Patent Application
20080309940
The invention relates to a method of imaging an interior of a turbid medium, said method comprising the following steps:
The invention also relates to a system for imaging an interior of a turbid medium comprising:
The invention also relates to a medical image acquisition system comprising:
An embodiment of a method, system, and medical image acquisition system of this kind is known from U.S. Pat. No. 6,327,488B1. The known method and systems can be used for imaging an interior of a turbid medium, such as biological tissues, using diffuse optical tomography. In medical diagnostics the method and systems may be used for imaging an interior of a female breast. A turbid medium, such as a breast, is accommodated inside a receiving volume. Transmission input light from a transmission light source is coupled into the receiving volume, with the transmission input light being chosen such that it is capable of propagating through the turbid medium. In diffuse optical tomography transmission input light having a wavelength within the range of 400 nm to 1400 nm is typically used. Transmission output light emanating from the receiving volume as a result of coupling transmission input light into the receiving volume is detected and used to reconstruct an image of an interior of the turbid medium. During the coupling of transmission input light into the receiving volume the turbid medium comprised in the receiving volume is surrounded by a matching medium. This matching medium has optical properties, such as its absorption coefficient, that are similar to the optical properties of the turbid medium under investigation. The matching medium is used to counteract optical boundary effects stemming from the optical coupling of the turbid medium to its surroundings and to prevent an optical short-circuit around the turbid medium inside the receiving volume. An optical short-circuit occurs when transmission output light is detected that has not been sufficiently scattered and attenuated inside the receiving volume but outside the turbid medium. In that case the intensity of the insufficiently scattered and attenuated detected transmission output light may dwarf the intensity of the detected transmission output light that has been scattered and attenuated through passage through the turbid medium. If a matching medium is used, a reference measurement may be performed without the turbid medium being comprised in the receiving volume.
It is a drawback of the known method and systems that it is not straightforward to determine the geometry of the turbid medium under investigation. Having data relating to the geometry of the turbid medium is desirable as the image reconstruction process is ill-posed. This means that for a certain set of detected signals, a plurality of reconstructed images can be made that all fit the same detected signals. The general solution to this problem is to gather as much information as possible during an examination and to use all this information in the reconstruction process.
It is an object of the invention to make it possible to optically obtain data relating to the geometry of the turbid medium under investigation. The obtained data can then be used to improve the image reconstruction process according to the opening paragraph. According to the invention this object is realized in that the method further comprises the following additional steps:
The invention is based on the recognition that the combination of light that is coupled into the receiving volume, a medium surrounding the turbid medium when light is coupled into the receiving volume, and the interface between the turbid medium and its surroundings in the receiving volume when light is coupled into the receiving volume allows to create a contrast between the turbid medium and its surroundings. A contrast between the turbid medium and its surroundings in turn makes it possible to detect the exterior of the turbid medium, resulting in additional data relating to the geometry of the turbid medium being obtained. This additional data establishes a boundary condition to be used in the image reconstruction process. This boundary condition in turn reduces the number of possible images that fit a particular set of detected signals, as not all possible images will satisfy the boundary condition established by the additional data.
An embodiment of the method according to the invention is characterized in that the combination of the geometry input light and the geometry medium is chosen such that the geometry medium is substantially transparent to the geometry input light and wherein the combination of the geometry input light and the interface is chosen such that at the interface the turbid medium is substantially opaque to the geometry input light relative to its surroundings. This embodiment has the advantage that it enables to directly image the exterior of the turbid medium at the wavelength of the geometry input light.
A further embodiment of the method according to the invention is characterized in that the geometry medium is a matching medium having optical properties of reducing boundary effects between the turbid medium and its surroundings for the transmission light. This embodiment implies that the wavelength of the geometry input light lies outside the wavelength range suitable for the transmission input light. After all, at the wavelength of the transmission input light the interface separating the turbid medium from its surroundings is difficult to distinguish because of the presence of the matching medium for this wavelength. This embodiment has the advantage that data relating to the exterior of the turbid medium can be obtained with the matching medium for the transmission input light in place.
A further embodiment of the method according to the invention is characterized in that the combination of the geometry input light and the geometry medium is chosen such that the geometry input light excites a fluorescent agent comprised in the geometry medium. This embodiment has the advantage that it provides an alternative to directly imaging the exterior of the turbid medium by providing a method for imaging the volume inside the receiving volume not occupied by the turbid medium.
