WO2008099314A2

WO2008099314A2 - Apparatus and method for determining high density shadows in projection data

Info

Publication number: WO2008099314A2
Application number: PCT/IB2008/050458
Authority: WO
Inventors: Thomas Koehler; Peter Forthmann; Holger Schmitt
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2007-02-13
Filing date: 2008-02-08
Publication date: 2008-08-21
Also published as: WO2008099314A3

Abstract

The invention relates to an apparatus for determining high density shadows in projection data, wherein the apparatus comprises a first projection data providing unit (1, 2, 3, 6, 7, 8) for providing first projection data, which correspond to a projection through an object having an amount of high density elements. The apparatus comprises further a second projection data providing unit (12) for providing second projection data, which correspond to a projection through a model of the object, in which the amount of high density elements is reduced. The apparatus further comprises a determination unit (13) for determining high density shadows in the first projection data by comparing the first projection data with the second projection data.

Description

Apparatus and method for determining high density shadows in projection data

FIELD OF THE INVENTION

The invention relates to an apparatus, a method and a computer program for determining high density shadows in projection data.

BACKGROUND OF THE INVENTION

WO 2006/039809 Al discloses a computed tomography system, which can be used for reconstructing an image of an object having metallic elements. The computed tomography system generates projection data, wherein a shadow in the projection data, which is caused by the metallic elements, is determined by thresholding. The determined metallic shadow is used by the computed tomography system for reducing metal artifacts in the reconstructed image of the object. This determination of metal shadows in the projection data by thresholding is inaccurate and, thus, metal artifacts are still present and pronounced in the reconstructed image.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus, a method and a computer program for determining high density shadows in projection data, wherein the determination of high density shadows in the projection data is more accurate.

In a first aspect of the present invention, an apparatus for determining high density shadows in projection data is presented, wherein the apparatus comprises a first projection data providing unit for providing first projection data, which correspond to a projection through an object having an amount of high density elements, a second projection data providing unit for providing second projection data, which correspond to a projection through a model of the object, in which the amount of high density elements is reduced, a determination unit for determining high density shadows in the first projection data by comparing the first projection data with the second projection data.

Since the first projection data correspond to a projection through an object having an amount of high density elements and since the second projection data correspond to a projection through a model of the object, in which the amount of high density elements is reduced, high density elements exist, which contribute to the first projection data, but which do not contribute to the second projection data. The invention is based on the idea that by comparing the first projection data and the second projection data regions within the projection data can be determined which differ from each other, because of the high density elements, which contribute only to the first projection data. These regions can easily and with a high accuracy be identified.

Projection data correspond to a projection through an object or through a model of the object, if the projection data depend on a projection through an object or through a model of the object. In particular, projection data correspond to a projection through an object or through a model of an object, if the projection data are generated by a projection through an object or through a model of the object by a real measurement or by simulation.

The first projection data providing unit is, for example, a computed tomography system, which measures projections through an object having an amount of high density elements. The first projection data providing unit can also be any other imaging system, which generates projections through an object having an amount of high density elements. The object is, for example, a head of a patient, which comprises dental metal, or a heart of a patient which comprises a cardiac pacemaker. The first projection data providing unit can also be a storage unit, in which first projection data are stored which have already been measured, for example, by a computed tomography system. Furthermore, the first projection data providing unit can also be a simulation unit, which simulates first projection data by simulating a forward projection through an object having an amount of high density elements. High density elements are elements, which have a density within a given density range or above a given density value. If the first projection data are provided by measuring projection values, which depend on X-ray radiation after having passed the object, i.e. if, for example, the first projection data providing unit is a computed tomography system or another X-ray system, and if the high density elements are metallic elements, the high density elements have preferentially a density above 2500 HU (Hounsfϊeld Units).

