CN112489849A - Filter device for spectral filtering of X-ray beams of a computed tomography apparatus - Google Patents
Filter device for spectral filtering of X-ray beams of a computed tomography apparatus Download PDFInfo
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Abstract
Embodiments of the present invention relate to a filter arrangement for spectral filtering of an X-ray beam of a computed tomography apparatus. In one aspect, the invention relates to a filter arrangement for spectral filtering of an X-ray fan beam of a computed tomography apparatus, the filter arrangement comprising: -a first filter area comprising a first spectral filter function, -a second filter area comprising a second spectral filter function, the second spectral filter function being different from the first spectral filter function, -the first filter area and the second filter area being consecutively arranged in a direction perpendicular to a central plane of the X-ray fan beam and adjacent to each other along a dividing line, -wherein the first filter area covers a first cross-sectional area of the X-ray fan beam and the second filter area covers a second cross-sectional area of the X-ray fan beam when the filter device is placed in a first position relative to the X-ray fan beam.
Description
Technical Field
In one aspect, the invention relates to a filter device for spectral filtering of an X-ray fan beam of a computed tomography apparatus. In another aspect, the present invention relates to a computed tomography apparatus. In another aspect, the invention relates to a method for providing spectral computed tomography data with a computed tomography apparatus. In a further aspect, the invention relates to a method for producing a filter device for spectral filtering of an X-ray fan beam of a computed tomography apparatus. In a further aspect, the invention relates to a method for locating defects in a filter arrangement for spectral filtering of an X-ray fan beam of a computed tomography apparatus.
Background
Compared to conventional single-energy Computed Tomography (CT) scanners, spectral computed tomography, for example in the form of dual-energy spectral computed tomography, allows for more accurate differentiation of chemical processes of tissue and disease processes based on different X-ray spectra.
In the known dual energy CT technique, a first example uses two X-ray tubes and two detectors to obtain simultaneous dual energy acquisition and data processing. An advantage of this example is that patient scans are performed very quickly, such as in cardiac imaging, which may minimize the risk of registration errors and respiratory motion artifacts. This design results in extremely complex hardware and data processing difficulties.
A second example uses a single X-ray tube that rapidly alternates tube voltage and a single detector that rapidly registers information from both low and high energies. The switching of high kilovolt voltages often leads to high requirements for the X-ray tube and the generator and to poor life performance thereof. Data processing has synchronization issues among low energy, high energy and patient motion.
A third example of spectral CT uses a layered detector, which is also referred to as a "sandwich" detector. The system consists of a single X-ray tube and a two-layer detector that detects two energy levels simultaneously. These layered semiconductor scintillators have considerable spectral overlap problems. The technical challenges and expense of this system are associated with dedicated detector hardware.
US10123756B2 discloses a slotted plate for limiting and shaping an X-ray fan beam, comprising at least one first slotted opening and two different X-ray filter regions for X-ray spectral discrimination of an incident X-ray beam, the two different X-ray filter regions being permanently arranged in the region of the at least one first slotted opening such that radiation components of the incident X-ray beam comprising different X-ray spectra that penetrate the at least one first slotted opening are simultaneously generatable.
Disclosure of Invention
The technical problem underlying the present invention is to facilitate single X-ray source spectral computed tomography with low requirements on the X-ray source and the detector. This problem is solved by the subject matter of each independent claim. The dependent claims relate to further aspects of the invention.
In one aspect, the invention relates to a filter arrangement for spectral filtering of an X-ray fan beam of a computed tomography apparatus, the filter arrangement comprising:
a first filter region comprising a first spectral filter function,
a second filter region comprising a second spectral filter function, the second spectral filter function being different from the first spectral filter function,
the first filter region and the second filter region are arranged consecutively in a direction perpendicular to a central plane of the X-ray fan beam and adjacent to each other along a dividing line,
-wherein the first filter area covers a first cross-sectional area of the X-ray fan beam and the second filter area covers a second cross-sectional area of the X-ray fan beam when the filter arrangement is placed in a first position relative to the X-ray fan beam.
In another aspect, the filter device further comprises:
a third filter region comprising a second spectral filter function,
the first filter region, the second filter region and the third filter region are arranged consecutively in a direction perpendicular to a central plane of the X-ray fan beam such that the second filter region is arranged between the first filter region and the third filter region,
-wherein when the filter arrangement is placed in the second position with respect to the X-ray fan beam, the second filter area covers a first cross-sectional area of the X-ray fan beam and the third filter area covers a second cross-sectional area of the X-ray fan beam, the first position and the second position being located consecutively in a direction perpendicular to a central plane of the X-ray fan beam.
The second filter region and the third filter region may be implemented as different regions of one and the same filter layer and/or merged seamlessly with each other.
The first cross-sectional area of the X-ray fan beam and the second cross-sectional area of the X-ray fan beam may be arranged consecutively in a direction perpendicular to a central plane of the X-ray fan beam and adjacent to each other. The first filter area and/or the first cross-sectional area of the X-ray fan beam may be planar and/or substantially perpendicular to a central plane of the X-ray fan beam. The second filter region and/or the second cross-sectional area of the X-ray fan beam may be planar and/or substantially perpendicular to a central plane of the X-ray fan beam. The third filter area and/or the first cross-sectional area of the X-ray fan beam may be planar and/or substantially perpendicular to a central plane of the X-ray fan beam.
