WO2008078231A1 - Imaging system for imaging substances present in an object of interest - Google Patents

Imaging system for imaging substances present in an object of interest Download PDF

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Publication number
WO2008078231A1
WO2008078231A1 PCT/IB2007/055105 IB2007055105W WO2008078231A1 WO 2008078231 A1 WO2008078231 A1 WO 2008078231A1 IB 2007055105 W IB2007055105 W IB 2007055105W WO 2008078231 A1 WO2008078231 A1 WO 2008078231A1
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Prior art keywords
substance
contribution
image
detection signals
interest
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PCT/IB2007/055105
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French (fr)
Inventor
Roland Proksa
Original Assignee
Koninklijke Philips Electronics N.V.
Philips Intellectual Property & Standards Gmbh
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Publication of WO2008078231A1 publication Critical patent/WO2008078231A1/en

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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/50—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment specially adapted for specific body parts; specially adapted for specific clinical applications
    • A61B6/504—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment specially adapted for specific body parts; specially adapted for specific clinical applications for diagnosis of blood vessels, e.g. by angiography
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
    • A61B6/03—Computed tomography [CT]
    • A61B6/032—Transmission computed tomography [CT]
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/42—Arrangements for detecting radiation specially adapted for radiation diagnosis
    • A61B6/4208—Arrangements for detecting radiation specially adapted for radiation diagnosis characterised by using a particular type of detector
    • A61B6/4241—Arrangements for detecting radiation specially adapted for radiation diagnosis characterised by using a particular type of detector using energy resolving detectors, e.g. photon counting
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/48—Diagnostic techniques
    • A61B6/481—Diagnostic techniques involving the use of contrast agents
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/48—Diagnostic techniques
    • A61B6/482—Diagnostic techniques involving multiple energy imaging
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
    • A61B6/027—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis characterised by the use of a particular data acquisition trajectory, e.g. helical or spiral

Definitions

  • Imaging system for imaging substances present in an object of interest
  • the present invention relates to an imaging system, an imaging method and a computer program for imaging of a substance present in an object of interest.
  • the invention relates further to an image generation device, an image generation method and a computer program for generating an image of a substance present in an object of interest.
  • a contrast agent into an object of interest and to image the contrast agent within the object by an imaging system, for example by a computed tomography system (CT system) or by a magnetic resonance system (MR system).
  • CT system computed tomography system
  • MR system magnetic resonance system
  • the contrast agent is chosen such that it can be imaged by the respective imaging system.
  • an iodine based contrast agent is used together with a CT system.
  • the imaging of the contrast agent within the object allows to image vessels, which might be present within the object.
  • Some known imaging system also provide the possibility to determine the flow of the contrast agent within the object.
  • an imaging system for imaging a substance present in an object of interest comprising:
  • a detection unit for detecting detection signals depending at least on the first substance and on the second substance
  • - a reconstruction unit for reconstructing at least an image of one of the first substance and of the second substance using the determined first substance contribution and the determined second substance contribution.
  • the basic idea of the invention is that the imaging system allows to detect detection signals depending at least on the first substance and on the second substance which are at the same time present within the object of interest, to determine at least the contribution of the first substance and the contribution of the second substance to the detection signals, and to reconstruct at least an image of the first substance and an image of the second substance using the determined contributions.
  • the imaging system according to the invention is able to reconstruct images of at least two substances using detection signals, which have been acquired, while at least the both substances are simultaneously present within the object of interest. Since the contributions of the first substance and the second substance have been determined by the calculation unit, the reconstruction unit can reconstruct images which only show the first substance or the second substance or which shows both.
  • the ability to image at least two substances simultaneously, which might have different properties increases the variability of the imaging system according to the invention in comparison to known imaging systems.
  • the imaging system comprises a detection unit for detecting the detection signals depending at least on the first substance and on the second substance.
  • a detection unit can be a detection unit which is adapted to detect detection signals which simultaneously depend at least on the first substance and on the second substance and/or to detect, in particular during a single scan operation, detection signals which only depend on the first substance and detection signals which only depend on the second substance.
  • the reconstruction unit is adapted to reconstruct an image at least of one of the first substance and the second substance using the determined first substance contribution and the determined second substance contribution.
  • a reconstruction unit can be a reconstruction unit, which is adapted such that it reconstructs an image comprising at least the first substance and the second substance, wherein the image might comprise means for distinguishing the first substance from the second substance within the image, wherein, for example, the first substance is presented having another color than the second substance within the image, or such that at least two images a reconstructed, wherein a first image shows the first substance and a second image shows the second substance.
  • the detection unit is adapted to detect detection signals depending on the object itself, wherein the calculation unit is adapted to determine an object contribution of the object itself to the detection signals, wherein the reconstruction unit is adapted to reconstruct the object itself using the determined object contribution.
  • the imaging system in this embodiment, is not only able to image the substances, but also the object itself. This allows for example to show the first substance and/or the second substance relative to the object in one or several reconstructed images.
  • the imaging system is a CT imaging system comprising a polychromatic X-ray source for emitting polychromatic X-ray radiation
  • the detection unit is an energy-resolving X-ray detector for detecting the X-ray radiation after passing through the object and for providing a plurality of energy-resolved detection signals
  • the calculation unit is adapted to determine the k-edge components at least of the first substance and of the second substance from the detection signals being the first substance contribution and the second substance contribution to the detection signals
  • the reconstruction unit is adapted to use the k-edge components at least of the first substance and the second substance for reconstructing.