If the first projection data are provided by projecting radiation through the object, the terms "density" and "density shadows" can have different meanings depending on the radiation used for projecting through the object. For example, if the radiation is X-ray radiation, the term "density" is or depends on the absorption of the object, and the "density shadow" is the shadow in the projection data caused by the absorption. In case of coherent scatter computed tomography the term "density" is or depends on the coherent scatter form factor, and the term "density shadow" is a shadow in the projection data caused by the coherent scatter form factor. Instead of X-ray radiation nuclear radiation can be used, for example, in a positron emission tomography. A high density shadow is a shadow in the projection data caused by the high density elements.

The second projection data providing unit can be any unit, which provides second projection data, which correspond to a projection through a model of the object, in which the amount of high density elements is reduced. The term "reduced" also includes the case in which the second projection data correspond to a projection through a model of the object without high density elements. Also the second projection data providing unit can be a storage unit, in which the second projection data are already stored. Furthermore, the second projection data providing unit can be adapted for averaging several simulated and/or measured projection data of similar objects, which have an reduced amount of high density elements or which have no high density elements at all, wherein the average projection data can be the second projection data.

In a preferred embodiment, the second projection data providing unit comprises a model provision unit for providing a model of the object, in which the amount of high density elements is reduced and a projection data calculation unit for calculating the second projection data by forward projecting through the provided model. Since in this preferred embodiment a model of the object, which comprises a reduced amount of high density elements, i.e. which comprises in particular no high density elements at all, is provided, second projection data can easily be calculated by forward projecting through the provided model. If the object is a head of a patient, the model is preferentially an anatomical model of the head.

It is further preferred that the second projection data providing unit further comprises a reconstruction unit for reconstructing an image of the object using the first projection data and a registration unit for registering the model of the object with the reconstructed image of the object. The registration unit can preferentially also adapt the model, i.e. for example, adapt the size and/or the shape and/or the position of the model and/or of components of the model, in order to register the model of the object with the reconstructed image of the object. The registration, which might include the above mentioned adaptation of the model, improves the effect that the difference between the first projection data and the second projection data is preferentially only caused by high density elements, which are present in the object, but which are not present in the model of the object.

It is further preferred that the determination unit for determining high density shadows in the first projection data comprises a difference unit for determining a difference between the first projection data and the second projection data and a threshold unit for determining whether the difference exceeds or goes below a predetermined threshold. This allows easily and accurately determining high density shadows by thresholding on difference projection data.

In a preferred embodiment, the apparatus for determining high density shadows in projection data further comprises an imaging unit for reconstructing an image of the object, wherein the imaging unit is adapted for reducing high density artifacts in the reconstructed image using the determined high density shadows. Since high density shadows have been determined with a high accuracy, artifacts caused by high density elements are strongly reduced or no more present in the reconstructed image.

In a further aspect of the invention a method for determining high density shadows in projection data is presented, wherein the method comprises following steps

- providing first projection data, which correspond to a projection through an object having an amount of high density elements,

- providing second projection data, which correspond to a projection through a model of the object, in which the amount of high density elements is reduced,

- determining high density shadows in the first projection data by comparing the first projection data with the second projection data.

In a further aspect of the present invention, a computer program for determining high density shadows in projection data is presented, wherein the computer program comprises program code means for causing an apparatus as claimed in claim 1 to carry out the steps of the method as claimed in claim 6, when the computer program is run on a computer controlling the apparatus.

It shall be understood that the apparatus of claim 1, the method of claim 6 and the computer program of claim 7 have similar and/or identical preferred embodiments as defined in the dependent claims. It shall be understood that preferred embodiments of the invention can also be any combination of the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings:

Fig. 1 shows schematically an embodiment of an apparatus for determining high density shadows in projection data, Fig. 2 shows schematically a second projection data providing unit of the apparatus for determining high density shadows in projection data,

Fig. 3 shows schematically a determination unit for determining high density shadows in projection data,

Fig. 4 shows a flowchart illustrating an embodiment of a method for determining high density shadows in projection data,

Fig. 5 shows a flowchart illustrating an embodiment of a determination of a second projection data, and

Fig. 6 shows a flowchart illustrating the determination of high density shadows from first projection data and second projection data.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows schematically an apparatus for determining high density shadows in projection data, which is in this embodiment a computed tomography system. The computed tomography system includes a gantry 1, which is capable of rotation about an axis of rotation R which extends parallel to the z direction. A radiation unit 2, which is in this embodiment an X-ray tube, is mounted on the gantry 1. The X-ray tube is provided with a collimator device 3 which forms a conical radiation beam 4 from the radiation emitted by the X-ray tube 2. In other embodiments, the collimator device 3 can be adapted for forming a radiation beam having another shape, for example, having a fan shape.