In another aspect, the filter device further comprises:
a fourth filter region comprising a first spectral filter function,
the first, second, third and fourth filter regions are arranged successively in a direction perpendicular to the centre plane of the X-ray fan beam such that the first filter region is arranged between the fourth and second filter regions,
-wherein when the filter arrangement is placed in a third position relative to the X-ray fan beam, the fourth filter area covers a first cross-sectional area of the X-ray fan beam and the first filter area covers a second cross-sectional area of the X-ray fan beam, the third position and the first position being positioned consecutively in a direction perpendicular to a centre plane of the X-ray fan beam, wherein the first position is positioned between the third position and the second position.
The second filter region and the third filter region may be realized as different regions of the same piece of filter material and/or merged seamlessly with each other. The first filter region and the fourth filter region may be realized as different regions of the same piece of filter material and/or merged seamlessly with each other.
On the other hand, when the filter arrangement is placed in the first position with respect to the X-ray fan beam, the first filter area and the second filter area together cover substantially the entire cross-sectional area of the X-ray fan beam, in particular the entire cross-sectional area of the X-ray fan beam. On the other hand, when the filter arrangement is placed in the second position with respect to the X-ray fan beam, the second filter region and the third filter region together cover substantially the entire cross-sectional area of the X-ray fan beam, in particular the entire cross-sectional area of the X-ray fan beam. In a further aspect, the fourth filter area and the first filter area together cover substantially the entire cross-sectional area of the X-ray fan beam, in particular the entire cross-sectional area of the X-ray fan beam, when the filter arrangement is placed in the third position relative to the X-ray fan beam.
On the other hand, when the filter arrangement is placed in the first position with respect to the X-ray fan beam, the first filter area and the second filter area together cover substantially the entire cross-sectional area of the X-ray fan beam, in particular the entire cross-sectional area of the X-ray fan beam in the area of the filter arrangement. In a further aspect, the second filter area and the third filter area together cover substantially the entire cross-sectional area of the X-ray fan beam, in particular the entire cross-sectional area of the X-ray fan beam in the area of the filter arrangement, when the filter arrangement is placed in the second position relative to the X-ray fan beam.
The term "substantially the entire cross-sectional area of the X-ray fan beam" may be understood as e.g. the entire cross-sectional area of the X-ray fan beam except for the following areas: at least one partial cross-sectional area of the X-ray fan beam which is not covered by the first filter area and which is not covered by the second filter area, in particular in the region of the dividing line, is due to at least one defect of the first filter area and/or at least one defect of the second filter area.
In another aspect, the split line is straight. In another aspect, the dividing line is positioned in a central plane of the X-ray fan beam when the filter arrangement is placed in the first position relative to the X-ray fan beam. In particular, the computed tomography apparatus is capable of providing dual energy spectral computed tomography data if the segmentation line is positioned in the central plane of the X-ray fan beam.
In another aspect, the filter device further includes a carrier (carrier) including steps formed along a line corresponding to the dividing line, each of the first and second filter regions being attached to the carrier such that the steps and the dividing line are overlapped in unison. In another aspect, the third filter region is attached to the carrier. The height of the step may be substantially equal, in particular equal, to the difference in thickness between the first filter region and the second filter region.
The height of the step may be equal to the difference in thickness between the first filter region and the second filter region. The difference in thickness between the first filter region and the second filter region can be compensated by a step such that the first filter region and the second filter region together form a uniform flat surface on the side facing away from the carrier. The sum of the thickness of the first filter region and the thickness of the carrier in the region of the first filter region may be equal to the sum of the thickness of the second filter region and the thickness of the carrier in the region of the second filter region.
The carrier supports the filter region of the filter device and is preferably stable in form and material composition, in particular with regard to exposure to X-ray radiation and rotational centrifugal forces. The material of the carrier may be chosen to reduce the influence of the carrier on the attenuation of the X-ray fan beam, for example using aluminium or an aluminium alloy, preferably aluminium or an aluminium alloy with a high homogeneity. High performance plastics such as PEEK (polyetheretherketone) and CF (carbon fibre) composites are also suitable for the carrier due to low density, high strength and X-ray resistance.
In another aspect, the first filter region is formed from a material selected from the group comprising gold, platinum, tungsten and tantalum, and/or the first filter region has a thickness of 10 to 300 microns, such as 20 to 200 microns, in particular 30 to 100 microns.
The first filter region is preferably composed of a material with a high atomic number and a high density for strong X-ray energy differences. Gold is the preferred metal element for the first filter region. Platinum, tungsten and tantalum are also suitable metals for the first filter region in terms of material processing and cost.
In another aspect, the first filter region is formed from a material selected from the group comprising tin, copper, titanium, molybdenum and silver, and/or the second filter region has a thickness of 100 microns to 6 mm, for example 200 microns to 4 mm, in particular 300 microns to 2 mm.
In another aspect, the second filter region is formed from a material selected from the group comprising tin, copper, titanium, molybdenum and silver, and/or the second filter region has a thickness of 100 microns to 6 mm, for example 200 microns to 4 mm, in particular 300 microns to 2 mm. Tin is the preferred metal for the second filter region. Copper, titanium, molybdenum and silver are also suitable metals for the second filter region in terms of material processing and cost.
