WO2010103440A1

WO2010103440A1 - Irradiating an absorption contrast agent contained in a turbid medium

Info

Publication number: WO2010103440A1
Application number: PCT/IB2010/050952
Authority: WO
Inventors: Bernhard J. Brendel; Tim Nielsen; Thomas Koehler; Ronny Ziegler
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2009-03-11
Filing date: 2010-03-05
Publication date: 2010-09-16

Abstract

The invention relates to a device and a method of irradiating an absorption contrast agent contained in a turbid medium (10). The absorption contrast agent has different absorption coefficients for light of a first wavelength (λ1) and light of a second wavelength (λ2), and the turbid medium (10) has similar optical properties for light of the first and second wavelengths (λ1, λ2). The turbid medium (10) is irradiated with light of the first wavelength (λ1) and light of the second wavelength (λ2) at the same source position, and an intensity of light having the first wavelength (λ1) emanating from the turbid medium (10) at at least one detection position, and an intensity of light having the second wavelength (λ2) emanating substantially simultaneously from the turbid medium (10) at the same detection position are measured. An image may be reconstructed from the measured intensities.

Description

Irradiating an absorption contrast agent contained in a turbid medium

FIELD OF THE INVENTION

The present invention relates to a method of irradiating an absorption contrast agent contained in a turbid medium, and to a device for irradiating an absorption contrast agent contained in a turbid medium.

BACKGROUND OF THE INVENTION

In the context of the present application, the term turbid medium is to be understood to mean a substance consisting of a material having a high light scattering coefficient, such as for example intralipid solution or biological tissue. Further, an absorption contrast agent is to be understood to mean a substance having a high light absorption coefficient for at least one wavelength of light, in particular, a higher light absorption coefficient than the turbid medium. Further, light is to be understood to mean electromagnetic radiation of a wavelength in the range from 180 nm to 1.400 nm. The term "optical properties" covers the reduced scattering coefficient and the absorption coefficient. Furthermore, "matching optical properties" is to be understood as having a similar reduced scattering coefficient and a similar absorption coefficient. Further, "similar optical properties" for different wavelengths is to be understood as the optical properties being mainly or substantially the same for said different wavelengths. Further, the term "substantially simultaneously" is to be understood as simultaneously or at least within a time interval during which the turbid medium remains substantially motionless as a whole and in parts. For example, motions of biological tissue may be caused by the pulsation of an artery or the heart and may include the flow of blood or other biological liquids.

In recent years, several methods and devices for examining turbid media, e.g. female breast tissue, have been developed. In particular, new devices for detection and analysis of breast cancer have been developed and existing technologies have been improved. Breast cancer is one of the most occurring types of cancer: in 2002, for example, more than 1.1 million women were diagnosed and over 410.000 women died of breast cancer worldwide. Several types of devices for imaging the interior of a turbid medium by use of light have been developed. Examples for such devices are mammography devices and devices for examining other parts of human or animal bodies. A prominent example for a method for imaging the interior of a turbid medium is Diffuse Optical Tomography (DOT). In particular, such devices are intended for the in vivo localization of inhomogeneities in breast tissue of a part of a breast of a female human body. A malignant tumor is an example for such an inhomogeneity. The devices are intended to detect such inhomogeneities when they are still small, so that for example carcinoma can be detected at an early stage. A particular advantage of such devices is that the patient does not have to be exposed to the risks of examination by means of ionizing radiation, as e.g. X-rays.

Such devices can be used to detect tumors based on the differences in their optical properties of healthy tissue and tumor tissue, e.g. caused by the differences in water, fat and blood content as well as blood oxygenation.

It is known to use diffuse optical tomography for fluorescence imaging. For example, the turbid medium is irradiated by laser light having a specific wavelength, and laser light emanating from the turbid medium is measured as well as fluorescence light. However, since the intensity of the fluorescence light is much lower than the intensity of the exiting laser light, detectors have to be very sensitive, and the measurements are noisy.