The radiation traverses an object (not shown), such as a patient, in particular a head of a patient, in a region of interest in a cylindrical examination zone 5. The object comprises high density elements. For example, the object might be a head of a patient comprising dental metals, like metallic dental fillings. After having traversed the examination zone 5, the X-ray beam 4 is incident on a detection unit 6, which is in this embodiment a two-dimensional detector and which is mounted on the gantry 1. In another embodiment, the detection unit can also be a one-dimensional detector. The gantry 1, the X-ray tube 2 with the collimator 3 and the detection unit 6 form a first projection data providing unit for providing first projection data, which correspond to a projection through an object having an amount of high density elements.

The gantry 1 is driven at a preferably constant but adjustable angular speed by a motor 7. A further motor 8 is provided for displacing the object, for example, the patient who is arranged on a patient table in the examination zone 5, parallel to the direction of the axis of rotation R or the z axis. These motors 7, 8 are controlled by a control unit 9, for instance, such that the radiation source 2 and the examination zone 5 move relative to each other along a helical trajectory. But, it is also preferred that the object or the examination zone 5 is not moved and that the X-ray tube 2 is rotated, i.e. that the X-ray tube 2 travels along a circular trajectory relative to the object. Also the motors 7, 8 can be regarded as components of a first projection data providing unit, which form together with a gantry 1, the X-ray tube 2 with the collimator 3 and the detection unit 6 preferentially a first projection data providing unit.

The data acquired by the detection unit 6 are provided to an imaging device 10 for reconstructing an image of the object, in which artifacts caused by high density elements are reduced or no more present in the image. The reconstructed image can be provided to a display unit 11 for displaying the reconstructed image. The imaging device 10 comprises a second projection data providing unit

12 for providing second projection data, which correspond to a projection through a model of the object, in which the amount of high density elements is reduced. In this embodiment, the second projection data correspond to a projection through a model of the object without any high density elements. The imaging device 10 further comprises a determination unit 13 for determining high density shadows in the first projection data by comparing the first projection data with the second projection data. The imaging device 10 also comprises a reconstruction unit 14 for reconstructing an image of the object, wherein the reconstruction unit 14 is adapted for reducing high density artifacts in the reconstructed image using the determined high density shadows. The second projection data providing unit 12 is schematically shown in more detail in Fig. 2. The second projection data providing unit 12 comprises a model provision unit 15 for providing a model of the object, wherein in the model of the object the amount of high density elements is reduced in comparison to the amount of high density elements present in the real object. In this embodiment, the model of the object does not comprise any high density elements.

The model provision unit 15 is preferentially a storage unit, in which models are stored. The model provision unit 15 provides the model, which corresponds to the object. For example, if the object is a head of a patient, an anatomical model of a head of a patient is preferentially used. A model of an object can, for example, be determined by reconstructing images of similar objects and by averaging over the reconstructed images. For example, if the object is a head of a male adult patient, a model can be used, which represents an average of reconstructed images of heads of male adult patients without having high density elements, for example, without having dental metals.

