WO2010102908A1

WO2010102908A1 - Filter for a computer tomograph

Info

Publication number: WO2010102908A1
Application number: PCT/EP2010/052449
Authority: WO
Inventors: Michael Grasruck; Gregor Jost; Ulrich Kühn; Bernhard Schmidt
Original assignee: Siemens Aktiengesellschaft; Bayer Schering Pharma Aktiengesellschaft
Priority date: 2009-03-11
Filing date: 2010-02-26
Publication date: 2010-09-16
Also published as: DE102009012631B4; DE102009012631A1

Abstract

The invention relates to a filter (14) by means of which an improved applicability of a computer tomography (1) for the executing of contrasting agent reinforced radiation therapy is attained. The filter (14), insertable in the emission path between an emission source (4) and an isocentric axis (I) of the computer tomograph (1), is sized or can be sized in regards to a varying radiation throughput coefficient (ß) that is location dependent within the filter surface (A) thereof for the x-ray radiation of the computer tomograph (1) such that the x-ray dose (D) deposited over the cross-section of a standard object (13') is substantially constant in the case of circulating irradiation through the filter (14) of said standard object (13') of specified geometry and x-ray absorption.

Description

description

Filter for a computer tomograph

The invention relates to a filter for a computer tomograph to be used in a radiotherapy.

Radiation therapy for the treatment of cancer patients today usually uses high-energy electromagnetic radiation with photon energies in the MeV range. An advantage of the high-energy radiation lies above all in a homogeneous energy absorption in the body and a favorable depth dose course, which is accompanied by a small proportion of scattered radiation.

Due to the small difference between the attenuation coefficients of customary materials (organic compounds, bone, iodine) in this energy range, however, a locally varying dose deposition can only be achieved by expensive methods, e.g. by multi-field irradiation.

On the other hand, organ or material-specific, and thus locally varying, dose deposition can be achieved comparatively easily by using low-energy X-ray radiation (in comparison to MeV radiation). Thus, e.g. When irradiated with X-rays generated with tube voltages in the range of 80-140 kV, iodine and soft tissue have significantly different energy absorption. This effect is exploited in so-called contrast enhanced radiation therapy with X-rays (CERT: contrast enhanced radiation therapy). In this method of irradiation, a contrast agent is administered to the patient before irradiation, which accumulates in the heavily perfused tumor tissue, and as a result leads to a local dose increase in the tumor during irradiation.

One of the advantages of contrast-enhanced radiotherapy lies in the fact that it has to be carried out conventional computed tomographs can be used, which are available in large numbers and at comparatively low acquisition and operating costs. The use of a conventional computed tomography scanner for irradiation therapy has the additional advantage that the computed tomography scanner can be used simultaneously with the irradiation for imaging and thus for monitoring the irradiation procedure.

Even when irradiated with an X-ray radiation field of homogeneous intensity, an inhomogeneous dose distribution often occurs due to the strong attenuation of the radiation by the body tissue, even if an approximately rotationally symmetric and materially homogeneous treatment object is irradiated rotationally symmetrically in a computer tomograph. This effect counteracts local dose enhancement by contrast agents and can completely or partially abolish the usefulness of contrast enhancement.

The invention has for its object to improve the applicability of a computed tomography for radiotherapy.

This object is achieved in accordance with the invention by a filter designed in a special way, which can be inserted into the radiation field between a radiation source and an isocentric axis of a computer tomograph. In this case, the filter is dimensioned or dimensioned in terms of its within the filter surface location-dependent radiation transmission coefficient for the X-ray of the computed tomography such that over the cross section of a standard object of predetermined geometry and predetermined, in particular homogeneous X-ray absorption, a substantially constant deposited X-ray dose is achieved when the standard object through the filter circumferentially, in particular rotationally symmetrically irradiated. The above object is further achieved according to the invention by a computer tomograph comprising this filter.

