Patent Application
20160270198
The present invention relates to an X-ray imaging device and to a method of operating such X-ray imaging device.
X-ray imaging is generally used in order to obtain information about internal structures of an object of interest. An X-ray imaging device comprises an X-ray source and an X-ray detector arranged at opposite sides of an observation volume in which the object of interest may be arranged during X-ray imaging. X-rays coming from the X-ray source are transmitted through the object. Due to different X-ray absorption properties of various internal structures of the object, information about the internal structure may be obtained based on a local distribution of X-ray intensities detected by the X-ray detector.
In medical X-ray imaging, the object of interest is typically a region of interest (ROI) of a human or animal body. As X-rays may damage living tissue of such body, it is of general interest to minimize an X-ray dose applied to the body during medical imaging.
In most conventional X-ray imaging devices, X-rays are transmitted through the region of interest homogeneously, i.e. an X-ray intensity applied to the ROI is substantially constant throughout the entire cross-section of the ROI. However, the ROI typically comprises various internal body structures having significantly differing X-ray absorption properties. For example, bones within the ROI show relatively strong X-ray absorbance whereas other tissue types such as muscle tissue or lung tissue show significantly lower X-ray absorbance. Accordingly, with homogeneous X-ray intensity being transmitted throughout the entire ROI, problems may arise in that portions of the ROI with structures having low X-ray absorbance are overexposed whereas portions with strong X-ray absorbance are underexposed.
For example, portions comprising mainly soft tissue like lung tissue hardly absorb X-rays such that a high intensity of X-rays is transmitted through these portions and detector pixels of an X-ray detector capturing the transmitted X-rays may be overexposed, i.e. the detector portions come into over-load conditions. In other words, in these soft tissue portions, the ROI is subject to an excessive patient X-ray dose. On the other hand, there are typically portions within the ROI comprising bones or other strongly X-ray absorbing structures. X-rays transmitted through these portions of the ROI are highly attenuated and detector pixels of an X-ray detector capturing X-rays transmitted through these highly X-ray absorbing portions may be underexposed, i.e. pixels of the X-ray detector in such portions receive only a very low X-ray intensity such that in such “starving” detector cells noise may have a significant impact onto a detector signal.
It may therefore be desired to adapt local intensities of X-rays transmitted through a region of interest to the properties of the internal structures comprised throughout this region of interest. In other words, it would be helpful to have localized and dynamic control of a primary photon flux of X-rays wherein this photon flux may be adapted for example upon a detector signal.
Dynamic attenuators have been proposed, but failed to be practical for fast modulation. For example, WO 0225671 describes an X-ray examination device including, inter alia, an X-ray filter having filter elements with an X-ray absorptivity which can be adjusted by controlling a quantity of X-ray absorbing liquid within individual filter elements. However, re-arranging liquid within filter elements is generally time-consuming and only suitable for low frequency image acquisition.
There may be a need for an imaging device enabling localized and dynamic control of a primary X-ray photon flux and a method for operating such X-ray imaging device. Particularly, there may be a need for an approach with which the primary X-ray photon flux may be influenced rapidly such that e.g. overexposure or underexposure of portions of an X-ray detector may be prevented during X-ray imaging.
Such needs may be met by the subject-matter of the independent claims. Further embodiments of the invention are defined in the dependent claims.
According to a first aspect of the present invention, an X-ray imaging device for imaging a region of interest of an object arranged within an observation volume is proposed. The X-ray imaging device comprises an X-ray source, a controllable transmission arrangement, an X-ray detector and a control device. The X-ray source is adapted for generating at least partially coherent X-radiation. The controllable transmission arrangement comprises an X-ray phase modifying unit, which comprises at least one sub-unit with periodic sub-structures and an X-ray absorbing unit, which comprises at least one sub-unit with periodic sub-structures both of which units being arranged within a beam path of X-rays emitted by the X-ray source one behind the other and spaced from each other. Therein, the X-ray phase modifying unit and the X-ray absorbing unit are arranged within the X-ray beam path in a region upstream of the observation volume. The X-ray phase modifying unit and/or the X-ray absorbing unit comprises at least one sub-unit with periodical sub-structure or preferably a multiplicity of sub-units. Therein, at least one sub-unit or preferably each of the sub-units comprises a steering means for displacing the sub-unit with respect to the other one of the X-ray phase modifying unit and the X-ray absorbing unit. The control device is adapted to controlling the steering means.
