JPS63121239A - Automatic strain compensator for image intensifier tube - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION BACKGROUND OF THE INVENTION The present invention relates to imaging apparatus, and more particularly to a method for eliminating distortion in an image tube caused by the interaction of moving charged particles with an external magnetic field. .

Although the present invention can be used in a wide range of applications in general,
In order to give a concrete example, the following description will be made regarding a case where the present invention is applied to a fluoroscopic line-of-line imaging device using an image intensifier tube.

The intensifier tube emits a pattern of electrons in response to the X-rays emitted by a large photocathode, accelerates these electrons toward a smaller anode, and causes them to collide with the anode at high speed. The electrons generated produce a corresponding pattern of light representing the output of the image intensifier tube. In addition to the brightness gain obtained by reducing the light-dark pattern from the large photocathode to the smaller anode, additional light output is obtained from the energy gained by accelerating electrons as they fly from the cathode to the anode. occurs.

Since electrons are charged particles, they are affected by electrostatic and magnetic fields. The presence of an electrostatic field between the cathode and anode produces the desired energy gain.

Furthermore, an electron lens incorporated into the image intensifier tube creates a field useful for electron focusing.

To prevent distortion of the output pattern, external magnetic fields are preferably removed or compensated.

The sides of the image tube are typically shielded with a high permeability material, such as a mu alloy. Shielding the cathode or anode end of the image tube is impractical as it would interfere with the necessary x-ray input or light output. As a result, external magnetic fields around the device can enter the image intensifier tube and disrupt the desired path of the electrons. This distorts the output image. The strain is proportional to the product of the sine of the angle between the field direction of the disturbing field and the path of the electrons and the strength of the disturbing field.

In many installations, the primary disturbing magnetic field is the Earth's magnetic field. Although the deviation and depression angle vary from place to place on the Earth's surface, the Earth's magnetic field remains constant in amplitude and direction at any particular location. For fixed image intensifier tubes, a compensating magnetic field of the amplitude and direction necessary to cancel the effects of the Earth's magnetic field can be applied. In one method, partial or complete cancellation can be achieved using one or more permanent magnets suitably placed in the vicinity of the image intensifier tube.

A more flexible method uses a compensation element that can be incorporated into or attached to the image intensifier tube and apply a control voltage to the compensation element to reduce or eliminate the effect of the Earth's magnetic field on the path of the electrons. A compensating magnetic field can be generated within the tube.

The compensation method described above is effective in compensating for the effects of magnetic fields in immovably fixed image intensifier tubes. Modern fluoroscopy systems allow the patient to remain stationary while the x-ray source and image intensifier tube are rotated as needed to obtain the desired image of the patient.

When rotating the X-ray source and image intensifier tube, the angle between the earth's magnetic field and the path of the electrons in the image intensifier tube is not fixed but can take any value. Prior art static compensation techniques not only do not work effectively, but in practice, various angular relationships occur between the compensation field and the electron travel, such that the compensation field increases the distortion rather than reducing it. Sometimes.

Another type of detection occurs when ferromagnetic materials are present in the vicinity of the fluoroscopic X-ray imaging device. Because these ferromagnetic materials can distort the Earth's magnetic field, compensation for the Earth's magnetic field based on the assumption that the magnitude and direction of the Earth's magnetic field are constant over all possible positions of the fluoroscopic X-ray imaging device is not sufficient. Large errors may occur.

One such predictable situation is that there are two
This is a case where three or more fluoroscopic X-ray imaging devices are arranged independently. Each device contains ferromagnetic material and can distort magnetic fields acting on other devices. Since each device can be placed independently, the resulting effects depend on the position of each device relative to the Earth's magnetic field and to each other.

Not only does its presence distort the Earth's magnetic field, but it also
The independently positionable fluoroscopic X-ray imaging device may change its magnetic influence depending on whether it is turned on or turned off. The same applies if other equipment, mobile or not, is in the vicinity of the imaging device. For example, applying power to any device capable of generating a magnetic field located in the vicinity of an imaging device may produce a resultant magnetic field that is significantly different than the Earth's magnetic field alone.

OBJECTS AND SUMMARY OF THE INVENTION One object of the invention is to compensate for magnetostriction in image intensifier tubes so as to overcome the disadvantages of the prior art.