The term "computed tomography" is here and below generally used as a synonym for an otherwise arbitrary irradiation device in which a radiation source for X-radiation is rotatable on a circular path about an isocentric axis, to which the beam path of the X-ray radiation, ie the central beam of the emitted radiation This is preferably, but not necessarily, a conventional computed tomography scanner commonly used to acquire X-ray images, in particular the X-ray detector present in a conventional computer tomograph for the present invention

Foremost application in an irradiation therapy at most of secondary importance and can therefore be omitted. As the X-ray radiation here and hereinafter electromagnetic brake radiation in an energy range of about 20 keV to 200 keV is referred to.

"Irradiation" refers to an irradiation form in which an irradiation object, in particular the standard object, is irradiated from a multiplicity of rotational positions distributed around the full circle, so that X-radiation irradiates the irradiation object at least substantially from all directions transversely to the isocentric axis A circulating irradiation is achieved, in particular, by rotating the X-ray tube of the computed tomography apparatus around the irradiation object under continuous irradiation Alternatively, a circulating irradiation can be realized by discontinuous irradiation of the irradiation object from a plurality of discrete rotational positions circulating irradiation is then designated when the energy radiated onto the irradiation object is substantially independent of the rotational position. The standard object serves in particular as a basis for the construction of the filter, as well as for the objective and repeatable evaluation of the respective filter properties. In this case, the standard object is preferably designed in each case with respect to its shape, extent and X-ray absorption properties such that it approximately represents a body part of a patient to be treated by means of radiation therapy. The properties of the standard object used thus vary depending on the particular application. For example, a substantially cylindrical standard object is used to construct a filter for irradiating a patient's head. For example, a standard elliptical shaped cross-sectional standard object serves as the basis for constructing a filter for radiotherapy of a thorax.

The filter is preferably constructed using a computer simulation, in particular a computer-aided optimization method, in which predetermined filter properties are varied in order to achieve the most homogeneous possible dose deposition in the standard object. The standard object is represented here by a virtual model with numerically specified physical properties. The filter constructed using the appropriate standard object, in the case of use, also produces a uniform dose distribution in the corresponding body part of a patient.

The filter is intentionally introduced into the beam path in such a way that its filter surface is always oriented substantially at right angles to the beam path or tangentially to the circular path circumscribed by the radiation source.

The location-dependent radiation transmission coefficient defines the proportion of the x-ray radiation incident on the filter, which transmits the filter at a specific location of its filter surface. Indirectly, the radiation permittivity thus how much the x-ray radiation is weakened by the filter at a certain location of its filter surface.

With regard to the dimensioning of the filter, the invention is based on the consideration that, when the standard object is irradiated with X-ray radiation, the distribution of the dose deposited over the object cross-section on the one hand depends on the attenuation curve of the X-radiation in the irradiation object, i. depends on the penetration depth of the X-ray radiation, and on the other hand on the width of the radiation field in relation to the width of the object. Upon irradiation of the object with a broad radiation field, it is recognized that the object's own X-ray attenuation regularly predominates. As a result, in the case of rotationally symmetrical irradiation of the object in its near-edge layers an increased dose is deposited, whereas the dose value decreases towards the center of the object. By contrast, in the case of circulating irradiation of the object with a narrowly focused radiation field (spot beam), it is recognized that an increased dose is deposited in the center of the object, especially since, during the rotation of the radiation field around the object, it is exposed to the x-ray radiation for longer than the layers close to the edge.

The filter according to the invention is now selectively dimensioned with respect to the X-ray radiation of a given computer tomograph, in particular a radiation field of predetermined spectral distribution and geometry, and with respect to the object properties predetermined by the standard object, such that the two effects described above compensate one another, whereby the substantially constant dose distribution is achieved in the object. The specific course of the location-dependent radiation transmission coefficient is therefore always different in detail for different standard objects and different radiation fields. Based on the above, but the invention for the particular application designed filter of the Be easily found by calculation, simulation and / or by empirical experiments.

A "constant" X-ray dose in the sense of the invention, or a "homogeneous" dose distribution, is preferably assumed when the locally deposited X-ray dose deviates from the dose average over the irradiated area of the object by a maximum of 5%.

