JPS60160599A - Tomograph - Google Patents

Abstract

PURPOSE:To obtain a picture of high quality by correcting changes in the quantity of X-rays reaching the surface of a film, which are caused by the distance changing with the move of an irradiation device, by means of changing the number of pulses or the pulse width of the pulsed irradiation. CONSTITUTION:A signal, which is proportional to the square of the distance between an X-ray tube 1 and a detection device 2, is inputted to a pulse-number setting circuit 6. The pulse-number setting circuit 16, including an A/D conversion circuit, generates a numerical output in inverse proportion to the input signal as the pulse-number setting output. A standard clock signal generating circuit 18 outputs the standard clock signal, which is counted by a counter contained in an irradiation signal generating circuit 17, and each time it reaches the numerical value of the pulse-number setting output given by the pulse-number setting circuit 16, an irradiation signal with a certain width is generated. Therefore, the irradiation intervals change in inverse proportion to the square of the distance between the X-ray tube and the film, and the quantity of X-rays reaching the film surface is maintained at a constant level.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention provides a radiation irradiation means and a two-dimensional radiation detection means that are arranged opposite to each other with a subject in between, and both of which are placed at the center of the tomographic plane of the subject. The present invention relates to an improvement in a tomography apparatus that obtains a shadow image of an arbitrary tomographic plane using transmitted radiation by moving symmetrically around a point to blur shadows in areas other than the tomographic plane.

(B) Prior art As shown in FIG. 1, in a normal parallel plane tomography apparatus, an X-ray tube and a film 2 are placed facing each other with a subject 4 on a table 3 in between. , the X-ray tube l and the film 2 are made movable on a plane 6.7 parallel to the tomographic plane 5 set at an arbitrary position in the body of the subject 4, and both are moved to the tomographic plane. It is configured to move symmetrically around the center point 0 of 5. Various loci of movement on the surface 6.7 are adopted, such as a straight line, a circle, an ellipse, a hybocycloidal trajectory, and a spiral trajectory.

Incidentally, in such a parallel plane type tomography apparatus, the distance between the X-ray tube 1 and the film 2 changes as the X-ray focal point moves. and.

The dose of X-rays reaching the surface of film 2 is the inverse of this distance 2
There is a relationship that changes with the power of the ratio. That is, the reached dose l
is I=/((ar)+L) However, it can be expressed as a=L/LO. Here, K is a constant, r is the distance III (that is, radius) from the normal passing through the center point O to the X-ray focal point, L is the distance between surfaces 6.7, and Lo is the distance between surfaces 5.6. It is. Therefore, when taking images under constant X-ray conditions, changes in the dose reaching the film surface due to movement of the X-ray tube may have an unfavorable effect on the shadow image produced on the film.

There is this inconvenience.

(c) Purpose This invention provides an improved tomography apparatus that can correct changes in the dose reaching the film surface due to changes in distance due to movement of radiation irradiation means and obtain images of excellent image quality. With the goal.

(D) Configuration In the tomography apparatus of the present invention, a radiation irradiation means for irradiating radiation in a large number of pulses and a two-dimensional radiation detection means are arranged facing each other with the object in between, and both are placed on the tomographic plane of the object. When moving symmetrically around the center point, the position of the radiation irradiation means or the two-dimensional radiation detection means is detected, the distance between them is determined from this position, and a signal related to this distance is generated in a pulsed manner. The radiation irradiation time per unit time is controlled by changing the pulse number or pulse width of the radiation.

(E) Embodiments In the embodiments shown below, the present invention is applied to a multi-orbit tomography apparatus that can take various orbits. In order to be able to take various trajectories, it is necessary to combine rotational motion and linear motion, so to express it schematically, as shown in FIG. This is held by the rotary motion mechanism 8 that rotates the linear motion mechanism 9. This second
Although not shown in the figure, the film 2 is connected to the X-ray tube l via a connecting lever that is rotatably supported around the center point 0 (see Figure 1), and the X-ray tube l is thinly wound. When the film 2 moves on various orbits such as an orbit, the film 2 moves symmetrically about a point O with respect to the movement of the X-ray tube 1. A mechanism for achieving such relative motion has been commonly used in tomography apparatuses, so its explanation will be omitted.

When the xII tube 1 moves in this manner, X-ray exposure is controlled by a control circuit as shown in FIG. That is, in FIG. 3, the X-ray tube power supply device 11 connected to the power supply 10 is for pulsed X-ray exposure and generates a pulsed high voltage. The X-ray tube is designated here by 12. The circuit to the left of the dotted line is the circuit added according to the present invention.

