Spectroscopic determination of analyte concentration

A spectroscopic apparatus for determining a concentration and/or spatial gradient of an analyte of a bodily fluid that provides determination of a position of a capillary vessel within a biological sample in order to focus spectroscopic excitation radiation to a volume that is in close proximity to the capillary vessel but does not overlap with the capillary vessel. The provided apparatus exploits the permeability of the vessel wall with respect to an analyte that is subject to analyte concentration determination. By means of biological transport processes, the concentration of an analyte of a bodily fluid located in the capillary vessel influences the concentration in the surrounding of the capillary vessel. Spectroscopic analysis of a volume outside the capillary vessel can therefore serve for a precise and reliable analyte concentration determination inside the capillary vessel.

The present invention relates to the field of spectroscopy and more particularly without limitation to non-invasive analyte concentration determination making use of optical imaging and spectroscopic techniques.

Usage of optical spectroscopic techniques for analytical purposes is as such known from the prior art. WO 02/057758 A1 and WO 02/057759 A1 describe spectroscopic analysis apparatuses for in vivo non-invasive spectroscopic analysis of the composition of blood flowing through a capillary vessel of a patient. The position of the capillary vessel is determined by an imaging system in order to identify a region of interest to which an excitation beam for the spectroscopic analysis has to be directed.

The imaging as well as the spectroscopic analysis both make use of a common focusing arrangement enabling imaging of a capillary vessel on the one hand and allowing focusing of a near infrared (NIR) laser beam into the capillary vessel for exciting a Raman spectrum on the other hand. Typically, the focusing arrangement is also used for collection of scattered radiation evolving from the Raman processes.

In vivo non-invasive spectroscopic analysis for determination of a concentration of a distinct analyte of blood is rather sensitive to the composition of the tissue into which the spectroscopic excitation radiation is directed. For instance, focusing an excitation beam into a blood stream or into a blood vessel, the resulting return radiation is severely affected by scattering processes with red blood cells. Furthermore, due to the aspect that there exists many different analytes within the blood, an obtained Raman signal inherently represents spectroscopic information of many constituents of the blood.

Also, the plurality of various Raman signals might become subject to interference, which further complicates the detection of a particular analyte or constituent of a bodily fluid, such as blood. The rather strong dependence of obtained spectroscopic signals on morphology or consistency of spectrally analyzed biological tissue therefore limits the reproducibility of analyte concentration determination.

The present invention therefore aims to provide a spectroscopic apparatus providing improved signal quality and an increased insensitivity towards morphology, structure and composition of investigated tissue.

The present invention provides a spectroscopic apparatus for determining a concentration of an analyte of a bodily fluid, which is inside a capillary vessel. The spectroscopic apparatus comprises an imaging system for determining the position of the capillary vessel, a radiation source for generating spectroscopic excitation radiation and a radiation guiding arrangement for directing excitation radiation into volume in close proximity to the capillary vessel but not overlapping with the capillary vessel. Further, the spectroscopic apparatus comprises a radiation detector for detecting return radiation emanating from the volume in response to excitation radiation irradiation. The apparatus further has a spectroscopic analysis unit providing spectral analysis of return radiation for determining the analyte concentration within the volume and/or within the capillary vessel. Hence, the invention makes effective use of the fact, that the vessel wall of the capillary vessel is at least semipermeable for the analyte whose concentration level has to be determined by means of the spectroscopic apparatus.

It is an advantage of the present invention that for concentration determination of an analyte of a bodily fluid spectroscopic excitation radiation is not directly directed into the bodily fluid or into a stream of the bodily fluid but into a region where only various constituents of the bodily fluid are present but not the fluid in its entirety. For instance, blood plasma leaks out a capillary vessels whereas various components of blood, like red and white blood cells remain inside a volume specified by the capillary vessel walls. The invention effectively exploits the permeability of the capillary vessel wall allowing for a biological transport process to take place resulting e.g. in a diffusion of the analyte of interest into tissue surrounding the capillary vessel.

This allows for a selective analyte concentration determination because only those analytes of the bodily fluid that are capable of penetrating through the capillary vessel wall and that may be subject to a biological transport process can effectively become subject to spectroscopic analysis. As a consequence, those analytes of the bodily fluid that are not capable to penetrate through the vessel wall cannot become subject to spectroscopic investigation according to the present invention.