SUMMARY OF THE INVENTION

It would be desirable to provide a method or a device, which allows to better image the interior of a turbid medium. It would also be desirable to provide a method or device for acquiring image data of a turbid medium that allows to detect differences between healthy tissue and tumor tissue with a higher specificity.

It would also be desirable to reduce the noise level of measurements of light emanating from the turbid medium. To better address one or more of these concerns, in a first aspect of the invention, a method of irradiating an absorption contrast agent contained in a turbid medium is provided, which absorption contrast agent has different absorption coefficients for light of a first and a second wavelength, and which turbid medium has similar optical properties for light of the first and second wavelengths, the method comprising the steps of: - irradiating the turbid medium at at least one source position with light of the first wavelength, and, substantially simultaneously, irradiating the turbid medium with light of the second wavelength at the same source position; and measuring an intensity of light having the first wavelength emanating from the turbid medium at at least one detection position, and measuring an intensity of light having the second wavelength emanating substantially simultaneously from the turbid medium at the same detection position. Thus, the irradiation step comprises, for each source position: irradiating the turbid medium with light of the first wavelength at a respective source position, and, substantially simultaneously, irradiating the turbid medium with light of the second wavelength at the same source position.

The measuring step may comprise, for each detection position: collecting light having the first wavelength emanating from the turbid medium at a respective detection position and, substantially simultaneously, collecting light having the second wavelength emanating from the turbid medium at the same detection position; and measuring the respective intensities of the collected light of the first and second wavelengths.

For example, the light of the first wavelength and the light of the second wavelength may be measured substantially simultaneously. Alternatively, the light of the first wavelength and the light of the second wavelength may be collected substantially simultaneously at the same detection position, and the intensity of the collected light may be measured subsequently for light having the first wavelength and light having the second wavelength.

For example, the absorption coefficients of the absorption contrast agent for light of the first wavelength and the second wavelength differ substantially; which is to be understood as differing by at least one order of magnitude (factor 10).

For example, the absorption contrast agent is a medical absorption contrast agent.

The term "an intensity of light having the first/second wavelength" is to be understood as the intensity of a frequency component of light, said frequency component substantially corresponding to the first or second wavelength, respectively. For example, the frequency component may comprise a narrow interval of frequencies.

For example, the intensities may be substantially simultaneously measured.

For example, the intensity of light of the first and second wavelength may be measured directly, or, for example, the intensity of the first wavelength may be measured directly and that of light of the second wavelength may be measured indirectly by measuring the intensity of light of both wavelengths and subtracting the measured values.

For example, the turbid medium is irradiated at the respective source position with light of the first wavelength and light of the second wavelength from the same direction. In the turbid medium, depending on the local concentration of the absorption contrast agent, light of the first wavelength and light of the second wavelength are subject to different light absorption coefficients of the absorption contrast agent.

For example, irradiating the turbid medium with light of a specific wavelength may be performed by irradiation the turbid medium with light comprising at least the specific wavelength. For example, other wavelengths may also be present.

Further useful details of the invention are indicated in the dependent claims.

In one embodiment, in the measuring step, a time interval between collecting light having the first wavelength emanating from the turbid medium at a respective detection position and collecting light having the second wavelength emanating from the turbid medium at the same detection position is less than 0.5 seconds, and wherein the respective intensity of the collected light having the first and the second wavelength is measured. For example, said time interval may be less than 0.1 s, less than 0.02 s, less than 0.005 s or less than 0.001 s. For example, the time interval may be less than, or may correspond to, twice the light collecting time of a single measurement at a single detection position for one of the first and second wavelength.

In one embodiment, in the irradiating step, a time interval between irradiating the turbid medium at a respective source position with light of the first wavelength and irradiating the turbid medium at the same source position with light of the second wavelength is less than 0.5 seconds. For example, said time interval may be less than 0.1 s, less than 0.02 s, less than 0.005 s, or less than 0.001 s.

