Multi X-ray imaging apparatus and control method therefor

An X-ray imaging apparatus includes a multi X-ray source which includes a plurality of X-ray focuses to generate X-rays by irradiating X-ray targets with electron beams, a detector which detects X-rays which have been emitted from the multi X-ray source and have reached a detection surface, and a moving mechanism for moving the multi X-ray source within a plane facing the detection surface. The X-ray imaging apparatus acquires a plurality of X-ray detection signals from the detector by causing the multi X-ray source to perform X-ray irradiation while shifting the positions of a plurality of X-ray focuses which the detector has relative to the detection surface by moving the multi X-ray source using the moving mechanism. The apparatus then generates an X-ray projection image based on the plurality of X-ray detection signals acquired by the detector.

TECHNICAL FIELD

The present invention relates to a multi X-ray imaging apparatus used for nondestructive X-ray imaging, diagnosis, and the like in the fields of medical equipment and industrial equipment using X-ray sources, and a control method for the apparatus.

BACKGROUND ART

A general X-ray tube uses a thermal electron source as an electron source. An X-ray tube of this type generates X-rays on the electron beam incident side by irradiating an X-ray target made of a bulk metal with thermal electrons which are emitted from a filament heated to a high temperature and spread widely. The generated X-rays are then used. A point source type X-ray tube therefore forms a pseudo point X-ray light source by obliquely extracting an elongated X-ray focus. The uniformity of an X-ray intensity distribution has been improved by spacing an X-ray light source apart from the position of an object.

Recently, a cold cathode multi electron source has been proposed as an electron source replacing this thermal electron source. In addition, as an application of this technique, a flat type multi X-ray generating apparatus with a devised method of extracting multi X-ray beams has been proposed (patent reference 1).

Furthermore, it has been proposed to use a multi X-ray source in the field of X-ray CT in which conventional point light source type X-ray tubes have been used. For example, there has been proposed a method of forming a three-dimensional CT image by measuring X-ray transmission data while rotating a combination of a multi X-ray source and a flat type two-dimensional sensor about the axis of an object and moving them along the axis (patent reference 2).

PRIOR ART REFERENCE

Patent References

Patent reference 1: Japanese Patent Application No. 2006-057846

When, however, an X-ray projection image is to be formed by using a multi X-ray source having a plurality of focuses, since the interval between the focuses of the multi X-ray source is about several mm, X-ray transmission data about an object becomes discrete. This makes it difficult to obtain a high-resolution two-dimensional transmission X-ray image.

The present invention has been made in consideration of the above problem, and has as its object to acquire a high-resolution transmission X-ray image by using a multi X-ray source.

SUMMARY OF THE INVENTION

In order to achieve the above object, an X-ray imaging apparatus according to an aspect of the present invention has the following arrangement. That is, the apparatus comprises:

a multi X-ray source which includes a plurality of X-ray focuses to generate X-rays by irradiating X-ray targets with electron beams;

a detector which detects X-rays which have been emitted from the multi X-ray source and have reached a detection surface;

moving means for moving the multi X-ray source within a plane facing the detection surface;

acquisition means for acquiring an X-ray detection signal from the detector for each irradiation by performing X-ray irradiation a plurality of number of times using the multi X-ray source while shifting the multi X-ray source relative to the detection surface by using the moving means; and

generating means for generating an X-ray projection image based on a plurality of X-ray detection signals acquired by the acquisition means.

In addition, in order to achieve the above object, a control method for an X-ray imaging apparatus according to an aspect of the present invention is a control method for an X-ray imaging apparatus including:

a multi X-ray source which includes a plurality of X-ray focuses to generate X-rays by irradiating X-ray targets with electron beams;

a detector which detects X-rays which have been emitted from the multi X-ray source and have reached a detection surface; and

moving means for moving the multi X-ray source within a plane facing the detection surface,

the control method comprising:

an acquisition step of acquiring X-ray detection signals from the detector for each irradiation by causing the multi X-ray source to perform X-ray irradiation a plurality of number of times while shifting the multi X-ray source relative to the detection surface by using the moving means; and

a generating step of generating an X-ray projection image based on a plurality of X-ray detection signals acquired in the acquisition step.