Another object of the present invention is to dynamically compensate for magnetostriction within the image intensifier tube to accommodate the changing relationship between the Earth's magnetic field and the electron path.

Yet another object of the present invention is to provide dynamic compensation for the image intensifier to accommodate changes in external ferromagnetic position.

Yet another object of the present invention is to provide dynamic compensation for the image intensifier tube to accommodate changes in the magnitude of the external magnetic field.

To summarize, the present invention provides a compensating element near the image intensifier tube to apply a compensating magnetic field, thereby suppressing a perturbative magnetic field (such as the above-mentioned earth's magnetic field).
i.e. an external magnetic field that disturbs the path of electrons in the image intensifier tube)
It is an object of the present invention to provide a dynamic compensation system for an image intensifier tube that cancels out the effects of.

A compensation current source changes the amplitude of the compensation current in response to the cosine of the angle between the tube axis and the perturbing magnetic field vector. Means are provided to compensate for local perturbations as well as magnetic field sources that vary in strength and angle within the range of motion of the image intensifier tube.

According to one embodiment of the present invention, an apparatus for dynamically compensating for distortion within an image intensifier tube is provided, the dynamic compensation apparatus comprising: an input surface and an output surface defining an axis of the tube; a compensating element located at the compensating element, means for applying a compensating signal to the compensating element, and means for varying the compensating signal as a function of the angle between the perturbing magnetic field and the axis of the tube.

The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings. Like reference numbers represent like elements in the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a simplified schematic diagram of a fluoroscopic imaging system 10. As shown in FIG. The X-ray source 12 is
A line beam is directed towards the image intensifier tube 16. A patient 18 or other object may be supported by conventional means in a position that blocks the x-ray beam from the x-ray source 12 toward the image intensifier tube 16. Any desired angular relationship between centerline 14 and patient 18 can be achieved using conventional rotational support devices (not shown), as shown by dashed circles 20 and 22.

Earth's magnetic field vector B24 shown in normal vector notation
Its size varies slightly depending on its location on the Earth's surface. The angle between the Earth's magnetic field vector B24 and the horizontal plane varies significantly over the Earth's surface.

Both the magnitude and direction of the earth's magnetic field vector B24 are determined by the magnitude and direction of the ferromagnetic material 26, for example, in the vicinity of the X-ray fluoroscopic imaging device 10.
affected by local perturbing objects such as . The ferromagnetic material 26 may distort the earth's magnetic field vector B 24 over a short range, such that the distortion of the output image depends on the distance between the image intensifier tube 16 and the ferromagnetic material 26, the distance between the centerline 14 and the ferromagnetic material 26, and so on. It varies depending on the angle between the axis 28 and the angle between the earth's magnetic field vector B24 and the axis 28.

Additionally, ferromagnetic material 26 may include elements (not shown) that change the magnetic field depending on whether it is turned on or turned off.

As shown in FIG. 2, the image intensifier tube 16 includes an envelope 30, which includes an input surface 32) an output surface 3;
4, and a side wall 36 to completely seal the internal parts. Scintillation layer 4 inside input surface 32
0 contains a fluorophore, which generates a pattern of light that corresponds to the pattern of incident X-rays. A photocathode layer 42 intimately coupled to scintillation layer 40 generates a pattern of electrons that corresponds to the pattern of light generated by scintillation layer 40 . Photocathode layer 42 is typically maintained at a negative reference voltage, such as ground potential. A focusing electrode 44 near the output surface 34 is maintained at a positive potential, eg, about 30 kilovolts, to accelerate electrons emitted from the photocathode layer 42 toward the output phosphor screen 46.

One or more additional focusing electrodes 48 and 50 are used with external conventional control circuitry to form an electronic lens;
Electrons emitted from photocathode layer 42 are caused to travel along controlled paths indicated by paths 54, 58 and 58. This forms an output image on output phosphor screen 46 that corresponds to the pattern of electrons emitted by photocathode layer 42 . Due to this electron pattern corresponding to the X-ray pattern hitting scintillation B40,
An output image is created on the output phosphor screen 46 representing a reduced image of the attenuation pattern on which the lines were illuminated. Acceleration of the electrons by the accelerating field of focusing electrode 44 increases the energy of the electrons so that a greater number of photons are emitted by output phosphor screen 46 than are generated by the x-rays impinging on scintillation layer 40 . In this way, the brightness of the image is increased or enhanced. Output phosphor screen 46 and output surface 34 are transparent so that the intensified image can be viewed from outside of image intensifier tube 16 or used for other purposes.