In another aspect, the second filter region is formed from a material selected from the group comprising gold, platinum, tungsten and tantalum, and/or the second filter region has a thickness of 10 to 300 microns, for example 20 to 200 microns, in particular 30 to 100 microns.
The third filter region may be formed of the same material as the second filter region. The third filter region may have the same dimensions, in particular the same thickness, as the second filter region. Preferably, the second filter region and the third filter region are regions of the same piece of filter material.
The fourth filter region may be formed of the same material as the first filter region. The fourth filter region may have the same dimensions, in particular the same thickness, as the first filter region. Preferably, the fourth filter region and the first filter region are regions of the same piece of filter material.
The purity of the material of the first filter region is sufficiently high to obtain an attenuation coefficient that is uniform within the first filter region. The layer thickness of the first filter region is required to be within a certain range with tight thickness tolerances in order to obtain a uniform attenuation of the X-ray fan beam by the first filter region. The purity of the material of the second filter region is sufficiently high to obtain an attenuation coefficient that is uniform within the second filter region. The layer thickness of the second filter region is required to be within a certain range with tight thickness tolerances in order to obtain a uniform attenuation of the X-ray fan beam by the second filter region.
The purity of the material of the third filter region is sufficiently high to obtain an attenuation coefficient that is uniform within the third filter region. The layer thickness of the third filter region is required to be within a certain range with tight thickness tolerances in order to obtain a uniform attenuation of the X-ray fan beam by the third filter region. The purity of the material of the fourth filter region is sufficiently high to obtain an attenuation coefficient that is uniform within the fourth filter region. The layer thickness of the fourth filter region is required to be within a certain range with tight thickness tolerances in order to obtain a uniform attenuation of the X-ray fan beam by the fourth filter region.
In yet another aspect, the invention relates to a computer tomography apparatus comprising:
an X-ray source and a detector, which are rotatably arranged with respect to a rotational axis of the computed tomography apparatus,
the invention relates to a filter arrangement arranged for spectral filtering of an X-ray fan beam emitted from an X-ray source and propagating towards a detector.
In another aspect, the filter arrangement is positioned between the X-ray source and a rotational axis of the computed tomography apparatus.
In another aspect, the rotational axis of the computed tomography device is perpendicular to the central plane of the X-ray fan beam. The central plane of the X-ray fan beam may comprise a central beam of the X-ray fan beam, which is perpendicular to the rotational axis of the computer tomography apparatus. In a further aspect, the rotational axis of the computer tomography apparatus and the central beam of the X-ray fan beam are positioned in a plane perpendicular to the central plane of the X-ray fan beam and/or perpendicular to the first filter region and/or the second filter region. In a further aspect, a central beam of the X-ray fan beam penetrates at least one of the first filter region, the second filter region and a dividing line between the first filter region and the second filter region.
In another aspect, each of the first and second filter regions covers an entire angular range of the detector relative to the X-ray source. In another aspect, each of the first filter region and the second filter region covers the entire angular range of the X-ray fan beam with a fan angle of the X-ray fan beam defined by the collimation of the X-ray fan beam, in particular with a maximum possible fan angle of the X-ray fan beam which can be provided by a collimator system of the computed tomography apparatus. In another aspect, each of the first filter region and the second filter region covers an angular range of the X-ray fan beam, including a central portion of the X-ray fan beam around a central beam of the X-ray fan beam. In yet another aspect, each of the first filter region and the second filter region covers an angular range of the detector, including a central portion of the detector around a central beam of the X-ray fan beam.
In a further aspect, the computer tomography apparatus further comprises a positioner for placing the filter device in a first position relative to the X-ray fan beam and/or for placing the filter device in a second position relative to the X-ray fan beam.
The positioner may comprise, for example, a linear drive device and/or a movable collimating slot plate to which the filter arrangement is attached. The positioner may position the filter arrangement by moving the filter arrangement in a direction perpendicular to the centre plane of the X-ray fan beam, in particular with high accuracy. This allows switching the computer tomography apparatus from a dual energy mode, in which the filter arrangement is in a first position with respect to the X-ray fan beam, to a single energy mode, in which the filter arrangement is in a second position with respect to the X-ray fan beam.
In dual energy mode, the X-ray fan beam is pre-filtered by two different spectral filter functions and split into a high energy half and a low energy half. During rotation of the gantry, the examination object (e.g. a patient on a table) may be moved in a direction substantially perpendicular to a central plane of the X-ray fan beam. After attenuation by the filter arrangement, the corresponding half of the detector may continuously record low-energy and high-energy spectra. Thus, sequential scanning may simultaneously acquire dual-energy projection data, particularly without a delay in switching between low and high energy modes of the X-ray source, and/or simultaneously reduce data registration problems due to breathing and/or other patient organ motion. In the single-energy mode, both static and sequential scans may be performed by single-energy filtering, e.g., for dedicated clinical use, such as low-dose and/or lung cancer screening, etc.
