WO2008146195A2

WO2008146195A2 - Device for imaging an interior of an optically turbid medium using diffuse optical tomography and ultrasound

Info

Publication number: WO2008146195A2
Application number: PCT/IB2008/051958
Authority: WO
Inventors: Robert R. Entrekin; Martinus B. Van Der Mark; Michael C. Van Beek; Levinus P. Bakker; Hendrikus A. C. M. Compen; Frank Dehn; Nijs C. Van Der Vaart; Jacobus A. J. M. Deckers; Jan F. Suijver
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-05-25
Filing date: 2008-05-19
Publication date: 2008-12-04
Also published as: WO2008146198A3; WO2008146195A3; WO2008146198A2

Abstract

The invention relates to a device for imaging the interior of a volume of an object using diffuse optical tomography and ultrasound. According to the invention a volume in which the object is accommodated and into which optical and ultrasound signals are sent is bound by a wall structure comprising entrance and exit positions for both the optical and ultrasound signals. In this way, the object can be positioned relative to the wall structure resulting in a well-defined measurement geometry. The invention improves the comparability of scans of an object that have been made at different points in time as the positioning of the object in a first scan can be precisely reproduced in a further scan relative to the wall structure.

Description

DEVICE FOR IMAGING AN INTERIOR OF AN OPTICALLY TURBID MEDIUM USING DIFFUSE OPTICAL TOMOGRAPHY AND ULTRASOUND

FIELD OF INVENTION

The invention relates to a device for imaging an interior of an optically turbid medium comprising:

(a) a diffuse optical tomography unit, with the diffuse optical tomography unit comprising:

-a light source for generating light to be coupled into the turbid medium; -a photodetector unit for detecting light emanating from the turbid medium;

(b) an ultrasound unit, with the ultrasound unit comprising:

-an ultrasound source for generating ultrasound waves to be coupled into the turbid medium;

-an ultrasound detector unit for detecting ultrasound waves emanating from the turbid medium.

The invention also relates to a medical image acquisition device for imaging an interior of an optically turbid medium.

The invention also relates to methods for imaging an interior of an optically turbid medium.

BACKGROUND OF THE INVENTION An embodiment of a device of this kind is known from N. Chen et al.,

'Simultaneous Near-Infra Red Diffuse Light and Ultrasound Imaging', Applied Optics, Vol. 40, issue 34, pp. 6367-6380, 2001. The device comprises a combined hand-held probe for simultaneous ultrasound and near-infrared diffuse light imaging and co-registration. A two- dimensional ultrasound array is deployed at the centre of the combined probe, and a plurality of optical fibers is deployed at the periphery. The probe has been used to reconstruct the absorption coefficient with the guidance of the co-registered ultrasound. In medical diagnostics, a device as described above may be used for imaging human tissue, such as a female breast. It is a drawback of the known device that it is hard to compare scans made at different moments in time.

SUMMARY OF THE INVENTION It is an object of the invention to provide a device for imaging an interior of an optically turbid medium that makes it easier to compare scans made at different moments in time. According to the invention this object is realized in that the device further comprises: a boundary for bounding a receiving volume for accommodating the turbid medium, with the boundary comprising: - at least one entrance position for light for coupling light from the light source into the receiving volume;

- at least one exit position for light for coupling light emanating from the receiving volume to the photodetector unit, with the light emanating from the receiving volume turbid medium as a result of coupling light from the light source into the receiving volume;

- at least one entrance position for ultrasound for coupling ultrasound waves from the ultrasound source to the receiving volume;

- at least one exit position for ultrasound for coupling ultrasound waves emanating from the receiving volume to the ultrasound detector unit, with the ultrasound waves emanating from the receiving volume as a result of coupling ultrasound waves from the ultrasound source into the receiving volume, with the at least one entrance position for light, the at least one exit position for light, the at least one entrance position for ultrasound, and the at least one exit position for ultrasound being arranged to have a fixed geometry relative to the receiving volume during a scan of the turbid medium.

The invention is based on the recognition that the boundary enables the turbid medium to be positioned relative to the boundary. This positioning can be reproduced well for scans that are made at different moments in time. As the boundary comprises the at least one entrance position for light, the at least one exit position for light, the at least one entrance position for ultrasound, and the at least one exit position for ultrasound, the position of the turbid medium relative to the aforementioned positions can also be reproduced well for scans that are made at different moments in time. Hence, comparing scans that have been made at different moments in time is made easier. It is a further additional advantage of the invention that the boundary may be used to stabilize the turbid medium during scanning. In general, a hand-held probe needs to be pressed against an object to be scanned which is virtually impossible without causing some movement of the object. Hence, the region that is scanned is better defined with a stabilizer than with a hand-held probe.

It is a further additional advantage of the invention that by choosing a boundary of the proper size the full volume of the turbid medium can be scanned. Choosing the size of the boundary as such that an object, for instance, a female breast fits inside the receiving volume bounded by the boundary, allows the full volume of the breast to be scanned. With a hand- held probe only selected regions of an object such as a female breast are typically scanned and not the whole object.

It is a further additional advantage of the invention that by choosing a boundary of the proper size no skilled operator is required. With a hand- held probe a skilled operator is needed to decide during a scan which regions of an object are of interest and which are not. With a device according to the invention, interpretation of scan data can be done after the scan has been completed instead of the interpretation being at least partially performed during the scan.

It is a further additional advantage of the invention that a device according to the invention enables scanning of a deformable optically turbid medium without the shape of the turbid medium changing during a scan. When scanning, for instance, a female breast, a hand- held probe needs to be placed in contact with the breast. As a result, the shape of the breast generally changes as a result of placing the hand- held probe into contact with the breast and moving the probe around. Hence, the process of scanning the breast changes the object that is to be scanned. Use of a boundary comprising at least one entrance position for light, at least one exit position for light, at least one entrance position for ultrasound, and at least one exit position for ultrasound solves this problem as the turbid medium is in a fixed position relative to the aforementioned positions during a scan. Consequently, the scanning geometry is better defined with a device according to the invention than with a hand-held probe. An embodiment of a device according to the invention wherein the boundary substantially forms a cup having an open side enabling at least a part of the turbid medium to enter the receiving volume. This embodiment has the advantage that a cuplike boundary substantially surrounds the turbid medium enabling scanning from almost any position. This embodiment has the further advantage that a cuplike boundary may be used to hold an optical coupling medium for enhancing the optical coupling of the turbid medium to its surroundings or an acoustic coupling medium for enhancing the acoustic coupling of the turbid medium to its surroundings. In general, the optical and acoustic characteristics of the turbid medium and the corresponding characteristics of the surroundings of the turbid medium do not match. This leads to reflections of light and ultrasound waves occurring at the boundaries between different mediums. Such boundary effects can be reduced by using coupling mediums. Moreover, an optical coupling medium may be used to provide sufficient scattering and attenuation for light that travels inside the receiving volume but outside the turbid medium. Without a coupling medium such light would experience less attenuation than light traveling through the turbid medium. At the photodetector unit the former light could dwarf the latter. This situation is called an optical short-circuit. Often, coupling mediums are liquids. Hence, a cup like boundary has the advantage that it can hold a coupling medium, especially when the medium is a liquid.

