WO2009019653A2

WO2009019653A2 - Imaging system comprising an optical two-way switch

Info

Publication number: WO2009019653A2
Application number: PCT/IB2008/053143
Authority: WO
Inventors: Michael C. Van Beek; Martinus B. Van Der Mark; Levinus P. Bakker; Marjolein Van Der Voort
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-08-09
Filing date: 2008-08-05
Publication date: 2009-02-12
Also published as: WO2009019653A3

Abstract

The invention relates to an optical imaging method for imaging an object comprising the following steps: - coupling light from a light source to an illumination position for illuminating the object, with the illumination position being selected from a plurality of illumination positions for illuminating the object using an optical switch, with the optical switch being optically coupled to the light source and to the plurality of illumination positions; - collecting light emanating from the object at least one collection position, with the light emanating from the object as a result of coupling light from the light source to the selected illumination position for illuminating the object. According to the invention the method is amended in that the method further comprises the following step: - coupling light collected at a collection position through the optical switch. The method allows to couple the process of receiving light from the object to be imaged to the process of coupling light from the light source to the object to be imaged. The invention also relates to an optical switch for use in the method.

Description

Optical two-way switch

FIELD OF THE INVENTION

The invention relates to an optical imaging method for imaging a volume comprising the following steps:

(a) coupling light from a light source to an illumination position for illuminating the volume, the illumination position being selected from a plurality of illumination positions for illuminating the volume using an optical switch, the optical switch being optically coupled to the light source and to the plurality of illumination positions;

(b) collecting light emanating from the volume at at least one collection position, the light emanating from the volume as a result of coupling light from the light source to the selected illumination position.

The invention also relates to an imaging system comprising: a light source for generating light to be coupled into a volume to be imaged; a plurality of illumination positions for coupling light from the light source into the volume; - at least one collection position for collecting light emanating from the volume as a result of coupling light from the light source into the volume; a photodetector unit for detecting light emanating from the volume; an optical switch, the optical switch being optically coupled to the light source and the plurality of illumination positions for coupling light from the light source to at least one illumination position selected from the plurality of illumination position. The invention also relates to an optical switch comprising: a source position for coupling light from a light source to at least one receiving position selected from a plurality of receiving positions; the plurality of receiving position.

BACKGROUND OF THE INVENTION

An embodiment of the known method, imaging system, and optical switch is known from US patent 6,327,488 Bl. Light from a light source is coupled into a volume to be imaged. The volume is bounded by a wall comprising a plurality of illumination positions for coupling light from the light source into the volume so that the volume can be illuminated with light from the light source. Using an optical switch, the light source is coupled to an illumination position selected from the plurality of illumination positions. The optical switch comprises a source position optically coupled to the light source and a plurality of receiving positions optically coupled to the plurality of illumination positions. By optically coupling the source position to a receiving position selected from the plurality of receiving positions, light from the light source is coupled to an illumination position selected from the plurality of illumination positions. Light emanating from the volume as a result of coupling light from the light source into the volume is collected at a plurality of collection positions, the plurality of collection positions being comprised in the wall bounding the volume. From the collection positions, collected light is coupled to a photodetector unit for detecting the collected light. Based on the detected light, an image of the volume is reconstructed.

SUMMARY OF THE INVENTION It is an object of the invention to provide a method for making more efficient use of the optical switch. According to the invention of this object is realised in that the method further comprises the following step: (c) coupling light collected at the collection position to the optical switch.

The invention is based on the recognition that coupling light collected at a collection position to the optical switch makes it possible to couple the process of collecting light from the volume to be imaged to the process of coupling light from the light source to the volume to be imaged. The process of collecting light from the volume may be coupled to the process of coupling light from the light source to the volume in various ways. One such way is that light collected at a specific collection position depends, for instance with respect to its intensity, on which specific illumination position is selected from the plurality of illumination positions. After all, the farther a specific collection position is located from a selected illumination position, the lower the intensity of light collected at the collection position will generally be. Another way is that, once an illumination position has been selected from the plurality of illumination positions, a non-selected illumination position may be used as a collection position. After all, in a system as described in the prior art, the illumination positions and the collection positions are merely optical couplings capable of coupling light both in a direction towards the volume to be imaged and in a direction away from the volume (one direction at a time). In both ways (which will be further elucidated in the embodiments) collecting light from the volume depends on the selection of an illumination position from the plurality of illumination positions. Hence, there is a relation between collecting light and selecting an illumination position. In selecting an illumination position from the plurality of illumination positions use is made of the optical switch. By coupling light collected at a collection position into the optical switch, the relation between collecting light from the volume to be imaged and selecting an illumination position can be used. This makes use of the optical switch more efficient compared to the prior art.