In a further aspect, the invention relates to a method for providing spectral computed tomography data using a computed tomography apparatus to which the invention relates, the method comprising:
placing the filter device in a first position relative to the X-ray fan beam such that the first filter area covers a first cross-sectional area of the X-ray fan beam and the second filter area covers a second cross-sectional area of the X-ray fan beam,
placing an examination zone of the examination object between a first detector area and a first filter area of the detector,
recording first projection data of an examination zone of the examination object via the first detector region on the basis of those X-rays of the X-ray fan beam which penetrate the first filter region and the examination zone of the examination object,
placing an examination zone of the examination object between a second detector area and a second filter area of the detector,
recording second projection data of the examination zone of the examination object via the second detector region on the basis of those X-rays of the X-ray fan beam which penetrate the second filter region and the examination zone of the examination object,
providing spectral computed tomography data based on the first projection data and the second projection data.
In a further aspect, the first detector region and the second detector region are arranged consecutively in a direction perpendicular to a central plane of the X-ray fan beam and/or adjacent to each other along a line along which a surface of the detector facing the X-ray source and the central plane of the X-ray fan beam intersect. The first projection data and the second projection data may be recorded simultaneously.
On the other hand, the examination zone of the examination object is continuously moved in a direction substantially perpendicular to the central plane of the X-ray fan beam, and during the continuous movement of the examination zone of the examination object, the examination zone of the examination object is placed in order first between the first detector region and the first filter region of the detector, and then placed between a second detector area and a second filter area of the detector, wherein during a continuous movement of an examination zone of an examination object the X-ray source and the detector are rotated about a rotational axis of the computed tomography apparatus, such that first projection data of an examination zone of the examination object and second projection data of an examination zone of the examination object are recorded in a corresponding helical acquisition, and/or such that each of the X-ray source and the detector follows a corresponding helical trajectory relative to an examination zone of the examination object.
In another aspect, the invention relates to a method for producing a filter arrangement for spectral filtering of an X-ray fan beam of a computed tomography apparatus, the method comprising:
providing the carrier, for example by machining or moulding,
attaching a first filter region comprising a first spectral filter function to a carrier,
attaching a second filter region comprising a second spectral filter function different from the first spectral filter function to the carrier such that the first filter region and the second filter region are arranged consecutively in a direction perpendicular to the centre plane of the X-ray fan beam and adjacent to each other along a splitting line,
-wherein the first filter area covers a first cross-sectional area of the X-ray fan beam and the second filter area covers a second cross-sectional area of the X-ray fan beam when the filter arrangement is placed in a first position relative to the X-ray fan beam.
The first filter region may be permanently attached to the carrier, for example by physical and/or chemical processing. The second filter region may be permanently attached to the carrier, for example by physical and/or chemical machining. The physical processing may be, for example, bonding and/or heating. The chemical process may be, for example, electroplating. The filter region made of tin can be produced, for example, by a rolling process.
In another aspect, the carrier includes a step formed along a line corresponding to the split line, wherein each of an edge of the first filter region and an edge of the second filter region is aligned with the step, thereby forming the split line between the edge of the first filter region and the edge of the second filter region such that the steps and the split line overlap in unison and/or such that a gap between the edge of the first filter region and the edge of the second filter region is minimized. The edge of the first filter region and/or the edge of the second filter region may be made, for example, by precision cutting to have a narrow straightness tolerance.
In another aspect, the invention relates to a method for locating defects in a filter arrangement according to the invention for spectral filtering of an X-ray fan beam of a computed tomography apparatus, the method comprising:
placing the filter device in a first position relative to the X-ray fan beam such that the first filter area covers a first cross-sectional area of the X-ray fan beam and the second filter area covers a second cross-sectional area of the X-ray fan beam,
continuously moving the focal spot of the X-ray source to each of a plurality of focal spot positions from which a fan-beam of X-rays is emitted,
for each of a plurality of focus positions, recording test projection data via a detector of the computed tomography apparatus based on those X-rays of the X-ray fan beam which have penetrated the filter device,
-locating defects in the filter arrangement based on the test projection data for each of the plurality of focus positions.
In another aspect, locating the defect in the filter arrangement based on the test projection data for each of the plurality of focus positions comprises:
determining a subtraction result by subtracting the reference projection data from the test projection data for at least one of the plurality of focus positions, thereby obtaining a subtraction result,
-determining a comparison result by comparing the subtraction result with at least one threshold value, based on which a defect in the filter arrangement is located.
The reference projection data may, for example, be selected from test projection data for each of a plurality of focus positions. For example, the reference projection data may be test projection data for a focus position different from at least one of the at least one focus position. In another aspect, the reference projection data is provided separately from the test projection data, e.g., based on a previous projection data acquisition, and/or based on a virtual model calculation of a filter device.
With the proposed filter device and method several benefits can be achieved. In particular, dual-energy spectral computed tomography data can be provided using a single X-ray source computed tomography device in a cost-effective setting that can be easily switched to a single-energy computed tomography mode, e.g., to a tin-filtration mode, for routine early detection screening, e.g., lung cancer screening, to obtain additional diagnostic information. Due to the additional X-ray filtering performed before the patient by the filter device, the patient is exposed to a reduced dose in both the dual energy mode and the single energy mode.
The individual embodiments or individual aspects and features thereof may be combined or interchanged with one another without explicit description, without limiting or enlarging the scope of the described invention, as long as such combination or interchange is meaningful and within the meaning of the present invention. Where applicable, advantages described with respect to one embodiment of the invention may also be advantageous with respect to other embodiments of the invention.
Reference is made to the following facts: the described method and the described apparatus are only preferred exemplary embodiments of the invention and a person skilled in the art may vary the invention without departing from the scope of the invention as specified by the claims.