For example, said time interval may be substantially equal to said measuring time interval mentioned above. In one embodiment, in the measuring step, the intensity of light having the first wavelength emanating from the turbid medium at least one detection position is measured, and the intensity of light having the second wavelength emanating simultaneously from the turbid medium at the same detection position is measured.

For example, light having the first wavelength and light having the second wavelength may be collected simultaneously, and the respective intensity of the collected light having the first and the second wavelength is measured.

For example, the light of the first wavelength and the light of the second wavelength may be measured simultaneously. Alternatively, the light of the first wavelength and the light of the second wavelength may be collected simultaneously at the same detection position, and the intensity of the collected light may be measured subsequently for light or having the first wavelength and light having the second wavelength.

For example, in the measuring step, the turbid medium is, at a respective source position, simultaneously irradiated with light of the first wavelength and light of the second wavelength.

In one embodiment, the method is a method of acquiring image data of an absorption contrast agent contained in a turbid medium, which absorption contrast agent has different absorption coefficients for light of a first and a second wavelength, and which turbid medium has similar optical properties for light of the first and second wavelengths, and wherein the method further comprises the step of reconstructing an image of the turbid medium based on the measured intensities. For example, the image is a tomographic image.

The use of an absorption contrast agent for acquiring image data of a turbid medium has a number of advantages, e.g. as compared to fluorescence imaging. Thus, in fluorescence imaging, the intensity of the fluorescence light is much lower than the intensity of the exiting laser light, imposing certain detector requirements. However, for imaging an absorption contrast agent, for example, only laser light is measured for at least two different wavelengths. Therefore, measurements with a lower noise level are possible and/or detector requirements can be relaxed, that is, the detectors do not have to be as sensitive as required for fluorescence imaging. Further, the concentration of a fluorescence contrast agent in blood and tissue is limited, because if the concentration exceeds a certain, low value, the emitted fluorescence light is reabsorbed by the surrounding fluorescence contrast agent to a high degree. This phenomenon is called self absorption. There is no such concentration limitation for an absorption contrast agent. Further, the spectrum of fluorescence light in fluorescence imaging is quite broad. Therefore, the measurement of the transmitted laser light may be corrupted by fluorescence light emitted at the same wavelength. If this is not corrected, artefacts appear in the reconstructed image. For imaging an absorption contrast agent, light sources with a narrow wavelength spectrum, e.g. laser light sources with different wavelengths may be used, thus avoiding these problems.

Further, in fluorescence imaging, a large fraction of the fluorescence light has a significantly different wavelength than the exiting laser light. Thus, due to wavelength depending optical properties of the tissue, scattering and absorption in the tissue are different for fluorescence light than for the laser light. This may lead to artefacts in the reconstructed images. For imaging an absorption contrast agent, for example, laser wavelengths can be chosen in a narrow wavelength interval, in which interval the turbid medium has similar optical properties, thereby avoiding said problems.

Measuring an intensity of light having the first wavelength and an intensity of light having the second wavelength emanating substantially simultaneously or simultaneously from the turbid medium at the same detection position has the further advantage that the measuring of light emanating at detection position is not corrupted by motion, because the light of both wavelengths is collected simultaneously or within a short time interval. Therefore, image artefacts due to motion in acquired image data will be reduced.

Thus, a reduced level of noise and artefacts in a reconstructed image may be achieved.

In one embodiment, the irradiating step and the measuring step are performed by subsequently irradiating the turbid medium at a plurality of different source positions and measuring the light emanating from the turbid medium at a plurality of different detection positions for each source position.

That is, for each combination of a source position and a detection position, the above described irradiating step and the above described measuring step are performed.

In one embodiment, the method further comprises the step of determining the ratio of the measured intensities for each detection position. For example, the intensity of light having the first wavelength is divided by the intensity of light having the second wavelength.