An embodiment of the method according to the invention, wherein in step (c) at least one non-selected illumination position is used as a collection position. This embodiment has the advantage that it allows to increase the number of light paths through the volume to be imaged, a light path being the path light takes between pairs of illumination and collection positions. Similarly, this embodiment has the advantage that it allows to reduce the overall number of illumination and collection positions, while maintaining the same number of light paths that can be set up through the volume to be imaged as compared to the prior art. For reconstruction of an image of an interior of the turbid medium it is advantageous to have a large number of light paths through the turbid medium. A large number of light paths can be obtained by having a plurality of illumination positions comprising a large number of illumination positions and a plurality of collection positions comprising a large number of collection positions. However, in a system as described in the prior art this would also increase the number of light guides used, which is undesirable from a manufacturability and ease-of-use point of view. When light is coupled from the switch to at least one illumination position selected from the plurality of illumination positions in a system as described above, non-selected illumination positions are left unused. These non-selected illumination positions can be used as collection positions. In this way the number of light paths can be increased while keeping the total number of illumination and collection positions constant. This increase can also be quantified as will now be shown. Suppose that in the system described above the plurality of illumination positions comprises n/2 illumination positions and that the plurality of collection positions comprises n/2 collection positions. The total number of illumination positions and collection positions then equals n and the number of light paths that can be set up then equals (n/2) x (n/2) = n²/4. If, however, according to the invention each illumination position can also be used as a collection position and vice versa, the number of light paths equals n (n-l)/2 = 1/2 (n²-n). Here, the factor 1/2 corrects for the fact that the situation in which a first, now combined, illumination/collection position is used as an illumination position and a second illumination/collection position as a collection position defines the same light path as the situation in which the second position is used as an illumination position and the first position as a collection position. It is noted that, compared to a situation according to the prior art in which there are two illumination positions into collection positions, the number of light paths increases by a factor 1.5 from four to six. For a larger number of illumination and collection positions the difference in the number of light paths approaches a factor of two.

Alternatively, use of a method according to this embodiment allows to reduce the total number of illumination and collection positions, while maintaining the same number of light paths that can be set up as compared to the prior art.

A further embodiment of the method according to the invention, wherein in step (c) light collected at the collection position is acted upon by an optical element, the position of the optical element in the optical switch depending on which illumination position is selected from the plurality of illumination positions. This embodiment has the advantage that it allows to detect collected light taking into account a possible relationship between a collection position optically coupled to the optical switch and an illumination position also optically coupled to the optical switch. Generally, the intensity of light collected at a specific collection position will decrease as the distance between the collection position and an illumination position increases. By making the position of an optical element, such as an optical filter, dependent on which illumination position is selected to be optically coupled to the source position, light collected at a collection position closer to an illumination position may be filtered stronger than light collected at a collection position lying farther from the illumination position.

The object of the invention is also realised with an imaging system comprising: a light source for generating light to be coupled into a volume to be imaged; - a plurality of illumination positions for coupling light from the light source into the volume; at least one collection position for collecting light emanating from the volume as result of coupling light from the light source into the volume; a photodetector unit for detecting light emanating from the volume; - an optical switch, the optical switch being:

(a) optically coupled to the light source and the plurality of illumination positions for coupling light from the light source to at least one illumination position selected from the plurality of illumination positions; (b) optically coupled to the collection position and the photodetector unit for coupling light emanating from the volume to the photodetector unit.

An imaging system would benefit from the method and optical switch according to any one of the embodiments. An embodiment of the imaging system according to the invention, wherein the collection position is a non-selected illumination position. This embodiment has the advantage that it allows the number of illumination positions to be reduced as compared to the prior art while maintaining the quality of a reconstructed image, or to increase the quality of a reconstructed image while keeping the number of illumination positions equal to that used in the prior art.

A further embodiment of the imaging system according to the invention, wherein the imaging system is a medical image acquisition system. This embodiment has the advantage that it allows the imaging system according to the invention to be used for a medical application. The object of the invention is also realised with an optical switch comprising: a source position for coupling light from a light source to at least one receiving position selected from a first plurality of receiving positions; the first plurality of receiving positions; at least one drain position for receiving light from a further light source from a receiving position selected from a second plurality of receiving positions; the second plurality of receiving positions.

An embodiment of the optical switch according to the invention, wherein: the source position has a numerical aperture that is smaller than or equal to the numerical aperture of any of the receiving positions comprised in the first plurality of receiving positions; the drain position has a numerical aperture that is equal to or larger than the numerical aperture any of the receiving positions comprised in the second plurality of receiving positions.

This embodiment has the advantage that a receiving position comprised in the first plurality of receiving positions having a numerical aperture equal to or larger than the numerical aperture of the source position improves the optical coupling of the receiving position and the source position as compared to the situation in which the numerical aperture of the receiving position would be smaller than the numerical aperture of the source position. The latter situation has a greater chance than the situation according to the embodiment of optical losses occurring when light is coupled from the source position to the receiving position. Similarly, a drain position having a numerical aperture equal to or larger than the numerical aperture of a receiving position comprised in the second plurality of receiving positions improves the optical coupling of the drain position and the receiving position as compared to a situation in which the numerical aperture of the drain position would be smaller than that of a receiving position comprised in the second plurality of receiving positions. The relation between the various numerical apertures mentioned in this embodiment is also indicative of the way in which light is supposed to go through the optical switch. In general light travels in the direction in which the numerical apertures of various positions increase or stay constant. Having light travel through the optical switch along a path along which the numerical apertures of various positions decrease would generally lead to optical losses occurring when coupling light from a position having a larger numerical aperture to a position having a smaller numerical aperture. Moreover, such a situation would hamper the reproducibility of coupling light from one position to another. Coupling light from a position with a smaller numerical aperture to a position with a larger numerical aperture provides margin as to the relative alignment of the two positions, because the larger numerical aperture can accommodate some misalignment of the position with the smaller numerical aperture relative to the position with the position with the larger numerical aperture. Despite a limited amount of misalignment or light from the position with the smaller numerical aperture will be coupled to the position with the larger numerical aperture.