Drawings
The invention will be described using example embodiments with reference to the drawings. The illustrations in the drawings are schematic and highly simplified and not necessarily drawn to scale.
Fig. 1 shows a filter arrangement for spectral filtering of an X-ray fan beam of a computed tomography apparatus.
Figure 2 shows a cross-sectional view of the filter device.
Fig. 3 shows the filter arrangement in a first position relative to the X-ray fan beam.
Fig. 4 shows the filter arrangement in a second position relative to the X-ray fan beam.
Fig. 5 shows a diagram illustrating a method for providing spectral computed tomography data with a computed tomography apparatus.
Fig. 6 shows a diagram illustrating a method for producing a filter arrangement.
Fig. 7 shows a diagram illustrating a method for locating defects in a filter device.
Fig. 8 shows a two-dimensional representation of the subtraction result for locating a defect in the filter device.
Fig. 9 shows a representation of the subtraction result for locating a defect in the filter device.
Fig. 10 shows a computer tomography apparatus comprising a filter device.
Detailed Description
Fig. 1 shows a filter arrangement 5 for spectral filtering of an X-ray fan beam 7 of a computed tomography apparatus 1, the filter arrangement 5 comprising:
a first filter area 51 comprising a first spectral filter function,
a second filter region 52 comprising a second spectral filter function, the second spectral filter function being different from the first spectral filter function,
the first filter region 51 and the second filter region 52 are arranged consecutively in a direction perpendicular to the central plane 7C of the X-ray fan beam 7 and adjacent to each other along a dividing line 54,
wherein the first filter area 51 covers a first cross-sectional area 71 of the X-ray fan beam 7 and the second filter area 52 covers a second cross-sectional area 72 of the X-ray fan beam 7 when the filter arrangement 5 is placed in a first position relative to the X-ray fan beam 7.
The filter device 5 further comprises:
a third filter area 53 comprising a second spectral filter function,
the first filter region 51, the second filter region 52 and the third filter region 53 are arranged successively in a direction perpendicular to the central plane 7C of the X-ray fan beam 7, such that the second filter region 52 is arranged between the first filter region 51 and the third filter region 53,
wherein the second filter area 52 covers a first cross-sectional area 71 of the X-ray fan beam 7 and the third filter area 53 covers a second cross-sectional area 72 of the X-ray fan beam 7 when the filter arrangement 5 is placed in a second position relative to the X-ray fan beam 7, the first position and the second position being successively positioned in a direction perpendicular to a central plane 7C of the X-ray fan beam 7.
The second filter region 52 and the third filter region 53 are formed from a single sheet of filter material. The boundary line 55 illustrates the boundary between the second filter region 52 and the third filter region 53 based only on the dimensions of the second filter region 52 and the third filter region 53 with respect to the geometry of the X-ray fan beam 7. The material composition of the filter material associated with the boundary line 55 does not change and the thickness does not change. Each of the dividing line 54 and the boundary line 55 is a straight line. The dividing line 54 and the boundary line 55 are parallel to each other.
The filter device 5 comprises through holes MB and MC for attaching the piece of filter material and/or carrier 5C forming the second filter area 52 and the third filter area 53 to the positioner 58.
Fig. 2 shows a cross-sectional view of a filter device 5 comprising a carrier 5C. The carrier 5C includes a step 56 formed along a line corresponding to the dividing line 54. Each of the first filter region 51 and the second filter region 52 is attached to the carrier 5C such that the step 56 and the dividing line 54 overlap in unison. First filter region 51 and second filter region 52 are directly adjacent to each other along a parting line 54.
The first filter region 51 is formed of a material selected from the group consisting of gold, platinum, tungsten, and tantalum, and has a thickness T1 of 30 to 100 micrometers. The second filter region 52 is formed of a material selected from the group consisting of tin, copper, titanium, molybdenum, and silver, and has a thickness T2 of 300 microns to 2 millimeters.
The height of the step 56 is equal to the difference in thickness between the first filter region 51 and the second filter region 52. The difference in thickness between the first filter region 51 and the second filter region 52 is compensated by the step 56, so that the first filter region 51 and the second filter region 52 together form a uniform flat surface on the side facing away from the carrier 5C. The sum of the thickness T1 of the first filter region 51 and the thickness TC1 of the carriers 5C in the region of the first filter region 51 is equal to the sum of the thickness T2 of the second filter region 52 and the thickness TC2 of the carriers 5C in the region of the second filter region 52.
Fig. 3 shows the filter arrangement 5 in a first position relative to the X-ray fan beam 7. When the filter arrangement 5 is placed in the first position with respect to the X-ray fan beam 7, the first filter area 51 and the second filter area 52 together cover substantially the entire cross-sectional area of the X-ray fan beam 7.
Each of the length of the first filter region 51, the length of the second filter region 52 and the length of the third filter region 53 allows to cover the entire extension of the X-ray fan beam 7 in the circumferential direction phi. Each of the width W1 of the first filter region 51, the width W2 of the second filter region 52 and the width W3 of the third filter region 53 allows to cover at least half of the entire extension of the X-ray fan beam 7 in a direction Z, which is parallel to the axis of rotation AR and perpendicular to the central plane 7C of the X-ray fan beam 7.