According to the present invention, it is possible to acquire a high-resolution transmission X-ray image by using a multi X-ray source.

Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference numerals designate the same or similar parts throughout the figures thereof.

DETAILED DESCRIPTION OF THE EMBODIMENTS

First Embodiment

FIG. 1is a view showing the arrangement of a multi X-ray source body10which has a plurality of X-ray focuses to generate X-rays by irradiating X-ray targets with electron beams. A multi electron beam generating unit12and transmissive targets13as X-ray targets are arranged in a vacuum chamber11of the multi X-ray source body10. The multi electron beam generating unit12includes an element substrate14and an element array16having a plurality of electron emitting elements15arrayed on the element substrate14. A driving signal unit17controls the driving of the electron emitting elements15. A lens electrode19and an anode electrode20are provided to control multi electron beams e generated from the electron emitting elements15. High voltages are applied to the electrodes19and20via high voltage introduction portions21and22. The lens electrode19is fixed to the element substrate14through an insulator18.

The transmissive targets13upon which electron beams e emitted from the electron emitting elements15impinge are discretely placed in correspondence with the plurality of electron emitting elements15to form X-ray focuses, respectively. In addition, the targets13are provided with vacuum chamber X-ray shield plates23made of a heavy metal. The vacuum chamber X-ray shield plates23are provided with X-ray extraction portions24. Wall portions25of the vacuum chamber11located in front of the X-ray extraction portions24are provided with X-ray extraction windows27including X-ray transmission films26.

The electron beams e emitted from the electron emitting elements15receive the lens effect of the lens electrode19, and are accelerated to the final potential level by portions of the transmissive targets13of the anode electrode20. X-ray beams x generated by the targets13pass through the X-ray extraction portions24and are extracted to the atmosphere through the X-ray extraction windows27. The multi X-ray source body10is provided with a scanning mechanism34for two-dimensionally scanning the multi X-ray source within a plane facing the detection surface of a detector35. The scanning mechanism34moves the position of the multi X-ray source in synchronism with the generation of X-rays from the multi X-ray source.

The detector35detects the X-rays which have been emitted from the multi X-ray source and have reached the detection surface. A controller300includes a CPU and ROM (not shown) and controls the overall X-ray imaging apparatus according to this embodiment, which includes the multi X-ray source body10and the detector35. That is, the controller300performs X-ray irradiation while shifting the multi X-ray source relative to the detection surface by causing the scanning mechanism34to move the multi X-ray source. In this manner, the controller300acquires a plurality of detection signals by acquiring X-ray detection signals from the detector35at the respective positions to which the multi X-ray source has been shifted. The controller300then generates an X-ray projection image based on these detection signals (X-ray transmission intensity data) and the positions of the multi X-ray source at the times when the detection signals have been acquired. The X-ray imaging operation of this embodiment will be described in detail below.

The electron emitting elements15are two-dimensionally arrayed on the element array16, as shown inFIG. 2. With recent advances in nanotechnology, it is possible to form a fine structure with nm (nanometer) size at a predetermined position by a device process. The electron emitting elements15are manufactured by this nanotechnology. The electron emission amounts of the electron emitting elements15are individually controlled by driving signals S1and S2(to be described later) via the driving signal unit17. That is, individually controlling the electron emission amounts of the element array16by using the driving signals S1and S2as matrix signals makes it possible to individually ON/OFF-control X-ray beams constituting a multi X-ray beam.

A cold cathode type electron emitting element can emit electrons by only applying a voltage of several 10 V to several kV to the electron emitting element. An X-ray generating apparatus using this electron emitting element as an electron source therefore need not heat the cathode and requires no wait time for the generation of X-rays. In addition, since no power is required for heating the cathode, a low-power-consumption X-ray source can be obtained even by using a multi X-ray source. Since currents for these electron emitting elements can be ON/OFF-controlled by high-speed driving operation using driving voltages, a multiarray type X-ray source can be manufactured, which selects an electron emitting element to be driven and performs high-speed response operation.