By using the filter according to the invention, a particularly effective implementation of a contrast-enhanced radiotherapy using a - in particular conventional - computed tomography is made possible in a comparatively simple manner. In particular, as a result of the filter and the uniform dose distribution effected thereby in the healthy tissue of a patient by using a contrast agent, a strong dose increase in the tumor to be treated is achieved with comparatively low radiation exposure of the remaining tissue. Thus, the tumor tissue can be treated specifically.

In principle, it is conceivable to change the radiation transmission coefficient by using different filter materials with different (volume) X-ray absorption coefficients depending on location. In an embodiment of the filter that is particularly easy to produce, the positional dependence of the radiation transmission coefficient is deviated exclusively from this by a material thickness of the filter that varies as a function of the location on the filter surface (filter thickness). In this embodiment, the filter is at least substantially made of a homogeneous X-ray absorbing material, in particular of Teflon or aluminum.

In an advantageous embodiment, the radiation transmission coefficient varies mirror-symmetrically with respect to a center plane oriented transversely to the filter surface, wherein it is maximum in the region of the center plane. If such a Filter is constructed of homogeneous filter material, also follows the filter thickness with respect to the center plane mirror-symmetric course, wherein in the region of the median plane, the filter thickness is minimal. The course of the filter thickness as a function of the distance from the median plane has approximately the shape of an inverse, ie "upside down" bell curve. "In other words, the minimum of the filter strength is flanked on both sides with increasing distance from the median plane by two approximately S-shaped curved Flank.

A numerical optimization of the filter shape for a given standard object and a given radiation field is simplified in an expedient embodiment of the invention in that the profile of the filter strength s (x) follows a - in the mathematically analytical sense - defined functional dependence, for example a function of the shape

- s (x) = s _m ax - (s _ma χ - s _min ) / (co sh (x / X ₀ )) "ode r - S (X) = Smax ^" (S _ma x "S _min ) ^* Θ Xp ("(x / X ₀ ) ² "),

in which

Sm _a x is a measure of the maximum filter strength at the edge of the filter, s _{min is} a measure of the minimum filter thickness in the area of the midplane, x inside the filter area is the distance of a location from the midplane, Xo is a normalization size over which the width of the bell shape is adjustable, and n is a natural number chosen, in particular, from an interval from 1 to about 10, via which the edge slope of the bell shape can be adjusted.

In a variant of the filter which is particularly easy to produce, for example by a CNC milling machine, the profile of the filter thickness is selected such that the surface of the filter can be made in a continuous or continuously differentiable manner in cross section. nedergesetzten arc and straight sections is formed.

For the synergetic use of the respective advantages, the two design variants of the filter described above are expediently combined in such a way that, when planning the filter design, the filter strength is first defined as function S (x) of the distance x to the center plane, and the parameters of this function in a computer simulation. with comparatively little numerical effort - be optimized for the given properties of the standard object and the radiation field, and that for - comparatively simple - production of the actual filter on the basis of the previously constructed

Filter designs this function S (x) is approximated in sections by continuously or continuously differentiable stringing together of circular arc and straight line sections.

In a preferred embodiment, the filter for X-radiation is dimensioned, which is generated by means of an applied tube voltage from an interval of about 80 kV to 140 kV, in particular about 120 kV.

In a simple embodiment, the filter is made in one piece from the X-ray absorbing material. Such a one-piece and therefore rigid mold filter is particularly useful in the irradiation of approximately rotationally symmetrical radiation objects, e.g. a head, arm or leg, suitable for use.

In an alternative embodiment, the filter is made in several pieces. In this case, the filter is expediently formed from two stacks of lamellae which in each case consist at least in an inner subsection of the X-ray-absorbing material. In this case, the individual lamellae of each stack end at different distances to the center plane, so that the sum of the lamellae of each stack formed filter strength in turn varies location dependent on the filter surface.