The dividing line 54 is positioned in the central plane 7C of the X-ray fan beam 7. Along a central plane 7C of the X-ray fan beam 7, the X-ray fan beam 7 is divided into two substantially equally sized halves 7A and 7B.
The X-ray source 6 and the detector 8 are rotatably arranged about a rotational axis AR of the computer tomography apparatus 1. The filter arrangement 5 is arranged for spectral filtering of an X-ray fan beam 7 emitted from the X-ray source 6 and propagating towards the detector 8. Each of the first filter region 51 and the second filter region 52 covers the entire angular range of the detector 8 with respect to the X-ray source 6.
The computer tomography apparatus 1 comprises the positioner 58 for placing the filter device 5 in a first position with respect to the X-ray fan beam 7 and for placing the filter device 5 in a second position with respect to the X-ray fan beam 7.
Fig. 4 shows the filter arrangement 5 in a second position relative to the X-ray fan beam 7. When the filter arrangement 5 is placed in the second position with respect to the X-ray fan beam 7, the second filter region 52 and the third filter region 53 together cover substantially the entire cross-sectional area of the X-ray fan beam 7. The boundary line 55 is positioned in the central plane 7C of the X-ray fan beam 7.
Fig. 5 shows a diagram illustrating a method for providing spectral computed tomography data with the computed tomography apparatus 1, the method comprising:
placing the filter device 5P 5 in a first position relative to the X-ray fan beam 7 such that the first filter area 51 covers the first cross-sectional area 71 of the X-ray fan beam 7 and the second filter area 52 covers the second cross-sectional area 72 of the X-ray fan beam 7,
placing the examination zone N of the examination object 13 PN1 between the first detector area 81 and the first filter area 51 of the detector 8,
recording R1 first projection data of an examination zone N of the examination object 13 via the first detector region 81 on the basis of those X-rays of the X-ray fan beam 7 which penetrate the first filter region 51 and the examination zone N of the examination object 13,
placing the examination zone N of the examination object 13 PN2 between the second detector region 82 and the second filter region 52 of the detector 8,
second projection data of the examination zone N of the examination object 13 are recorded R2 via the second detector region 82 on the basis of those X-rays of the X-ray fan beam 7 which penetrate the second filter region 52 and the examination zone N of the examination object 13.
Providing PS-spectral computed tomography data based on the first projection data and the second projection data.
The examination zone N of the examination object 13 is continuously moved in a direction substantially perpendicular to the central plane 7C of the X-ray fan beam 7 and during the continuous movement the examination zone N of the examination object 13 is placed in order first between the first detector region 81 and the first filter region 51 of the detector 8 and then between the second detector region 82 and the second filter region 52 of the detector 8. During the continuous movement of the examination zone N of the examination object 13, the X-ray source 6 and the detector 8 are rotated about the rotational axis AR of the computer tomography apparatus 1 such that each of the first projection data of the examination zone N of the examination object 13 and the second projection data of the examination zone N of the examination object 13 is recorded in a corresponding helical acquisition.
Fig. 6 shows a diagram illustrating a method for producing a filter arrangement 5 for spectral filtering of an X-ray fan beam 7 of a computed tomography apparatus 1, the method comprising:
-providing a PC carrier 5C,
attaching A51 a first filter region 51 comprising a first spectral filter function to a carrier 5C,
attaching A52 a second filter region 52 comprising a second spectral filter function different from the first spectral filter function to the carrier 5C such that the first filter region 51 and the second filter region 52 are arranged consecutively in a direction perpendicular to the central plane 7C of the X-ray fan beam 7 and adjacent to each other along a splitting line 54,
wherein the first filter area 51 covers a first cross-sectional area 71 of the X-ray fan beam 7 and the second filter area 52 covers a second cross-sectional area 72 of the X-ray fan beam 7 when the filter arrangement 5 is placed in a first position relative to the X-ray fan beam 7.
The carrier 5C includes a step 56 formed along a line corresponding to the dividing line 54. Each of the edges of first filter area 51 and the edges of second filter area 52 are aligned with step 56, thereby forming a dividing line 56 between the edges of first filter area 51 and the edges of second filter area 52 such that step 56 and dividing line 54 overlap uniformly.
Fig. 7 shows a diagram illustrating a method for locating defects in a filter arrangement 5, which filter arrangement 6 is used for spectral filtering of an X-ray fan beam 7 of a computed tomography apparatus 1, which method comprises:
placing the filter device 5 at a first position PT with respect to the X-ray fan beam 7 such that the first filter area 51 covers a first cross-sectional area 71 of the X-ray fan beam 7 and the second filter area 52 covers a second cross-sectional area 72 of the X-ray fan beam 7,
continuously moving the focal spot of the X-ray source 6 to each of a plurality of focal spot positions from which the X-ray fan beam 7 is emitted by the X-ray source 6,
for each of a plurality of focus positions, recording RT test projection data via a detector 8 of the computed tomography apparatus 1 based on those X-rays of the X-ray fan beam 7 which have penetrated the filter device 5,
-locating defects in the T5 filter device 5 based on the test projection data for each of the plurality of focus positions.
Locating defects in the T5 filter device 5 based on the test projection data for each of the plurality of focus positions includes:
determining a subtraction result by subtracting the reference projection data from the test projection data for at least one of the plurality of focus positions, thereby obtaining a subtraction result,
-determining a comparison result by comparing the subtraction result with at least one threshold value TH, TL, based on which a defect in the filter device 5 is located.