  • the k-edge components are the contributions of the respective substance to the detection signals.
  • the contributions of the k-edge components to the detections signals can be determined with high quality by using the CT imaging system of this embodiment.
  • the images, which have been reconstructed using this contributions of the k-edge components to the detection signals have an improved quality.
  • the reconstruction unit can be adapted to use the k-edge component of the first substance to reconstruct an image of the first substance and to use the k-edge component of the second substance to reconstruct an image of the second substance.
  • the calculation unit can be adapted to use a model which takes account of the emission spectrum of the X-ray source and the spectral sensitivity of the X-ray detector. Furthermore, the calculation unit can be adapted to separate contributions of the Compton effect and of the photo-electric effect from the detection signals. These adaptations of the calculation unit further improve the quality of the reconstructed image.
  • the calculation unit is adapted to determine the k-edge components at least of the first substance and the second substance by solving a system of equations for the plurality of energy-resolved detection signals, using a model for the detection signals describing a detection signal as a combination of the k-edge effects, the Compton effect and the photo-electric effect, each effect contributing with a corresponding component to the detection signal, wherein the reconstruction unit is adapted to reconstruct at least a k-edge image of the first substance and a k-edge image of the second substance from the determined k-edge components at least of the first substance and the second substance.
  • the calculation unit can be adapted to determine the photo-electric effect component and/or the Compton effect component by solving the system of equations for the plurality of energy resolved detection signals, and the reconstruction unit can be adapted to reconstruct a photo-electric effect image and/or a Compton effect image from the determined photo-electric effect components and/or the Compton effect components.
  • the photo-electric effect image and the Compton effect image can show additional information with respect to the object and/or the substances within the object, which could not be derived from images, which have been reconstructed using the k-components of the detection signals and/or using the object contribution to the detection signals.
  • an imaging method for imaging a substance present in an object of interest wherein at least a first substance and a second substance are present in the object of interest at the same time, wherein the imaging method comprises the steps of: - detecting detection signals depending at least on the first substance and on the second substance,
  • a computer program for imaging a substance present in an object of interest wherein at least a first substance and a second substance are present in the object of interest at the same time and wherein the computer program comprises program code means for causing a computer to carry out the steps of the imaging method according to the invention when the computer program is carried out on a computer controlling an imaging system according to the invention.
  • an image generation device for generating an image of a substance present in an object of interest, wherein at least a first substance and a second substance are present in the object of interest at the same time, the image generation device being provided with detection signals depending at least on the first substance and the second substance, comprising:
  • an image generation method for generating an image of an substance present in an object of interest wherein at least a first substance and a second substance are present in the object of interest at the same time, the image generation method being provided with detection signals depending at least on the first substance and the second substance, the image generation method comprises the steps of:
  • a computer program for generating an image of an substance present in an object of interest wherein at least a first substance and a second substance are present in the object of interest at the same time and wherein the computer program comprises program code means for causing a computer of an image generation device according to the invention to carry out the steps of an image generation method according to the invention when said computer program is carried out on a computer.
  • Fig. 1 shows a diagrammatic representation of an imaging system in accordance with the invention
  • Fig. 2 shows an exemplary emission spectrum of a polychromatic X-ray source
  • Fig. 3 shows exemplary spectra of the photo-electric effect, the Compton effect, and of substances within an object of interest.
  • the imaging system shown in Fig. 1 is a computed tomography system (CT system).
  • the CT system includes a gantry which is capable of rotation about an axis of ration R which extends parallel to the z direction.
  • the radiation source 2 for example an X-ray tube emitting polychromatic X-ray radiation, is mounted on the gantry 1.
  • the X-ray source is provided with a collimator device 3 which forms in this embodiment a conical radiation beam 4 from the radiation produced by the X-ray source 2.
  • the radiation traverses an object (not shown), such as a patient, in a region of interest in an examination zone 5, which is in this embodiment cylindrical.
  • the X-ray beam 4 is incident on an energy-resolving X-ray detector unit 6, in this embodiment a two-dimensional detector, which is mounted on the gantry 1.
  • 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 a 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 another along a helical trajectory.
  • the object or the examination zone 5 is not moved, but that only the X-ray source 2 is rotated.
  • the data acquired by the detector unit 6 are provided to an image generation device 10 for generating an image at least of one of a first substance and a second substance within the object (for example within the patient).
  • the reconstructed image can finally be provided to a display 11 for displaying the image.
  • the image generation device is preferably controlled by the control unit 9.
  • these substances are preferentially contrast agents, for example based on gadolinium and/or iodine.
  • the substances comprise preferentially different properties such that an image of the first substance comprises information which is different from the information contained in an image of the second substance.
  • the first substance can be a substance which is present within the object without attaching to parts of the object
  • the second substance can be a substance which attaches to some parts of the object being normally not visible in the imaging system, in particular in the CT system. If such two substances are present within the object, the image of the first substance shows for example the vessels within the object in which the first substance is present. Furthermore, the image of the second substance shows the parts of the object, to which the second substance attaches.
  • the first substance is preferentially a contrast agent based on iodine
  • the second substance is preferentially a contrast agent based on gadolinium.
  • the gadolinium based contrast agent can for example be the contrast agent named "EP2104R" which has been developed by the firm EPIX Pharmaceuticals, Inc..
  • Iodine has a good contrast for X-rays and can for example be used in a human heart.
  • the contrast agent EP2104R binds fibrin, whereas the contrast agent based on iodine does not attached to a part of a human body, in particular does not attached to a part of the human heart.