A further embodiment of the optical switch according to the invention, wherein the switch further comprises an optical element in the light path of light passing a drain position, the position of the optical element in the switch depending on which receiving position is selected from the first plurality of receiving positions. This embodiment has the advantage that the optical switch forms a convenient platform for having an optical element act upon light travelling in the direction from the source position towards a receiving position comprised in the first plurality of receiving positions or in the direction from a receiving position comprised in the second plurality of receiving positions towards a drain position. Examples of an optical element are: an optical fibre, a filter, a lens, a mirror, a polariser, a grating, a stop, a diffuser, etc.

A further embodiment of the optical switch according to the invention, wherein the optical element is an optical filter. This embodiment has the advantage that an optical filter may be used, for instance, to separate excitation light generated by a light source optically coupled to the optical switch from fluorescence light collected from a fluorescent agent at a collection position optically coupled to the optical switch and having been generated in response to the excitation light acting on the fluorescent agent.

A further embodiment of the optical switch according to the invention, wherein the second plurality of receiving positions is a subset of the first plurality of receiving positions. This embodiment has the advantage that it allows the total number of receiving positions to be reduced as compared to the situation in which there are two separate pluralities of receiving positions. Consequently, the number of illumination positions optically coupled to the optical switch may be reduced, while maintaining the quality of a reconstructed image obtained using the optical switch. Similarly, the total number of receiving positions may be maintained, allowing the quality of a reconstructed image obtained using the optical switch to be increased, while maintaining the number of illumination positions optically coupled to the optical switch.

A further embodiment of the optical switch according to the invention, wherein the second plurality of receiving positions and the first plurality of receiving positions coincide with each other. This embodiment has the advantage that all receiving positions can be used to couple light from the source position to a receiving position or from a receiving position to a drain position.

A further embodiment of the optical switch according to the invention, wherein the switch comprises: - a source segment comprising:

(a) the source position;

(b) the drain positions; a receiving segment comprising:

(c) the first plurality of receiving positions; (d) the second plurality of receiving positions, the source segment and the receiving segment being arranged to be optically coupled to each other and being movable relative to each other.

This embodiment has the advantage that enables an easy way of coupling the source position to a receiving position selected from the first plurality of receiving positions and a drain position to a receiving position selected from the second plurality of receiving positions. Coupling can be achieved, for instance, by placing the source segment and the receiving segment into contact with each other at an interface, the interface comprising the source position, the drain position, and both pluralities of receiving positions, and having the various positions translate relative to each other along the interface. A further embodiment of the optical switch according to the invention, wherein the source segment and the receiving segment are rotatable relative to each other. This embodiment has the advantage that an optical switch comprising segments that are rotatable relative to each other can be made more compact as compared to an optical switch in which the segments translate relative to each other. Consequently, an optical switch according to this embodiment is easier to use.

A further embodiment of the optical switch according to the invention, wherein the source segment comprises an optical element arranged to act on light passing a drain position comprised in the source segment. This embodiment has the advantage that the source segment forms a convenient platform for locating an optical element. Moreover, an optical element comprised in the source segment has a fixed position relative to the source position comprised in the source segment. This is beneficial if, for instance, neighbouring receiving positions on the optical switch are coupled to neighbouring illumination/collection positions on, for instance, a boundary bounding a volume to be imaged (here reference is made to the international patent application PCT/IB2006/054061 in which mapping the position of illumination positions on, for instance, a boundary bounding a volume to be imaged to the position of receiving positions on an optical switch is explained in detail). Under this condition it can be expected that a drain position closer to the source position will receive more light than the drain position farther from the source position. Consequently, starting from the source position and going to a drain position that is increasingly farther away from the source position, an increasingly weak attenuation filter may be used in order to bring the intensity of light received at all drain positions within a predefined range. This in turn limits the dynamic range requirements put on a photodetector unit for detecting light that has been filtered as described above. A further embodiment of the optical switch according to the invention, wherein the switch further comprises an exit segment, the exit segment comprising an exit position arranged to be optically coupled to a receiving position comprised in the second plurality of receiving positions through a drain position; the exit segment and the receiving segment have a fixed geometry relative to each other; the exit segment and the source segment are movable relative to each other. This embodiment has the advantage that light guides optically coupled to the exit segment and the receiving segment need not follow the movements of the source segment, thus avoiding unnecessary bending and associated strain occurring in light guides, such as optical fibres.

A further embodiment of the optical switch according to the invention, wherein the exit segment and the source segment are rotatable relative to each other. This embodiment has the advantage that an optical switch comprising segments that are rotatable relative to each other can be made more compact as compared to an optical switching in which the segments translate relative to each other. Consequently, an optical switch according to this embodiment is easier to use.

A further embodiment of the optical switch according to the invention, wherein the switch further comprises an optical element segment, the optical element segment comprising an optical element arranged to act on light passing a drain position, the optical element segment and the optical element segment and the source segment being movable relative to each other. This embodiment has the advantage that an optical element segment enables the use of an optical element without the optical element being permanently coupled to a specific position relative to the source position comprised in the optical switch.

A further embodiment of the optical switch according to the invention, wherein optical element segment and the source segment are rotatable relative to each other. This embodiment has the advantage that an optical switch comprising segments that are rotatable relative to each other can be made more compact as compared to an optical switch in which the segments translate relative to each other. Moreover, being able to rotate the optical element segment and the source segment relative to each other allows an optical element to be easily optically coupled to the source position or a drain position.

