JPH09245701A - Method and device for forming cycling x-ray beam used for high-speed computer tomogarphy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device for forming X-ray beam, and to perform the high-speed sampling over a polar coordinate angle of 2π for computer tomography. SOLUTION: This device forms a cycling X-ray beam used for high-speed computer tomography. A solenoid coil 40 is provided to extend into a beam guiding channel and follows this beam guiding channel in a negative direction. Thereby, an electron beam e' is focused inside the solenoid coil 40 in the beam guiding channel so that an anode device 30 is contained inside the solenoid coil 40.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and method for forming a circulating X-ray beam for high speed computer tomography.

[0002]

2. Description of the Related Art In a conventional computer tomography apparatus, a movable X-ray source and an X-ray detector are used, and these are provided so as to be rotatable around a region to be inspected. This is because the scanning is gradually performed around the region to be inspected, and scanning at successive observation angles is obtained at the polar angle on the rotation plane.
This type of device was relatively slow due to mechanical adjustments. Mechanical adjustment was needed only for circulation. This is a drawback. This is because the imaging time becomes uncomfortably long in a patient who has to image a large number of consecutive slices.

There is also a need for relatively fast sampling times. This is because the movement process should be imaged by computer tomography. However, this is only possible if the sampling time is smaller than the typical time unit of motion to be detected.

The promotion of the acquisition of tomographic images has already been achieved with so-called electron beam tomography. In this device, mechanical swiveling of the beam source and the detector is omitted. The known device has an electron beam source, which forms a horizontally traversing electron beam of a predetermined energy and has a controllable deflection device. This deflecting device gradually deflects the electron beam to successive points at a semi-ring shaped anode. A detector half ring is arranged opposite the anode half ring. The electron beam is guided on the anode half ring by means of an electromagnetic deflection device, and successive recordings are made by the detector half ring over the polar angular region of Φ.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an X-ray beam forming apparatus for high speed sampling, which apparatus for high speed sampling over a 2π polar angle region for computer tomography. To Another subject of the invention is also a corresponding method.

[0006]

According to the present invention, there is provided an apparatus for forming a circulating X-ray beam for high speed computer tomography, comprising a beam guiding channel, a guiding magnetic field forming means, an emission magnetic field forming means and a control means. Means for extending the beam guiding channel spirally in one plane over a polar angle of at least 2π, and the guiding magnetic field forming means extends in the beam guiding channel perpendicular to the plane. Forming an existing axial magnetic field, the intensity of said axial magnetic field increasing along the helical beam guiding channel such that an electron beam of a given energy is directed substantially centrally within the beam guiding channel, The emission field forming means forms a magnetic field in the Φ-direction selectable region of the beam guiding channel, the magnetic field extending substantially radially in the plane. The electron beam passing through the beam guiding channel is deflected from the plane and is focused in the Φ-direction onto the anode device, which is substantially ring-shaped and concentric with the beam guiding channel. In the anode device, an X-ray beam is formed at a collision position of an electron beam, and the control unit causes the emission magnetic field forming unit to direct a selectable region in which a radial magnetic field is formed to a beam guide channel in the Φ-direction. The problem is solved by a circulating X-ray beam forming device that controls to circulate along.

The method of the invention is also a method of forming an X-ray beam for computer tomography, wherein the X-ray beam is centered and circulates over a polar angle of 2π. Forming an electron beam of energy; -injecting the electron beam into a beam guide, the beam guide guiding the electron beam into a spiral orbit by an axial magnetic field; At a selected location, the X-rays are deflected from the impingement point of the electron beam toward the center of the substantially ring-shaped anode device by deflecting them toward the substantially ring-shaped anode device arranged above the beam guide portion. Forming the beam, and-changing a selected location along the spiral trajectory at which the electron beam is deflected out of the plane, so that the electron beam is substantially phosphorous. Correspondingly changing the point of impact on the rugged anode device.

[0008] Advantageous embodiments of the invention are described in the dependent claims.