  • the object is a human heart and an iodine based contrast agent and the contrast agent EP2104R are present within the human heart
  • the vessels within the human heart, in which the iodine based contrast agent is present can be determined from an image data set showing the first substance, i.e. the iodine based contrast agent, and fibrin can be visualized by reconstructing an image of the second contrast agent, which has, in this example, attached to fibrin within the human heart.
  • the imaging system according to the invention is adapted to generate images of a first substance and a second substance, wherein both substances are present within the object at the same time and wherein the first substance can be distinguished from the second substance within the generated images, the vessels (coronary angiography) and fibrin can be determined within one imaging operation step, in particular, if the imaging system is a CT system, within one CT scan.
  • an image which shows a substance within an object having the property to attach to parts of the object, shall only show the part of the substance, which has been attached to parts of the object
  • the imaging method in accordance with the invention will be started, after the part of the substance, which has not been attached to a part of the object, has washed out of the object.
  • the imaging method is performed preferentially two to three hours after administrating EP2104R to the object, which is in this example a human heart.
  • the gantry 1 is started and rotates around the examination zone 5, in which an object, for example a patient, is positioned, in which at least two substances, a first substance and a second substance, are present. Furthermore, the object can be displaced parallel to the direction of the axis of rotation R. If the object is displaced, for example by moving a patient table parallel to the direction of the axis of rotation R, the X-ray radiation source 2 travels along a helical trajectory relative to the object. If the object is not displaced, the X-ray source 2 travels along a circular trajectory relative to the object.
  • the radiation emitted from the X- ray source 2 traverses the examination zone 5 and the object, and the light, which has traversed the examination zone 5 and the object, is acquired by the detection unit 6.
  • the detection unit 6 converts the acquired X-ray radiation into electrical signals, i.e. detection signals, which are transmitted to the image generation device 10.
  • the image generation device 10 comprises preferentially a calculation unit 12 and a reconstruction unit 13.
  • the calculation unit 12 determines the contribution of the first substance to the detection signals, i.e. the first substance contribution, and the contribution of the second substance to the detection signals, i.e. the second substance contribution.
  • the X-ray source 2 emits polychromatic X-ray radiation with an emission spectrum T(E).
  • the emission spectrum T(E) is exemplary shown in Fig. 2.
  • the detector unit 6 comprises several detector channels.
  • the detection signal of the z-th detector channel is indicated by d l and can be described by following equation:
  • D 1 (E) is the spectral sensitivity of the z-th detector channel.
  • pphoto, pcompton , pk-edgei and pk-ed g e2 are the density length products of the photo-electric effect, the Compton effect, the k-edge effect of the first substance and the k-edge effect of the second substance, respectively.
  • the energy dependent absorption spectra of the photo-electric effect, the Compton effect and the k-edge effect of the first substance and the k-edge effect of the second substance are indicated by P(E), C(E), K 1 (E) and K 2 (E), respectively. These energy dependent absorption spectra are exemplary shown in arbitrary units in Fig. 3.
  • the input to the image generation device 10 are the energy-resolved detection signals d l for a plurality, at minimum 4, energy bins.
  • the emission spectrum T(E) from the X-ray source 2 is know, for example from earlier measurements of the emission spectrum. Also the spectral sensitivity D 1 (E) is known, for example from earlier sensitivity measurements.
  • the absorption spectra P(E), C(E), Kj(E) and K 2 (E) are known in the state of the art. In particular, P(E) can be E ⁇ 3/2 and C(E) can be
  • the resulting density length products pk-edgei and pk-ed g e2 are the first substance contribution and the second substance contribution, respectively.
  • the first substance contribution is used to reconstruct a first image showing the first substance and the second substance contribution is used to reconstruct a second image showing the second substance.
  • the photo-electric density length product p p h o to can be used to reconstruct a photo-electric image
  • the Compton effect density length product pcompton can be used to reconstruct a Compton effect image.
  • the Compton effect image and the photo-electric effect image show the object itself. These four images can be shown one by one, or they can be mixed, for example, a final image can show the first substance, the second substance and the Compton effect image and/or the photo-electric effect image. It is also possible to reconstruct one image showing one, some or all of the four components.
  • the reconstruction unit 13 can use conventional reconstruction algorithms, for example, a filtered backprojection algorithm or an iterative reconstruction algorithm.
  • the present invention allows a direct measurement of a contrast agent injected into a patient. Many different applications in clinical practice are thus possible without the need for high technical efforts, such as a monochromatic X-ray source.
  • the above mentioned contrast agent EP2104R could have been injected into a patient, and after a proper wash-out time, for example two to three hours, a second conventional iodine based contrast agent could have been injected into the patient.
  • the imaging system in accordance with the invention allows to distinguish the two contrast agents and the remaining absorption, it is possible to get at least three important measures, which are, for example, a fibrin image, the vessel structure and the absorption image of the patient, with a single computed tomography scan.
  • the image showing the contrast agent EP2104R shows, for example, fibrin, which could be an indication of coronary thrombosis and soft plaque.
  • the image showing the conventional iodine based contrast agent can be used to show a coronary angiography. Since the remaining absorption image does not include the contrast agents, it can be used for example for calcium scoring. Thus, it is possible to perform these three measurements with a single CT scan. It is not necessary to have a first CT scan for imaging the first contrast agent, a second CT scan for imaging the second contrast agent, and third CT scan for imaging the remaining absorption image, i.e. it is not necessary to perform three CT scans. It reduces the dose applied to a patient strongly.