According to a second aspect of the invention, a method of operating an X-ray imaging device according to the above-mentioned first aspect is proposed. The method comprises, inter alia, controlling each of the steering means such as to displace the sub-unit(s) of one of the periodical X-ray phase modifying unit and/or the X-ray absorbing unit with respect to the other one of the X-ray phase modifying unit and the X-ray absorbing unit.
For example, according to an embodiment, the control device is adapted to controlling the steering means based on information received from the X-ray detector. Accordingly, based on such X-ray detector information, it may be determined whether specific regions or portions of the X-ray detector tend to be overexposed or underexposed and the controllable transmission arrangement may be controlled accordingly in order to adapt local X-ray transmittance.
For example, according to an embodiment, the control device may be adapted to controlling each of the steering means based on information about accumulated X-ray intensity received by the X-ray detector in a pixel area associated to a respective sub-grating.
According to an embodiment, each sub-unit extends within a plane and each of the steering means is adapted for displacing an associated sub-unit in a direction within this plane. In other words, for example in an embodiment where the sub-units are provided with sub-gratings, the steering means should be adapted for displacing e.g. a planar sub-grating in a lateral direction. By moving the associated grating laterally, the units comprised in the controllable transmission arrangement may be arranged such that the interference patterns generated by a grating forming the periodical X-ray phase modifying unit are positioned in a desired manner with respect to the X-ray absorbing unit created by the third grating, thereby enabling controlling of the transmitted X-ray intensity.
According to an embodiment, the steering means may be adapted for displacing the sub-units using forces induced by electric fields. Based on such electric fields, forces may be generated very rapidly such that a steering means may displace an associated sub-unit very rapidly. For example, displacement rates of up to 5 kHz may be achieved. For example, the steering means may be driven using piezo-elements. Generally, steering means may operate similarly as DLP (Digital Light Processing) devices, using electric forces acting on micromechanical movable elements.
It shall be noted that various possible features and advantages of embodiments of the invention are described herein partly with respect to an X-ray imaging device and partly with respect to a method of operating such device. One skilled in the art will realize that the various features may be combined, exchanged or replaced throughout the described embodiments in suitable ways and particularly features described with respect to the device may be applied to the method, and vice versa, in analog manners.
In the following, embodiments of the present invention will be described with respect to the enclosed drawings. However, neither the description nor the drawings shall be interpreted as limiting the scope of the invention.
The figures are only schematical and not to scale. Generally, same reference signs are used for same or similar features throughout the figures.
Without limiting the scope of the invention in any way, concepts underlying embodiments of the present invention may be understood as being based on the following ideas and recognitions:
In the proposed X-ray imaging device, a controllable transmission arrangement comprises at least two units such as a X-ray phase modifying unit, consisting of at least one sub-unit with periodical structures which generate an interference pattern for X-radiation and an X-ray absorbing unit consisting of at least one sub-unit with periodical structures, which are matched with the interference pattern and attenuate the X-radiation. In embodiments, each of these two units may comprise a grating wherein the gratings are arranged serially within an X-ray beam path and may be used to controllably attenuate a local X-ray photon flux emitted by the X-ray source before reaching the observation volume in which for example a body part of a patient is arranged. Therein, the gratings may be arranged similarly, but may be operated differently, as in a Talbot-Laue interferometer. An optional first grating being for example part of the X-ray source and being e.g. arranged closest to an X-ray tube may be adapted and serve as a source grating. Such source grating may comprise an, e.g. periodic, pattern of highly X-ray absorbing regions and less X-ray absorbing regions. X-rays coming from the X-ray tube and being transmitted through such source grating will be influenced such that the source grating creates an array of individual coherent X-ray sources. Alternatively, a coherent X-ray source such as a synchrotron or small dimensioned coherent X-ray emitting spots may be used. A second grating which is arranged downstream of the first grating is adapted and may serve as a phase grating. Such phase grating generally shows minor X-ray absorbance but comprises a pattern of regions differently influencing a phase of transmitted X-rays. The key characteristic of this device is the, at least local, periodic alteration of the phase of the X-ray waves. Usually it consists of an etched sheet of silicon. But, it may also have a fluid nature or a combination of fluid and gaseous nature. Accordingly, when being transmitted through the phase grating, coherent X-rays form an interference pattern downstream of the phase grating. A third grating which is arranged downstream of the second grating is adapted as and operates as an absorber grating. Such absorber grating comprises a pattern of different regions having different X-ray absorbance similarly as in the source grating.