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 an optical imaging method for imaging a volume according to the invention;

Fig. 2 schematically shows an embodiment of an imaging system for imaging an interior of a volume according to the invention;

Fig. 3 schematically shows an embodiment of a medical image acquisition system according to the invention;

Fig. 4 schematically shows an embodiment of an optical switch comprising a dynamic attenuator; Fig. 5 schematically shows an embodiment of an optical switch comprising a moving detector;

Fig. 6 schematically shows an embodiment of an optical switch according to the invention comprising a source segment and a receiving segment; Fig. 7 schematically shows a further embodiment of an optical switch according to the invention comprising an exit segment;

Fig. 8 schematically shows a further embodiment of an optical switch according to the invention;

Fig. 9 schematically shows a further embodiment of an optical switch according to the invention;

Fig. 10 schematically shows a further embodiment of an optical switch according to the invention comprising an optical element segment;

DETAILED DESCRIPTION OF EMBODIMENTS Fig. 1 schematically shows an embodiment of an optical imaging method for imaging a volume according to the invention. The method 5 first comprises a step 10 in which light from a light source is coupled to an illumination position for illuminating the volume, the illumination position being selected from a plurality of illumination positions for illuminating the volume using an optical switch, the optical switch being optically coupled to the light source into the plurality of illumination positions. When imaging, for instance, an interior of a female breast, the breast may be accommodated in a receiving volume. The receiving volume may be bounded by a cuplike receptacle the wall of which comprises the plurality of illumination positions. The plurality of illumination positions is optically coupled to an optical switch that selectively couples light from a light source (to which the optical switches also optically coupled) to an illumination position selected from the plurality of illumination positions. Next, in step 15, light emanating from the volume is collected at at least one collection position, the light emanating from the volume as a result of coupling light from the light source to the selected illumination position. In the above-mentioned example of imaging a female breast, the collection position may be comprised in the wall of the cuplike receptacle just as the plurality of illumination positions. In step 17, light collected at the collection position is then coupled to the optical switch. According to one embodiment of the invention, an illumination position not selected to illuminate the volume at a certain moment may be used as a collection position. This collection position is optically coupled to the optical switch. By using a non-selected illumination position for collecting light emanating from the volume to be imaged, the number of light paths through the volume that can be set up can be increased, while keeping the total number of (now combined) illumination/collection positions constant. Alternatively, the total number of combined illumination/collection positions and light guides that may be coupled to these positions can be decreased, while keeping the total number of light paths that can be set up through the volume constant. According to another embodiment of the invention that dedicated collection position is used, dedicated meaning that the collection position cannot be used as an illumination position. Fig. 2 schematically shows an embodiment of a system for imaging an interior of a volume suitable for the method shown in fig. 1. Fig. 3 schematically shows an embodiment of a medical image acquisition system suitable for the method shown in fig. 1. Fig. 2 schematically shows an embodiment of an imaging system for imaging an interior of a volume according to the invention. The system 170 comprises a light source 175 for generating light to be coupled into a receiving volume 180 for accommodating a turbid medium 185. The system 170 further comprises a receptacle 190 bounding the receiving volume 180. The receptacle 190 comprises a plurality of combined illumination/collection positions 195 for coupling light into and collecting light from the receiving volume 180 according to an embodiment of the invention. Light collected from the receiving volume 180 emanates from the receiving volume 180 as a result of coupling light from the light source 175 to an illumination position selected from the plurality of illumination/collection positions 195. To successively couple light from the light source 175 to at least one illumination/collection position selected from the plurality of illumination/collection positions 195 the system 170 comprises an optical switch 200 according to the invention. The coupling of light from the light source 175 to, in this example, one illumination/collection illumination position chosen from the plurality of illumination/collection positions 195 is indicated by the dashed line 205. According to the invention, the optical switch 200 is arranged such that non-selected illumination/collection positions from the plurality of illumination/collection positions 195 are used to collect light from the receiving volume 180. This has been indicated by the plurality of dashed lines 210. Light collected from the receiving volume 180 is coupled to a photodetector unit 215 for detection. Based on the detected light, image reconstruction unit 220, which is coupled to the photodetector unit 215 through coupling 221, is used to reconstruct an image of an interior of the receiving volume 180 comprising the turbid medium 185. Light guides 223 are used to optically couple the light source 175, the photodetector unit 215, and the plurality of illumination/collection positions 195 to the optical switch 200. In medical diagnostics, a system like the system 170 may be used to image, for instance, an interior of a female breast using diffuse optical tomography. Light from the light source 175 then has a wavelength that typically lies within the range of 400 nm to 1400 nm. In fig. 2 the receiving volume 180 is bound by a receptacle 190. However, this need not always be the case. Another embodiment of a device for imaging an interior of a turbid medium is that of a handheld device that may, for instance, be pressed against a side of a turbid medium. In that case, the receiving volume is the volume occupied by the part of the turbid medium from which light is detected as a result of irradiating the turbid medium. Moreover, instead of a boundary like the cuplike receptacle 190, the receiving volume 180 may be bound by compression surfaces (not shown in fig. 2) for compressing an object to be imaged. As is clear from fig. 2, designs of an imaging system like the system 170 are conceivable in which the light guides 223 are left out such that the plurality of receiving positions 203 comprising the optical switch 200 coincide with the plurality of illumination/collection positions 195 comprised in the receptacle 190. Fig. 3 schematically shows an embodiment of a medical image acquisition system according to the invention. The medical image acquisition system 225 comprises the elements comprised in the system 170 shown in fig. 2. In fig. 3 this is indicated by the dashed rectangle 230. In addition to the elements already comprised in the system 170, the medical image acquisition system 225 further comprises a screen 235 for displaying an image reconstructed using image reconstruction unit 220 and an operator interface 240, for instance, a keyboard allowing an operator to interact with the medical image acquisition system 225. Fig. 4 schematically shows an embodiment of an optical switch comprising a dynamic attenuator. The optical switch 400 comprises an optical fibre 405 having a first end 410 and a second end 415, the first end 410 being optically coupled to a light source (not shown in fig. 4) and the second end 415 acting as a source position for coupling light from the light source to a receiving position comprised in a first plurality of receiving positions 420. The first plurality of receiving positions 420 is comprised in a first ring 425, the first ring 425 and the second end 415 of the optical fibre being rotatable relative to each other. The optical switch 400 further comprises a second plurality of receiving positions 430 comprised in a second ring 435. The second plurality of receiving positions 430 is optically coupled to a plurality of dedicated collection positions (not shown in fig. 4). Dedicated means that the collection positions cannot be used as illumination positions. The fact that the collection positions are dedicated is reflected in the optical switch 400 in that the optical fibre 405 is optically coupled to a receiving position 440 comprised in the first plurality of receiving positions 420 and not to a receiving position comprised in the second plurality of receiving positions 430. The first plurality of receiving positions 420 is used to couple light from the light source to the plurality of illumination positions, whereas the second plurality of receiving positions 430 is used to couple light from the plurality of collection positions to a plurality of detector positions comprised in a photodetector unit (not shown in fig. 4) for detecting collected light. The optical switch 400 still further comprises a dynamic attenuator comprising optical filters 445 of different strengths, filters of different strengths being optically coupled to different receiving positions comprised in the second plurality of receiving positions 430. According to an embodiment of the invention, the position of a filter of a specific strength relative to a specific receiving position depends on which receiving position of the first plurality of receiving positions 420 is optically coupled to the source position 415. In an imaging system as shown in fig. 2 and fig. 3, diffuse optical tomography may be used as an imaging technology to image an interior of, for instance, a female breast. When imaging an interior of a female breast using diffuse optical tomography and a cuplike receptacle like the receptacle 190 shown in fig. 2 and fig. 3, the dynamic range of collected light easily spans 12 orders of magnitude. One of the causes of this large dynamic range is the fact that the distance between a collection position and the source position generally varies