  • preferred substances which are imaged by the imaging system according to the invention, comprise a k-edge in the energy range of the emission spectrum T(E) of the radiation source 2.
  • any imaging system can be used, for example a magnetic resonance imaging system or an ultra sonic imaging system, which allows to detect detection signals depending at least on a first substance and a second substance within an object, wherein both substances are present within the object at the same time, which comprise a calculation unit for determining at least the first substance contribution and the second substance contribution to the detection signals, and which reconstructs an image at least of one of the first substance and the second substance using the determined first substance contribution and the determined second substance contribution.
  • a magnetic resonance imaging system or an ultra sonic imaging system which allows to detect detection signals depending at least on a first substance and a second substance within an object, wherein both substances are present within the object at the same time, which comprise a calculation unit for determining at least the first substance contribution and the second substance contribution to the detection signals, and which reconstructs an image at least of one of the first substance and the second substance using the determined first substance contribution and the determined second substance contribution.
  • a single scan can provide several object properties, and it is not necessary to perform several scans to obtain these properties, i.e., in accordance with the invention, the same information about an object can be obtained with a reduced number of scans.
  • the information contained in a reconstructed image of an object is not limited to the properties of one substance, because, in accordance with the invention, several substances, which are present within the object and which can comprise different properties, can be detected, the contributions of the different substances to the detection signals can be determined by the calculation unit and images, which distinguish between the several substances, are reconstructed. This increases the variability of imaging systems, which image substances present within an object.
  • the invention is not limited to the imaging of substances within a patient. Also substances within other objects, for example within devices or components of devices or within materials can be imaged for example for testing purposes.
  • the above described imaging system is a CT imaging system which comprises a cone beam.
  • the invention is not limited to a CT system, and in particular not to a CT system comprising a cone beam.
  • a fan beam or other beam shapes can be used instead of the cone beam.
  • the above mentioned imaging system is adapted to generate images of two substance which are present within an object at the same time.
  • the invention is not limited to an imaging system which is adapted to generate images of only two substances present within the object at the same time.
  • the imaging system according to the invention can also be adapted to generate images of more than two substances, which are present within the object at the same time.
  • equation (1) comprises additional terms pk-ed g e3 K ⁇ (E) ...pk-ed g e(2 + ⁇ O KN + 2 (E), if TV additional substances are used.
  • the above described CT imaging system generates images of a contrast agent.
  • the invention is not limited to the imaging of a contrast agent.
  • the imaging system in accordance with the invention can also be adapted to generate images of a substance within the object which is not a contrast agent, but which produces a detection signal in the detection unit.
  • 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.
  • 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.

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Abstract

The present invention relates to an imaging system for imaging a substance present in an object of interest. In the object of interest are at least a first substance and second substance at the same time present. The imaging system comprises a detection unit (6) for detecting detection signals depending at least on the first substance and on the second substance, and a 5 calculation unit (12) for determining at least a first substance contribution and a second substance contribution to the detection signals. The imaging system comprises further a reconstruction unit (13) for reconstructing an image at least of one of the first substance and of the second substance using the determined first substance contribution and the determined second substance contribution.

Description

Imaging system for imaging substances present in an object of interest
FIELD OF THE INVENTION
The present invention relates to an imaging system, an imaging method and a computer program for imaging of a substance present in an object of interest. The invention relates further to an image generation device, an image generation method and a computer program for generating an image of a substance present in an object of interest.
BACKGROUND OF THE INVENTION
It is known to introduce a contrast agent into an object of interest and to image the contrast agent within the object by an imaging system, for example by a computed tomography system (CT system) or by a magnetic resonance system (MR system). The contrast agent is chosen such that it can be imaged by the respective imaging system. For example, an iodine based contrast agent is used together with a CT system. The imaging of the contrast agent within the object allows to image vessels, which might be present within the object. Some known imaging system also provide the possibility to determine the flow of the contrast agent within the object.
These known imaging systems detect and reconstruct only one contrast agent present in the object of interest. The imaging is therefore limited by the properties of this single contrast agent. For example, if the contrast agent is only temporarily present within the object, the time interval, during which the contrast agent can be visualized, is limited by the residence time within the object. Furthermore, if the substance, for example, a contrast agent, is only present within and/or attaches only to certain parts of the object, only these certain parts and/or the attachment to these certain parts of the object can be visualized. Therefore, the use of known imaging systems, which can image a single substance within an object, is limited by the properties of the substance, i.e. by the ability of the imaging system to image a single substance only. SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an imaging system which allows more variability, when imaging a substance within an object.
In a first aspect of the present invention an imaging system for imaging a substance present in an object of interest, is provided, wherein at least a first substance and a second substance are present in the object of interest at the same time, comprising:
- a detection unit for detecting detection signals depending at least on the first substance and on the second substance,
- a calculation unit for determining at least a first substance contribution of the first substance and a second substance contribution of the second substance to the detection signals,
- a reconstruction unit for reconstructing at least an image of one of the first substance and of the second substance using the determined first substance contribution and the determined second substance contribution.
The basic idea of the invention is that the imaging system allows to detect detection signals depending at least on the first substance and on the second substance which are at the same time present within the object of interest, to determine at least the contribution of the first substance and the contribution of the second substance to the detection signals, and to reconstruct at least an image of the first substance and an image of the second substance using the determined contributions. Thus, the imaging system according to the invention is able to reconstruct images of at least two substances using detection signals, which have been acquired, while at least the both substances are simultaneously present within the object of interest. Since the contributions of the first substance and the second substance have been determined by the calculation unit, the reconstruction unit can reconstruct images which only show the first substance or the second substance or which shows both. The ability to image at least two substances simultaneously, which might have different properties, increases the variability of the imaging system according to the invention in comparison to known imaging systems.