The second protection data providing unit 12 further comprises a reconstruction unit 16 for reconstructing an image of the object using the first projection data. In this embodiment, the first projection data are the projection data acquired by the detection unit 6. The image of the object can be reconstructed by using a filtered backprojection, but also other computed tomography reconstruction methods can be used. The second projection data providing unit 12 also comprises a registration unit 17 for registering the model of the object with the reconstructed image of the object. In particular, the registration unit is adapted such that the model is adapted to the reconstructed image of the object, i.e. the size and/or the shape and/or the position of the object and/or of components of the object are adapted to the reconstructed image. This registered and preferentially adapted model substantially represents the object as it would be without high density elements. The second projection data providing unit 12 further comprises a projection data calculation unit 18 for calculating the second projection data by forward projecting through the provided model. The forward projection is performed in the acquisition geometry, which was used for acquiring the first projection data. Fig. 3 shows schematically the determination unit 13 in more detail. The determination unit 13 comprises a difference unit 19 for determining a difference between the first projection data and the second projection data and a threshold unit 20 for determining whether the difference exceeds or goes below a predetermined threshold. The threshold is preferentially determined by calibration, for example, by using phantoms, wherein the distribution of high density elements within the phantoms is known, either by measurements or by simulations.

In the following, an embodiment of a method for determining high density shadows in the projection data will be described in more detail with reference to a flowchart shown in Fig. 4. In step 101 first projection data are provided by the first projection data providing unit, which comprises in this embodiment the X-ray tube 2, the collimator 3, the detection unit, the gantry 1 and the motors 7, 8. The X-ray tube 2 rotates around the axis of rotation R or the z direction, and the object is moved, i.e. the X-ray tube 2 travels along a circular trajectory around the object. In another embodiment, the X-ray tube 2 can move along another trajectory, for example, along a helical trajectory, relative to the object. The X-ray tube 2 emits X-ray radiation traversing the object in which high density elements are present. The object is, for example, a head of a patient comprising dental metals. The X-ray radiation, which has traversed the object and the high density elements within the object, is detected by the detection unit 6, which generates the first projection data. In step 102 second projection data are provided, which correspond to a projection through a model of the object, wherein the amount of high density elements is reduced. As already mentioned above, in this embodiment second projection data are provided, which correspond to a projection through a model of the object without any high density elements. This provision of second projection data will be explained in more detail with reference to a flowchart shown in Fig. 5.

In step 201, the model provision unit 15 provides a model of the object having the reduced amount of high density elements, i.e. in this embodiment having no high density elements at all. In step 202, an image of the object is reconstructed using the first projection data by the reconstruction unit 16.

The reconstructed image and the model provided by the model provision unit 15 are transmitted to the registration unit 17, which in step 203 registers the model of the object with the reconstructed image of the object. In this embodiment, the registration also includes an adaptation of the model to the reconstructed image of the object. In step 204, a forward projection is performed through the registered and preferentially adapted model of the object in the same acquisition geometry, which has been used in step 101 for providing the first projection data. The projection data resulting from this forward projection are the determined second projection data.

Referring again to Fig. 4, in step 103 the determination unit 13 determines high density shadows in the first projection data by comparing the first projection data with the second projection data. This determination of the high density shadows will in the following be explained in more detail with reference to a flowchart shown in Fig. 6.

In step 301, the difference unit 19 determines a difference between the first projection data on the second projection data resulting in difference projection data. Since the first projection data and the second projection data are substantially similar, except for the projection data caused by the high density elements, which are present in the object but not in the model, the difference projection data have in general small values and in regions, in which contributions of high density elements of the object, but not of the model, are present, larger absolute difference projection data values occur. Thus, the high density shadows can be determined by determining the part or the parts of the difference projection data, which exceed or go below a predetermined threshold, respectively, depending on the sign of the difference projection data values. This determination is performed by the threshold unit 20 in step 203. The threshold value is predetermined and can, for example, be determined by calibration using objects having known high density element distributions.

Referring again to Fig. 4, in step 104 the imaging unit 14 reconstructs an image of the object, wherein the imaging unit 14 reduces high density artifacts in the reconstructed image using the high density shadows determined in step 103. This reconstruction of an image of the object and this reduction of high density artifacts in the reconstructed image can, for example, be performed by replacing projection data values within the determined high density shadows by interpolated projection data values, which are interpolated using the projection data values adjacent to the respective high density shadow. In another embodiment, for example, the determined high density shadows and the first projection data can be used for reconstructing an image of the object having reduced high density artifacts as disclosed in WO 2005/008586 A2, which is herewith incorporated by reference. Also other reconstruction methods can be used, which require the high density shadows and projection data for reconstructing an image of the object having reduced high density artifacts. The determined high density shadows can also be used for the high density specific beam hardening correction, in particular for a metal specific beam hardening correction.