from collection position to collection position. This distance typically varies from approximately 8 mm to 130 mm. Generally, the intensity of light collected at a collection position closer to the source position will be higher than the intensity of light collected at a collection position farther from the source position. As a result of the large dynamic range, each detector comprised in the plurality of detectors need to be able to cope with the entire dynamic range. Usually this means that expensive detectors are required. If the distance between a collection position and the source position would not vary from collection position to collection position, the dynamic range per detector would be reduced significantly because collection positions close to the source position would only 'see' higher intensity light, whereas collection positions farther from the source position would only 'see' lower intensity light. The optical switch 400 shown in fig. 4 mimics the above-mentioned effect of reducing the dynamic range. A receiving position optically coupled to a collection position that is close to an illumination position optically coupled to the source position is optically coupled to a filter having a high attenuation factor. Other receiving positions optically coupled to collection positions that lie increasingly farther from the source position are optically coupled to filters having increasingly low attenuation factors. The dynamic attenuator comprising the different filters moved along with the rotation of the source position 415 and the first ring 425 relative to each other. Fig. 5 schematically shows an embodiment of an optical switch comprising a moving detector. Referring to the discussion of using diffuse optical tomography for imaging a female breast, or more general biological tissue, given in relation to fig. 4, the intensities of collected light may not only span a large dynamic range. The intensity of collected light and, hence, the strength of signals based on collected light may be very low, especially when the optical path through the tissue is long. Detecting very low intensities usually requires the use of expensive detectors. In a diffuse optical tomography imaging system as shown in fig. 2 and fig. 3 this may not be realistic as the plurality of collection positions and, hence, the plurality of detectors typically comprise tens or even hundreds of positions. Moreover, additional information about tissue properties can be obtained by detecting collected light in a time-resolve way. Time-resolved detection usually also requires use of expensive detectors making the use of tens or even hundreds of detectors undesirable. Just as in fig. 4, the optical switch 500 shown in fig. 5 comprises an optical fibre 505 comprising a first end 510 optically coupled to a light source such as the laser (not shown in fig. 5) and a second end 515 that acts as a source position. The optical switch 500 further comprises a ring 520 comprising a plurality of receiving positions 525. The plurality of receiving positions 525 is optically coupled to a plurality of illumination/collection positions (not shown in fig. 5) and can be used to communicate light both in a direction from the light source towards an illumination position and in a direction from a collection position towards the optical switch 500. The relative positions of the receiving positions 525 on the ring 520 reflect the relative positions of the collection positions. Here, the reader is referred to international patent application PCT/IB2006/054061. In this patent application correlating the relative positions of collection positions to that of receiving positions is discussed in detail. Further still, the optical switch 500 comprises an optical fibre 530 comprising a first end 535 optically coupled to one of the receiving positions comprised in the plurality of receiving positions 525 and a second end 540 optically coupled to a photodetector (not shown in fig. 5). Alternatively, the photodetector can be mounted directly on a receiving position, without the need for a connecting optical fibre 530. The photodetector may measure the intensity of collected light, but may also be a spectrometer or a phase sensitive detector. The first end 535 of the optical fibre 530 and the plurality of receiving positions 525 on the one hand and the second end 515 of the optical fibre 505 and the plurality of receiving positions 525 on the other hand are rotatable relative to each other as indicated by the dotted double headed arrow. As the relative positions of the receiving positions on the ring 520 reflect the relative positions of the collection positions, the relative position of the collection position optically coupled to the photodetector and the illumination position optically coupled to the source position 515 is fixed. This relative position can, for instance, be such that the optical path that the photodetector probes is always a long part, which generally results in small signal strengths. Fig. 6 schematically shows an embodiment of an optical switch according to the invention comprising a source segment and a receiving segment. The optical switch 20 comprises a source segment 25 and a receiving segment 30. The source segment 25 comprises a source position 35 and a drain position 40 (as such, the optical fibre 505 and the optical fibre 530 shown in fig. 5 together form a source segment). The receiving segment 30 comprises a plurality of receiving positions 45 (as such, the ring 425 in the ring 435 shown in fig. 4 together form a receiving segment) For instance in a system for imaging an interior of a turbid medium, the source position 35 may be coupled to a light source (not shown in fig. 6) using light guide 50. The source position 35 is coupled to receiving position 55, which was selected from the plurality of receiving positions 45. In a system for imaging an interior of an optically turbid medium such as biological tissue (such as the system shown in figs. 2 and 3), the selected receiving position 55 may be coupled to a receiving volume for accommodating the turbid medium (not shown in fig. 6) using light guide 60. Light guide 65 may then be used to couple light from the receiving volume to the non-selected receiving position 70. According to the invention, the non-selected receiving position 70 is used to couple light from the receiving volume to the drain position 40 comprised in the source segment 25. The receiving volume with light emanating from it then acts as a further light source in addition to the light source that was mentioned previously. Still following the example of a system for imaging an interior of a turbid medium, light guide 80 may then be used to couple the drain position 40 to a photodetector unit (not shown in fig. 6) for detecting light coupled out of the receiving volume. The source segment 25 and the receiving segment 30 can be rotated relative to each other along an axis 85. By rotating the source segment 25 and the receiving segment 30 relative to each other, the source position 35 is able to successively select a receiving position from the plurality of receiving positions 45. Assuming that the relative motion of the source segment 25 and the receiving segment 30 is such that the source position 35 moves from a location opposite the receiving position 55 to a location opposite a further receiving position (not shown in fig. 6), the drain position 40 moves with the source position 35 relative to the plurality of receiving positions 45 comprised in the receiving segment 30 in order to couple light from a further non-selected receiving position (not shown in fig. 6) to the photodetector unit. Similarly, a further drain position (not shown in fig. 6) moves into the position previously occupied by the source position 35. Light emanating from the receiving position 55, which, according to the invention, is now used to couple light out of instead of into the receiving volume, is coupled to the photodetector unit using this further drain position. The source segment 25 shown in fig. 6 and in figs. 6-9 may comprise an optical element such as an optical fibre, an optical filter, a lens, a mirror, a polariser, a grating, a stop, a diffuser, etc to act on light received from, for instance, a receiving volume (not shown in fig. 6) or on light to be coupled to the source position 35. An optical filter, for instance, is handy if, for instance, light emanating from the receiving volume comprises two components that can be separated through use of the optical filter. This is the case, for instance, if light emanating from the receiving volume comprises excitation light used to excite a fluorescent agent comprised in the receiving volume and the fluorescence light emanating from the agent as a result of irradiation of the agent with the excitation light. It should be realised that an optical element comprised in the source segment 25 and arranged to act on light received from, for instance, a receiving volume is not fixed in relation to a specific receiving position from the plurality of receiving positions 45 but to the source position 35. Thus, it is possible to use, for instance, neutral density filters with a high optical density in relation to drain positions that are closer to the source position and no filters, or filters with a lower optical density, in relation to drain positions that are farther from the source position. This reduces the required dynamic range of a photodetector unit that might be used to detect light coupled through the drain positions (see fig. 4). Another possibility is to couple optical elements, such as filters or a photodetector (see fig. 5), to some drain positions and not to couple optical elements to other drain positions. If the optical elements are fluorescence filters, simultaneous detection of fluorescence and non-fluorescence light becomes possible. Finally, it is noteworthy that the diameter of the light guide 50 is smaller than the diameters of the light guides 60, 65 and 80 which are all equal. Moreover, the numerical aperture at the source position 35 is smaller than the numerical aperture at the receiving position 55. In this way, optical losses that might occur when coupling light from the source position 35 to the receiving position 55 are reduced. Similarly, the numerical aperture at the drain position 40 might be chosen to be larger than the numerical aperture at the receiving position 70. As stated however, in fig. 6 of the numerical aperture at the drain position 40 equals that at the receiving position 70.