For example, when the method is a method of acquiring image data of an absorption contrast agent contained in a turbid medium, the image of the turbid medium may be reconstructed based on the ratio of the measured intensities.

Dividing two simultaneously or substantially simultaneously acquired measurement values has the advantage that relative noise, e.g. due to variations of coupling factors of source and detector light fibers cancel out, thus reducing the noise level in the data used for reconstruction. For example, for each source position, light emanating from the turbid medium at a plurality of different detection positions may be collected simultaneously, and the respective intensities of the collected light may be measured.

Thus, when an image is reconstructed based on the measured intensities, image artefacts due to motion may be reduced. In one embodiment, the measuring step may comprise: collecting light having the first wavelength and the second wavelength emanating from the turbid medium at a respective detection position; splitting the light collected at the second spatial position into at least two optical channels; and measuring the respective intensity of the split light in the respective optical channels.

For example, splitting the connected light may comprise separating light having the first wavelength from light having the second wavelength. Thus, the light of the respective wavelength may be collected or measured simultaneously in the respective optical channels.

In one embodiment, the first and the second wavelength differ by less than 100 nm.

In one embodiment, an absorption coefficient of the absorption contrast agent for light of the first wavelength differs from an absorption coefficient of the absorption contrast agent for light of the second wavelength by at least a factor of 10. For example, the absorption coefficients may differ by at least a factor of 100 or at least a factor of 1000.

When the absorption coefficient of the absorption contrast agent has a very distinct dependence on the wavelength in the near infrared region (NIR), this is advantageous for imaging the absorption contrast agent by measuring the light transported through the turbid medium at two wavelengths for which the optical properties of the tissue stay mainly the same, while the optical properties of the absorption contrast agent vary drastically. The ratio of the two measurements is a quantity describing the absorption of the light due to the absorption contrast agent, compensated for the absorption of the tissue. For example, the absorption of the absorption contrast agent may decrease by several orders of magnitude over a quite narrow wavelength interval of, for example, about 50 nm, in which interval the intrinsic absorption and scattering properties of the tissue stay mainly the same or unchanged. One example of a suitable absorption contrast agent is a photosensitizer such as lutetium texaphyrin (LuT ex). In a further aspect of the invention, a device for irradiating an absorption contrast agent contained in a turbid medium is provided, which absorption contrast has different absorption coefficients for light of a first and a second wavelength, and which turbid medium has similar optical properties for light of the first and second wavelengths; the device comprising: a receiving volume for receiving a turbid medium to be irradiated; at least one light source for irradiating an interior of the receiving volume; at least one detection unit for measuring an intensity of light emanating from the interior of the receiving volume; and - a control unit for controlling the device for irradiating an absorption contrast agent contained in a turbid medium; the at least one light source being arranged to provide light of the first wavelength and, substantially simultaneously, light of the second wavelength; wherein the control unit is adapted to control the device for irradiating an absorption contrast agent contained in a turbid medium such that: the turbid medium is irradiated at at least one source position with light of the first wavelength and, substantially simultaneously, the turbid medium is irradiated with light of the second wavelength at the same source position; an intensity of light having the first wavelength emanating from the turbid medium at at least one detection position is measured, and an intensity of light having the second wavelength emanating substantially simultaneously from the turbid medium at the same detection position is measured.

In one embodiment, the at least one detecting unit is arranged to measure the intensity of light having the first wavelength emanating from the turbid medium at at least one detection position and to measure the intensity of light having the second wavelength emanating simultaneously from the turbid medium at the same detection position.

For example, the detecting unit may comprise at least a first detector and a second detector and a wavelength separator for separating light having the first wavelength emanating at a respective detection position and the light having the second wavelength emanating at the same detection position. For example, the first detector may be arranged to measure the intensity of light having the first wavelength, and the second detector may be arranged for measuring the intensity of light having the second wavelength.

