The invention relates to a device for imaging an interior of a turbid medium, said device comprising a measurement volume for accommodating the turbid medium, said measurement volume comprising a number of sources capable of communicating light, said sources comprising a preferred source, capable of communicating preferred light and a further source, capable of communicating further light, said device further comprising a detection unit capable of detecting composed light comprising a preferred component comprising at least a part of the preferred light and a further component comprising at least a part of the further light. The term ‘light’ is understood to cover the entire electromagnetic spectrum.
The invention also relates to a medical image acquisition device comprising the device.
The invention also relates to a selection unit for coupling a light generator to the preferred source and for choosing the preferred source from the number of sources.
An embodiment of a device for imaging an interior of a turbid medium of this kind is known from U.S. Pat. No. 6,327,488 B1. The known device can be used for imaging an interior of a turbid medium, such as biological tissues. In medical diagnostics the device may be used for imaging an interior of a female breast. The measurement volume receives a turbid medium, such as a breast. The measurement volume may be bound by a holder having only one open side, with the open side being bound by an edge portion. This edge portion may be provided with an elastically deformable sealing ring. Such a holder is known from U.S. Pat. No. 6,480,281 B1. Light is applied to the turbid medium by communicating the light into the measurement volume via the preferred source, said preferred source being successively chosen from the number of sources. Light emanating from the measurement volume via further sources selected from the number of sources is detected by a detector unit and is used to derive an image of the interior of the turbid medium.
It is a drawback of the known device that the presence of the further component in the composed light hampers the detection of the preferred component that is also present in the composed light.
It is an object of the invention to counteract the effect said further component in the composed light may have on detecting the preferred component also present in the composed light. According to the invention this object is realized in that the preferred source and the further source are located such that a path followed by the preferred component from the preferred source to the detector unit and a path followed by the further component from the further source to the detector unit are substantially the same. The invention is based on the recognition that light following essentially the same path will be similarly affected by outside factors, such as attenuation. Consequently, if the intensities of light at two sources are in a certain proportion, this proportion will be maintained at the end of the paths, even if the light is attenuated. As a result of the above, paths of light should be chosen such that the proportion of intensities is maintained at the end of the paths, if the proportion of intensities is acceptable at the beginning of the paths. Analogously, paths of light should be chosen such that an acceptable proportion of intensities is obtained at the end of the paths, if the proportion of intensities is not acceptable at the beginning of the paths. Unacceptable proportions of intensities may arise if paths of light are not substantially the same, resulting in light following one path being attenuated stronger than light following another path. Consequently, the choosing of paths may involve relocating the beginnings of the original paths. An essential feature of the drawback of the known device is that at least a part of the preferred light and at least a part of the further light are detected as components of composed light. Therefore, there must be ways for at least a part of the preferred light and at least a part of the further light of arriving at the same position. In a device for imaging an interior of a turbid medium these ways include crosstalk between the paths taken by at least a part of the preferred light and by at least a part of the further light as well as the simultaneous application of light from multiple light sources to the turbid medium. With regard to the issue of crosstalk, the known device comprises a light source, a measurement volume for accommodating the turbid medium bound by a wall comprising a plurality of openings, a selection unit for coupling the light source to an opening in the wall bounding the measurement volume, said opening being successively chosen from the plurality of openings, a photodetector unit comprising multiple detector locations, and light guides for coupling the light source to the selection unit, the selection unit to openings in the wall bounding the measurement volume, and further openings in the wall bounding the measurement volume to the multiple detector locations in the photodetector unit. Light guides may be connected to each other using a connector unit comprising an entrance and an exit element for coupling a plurality of light guides simultaneously.
In locations where light guides are positioned in each other's neighborhood crosstalk can occur between light guides. In the known device these locations include the selection unit, the connector units, and the photodetector unit.
The selection unit is a potential source of crosstalk, because it couples a light guide coupled to the light source to a further light guide chosen from a plurality of further light guides coupled to openings in the wall bounding the measurement volume. As a number of the further light guides coupled to openings in the wall bounding the measurement volume are located in each other's neighborhood on the selection unit, there is the risk that at least a part of the light coming from the light source does not enter or stay in the chosen further light guide coupled to an opening in the wall bounding the measurement volume that is chosen from the plurality of further light guides, but enters another one of the further light guides that is in the neighborhood of the chosen further light guide. A connector unit is a potential source of crosstalk, because it comprises an entrance element comprising multiple light guides located in each other's neighborhood and an exit element comprising further multiple light guides, also located in each other's neighborhood, wherein light communicated by a light guide in the entrance element must be communicated to a light guide in the exit element that is located opposite the light guide in the entrance element. Analogous to the situation with the selection unit, at least a part of the light communicated by a light guide in the entrance element of a connector unit may be communicated to a light guide in the exit element of the connector unit that is not located opposite the light guide in the entrance element, but that is located in the neighborhood of the light guide in the exit element that is opposite the light guide in the entrance element.