A further embodiment of the method according to the invention is characterized in that the geometry medium is a matching medium having optical properties of reducing boundary effects between the turbid medium and its surroundings for the transmission light. This embodiment has the advantage that it enables the use of a fluorescent agent according to the previous embodiment with a matching medium for the transmission input light in place.
A further embodiment of the method according to the invention is characterized in that the transmission input light and the geometry input light are at the same. This embodiment has the advantage that a single light source can be used both for obtaining an image of an interior of the turbid medium and obtaining data relating to the exterior of the turbid medium.
A further embodiment of the method according to the invention is characterized in that the method further comprises a step of enhancing the contrast between the turbid medium and its surroundings by accommodating a contrast enhancer at the interface between the turbid medium and its surroundings. This embodiment has the advantage that the interface between the turbid medium and its surroundings, and hence the exterior shape of the turbid medium, can be distinguished better if the contrast between the turbid medium and its surroundings is enhanced.
A further embodiment of the method according to the invention is characterized in that the contrast enhancer is chosen for at least partially reflecting geometry input light. This embodiment has the advantage that by covering a surface of the turbid medium with a contrast enhancer that at least partially reflects geometry input light, the contrast between the surface of the turbid medium and its surroundings at the wavelength of the geometry input light is enhanced. The exterior of the turbid medium becomes better visible at the wavelength of the geometry input light.
A further embodiment of the method according to the invention is characterized in that the contrast enhancer is chosen for at least partially absorbing geometry input light. This embodiment has the advantage that it provides an alternative way, compared to the previous embodiment, of enhancing the contrast between the turbid medium and its surroundings. Instead of improving the visibility of the exterior of the turbid medium at the wavelength of geometry input light, the contrast between the contour of the turbid medium and its surroundings is enhanced.
A further embodiment of the method according to the invention is characterized in that the contrast enhancer is chosen for emitting fluorescence light in response to at least a part of the geometry input light. This embodiment has the advantage that covering a surface of the turbid medium with a contrast enhancer comprising a fluorescent agent enables direct imaging of the external shape of the surface at the wavelength of the fluorescence light emitted by the fluorescent agent. Moreover, whereas light that is reflected at or near the surface of the turbid medium passes through the measurement volume twice, once prior to and once after reflection, the fluorescent light only passes through the measurement volume once as it goes from the turbid medium to a detection position. This makes image reconstruction easier.
The object of the invention is further realized with a system for imaging an interior of a turbid medium comprising:
for carrying out the method according to any one of the previous embodiments.
A system for imaging an interior of a turbid medium would benefit from any of the previous embodiments of the method according to the invention.
An embodiment of the system for imaging an interior of a turbid medium according to the invention is characterized in that the system for imaging interior of a turbid medium further comprises a contrast enhancer for enhancing the contrast between the turbid medium and its surroundings. This embodiment has the advantage that the interface between the turbid medium and its surroundings, and hence the exterior shape of the turbid medium, can be distinguished better if the contrast between the turbid medium and its surroundings is enhanced.
A further embodiment of the system for imaging an interior of a turbid medium according to the invention is characterized in that the transmission photodetector unit and the contrast photodetector unit are comprised in a single photodetector unit. This embodiment has the advantage that there is no need for separate transmission photodetector and contrast photodetector units.
The object of the invention is further realized with a medical image acquisition system comprising:
A medical image acquisition system would benefit from any of the previous embodiments of the method according to the invention.
An embodiment of the medical image acquisition system according to the invention is characterized in that the medical image acquisition system further comprises a contrast enhancer for enhancing the contrast between the turbid medium and its surroundings. This embodiment has the advantage that the interface between the turbid medium and its surroundings, and hence the exterior shape of the turbid medium, can be distinguished better if the contrast between the turbid medium and its surroundings is enhanced.
A further embodiment of the medical image acquisition system according to the invention is characterized in that the transmission photodetector unit and the contrast photodetector unit are comprised in a single photodetector unit. This embodiment has the advantage that there is no need for separate transmission photodetector and contrast photodetector units.