Fig. 8 shows a two-dimensional representation of the subtraction result for locating a defect in the filter device 5.
The axis DCN represents the detector channel number. Axis DRN represents the detector row number. The subtraction results for test projection data recorded on the basis of those X-rays of the X-ray fan beam 7 which have penetrated the first filter region 52 are shown in lines 1 to 32. The subtraction results for test projection data recorded on the basis of those X-rays of the X-ray fan beam 7 which have penetrated the second filter region 52 are shown in lines 33 to 64. Since the size of the focus is not infinitely small, the results shown in rows 30 to 35 can be considered as results obtained based on hybrid filtering.
Fig. 9 shows a representation of the subtraction results S1, S2, S3 for locating defects in the filter device 5. Each of the subtraction results S1, S2, S3 is determined for a corresponding focus position of three different focus positions. The axis Y represents the attenuation values. A peak S having an abnormally high value of the subtraction result exceeding the threshold TH indicates that there is a defect in the filter device 5 in the vicinity of the dividing line 54.
Fig. 10 shows a computer tomography apparatus 1 comprising a filter device 5. The computer tomography apparatus 1 comprises a gantry 20 with a support frame 21, a tilt frame 22 and a rotation frame 24. The X-ray source 6 and the detector 8 are mounted on a rotating frame 24 for rotation about the acquisition portion 4, the acquisition portion 4 being positioned within the tunnel-shaped opening 9. The computer tomography apparatus 1 further comprises a patient handling system 10 for movably supporting the patient 13 such that at least a portion of the patient's body can be placed in the acquisition portion 4 in the tunnel-shaped opening 9 so as to be penetrated by the X-ray fan beam.
The computer tomography apparatus 1 further comprises: a control unit 30, the control unit 30 including a computer readable medium 32; an image reconstruction unit 34; a processor 35; a positioner control unit 36 for controlling the positioner 58; an input unit 38 and an output unit 39. The input unit 38 may comprise means for inputting data, such as a keyboard and/or a touch-sensitive surface. The output unit 39 may comprise means for outputting data, such as a display.
Claims (15)
1. A filter arrangement (5) for spectral filtering of an X-ray fan beam (7) of a computed tomography apparatus (1), the filter arrangement (5) comprising:
a first filter region (51) comprising a first spectral filter function,
a second filter region (52) comprising a second spectral filter function, the second spectral filter function being different from the first spectral filter function,
-the first filter region (51) and the second filter region (52) are arranged consecutively in a direction perpendicular to a central plane (7C) of the X-ray fan beam (7) and adjacent to each other along a dividing line (54),
-wherein the first filter area (51) covers a first cross-sectional area (71) of the X-ray fan beam (7) and the second filter area (52) covers a second cross-sectional area (72) of the X-ray fan beam (7) when the filter arrangement (5) is placed in a first position relative to the X-ray fan beam (7).
2. The filter device (5) according to claim 1, further comprising:
a third filter region (53) comprising the second spectral filter function,
-the first filter region (51), the second filter region (52) and the third filter region (53) are arranged consecutively in the direction perpendicular to the central plane (7C) of the X-ray fan beam (7) such that the second filter region (52) is arranged between the first filter region (51) and the third filter region (53),
-wherein the second filter area (52) covers the first cross-sectional area (71) of the X-ray fan beam (7) and the third filter area (53) covers the second cross-sectional area (72) of the X-ray fan beam (7) when the filter arrangement (5) is placed in a second position relative to the X-ray fan beam (7), the first and second positions being successively positioned in the direction perpendicular to a central plane (7C) of the X-ray fan beam (7).
3. The filter device (5) according to claim 1 or 2, wherein the first filter area (51) and the second filter area (52) together cover substantially the entire cross-sectional area of the X-ray fan beam (7) when the filter device (5) is placed in the first position relative to the X-ray fan beam (7).
4. The filter device (5) as defined in any one of claims 1 to 3, the division line (54) being straight and/or the division line (54) being positioned within the central plane (7C) of the X-ray fan beam (7) when the filter device (5) is placed in the first position relative to the X-ray fan beam (7).
5. The filter device (5) according to any one of claims 1 to 4, further comprising a carrier (5C),
-the carrier (5C) comprises a step (56), the step (56) being formed along a line corresponding to the dividing line (54),
-each of said first filter region (51) and said second filter region (52) is attached to said carrier (5C) such that said step (56) and said dividing line (54) are congruent overlapping.
6. Filter device (5) according to one of the claims 1 to 5,
-wherein the first filter region is formed of a material selected from the group comprising gold, platinum, tungsten and tantalum, and/or
-wherein the first filter region has a thickness of 10 to 300 microns, and/or
-wherein the second filter region is formed of a material selected from the group comprising tin, copper, titanium, molybdenum and silver, and/or
-wherein the second filter region has a thickness of 100 microns to 6 millimeters.
7. A computed tomography apparatus (1) comprising:
an X-ray source (6) and a detector (8), the X-ray source (6) and the detector (8) being rotatably arranged with respect to a rotational Axis (AR) of the computed tomography apparatus (1),
-a filter device (5) according to any one of claims 1 to 6, the filter device (5) being arranged for spectral filtering of an X-ray fan beam (7), the X-ray fan beam (7) being emitted from the X-ray source (6) and propagating towards the detector (8).