The photodetector unit is a potential source of crosstalk, because it comprises a plurality of detector locations located in each other's neighborhood that are coupled to openings in the wall bounding the measurement volume using light guides. At least part of the light exiting a light guide that is coupled to a certain detector location may stray unto another detector location in the neighborhood of the first detector location.
As far as the detection of composed light is concerned with respect to the known device, the locations where light enters the measurement volume may be regarded as the sources of components of the composed detected light, as long as crosstalk occurs before light enters the measurement volume. In that case, there is a preferred source, communicating light directly from the light source, and at least one further source, communicating light that has undergone crosstalk. By choosing the location of the preferred source and the further source such that the paths of the preferred component and the further component that are detected as components of composed light at a single detector location are substantially the same, the presence of the further component in the composed light no longer hampers the proper detection of the preferred component. Light from the preferred source and the at least one further source will experience essentially the same attenuation by the turbid medium, thus maintaining the initial proportion of their intensities. As crosstalk usually involves only a small fraction of light, this proportion will be such that the presence of light that has undergone crosstalk at the detector location will no longer hamper proper measurement. A similar situation arises if crosstalk occurs after light has exited the measurement volume. However, in this case the location of the preferred source is the location where the light one wants to detect exits the measurement volume. The location of the further source is the location where the light, at least a part of which will experience crosstalk between the measurement volume and the detector location, exits the measurement volume. Therefore, if crosstalk occurs before light enters the measurement volume, the preferred source and a further source are the locations where light enters the measurement volume. If crosstalk occurs after light has exited the measurement volume, the preferred source and a further source are the locations where light exits the measurement volume. Although in the latter case the preferred source and a further source are not sources in the sense that light enters the measurement volume at these locations (in fact, light exits the measurement volume at these locations), they are sources in the sense that the preferred component and the further component that are detected as components of composed light at a detection unit can be regarded as parts of light originating from these locations.
With regard to the issue of simultaneously using light from multiple light sources, the known device could conceivably be adapted such that a turbid medium inside the measurement volume is not irradiated with light from a single light source, but with light from at least two light sources. In such a situation the light emitted by different light sources may have different wavelengths. As, for instance, at least a part of the light emitted by one light source and at least a part of the light emitted by another light source may exit the measurement volume through a single opening in the wall bounding the measurement volume coupled to a single detector location, the use of at least two light sources holds the risk of detecting composed light wherein a further component hampers the detection of a preferred component. If the different light sources emit light with different wavelengths, using optical filtering is not always sufficient to solve the problem. A situation is conceivable in which at least part of the light emitted by one light source is detected only after traversing the turbid medium, whereas at least a part of the light emitted by another light source is detected without the light having traversed the turbid medium, for instance because the second light source is located in the neighborhood of the opening in the wall bounding the measurement volume coupled to the detector location. In that case the intensity of the preferred component in the detected composed light may be so small, due to attenuation by the turbid medium, that, even after filtering, the intensity of a further component present in the detected composed light, including the accompanying noise, may be too large for proper detection of the preferred component. Of course, combining the preferred component and a further component in composed light may also be the result of crosstalk. As far as the detection of composed light is concerned in the case of multiple light sources, the locations where light enters the measurement volume may be regarded as the sources of components of the composed detected light. In that case, there is a preferred source, communicating light directly from one light source, and at least one further source, communicating light from another light source. By choosing the location of the preferred source and the at least one further source such that the paths of the part of the light communicated by the preferred source and the part of the light communicated by the at least one further source that are detected as components of composed light at a single detector location are essentially the same, the presence of the part of the light communicated by the at least one further source that is detected as a component of composed light at a single detector location no longer hampers the proper detection of the part of the light communicated by the preferred source that is detected as a component of composed light at that detector location.