In one embodiment, the device is a device for acquiring image data of an absorption contrast agent contained in a turbid medium, and the control unit is adapted to control the device for irradiating an absorption contrast agent contained in a turbid medium such that an image of the turbid medium is reconstructed based on the measured intensities. For example, the device is a medical image acquisition device. For example, the device is a medical diagnostic device. In a further aspect of the invention, there is provided a computer program or computer program product for performing the steps of the method as described above when executed on a computer, e.g. a computer of a control unit of a device as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 schematically shows a receiving portion of a device for acquiring image data of an absorption contrast agent contained in a turbid medium, with a turbid medium and an optically matching medium located therein; and

Fig. 2 schematically shows the optical connection of the receiving portion and light sources and detectors, respectively.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will now be described with reference to Figs. 1 and 2. In the embodiment, the device for acquiring image data of an absorption contrast agent contained in a turbid medium is formed by a device for a diffuse optical tomography (DOT), in particular by a mammography device. Since the overall construction of such a device is known for a skilled person, no detailed description of the device will be given.

In the device of the embodiment, the turbid medium 10 to be examined is a female human breast. The device is provided with a table, on which a patient is placed. In the table, a receiving portion 12 enclosing a measuring volume and arranged to receive the turbid medium 10 is provided, as is schematically indicated in Fig. 1. The receiving portion 12 has a cup-like shape with rotational symmetry with respect to a vertical axis Z and is provided with an opening 14. As can be seen in Fig. 1, the turbid medium 10 to be examined, i.e. the breast, is placed in the receiving portion 12 such that it freely hangs in the receiving portion 12 from the side of the opening 14. The inner surface of the receiving portion 12 facing the turbid medium 10 is provided with a plurality of ends of light guides 16 formed by optically guiding fibers connecting to a first light source 20, a second light source 22, and to a plurality of detection units 24 as will be described below. These ends of the light guides 16 are distributed on the inner surface of the receptacle 12 such that the receptacle 12 provided with the light guides 16 still comprises substantially rotational symmetry. For example, the ends of the light guides 16 on the side of the receiving portion 12 are arranged on a plurality of rings 16- 1^st, 16-2^nd,...., 16-n^ώ, which are positioned in planes perpendicular to the axis Z, as is schematically indicated in Fig. 1. One each ring, ends of a plurality of light guides 16 are distributed about the circumference of the receiving portion 12.

For example, in the device according to the embodiment, 512 light guides 16 are provided, the ends of which are distributed on the receiving portion 12. The light guides 16 may be formed by optically guiding fibers. In the embodiment, half of these light guides 16 are connected to an array of detecting units 24. The other half of the light guides 16 is connected to a switch 26 capable to direct light, which is input into the switch 26 from the light sources 20, 22 through an optical connector 28, into either one of the 256 light guides connected to the switch 26. It should be noted that the number of light guides 16 is not limited to the number described above. Further, more or less than half of the light guides 16 may be connected to the detecting units 24.

The size of the receiving portion 12 is such that a space remains between the inner surface of the receiving portion 12 and the turbid medium 10. The receiving portion 12 is structured to receive an optically matching medium 18, which serves to provide optical coupling between the turbid medium 10 to be imaged and the inner surface of the receiving portion 12. The optically matching medium further serves to prevent optical short-cutting between the light guides 16 coming from the light sources 20, 22 and the light guides 16 coupling to the detecting units 24. Furthermore, the optically matching 18 serves to counteract boundary effects in the reconstructed image which are coursed by the difference in optical contrast between the interior of the turbid medium 10 and the remaining space in the receiving portion 12. For this purpose, the optically matching medium 18 is provided with optical properties, which substantially match the optical properties of the turbid medium 10 to be examined.

In the embodiment, the first and second light sources 20, 22 are formed by lasers, which respectively emit light of a first wavelength X₁ and a second wavelength X₂ in the near infrared region, and the wavelengths X₁ , X₂ of the lasers differ by, e.g., 50 nm. Light of both lasers is coupled by the optical connector 28 and is input to the switch 26. The optical properties of the turbid medium 10 are mainly the same for light of both wavelengths. Furthermore, the optical properties of the optically matching medium 18 are mainly the same for light of both wavelengths.