When multi X-ray beams are actually formed, members serving as shield slits for limiting the radiation angles of X-rays generated at the respective X-ray focuses are required near the positions of the X-ray focuses. Therefore, an interval of several mm or more is required in the multi X-ray source.

FIG. 3is a view schematically showing an example of a scanning type multi X-ray source30according to this embodiment, which includes a multi X-ray source31having 12×12 X-ray focuses (to be also referred to as X-ray sources hereinafter) arrayed at 20-mm intervals. The multi X-ray source31includes multi X-ray units Bij each having an array of 3×3 X-ray sources33. These multi X-ray units are arrayed in a 4×4 matrix. As shown inFIGS. 1 and 2, each X-ray source33includes the electron emitting element15and the target13. Note that the multi X-ray source31is controlled such that one X-ray source of each multi X-ray unit32generates X-rays in one X-ray irradiation. In each multi X-ray unit32, the X-ray focuses of the radiation source array are sequentially scanned. The scanning mechanism34is provided to move the overall multi X-ray source31. The scanning mechanism34can move the overall multi X-ray source31throughout at least the distance between the focuses of the multi X-ray source, that is, throughout the distance between adjacent X-ray focuses. Moving the multi X-ray source31using the scanning mechanism34is equivalent to moving the multi X-ray source body10in this embodiment.

Note that the controller300includes a CPU and ROM (not shown) and controls the overall scanning type multi X-ray source30, as described above. The controller300controls the multi X-ray source31, the scanning mechanism34, and the detector35by executing predetermined control programs, as will be described below, thereby executing X-ray imaging.

FIG. 4is an enlarged view of the positions of the X-ray focuses of the multi X-ray unit Bij described above, more specifically, a case in which the position of a generated X-ray source sequentially moves in the order of m(1,1), m(1,2), m(1,3), m(2,1), . . . .FIG. 5shows how the X-ray beams generated by X-ray sources of a multi X-ray unit spread.FIG. 5is a view showing how the multi X-ray source31generates X-rays, when viewed from the side, and an array of multi X-ray units B11to B14is indicated. In each multi X-ray unit, X-ray sources m(1,1), m(1,2), and m(1,3), which are the X-ray sources in the unit, are arrayed. In this case, reference symbols x1, x2, and x3denote X-ray beams emitted from the respective X-ray sources.

When the X-ray sources m(1,1) are to generate X-rays, the X-ray sources m(1,1) at the positions of the respective multi X-ray units are ready for the generation of X-rays. The divergence angles of the X-ray beams generated from the X-ray sources m(1,1) of all the multi X-ray units are controlled to prevent interference with each other on the detector35. This applies to the remaining X-ray sources of the respective multi X-ray units. That is, in this embodiment, all the multi X-ray units simultaneously drive X-ray sources in the order of X-ray sources m(1,1)→X-ray sources m(1,3), X-ray sources m(2,1)→X-ray sources m(2,3), X-ray sources m(3,1)→X-ray sources m(3,3). In other words, the plurality of X-ray sources33of the multi X-ray source31are divided into groups so as to form groups according to the X-ray focuses of X-rays which do not interfere with each other on the detection surface of the detector35even if the X-ray sources are made to simultaneously generate X-rays. That is, in this embodiment, the X-ray sources are grouped such that the X-ray sources m(1,1) of the respective units belong to the first group, and the X-ray sources m(1,2) of the respective units belong to the second group. The plurality of X-ray sources33are then driven for each group to generate X-rays.

When X-ray irradiation from the X-ray source m(1,1) is complete and X-ray irradiation from the next X-ray source starts, an X-ray detection signal obtained by this X-ray beam is stored as image data in the memory (not shown) of the controller. In addition, the position of the multi X-ray source31at this time is held in the memory for the generation of a projection image. Thereafter, the next X-ray source m(1,2) is driven to perform X-ray irradiation. While the X-ray sources33are sequentially turned on in each multi X-ray unit32, the X-ray transmission image data of an object36is acquired via the detector35.

