13
...Motor, 14...Common axis of rotation, 20...Patient, 21,42...X-ray source, 22,43...X
-ray detector, 23, 44...X-ray detection element, 2
4, 41...X-ray source and detector moving path, 2
5... Integration circuit, 26... Correction circuit, 27... Logarithmic amplifier, 28... Memory, 29... Computer, 30... Television monitor, 45... Diaphragm, 48... Diaphragm.

Claims (1)

[Scope of Claims] 1. An X-ray source for generating a flat fan-shaped X-ray beam, the X-ray source being substantially fixed relative to the X-ray source body during operation. an X-ray detector having a radiation region and further comprising a number of detection elements arranged along an arc to receive the X-ray radiation from said fan beam after passing through the object under diagnosis; X of each of the detection elements
- The line detection area is made relatively small, and the first and second
each rotatable support member is rotatably supported by a fixed support means such that the first and second rotatable support members are of a common axis oriented at right angles to the plane of the fan beam. the X-ray source is fixed to the first rotating support member such that the X-ray source is rotatable about the first rotational support member such that the fan beam is oriented generally toward the common axis; fixing the X-ray detector to the second rotating support member such that the distance between the X-ray detector and the common axis is approximately equal to the distance between the X-ray emitting region of the X-ray source and the common axis; The computer tomography apparatus further comprises means for rotating the first and second rotational support members about the common axis at the same angular velocity but in opposite rotational directions. equipment. 2. An apparatus for computer tomography according to claim 1, wherein the X-ray detector is constructed with a large number of detection elements arranged to form one closed circle. 3. The device according to claim 1 or 2, wherein the device is provided with a number of X-ray sources, and these
- An apparatus for computer tomography, characterized in that a radiation source is mounted on the first rotary support member in an arc centered on the common axis. 4. In the device according to any one of claims 1 to 3, when the first and second rotation support members rotate around the common axis, each detection element and the detection X- corresponding to the element
Apparatus for computer tomography, characterized in that it is provided with an electrical circuit 26 for correcting the output signal provided by each detection element so as to compensate for variations in the distance to the source.