As mentioned above, the imaging system according to the invention comprises a detection unit for detecting the detection signals depending at least on the first substance and on the second substance. Such a detection unit can be a detection unit which is adapted to detect detection signals which simultaneously depend at least on the first substance and on the second substance and/or to detect, in particular during a single scan operation, detection signals which only depend on the first substance and detection signals which only depend on the second substance.
As also already mentioned above, the reconstruction unit is adapted to reconstruct an image at least of one of the first substance and the second substance using the determined first substance contribution and the determined second substance contribution. Such a reconstruction unit can be a reconstruction unit, which is adapted such that it reconstructs an image comprising at least the first substance and the second substance, wherein the image might comprise means for distinguishing the first substance from the second substance within the image, wherein, for example, the first substance is presented having another color than the second substance within the image, or such that at least two images a reconstructed, wherein a first image shows the first substance and a second image shows the second substance.
In an embodiment, the detection unit is adapted to detect detection signals depending on the object itself, wherein the calculation unit is adapted to determine an object contribution of the object itself to the detection signals, wherein the reconstruction unit is adapted to reconstruct the object itself using the determined object contribution.
Since the calculation unit can be adapted to determine the object contribution to the detection signals and since the reconstruction unit can be adapted to reconstruct the object itself using this object contribution, the imaging system, in this embodiment, is not only able to image the substances, but also the object itself. This allows for example to show the first substance and/or the second substance relative to the object in one or several reconstructed images.
In an embodiment, the imaging system is a CT imaging system comprising a polychromatic X-ray source for emitting polychromatic X-ray radiation, wherein the detection unit is an energy-resolving X-ray detector for detecting the X-ray radiation after passing through the object and for providing a plurality of energy-resolved detection signals, wherein the calculation unit is adapted to determine the k-edge components at least of the first substance and of the second substance from the detection signals being the first substance contribution and the second substance contribution to the detection signals, wherein the reconstruction unit is adapted to use the k-edge components at least of the first substance and the second substance for reconstructing.
The k-edge components are the contributions of the respective substance to the detection signals. The contributions of the k-edge components to the detections signals can be determined with high quality by using the CT imaging system of this embodiment. Thus, the images, which have been reconstructed using this contributions of the k-edge components to the detection signals, have an improved quality.
The reconstruction unit can be adapted to use the k-edge component of the first substance to reconstruct an image of the first substance and to use the k-edge component of the second substance to reconstruct an image of the second substance.
The calculation unit can be adapted to use a model which takes account of the emission spectrum of the X-ray source and the spectral sensitivity of the X-ray detector. Furthermore, the calculation unit can be adapted to separate contributions of the Compton effect and of the photo-electric effect from the detection signals. These adaptations of the calculation unit further improve the quality of the reconstructed image.
It is preferred that the calculation unit is adapted to determine the k-edge components at least of the first substance and the second substance by solving a system of equations for the plurality of energy-resolved detection signals, using a model for the detection signals describing a detection signal as a combination of the k-edge effects, the Compton effect and the photo-electric effect, each effect contributing with a corresponding component to the detection signal, wherein the reconstruction unit is adapted to reconstruct at least a k-edge image of the first substance and a k-edge image of the second substance from the determined k-edge components at least of the first substance and the second substance. The calculation unit can be adapted to determine the photo-electric effect component and/or the Compton effect component by solving the system of equations for the plurality of energy resolved detection signals, and the reconstruction unit can be adapted to reconstruct a photo-electric effect image and/or a Compton effect image from the determined photo-electric effect components and/or the Compton effect components. The photo-electric effect image and the Compton effect image can show additional information with respect to the object and/or the substances within the object, which could not be derived from images, which have been reconstructed using the k-components of the detection signals and/or using the object contribution to the detection signals.
In a further aspect of the present invention, an imaging method for imaging a substance present in an object of interest is provided, wherein at least a first substance and a second substance are present in the object of interest at the same time, wherein the imaging method comprises the steps of: - detecting detection signals depending at least on the first substance and on the second substance,
- determining at least a first substance contribution of the first substance and a second substance contribution of the second substance to the detection signals,
- reconstructing an image at least of one of the first substance and the second substance using the determined first substance contribution and the determined second substance contribution
In a further aspect of the present invention a computer program for imaging a substance present in an object of interest is provided, wherein at least a first substance and a second substance are present in the object of interest at the same time and wherein the computer program comprises program code means for causing a computer to carry out the steps of the imaging method according to the invention when the computer program is carried out on a computer controlling an imaging system according to the invention.
In a further aspect of the present invention an image generation device for generating an image of a substance present in an object of interest is provided, wherein at least a first substance and a second substance are present in the object of interest at the same time, the image generation device being provided with detection signals depending at least on the first substance and the second substance, comprising:
- a calculation unit for determining at least a first substance contribution of the first substance and a second substance contribution of the second substance to the detection signals,
- a reconstruction unit for reconstructing an image at least of one of the first substance and of the second substance using the determined first substance contribution and the determined second substance contribution. In a further aspect of the present invention an image generation method for generating an image of an substance present in an object of interest is provided, wherein at least a first substance and a second substance are present in the object of interest at the same time, the image generation method being provided with detection signals depending at least on the first substance and the second substance, the image generation method comprises the steps of:
- determining at least a first substance contribution of the first substance and a second substance contribution of the second substance to the detection signals,
- reconstructing an image at least of one of the first substance and of the second substance using the determined first substance contribution and the determined second substance contribution.