The described lamellar construction has the particular advantage that it can be used very flexibly. In particular, such lamellar filters can be produced quickly and with the simplest means. In addition, if the lamellae of a stack are not permanently connected to one another, the filter can be adapted to different applications and different underlying standard objects by moving the lamellae towards one another.

In a particularly advantageous embodiment, at least one blade of each stack is motor-displaced. This makes it possible to adapt the filter geometry automatically - without manual conversion effort, and thus saving time - to different applications. In addition, the geometry of the filter can also be changed during circulation of the radiation source around the standard object. As a result, this filter is also particularly suitable for achieving a substantially homogeneous dose distribution in the case of a non-rotationally symmetrical standard or treatment object.

In connection with the filter variant described above, the associated computer tomograph expediently comprises drive means for the motorized displacement of the corresponding slats. The drive means, which are formed for example by piezo actuators, can also be integrated in the filter. The computer tomograph further comprises a control unit for controlling the drive means in accordance with a stored profile, which specifies the positioning of the slats with respect to the median plane.

The profile specifies the positioning of the slats for a specific application either constant or varying depending on the rotation angle of the computer tomograph. The latter profile variant thus realizes a method for controlling the slats, in which the shape of the Filters, in particular the width and / or edge slope of the bell-shaped curve of the filter thickness, is continuously changed depending on the rotation angle of the radiation source.

Embodiments of the invention will be explained in more detail with reference to a drawing. Show:

1 shows a schematic representation of a computer tomograph that can be used for radiation therapy with a filter optimized for uniform dose deposition in a standard object, FIG. 2 shows a first view in a three-dimensional representation

3 shows a cross section through the filter according to FIG. 2, FIG.

4 shows a simulated dose distribution in a cylindrical standard object with rotationally symmetrical irradiation of this object using the filter according to FIG. 2, and FIG. 5 shows a second embodiment of the filter according to FIG.

Corresponding parts and sizes are always provided with the same reference numerals in all figures.

FIG. 1 shows, roughly schematically, a computer tomograph 1 which can be used for carrying out a contrast agent-enhanced radiation therapy.

The computer tomograph 1 comprises a carrier 2, on which an annular gantry 3 is rotatably mounted. At the gantry 3 in comparison to each other serving as a radiation source X-ray tube 4 and an (X-ray) detector 5 are mounted. The detector 5 is used in imaging use of the computed tomography scanner 1 for recording x-ray images. picture pictures. In contrast, in connection with the therapeutic use of the computed tomograph 1, which is in the foreground here, it is of no significance and is therefore only indicated by dots in FIG.

The x-ray tube 4 is rotatable together with the gantry 3 about an isocentric axis I. In this case, the x-ray tube 4 is oriented such that a central ray 6 of a fan-shaped (x-ray) radiation field 7 generated by it always falls on the opposite detector 5 through the isocentric axis I, independently of the rotational position of the x-ray tube 4.

The direction of the central beam 6 is referred to below as the radiation direction 8. The isocentric axis I is aligned along a horizontal longitudinal direction 9. The rotational position of the X-ray tube 5 is defined below by specifying a rotation angle α, which is formed between the radiation direction 8 and the vertical direction 10. The direction tangential to the pivoting circle K of the x-ray tube 4 is referred to below as the transverse direction 11.

The computer tomograph 1 is assigned a patient couch 12. In order to carry out an irradiation, an irradiation object 13 mounted on the patient couch 12 is positioned approximately centered with the isocentric axis I in the interior of the gantry 3. In the case of medical application, the irradiation object 13 is a body part of a patient. The irradiation object 13 is irradiated in this case usually rotationally symmetrical.

In the direction of radiation 8, the X-ray tube 4 is connected substantially immediately adjacent to a filter 14 made of an X-ray absorbing material. The filter 14 is embodied such that in a standard object 13 ^λ positioned on the patient couch 12 instead of the irradiation object 13, an X-ray dose D which is uniformly distributed substantially over its cross-sectional area is deposited, if this Standard object 13 ^λ is irradiated rotationally symmetrically through the filter 14 therethrough.