[0009]

According to the method of the present invention, an electron beam having a predetermined energy is formed, and the electron beam is guided in one plane by a beam guide portion having an axial magnetic field on a spiral orbit, At that time, a polar angle of at least 2π can be passed. A radial magnetic field is created at selectable locations along the helical beam guide. This magnetic field extends in the plane of the spiral orbit and deflects the electron beam through the beam guide vertically from said plane, focusing in the Φ-direction, substantially ring-shaped and concentric with the spiral orbit. Aim at the anode device. This anode device forms an X-ray beam at the point of collision of the electron beam. The diaphragm here selects a beam bundle which is directed towards the center of the substantially ring-shaped anode device. Subsequently, the location where the radial magnetic field is formed is moved along the spiral trajectory. This correspondingly changes where the electron beam is deflected from the spiral trajectory and where the correspondingly deflected electron beam strikes the anode device. By circular scanning of the deflecting points, i.e. circular scanning of the radial magnetic field along a spiral trajectory, the impinging points of the deflected electron beam also change correspondingly over a polar angle .PHI. The inside of the anode device is scanned.
As a result, the impingement point of the electron beam also moves on the substantially ring-shaped anode device, so that the sorted beam bundle is correspondingly centered and circulates swirling over the polar angle of the anode device.

The device according to the invention comprises a beam guiding channel, which is at least two in one plane.
It extends over a polar angle Φ of π. Guide field forming means are provided, which means in the beam guide channel,
An axial magnetic field is formed that extends perpendicular to the plane. The strength of this axial magnetic field increases along the spiral beam guiding channel as follows. That is, the electron beam of a given energy is enhanced to be directed along the spiral trajectory substantially at the center of the beam guiding channel. Emission field forming means are also provided, by means of which a radial magnetic field is formed in the selectable region of the beam guiding channel with respect to the Φ position. This magnetic field extends substantially in the plane of the helix and deflects the electron beam, which extends in the beam guiding channel, from said plane and focuses it in the Φ-direction towards the anode device. The anode device is substantially ring-shaped and concentric with the beam guiding channel. This anode device has X at the collision point of the electron beam.
Form a line beam. The emission magnetic field forming means is controlled by the control means as follows. That is, the position of a predetermined region where the radial magnetic field is formed is controlled so as to move while scanning along the spiral trajectory. Corresponding to the deflection of the electron beam moved along the spiral trajectory, the impinging point of the deflected electron beam on the substantially ring-shaped anode device also moves, so that the generation point of the X-ray beam also changes. It is moved while circulating around a substantially ring-shaped anode device. This causes the selected beam bundle to circulate and illuminate the center of the anode device from all sides.

The method and apparatus of the invention enable very fast sampling by the X-ray beam over a polar angle of 2Φ, eliminating the mechanical movement of each.

In a preferred embodiment, the guide field forming means comprises two metal plates arranged in a spiral and conductor windings wound around the metal plates in the Φ-direction. This metal plate surrounds the beam guiding channel. Here, it is possible to supply electric currents in opposite directions to the conductor windings arranged on the inner spiral metal plate and the conductor windings arranged on the outer metal plate. this is,
This is for forming an axial magnetic field between the metal plates in the beam guiding channel.

In another advantageous embodiment, the strength of the magnetic field is increased along the spiral trajectory from outside to inside as follows. That is, the height of the metal plate is reduced from the outside to the inside along the spiral circulation. This causes the magnetic field in the axial direction to increase from the outside to the inside when the current is constant.

In another advantageous embodiment, the conductor winding is constructed as follows. That is, the inner metal plate is configured to have a relatively higher current occupancy density than that of the outer metal plate. This causes the axial magnetic field to decrease radially from the inside to the outside. Due to this radial magnetic field gradient, according to the weak focusing method,
Focusing of the circulating electron beam is obtained.

In an alternative embodiment, the expansion of the electron beam is counteracted by a solenoid coil. The solenoid coil follows the helix of the beam guiding channel and is arranged to reach within this channel. This is because the magnetic field is formed in the Φ− direction. The solenoid coil projects beyond the beam guiding channel and also surrounds the anode device. This eliminates the need for the deflected electron beam to pass through the solenoid coil to reach the anode device. This eliminates significant beam obstruction and inoperable heating of the solenoid coil. Instead of the deflected electron beam reaching the anode unimpeded, the X-ray beam formed at the impact point of the anode must pass through the solenoid coil. This is so as to hit the center of the anode device. This configuration is advantageous because the X-ray beam contains only a small part of the energy of the electron beam and the working cross section of the material of the solenoid coil and also of the X-ray beam is relatively small.