A further embodiment of a device according to the invention wherein the boundary comprises compression surfaces for compressing the turbid medium. This embodiment has the advantage that a boundary according to the invention comprising compression surfaces enables scanning of a turbid medium in a fixed geometry, while, when the turbid medium is deformable, creating a region in which the turbid medium has a well- defined thickness. If the compression surfaces are two parallel plates this well-defined thickness is constant. For diffuse optical tomography a constant thickness has the advantage that light attenuation is limited in a range that is smaller than it would be if the turbid medium were not compressed.

A further embodiment of a device according to the invention wherein the ultrasound unit comprises a single ultrasound source, with the ultrasound source being comprised in the boundary, and with the ultrasound source at least partially facing the open side. This embodiment has the advantage that as the degree to which the ultrasound source faces the open side increases, the degree to which ultrasound waves reflect from the boundary decreases as the general scanning direction will increasingly be in the direction of the opening. A further embodiment of a device according to the invention wherein the ultrasound unit comprises a plurality of ultrasound sources, with the plurality of ultrasound sources being comprised in the boundary. This embodiment has the advantage that it enables tomographic scanning of an object comprised in the volume bounded by the boundary. A further embodiment of a device according to the invention wherein the device further comprises a multimodality image reconstruction unit for reconstructing a combined image of an interior of the turbid medium based both on detected light and detected ultrasound waves. This embodiment has the advantage that the quality of the combined image (possibly an overlay) based both on the further imaging modality and on ultrasound signals received by at least one ultrasound transducer benefits from any one of the previous embodiments. With diffuse optical tomography the resolution of a reconstructed image is typically lower than the resolution of an ultrasound image. Furthermore, ultrasound is more suitable for imaging structures, whereas diffuse optical tomography allows the functional imaging of objects such as human tissues. Consequently, reconstruction of an image based on a scan that has been made using one imaging modality may benefit from using on a scan using the other imaging modality. Reconstructing an image based on both diffuse optical tomography and ultrasound would improve the resolution of the image as compared to the resolution of a diffuse optical tomography image (for instance a diffuse optical tomography image of which the resolution is enhanced using an ultrasound scan) and could also be used to combine the strengths of both imaging modalities (for instance an image in which both structural and functional features are displayed). Moreover, structural information relating to an object, such as human tissue, obtained from an ultrasound scan may be used in setting boundary conditions for the reconstruction of an optical image of the same object. This would improve the accuracy of the reconstruction of the optical image. Moreover still, using data obtained from an ultrasound scan in the reconstruction of an optical image allows a better estimate of the optical properties of the object imaged and hence of its composition. This is beneficial when imaging, for instance, an interior of a female breast. The object of the invention is also realized with a medical image acquisition system, characterized in that the medical image acquisition device comprises a device according to any one of the previous embodiments. A medical image acquisition system benefits from any one of the previous embodiments.

The object of the invention is also realized with a method for imaging an interior of an optically turbid medium comprising the following steps: coupling light from a light source into a receiving volume for accommodating the turbid medium using an entrance position for light successively chosen from a plurality of entrance positions for light comprised in a boundary bounding the receiving volume, with the plurality of entrance positions for light being arranged to have a fixed geometry relative to the receiving volume during a scan of the turbid medium; coupling light emanating from the receiving volume to a photodetector unit, with the light emanating from the receiving volume as a result of coupling light from the light source into the receiving volume using a plurality of exit positions for light comprised in the boundary, with the plurality of exit positions for light being arranged to have a fixed geometry relative to the receiving volume during a scan of the turbid medium; coupling omnidirectional ultrasound waves from an ultrasound source into the receiving volume using an entrance position for ultrasound successively chosen from a plurality of entrance positions for ultrasound comprised in the boundary, with the plurality of entrance positions for ultrasound being arranged to have a fixed geometry relative to the receiving volume during a scan of the turbid medium; coupling ultrasound waves emanating from the receiving volume to an ultrasound detector unit, with the ultrasound waves emanating from the receiving volume as a result of coupling ultrasound waves from the ultrasound source into the receiving volume using a plurality of exit positions for ultrasound comprised in the boundary, with the plurality of exit positions for ultrasound being arranged to have a fixed geometry relative to the receiving volume during a scan of the turbid medium.

This method has the advantage that optical and ultrasound data can be obtained simultaneously, with the positions of the entrance and exit positions for light and the entrance and exit positions for ultrasound being well-defined in relation to the receiving volume and hence well reproducible for future scans. This method also has the advantage that coupling omnidirectional ultrasound waves into and out of the receiving volume enables quick scanning of an object under investigation.

The object of the invention it also realized with a method for imaging an interior of an optically turbid medium comprising the following steps: coupling light from a light source into a receiving volume for accommodating the turbid medium using an entrance position for light successively chosen from a plurality of entrance positions for light comprised in a boundary bounding the receiving volume, with the plurality of entrance positions for light being arranged to have a fixed geometry relative to the receiving volume during a scan of the turbid medium; coupling light emanating from the receiving volume to a photodetector unit, with the light emanating from the receiving volume as a result of coupling light from the light source into the receiving volume using a plurality of exit positions for light comprised in the boundary, with the plurality of exit positions for light being arranged to have a fixed geometry relative to the receiving volume during a scan of the turbid medium; coupling ultrasound waves from an ultrasound source into the receiving volume along a well-defined path, using an entrance position for ultrasound chosen from a plurality of entrance positions for ultrasound comprised in the boundary, with the plurality of entrance positions for ultrasound being arranged to have a fixed geometry relative to the receiving volume during a scan of the turbid medium; coupling ultrasound waves emanating from the receiving volume along the well-defined path in the direction of the entrance position for ultrasound to an ultrasound detector unit.

This method has the advantage that allows quick scanning of an object under investigation by a using a plurality of focused single element ultrasound transducers for emitting and detecting ultrasound waves along well-defined paths the direction of which cannot be steered. The directions along which ultrasound waves are emitted and detected form a grid enabling quick scanning.

An embodiment of the method according to the invention, wherein the direction of the well-defined path is steerable. This embodiment has the advantage that coupling ultrasound waves into the receiving volume along a well-defined path the direction of which (defined by the propagation direction of the ultrasound waves traveling along the well-defined path) is steerable allows the scanning of a part of an object under investigation. This improves the scanning resolution compared to a situation in which the direction of the well-defined path cannot be steered. Scans of different parts of the object obtained by coupling ultrasound waves into the receiving volume from different entrance positions chosen from the plurality of entrance positions can be used to obtain an image of the entire object. If multiple entrance positions are used simultaneously it is important that the different well-defined paths do not cross. Steering of the direction of the well-defined path can be achieved by using a phased array ultrasound transducer or by using a single element ultrasound transducer combined with a fluid focus lens.