These and other aspects of the invention will be further elucidated and described with reference to the drawings, in which:
According to the invention, geometry input light from a geometry light source is coupled into the receiving volume, with the receiving volume comprising the turbid medium, with the receiving volume further than comprising a geometry medium for surrounding the turbid medium during coupling of geometry input light into the receiving volume, and with the combination of the geometry input light, the geometry medium, and the interface being chosen for creating a contrast between the turbid medium and its surroundings. This is done in step 215. A number of combinations is especially advantageous, as will be discussed below. Next, in step 220 the contrast created between the turbid medium and its surroundings is detected. In step 225 the detected contrast is used in reconstructing an image of an interior of the turbid medium. In this step the transmission output light detected in step 210 is used as well.
As mentioned, a number of combinations of geometry input light, geometry medium, and interface between the turbid medium and its surroundings is especially advantageous for creating the contrast between the turbid medium and its surroundings. This will now be further elucidated.
A first especially advantageous combination is one in which the combination of the geometry input light and the geometry medium is chosen such that the geometry medium substantially transparent to the geometry input light and wherein the combination of the geometry input light and the interface is chosen such that at the interface the turbid medium is substantially opaque to the geometry input light relative to its surroundings. One way to realize such a combination is to replace the matching medium used in step 205 with a geometry medium that is substantially transparent to the transmission input light used in step 205. In this way, at least a part of the optical discontinuity at the interface between the turbid medium and its surroundings that is removed through the use of the matching medium is reintroduced. The optical discontinuity in turn results in reflection occurring at the interface. If the reflection is sufficient at the surface of the turbid medium, this option allows direct imaging of the exterior of the turbid medium using the same light source as is used for obtaining an image of an interior of the turbid medium. In one medical application of the method, one in which the method is used for imaging an interior of a female breast, the transmission input light typically has a wavelength within the range of 400 and nm to 1400 nm. An example of a suitable geometry medium that is substantially transparent to transmission input light having a wavelength within this range is water. Although this option is especially advantageous when using transmission input light as geometry input light, this option works for all wavelengths to which the geometry medium is substantially transparent and to which the interface is substantially opaque. In the example of the medical application in which the method is used for imaging an interior of a female breast, water or a water-based substance could be used as a geometry medium. Then, the geometry medium would be substantially transparent to, for instance, blue or green light. At the same time the surface of the breast were then be sufficiently opaque to the blue or green light to allow the data relating to the exterior of the breast to be obtained. Clearly, this option enables both the direct imaging of an exterior of the turbid medium and obtaining shadow images of the turbid medium from which data relating to the exterior of the turbid medium can be deduced. In replacing the matching medium with the geometry medium it must be realized that the matching medium may exert a force on the turbid medium under investigation. If, for instance, the matching medium is a fluid, the turbid medium under investigation will experience a buoyancy force. Clearly, it is intended that the geometry of the turbid medium when coupling transmission input light into the receiving volume is the same as when geometry input light is coupled into the receiving volume. Hence, the geometry medium taking the place of the matching medium will have to exert a similar force on the turbid medium as did the matching medium. A possible way to achieve this is to replace a fluidic matching medium with a fluidic geometry medium of the same density. Above, water was mentioned as an example of a suitable geometry medium that is transparent within a wavelength range of 400 nm to 1400 nm. Should the matching medium have a higher density than water substances like salts could be added to the geometry medium to increase its density. Should the matching medium have a density lower than that of water, a transparent oil could be used as a geometry medium with additional substances added if required to match the density of the oil to that of the matching medium.