Although in the above described embodiments the object is preferentially a head of a patient, the invention is not limited to an object being a head of a patient. For example, the object can be a heart of a patient comprising a cardiac pacemaker or a technical object, for example, made of plastic material including metallic elements.

Although the apparatus for determining high density shadows and projection data has been described as comprising a lot of components and units of a computed tomography system, the invention is not limited to an apparatus being a computed tomography system. For example, a processing device, which only comprises a first projection data providing unit, which might be a storage unit, a second data projection providing unit, which might also be a storage unit, and a determination unit for determining high density shadows in the first projection data by comparing the first projection data with the second projection data are within the scope of the invention. The apparatus in accordance with the invention can also be any other system or can be a part of any other system which uses and/or generates projection data by projecting through an object having an amount of high density elements. For example, the apparatus in accordance with the invention can be a nuclear imaging system or can be a part of a nuclear imaging system.

The invention is not limited to the determination of only several high density shadows. The apparatus in accordance with the invention can also determine only one high density shadow in the projection data. Some or all of the above described units can be implemented by one or several units. For example, the reconstruction unit 16 and the imaging unit 14 can be implemented as one unit. Furthermore, the units performing calculations and/or determinations like, for example, the determination unit 13, the reconstruction unit 16, the registration unit 17, the calculation unit 18, the difference unit 19, the threshold unit 20, and the imaging unit 14 can be program code means, which perform the respective functions, and which can be run on a computer system, or dedicated hardware, which perform the respective functions. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention from the study of the drawing, the disclosure, and the appended claims. While the invention has been illustrated and described in detail in the drawings and in the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments.

In the claims the word "comprising" does not exclude other elements or steps and the indefinite article "a" or "an" does not exclude a plurality. A computer program may be stored, distributed on a suitable medium, such as an optical storage medium or a solid state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems.

Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims

CLAIMS:

1. An apparatus for determining high density shadows in projection data, wherein the apparatus comprises

- a first projection data providing unit (1, 2, 3, 6, 7, 8) for providing first projection data, which correspond to a projection through an object having an amount of high density elements,

- a second projection data providing unit (12) for providing second projection data, which correspond to a projection through a model of the object, in which the amount of high density elements is reduced,

- a determination unit (13) for determining high density shadows in the first projection data by comparing the first projection data with the second projection data.

2. The apparatus as claimed in claim 1, wherein the second projection data providing unit comprises

- a model provision unit (15) for providing a model of the object, wherein the amount of high density elements is reduced,

- a projection data calculation unit (18) for calculating the second projection data by forward projecting through the provided model.

3. The apparatus as claimed in claim 2, wherein the second projection data providing unit further comprises

- a reconstruction unit (16) for reconstructing an image of the object using the first projection data,

- a registration unit (17) for registering the model of the object with the reconstructed image of the object.

4. The apparatus as claimed in claim 1, wherein the determination unit (13) for determining high density shadows in the first projection data comprises

- a difference unit (19) for determining a difference between the first projection data and the second projection data,

- a threshold unit (20) for determining whether the difference exceeds or goes below a predetermined threshold.

5. The apparatus as claimed in claim 1, further comprising an imaging unit (14) for reconstructing an image of the object, wherein the imaging unit is adapted for reducing high density artifacts in the reconstructed image using the determined high density shadows.

6. A method for determining high density shadows in projection data, wherein the method comprises following steps

- providing second projection data, which correspond to a projection through a model of the object, in which the amount of high density elements is reduced, - determining high density shadows in the first projection data by comparing the first projection data with the second projection data.

7. A computer program for determining high density shadows in projection data, comprising program code means for causing an apparatus as defined in claim 1 to carry out the steps of the method as claimed in claim 6, when the computer program is run on a computer controlling the apparatus.