The orbital radius r of the X-ray tube is determined by a detector 1 such as a potentiometer.
3, and a signal (ar) proportional to the radius r is fed to the signal (ar), which is proportional to the radius r, to obtain (ar)2. On the other hand, L corresponding to the height L
The height setting circuit 15 is set in advance so as to generate an output of 2, and a signal of (ar)2+L21, which is the sum of (ar)2 and L2, is input to the pulse number setting circuit 16. This pulse number setting circuit 16 is an A/D
It includes a conversion circuit and produces a numerical output inversely proportional to the input signal as a pulse number setting output. Reference clock generation circuit 18
A clock signal is output as shown in FIG. 4A (or FIG. 5A) from Each time the pulse number setting output value is reached, an exposure signal of a constant width is generated as shown in FIGS. 4B and 4C. Therefore, the exposure interval changes in inverse proportion to the square of the distance between the X-ray tube and the film (1 (a r)" + L2), and the smaller this distance, the wider the interval (see Figure 4). B), the larger the distance, the narrower it becomes (Fig. 4 C). Therefore, when the distance is short, the number of pulses per unit time is small and the X-ray exposure time is shortened, and when the distance is long, the number of pulses per unit time is large. This increases the X-ray exposure time.

In this example, the pulse width of the pulsed X-ray is kept constant and the X-ray exposure time per unit time is changed by changing the number of pulses, but the pulse width may be changed while keeping the number of pulses constant. To do this, pulse number setting circuit 1
6, a pulse width setting circuit that generates a pulse width setting output with a numerical value proportional to the input signal 2 ((ar) + L) is used, and the exposure signal generation circuit 17 counts the clock signal and outputs the pulse width setting. It is configured to generate an exposure signal (FIG. 5B, C) with a pulse width corresponding to the pulse width.

2 (Car)+L) In other words, the pulse width of the exposure signal of the exposure signal generation circuit 17 changes in proportion to the square of the distance between the X-ray tube and the film, and the clock signal changes as shown in FIG. 5A. If this distance is small, the pulse width becomes narrow as shown in FIG. 5B, and if it is large, the pulse width becomes wide as shown in FIG. 5C.

By performing pulsed X-ray irradiation in this way and controlling the number of pulses or pulse width, the X-ray irradiation time per unit time is changed in proportion to the square of the distance. The delivered dose can be kept constant regardless of changes in the distance between the X-ray tube and the film. In other words, the above-mentioned formula expressing the achieved dose I is such that Ko is a constant and
If mA is the X-ray tube current, S is the X-ray exposure time, and KV is the X-ray tube voltage, it can be rewritten as follows.

I = K'-mA-3-KVN/((a r)"+
L2) However, a=L/L.

Therefore, the X-ray exposure time S is controlled in proportion to the square of the variable r as shown in the following formula, and S = ″・((a
r)+L) However, if K T+ is a constant and other X-ray conditions are constant, the dose I reaching the film surface is I = K'eK" @mA*KVN, and the radius r
In other words, it is unrelated to changes in distance.

Note that although an example in which the number of X-ray pulses is controlled and an example in which the width of the X-ray pulses is controlled are shown above, both may be used in combination.

In addition, in the above example, the X-ray exposure time per unit time is controlled so that the dose reaching the film surface is always constant, but clinically, the dose reaching the film is not constant and may be adjusted somewhat. There may be cases where it is better, and the configuration may be such that control is performed in response to such cases.

(f) Effects According to the present invention, the distance between the radiation irradiation means that irradiates radiation in the form of many pulses and the two-dimensional radiation detection means is determined, and a signal related to this distance is used to determine the number of pulses of the pulsed radiation or Since the radiation irradiation time per unit time is controlled by changing the pulse width, it is possible to compensate for fluctuations in the dose reaching the film surface due to changes in distance, and as a result, images with excellent image quality can be obtained. In addition, it is easy to put it into practical use because it is only necessary to add a few control circuits to the configuration.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram for explaining the operation of the tomography apparatus,
FIG. 2 is a perspective view schematically showing a movement mechanism of a tomography apparatus according to an embodiment of the present invention, FIG. 3 is a block diagram showing a control circuit of the same embodiment, and FIGS. 4A, B, C, and 5A, B, and C are waveform diagrams for explaining the operation, respectively. 1.12... X-ray tube 2... Film 3... Table 4... Subject 5... Fault plane 8... Rotating motion mechanism 9... Linear motion mechanism 10... Power supply 11...xm, tube power supply device
13... Radius detector 14... Square circuit 15...
Height setting circuit 16...Pulse number setting circuit 17...Exposure signal generation circuit 18...Reference clock generation circuit Applicant: Shimadzu Corporation

Claims (1)

[Claims] (1) A radiation irradiation means that irradiates the subject with radiation in the form of many pulses, and a two-dimensional radiation detection means that is arranged opposite to the radiation irradiation means with the subject in between so that the radiation that has passed through the subject is incident. a moving means for moving the radiation irradiation means and the two-dimensional radiation detection means symmetrically about a center point on a tomographic plane set in the subject; and a position of the radiation irradiation means or the two-dimensional radiation detection means. means for detecting a distance between the radiation irradiation means and the two-dimensional radiation detection means from the detected position; and means for determining the number of pulses or the pulse width of the pulsed radiation using a signal related to the distance. A tomography apparatus comprising a control means for controlling radiation irradiation time per unit time by changing the radiation irradiation time.