A second especially advantageous combination builds on the first one, but this time the geometry medium is a matching medium having optical properties of reducing boundary effects between the turbid medium and its surroundings for the transmission input light. In this second option, the geometry medium is no longer transparent to the transmission input light. Therefore, the transmission input light and the geometry input light can no longer be the same as was still a possibility in the first option. In the second option the geometry input light will have to have a wavelength that is different from the wavelength of the transmission input light. However, in the second option it is possible to obtain both data relating to an interior of the turbid medium and data relating to the exterior of the turbid medium without the need to replace the matching medium. Hence, the second option has a low impact on the measurement procedure followed in the known method. Clearly, this option enables both the direct imaging of an exterior of the turbid medium and obtaining shadow images of the turbid medium from which data relating to the exterior of the turbid medium can be deduced. It was already mentioned that in one medical application of the method, one in which the method is used for imaging an interior of a female breast, the transmission input light typically has a wavelength within the range of 400 nm to 1400 nm. Examples of suitable matching mediums for this wavelength range are mentioned in the discussion of
A third especially advantageous combination is one in which the combination of the geometry input light and the geometry medium is chosen such that the geometry input light excites a fluorescent agent comprised in the geometry medium. As a result, the fluorescent agent emits fluorescence light. This in turn creates a contrast between the turbid medium and its surroundings by creating a first region in which fluorescence light is generated in response to the geometry input light and a second region, this region being the turbid medium, in which this is not the case. Whereas in the first and second option is the exterior of the turbid medium was directly imaged, the third option enables determining the exterior shape of the turbid medium by determining the region in the receiving volume that does not emit fluorescence light in response to the geometry input light. In this sense, the third option proposes to make a negative image of the turbid medium whereas the first and second option is proposed to make a positive image. Excitation of the fluorescent agent may be achieved by geometry input light having a wavelength within the normal wavelength range of the transmission input light or by geometry input light having a wavelength outside the wavelength range of the transmission input light. In the example of the medical application of the method mentioned earlier in which an interior of a female breast is imaged, the normal wavelength range of transmission input light typically lies within the range of 400 nm to 1400 nm. If the transmission input light and the geometry input light are the same, that is if the geometry input light has a wavelength within the normal wavelength range of the transmission input light, then there is no need for a separate transmission light source and geometry light source.
A fourth especially advantageous option builds on the third option, but now the geometry medium is a matching medium having optical properties of reducing boundary effects between the turbid medium and its surroundings for the transmission input light. This option combines the use of a fluorescent geometry medium with the use of a matching medium used in step 205. In this way, there is no need to replace the matching medium with a fluorescent geometry medium that does not have matching properties for the transmission input light. Excitation of the fluorescent agent may be achieved by geometry input light having a wavelength within the normal wavelength range of the transmission input light or by geometry input light having a wavelength outside the wavelength range of the transmission input light. If the transmission input light and the geometry input light are the same, that is if the geometry input light has a wavelength within the normal wavelength range of the transmission input light, then there is no need for a separate transmission light source and geometry light source. It was already mentioned that in one medical application, one in which an interior of a female breast is imaged, the wavelength of the transmission input light typically lies within the range of 400 nm to 1400 nm. Examples of suitable matching mediums for imaging an interior of a female breast are given in the discussion relating to
A fifth especially advantageous combination is one in which the contrast between the turbid medium and its surroundings created by the combination is enhanced by accommodating a contrast enhanced at the interface between the turbid medium and its surroundings. Enhancing the contrast makes the turbid medium better distinguishable from its surroundings.
A first advantageous enhancement method is to choose the contrast enhancer such that at least a part of the light arriving at the contrast enhancer is reflected. In this way, the visibility of the turbid medium at the wavelength of the light reaching the contrast enhancer, for instance, the geometry input light is improved. For the medical application of the method according to the invention in which an interior of a female breast is imaged, an example of a suitable contrast enhancer is blue body paint.
A second advantageous enhancement method is to choose the contrast enhancer such that at least a part of a light arriving at the contrast enhancer is absorbed. In this way, the turbid medium becomes darker at the wavelength of the light reaching the contrast enhancer, for instance, the geometry input light. Hence, the contrast between the turbid medium and its surroundings is enhanced. For the medical application of the method according to the invention in which an interior of a female breast is imaged, an example of a suitable contrast enhancer is a body paint containing the dye known as brilliant black.
A third advantageous enhancement method is to choose the contrast enhancer of such that it emits fluorescence light in response to at least a part of the light arriving at the contrast enhancer. In this way, the contour of the turbid medium becomes fluorescent as a result of which the external shape of the turbid medium becomes visible at the wavelength of the fluorescence light emitted by the contrast enhancer. This option is especially advantageous if the light exciting the fluorescent agent comprised in the contrast enhancer and the transmission input light are the same. Then, an interior of the turbid medium and the exterior of the turbid medium can be probed in a single measurement with a part of the light exciting the contrast enhancer and another part of the light passing through the turbid medium. For the medical application of the method according to the invention in which an interior of a female breast is imaged, an example of a suitable contrast enhancer is a body paint containing Alexa Fluor 430 or dyes with spectrums similar to that of Alexei Fluor 430.
Clearly, the sequence of steps shown in
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 |
|---|---|---|---|
| 05111165.6 | Nov 2005 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2006/054339 | 11/20/2006 | WO | 00 | 5/22/2008 |