The object of the invention is also realized with a method for imaging an interior of an optically turbid medium comprising the following steps: coupling light from a light source into a receiving volume for accommodating the turbid medium using an entrance position for light successively chosen from a plurality of entrance positions for light comprised in a boundary bounding the receiving volume, with the plurality of entrance positions for light being arranged to have a fixed geometry relative to the receiving volume during a scan of the turbid medium; coupling light emanating from the receiving volume to a photodetector unit, with the light emanating from the receiving volume as a result of coupling light from the light source into the receiving volume using a plurality of exit positions for light comprised in the boundary, with the plurality of exit positions for light being arranged to have a fixed geometry relative to the receiving volume during a scan of the turbid medium; coupling ultrasound waves from an ultrasound source into the receiving volume along a first well-defined path, using an entrance position for ultrasound chosen from a plurality of entrance positions for ultrasound comprised in the boundary, with the plurality of entrance positions for ultrasound being arranged to have a fixed geometry relative to the receiving volume during a scan of the turbid medium; coupling ultrasound waves emanating from the receiving volume along a further well-defined path to an ultrasound detector unit, with the first well-defined path and the further well-defined path partially overlapping.

An embodiment of the method according to the invention, wherein the direction of at least one of the well-defined path and the further well-defined path is steerable.

An embodiment of the method according to the invention, wherein the step of coupling ultrasound waves from an ultrasound source into the receiving volume and the step of coupling ultrasound waves emanating from the receiving volume to an ultrasound detector unit are carried out at a plurality of positions comprised in the boundary simultaneously. This embodiment has the advantage that it enables faster scanning of an object accommodated in the receiving volume.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be further elucidated and described with reference to the drawings, in which: Fig. 1 schematically shows an embodiment of a device for imaging an interior of an optically turbid medium according to the invention;

Fig. 2 schematically shows a number of embodiments of a cuplike wall structure being used for ultrasound tomography; Fig. 3 schematically shows the point spread function produced by a typical ID ultrasound transducer array;

Fig. 4 schematically shows an embodiment of a combined ultrasound transducer for making scans with different spatial resolution patterns;

Fig. 5 shows spatial resolution patterns measured in the elevation-azimuth plane for different rotational positions of a combined ultrasound transducer;

Fig. 6 schematically shows an embodiment of a cup comprising four ultrasound transducers aligned using face normals of a regular tetrahedron;

Fig. 7 schematically shows the overlapping spatial resolution patterns of four ultrasound transducers aligned using face normals of a regular tetrahedron; Fig. 8 schematically shows the overlap of the fields of view of four ultrasound transducers aligned using face normals of a regular tetrahedron;

Fig. 9 schematically shows an embodiment of a medical image acquisition system comprising an ultrasound device according to the invention;

Fig. 10 schematically shows an embodiment of a method for making an ultrasound scan of a volume using 3-D spatial compounding.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 schematically shows an embodiment of a device for imaging an interior of an optically turbid medium according to the invention. The device 170 comprises a diffuse optical tomography unit, with the diffuse optical tomography unit comprising a light source 195 and a photodetector unit 215. The device 170 further comprises an ultrasound unit, with the ultrasound unit comprising an ultrasound source, which is comprised in an ultrasound probe 175, and an ultrasound detector unit, which in the example of fig. 1, is also comprised in the ultrasound probe 175. Hence, the ultrasound probe 175 functions as both an entrance position for ultrasound and an exit position for ultrasound. This dual function can be obtained by comprising a sending and receiving ultrasound transducer in the ultrasound probe 175. The device 170 still further comprises a multimodality image reconstruction unit 230 for reconstructing an image of an interior of a scanned object based either on light detected by the photodetector unit 215 or on ultrasound waves detected by the ultrasound detector unit comprised in the ultrasound probe 175 or on both. The device 170 still further comprises a receiving volume 180 for accommodating an optically turbid medium 185. According to the invention, the receiving volume 180 is bounded by a cuplike boundary 190. An advantage of using a cuplike boundary 190 is that it allows the positioning of a possibly combined ultrasound probe 175 such that it faces the opening of the cup. From this position, the ultrasound probe 175 can scan a substantial part of the receiving volume 180 bounded by the cup with a first spatial resolution pattern, rotate through a predefined angle, and then make a further scan of at least a part of the receiving volume 180 scanned in the first scan with a further spatial resolution pattern that is different from the first space resolution pattern. As the ultrasound probe 175 faces the opening of the cup reflections of ultrasound waves of the sides of the cup facing the receiving volume 180 bounded by the cup are reduced. In another embodiment according to the invention, the boundary does not have a cuplike shape, but comprises compression surfaces for compressing a turbid medium 185 instead. With the compression surfaces, a turbid medium is accommodated between the surfaces after which the distance between the surfaces is decreased in order to compress the turbid medium between the surfaces. Returning to the cuplike boundary 190 shown in fig. 1, light from the light source 195 is coupled into the receiving volume 180 through use of the selection unit 200 after the turbid medium 185 is accommodated in the receiving volume 180. The light from the light source 195 is chosen such that it can propagate through the turbid medium 185 without causing fluorescence in the turbid medium 185. In medical diagnostics, where the device 170 may be used as a medical image acquisition system for imaging, for instance, an interior of a female breast, light having a wavelength within the range of, for instance, 400 nm to 1400 nm is suitable for this purpose. Alternatively, the light from the light source 195 may be chosen such that it can propagate through the turbid medium 185 and excite a fluorescent agent comprised in the turbid medium 185. The selection unit 200 is used to successively select an entrance position for light from a plurality of entrance positions for light 205. Light emanating from the receiving volume 180 as a result of coupling light from the light source 195 into the receiving volume 180 exits the receiving volume 180 using a plurality of exit positions for light 210. For the sake of clarity, the entrance positions for light 205 and the exit positions for light 210 has been positioned at opposite sides of the boundary 190. In reality, however, both types of position may be spread around the receiving volume 180. Light emanating from the receiving volume 180 is detected through use of the photodetector unit 215. The plurality of entrance positions for light is 205 are optically coupled to the selection unit 200 using light guides 220. The plurality of exit positions for light 210 is optically coupled to the photodetector unit 215 using light guides 225. Ultrasound waves from the ultrasound probe 175 are coupled into the receiving volume 180. The ultrasound waves are emitted such that at least a substantial part of the receiving volume 180 comprising the turbid medium 185 is scanned. Ultrasound waves emanating from the receiving volume 180 as a result of coupling ultrasound waves into the receiving volume 180 are detected through use of the ultrasound detector unit comprised in the ultrasound probe 175. The multimodality image reconstruction unit 230 is used to reconstruct an image of an interior of the turbid medium 185 based on either ultrasound waves detected through use of the ultrasound detector unit comprised in the ultrasound probe 175 or on light detected using the photodetector unit 215 or on both. Inside the receiving volume 180, the turbid medium 185 may be surrounded by a matching medium 245. The matching medium 245 has optical properties, such as an absorption coefficient, similar to those of the turbid medium 185. In this way, boundary effects stemming from coupling light from the light source 195 into and out of the turbid medium 185 are reduced and optical short-circuits around the turbid medium 185 prevented. An optical short-circuit arises if light that has traveled through the receiving volume 180 but outside the turbid medium 185 has been attenuated less than light that has traveled through the turbid medium 185. In that case, the former light may dwarf the latter light at the photodetector unit 215. Similarly, the turbid medium 185 may be surrounded in the receiving volume 180 by a matching medium 245 having acoustic properties, such as the speed of sound, similar to those of the turbid medium 185. In this way, boundary effects stemming from coupling ultrasound waves into and out of the turbid medium 185 are reduced.