An embodiment of the device according to the invention is characterized in that the preferred source and the further source are located such that the preferred source and the further source are adjacent. If the preferred source and the further source communicate light into the measurement volume, adjacent indicates that there are no further sources between the preferred source and the further source that communicate light into the measurement volume. If, on the other hand, the preferred source and the further source communicate light out of the measurement volume, adjacent indicates that there are no further sources between the preferred source and the further source that communicate light out of the measurement volume. This embodiment is the most rigorous implementation of the invention and has the advantage of being easy to implement. Locating the preferred source and the further source in adjacent positions maximizes the similarity between the paths taken by at least a part of the preferred light from the preferred source to the detection unit and by at least a part of the further light from the further source to the detection unit. However, as the problem solved by the invention has its origin in the detection of composed light comprising components that have been attenuated differently with a further component hampering the detection of the preferred component, the invention need not always be implemented in its most rigorous form. As the preferred source and a further source are located in increasingly similar positions, there may come a point at which the attenuation of the light communicated by the preferred source and the further source becomes such that the detection of the preferred component is no longer hampered by the presence in the composed light of a further component that stems from the further source, although at this point the preferred source and the further source need not be in adjacent positions.
A further embodiment of the device according to the invention is characterized in that the preferred light and the further light have wavelengths in the range from 400 to 1400 nanometers. This embodiment has the advantage that light with a wavelength in this range can penetrate biological tissues, such as female breasts, without some of the disadvantages of, for instance, x-rays, such as the use of ionizing radiation.
A further embodiment of the device according to the invention is characterized in that the device further comprises a selection unit for coupling a light generator to the preferred source, and for choosing said preferred source from the number of sources, said number of sources comprising subsets and said selection unit comprising an entrance element for receiving light from the light generator and an exit element comprising a number of exit locations for communicating the light from the light source to the number of sources, said exit locations comprising subsets, with the entrance and exit elements being displaceable relative to each other and with the exit locations on the exit element being arranged such that the subsets of exit locations correspond to the subsets of sources. The use of a selection unit has the advantage that radiation from a light source can be easily coupled to a preferred source communicating light into the measurement volume, said preferred source being chosen from the number of sources. However, the use of a selection unit also introduces a potential source of crosstalk. As the occurrence of crosstalk is related to the relative positioning of the exit locations on the exit element of the selection unit and as the invention concerns, among other things, the relative positioning of sources in the measurement volume with the advantage that the effect of the crosstalk on the detected composed light is reduced, the invention implies a mapping of said sources in the measurement volume to said exit locations on the exit element, with subsets of sources in the measurement volume corresponding to subsets of exit locations on the exit element. Various special arrangements present various benefits that will be discussed later.
A further embodiment of the device according to the invention is characterized in that the subsets of exit locations on the exit element of the selection unit are arranged in concentric circles. If the sources capable of communicating light into the measurement volume lie in parallel planes with the sources belonging to a single plane corresponding to a single circle, this embodiment has the advantage that it represents a ‘real’ mapping in that all exit locations on the selection unit that are geometrical neighbors correspond to neighboring sources in the measurement volume. This embodiment has the further advantage that no boundaries are required on the selection unit to prevent crosstalk.
A further embodiment of the device according to the invention is characterized in that the subsets of exit locations on the exit element of the selection unit are arranged to form consecutive segments of a single circle, with each segment corresponding to a subset of sources. This embodiment has the advantage of simplicity, a single degree of freedom along the axis of symmetry, easy assembly, and high symmetry.
A further embodiment of the device according to the invention is characterized in that the subsets of exit locations on the exit element of the selection unit are arranged in a spiral. This embodiment has the advantage that coupling a light source to a selected exit location on the spiral can be easily implemented mechanically.
A further embodiment of the device according to the invention is characterized in that the exit element of the selection unit comprises a light barrier between adjacent exit locations that correspond to non-adjacent sources. This embodiment allows greater freedom in coupling exit locations on the selection unit to sources in the measurement volume.
A further embodiment of the device according to the invention is characterized in that the exit element of the selection unit comprises a light barrier for optically separating at least two of the subsets of exit locations. One of the benefits of this embodiment is that it allows the use of an entrance element of the selection unit that is simultaneously coupled to multiple light generators. Another benefit of this embodiment is that it allows greater freedom in coupling light guides to sources in the measurement volume, because light guides that are geometrical neighbors on the selection unit, but that are separated by a barrier on the selection unit aimed at preventing crosstalk, are not optical neighbors on the selection unit and need not be coupled to sources in the measurement volume that are geometrical neighbors.