The standard object 13 ^λ is selected for each application in terms of its geometry and X-ray absorption such that it approximately corresponds to the average geometry and X-ray absorption of the irradiation object 13 to be irradiated in this application. Accordingly, the properties of the filter 14 also vary from application to application.

FIG 2 and FIG 3 show an embodiment of the filter 14, which is designed for irradiation in the head area. This filter 14 was based on a circular cylindrical standard object 13 ^λ made of Teflon or aluminum with a

Diameter of d = 20 cm and a homogeneous (volume) X-ray absorption coefficient of about 0.3 cm ^"1 or 0.5 cm ^{" 1} (designed for 120 kV tube voltage).

The filter 14 shown in FIG. 2 is formed in one piece from polytetrafluoroethylene (PTFE) and has a flat, cuboid-shaped outer contour. In the intended installation situation, the filter 14 is aligned transversely to the radiation direction 8 with a filter surface A spanned by the longitudinal direction 9 and the transverse direction 11. A in the installation situation of the X-ray tube 4 facing front 20 of the filter 14 is flat. In contrast, a central indentation 22 is introduced into a reverse side 21 of the filter 14 opposite thereto.

The indentation 22 has a constant profile in the longitudinal direction 9 and is mirror-symmetrical with respect to a center plane 23 spanned between the radiation direction 8 and the longitudinal direction 9. The filter 14 thus has a hereinafter referred to as filter thickness s expansion in

Radiation direction 8, which is minimal in the region of the center plane 23 and increases continuously with increasing distance x to the center plane 23. In the cross section according to FIG. For example, the shape of a bell curve is approx. In the minimum, ie in the region of the center plane 23, the filter strength s has approximately a minimum value of s _min = 3 mm. For large distances x from the center plane 23, specifically for x> 25 mm, the filter strength s reaches a maximum value of s _ma χ = 49 mm. A half-value strength of Si _/ 2 = 26 mm reaches the filter thickness s in each case at a distance Xy ₂ ~ 12 mm.

The rear side 21 of the filter 14 has in the region of the in-book 22 in cross section according to FIG 3 a continuous composed of circular arc sections (radii) and straight sections course. The resulting dependence of the filter strength s on the distance x follows this approximately a function

s (x) = Smax - (s _ma χ - s _min ) - exp (- (x / X ₀ ) ² ") GLG 1

with Smax = 49 mm, s _min = 3 mm, x ₀ = 13 and n = 2.

The filter 14 is designed for X-radiation which is generated by applying a tube voltage of about 120 kV.

In proper positioning of the filter 14 is centered with respect to the radiation field 7 aligned. The central ray 6 of the X-ray radiation thus passes through the filter 14 approximately in the center plane 23. As the filter strength s increases, the proportion of X-ray radiation transmitting the filter 14 at a certain distance x decreases exponentially. As a result of the varying filter strength s (x), a radiation transmission coefficient β (x) which likewise varies as a function of the distance x is thus determined. The radiation transmission coefficient β (x) generally designates that portion of the x-radiation incident on the filter 14 at a given distance x, which transmits the filter, ie which is not absorbed in the filter 14. The construction of the filter 14, in particular the determination of the shape of the indentation 22, takes place using a computer simulation.

For this purpose, the standard object 13 ^λ , the radiation field 7 and the filter 14 arranged therein are modeled as virtual (ie numerical) models. These models include information on the geometry of the standard object 13 ^λ , the radiation field 7 and the filter 14 and information on their relative position to each other. The models representing the standard object 13 ^λ and the filter 14 additionally include an indication of the respective (volume) X-ray absorption coefficient. In the context of the model representing the radiation field 7, the X-ray spectrum is additionally specified.

Based on the thus predetermined models, a rotationally symmetrical irradiation path of the computer tomograph 1 is simulated. In this case, the X-ray dose D deposited in total in the standard object 13 ^{λ is} spatially resolved over the cross-sectional area of the standard object 13 ^λ .