The quality of an image of an interior of the turbid medium 185 reconstructed using the device 170 can be improved based on the recognition that the spatial resolution of an ultrasound transducer is generally anisotropic. For an ultrasound transducer arranged for scanning a volume, for instance, the spatial resolution is generally different in the axial, azimuth, and elevation directions. This is because the size of the transducer aperture and the means of focusing the ultrasound beam generated by the transducer are likewise different in different directions. Typically, the transducer aperture is larger in the azimuth dimension as compared to the elevation dimension, and is dynamically focused using a 'beam former' that acts as an electronically variable lens. The transducer aperture is typically smaller in the elevation dimension as compared to the azimuth dimension and has a fixed focus using a physical cylindrical lens. The above results in a resolution in the azimuth dimension that is typically better than the resolution in the elevation dimension. Furthermore, even if the transducer aperture was equally sized and focused in azimuth and elevation, the axial resolution is typically better than the resolution in either the azimuth or elevation dimension. The spatial resolution is characterized by the point spread function, the three- dimensional graphical interpretation of which is the resolution of an ultrasound device in three dimensions. In the example of the transducer described above, the graphical rendering of the point spread function would typically be that of a flattened rugby ball-like object, with the resolution in the elevation dimension defining the length of the point spread function, with the resolution in the azimuth dimension defining the width of the point spread function, and with the axial resolution defining the depth of the point spread function. From the above it is clear that the point spread function relates to a spatial resolution pattern that has a certain shape, for instance, the rugby ball-like shape. However, the spatial resolution pattern also has a spatial orientation in a volume to be scanned depending on, for instance, the orientation of an ultrasound transducer relative to the volume that the transducer is to scan. After all, the point spread function will move with the ultrasound transducer because the point spread function is defined by the properties of the transducer. Hence, it is possible to obtain a first scan of a volume with a first spatial resolution pattern and a further scan of at least a part of the same volume with a further spatial resolution pattern that is different from the first spatial resolution pattern. The spatial resolution patterns of the various scans will overlap, but will not be congruent. Consequently, combining at least two scans with different spatial resolution patterns through three- dimensional spatial compounding, for instance, some kind of averaging, will result in a compounded spatial resolution pattern to which regions of the spatial resolution patterns that do not have overlap contribute less than regions that do overlap. By combining an increasing number of scans, with, for instance, each scan having been made with a different orientation of the ultrasound transducer used for the scans relative to the volume that is scanned, the resulting compounded spatial resolution pattern will become increasingly more isotropic. A more isotropic compounded spatial resolution pattern will result in an improved quality of a reconstructed image of the volume scanned. This is true because the volume is effectively scanned with a resolution that is increasingly similar in all dimensions, as an increasing number of scans having different spatial resolution patterns is compounded.

On the basis of the above, a device, like the device 170 shown in fig. 1, arranged for reconstructing an image with improved quality comprises an ultrasound device that can be described as: An ultrasound device comprising at least one ultrasound transducer arranged for making a scan of at least a first part of a volume by at least receiving ultrasound signals from the volume, with the at least one ultrasound transducer comprising: a first-scan ultrasound transducer arranged for making a first scan of at least a first part of the volume with the first scan being made with a first-scan spatial resolution pattern, and with the first-scan ultrasound transducer being arranged for producing first-scan image signals based on the ultrasound signals received by the first-scan ultrasound transducer, with the at least one ultrasound transducer further comprising: at least one further-scan ultrasound transducer arranged for making a further scan of at least a further part of the volume by at least receiving ultrasound signals from the volume, with the further part of the volume at least partially overlapping with the first part of the volume, with the further scan being made with a further-scan spatial resolution pattern, with the further-scan spatial resolution pattern being different from the first-scan spatial resolution pattern, and with the further-scan ultrasound transducer being arranged for producing further-scan image signals based on the ultrasound signals received by the further- scan ultrasound transducer; and with the ultrasound device further comprising: a 3-D spatial compounding unit for spatially compounding the first scan and the further scan based on the first-scan image signals and the further-scan image signals. A device like the device 170, with or without being arranged to provide an image with improved quality based on the above-mentioned recognition, may be used as a medical image acquisition system for imaging, for instance, an interior of a female breast. Improving the quality of a reconstructed image and the recognition on which this improvement is based will be further discussed in relation to figs. 2-9. Fig. 2 schematically shows a number of embodiments of a cuplike wall structure being used for ultrasound tomography. Figs. 2a-c all schematically show a cross- section of a cuplike boundary 190 comprising an object to be studied. In fig. 2 is the object to be studied is a female breast 600 comprising a lesion 603. Ultrasound probes 605 (indicated in fig. 2a only for clarity) are distributed along the wall of the boundary 190. In figs. 2a-c the ultrasound probes 605 appear to be positioned at opposite sides of the cuplike boundary 190. However, this is the result of figs. 2a-c showing a cross-section of the cuplike boundary 190. In reality, the ultrasound probes 605 may be distributed along the entire surface of the cuplike boundary 190. In a first embodiment of ultrasound tomography shown in fig. 2a, a plurality of ultrasound probes 605 is used to provide an omnidirectional ultrasound radiation pattern as indicated by the concentric circles 610. Sequentially one of the probes 605, in the case of fig. 2a probe 615, is used as a source and all other probes 605 (possibly including also the source probe 615) are used as detectors. From attenuation in time of flight of ultrasound signals between all source-detector pairs, three-dimensional maps of attenuation and speed of sound corresponding to the internal structure of the breast 600 can be calculated using an image reconstruction algorithm. This embodiment is the ultrasound analogue of regular diffuse optical tomography technology. In a second embodiment of ultrasound tomography shown in fig. 2b, a plurality of ultrasound probes 605 is used to emit and detect ultrasound energy along a well- defined path (beam). One ultrasound probe, in the case of fig. 2b probe 620, acts as a source and as a detector for one path. This beam is created by a multi-element phased array ultrasound transducer or by a single element transducer combined with a fluid focus lens. Both the phased array and the fluid focus lens are capable of beam steering. In a preferred embodiment, multiple probes out of the plurality of probes 605 measure simultaneously, each along a unique path. If multiple ultrasound probes are used to measure simultaneously, it is important that the different paths do not cross. In the second embodiment, every probe out of the plurality of probes 605 can image a partial volume of the breast 600. A reconstruction algorithm is used to combine all separate images to a complete image of the breast 600.