In contrast non-invasive spectroscopic analysis making use of directly focusing of excitation radiation into a capillary vessel, the inventive procedure effectively prevents that those analytes of the bodily fluid that are not capable to penetrate through the vessel wall do inherently not contribute to the spectroscopic signal that is detectable by means of the detector. Hence, by virtue of the hindered penetration, these analytes no longer have an impact to the spectrum of the return radiation and therefore do no longer affect spectroscopic analyte concentration determination.

Exploiting the aspect that the analyte of interest is capable to penetrate through the vessel wall, the entire spectroscopic procedure does no longer have to be performed inside the capillary vessel itself. Moreover, scattering effects of e.g. red blood cells as well as interference of Raman signals of various constituents of the blood can be reduced to a minimum. This provides an increased signal to noise ratio and improved sensitivity and hence an improved accuracy of the entire spectroscopic analysis.

According to a preferred embodiment, the spectroscopic apparatus further comprises a control unit that is adapted to determine the position of the volume with respect to the determined position of the capillary vessel. Preferably, the control unit is adapted to determine the position of the volume in response to receive an input from the imaging system. The imaging system, which is preferably implemented as an optical image acquisition system, provides position or location information of a capillary vessel that contains the bodily fluid. By means of optical image acquisition and image processing, various parameters specifying an absolute or relative position of the capillary vessel as well as specifying its morphology or geometric structure can be obtained. This information gathered by the imaging system is typically processed by the control unit in order to determine an absolute or relative position as well as a circumference or size of the volume.

The volume determined by means of the control unit typically specifies an inspection volume of the sample, which defines a volume into which excitation radiation is directed into. A portion of the excitation volume from which emanating return radiation is inspected is typically denoted as detection volume and is entirely included in the excitation volume. Hence, the detection volume can completely coincide with the excitation volume but may also specify a smaller volume. In general, the circumference as well as the position of the detection volume can be arbitrarily and independently modified within the circumference of the excitation volume, e.g. by changing a size of a pinhole of a confocal detection arrangement.

Preferably, the control unit autonomously determines position, circumference and structure of the volume in response to parameters obtained from the imaging system or from image processing means. Typically, the position of the volume is determined with respect to the position of the capillary vessel, i.e. the determined position or location of the capillary vessel. For instance, the position of the volume may be specified by a predetermined distance from the capillary vessel not exceeding a predefined threshold. In this way it is effectively guaranteed, that the volume to which spectroscopic excitation radiation is applied, does not overlap with the capillary vessel but is in close proximity to the capillary vessel, such that the concentration of analytes penetrating through the vessel wall does not drop below a minimum detection threshold.

According to a further preferred embodiment of the invention, the radiation guiding arrangement comprises a focusing arrangement for focusing the excitation radiation into the volume. The focusing arrangement additionally provides varying of the focal spot size of the excitation radiation in the volume. In this way either focused or non-focused radiation can be applied to the volume, thus allowing to spectrally investigate a region of variable size. Depending on the analyte of interest and the general properties of the tissue to which the excitation radiation is directed, a larger or smaller focal spot size might be beneficial. Generally, a smaller focal spot size allows for higher radiation intensity in the volume and therefore inherently provides a rather large intensity of the scattered signal. However, focusing the excitation radiation to a rather small spot size, the morphology and internal structure of the tissue surrounding the capillary vessel play a more predominant role. For instance, when applied to bodily tissue, the spectrum of scattered radiation may strongly depend on whether the excitation radiation is focused into the inner part of a cell or to a cell membrane. Consequently, by enlarging the focal spot size of the excitation radiation, aspects of the morphology or internal geometric structure only have a minor impact on the detectable spectroscopic signals. In such a case, the obtained spectrum represents an average of various spectroscopic signals obtained from different biological structures that are located inside the volume. Enlarging of the focal spot size therefore provides an increased insensitivity of the spectroscopic analysis towards the structure of the bodily tissue.