The X-ray transmission image data obtained in the above manner is an X-ray image from positions spaced apart from each other by the interval between the X-ray sources (20 mm in this case). For this reason, when a transmission X-ray image is reproduced from these image data, X-rays obliquely and discretely strike the object36. When, therefore, these data are converted into a projection image, a high-quality projection image cannot be expected because the image data have defects.

In this embodiment, therefore, in order to implement proximity projection imaging with high image quality by eliminating such data defects between X-ray sources, X-ray projection data are acquired by performing X-ray irradiation a plurality of number of times while finely moving the position of the multi X-ray source31inFIG. 3using the scanning mechanism34. Using the scanning type multi X-ray source30described above will acquire X-ray projection data between the respective multi X-ray sources (e.g., between m(1,1) and m(1,2) of the multi X-ray source). It is therefore possible to implement a proximity projection imaging apparatus which can acquire high-resolution images.

The manner of how X-ray imaging is actually performed while X-ray sources are moved by using the scanning type multi X-ray source30will be described with reference toFIG. 6. Consider first one X-ray source m(k,1) of the X-ray source array in a multi X-ray unit. First of all, when the multi X-ray source m(k,1) emits X-rays at a position p1, detectors d0to d9detect the transmitted X-rays. The scanning mechanism34then moves the position of the multi X-ray source to p2. When the multi X-ray source m(k,1) emits X-rays at a position p2after the movement, detectors d1to d10detect the transmitted X-rays. In this manner, transmission X-ray data are acquired while the multi X-ray source is repeatedly moved to the adjacent multi X-ray source from p1to p10, and the X-ray sources of the radiation source array in each unit repeatedly emit X-rays.

FIG. 7shows the temporal operation of each X-ray source of the radiation source array in each unit in association with the above X-ray irradiation method. The X-ray source m(1,1) is turned on at the position p1for a time Δt. Subsequently, the X-ray sources are sequentially switched and turned on up to the X-ray source m(3,3). The position of the multi X-ray source31then moves from p1to p2, and X-rays are repeatedly turned on in the same manner.

The flowchart ofFIG. 12summarizes, as follows, the imaging operation of the X-ray imaging apparatus according to the first embodiment described above. Assume that X-ray sources are arrayed in a 3×3 matrix in the multi X-ray unit32, as shown inFIG. 4, and both kmax and lmax in the flowchart are 3.

The controller300moves the multi X-ray source31to a reference position having a predetermined positional relationship with the detector35by using the scanning mechanism34(step S101). First of all, the controller300selects the X-ray source m(1,1) of each multi X-ray unit, and executes the acquisition processing of making these X-ray sources simultaneously generate X-rays and making the detector35obtain X-ray image information (steps S102and S103). Thereafter, the controller300sequentially selects and drives the X-ray sources m(1,2) and m(1,3) and repeats the above acquisition processing. That is, the controller300sequentially drives the X-ray sources m(1,2) and m(1,3) of each multi X-ray unit, and obtains the respective pieces of X-ray image information by using the detector35(steps S103to S105).

Subsequently, the controller300repeats steps S103to S105described above with respect to k=2. That is, the controller300sequentially drives the X-ray sources m(2,1) to m(2,3) of each multi X-ray unit, and obtains X-ray image information by using the detector35(steps S106and S107). Likewise, the controller300repeats steps S103to S105described above with respect to k=3. That is, the controller300sequentially drives the X-ray sources m(3,1) to m(3,3) of each multi X-ray unit, and obtains X-ray image information by using the detector35(steps S106and S107). Upon completing the acquisition processing using all the X-ray sources in each multi X-ray unit, the controller300moves the multi X-ray source31from, for example, p1to p2inFIG. 6by using the scanning mechanism34. The controller300then repeats the processing in steps S102to5107described above at the position p2(steps S108and S109).

When the multi X-ray source31reaches the position p10and X-ray irradiation and detection at the position p10are completed by repeating the above processing, the process advances from step S108to step S110. In step S110, the controller300obtains an X-ray projection image by performing image generation using the X-ray image information acquired in step S103.