An absorption contrast agent, for example lutetium texaphyrin (LuTex) is present in the turbid medium 10 at varying concentration. For example, the absorption contrast agent has been administered to the patient. For example, the absorption contrast agent is mainly concentrated in specific biological structures of the biological tissue. The absorption contrast agent has different absorption coefficients for lights of the first and the second wavelength. For example, the absorption coefficients differ by several orders of magnitude.

In the embodiment, each of the detecting units 24 is formed by a wavelength separator 30 connected to the respective light guide 16, and a first detector 32 and a second detector 34.

In the embodiment, the wavelength separator 30 is formed by light splitting means 36, which splits the light from a light guide 16 into two optical channels, and optical filters 40, 42 arranged in the respective channels. For example, the optical filter 40 lets light pass having the first wavelength X₁ of the first light source 20, and the optical filter 42 lets light pass having the second wavelength λ₂ of the second light source 22. For example, the light splitting means 36 are formed by a branching of the light guide 16.

In the present embodiment, the light sources 20, 22 are formed by lasers. However, more than one first light source and more than one second light source may be provided the light of which can be directed into selected light guides 16 by the switch 26.

In the present embodiment, the optical connector 28 connects the light sources 20, 22 to the switch 26. However, the light sources 20, 22 may be directly connected to the switch 26, and the switch 26 may be adapted to couple light from the first light source 20 and the second light source 22 simultaneously into the same selected light guide 16. The device is structured such that light from the light sources 20, 22 can be subsequently directed to the turbid medium 10 from different directions, and light emanating from the turbid medium 10 can be detected by a plurality of detecting units 24, the corresponding light guides 16 of which are distributed on the inner surface of the receptacle 12. The device comprises a control unit 44 which controls the switch 26 and which reconstructs an image of the absorption contrast agent and, thus, of the interior of the turbid medium 10, based on the signals from the detectors 32, 34. For the reconstruction, the signals sampled during a scan in which the light is directed to the turbid medium 10 from different directions are used. For reasons of simplicity, these elements of the device for acquiring image data of an absorption contrast agent contained in a turbid medium are only schematically indicated in Fig. 2. In Fig. 2, the control unit comprises the light sources 20, 22, the optical connector 28, the switch 26, and the detecting units 24. In the embodiment, for example, the light from the light sources 20, 22 is subsequently directed into different light guides 16, wherein, for each light guide 16, light of the first light source 20 and light of the second light source 22 is simultaneously directed into the same light guide 16. In the embodiment, the light that emanates from the receiving portion 12 in response is detected by the plurality of detecting units 24 in each case, e.g. 256 detecting units. For each light guide 16 connected to a detecting unit 24, light having the first wavelength X₁ and light having the second wavelength X₂ is simultaneously collected and detected by the detectors 32, 34. Thus, the respective intensities of light having the first wavelength X₁ and light having the second wavelength X₂ are measured by the individual detectors 32, 34. In the embodiment, the control unit 44, for each combination of a light guide irradiating the turbid medium 10 and a light guide 16 collecting light emanating from the turbid medium, the control unit 44 calculates the ratio of the respective intensities of light having the first wavelength and light having the second wavelength. After the scan, the image of the turbid medium is reconstructed based on the ratios of the measured intensities. Thus, relative wavelength independent noise, which is identical in measurements of both wavelengths at the same detection position, cancels out when the ratio is taken. Thus, image artefacts due to motion are reduced in the reconstructed images.

