JPH0415576B2 - - Google Patents

Description

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

Although the present invention can be used in a wide range of applications in general, for specific purposes of illustration, the following description will be directed to a case where the invention is applied to a fluoroscopic line-of-line imaging device using an image intensifier tube.

The X-ray image shows that the intensifier tube emits a pattern of electrons in response to the X-rays emitted by the large photocathode, accelerates these electrons toward the smaller anode, and causes them to collide with the anode at high speed. The electrons 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 a large photocathode to a smaller anode,
Additional light output results from the energy gained by accelerating electrons as they fly from the cathode to the anode.

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 Miu 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 involves the use of a compensation element that can be incorporated into or attached to the image intensifier tube, and a control voltage applied to the compensation element, which serves to reduce or eliminate the influence 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 problem arises 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, the Earth's magnetic field is 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. Compensation for can result in fairly large errors. One such foreseeable situation is when two or more fluoroscopic x-ray imaging devices are placed independently around a patient. 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.

In addition to distorting the Earth's magnetic field by its presence, the second independently positionable fluoroscopic x-ray imaging device may change its magnetic influence depending on whether it is turned on or not. 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 drawbacks 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.

In summary, the present invention provides a compensating element in the vicinity of the image intensifier tube to apply a compensating magnetic field to prevent perturbing magnetic fields (i.e., external disturbances that disturb the path of electrons in the image intensifier tube), such as the earth's magnetic field described above. An object of the present invention is to provide a dynamic compensation system for an image intensifier tube that cancels out the effects of magnetic fields. 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 connected to the tube centerline 14
An X-ray beam represented by is sent toward the image intensifier tube 16. A patient 18 is placed in a position that blocks the X-ray beam from the X-ray source 12 toward the image intensifier tube 16.
or other objects may be supported by conventional means. As shown by dashed circles 20 and 22,
A conventional rotational support device (not shown) can be used to achieve the desired angular relationship between centerline 14 and patient 18.

The magnitude of the earth's magnetic field vector B24, expressed in ordinary vector notation, varies slightly depending on the 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 geomagnetic field vector notation B24 are affected by local perturbing objects, such as ferromagnetic material 26, in the vicinity of the fluoroscopic imaging device 10. The ferromagnetic material 26 can distort the Earth's magnetic field vector B24 over a short range, thereby distorting the output image
and the ferromagnetic material 26, the angle between the centerline 14 and the axis 28 of the ferromagnetic material 26, 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, image intensifier tube 16 includes an envelope 30 having an input surface 32, an output surface 34, and side walls 36 to completely enclose the internal components. A scintillation layer 40 inside input surface 32 includes a phosphor, which produces a pattern of light that corresponds to a pattern of incident x-rays. A photocathode layer 42 closely 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. Focusing electrode 4 near output surface 34
2 is maintained at a positive potential, for example about 30 kilovolts, to accelerate the electrons emitted from the photocathode layer 42 towards the output phosphor screen 46.

One or more additional focusing electrodes 48 and 50 are used in conjunction with external conventional control circuitry to form an electron lens to direct electrons emitted from the photocathode layer 42 to controlled paths 54, 56, and 58. Proceed along the route. This forms an output image on output phosphor screen 46 that corresponds to the pattern of electrons emitted by photocathode layer 42 . This pattern of electrons, corresponding to the pattern of x-rays impinging on the scintillation layer 40, creates an output image on the output phosphor screen 46 that represents a reduced image of the attenuated pattern that was exposed to the x-rays. Acceleration of the electrons by the accelerating field of the focusing electrode 44 increases the energy of the electrons so that a greater number of photons are directed to the output phosphor screen 46 than are generated by the X-rays impinging on the scintillation layer 40. It is then released. 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 and used for other purposes.

A magnetic shield layer 60 is arranged on the side wall 36,
This prevents the output image from being distorted by the earth's magnetic field vector B24. It is not practicable to stretch the magnetic shield layer 60 over the input surface 32 and output surface 34. This is because the necessary incidence of X-rays and transmission of the output image will be hindered. Therefore, depending on the angle between centerline 14 and earth's magnetic field vector B24, earth's magnetic field vector B24 may enter through unshielded apertures in input face 32 and output face 34, causing paths 54, 56.
and 58 may be disturbed.

In Figure 3, the earth's magnetic field vector B24 is located at the center line 14.
This shows the worst case scenario. 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 proportional to the product of the magnitude of the earth's magnetic field vector B24 and 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. The paths 54 and 58 are at an angle a with respect to the earth's magnetic field vector B24.
and -a, so 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 increases as the distance from the center 68 of the straight line 64 increases.
Focusing electrodes 48 and 50 (third
The electric field generated by the output phosphor screen 46 tends to dampen the perturbations of the electrons on the path terminating closest to the edge of the output phosphor screen 46.