As described above, the first embodiment can acquire the data of a high-quality X-ray projection image using the multi X-ray source by causing the multi X-ray source to perform X-ray irradiation (scanning) while moving the multi X-ray source within the plane of the multi X-ray source and in the range of the X-ray source intervals. That is, since high-resolution X-ray imaging can be performed in spite of the fact that the multi X-ray source is placed near the two-dimensional flat type detector, a high-resolution and compact X-ray projection imaging apparatus can be obtained. In addition, since imaging is performed while X-ray sources are placed near the detector, the power of X-rays can be efficiently used. This can obtain a low-cost X-ray apparatus with reduced leakage X-rays to the surroundings. In addition, since the X-ray sources of the multi X-ray source which are to be simultaneously driven are selected so as to avoid interference of X-rays from the respective X-ray sources on the detection surface, interference of X-rays from different X-ray sources can be prevented, and an X-ray image with higher resolution can be obtained.

Second Embodiment

The intensity of X-rays obtained by the X-ray sources of the radiation source array in each multi X-ray unit depends on the melting point of an X-ray target material or its cooling system, an accelerating voltage for electron beams, a current value, a focus size, an irradiation time, an X-ray extraction method, and the like.

Since the maximum X-ray power of a conventional X-ray tube is determined by the temperature limit of an X-ray target material to be used, thermal diffusion is performed by mechanically rotating the X-ray target so as to sequentially move the irradiation position, thereby extracting a higher X-ray power. In contrast to this, the scheme used by this embodiment performs thermal diffusion of an X-ray target by electrically scanning the position of the multi electron source to allow the injection of higher X-ray power. These specific examples will be described with reference toFIG. 8.

FIG. 8shows temporal changes in the temperature of the surface of an X-ray target when the target is irradiated with an electron beam. “A” and “B” inFIG. 8respectively represent examples of the waveform of an electron current (“8B” inFIG. 8) at the time of the generation of a rectangular pulse signal as a driving signal for an electron source, and changes in the surface temperature of the X-ray target (“8A” inFIG. 8). A temperature Tm of an X-ray target quickly rises for 1 ms during which X-rays are generated by pulse currents, and the surface temperature of the X-ray target is restored to an initial temperature state “To” owing to heat conduction to the peripheral structure for about 9 ms from the time when a pulse current is turned off. In this embodiment, in order to effectively generate X-rays from the multi X-ray source, one X-ray source of each multi X-ray unit32inFIG. 3is always turned on. The time during which all the X-ray sources of the radiation source array in each basic unit are OFF is set to a cooling time for the surface temperature of the X-ray target. This makes it possible to form a high-power X-ray source exploiting the characteristics of the multi X-ray source.

In order to safely operating such a multi X-ray source while extracting power from it, it is important to manage the setting about an X-ray irradiation time Δt of each radiation source array in each basic unit shown inFIG. 7so as not to exceed the temperature limit of the X-ray target.

For example, the surface temperature Tm of the X-ray target is determined by parameters including the current, voltage, and Δt of the multi X-ray source in association with a temperature allowable value Tmax of the X-ray target. Therefore, Δt is set to hold
Tm=T(voltage,current,Δt)
Tm<Tmax
The safety of the multi X-ray source can be improved by having these data in advance as a function or data, and by determining a maximum mAs value to be applied to the X-ray target. In addition, the time interval of generation of X-rays from one X-ray source, that is, the time interval of irradiation of electron beams, is at least the period during which the temperature of the X-ray target which has been raised by the irradiation of an electron beam lowers to the first temperature or lower (To or lower).

Note that if the X-ray dose required for an X-ray projection image exceeds this mAs value, it is possible to make setting so as to cause the multi X-ray source to automatically repeat necessary X-ray irradiation at the same position. Using this method can perform imaging while the performance of the multi X-ray source is maximized.

Third Embodiment

The first embodiment has exemplified the projection imaging method in a case in which the multi X-ray source having the two-dimensional array is used. The third embodiment will exemplify an imaging apparatus and an imaging method with reference toFIG. 9in a case in which a multi X-ray source having a one-dimensional array is used.