A general idea underlying such embodiment is to enable relative displacement of the second grating with respect to the third grating using steering means such that the second and third gratings may be displaceably arranged with respect to each other either in a relative position where an interference pattern downstream of the second grating is mainly blocked by the third pattern or, alternatively, in a relative position where such interference X-ray pattern is mainly transmitted through the third grating.
In other words, by enabling relative displacement of the second and third gratings with respect to each other, X-ray transmittance through the entire arrangement of the three gratings may be controlled.
Therein, in order to not only allow influencing an overall X-ray transmittance through the region of interest, at least one of the second and third gratings is divided into a multiplicity of sub-gratings wherein each sub-grating may be independently displaced using an associated steering means. Accordingly, for each area corresponding to a sub-grating, X-ray transmittance may be controlled independently by suitably controlling the associated steering means. If a fluid phase grating is being used, the steering means may be a device that alters the phase shift by creating locally periodic regions of higher and smaller phase shift, including generation of gaseous regions or even vacuum regions.
The X-ray source 6 may comprise a conventional X-ray tube 5. The X-ray source 6 is adapted for emitting an X-ray beam 8 along a beam path 9 in a direction towards the X-ray detector 7. The aperture 11 limits the X-ray beam 8 to a certain angle. The path 9 of the X-ray beam traverses the observation volume 3 before reaching a detection surface of the X-ray detector 7. This detection surface may comprise a multiplicity of detector elements arranged in a two-dimensional matrix, wherein these detector elements are also referred to as pixels 19. It may be mentioned that, in alternative embodiments, different X-ray detectors may be used such as for example line detectors in which multiple detector elements or pixels are arranged along a line, i.e. one-dimensionally, and which are typically operated in a scanning manner.
The first, second and third gratings 13, 15, 17 are arranged serially within the beam path 9, i.e. one behind the other, such that the X-ray beam 8 first transmit through the first grating 13, then through the second grating 15 and finally through the third grating 17. Therein, the gratings 13, 15, 17 are arranged spaced from each other, i.e. they have no direct mechanical contact to each other. For example, the second grating 15 may be spaced from the first grating 13 by a distance d1 of for example between 5 and 50 cm, preferably between 5 and 20 cm. The third grating 17 may be arranged at a distance d2 of for example between 5 and 50 cm, preferably between 5 and 20 cm, with respect to the second grating 15. Generally, the distances d1 and d2 are significantly smaller than the length 1 of the observation volume 3.
An arrangement comprising the first, second and third gratings 13, 15, 17 may be similar to an arrangement of gratings within a Talbot-Laue interferometer. However, as will be described in further detail below, the arrangement 12 of gratings 13, 15, 17 will be used and operate differently than in a Talbot-Laue interferometer. One main difference may be seen in the fact that all of the first, second and third gratings 13, 15, 17 are arranged upstream of the observation volume 3 whereas in a Talbot-Laue interferometer, a first grating is generally arranged upstream of an observation volume whereas two further gratings are arranged downstream of such observation volume. While details of a Talbot-Laue interferometer shall not be discussed within the present description, reference may be made to Pfeiffer et al.: “Phase retrieval and differential phase-contrast imaging with low-brilliance X-ray sources”, Nature Physics, volume 2, April 2006.