With rotationally symmetrical irradiation and a rotationally symmetrical standard object 13 ^λ , the profile of the X-ray ^dose deposited in the standard object 13 ^{λ is} always rotationally symmetrical. In this case, by the computer simulation, the deposited X-ray dose D is calculated as a function of the distance r from the (cross-sectional area) center 24 of the standard object 13 ^λ .

In the context of the model representing the filter 14, the rear side 21 of the filter 14 is described by GLG 1. The parameters of this function are varied as part of the computer simulation until a predetermined optimization criterion is met. In particular, it is specified as an optimization criterion that for an optimized parameter set of GLG 1, the standard deviation of the dose distribution D (r) calculated for this purpose must be less than 5% of the dose mean value D _ave . A dose distribution D (r) calculated according to the method described above for the filter 14 according to FIGS. 2 and 3, which fulfills the above-mentioned optimization criterion, is shown in FIG.

After optimized parameters for GLG 1, and thus an optimized curve of the filter strength s as a function of the distance x from the center plane 23 are found, the correspondingly parameterized GLG 1 is piecewise steadily approximated by arc sections each having a constant radius, and thus an optimized curvature course determined for the back 21 of the filter 14.

Based on this curvature of the filter 14 is then milled by means of a CNC milling machine from a solid PTFE block.

Filters of the type shown with reference to FIGS. 2 and 3 are basically also suitable for the irradiation of non-rotationally symmetric, e.g. elliptical irradiation objects 13 suitable. In order to achieve a uniform dose deposition in the case of circulating irradiation despite a rigid filter, the radiation intensity and / or the radiation spectrum are expediently changed in this case by varying the tube current and / or the tube voltage during the circulation of the x-ray tube. The filter is designed based on the corresponding tube current or tube voltage curve.

5 shows a second embodiment of the filter 14 is shown. In contrast to the embodiment described above, the filter 14, which is embodied here in several parts, comprises a frame 30 in which a plurality of lamellae 31 made of the X-ray-absorbing material, here again PTFE, are held. The fins 31 are arranged in two stacks 32 and 33, which face each other on different sides of the median plane 23. The blades 31 of the same pels 32 and 33 are arranged in each case in the radiation direction 8 behind one another and at least partially overlapping. The X-radiation propagating in the radiation direction 8 must therefore penetrate all or at least a majority of the fins 31 of a stack 32 or 33 at least in a partial area of the filter surface A. The filter thickness s is thus determined by the number of lamellae 31, which must penetrate the x-ray radiation at a specific location of the filter surface A.

The lamellae 31 of each stack 32 and 33 are each slidably guided in associated receptacles 34 and 35 of the frame 30. Each lamella 31 is here coupled separately with an associated drive 36 in the form of a piezoceramic actuator, so that the lamella 31 can be displaced transversely to the center plane 23 by actuation of the drive 36. The filter 14 is further associated with a control unit 37, by means of which the actuators 36 are controlled programmatically to move the slats 31. The control unit 37 is embodied in particular as a software module and integrated, for example, in the control software of the computed tomography system 1. The drives 36 are preferably integrated with the remaining components of the filter 14 to form a coherent assembly. Alternatively, however, it may also be provided that the drives 36 are present separately from the actual filter 14, for example as integral components of a filter holder of the computer tomograph 1.

For the application of the computed tomography system 1 for irradiation therapy, the individual lamellae 31 are displaced according to a profile stored in the control unit 37 in such a way that the lamellae of the same stack 32 or 33 have a varying distance from the center plane 23 and thus a course according to the invention the filter strength s as a function of the distance x from the center plane. In the setting according to the invention of its fins 31, the filter 14 again has the pronounced indentation 22 in the area of the center plane 23. For irradiations in the head region of a patient, the fins 31 are adjusted such that the profile of the filter 14 shown in FIG. 3 is approximately simulated.