An elongated detector42is placed at a position facing a multi X-ray source40having a one-dimensional array. An object is placed in a direction parallel to the multi X-ray source40with the one-dimensional array and the detector42. The multi X-ray source40includes a one-dimensional array multi X-ray unit and X-ray sources constituting a radiation source array in a one-dimensional unit. In the case ofFIG. 9, each multi X-ray unit includes X-ray sources m(1,1), m(1,2), and m(1,3).

When the multi X-ray source is made to generate X-rays by repeating the operation of sequentially turning on X-ray sources m(1,1), m(1,2), and m(1,3) and moving the multi X-ray source40in the array direction, thereby acquiring X-ray projection data. At the stage at which the multi X-ray source40is scanned by a width corresponding to the array interval of X-ray sources, an object is moved in a direction perpendicular to the array direction41of the X-ray sources in the multi X-ray source40. Note that in place of moving the object, it is possible to use a method of simultaneously moving the multi X-ray source40and the detector42in a direction perpendicular to the array direction of the X-ray sources.

FIG. 10is a view showing an example of an arrangement configured to move the multi X-ray source40and the detector42together. In this case, the multi X-ray source40and the detector42are fixed by a support unit43. A driving unit45on a base44moves the multi X-ray source40and the detector42in synchronism with the generation of X-rays from X-ray sources.

As described above, the third embodiment uses the multi X-ray source40having a one-dimensional array of a plurality of X-ray focuses. A controller300causes the multi X-ray source40to emit X-rays while causing a scanning mechanism34to move a multi X-ray source40in an array direction41of the plurality of X-ray focuses, thereby acquiring X-ray detection signals using the detector42. The controller300then obtains two-dimensional X-ray image information by repeating this processing while moving the multi X-ray source40in a direction perpendicular to the array direction41. Note that if the detector42has a sufficiently large detection area, it is possible to move only the multi X-ray source40in the direction perpendicular to the array direction41of the multi X-ray source40. The arrangement shown inFIGS. 9 and 10is configured to move both the multi X-ray source40and the detector42in the direction perpendicular to the array direction41because the detection surface of the detector42corresponds to an area of the multi X-ray source40which corresponds to one irradiation.

As described above, according to the third embodiment, it is possible to form an X-ray projection apparatus using a very compact, low-cost scanning type multi X-ray source by using a one-dimensional multi X-ray source.

Fourth Embodiment

FIG. 11shows a method of obtaining high-quality X-ray projection data in the driving mechanism of a scanning type multi X-ray source. In order to obtain a high-resolution X-ray image, it is necessary to set the accuracy of attitude control of the multi X-ray source to several 10 μm or less. A simple method of implementing this is to attach a position detector38using an optical means to a scanning type multi X-ray source30according to the fourth embodiment, in addition to a scanning mechanism34for a multi X-ray source31, to read the position of the multi X-ray source31. The position detector38reads the position of the multi X-ray source31at the time of X-ray irradiation. It is possible to convert X-ray transmission intensity data (detection signal) into a high-resolution projection image by using this position data as X-ray source position correction data at the time of conversion from the X-ray transmission intensity data into the X-ray projection image.

As has been described above, according to each embodiment described above, it is possible to convert the transmission X-ray data acquired by the scanning type multi X-ray apparatus into an X-ray projection image by using a conventional method of obtaining a tomogram. This makes it possible to provide a compact X-ray projection imaging apparatus which exploits the characteristics of a multi X-ray source and can acquire a high-resolution image.

Other Embodiments

In addition, the present invention is implemented by executing the following processing. That is, this processing is to supply software (program) for implementing the functions of the above embodiments to a system or apparatus via a network or various kinds of storage media and cause the computer (or CPU, MPU, or the like) of the system or apparatus to read out and execute the program.

This application claims the benefit of Japanese Patent Application No. 2008-239754, filed Sep. 18, 2008, which is hereby incorporated by reference herein in its entirety.