Generally, a grating may be understood as a structure with a repetitive pattern of areas with significantly differing properties. For example, such areas may differ with respect to X-ray absorbance thereby enabling influencing a transmission of an X-ray beam through the grating. Alternatively or in addition, the areas may differ with respect to X-ray propagation velocity thereby enabling influencing phase characteristics of an X-ray beam through the grating. Generally the gratings 13, 15, 17 described herein may have structural dimensions in a micrometer range, with structures or a periodicity of the gratings being e.g. less than 50 μm, preferably less than 20 μm or less than 10 μm.
An example of a second grating 15 is shown in
An example of a third grating 17 is shown in
A purpose of such third grating 17 or absorber grating within the arrangement 12 may be seen in enabling selectively and controllingly transmitting X-rays which, after having passed the second grating 15 or phase grating, arrive at the third grating 17 with an interference pattern having a certain periodicity pi. Generally, the periodicity pi of the interference pattern depends on the periodicity p1 of the second grating 15 as well as on the distance d2 between the second grating 15 and the third grating 17. Accordingly, it may be advantageous to provide the third grating 17 with a periodicity p2 substantially corresponding to the periodicity pi of the interference pattern at the location of the third grating 17.
With a geometry having such periodicity p2, the third grating 17 may either be arranged such that regions of maximum X-ray intensity within the interference pattern coincide with the highly X-ray absorbing second blocks 35 and are therefore strongly attenuated, or such that they coincide with the weakly X-ray absorbing first blocks 33 and then they are only weakly attenuated upon transmission through the third grating 17.
Accordingly, whether or not the X-ray beam 8 is strongly attenuated upon transmission through the arrangement 12 of first, second and third gratings 13, 15, 17 mainly depends on the relative positioning of the second and third gratings 15, 17.
It may be noted that each of the gratings 13, 15, 17 and their sub-gratings may be manufactured using various manufacturing techniques. For example, the gratings may be manufactured as arrays of micrometer sized MEMS (micro electro-mechanical systems) structures. Such MEMS structures may be fabricated using for example approved silicon manufacturing technologies such as photolithography and selective etching processes.
Furthermore, it may be noted that, while the first grating 13 is shown with strongly X-ray absorbing blocks 25 only and the third grating 17 is shown with both strongly X-ray absorbing blocks 35 as well as weakly X-ray absorbing blocks 33, each of these first and third gratings 17 may or may not have weakly X-ray absorbing blocks. For example, such weakly X-ray absorbing blocks may be formed in a silicon substrate and spaces between neighboring silicon fingers may then be filled with a highly X-ray absorbing material such as gold, thereby forming the X-ray absorbing blocks 25, 35.
Therein, the steering means 39 may be adapted and arranged such that it displaces the associated sub-grating 37 laterally as shown with the arrows 41, i.e. within a plane of the planar sub-grating 37 and preferably in a direction of the periodicity of the sub-grating 37, i.e. transverse to a direction of the fingers of the blocks 29. Thus, using the means steering means 39, each of the sub-gratings 37 may be controllably displaced within the plane of the second grating 15 and may therefore be displaced relative to the neighboring third grating 17.
An operating principle underlying embodiments of the presented X-ray imaging device 1 as well as an exemplary method for operating such device will be explained in further detail with reference to
A distribution of X-ray intensity I within such interference pattern behind the second grating 15 is schematically shown in
As explained above with reference to
Accordingly, each of a multiplicity of sub-gratings 37, 37′, 37″, 37′″ may be specifically positioned such that an interference pattern generated upon X-ray transmission through the respective sub-grating is created such that its intensity maxima 45 spatially coincide with the highly absorbing blocks 35 of the subsequent third grating 17 (as shown for the uppest sub-grating 37 in
For example, as shown in
Accordingly, in order to minimize an X-ray dose supplied to the patient and, at the same time, avoid overexposure and/or underexposure of regions of the X-ray detector 7, information about accumulated X-ray intensity received by the X-ray detector in an area comprising one or more pixels 19 associated to one of the respective sub-gratings 37, 37′, 37″, 37′″ of the second grating 15 may be used in order to specifically position this sub-grating 37, 37′, 37″, 37′″.