There are basically two alternative embodiments for the anode device. The anode device can be covered by a spiral of beam guiding channels and arranged on top of it and concentric to it. In this embodiment, the anode device forms a tape-shaped metal surface. This metal surface, like the beam guiding channel, is also helical and is located on top of it. In this case, the electron beam has to be deflected upwards in order to hit the metal tape of the anode device. The substantially ring-shaped anode is not closed in this example and has a crack at one point on its outer circumference. In an alternative embodiment, the anode device is constructed as a closed circular ring. In this case, a correction magnetic field is especially necessary. This is so that the electron beam deflected from the predetermined position of the spiral beam guiding channel hits the circular ring-shaped anode surface. This is for example
This can be done by a compensating magnetic field created between the beam guiding channel and the anode device.

In either case, the tape-shaped metal surface of the anode device is preferably slightly tilted with respect to the plane of the substantially ring-shaped anode device. Thereby, the metal surface has a very small working extent with respect to the plane of the anode device, as seen from the center of the anode device. This allows the source of the X-ray beam to be very small in terms of divergence. A diaphragm selects a beam that is directed toward the center of the anode device. However, this beam is slightly tilted with respect to the plane of the anode device. This causes the beam to reach a detector ring located adjacent to the anode device.

The emission field for deflecting the electron beam is preferably formed as follows. That is, a large number of conductor windings are arranged on both sides of the beam guiding channel along the entire length of this guiding channel. This is achieved, for example, by winding the conductor winding around a metal plate. As a result, the surface normal of each winding extends substantially in the e direction. The conductor windings can be individually and selectively supplied with current. This is done, for example, by connecting one transistor to the conductor winding. On one side of the beam guiding channel, current is supplied to one or more adjacent conductor windings. The corresponding conductor windings are opposite. Current is supplied in the opposite direction on the other side of the beam guiding channel. This creates a radial magnetic field in the beam guiding channel. When a current is supplied to a plurality of conductor windings that are consecutive in the Φ-direction, the radial magnetic field rises linearly in the region of the conductor windings of the term. The Φ position of the radial magnetic field is moved along the spiral trajectory by the control means. This is done by sequentially connecting the feeding point and the take-out point of the conductor winding. This causes the radial magnetic field to circulate along the spiral trajectory and correspondingly at the point where the electron beam is deflected from the spiral plane.

[0019]

The present invention will be described in detail below.

A spiral trajectory is schematically shown in FIG. The electron beam e is guided by the beam guiding channel on the spiral orbit. At the point of incidence on the beam guiding channel, the initial radius of the spiral trajectory is R + W. This radius decreases to RW after one revolution along the orbit. Typical values here are 0.5 to 1 m for R and about 2.5 cm for W. The energy of the electron beam can be, for example, 140 keV, and the current intensity of the beam is 1A. The electron beam e is guided in the illustrated spiral trajectory by an axial magnetic field B in the beam guiding channel.

The magnetic field B is formed by the conductor windings 1, 3 circulating in the Φ-direction in the beam guiding channel. The beam guiding channel is closed by iron plates 2, 4 which follow the spiral track on both sides. Electric currents in opposite directions are supplied to the conductor windings 1 to 3 which circulate inside the iron plates 2 and 4, respectively. As a result, an axial magnetic field B is formed between the iron plates 2 and 4. The course of this magnetic field is shown schematically in FIG. Since the current occupation density in the inner iron plate 4 is different from the current occupation density in the outer iron plate 2,
The magnetic field created is not constant and decreases radially as shown at the bottom of FIG. This magnetic field gradient counteracts the spread of the electron beam e according to the weak focusing method. The difference in the current occupation density between the inner iron plate and the outer iron plate can be formed as follows, for example. That is, as shown in FIG. 2, the outer iron plate 2 has a relatively large height. This results in a relatively small current density for the same number of conductor windings.

In order to guide the electron beam e on the spiral orbit, the axial guiding magnetic field B must increase along the spiral orbit. This can be done, for example, by gradually reducing the height of the metal plates 2, 4 along a spiral trajectory. As a result, a corresponding increase in the current occupation density,
A corresponding increase in the axial guidance field B occurs.