In a third embodiment of ultrasound tomography shown in fig. 2c, a plurality of ultrasound probes 605 is used to emit ultrasound energy along a well-defined path (beam 625) and also detect ultrasound energy along a wall-defined direction 630, with the ultrasound energy originating from a detection region 635. One ultrasound probe, in the case of fig. 2c probe 640, acts as a source and another ultrasound probe, in the case of fig. 2c probe 645, as a detector. Only the volume where the beam and the detection region overlap is probed. In a preferred embodiment, multiple volumes are measured simultaneously, by crossing multiple detection regions and different positions with a single source beam. In this embodiment, every source-detector pair can image a partial volume of the breast 600. Different volumes can be probed by steering the beam and detect a region into a different direction (for instance by using a phased array ultrasound transducer or a single element ultrasound transducer combined with a fluid focus lens) or by using different source-detector pairs. A reconstruction algorithm is used to combine all separate images to a complete image of the breast 600. In a preferred embodiment, the source ultrasound probe 605 is also used for detection.

In a fourth embodiment of ultrasound tomography (not shown in fig. 2), multiple focused single element ultrasound transducers are used in the ultrasound probes 605. These ultrasound transducers can be pre-shaped transducers or flat transducers combined with an ultrasound lens. The emitted ultrasound beams and the ultrasound detection region cannot be scanned. The beams emitted by these ultrasound transducers form a grid. This fourth embodiment can be considered as a simple and inexpensive version of the second and/or third embodiment. As an alternative to a cup like boundary, another possibility is a boundary comprising compression surfaces (not shown in fig. 2). An object to be studied can be positioned between the compression surfaces after which the surfaces are moved closer to each other to apply some pressure to the object. In this way, the object can be positioned and stabilized. If the object is deformable under pressure, as is the case with a female breast, use of parallel compression surfaces can also be used to create a region in the object under investigation that has a constant thickness. For optical measurements such an arrangement has the advantage that the attenuation of light through passage of light through the object is limited to a certain range as the thickness of the object under investigation is limited to a certain range. Fig. 3 schematically shows the point spread function produced by a typical ID ultrasound transducer array. Fig. 3 elaborates on improving the quality of a reconstructed image as discussed in relation to fig. 1. Fig. 3b schematically shows a typical ID ultrasound transducer array as a series of vertical lines 5. The transducer aperture is larger in the azimuth dimension than in the elevation dimension. Typically, the transducer aperture in the azimuth dimension is dynamically focused using a 'beam former' that acts as an electronically variable lens. Similarly, in the elevation dimension, the transducer aperture typically has a fixed focus through use of a physical cylindrical lens. In fig. 3b the transducer aperture in the azimuth dimension is indicated by the number 10, whereas the transducer aperture in the elevation dimension is indicated by the number 15. The point spread function of the ultrasound transducer 5 is represented by the rugby ball- like object 20 in front of the transducer aperture. Figs. 2a, 2c, and 2d schematically show the point spread function 20 as seen from different directions, with fig. 3a showing a top view, with fig. 3c showing a front view, and with fig. 3d showing a side view. The differences in aperture size and focusing in the azimuth and elevation dimensions discussed above, typically result in the spatial resolution in the azimuth dimension 25 being better than the spatial resolution in the elevation dimension 30. However, even if the transducer aperture was equally sized and focused in the azimuth and elevation dimensions, the axial resolution 35 is typically better than the spatial resolution in either the azimuth or elevation dimension. The axial resolution 35 is independently determined by the transducer bandwidth and the resulting pulse length, regardless of aperture size. From fig. 3 the anisotropy of the point spread function of a typical ultrasound transducer array is clear. It is also clear that the point spread function being a characteristic of the ultrasound transducer moves with a transducer when the transducer itself is moved, for instance, for the scanning of a volume. Hence, it is possible to obtain different scans of a volume with different scans having been made with their respective point spread functions being oriented differently.

Thus, the spatial resolution patterns being formed by the respective point spread functions in the various orientations will, in general, partially overlapped but not be congruent.

Fig. 4 schematically shows an embodiment of a combined ultrasound transducer for making scans with different spatial resolution patterns. Fig. 4 elaborates on improving the quality of a reconstructed image as discussed in relation to fig. 1. The combined ultrasound source 40 comprises a scanner head 45 comprising, for instance, a curved linear array ultrasound transducer capable of making a sector scan in the azimuth dimension through electronic steering of the array. This is shown in fig. 4a in which the sector scan lies in the plane of the drawing. The combined ultrasound source 40 further comprises an axis 50 around which the scanner head 45 can mechanically pivot. In this way, the ultrasound source 40 can scan a volume through electronic steering in the azimuth dimension and mechanical steering in the elevation dimension, with the elevation dimension being perpendicular to the azimuth dimension. At each possible position relative to the pivot axis 50 a sector scan can be made in the azimuth dimension. The pivoting of the scanner head 45 around the pivoting axis 50 is schematically shown in fig. 4b that shows a side view of the combined ultrasound source 40 shown in fig. 4a. The combined ultrasound source 40 is arranged to be rotatable as a whole around an axis 55. At each angular position relative to the axis 55 the combined ultrasound source 40 can scan a volume through electronic and mechanical steering of the scanner head 45 comprising the ultrasound transducer. The incremental angle A between each angular position relative to the axis 55 at which one out of a series of N scans is made may be determined by the formula A = 180°/N. Here, N can be extended to arbitrary positive integers. When, for example, N equals four, the incremental angle A equals 45°. This means that the combined ultrasound source 40 will make a volume scan at 0°, 45°, 90° and 135° relative to the axis 55 and relative to an arbitrary starting point relative to this axis. As in fig. 4 the scanned volume is symmetric relative to the axis 55, scans at angular positions exceeding 180° are not required because these areas are already covered by scans made at angular positions below 180°. As explained in relation to fig. 3, the point spread function of an ultrasound transducer will move with the transducer when the transducer itself is moved. Hence, the combined ultrasound source 40, being rotatable around the axis 55, allows different scans of a single volume to be obtained with different scans being made with different spatial resolution patterns, that is with the point spread functions of different scans having different spatial orientations. Consequently, the combined ultrasound source 40 comprises a first-scan ultrasound transducer and a further-scan ultrasound transducer in a single, combined ultrasound transducer. The spatial resolution patterns of scans, having been made at different positions relative to the axis 55, will partially overlap. By combining different scans through 3-D spatial compounding, for instance, through averaging, the overlapping regions will contribute more to the compounded spatial resolution pattern than non-overlapping regions. Consequently, the compounded spatial resolution pattern that can be obtained through use of the combined ultrasound source 40 will show an increased isotropy in a plane perpendicular to the axis 55 as compared to the spatial resolution pattern in scans according to the prior art. As in the direction of the axis 55 there is only one look angle, the isotropy of the compounded spatial resolution pattern in this direction will be substantially comparable to the isotropy that can be obtained in scans according to the prior art. The isotropy of the compounded spatial resolution pattern in the direction of the axis 55 can be improved by using not one, but a plurality of combined ultrasound transducers having different look angles and by compounding the scans obtained from the plurality of combined ultrasound transducers. In fig. 4b the combined ultrasound source 40 is comprised in a cup 60 suitable for imaging an interior of a female breast. Through electronic and mechanical steering of the combined ultrasound source 40, the transducer is capable of scanning a substantial part of the volume defined by the cup 60. A cup like boundary similar to the cup 60 was already shown in fig. 1 in which the device 170 comprised a boundary 190 that substantially forms a cup and in fig. 2.