A further embodiment of the device according to the invention is characterized in that the entrance element of the selection unit comprises N entrance locations optically coupled to the light generator, said N entrance locations being arranged such that they form the corners of a first N-sided polygon and wherein the subsets of exit locations on the exit element of the selection unit are arranged such that each subset forms the corners of a second N-sided polygon with said second N-sided polygons being arranged in a grid of second N-sided polygons and with the said second N-sided polygons being congruent with the first N-sided polygon. Alternatively, overlapping grids of N-sided polygons may be used. This embodiment allows the easy use of an entrance element of the selection unit comprising N entrance locations coupled to N light sources that may be selected simultaneously. The high degree of symmetry of the grid structure offers flexibility in selecting sets of sources in the measurement volume.
According to the invention the medical image acquisition device comprises the device according to any of the previous embodiments.
According to the invention the selection unit is arranged for coupling a light generator to a preferred source and for choosing the preferred source from a number of sources, the number of sources comprising subsets and the selection unit comprising an entrance element for receiving light from the light generator and an exit element comprising a number of exit locations for communicating the light from the light source to the number of sources, the exit locations comprising subsets, with the entrance element and exit element being displaceable relative to each other and with the exit locations on the exit element being arranged such that the subsets of exit locations correspond to the subsets of sources.
These and other aspects of the invention will be further elucidated and described with reference to the drawings, in which:
a, 2b, and 2c show the positioning of a preferred source and a further source relative to each other,
a and 6b show two possible arrangements of barriers aimed at preventing crosstalk,
In medical diagnostics a device such as device 1 may be used for imaging the interior of biological tissues, such as a female breast. In the latter case, the device may look and work as follows. The measurement volume 15 is bound by a wall 20, which forms a cup in which a breast may be positioned. The space between the breast and the cup surface is then filled with a matching fluid, the optical properties of which closely match the optical properties of the breast or of an average breast. A large number of light guides 30a and 30b, for instance 510, is connected to the cup 20. These light guides 30a and 30b may be optical fibers. Half of the light guides, light guides 30a, are connected to a selection unit 35. The other half of the light guides, light guides 30b, are connected to a photodetector unit 10. The selection unit 35 can direct light from three different light sources, for instance light sources 5a, 5b, and 5c, which may be lasers, into any one of, for instance, 256 light guides 30a. 255 light guides 30a are coupled to the cup 20, whereas one light guide 30a is coupled directly to a detection light guide 30b. In this way, any of the, in this example, 255 light guides 30a can provide a conical light beam in the cup 20. By properly switching the selection unit 35, the light guides 30a will emit a conical light beam one after the other. The light from the selected light guide 30a is scattered and attenuated by the matching fluid and the breast, and is detected by, again in this example, 255 detectors on the photodetector unit 10. The scattering of light in breast tissue is strong, which means that only a limited amount of photons can traverse the breast compared to the reflected (or backscattered) light. Therefore, the detectors should cover a large dynamical range (about nine orders of magnitude). Photodiodes may be used as detectors. The front-end detector electronics then consists of these photodiodes and an amplifier. The gain factor of the amplifier can be switched between several values. The device 1 first measures at the lowest amplification and increases the amplification if necessary. A computer controls the detectors. This computer also controls the light sources, in these example light sources 5a, 5b, and 5c, the selection unit 35 and a pump system. All elements are mounted into a structure resembling a bed. The measurement starts with a measurement of a cup 20 filled completely with the matching fluid. This is the calibration measurement. After this calibration measurement, a breast is immersed in the fluid and the measurement procedure is carried out again. In this example, both the calibration and the breast measurement consist of 255×255 detector signals for each of the three light sources 5a, 5b, and 5c. The signals can be converted into a three-dimensional image using a process called image reconstruction. This reconstruction process, which is based on, for example, an algebraic reconstruction technique or a finite element method finds the most likely solution to the inverse problem, that is finding an image that correctly fits the measured data.
a, 2b, and 2c show the positioning of a preferred source and a further source relative to each other.
b shows a situation that is similar to the one described in
c shows the use of two light sources simultaneously. The preferred source and the further source are now formed by the positions 25a at which light from the two light sources enters the measurement volume 15. If the preferred source is positioned opposite the exit position for light 25b that communicates light to the photodetector unit 10 and if the further source is positioned near the exit position for light 25b, a situation not shown in
a and 6b show two possible arrangements of optical barriers. Depicted in
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.
Number | Date | Country | Kind |
---|---|---|---|
05110977.5 | Nov 2005 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/IB06/54061 | 11/2/2006 | WO | 00 | 5/16/2008 |