An emission magnetic field forming means is provided so that the electron beam e can be deflected upward from the spiral orbit at an arbitrary point Φ. This emission field forming means is shown schematically in FIG. Shown is a cross section of the beam guiding channel. In this beam guiding channel, the iron plates 2, 4 are provided with current winding turns 10, 12 on either side of the beam guiding channel. The plane of the current winding turns 10, 12 is substantially perpendicular to the spiral trajectory. The current winding turns 10, 12 can be driven locally, which makes it possible to selectively supply current in the region of the very small current winding turns 10, 12 in the Φ-direction. In the extreme case, only one current winding turn can be supplied. In the illustrated embodiment, the predetermined Φ
In position, the current winding turns 10 to 12 are supplied with currents i to -i and some of the current winding turns are further supplied with currents i to i in the opposite direction in the Φ-direction. Therefore, the current flows only through the closed current winding turn, and does not flow in the other current winding turns. The opposing current winding turns 10, 12 form a magnetic field Br by currents in opposite directions. This magnetic field extends substantially radially with respect to the plane of the helix and its amplitude is shown diagrammatically at the bottom of FIG. Radial magnetic field B
r rises linearly in the region of the current winding turns fed with current. In order to avoid a relatively large magnetic field in the Φ-direction of the iron plate, it is advantageous to form the radial magnetic field Br at a remote location on the outer circumference with opposite polarities. This magnetic field is formed in FIG. 3 by currents in opposite directions at remote locations on the outer circumference.

The effect of the emission magnetic field formed as shown in FIG. 3 is shown in FIG. At a position in the radial direction Br, the electron beam e is (b
It is deflected in the − direction and the Z- direction. In order to achieve such deflection, the radial magnetic field amplitude must typically be significantly larger than the axial magnetic field amplitude. For example, the axial magnetic field Bz is 30 G and the radial emission magnetic field Br is 110 G. When emitting from the Br magnetic field,
The beam e'is focused in the Φ-direction (edge focusing) and is deflected to the anode. Anode device 30
The metal surface of which the electron beam e'is deflected is slightly tilted with respect to the anode plane. This causes the metal surface 30 to have a very small Z-spread when viewed from the center of the substantially ring shaped anode device. In this way,
The source size of the X-ray beam is very well defined by the small spread in the Z-direction and the good focusing of the electron beam e'in the Φ-direction.

By moving in the region of the current winding turn to which the current is fed as shown in FIG. 3, the deflection point of the electron beam e is changed in the Φ-direction along the spiral orbit.
Correspondingly, the point of impact of the deflected electron beam e'on the metal surface of the anode changes, and the point of generation of the X-ray beam also changes (b-direction).
The 12 turn spacing can be less than 1 mm. This allows the focus on the anode to be
You can move in any direction. Current winding turns 10, 1
For two local current supplies, each current winding turn 10, 12 must basically be provided with a controllable device for current supply. This can be achieved, for example, by means of transistors.

The configuration for the anode device is basically 2
There are two examples. In one embodiment, the tape-shaped metal surface follows the helical anode 30 of the beam guiding channel. In this case, the tape-shaped metal surface 30 has a spiral shape and jumps by a value of 2 W at a corresponding point on the outer periphery thereof. However, this is not a problem for typical dimensions less than 10% of the radius.

In an alternative embodiment, an anode 30 having a circular ring-shaped metal surface is used. In this case, for each deflection point of the electron beam e along the spiral electron orbit, a radial beam deflection depending on Φ is ensured in order to ensure that the deflected electron beam e ′ hits the anode ring. is necessary. This is achieved, for example, by a correction field. The correction field is achieved by providing specially configured current winding turns 11, 13 along the spiral trajectory. This is shown in FIG. Alternatively,
The slope of the current winding turns 10, 12 shown in FIG. 3 can be varied to obtain the required correction, depending on Φ. This is shown in FIG. The result of such a correction is schematically shown in FIG. Here, the beam guiding channel is shown in cross section in the region of overlap of the start and end regions. Here, the starting region of the beam guiding channel has a radius R + W and the ending region has a radius R−W.
It is. The ring-shaped anode has a radius R. The deflected electron beam e ′ is caused to hit the anode ring in any case by the correction magnetic field forming means 101, 121 or 11, 13.

In the case of non-relativistic particle beams, large beam currents result in axially and radially defocused space charge effects. This effect is greater the smaller the beam cross section. The weak focusing initially described here compensates for space charge effects when the beam current is 1 A, the beam energy is 140 KeV and the round beam cross section has a radius of 1 cm. If the energy and beam cross section are relatively small, the current must be reduced.

The space charge effect described above occurs at full intensity only when the vacuum is complete or artificial ion pumping. In the device described here, due to the space-charge electric field of the beam, no significant ionic action occurs with the guiding magnetic field. In the unlikely event that ion accumulation can be easily avoided by the additional radial electric field.