In the embodiment, during a scan, measurements of a large number of detector signals, forming a measurement set of data, are acquired. For example, in the embodiment, 2 x 256 x 256 detector signals, or 256 x 256 ratios of measured intensities, form the measurement set of data, since 2 x 256 detectors and corresponding 256 light guides 16 are provided, and the light can be directed to the interior of the receiving portion 12 via 256 different light guides 16. The measured detector signals forming the measurement set of data can then be converted into a three-dimensional image of the interior of the turbid medium using a process called image reconstruction.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiment. Variations to the disclosed embodiment can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. For example, although it has been described that a plurality of light guides 16 is provided in the receiving portion for connecting to the detecting units 24, the detecting units 24 may also be directly arranged in the receiving portion 12 without light guides 16 interposed. Although a plurality of detecting units 24 is described throughout the specification, only one detecting unit 24 may be provided which is connected to a plurality of detection positions in the receiving portion 12, i.e. ends of light guides 16, by means of the switch. However, use of a plurality of detecting units 24 is preferred.

Although it has been described that each detecting unit 24 comprises a wavelength separator 30, instead a common wavelength separator 30 may be provided for all detecting units 24.

Further, for example, the light splitting means 36 may comprise a semi- transparent mirror for splitting the light.

Further, for example, the wavelength separator 30 may comprise a mirror that reflects and transmit light depending on its wavelength. For example, light of the first wavelength may be mainly reflected, and light of the second wavelength may be mainly transmitted by said mirror.

The receiving portion 12 does not necessarily have to be formed by a cup-like receptacle as described above. It is also possible to realize the receiving portion 12 by compression surfaces between which the turbid medium is compressed. In this case, the device may comprise an optically transparent, flexible bag for accommodating the turbid medium and the optically matching medium, wherein the flexible bag and its contents, i.e. the turbid medium and the optically matching medium, are compressed between the compression surfaces. In such embodiment the surfaces are arranged mostly in parallel having a plurality of rows of light guides 16. Thus, the construction of the receiving portion could be simplified. Moreover, the position of the surfaces to each other could be made flexible. Thus, by moving the surfaces the receiving portion could be adapted to the size of turbid media.

Further, for example, instead of simultaneously collecting light having the first wavelength and light having the second wavelength, a time interval between collecting light having the first wavelength via a light guide 16 and collecting light having the second wavelength via the same light guide 16 may exist and, for example, may be less than 0,5 seconds. For example, the detecting unit 24 may comprise only a single detector. Moreover, a wavelength separator 30 may be dispensed with. Thus, light having the first wavelength and light having the second wavelength may be collected and measured subsequently within said time interval. Furthermore, all the disclosed elements and features of the described method or the described device can be combined with, or substituted for, the disclosed elements and features of the described device or the described method, except where such elements or features are mutually exclusive. 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.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. Method of irradiating an absorption contrast agent contained in a turbid medium (10), which absorption contrast agent has different absorption coefficients for light of a first wavelength (X₁) and light of a second wavelength (λ₂), and which turbid medium (10) has similar optical properties for light of the first and second wavelengths (X₁ , λ₂); the method comprising the steps of: irradiating the turbid medium (10) at at least one source position with light of the first wavelength (X₁), and, substantially simultaneously, irradiating the turbid medium (10) with light of the second wavelength (X₂) at the same source position; and measuring an intensity of light having the first wavelength (Xi) emanating from the turbid medium (10) at at least one detection position, and measuring an intensity of light having the second wavelength (X₂) emanating substantially simultaneously from the turbid medium (10) at the same detection position.

2. The method as claimed in claim 1, wherein, in the measuring step, a time interval between collecting light having the first wavelength (Xi) emanating from the turbid medium (10) at a respective detection position and collecting light having the second wavelength (X₂) emanating from the turbid medium (10) at the same detection position is less than 0.5 seconds, and wherein the respective intensity of the collected light having the first and the second wavelength (X₁, X₂) is measured.

3. The method as claimed in claim 1, wherein, in the measuring step, the intensity of light having the first wavelength (Xi) emanating from the turbid medium (10) at at least one detection position is measured, and the intensity of light having the second wavelength (X₂) emanating simultaneously from the turbid medium (10) at the same detection position is measured.