In contrast to the rigid embodiment of the filter 14 according to FIGS. 2 and 3, the embodiment according to FIG. 5 can be used for different applications, in particular for the irradiation of different body parts. For this purpose, several profiles for setting the slats 31 are stored in the control unit 37, which are provided for one purpose, and were created on the basis of different standard objects 13 ^λ . From these profiles can be selected by a user of the computed tomography 1 software depending on the desired application, the appropriate profile. Due to this selection, the control unit 37 then adjusts the filter 14 accordingly.

The control unit 37 is moreover configured to adjust the position of the lamellae 31 during the rotation of the x-ray tube 4. The control unit 37 is supplied for this purpose the rotation angle α as an input variable. In addition, at least one profile stored in the control unit 37 contains sets of settings for the slats 31 as a function of the angle of rotation α. The rotation angle-dependent variation of the slat position is particularly advantageous for the irradiation of distinctly non-rotationally symmetrical irradiation objects 13, for example the thorax of a patient. When generating an associated rotation angle-dependent profile, a corresponding non-rotationally symmetrical standard object 13 ^λ is used.

Claims

claims

1. Filter (14) for positioning between a radiation source (4) and an isocentric axis (I) of a computer tomograph (1) with respect to its within the filter surface (A) location-dependent radiation transmission coefficient (ß) for the X-ray radiation of the computer tomograph (1 ) is dimensioned or dimensioned in such a way that the X-ray dose (D) deposited over the cross section of the standard object (13 ^λ ) is substantially constant when the standard object (13 ^λ ) of predetermined geometry and X-ray absorption through the filter (14) is irradiated.

2. Filter (14) according to claim 1, which consists essentially of a homogeneous X-ray absorbing material, wherein the location dependence of the radiation transmission coefficient (ß) by a depending on the location (x) on the filter surface (A) varying filter strength (s) is determined ,

3. Filter (14) according to claim 1 or 2, wherein the location dependence of the radiation transmission coefficient (ß) for a means of a tube voltage from an interval of about 80 to 140 kV, in particular about 120 kV, generated Röntgen radiation is calculated.

4. Filter (14) according to one of claims 1 to 3, wherein the radiation transmission coefficient (ß) with respect to a transversely to the filter surface (A) oriented center plane (23) mirror symmetrically varies and in the region of the median plane (23) is maximum.

5. Filter (14) according to claims 2 and 4, wherein the filter thickness (s) with respect to the central plane (23) varies mirror-symmetrically and in the region of the median plane (23) is minimal.

6. Filter (14) according to claim 5, wherein the filter thickness (s) in cross-section transverse to the median plane (23) has a continuous course in the manner of an inverse bell curve.

7. Filter (14) one of claims 1 to 6, wherein in the standard object (13 ^λ ) deposited X-ray dose (D) deviates from its average value (D _ave ) by a maximum of 5%.

8. Filter (14) according to any one of claims 1 to 7, which is integrally formed from a X-ray absorbing material.

9. Filter (14) according to one of claims 1 to 8, which is formed of two stacks (32,33) of lamellae (31) of X-ray absorbing material, wherein the lamellae (31) within the same stack (32,33) has a varying Distance to the median plane (23) have.

10. Filter (14) according to claim 9, wherein at least a part of the slats (31) of each stack (32,33) for changing the distance of these slats (31) to the center plane (23) is displaceable.

11. Computer tomograph (1) with one around an isocentric

Axis (I) rotatable radiation source (4) for emission of X-radiation, and with a between the radiation source (4) and the isocentric axis (I) arranged filter (14) according to one of claims 1 to 10.

12. Computer tomograph (1) according to claim 11, with a filter (14) according to claim 10, with drive means (36) for displacing the slats (31) and with a control unit (37) for controlling the drive means (36) in accordance with a deposited profile, which dictates the positioning of the slats (31) with respect to the median plane (23).

3. computed tomography (1) according to claim 12, wherein the profile, the positioning of the slats (31) varies depending on a rotation angle (α) of the radiation source (4).