The device described thus far is current limited as described above because of the space charge effect of the circulating electron beam and the weak focusing that limits the effect.

This limitation is almost eliminated by using a longitudinal solenoid field rather than using a weak gradient field (see FIG. 2) for beam focusing of the electron beam. In an embodiment of the device according to the invention, the electron beam e is focused by a longitudinal solenoid field. This is shown in FIG. Beam guide channel steel plates 2, 4
A solenoid 40 that follows a spiral orbit is disposed between the two. An electron beam e and an anode are arranged in the cross section of this. This eliminates the problem of the anode also being inside the solenoid 40 and extracting the electron beam e'through the winding turns of the solenoid. Instead, the x-ray beam formed on the metal surface of the anode must pass through the very thin wire of the solenoid. However, the thermal loading of the wire can be tolerated.

Also in the embodiment shown in FIG. 8, the emission magnetic field can be formed as shown in FIGS. 3, 4a and 4b. On the other hand, the guide magnetic field B changing in the radial direction shown in FIG.
A constant magnetic field can be used instead of r. Because weak focusing is no longer needed.

The basic idea of the present invention is that in a device for forming a circulating X-ray beam for high speed computer tomography, a solenoid coil 40 reaching a beam guiding channel.
And the solenoid coil follows the beam guiding channel in the Φ-direction, thereby focusing the electron beam e ′ inside the solenoid coil 40 in the beam guiding channel, and the anode device 30 inside the solenoid coil 40. To be there.

The device according to the invention can have circular or approximately circular beam guiding channels as well as helical beam guiding channels.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the trajectory of an electron beam in a beam guiding channel.

FIG. 2 is a schematic view showing a cross section of a beam guiding channel.

FIG. 3 is a partial perspective view of a beam guiding channel.

FIG. 4a is a side view of the beam guiding channel and b is a cross-sectional view of its RZ-plane.

FIG. 5 is a perspective view of a modified embodiment of the beam guiding channel.

FIG. 6 is a cross-sectional view of an alternative embodiment of a beam guiding channel and anode device.

FIG. 7 is a cross-sectional view of the beam guiding channel in the region where the starting region and the ending region of the spiral beam guiding channel overlap.

FIG. 8 is a schematic view of another embodiment of the present invention.

[Explanation of symbols]

 1, 3 Conductor winding 2, 4 Iron plate 10, 12 Current winding turn 30 Anode

Claims (17)