Fig. 5 shows spatial resolution patterns measured in the elevation-azimuth plane for different rotational positions of a combined ultrasound transducer. Fig. 5 elaborates on improving the quality of a reconstructed image as discussed in relation to fig. 1. The spatial resolution patterns 65, 70, 75, and 80 shown in figs. 4a-4d respectively have been measured at 0°, 45°, 90°, and 135° relative to an arbitrary starting point and relative to a rotational axis that is perpendicular to the elevation-azimuth plane (see also the discussion in relation to fig. 4). The various spatial resolution patterns shown are oriented differently relative to a single coordinate system because the point spread function moves with the ultrasound transducer as the transducer is rotated. Fig. 5e shows the compounded spatial resolution pattern 85 obtained by spatially compounding figs. 4a-4d. The central regions of the various spatial resolution patterns overlap, whereas the more outward lying regions do not. The central regions contribute more to the compounded spatial resolution pattern than the more outward lying regions resulting in a more isotropic spatial resolution pattern being obtained as compared to prior art.

Fig. 6 schematically shows an embodiment of a cup comprising four ultrasound transducers aligned using face normals of a regular tetrahedron. Fig. 6 elaborates on improving the quality of a reconstructed image as discussed in relation to fig. 1. Four ultrasound transducers numbered 90, 95, 100, and 105 respectively are mounted on a cup 110 comprising one half of a hollow sphere. Each of these four transducers is rotatable around a longitudinal axis 115 (shown only for transducer 90 for clarity). This principle was already discussed in relation to fig. 4 in which the combined ultrasound source 40 was rotatable along an axis 55. In relation to the same figure, the use of a plurality of combined ultrasound transducers was also already mentioned. Each transducer is arranged such that from it a compounded scan can be obtained with a compounded spatial resolution pattern that has improved isotropy in the elevation-azimuth plane as compared to prior art (see also the discussion in relation to fig. 5). Each transducer is mounted on the cup 110 at a location determined by the intersection of the surface of the hemispherical cup 110 that faces the volume to be scanned with a line defined by face normals of a regular polyhedron that is concentric with the imaginary sphere of which the cup 110 forms one half. For each of the ultrasound transducers 90, 95, 100, and 105 this line is indicated in fig. 6 by the arrows 120, 125, 130, and 135 respectively. These lines are mutually separated by angles of 120°. They also represent the central ultrasound beam emitted by each of the four transducers, which converge at the origin of the aforementioned imaginary sphere. Because of the planar symmetry of the point spread function associated with ultrasound transducer 95 relative to a plane perpendicular to arrow 125 (see also fig. 7), ultrasound transducer 95 can be comprised in the cup 110 with its general scanning direction facing the opening defined by the cup 110. Arranging a number of ultrasound transducers in accordance with face normals of a regular polyhedron introduces additional symmetry in the combined spatial resolution pattern obtained after compounding the scans made by the various ultrasound transducers as compared to a less regular arrangement of transducers. Fig. 7 schematically shows the overlapping spatial resolution patterns of four ultrasound transducers aligned using face normals of a regular tetrahedron. Fig. 7 elaborates on improving the quality of a reconstructed image as discussed in relation to fig. 1. An arrangement comprising four ultrasound transducers aligned using face normals of a regular tetrahedron was already discussed in relation to fig. 6. From each ultrasound transducer a compounded spatial resolution pattern is obtained that, starting from a rugby ball-like point spread function, looks more or less like a disk with rounded edges. This disk is obtained by combining several rugby ball-like point spread functions with different orientations. Fig. 7a essentially shows the arrangement already shown in fig. 6. In fig. 7b the compounded spatial resolution patterns obtained from ultrasound transducers 90, 95, 100, and 105, that is the discs, have been indicated by the numbers 140, 145, 150, and 155 respectively. When scans from all four transducers are additionally registered and compounded, for instance, by averaging, the resulting overall spatial resolution pattern will require additional symmetry and become substantially isotropic in three dimensions, as simulated in fig. 7d. The central region in this figure where all spatial resolution patterns overlap is the region that is reinforced by spatial compounding, and the fainter, outer regions are suppressed. The net result is a composite spatial resolution pattern that is highly symmetric and substantially isotropic in all three dimensions and whose size approaches the axial resolution of the point spread function of the original anisotropic point spread function (see also fig. 3). This results in a significant improvement in spatial resolution within at least a part of the volume that scanned where the images from all four transducers overlap. In this overlap region there is also a profound reduction of speckle noise, in this case on the other of the factor of Vl 6, assuming that a total of four volume scans from each of the four transducers were compounded to form the composite space resolution pattern. Fig. 8 schematically shows the overlap of the fields of view of four ultrasound transducers aligned using face normals of a regular tetrahedron. Fig. 8 elaborates on improving the quality of a reconstructed image as discussed in relation to fig. 1. An arrangement comprising four ultrasound transducers aligned using face normals of a regular tetrahedron was already discussed in relation to fig. 5. Assuming that each transducer will scan a pyramid- like field of view having an apex measuring 130°x 130° in the azimuth and elevation dimensions, the scans from the four transducers will substantially overlap inside the cup 110. The maximum beneficial effect from the invention will occur in the central region 160 shown in figs. 7a and 7b. There is an additional region in which three out of four scans will overlap, shown as region 165 in figs. 7c and 7d. The compounded spatial resolution pattern in region 165 will be less symmetric and less isotropic than in the central region 160 and speckle will be reduced less because only 12 of the assumed 16 (see the discussion relation to fig. 7) scans will overlap. However, the effects will still be beneficial. Most of the remaining volume of the cup 110 will be covered by overlapping scans of two transducers, where the effect of the invention will be further reduced, but still worthwhile. It will be obvious to one skilled in the art that the same benefits of this embodiment could be achieved by using mechanical 3-D ultrasound transducers, which rotate or pivot a ID linear, curvilinear, or phased array transducer to scan a volume or with a '2D' matrix linear, matrix curvilinear, or matrix phased array transducer. Also, the cup 110 does not have to be hemispherical, but could have an arbitrary shape. It will also be recognized that other numbers of transducers in different geometric arrangements are also possible according to this invention and would give beneficial effect similar to those described above. For example, instead of four transducers arranged in a tetrahedral pattern, three test users could be arranged in a cubic pattern, where the transducers are lined with three face normals that are mutually separated by 90° angles. This arrangement would also produce a compounded spatial resolution pattern with symmetry and isotropy in three dimensions. Likewise, the number of transducers can be increased and they can be arranged in accordance with the patterns defined by face normals of regular polyhedrons and according to the method of the invention create a compounded space resolution pattern with symmetry and isotropy in three dimensions. Reducing the number of transducers to only two, which could be mounted, for instance, at right angles to each other, would provide improvement, but would not have the desirable three-dimensional symmetry and isotropy of the tetrahedral arrangement described above. This invention may optionally be combined with means for coal acquisition and/or post acquisition image registration and fusion of diffuse optical tomography and/or photoacoustic measurements within the cup. This invention may optionally be combined with means for court requisition and/or post acquisition image registration and fusion with MRI, CT, x-ray, or other imaging modalities. Finally, it will be obvious that this invention can be extended to form high-resolution three-dimensional ultrasound images of various human body parts, including a female breast, and extremities, small animals, or any animate or inanimate object as deemed useful.