E.g. in the example shown in
All other sub-gratings 37′, 37″, 37′″ may first be arranged such that a maximum intensity of X-rays is transmitted through the arrangement 12 of gratings 13, 15, 17 and X-ray intensity is accumulated within the pixels 19 of the detector 7. The information about the accumulated X-ray intensity is continuously monitored and as soon as it is recognized that partial areas of the detector 7 tend to be over-exposed, such information may be used in a control device 21 (see
The control device 21 may then control steering means 39 of sub-gratings 37′, 37′″ associated to the nearly overexposed detector portions such that at least part of the intensity maxima 45 spatially coincide with the highly absorbing blocks 35 of the third grating 17 and therefore portions of the X-ray beam 8 are blocked in the respective areas.
In contrast hereto, areas of the detector 7 downstream of highly X-ray absorbing structures such as the backbone 51 within the body 49 will not tend to be overexposed and thus the control device 21 will control the steering means 39 of an associated sub-grating 37″ such that intensity maxima 45 of a generated interference pattern coincide with the weakly absorbing blocks 33 of the third grating 17 and are therefore maximally transmitted.
It may be noted that in order to be able to control X-ray transmittance through the arrangement 12, it may be only necessary to be able to control the relative positioning of the second and third gratings. Accordingly, in principle, instead of or in addition to moving sub-gratings of the second grating 15, the third grating 17 or sub-gratings thereof may be actively moved using steering means.
However, it may generally be easier to move sub-gratings 37 of the second grating 15 as these sub-gratings 37 are typically lighter than optional sub-gratings of the third grating 17 as this third grating 17 comprises highly X-ray absorbing blocks 35 of heavy materials such as gold.
In a specific embodiment of the presented X-ray imaging device 1, the X-ray source 6 may be adapted to emit a poly-chromatic spectrum of X-rays. In such embodiment, the control device 21 may be adapted to controlling the steering means 39 such that, during an X-ray imaging acquisition, sub-gratings 37 are displaced such that varying X-ray spectra are transmitted through the arrangement 12 of first, second and third gratings 13, 15, 17. Such embodiment may be seen as based on the idea that a Talbot-pattern is mainly dependent on a wavelength of transmitted X-rays and therefore attenuation of X-rays within the pre-patient arrangement 12 of gratings 13, 15, 17 is generally maximal only for a specific (“design-”) wavelength and grating setting. E.g., when using a poly-chromatic X-ray beam, photons with other wavelengths will at least partly pass the arrangement 12. Accordingly, by modulating the grating structure and positioning as explained above, a specific object of interest such as e.g. a vessel structure within a patient may be illuminated with photons of different mean energy.
In a further specific embodiment, the periodical X-ray phase modifying unit 15 comprises an array of structures or gratings such as e.g. sub-millimeter sized pieces of etched silicon. Such array may comprise e.g. one or several sources of ultrasound waves. For example, such sources may be provided using modern cMut-sources which may be formed in a MEMS as an array of ultrasound sources. Thus, the array may be attached to localized ultrasound transducers. Such array may be immersed in a liquid. Upon activation of the localized transducers, a periodical pattern of cavitations may be generated in the liquid, which may act as localized periodic elements to alter or modify a phase shift in transmitted X-rays. In other words, the cavitations generally comprise a different index of refraction compared to surrounding liquid such that a period pattern of refraction index changes is created. Due to the phase shift occurring upon transmission of X-rays through such pattern, an interference pattern may form and beam intensity behind the array will be altered locally. With the periodical X-ray absorbing unit 17 being arranged specifically with respect to such interference pattern, the intensity of X-rays transmitted through the entire controllable transmission arrangement 12 may be controlled.
| Number | Date | Country | Kind |
|---|---|---|---|
| 13191590.2 | Nov 2013 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/072474 | 10/21/2014 | WO | 00 |