In a further aspect of the present invention a computer program for generating an image of an substance present in an object of interest is provided, wherein at least a first substance and a second substance are present in the object of interest at the same time and wherein the computer program comprises program code means for causing a computer of an image generation device according to the invention to carry out the steps of an image generation method according to the invention when said computer program is carried out on a computer.
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 with respect to the drawings in which:
Fig. 1 shows a diagrammatic representation of an imaging system in accordance with the invention,
Fig. 2 shows an exemplary emission spectrum of a polychromatic X-ray source,
Fig. 3 shows exemplary spectra of the photo-electric effect, the Compton effect, and of substances within an object of interest.
The imaging system shown in Fig. 1 is a computed tomography system (CT system). The CT system includes a gantry which is capable of rotation about an axis of ration R which extends parallel to the z direction. The radiation source 2, for example an X-ray tube emitting polychromatic X-ray radiation, is mounted on the gantry 1. The X-ray source is provided with a collimator device 3 which forms in this embodiment a conical radiation beam 4 from the radiation produced by the X-ray source 2. The radiation traverses an object (not shown), such as a patient, in a region of interest in an examination zone 5, which is in this embodiment cylindrical. After having traversed the examination zone 5, the X-ray beam 4 is incident on an energy-resolving X-ray detector unit 6, in this embodiment a two-dimensional detector, which is mounted on the gantry 1.
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 a 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 another along a helical trajectory. However, it is also possible that the object or the examination zone 5 is not moved, but that only the X-ray source 2 is rotated.
The data acquired by the detector unit 6 are provided to an image generation device 10 for generating an image at least of one of a first substance and a second substance within the object (for example within the patient).
The reconstructed image can finally be provided to a display 11 for displaying the image. Also the image generation device is preferably controlled by the control unit 9.
These substances are preferentially contrast agents, for example based on gadolinium and/or iodine. Furthermore, the substances comprise preferentially different properties such that an image of the first substance comprises information which is different from the information contained in an image of the second substance. For example, the first substance can be a substance which is present within the object without attaching to parts of the object, whereas the second substance can be a substance which attaches to some parts of the object being normally not visible in the imaging system, in particular in the CT system. If such two substances are present within the object, the image of the first substance shows for example the vessels within the object in which the first substance is present. Furthermore, the image of the second substance shows the parts of the object, to which the second substance attaches. As already mentioned above, the first substance is preferentially a contrast agent based on iodine, and the second substance is preferentially a contrast agent based on gadolinium. The gadolinium based contrast agent can for example be the contrast agent named "EP2104R" which has been developed by the firm EPIX Pharmaceuticals, Inc.. Iodine has a good contrast for X-rays and can for example be used in a human heart. The contrast agent EP2104R binds fibrin, whereas the contrast agent based on iodine does not attached to a part of a human body, in particular does not attached to a part of the human heart. Therefore, if the object is a human heart and an iodine based contrast agent and the contrast agent EP2104R are present within the human heart, the vessels within the human heart, in which the iodine based contrast agent is present, can be determined from an image data set showing the first substance, i.e. the iodine based contrast agent, and fibrin can be visualized by reconstructing an image of the second contrast agent, which has, in this example, attached to fibrin within the human heart.
Since the imaging system according to the invention is adapted to generate images of a first substance and a second substance, wherein both substances are present within the object at the same time and wherein the first substance can be distinguished from the second substance within the generated images, the vessels (coronary angiography) and fibrin can be determined within one imaging operation step, in particular, if the imaging system is a CT system, within one CT scan.
If an image, which shows a substance within an object having the property to attach to parts of the object, shall only show the part of the substance, which has been attached to parts of the object, the imaging method in accordance with the invention, which will be described in more detail further below, will be started, after the part of the substance, which has not been attached to a part of the object, has washed out of the object. For example, if EP2104R is used as such a substance, the imaging method is performed preferentially two to three hours after administrating EP2104R to the object, which is in this example a human heart.
An embodiment of an imaging method in accordance with the invention will now be explained in more detail.
The gantry 1 is started and rotates around the examination zone 5, in which an object, for example a patient, is positioned, in which at least two substances, a first substance and a second substance, are present. Furthermore, the object can be displaced parallel to the direction of the axis of rotation R. If the object is displaced, for example by moving a patient table parallel to the direction of the axis of rotation R, the X-ray radiation source 2 travels along a helical trajectory relative to the object. If the object is not displaced, the X-ray source 2 travels along a circular trajectory relative to the object. The radiation emitted from the X- ray source 2 traverses the examination zone 5 and the object, and the light, which has traversed the examination zone 5 and the object, is acquired by the detection unit 6. The detection unit 6 converts the acquired X-ray radiation into electrical signals, i.e. detection signals, which are transmitted to the image generation device 10.
The image generation device 10 comprises preferentially a calculation unit 12 and a reconstruction unit 13. In step 102 the calculation unit 12 determines the contribution of the first substance to the detection signals, i.e. the first substance contribution, and the contribution of the second substance to the detection signals, i.e. the second substance contribution.