Fig. 7 schematically shows a further embodiment of an optical switch according to the invention comprising an exit element. The optical switch 90 comprises a number of elements already shown in fig. 6. As far as relevant, these elements are indicated in figs. 6-10 using the same numbers. However, according to an embodiment of an optical switch according to the invention, the optical switch 90 shown in fig. 7 further comprises an exit segment 90 comprising an exit position. In fact two exit positions, exit position 95 and exit position 100, are shown in fig. 7. In the situation shown in fig. 7, the drain position 40 is coupled to the exit position 95 using light guide 105. This exit position 95 may be coupled to, for instance, a photodetector unit (not shown in fig. 7) using light guide 110. The exit position 100 is currently not coupled to the receiving position 55 as the source position 35 is currently coupled to the receiving position 55. Exit position 100 may be coupled to, for instance, a photodetector unit (not shown in fig. 7) using light guide 115. In the embodiment shown in fig. 7, the source segment 25 can be rotated along the axis 85 relative to the receiving segment 30 and the exit segment 90. Receiving segment 30 and exit segment 90, however, have fixed positions relative to each other. In other words, the source segment 25 can be rotated 'between' the receiving segment 30 and the exit segment 90, the two of which remain stationary relative to each other. Similar to the explanation in relation to fig. 6, rotating the source segment 25 relative to the receiving segment 30 and the exit segment 90 enables the source position 35 to be successively coupled to a receiving position selected from the plurality of receiving positions 45. At the same time, rotating the source segment 25 changes a receiving position previously used to couple light into, for instance, a receiving volume into a position used to couple light out of the receiving volume and vice versa. This embodiment has the advantage that light guides 60 and 65 coupled to, for instance, a receiving volume (not shown in fig. 7) and light guides 110 and 115 coupled to, for instance, a photodetector unit (not shown in fig. 7) remain stationary and do not follow the rotation of the source segment 25. Consequently, operation of the switch 90 does not require the light guides to move so that they do not experience operation-induced strain. Light guides 50, 60, 65, 105, 110 and 115 may be optical fibres and coupling two optical fibres to each other may be accomplished by simply placing the fibres in close proximity. Light guides 60, 65, 105, 110, and 115 are used for communicating light received from, for instance, a receiving volume (not shown in fig. 7). Therefore, these light guides have a large diameter (for instance 1 mm) and a large numerical aperture (NA = 0.5). Light guide 50 is used to communicate light from, for instance, a light source (not shown in fig. 7). Therefore, a smaller diameter and a smaller numerical aperture (NA = 0.4) are sufficient. Consequently, it is possible to place the distal end of the light guide 50 in close proximity of the proximal end of light guide 60 or light guide 65 (depending on the position of the source segment 25 relative to the receiving segment 30) in such a way that the surface and the numerical aperture of light guide 60 or light guide 65 comprised the surface and the numerical aperture of the light guide 50. This way of optically coupling light guides is described in detail in US patent 6,327,488. A similar observation regarding the coupling of light guides goes for the situations shown in fig. 6 and figs. 8-10.