A magnetic shield layer 60 is disposed on the side wall 36 to prevent the output image from being distorted by the earth's magnetic field vector B24. The magnetic shield layer 60 is extended to form the input surface 32 and the output surface 3.
4 is not feasible. This is because the necessary incidence of X-rays and transmission of the output image will be hindered. Therefore, the center line 14 and the earth's magnetic field vector B
24, the earth's magnetic field vector B24 may enter through unshielded apertures in the input and output surfaces 32 and 34, disrupting the path of electrons traveling along paths 54, 56 and 58. Sometimes.

FIG. 3 shows the worst case where the earth's magnetic field vector B24 is aligned with the center line 14. In this case, the earth's magnetic field vector B24 has maximum access to the unshielded input and output surfaces 32 and 34. The amount by which the electron path is disturbed by the earth's magnetic field vector B24 is the earth's magnetic field vector B.
24 times the sine of the angle between them. If the angle between the earth's magnetic field vector B24 and path 56 is zero, then the perturbation or disturbance in path 56 will also be zero. Electron velocity vector 62 along path 54 or 58 makes an angle a with earth's magnetic field vector B24. As a result, the output image formed on output phosphor screen 46 is distorted as paths 54 and 58 are perturbed by Earth's magnetic field vector B24.

Referring now to FIGS. 4 and 5, if the X-ray pattern without distortion is composed of one horizontal line, the output image also constitutes a straight line 64. If the relationship of FIG. 3 exists, the central path 56 is undisturbed because there is a zero angle between this line and the Earth's magnetic field vector B24.

Since paths 54 and 58 make angles a and -a with respect to the earth's magnetic field vector B24, the sines of these angles are not zero. As a result, the straight line 64 is distorted in opposite directions on either side of the center 68, resulting in an S-shaped curve 66, as shown in FIG. It can be seen that the shape of the S-shaped curve 66 does not comply with the above-mentioned instruction that the distortion becomes large in the portion far from the center 68 of the straight line 64.

The electric field generated by focusing electrodes 48 and 50 (FIG. 3) near sidewall 36 tends to dampen perturbations of the electrons on the path that terminates closest to the edge of output phosphor screen 46.

As is clear from the above explanation, the earth's magnetic field vector B2
4 on the path of electrons within the image intensifier tube 16 depends on the angle between the earth's magnetic field vector B 24 and the centerline 14. When these angles are perpendicular to each other,
Image intensifier tube 16 is substantially completely shielded from Earth's magnetic field vector B24 by an intervening magnetic shield layer 60 (FIG. 2). When these angles are equal, the influence of the earth's magnetic field vector B24 is maximum. For the purpose of the invention, the magnitude of the influence of the perturbation is determined by the earth's magnetic field vector B24
It is sufficient to assume that it is proportional to the cosine of the angle between and the centerline 14.

Compensation coil 70 (FIG. 2) receives a compensation current.

The amplitude of this compensation current is such that the earth's magnetic field vector B24 is on path 5.
4 and 58 (and intermediate paths not shown) to eliminate distortion of the output image. The compensation current has a maximum amplitude CCO sufficient to compensate for the illustrated case where the angle between the earth's magnetic field vector B24 and the centerline 14 is zero (ie, cosine -1 of zero).

At other angles, the amplitude CC of the compensation current is determined by:

CC-CC0(COS a) where a is the angle between the earth's magnetic field vector B24 and the centerline 14 of the image intensifier tube 16.

Although compensation coil 70 is shown within envelope 30, in alternative embodiments compensation coil 70 may be located outside envelope 30 near input surface 32.

Next, FIG. 6 shows the compensation current generator 72. This is a conventional angle measuring device 7 for measuring the angle a defined by the centerline 14 (FIG. 2) with respect to any suitable coordinate system.
Contains 4. The measured angle a is provided to the earth's magnetic field compensator 76. The earth's magnetic field compensator 76 receives the earth's magnetic field vector B2.
It also receives information about the amplitude and angle of 4. Earth's magnetic field compensator 76 generates a signal proportional to the cosine of the angle between earth's magnetic field vector B 24 and centerline 14 and provides this signal to adder 78 . When only the influence of the earth's magnetic field vector B24 is to be compensated for, the output of the earth's magnetic field compensator 76 is sent to the adder 7.
8 may be bypassed and applied directly to compensation coil 70 via line 79.