The X-ray source 2 emits polychromatic X-ray radiation with an emission spectrum T(E). The emission spectrum T(E) is exemplary shown in Fig. 2. The detector unit 6 comprises several detector channels. The detection signal of the z-th detector channel is indicated by dl and can be described by following equation:
d, =
Figure imgf000011_0001
+ P^2K2 (E))) (1)
In this equation, D1(E) is the spectral sensitivity of the z-th detector channel. Furthermore, pphoto, pcompton , pk-edgei and pk-edge2 are the density length products of the photo-electric effect, the Compton effect, the k-edge effect of the first substance and the k-edge effect of the second substance, respectively. The energy dependent absorption spectra of the photo-electric effect, the Compton effect and the k-edge effect of the first substance and the k-edge effect of the second substance are indicated by P(E), C(E), K1(E) and K2(E), respectively. These energy dependent absorption spectra are exemplary shown in arbitrary units in Fig. 3.
The input to the image generation device 10 are the energy-resolved detection signals dl for a plurality, at minimum 4, energy bins. The emission spectrum T(E) from the X-ray source 2 is know, for example from earlier measurements of the emission spectrum. Also the spectral sensitivity D1(E) is known, for example from earlier sensitivity measurements. The absorption spectra P(E), C(E), Kj(E) and K2(E) are known in the state of the art. In particular, P(E) can be E ~3/2 and C(E) can be
r . . 1 + α [ 2 ( 1 + α ) 1 . „ . 1 + 3α
+ -Mn( I + 2α ) -
2α (1 + 2 α )2 with α = E / m e. (2)
The last equation is the well known integrated Klein-Nishina function.
Since the energy dependent functions in equation (1) and the detection signals dl are known and since at least four detection signals
Figure imgf000012_0001
are available for at least four energy bins
Figure imgf000012_0002
a system of at least four equations is formed having four unknowns which can thus be solved with known mathematical methods in the calculation unit 12.
If more than four energy bins are available, it is preferred to use a maximum likelihood approach that takes the noise statistics of the measurements into account. The resulting density length products pk-edgei and pk-edge2 are the first substance contribution and the second substance contribution, respectively.
The first substance contribution is used to reconstruct a first image showing the first substance and the second substance contribution is used to reconstruct a second image showing the second substance. Furthermore, the photo-electric density length product pphoto can be used to reconstruct a photo-electric image, and the Compton effect density length product pcompton can be used to reconstruct a Compton effect image. The Compton effect image and the photo-electric effect image show the object itself. These four images can be shown one by one, or they can be mixed, for example, a final image can show the first substance, the second substance and the Compton effect image and/or the photo-electric effect image. It is also possible to reconstruct one image showing one, some or all of the four components. The reconstruction unit 13 can use conventional reconstruction algorithms, for example, a filtered backprojection algorithm or an iterative reconstruction algorithm.
The present invention allows a direct measurement of a contrast agent injected into a patient. Many different applications in clinical practice are thus possible without the need for high technical efforts, such as a monochromatic X-ray source.
For example, the above mentioned contrast agent EP2104R could have been injected into a patient, and after a proper wash-out time, for example two to three hours, a second conventional iodine based contrast agent could have been injected into the patient. Since the imaging system in accordance with the invention allows to distinguish the two contrast agents and the remaining absorption, it is possible to get at least three important measures, which are, for example, a fibrin image, the vessel structure and the absorption image of the patient, with a single computed tomography scan. Furthermore, the image showing the contrast agent EP2104R shows, for example, fibrin, which could be an indication of coronary thrombosis and soft plaque. The image showing the conventional iodine based contrast agent can be used to show a coronary angiography. Since the remaining absorption image does not include the contrast agents, it can be used for example for calcium scoring. Thus, it is possible to perform these three measurements with a single CT scan. It is not necessary to have a first CT scan for imaging the first contrast agent, a second CT scan for imaging the second contrast agent, and third CT scan for imaging the remaining absorption image, i.e. it is not necessary to perform three CT scans. It reduces the dose applied to a patient strongly.
In this embodiment, preferred substances, which are imaged by the imaging system according to the invention, comprise a k-edge in the energy range of the emission spectrum T(E) of the radiation source 2.
Although the invention has been described in more detail with respect to a CT system, the imaging system in accordance with the invention is not limited to such a CT system. In accordance with the invention any imaging system can be used, for example a magnetic resonance imaging system or an ultra sonic imaging system, which allows to detect detection signals depending at least on a first substance and a second substance within an object, wherein both substances are present within the object at the same time, which comprise a calculation unit for determining at least the first substance contribution and the second substance contribution to the detection signals, and which reconstructs an image at least of one of the first substance and the second substance using the determined first substance contribution and the determined second substance contribution. Such an imaging system in accordance with the invention allows to determine different properties of the object related to the different substances imaged in the reconstructed images. Thus a single scan can provide several object properties, and it is not necessary to perform several scans to obtain these properties, i.e., in accordance with the invention, the same information about an object can be obtained with a reduced number of scans. Furthermore, the information contained in a reconstructed image of an object is not limited to the properties of one substance, because, in accordance with the invention, several substances, which are present within the object and which can comprise different properties, can be detected, the contributions of the different substances to the detection signals can be determined by the calculation unit and images, which distinguish between the several substances, are reconstructed. This increases the variability of imaging systems, which image substances present within an object.
The invention is not limited to the imaging of substances within a patient. Also substances within other objects, for example within devices or components of devices or within materials can be imaged for example for testing purposes.
Furthermore, the above described imaging system is a CT imaging system which comprises a cone beam. The invention is not limited to a CT system, and in particular not to a CT system comprising a cone beam. For example, also a fan beam or other beam shapes can be used instead of the cone beam.