Fig. 8 schematically shows a further embodiment of an optical switch according to the invention. The optical switch 120 comprises a number of elements already shown in fig. 7. For clarity only elements relevant to the present discussion are numbered. For an explanation of unnumbered elements reference is made to fig. 6. However, whereas light guide 50 in fig. 7 is bent, light guide 125 in fig. 8 is not bent. The different diameters and numerical apertures of light guide 125 and light guide 60 ensure good optical coupling. Fig. 9 schematically shows a further embodiment of a switch according to the invention. The optical switch 130 comprises a number of elements already shown in fig. 7. For clarity only elements relevant to the present discussion are numbered. For an explanation of unnumbered elements references made to fig. 7. However, whereas the optical switch 90 in fig. 7 comprises a single-element light guide 50 for coupling light from, for instance, a light source to the source position 35, the optical switch 130 in fig. 9 comprises a light guide 135 and an optical system 140 (indicated by the dashed rectangle) for this purpose. The optical system 140 comprises mirror 145, mirror 150, and lens 155 for communicating light received by the light guide 135 to the source position 35. This embodiment has the advantage that no close proximity is required between the light guides and the rotatable source segment 25. Moreover, the optical switch 130 may comprise optical elements in a way similar to that shown in fig. 10.

Fig. 10 schematically shows a further embodiment of an optical switch according to the invention comprising an optical element segment. The optical switch 160 comprises a number of elements already shown in fig. 7. For clarity only elements relevant to the present discussion are numbered. For an explanation of unnumbered elements references made to fig. 7. According to an embodiment of an optical switch according to the invention, the optical switch 160 comprises an optical element segment 165. The optical elements segment 165 may comprise an optical element such as an optical fibre, an optical filter, a lens, a mirror, a polariser, a grating, a stop, a diffuser, etc to act on light received from, for instance, a receiving volume (not shown in fig. 10). In fig. 10 line 167 and line 169 schematically indicate two optical filters as an example of an optical element comprised in the optical element segment 165. If, the number of positions on the optical element segment 165 for comprising an optical element is double the number of exit positions, half of the positions on the optical element segment 165 can have filters, whereas the other half does not have filters. By rotating the optical element segment 165 along the axis 85 and using a photodetector unit (not shown in fig. 10), it is possible to obtain filtered and unfiltered measurements

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. 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. An optical imaging method (5) for imaging a volume (180) comprising the following steps:

(a) coupling light from a light source (175) to an illumination position for illuminating the volume (180), the illumination position being selected from a plurality of illumination positions (195) for illuminating the volume (180) using an optical switch (20, 90, 120, 130, 160, 200, 400, 500), the optical switch (20, 90, 120, 130, 160, 200, 400, 500) being optically coupled to the light source (175) and to the plurality of illumination positions (195);

(b) collecting light emanating from the volume (180) at at least one collection position (195), the light emanating from the volume (180) as a result of coupling light from the light source (175) to the selected illumination position;

(c) coupling light collected at the collection position ( 195) to the optical switch (20, 90, 120, 130, 160, 200, 400, 500).