[Claims] 1. An apparatus for forming a circulating X-ray beam for high speed computer tomography, comprising a beam guiding channel, a guiding magnetic field forming means (1, 2, 3, 4) and an emission magnetic field forming means (10, 12). And a control means, wherein the beam guiding channel spirally extends in one plane over a polar angle of at least 2π, and the guiding magnetic field forming means defines a beam guiding channel in the beam guiding channel with respect to the plane. Forming a vertically extending axial magnetic field (Bz), the intensity of said axial magnetic field (Bz) is such that an electron beam (e) of a given energy is directed substantially centrally within the beam guiding channel. A magnetic field which increases along the helical beam guiding channel and extends substantially radially in said plane into the Φ-selectable region of the beam guiding channel by said emission field forming means ( r) is formed, whereby the electron beam passing in the beam guiding channel is deflected from said plane and is focused in the Φ-direction onto the anode device, said anode device being substantially ring-shaped and beam guiding. An X-ray beam is formed concentrically with the channel, where the anode device collides with the electron beam (e ′), and the control means can select the emission magnetic field forming means to form a radial magnetic field. An apparatus for forming a circulating X-ray beam, characterized in that the different regions are controlled to circulate along the beam guiding channel in the Φ-direction. 2. The guide magnetic field forming means comprises two metal plates (2, 4) spirally extending in the Φ-direction perpendicular to the spiral plane, and Φ in the metal plates (2, 4). A conductor winding (1, 3) extending in the negative direction, said metal plate enclosing a beam guiding channel therein, a conductor winding arranged on an inner spiral metal plate (4) and an outer spiral Device according to claim 1, characterized in that a current is supplied in the opposite direction to the conductor windings arranged on the metal plate (2), whereby a magnetic directional magnetic field (Bz) is formed in the beam guiding channel. 3. The height of the metal plates (2, 4) decreases from the outer side to the inner side along the outer circumference of the spiral, whereby the strength of the axial magnetic field (Bz) is reduced by the conductor windings (1, 3) If the current in is constant, it increases along the spiral from the outside to the inside,
An apparatus according to claim 2. 4. Conductor windings (1, 3) extending in the Φ-direction.
Are configured such that the current occupancy density in the inner metal plate (4) is greater than the current occupancy density in the outer metal plate, so that the axial magnetic field (B) is radially from the inside to the outside, the beam guiding channel. Claim 2 which decreases within
Or the apparatus according to 3. 5. A solenoid coil (40) is provided reaching the beam guiding channel, the solenoid coil spirally following the beam guiding channel in the Φ-direction, whereby the electron beam (e) is directed to the beam. Device according to any one of the preceding claims, wherein the anode device (30) is focused inside the solenoid coil (40) in the guide channel and the anode device (30) is arranged in the solenoid coil (40). 6. The anode device (30) has a tape-shaped metal surface, said metal surface according to the spiral shape of the beam guiding channel.
Device according to any one of claims 1 to 5, arranged concentrically to the upper part of the beam guiding channel. 7. The anode device (30) has a circular ring-shaped metal surface, the diameter of the metal surface approximately corresponding to the mean diameter of the spiral beam guiding channel, the metal surface being the beam guiding channel. The correction field forming means (11, 13), which is arranged concentrically with respect to this on the upper part of the electron beam deflecting electron beam (e ') from the beam guiding channel, forms a circular ring-shaped metal of the anode device (30). 6. A device according to any one of claims 1 to 5, provided for directing to a surface. 8. The metal surface of the anode device (30) is tilted with respect to the plane of the substantially ring-shaped anode device, whereby the metal surface is visible from the center of the anode device. Or the device according to 7. 9. The emission magnetic field forming means comprises a large number of conductor winding turns (10, 12) on both sides of the beam guiding channel over the entire length of the channel, and selectively supplies a current to the conductor winding turns in the Φ-direction. Means for supplying, each said conductor winding turn being arranged respectively such that its surface normal extends substantially in the Φ-direction. The device according to claim 1. 10. Device according to claim 9, characterized in that the conductor winding turns (10, 12) of the emission field forming means are wound around the metal plates (2, 4). 11. A device according to claim 9, wherein a transistor is connected to each of the conductor winding turns (10, 12), and a current is selectively supplied to each conductor winding turn. 12. The emission magnetic field forming means comprises a large number of conductor winding turns (10, 12) on both sides of the beam guiding channel over the entire length of the channel and Φ− on the conductor winding turns.
Means for selectively supplying an electric current in a direction, the conductor winding turns each being arranged such that their surface normals are inclined with respect to the spiral plane, whereby at the same time Device according to claim 7, characterized in that an electron beam (e '), which is used as a correction field forming means and which is axially deflected from the beam guiding channel, is directed onto the circular ring-shaped metal surface of the anode device (30). 13. A method of forming an X-ray beam for computer tomography, wherein the X-ray beam is directed to the center and circulates over a polar angle of 2π: an electron beam of a predetermined energy ( e) forming: -injecting the electron beam (e) into a beam guide, the beam guide directing the electron beam into a spiral orbit by an axial magnetic field (Br); To a substantially ring-shaped anode device disposed at the upper part of the beam guide portion at a selected position in the direction from the collision point of the electron beam (e ′) to the substantially ring-shaped anode device. Forming an X-ray beam directed at the center of the electron beam, e.g. Substantially X-ray beam forming method characterized by having a step of changing accordingly the collision point to the ring-shaped anode (30) of. 14. An apparatus for forming a circulating X-ray beam for high speed computer tomography, comprising a beam guiding channel and a guiding magnetic field forming means (1, 2, 3, 4), said beam guiding channel comprising: Extending over a polar angle of at least 2π, said guiding field forming means forms a magnetic field (Bz) in the beam guiding channel such that the electron beam (e) is substantially centered in the beam guiding channel at a given energy. Solenoid coil guided to the beam guiding channel (40)
A solenoid coil follows the beam guiding channel, and the anode device (30) is arranged inside the solenoid coil (40). 15. The device according to claim 14, wherein the beam guiding channels are configured in a spiral shape. 16. The apparatus of claim 14, wherein the beam guiding channels are helically configured and extend in one plane. 17. The apparatus according to claim 14, wherein the beam guiding channel is configured at least approximately in the shape of a circular ring and extends over a polar angle of at least 2π.