As is clear from the above description, the influence of the earth's magnetic field vector B24 on the path of electrons within the image intensifier tube 16 depends on the angle between the earth's magnetic field vector B24 and the center line 14. When these angles are perpendicular to each other, the image intensifier tube 16
is almost completely shielded from the earth's magnetic field vector B24 by the 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 purposes of the invention, it is sufficient to assume that the magnitude of the perturbation effect is proportional to the cosine of the angle between the earth's magnetic field vector B24 and the centerline 14.

Compensation coil 70 (FIG. 2) receives a compensation current. The amplitude of this compensation current is the earth's magnetic field vector B2
4 is determined to compensate for the effects on paths 54 and 58 (and intermediate paths not shown) to eliminate distortion of the output image. The compensation current has a maximum amplitude CCD sufficient to compensate for the illustrated case where the angle between the earth's magnetic field vector B 24 and the centerline 14 is zero (ie, cosine of zero=1).

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

CC=CCO(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 the centerline 14 (second
It includes a conventional angle measuring device 74 for measuring the angle a defined by FIG. The measured angle a is provided to the earth's magnetic field compensator 76. The geomagnetic field compensator 76 also receives information about the amplitude and angle of the geomagnetic field vector B24. Earth's magnetic field compensator 76
generates a signal proportional to the cosine of the angle between the earth's magnetic field vector B 24 and centerline 14 and provides this signal to adder 78 . When compensating only the influence of the earth's magnetic field vector B24, the earth's magnetic field compensator 7
The output of 6 may bypass summer 78 and be 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 one or more other perturbing magnetic fields that differ in magnitude and direction from the earth's magnetic field vector B24, such as perturbing magnetic field O82 (FIG. 1) acting along axis 28. Unlike the earth's magnetic field vector B24, the other magnetic field O82 may vary in both amplitude and angle depending on its location within the range of motion of the image intensifier tube 16. Furthermore, the magnetic field O8
The amplitude and angle of 2, or even its presence, may depend on external factors, such as the mode of operation of the external device, such as whether it is turned on or not. Therefore, additional parameters of the perturbing magnetic field O82 may be required to be applied to the compensator 80 via line 84. A source of such additional data can be obtained from continuous measurements or by taking a single measurement, storing the results, and then reading out the required values as needed. In some cases, earth magnetic field compensator 76
The functions of compensator 80 and compensator 80 may be performed from a stored value for the measured value of 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. Alternatively, a lookup table (not shown) for the compensation current applied to line 79 may be created in relation to the value of angle a. When the value of angle a is measured, the lookup table is accessed to determine the corresponding value of the compensation current.

Variations in the perturbing magnetic field O82 caused by limited range effects or differences between when an external device is on and off can be accommodated in the equation or lookup table. . The constants and variables of the equations or values in the lookup table relating to the perturbing magnetic field O82 may be measured once and stored for subsequent continuous use.

Although the embodiments of the present invention have been described with reference to the drawings,
The invention is not limited to these specific embodiments, and those skilled in the art will appreciate that various changes and modifications can be made without departing from the scope or spirit of the invention as defined in the claims. I want to be understood.

[Brief explanation of drawings]

FIG. 1 is a highly simplified diagram of a fluoroscopic imaging apparatus to which the present invention may be applied. Second
The figure 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 (1)

[Claims] 1. A distortion compensation device for dynamically compensating for distortion in an image intensifier tube, comprising: an image intensifier tube including an input surface and an output surface that define a tube axis; a compensation element provided in the tube; A distortion compensation device, comprising compensation signal applying means for applying a compensation signal to the compensation element, and compensation signal changing means for varying the compensation signal in relation to the angle between the perturbing magnetic field and the axis. 2. The distortion compensation device 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. A distortion compensator according to claim 3, wherein said perturbing magnetic field includes at least one additional magnetic field, said at least one additional magnetic field being within a range of positions of said image intensifier tube. A strain compensator comprising a magnetic field whose amplitude and/or angle vary at least one of the following. 5. The distortion compensation device according to claim 4, wherein 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. Device. 6. The distortion compensator according to claim 4, wherein the compensation signal applying means and the compensation signal changing means include a lookup table for associating the angle of the axis with the compensation signal. 7. The distortion compensation device according to claim 3, wherein 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. Device. 8. The distortion compensator according to claim 3, wherein the compensation signal applying means and the compensation signal changing means include a lookup table for associating the angle of the axis with the compensation signal.