2. A method (5) as claimed in claim 1, wherein in step (c) at least one non- selected illumination position (195) is used as a collection position (195).

3. A method (5) as claimed in claim 1, wherein in step (c) light collected at the collection position (195) is acted upon by an optical element (167, 169), the position of the optical element (167, 169) in the optical switch (20, 90, 120, 130, 160, 200, 400, 500) depending on which illumination position is selected from the plurality of illumination positions (195).

4. An imaging system (170, 225) comprising: - a light source (175) for generating light to be coupled into a volume (180) to be imaged; a plurality of illumination positions (195) for coupling light from the light source (175) into the volume (180); at least one collection position (195) for collecting light emanating from the volume (180) as a result of coupling light from the light source (175) into the volume (180); a photodetector unit (215) for detecting light emanating from the volume (180); an optical switch (20, 90, 120, 130, 160, 200, 400, 500), the optical switch (20, 90, 120, 130, 160, 200, 400, 500) being:

(a) optically coupled to the light source (175) and the plurality of illumination positions (195) for coupling light from the light source (175) to at least one illumination position selected from the plurality of illumination positions (195);

(b) optically coupled to the collection position (195) and the photodetector unit (215) for coupling light emanating from the volume ( 180) to the photodetector unit (215).

5. An imaging system (170, 225) as claimed in claim 4, wherein the collection position (195) is a non-selected illumination position.

6. An imaging system (170, 225) as claimed in claims 4-5, wherein the imaging system (170, 225) is a medical image acquisition system (225).

7. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) comprising: a source position (35, 415, 515) for coupling light from a light source (175) to at least one receiving position selected from a first plurality of receiving positions (45, 420, 525); the first plurality of receiving positions (45, 420, 525); at least one drain position (40, 535) for receiving light from a further light source from a receiving position selected from a second plurality of receiving positions (430, 525); the second plurality of receiving positions (430, 525).

8. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) as claimed in claim 7, wherein: - the source position (35, 415, 515) has a numerical aperture that is smaller than or equal to the numerical aperture of any of the receiving positions comprised in the first plurality of receiving positions (45, 420, 525); the drain position (40, 535) has a numerical aperture that is equal to or larger than the numerical aperture any of the receiving positions comprised in the second plurality of receiving positions (430, 525).

9. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) as claimed in claims 7-8, wherein the switch (20, 90, 120, 130, 160, 200, 400, 500) further comprises an optical element (445) in the light path of light passing a drain position (40, 535), the position of the optical element (445) in the switch (20, 90, 120, 130, 160, 200, 400, 500) depending on which receiving position is selected from the first plurality of receiving positions (45, 420, 535).

10. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) as claimed in claim 9, wherein the optical element (445) is an optical filter.

11. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) as claimed in claims 7-10, wherein the second plurality of receiving positions (430, 525) is a subset of the first plurality of receiving positions (45, 420, 525).

12. An optical switch (20, 90, 120, 30, 160, 200, 400, 500) as claimed in claim 11, wherein the second plurality of receiving positions (430, 525) and the first plurality of receiving positions (45, 420, 525) coincide with each other.

13. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) as claimed in claims 7-12, wherein the switch (20, 90, 120, 130, 160, 200, 400, 500) comprises: a source segment (25) comprising: (a) the source position (35);

(b) the at least one drain position (40); a receiving segment (30) comprising:

(c) the first plurality of receiving positions (45, 420, 525);

(d) the second plurality of receiving positions (430, 525), the source segment (25) and the receiving segment (30) being arranged to be optically coupled to each other and being movable relative to each other.

14. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) as claimed in claim

13, wherein the source segment (25) and the receiving segment (30) are rotatable relative to each other.

15. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) as claimed in claims

13-14, wherein the source segment (25) comprises an optical element (167, 169) arranged to act on light passing a drain position (40, 535) comprised in the source segment (25).

16. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) as claimed in claims 13- 15, wherein: the switch (20, 90, 120, 130, 160, 200) further comprises an exit segment (90), the exit segment (90) comprising an exit position (95, 100) arranged to be optically coupled to a receiving position comprised in the second plurality of receiving positions (430, 525) through a drain position (40); - the exit segment (90) and the receiving segment (30) have a fixed geometry relative to each other; the exit segment (90) and the source segment (35) are movable relative to each other.

17. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) as claimed in claim

16, wherein the exit segment (90) and the source segment (35) are rotatable relative to each other.

18. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) as claimed in claims 13-17, wherein the switch (20, 90, 120, 130, 160, 200) further comprises an optical element segment (165), the optical element segment (165) comprising an optical element arranged to act on light passing a drain position (40), the optical element segment (165) and the optical element segment (165) and the source segment (25) being movable relative to each other.

19. An optical switch (20, 90, 120, 130, 160, 200, 400, 500) as claimed in claim

18, wherein the optical element segment (165) and the source segment (25) are rotatable relative to each other.