4. The method as claimed in one of the claims 1 to 3, wherein the method is a method of acquiring image data of an absorption contrast agent contained in a turbid medium (10), which absorption contrast agent has different absorption coefficients for light of a first wavelength (X₁) and light of a second wavelength (λ₂), and which turbid medium (10) has similar optical properties for light of the first and second wavelengths (X₁ , λ₂); and wherein the method further comprises the step of: - reconstructing an image of the turbid medium (10) based on the measured intensities.

5. The method as claimed in one of the claims 1 to 4, wherein the irradiating step and the measuring step are performed by subsequently irradiating the turbid medium (10) at a plurality of different source positions and measuring the light emanating from the turbid medium (10) at a plurality of different detection positions for each source position.

6. The method as claimed in one of the claims 1 to 5, further comprising the step of determining the ratio of the measured intensities for each detection position.

7. The method as claimed in one of the claims 1 to 6, wherein an absorption coefficient of the absorption contrast agent for light of the first wavelength (X₁) differs from an absorption coefficient of the absorption contrast agent for light of the second wavelength (X₂) by at least a factor of 10.

8. A device for irradiating an absorption contrast agent contained in a turbid medium, which absorption contrast agent has different absorption coefficients for light of a first wavelength (Xi) and light of a second wavelength (X₂), and which turbid medium (10) has similar optical properties for light of the first and second wavelengths (X₁, X₂); the device comprising: a receiving volume (12) for receiving a turbid medium (10) to be irradiated; at least one light source (20; 22) for irradiating an interior of the receiving volume; - at least one detecting unit (24) for measuring an intensity of light emanating from the interior of the receiving volume (12); and a control unit (44) for controlling the device for irradiating an absorption contrast agent contained in a turbid medium; the at least one light source (20; 22) being arranged to provide light of the first wavelength (X₁) and, substantially simultaneously, light of the second wavelength (λ₂); wherein the control unit (44) is adapted to control the device for irradiating an absorption contrast agent contained in a turbid medium such that: - the turbid medium (10) is irradiated at at least one source position with light of the first wavelength (X₁), and, substantially simultaneously, the turbid medium (10) is irradiated with light of the second wavelength (λ₂) at the same source position; and an intensity of light having the first wavelength (X₁) emanating from the turbid medium (10) at at least one detection position is measured, and an intensity of light having the second wavelength (X₂) emanating substantially simultaneously from the turbid medium (10) at the same detection position is measured.

9. The device as claimed in claim 8, wherein the at least one detecting unit (24) is arranged to measure the intensity of light having the first wavelength (Xi) emanating from the turbid medium (10) at at least one detection position and to measure the intensity of light having the second wavelength (X₂) emanating simultaneously from the turbid medium (10) at the same detection position.

10. The device as claimed in claim 8 or 9, wherein the device is a device for acquiring image data of an absorption contrast agent contained in a turbid medium, and wherein the control unit (44) is adapted to control the device for irradiating an absorption contrast agent contained in a turbid medium such that an image of the turbid medium (10) is reconstructed based on the measured intensities.

11. The device as claimed in any one of claims 8 to 10, wherein the device is a medical image acquisition device.

12. A computer program or computer program product for performing, when being executed on a computer, the steps of: - irradiating a turbid medium (10) at at least one source position with light of a first wavelength (Xi), and, substantially simultaneously, irradiating the turbid medium (10) with light of a second wavelength (X₂) at the same source position; and measuring an intensity of light having the first wavelength (Xi) emanating from the turbid medium (10) at at least one detection position, and measuring an intensity of light having the second wavelength (λ₂) emanating substantially simultaneously from the turbid medium (10) at the same detection position.

13. Data carrier including a computer program as claimed in claim 12.

14. Computer for executing a computer program as claimed in claim 12.