8. The computed tomography apparatus (1) according to claim 7,
-wherein each of the first filter region (51) and the second filter region (52) covers the entire angular range of the detector (8) with respect to the X-ray source (6).
9. The computed tomography apparatus (1) according to claim 7 or 8, further comprising a positioner (58) for placing the filter device (5) in the first position relative to the X-ray fan beam (7) and/or for placing the filter device (5) in the second position relative to the X-ray fan beam (7).
10. A method for providing spectral computed tomography data with a computed tomography apparatus (1) according to any of claims 7 to 9, the method comprising:
-placing (P5) the filter device (5) in the first position relative to the X-ray fan beam (7) such that the first filter area (51) covers the first cross-sectional area (71) of the X-ray fan beam (7) and the second filter area (52) covers the second cross-sectional area (72) of the X-ray fan beam (7),
-placing (PN1) an examination zone (N) of an examination object (13) between a first detector area (81) of the detector (8) and the first filter area (51),
-recording (R1) first projection data of the examination zone (N) of the examination object (13) via the first detector region (81) based on those X-rays of the X-ray fan beam (7) which penetrate the first filter region (51) and the examination zone (N) of the examination object (13),
-placing (PN2) the examination zone (N) of the examination object (13) between a second detector region (82) of the detector (8) and the second filter region (52),
-recording (R2) second projection data of the examination zone (N) of the examination object (13) via the second detector region (82) based on those X-rays of the X-ray fan beam (7) which penetrate the second filter region (52) and the examination zone (N) of the examination object (13),
-Providing (PS) spectral computed tomography data based on the first projection data and the second projection data.
11. The method of claim 10, wherein the first and second light sources are selected from the group consisting of,
-wherein the examination zone (N) of the examination object (13) is continuously moved in a direction substantially perpendicular to a central plane (7C) of the X-ray fan beam (7) and during the continuous movement the examination zone (N) of the examination object (13) is placed in order first between the first detector region (81) and the first filter region (51) of the detector (8) and then between the second detector region (82) and the second filter region (52) of the detector (8),
-wherein during the continuous movement of the examination zone (N) of the examination object (13), the X-ray source (6) and the detector (8) are rotated about the rotational Axis (AR) of the computed tomography apparatus (1) such that each of the first projection data of the examination zone (N) of the examination object (13) and the second projection data of the examination zone (N) of the examination object (13) is recorded in a corresponding helical acquisition.
12. A method for producing a filter arrangement (5), which filter arrangement (5) is used for spectral filtering of an X-ray fan beam (7) of a computed tomography apparatus (1), which method comprises:
-Providing (PC) a carrier (5C),
-attaching (A51) a first filter region (51) comprising a first spectral filter function to the carrier (5C),
-attaching (A52) a second filter region (52) comprising a second spectral filter function to the carrier (5C), the second spectral filter function being different from the first spectral filter function such that the first filter region (51) and the second filter region (52) are arranged consecutively in a direction perpendicular to a central plane (7C) of the X-ray fan beam (7) and adjacent to each other along a splitting line (54),
-wherein the first filter area (51) covers a first cross-sectional area (71) of the X-ray fan beam (7) and the second filter area (52) covers a second cross-sectional area (72) of the X-ray fan beam (7) when the filter arrangement (5) is placed in a first position relative to the X-ray fan beam (7).
13. The method of claim 12, wherein the first and second light sources are selected from the group consisting of,
-the carrier (5C) comprises a step (56), the step (56) being formed along a line corresponding to the dividing line (54),
-wherein each of an edge of the first filter area (51) and an edge of the second filter area (52) is aligned with the step (56), thereby forming the dividing line (56) between the edge of the first filter area (51) and the edge of the second filter area (52) such that the step (56) and the dividing line (54) overlap in unison.
14. A method for locating defects in a filter arrangement (5) according to any one of claims 1 to 6, the filter arrangement (5) being used for spectral filtering of an X-ray fan beam (7) of a computed tomography apparatus (1), the method comprising:
-Placing (PT) the filter device (5) in the first position relative to the X-ray fan beam (7) such that the first filter area (51) covers the first cross-sectional area (71) of the X-ray fan beam (7) and the second filter area (52) covers the second cross-sectional area (72) of the X-ray fan beam (7),
-continuously Moving (MF) a focal spot of the X-ray source (6) to each of a plurality of focal spot positions, the X-ray fan beam (7) being emitted from the focal spot of the X-ray source (6),
-Recording (RT) test projection data via the detector (8) of the computed tomography apparatus (1) based on those X-rays of the X-ray fan beam (7) which penetrate the filter device (5) for each of the plurality of focus positions,
-locating (T5) the defect in the filter arrangement (5) based on the test projection data for each of the plurality of focus positions.
15. The method of claim 14, wherein the first and second light sources are selected from the group consisting of,
-locating (T5) the defect in the filter arrangement (5) based on the test projection data for each of the plurality of focus positions comprises:
determining a subtraction result by subtracting reference projection data from the test projection data for at least one of the plurality of focus positions, thereby obtaining a subtraction result,
-determining a comparison result by comparing the subtraction result with at least one threshold value (TH, TL), on the basis of which the defect in the filter device (5) is located.
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