Fig. 9 schematically shows an embodiment of a medical image acquisition system comprising an ultrasound device according to the invention. Fig. 9 elaborates on improving the quality of a reconstructed image as discussed in relation to fig. 1. The medical image acquisition system 250 comprises the elements of the system 170 shown in fig. 1 as indicated by the dashed square. Additionally, the medical image acquisition system 250 further comprises a screen 255 for displaying a reconstructed image of an interior of the turbid medium 185 and an operator interface 260, such as a keyboard, allowing an operator to interact with a the medical image acquisition system 250. Ultrasound images and diffuse optical tomography images may be displayed independently of one another. Alternatively, a combined image based on both ultrasound signals received by the ultrasound transducer 175 and on light detected by the photodetector unit 215 may be displayed.

Fig. 10 schematically shows an embodiment of a method for making an ultrasound scan of a volume using 3-D spatial compounding. Fig. 10 elaborates on improving the quality of a reconstructed image as discussed in relation to fig. 1. First in the method 263, in step 265, a first scan of at least a first part of the volume is made. This first scan is obtained by at least receiving ultrasound signals from the volume using a first-scan ultrasound transducer, with the first scan being made with a first-scan spatial resolution pattern and with the first-scan ultrasound transducer being arranged for producing first-scan image signals based on the ultrasound signals received by the first-scan ultrasound transducer. According to the invention the method further comprises step 270 in which a further scan is made of at least a further part of the volume by at least receiving ultrasound signals from the volume using at least one further-scan ultrasound transducer, with the further part of the volume at least partially overlapping with the first part of the volume, with the further scan being made with a further-scan spatial resolution pattern, with the further-scan spatial resolution pattern being different from the first-scan spatial resolution pattern, and with the further-scan ultrasound transducer being arranged for producing further-scan image signals based on the ultrasound signals received by the further-scan ultrasound transducer. The further scan may be obtained by rotating an ultrasound transducer as was, for instance, for instance described in relation to fig. 4. Another way of obtaining the further scan is to use a plurality of ultrasound transducers that are oriented differently with respect to the common volume that is scanned by all transducers. Still another way of obtaining the further scan is to use a plurality of rotatable ultrasound transducers. Through the rotation of each individual ultrasound transducer and acquiring a volume scan at a number of rotational positions, the isotropy of the spatial resolution pattern of the compounded scan that results from compounding the number of scans can be improved in a plane perpendicular to the rotation axis as compared to the isotropy of the spatial resolution pattern of each individual scan. By having multiple ultrasound transducers of which the rotational axes lie in spatially independent directions and combining scans obtained from these ultrasound transducers, a compounded spatial resolution pattern can be obtained with improved isotropy in the direction of the rotational axes. Next, in step 275, and still according to the invention, the first scan and the at least one further scan are 3-D spatially compounded to obtain the compounded scan, as discussed above, based on the first-scan image signals and the further- scan image signals using a 3-D spatial compounding unit. In step 280 a further modality scan is made of at least a third part of the volume, with the third part of the volume at least partially overlapping with the first and a further part of the volume and with a further modality scan being made with a further imaging modality. This further imaging modality may, for instance, be chosen from the group comprising: diffuse optical tomography imaging, MRI, CT, and x-ray. Interpretation of data obtained from the further imaging modality benefits from the availability of improved ultrasound data. A further imaging modality may further benefit from the invention if an image of an interior of the volume is reconstructed based on both the further imaging modality and ultrasound signals received by at least one ultrasound transducer using a multimodality image reconstruction unit according to the invention. Ultrasound data may, for instance, be used to enhance the resolution of a diffuse optical tomography image in which, because of the technology used, the resolution is typically limited. Moreover, diffuse optical tomography is suitable for the functional imaging of, for instance, human tissue. Diffuse optical tomography may, for instance, be used to determine the oxygenation in human tissue such as a female breast. Similarly, other imaging technologies, such as MRI, CT, and x-ray, also have their own particular strengths.