The above mentioned imaging system is adapted to generate images of two substance which are present within an object at the same time. The invention is not limited to an imaging system which is adapted to generate images of only two substances present within the object at the same time. The imaging system according to the invention can also be adapted to generate images of more than two substances, which are present within the object at the same time. In this case, equation (1) comprises additional terms pk-edge3 K^(E) ...pk-edge(2 + ΛO KN + 2(E), if TV additional substances are used.
The above described CT imaging system generates images of a contrast agent. The invention is not limited to the imaging of a contrast agent. The imaging system in accordance with the invention can also be adapted to generate images of a substance within the object which is not a contrast agent, but which produces a detection signal in the detection unit.
While the invention has been illustrated and described in detail in the drawings and 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.
Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.
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.

Claims

CLAIMS:
1. Imaging system for imaging a substance present in an object of interest, wherein at least a first substance and a second substance are present in the object of interest at the same time, comprising:
- a detection unit (6) for detecting detection signals depending at least on the first substance and on the second substance,
- a calculation unit (12) for determining at least a first substance contribution of the first substance and a second substance contribution of the second substance to the detection signals,
- a reconstruction unit (13) for reconstructing an image at least of one of the first substance and of the second substance using the determined first substance contribution and the determined second substance contribution.
2. Imaging system as claimed in claim 1, wherein the detection unit (6) is adapted to detect detection signals depending on the object itself, wherein the calculation unit (12) is adapted to determine an object contribution of the object itself to the detection signals, wherein the reconstruction unit (13) is adapted to reconstruct the object itself using the determined object contribution.
3. Imaging system as claimed in claim 1, wherein the imaging system is a CT imaging system comprising a polychromatic X-ray source (2) for emitting polychromatic X-ray radiation, wherein the detection unit (6) is an energy-resolving X-ray detector (6) for detecting the X- ray radiation after passing through the object and for providing a plurality of energy-resolved detection signals, wherein the calculation unit (12) is adapted to determine the k-edge components at least of the first substance and of the second substance from the detection signals being the first substance contribution and the second substance contribution to the detection signals, wherein the reconstruction unit (13) is adapted to use the k-edge components at least of the first substance and the second substance for reconstructing.
4. Imaging system as claimed in claim 3, wherein the calculation unit (12) is adapted to use a model which takes account of the emission spectrum (T(E)) of the X-ray source (2) and the spectral sensitivity of the X-ray detector (6).
5. Imaging system as claimed in claim 3, wherein the calculation unit (12) is adapted to separate contributions of the Compton effect and the photo-electric effect from the detection signals.
6. Imaging system as claimed in claim 5, wherein the calculation unit (12) is adapted to determine the k-edge components at least of the first substance and the second substance by solving a system of equations for the plurality of energy-resolved detection signals, using a model for the detection signals describing a detection signal as a combination of the k-edge effects, the Compton effect and the photoelectric effect, each effect contributing with a corresponding component to the detection signal, wherein the reconstruction unit (13) is adapted to reconstruct at least a k-edge image of the first substance and a k-edge image of the second substance from the determined k-edge components at least of the first substance and the second substance.
7. Imaging system as claimed in claim 6, wherein the calculation unit (12) is adapted to determine the photo-electric effect component and/or the Compton effect component by solving the system of equations for the plurality of energy resolved detection signals, and wherein said reconstruction unit (13) is adapted to reconstruct a photo-electric effect image and/or a Compton effect image from the determined photo-electric effect components and/or the Compton effect components.
8. Imaging method for imaging a substance present in an object of interest, wherein at least a first substance and a second substance are present in the object of interest at the same time, the imaging method comprising the steps of: - detecting detection signals depending at least on the first substance and on the second substance,
- determining at least a first substance contribution of the first substance and a second substance contribution of the second substance to the detection signals, - reconstructing an image at least of one of the first substance and the second substance using the determined first substance contribution and the determined second substance contribution.
9. Computer program for imaging a substance present in an object of interest, wherein at least a first substance and a second substance are present in the object of interest at the same time, the computer program comprising program code means for causing a computer to carry out the steps of the method as claimed in claim 8 when the computer program is carried out on a computer controlling an imaging system as claimed in claim 1.
10. Image generation device for generating an image of a substance present in an object of interest, wherein at least a first substance and a second substance are present in the object of interest at the same time the image generation device (10) being provided with detection signals depending at least on the first substance and the second substance, comprising: - a calculation unit (12) for determining at least a first substance contribution of the first substance and a second substance contribution of the second substance to the detection signals,
- a reconstruction unit (13) for reconstructing an image at least of one of the first substance and of the second substance using the determined first substance contribution and the determined second substance contribution.
11. Image generation method for generating an image of a substance present in an object of interest, wherein at least a first substance and a second substance are present in the object of interest at the same time , the image generation method being provided with detection signals depending at least on the first substance and the second substance, the image generation method comprises the steps of:
- determining at least a first substance contribution of the first substance and a second substance contribution of the second substance to the detection signals, - reconstructing an image at least of one of the first substance and of the second substance using the determined first substance contribution and the determined second substance contribution.
12. Computer program for generating an image of a substance present in an object of interest, wherein at least a first substance and a second substance are present in the object of interest at the same time, the computer program comprising program code means for causing a computer of the image generation device (10) as claimed in claim 10 to carry out the steps of the method as claimed in claim 11 when said computer program is carried out on a computer.
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