Compensation current generator 72 also includes a compensator 80 .

This compensator 80 compensates for the effects of the earth's magnetic field vector B24 and one or more other perturbing magnetic fields of different magnitudes and directions, such as the perturbing magnetic field 0B2C (FIG. 1) acting along the axis 28. Unlike the earth's magnetic field vector B24, the other magnetic field 082 may vary in both amplitude and angle depending on its location within the range of motion of the image intensifier tube 16. Furthermore, the amplitude and angle of the magnetic field 082, or even its presence, may depend on external factors, such as the mode of operation of an external device, such as whether it is turned on or not. Therefore, additional parameters of perturbing magnetic field 082 may be required to apply to compensator 80 via line 84. A source of such additional data can be obtained from a continuous n1 constant, or by measuring once and storing the results and then reading out the desired values as needed. In some cases, a geomagnetic field compensator 76
The function of the compensator 80 may also be performed from a stored value for the measured value of the angle a. The stored values may be in the form of one or more equations whose values are determined by the existing value of angle a. Instead, for the value of the compensation current applied to line 79 (
A look-up table (not shown) may be created in relation to the value of angle a. When the value of angle a is measured, a look-up table is accessed to determine the corresponding value of compensation current.

Variations in the perturbing magnetic field 082 caused by limited range effects or differences between when an external device is on and off can be accommodated in an equation or lookup table. The equation constants and variables associated with the perturbing magnetic field 082, or values in a look-up table, may be measured and stored at one time for subsequent continuous use.

Although the present invention has been described in detail with reference to the drawings, the present invention is not limited to these specific embodiments, and those skilled in the art will be able to understand the scope and spirit of the present invention as defined in the claims. It should be understood that various changes and modifications may be made without further modification.

[Brief explanation of the drawing]

FIG. 1 is a highly simplified diagram of a fluoroscopic imaging apparatus to which the present invention may be applied. FIG. 2 is a cross-sectional view of the image intensifier tube. FIG. 3 is a simplified top view of the image intensifier, and also shows the directions of the magnetic and velocity vectors to illustrate the sources of image distortion. FIG. 4 is a front view of the output fluorescent screen of FIGS. 2 and 3 without distortion. FIG. 5 is a front view corresponding to FIG. 4 showing the distortion of a single bright line across the output fluorescent screen. FIG. 6 is a block diagram of a compensation current generator according to one embodiment of the present invention. [Explanation of main symbols] 16... Image intensifier tube, 32... Input surface, 34...
... Output surface, 70... Compensation coil, 72... Compensation current generator.

Claims (8)

[Claims] (1) In a distortion compensation device for dynamically compensating for distortion in an image intensifier tube, the image intensifier tube includes an input surface and an output surface that define the tube axis;
a compensation element provided in the tube; compensation signal applying means for applying a compensation signal to the compensation element;
and compensation signal modification means for varying the compensation signal in relation to the angle between the perturbing magnetic field and the axis. (2) The distortion compensator according to claim (1), wherein the compensation signal is changed according to the cosine of the angle. (3) A tube strain compensator according to claim (1), wherein the perturbing magnetic field includes the earth's magnetic field. (4) In the distortion compensator according to claim (3), the perturbing magnetic field includes at least one additional magnetic field, and the at least one additional magnetic field is located at a position of the image intensifier tube. A strain compensator comprising a magnetic field whose amplitude and/or angle vary over a range of . (5) In the distortion compensator according to claim (4), the compensation signal applying means and the compensation signal changing means include a set of equations relating the angle of the axis to the compensation signal. Distortion compensation device. (6) In the distortion compensation device according to claim (4), the compensation signal applying means and the compensation signal changing means include a look-up table that associates the angle of the axis with the compensation signal. Distortion compensation device. (7) In the distortion compensator according to claim (3), the compensation signal applying means and the compensation signal changing means include a set of equations relating the angle of the axis to the compensation signal. Distortion compensation device. (8) In the distortion compensator according to claim (3), the compensation signal applying means and the compensation signal changing means include a lookup table that associates the angle of the axis with the compensation signal. Distortion compensation device.