Ultrasound technology, in its turn of , is more suitable to image structures than diffuse optical tomography. By basing the reconstruction of an image on both imaging modalities, the strengths of both technologies can be combined. In fig. 10 this is done in step 285.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the system claims enumerating several means, several of these means can be embodied by one and the same item of computer readable software or hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A device (170) for imaging an interior of an optically turbid medium (185) comprising:

(a) a diffuse optical tomography unit, with the diffuse optical tomography unit comprising: - a light source (195) for generating light to be coupled into the turbid medium

(185);

- a photodetector unit (215) for detecting light emanating from the turbid medium (185);

(b) an ultrasound unit, with the ultrasound unit comprising: - at least one ultrasound source (175) for generating ultrasound waves to be coupled into the turbid medium (185);

- an ultrasound detector unit (175) for detecting ultrasound waves emanating from the turbid medium (185); and with the device (170) further comprising: - a boundary (190) for bounding a receiving volume (180) for accommodating the turbid medium (185), with the boundary (190) comprising:

- at least one entrance position for light (205) for coupling light from the light source (195) into the receiving volume (180);

- at least one exit position for light (210) for coupling light emanating from the receiving volume (180) to the photodetector unit (215), with the light emanating from the receiving volume turbid medium (180) as a result of coupling light from the light source (195) into the receiving volume (180);

- at least one entrance position for ultrasound for coupling ultrasound waves from the ultrasound source (175) to the receiving volume (180); - at least one exit position for ultrasound for coupling ultrasound waves emanating from the receiving volume (180) to the ultrasound detector unit (175), with the ultrasound waves emanating from the receiving volume (180) as a result of coupling ultrasound waves from the ultrasound source (175) into the receiving volume (180), with the at least one entrance position for light (205), the at least one exit position for light (210), the at least one entrance position for ultrasound, and the at least one exit position for ultrasound being arranged to have a fixed geometry relative to the receiving volume (180) during a scan of the turbid medium (185).

2. A device (170) as claimed in claim 1, wherein the boundary (190) substantially forms a cup having an open side enabling at least a part of the turbid medium (185) to enter the receiving volume (180).

3. A device (170) as claimed in claim 1, wherein the boundary (190) comprises compression surfaces for compressing the turbid medium (185).

4. A device (170) as claimed in claim 2, wherein the ultrasound unit comprises a single ultrasound source (175), with the ultrasound source (175) being comprised in the boundary (190), and with the ultrasound source (175) at least partially facing the open side.

5. A device (170) as claimed in claims 1-3, wherein the ultrasound unit comprises a plurality of ultrasound sources, with the plurality of ultrasound sources being comprised in the boundary (190).

6. A device (170) as claimed in claims 1-5, wherein the device (170) further comprises a multimodality image reconstruction unit (230) for reconstructing a combined image of an interior of the turbid medium (185) based both on detected light and detected ultrasound waves.

7. A medical image acquisition device (190) for imaging an interior of an optically turbid medium (185), wherein the medical image acquisition device (190) comprises a device (170) according to any one of claims 1-6.

8. A method for imaging an interior of an optically turbid medium (185) comprising the following steps: coupling light from a light source (195) into a receiving volume (180) for accommodating the turbid medium (185) using an entrance position for light (205) successively chosen from a plurality of entrance positions for light comprised in a boundary (190) bounding the receiving volume (180), with the plurality of entrance positions for light (205) being arranged to have a fixed geometry relative to the receiving volume (180) during a scan of the turbid medium (185); coupling light emanating from the receiving volume (180) to a photodetector unit (215), with the light emanating from the receiving volume (180) as a result of coupling light from the light source (195) into the receiving volume (180) using a plurality of exit positions for light (210) comprised in the boundary (190), with the plurality of exit positions for light (210) being arranged to have a fixed geometry relative to the receiving volume (180) during a scan of the turbid medium (185); coupling omnidirectional ultrasound waves from an ultrasound source (175) into the receiving volume (180) using an entrance position for ultrasound successively chosen from a plurality of entrance positions for ultrasound comprised in the boundary (190), with the plurality of entrance positions for ultrasound being arranged to have a fixed geometry relative to the receiving volume (180) during a scan of the turbid medium (185); coupling ultrasound waves emanating from the receiving volume (180) to an ultrasound detector unit (175), with the ultrasound waves emanating from the receiving volume (180) as a result of coupling ultrasound waves from the ultrasound source (175) into the receiving volume (180) using a plurality of exit positions for ultrasound comprised in the boundary (190), with the plurality of exit positions for ultrasound being arranged to have a fixed geometry relative to the receiving volume (180) during a scan of the turbid medium (185).

9. A method for imaging an interior of an optically turbid medium (185) comprising the following steps: coupling light from a light source (195) into a receiving volume (180) for accommodating the turbid medium (185) using an entrance position for light (205) successively chosen from a plurality of entrance positions for light comprised in a boundary (190) bounding the receiving volume (180), with the plurality of entrance positions for light (205) being arranged to have a fixed geometry relative to the receiving volume (180) during a scan of the turbid medium (185); - coupling light emanating from the receiving volume (180) to a photodetector unit (215), with the light emanating from the receiving volume (180) as a result of coupling light from the light source (195) into the receiving volume (180) using a plurality of exit positions for light (210) comprised in the boundary (190), with the plurality of exit positions for light (210) being arranged to have a fixed geometry relative to the receiving volume (180) during a scan of the turbid medium (185); coupling ultrasound waves from an ultrasound source (175) into the receiving volume (180) along a well-defined path, using an entrance position for ultrasound chosen from a plurality of entrance positions for ultrasound comprised in the boundary (190), with the plurality of entrance positions for ultrasound being arranged to have a fixed geometry relative to the receiving volume (180) during a scan of the turbid medium (185); coupling ultrasound waves emanating from the receiving volume (180) along the well-defined path in the direction of the entrance position for ultrasound to an ultrasound detector unit (175).

10. A method as claimed in claim 9, wherein the direction of the well-defined path is steerable.

11. A method for imaging an interior of an optically turbid medium (185) comprising the following steps: coupling light from a light source (195) into a receiving volume (180) for accommodating the turbid medium (185) using an entrance position for light (205) successively chosen from a plurality of entrance positions for light comprised in a boundary (190) bounding the receiving volume (180), with the plurality of entrance positions for light (205) being arranged to have a fixed geometry relative to the receiving volume (180) during a scan of the turbid medium (185); coupling light emanating from the receiving volume (180) to a photodetector unit (215), with the light emanating from the receiving volume (180) as a result of coupling light from the light source (195) into the receiving volume (180) using a plurality of exit positions for light (210) comprised in the boundary (190), with the plurality of exit positions for light (210) being arranged to have a fixed geometry relative to the receiving volume (180) during a scan of the turbid medium (185); coupling ultrasound waves from an ultrasound source (175) into the receiving volume (180) along a first well-defined path, using an entrance position for ultrasound chosen from a plurality of entrance positions for ultrasound comprised in the boundary (190), with the plurality of entrance positions for ultrasound being arranged to have a fixed geometry relative to the receiving volume (180) during a scan of the turbid medium (185); coupling ultrasound waves emanating from the receiving volume (180) along a further well-defined path to an ultrasound detector unit (175), with the first well-defined path and the further well-defined path partially overlapping

12. A method as claimed in claim 11, wherein the direction of at least one of the well-defined path and the further well-defined path is steerable.

13. A method as claimed in claims 9-12, wherein the step of coupling ultrasound waves from an ultrasound source (175) into the receiving volume (180) and the step of coupling ultrasound waves emanating from the receiving volume (180) to an ultrasound detector unit (175) are carried out at a plurality of positions comprised in the boundary (190) simultaneously.