According to a further preferred embodiment of the invention, the volume is moveable with respect to the capillary vessel during the detection of return radiation. For instance, during spectroscopic analysis, i.e. application of excitation radiation into the volume, the volume can be moved, which generally provides dynamic spectroscopic analysis of the vicinity of the capillary vessel. For example, the volume can be moved in such a way that the distance between volume and the capillary vessel varies. In such a configuration, the spectroscopic apparatus provides to determine a spatial concentration gradient of the analyte in the bodily tissue surrounding the capillary vessel. In another constellation, the volume might be moved with respect to the position of the capillary vessel at a constant distance to the capillary vessel.

For instance, if the capillary vessel is an elongated blood vessel, the volume may be moved along the direction of elongation of the capillary vessel. This allows to determine the analyte concentration at numerous locations inside the bodily tissue, each of which having the same distance to the capillary vessel. In this way spatial inhomogeneities of the tissue and/or the capillary vessel or vessel wall can be effectively compensated.

In particular, when moving the volume in such a way that the distance between capillary vessel and the inspection volume changes, the movement should preferably be performed on a timescale which is below the timeframe of diffusion processes inside the bodily tissue. Otherwise temporal fluctuations in the concentration of the analyte may falsify the obtained results. Therefore, an increase of the distance between first and volumes during a spectroscopic analysis shall be performed on a timescale that is smaller than the timescale on which the concentration of the analyte of the bodily fluid typically changes. For instance when determining the concentration of blood glucose in the vicinity of a blood vessel the diffusion time of glucose in the respective tissue always has to be taken into account. Depending on the area or body part to which the spectroscopic analysis is applied, the diffusion time of the analyte may strongly vary. The diffusion time of glucose is governed by the so called glucose transporters, which are tissue specific membrane proteins that enable transport of glucose through cell membranes of cells forming the tissue surrounding e.g. blood capillaries.

As already described above, glucose can diffuse freely through the capillary walls into the interstitial fluid between the cells of bodily tissue. From Einstein's relation the average time that a glucose molecule needs to diffuse over a distance of 100 μm is estimated to be around 5 s, when assuming a diffusion coefficient of 1*10−9 m2/s as measured in water. Since glucose is presumably transported paracellular, i.e. not through the cells but around them, the transport distance may be considerably longer than the measured dimensions of the tissue. Further, a net transport of glucose through the tissue only occurs in the case of a concentration gradient. As an example, applying Fick's law and assuming a concentration difference of 1 mM and a diffusion distance of 100 μm, the glucose flux can be estimated to be about 6 molecules/s, if the capillary surface is around 1 μm2.

According to Stryer L., Biochemistry 4thedition, W.H. Freeman and Company, New York 1995 there exists a variety of glucose transporters, denoted as GLUT that are particularly adapted for glucose transportation in various kinds of tissue. For instance, GLUT 1 provides glucose transportation for nearly all mammalian cells, erythrocytes, placenta, or fetal tissue. GLUT 2 is particularly relevant for glucose transportation in liver, kidney, intestine and pancreatic β-cell. GLUT 3 is provides glucose transportation in the brain and GLUT 4 serves to transport glucose in skeletal muscles, cardiac muscles, and in adipose (fat) tissue.

According to a further preferred embodiment of the invention, the capillary vessel comprises a blood vessel and the analyte is blood glucose. In this way the spectroscopic apparatus is particularly operable to determine blood glucose concentration of blood flowing through blood vessels of a person or an animal. Generally, the spectroscopic apparatus provides in vivo non-invasive blood glucose concentration making use of a spectroscopic analysis performed in tissue surrounding a capillary blood vessel.

According to a further preferred embodiment of the invention, the spectroscopic analysis unit is further adapted to determine the analyte concentration by making use of distance information between the capillary vessel and the volume. Making use of an appropriate calibration of the spectroscopic apparatus by determining the glucose concentration in the vicinity of a blood vessel, also the glucose concentration in the capillary vessel can be derived. Having knowledge of the glucose or analyte transport properties of the surrounding tissue and having knowledge of the distance between capillary vessel and the volume, determination of a glucose or analyte concentration within the volume is generally sufficient for a precise and reliable determination of the glucose concentration of the bodily fluid flowing inside the capillary vessel.

In another aspect the invention provides a method of determining a concentration of an analyte of a bodily fluid that is located in a first volume which is confined by a capillary vessel wall of a biological sample. The capillary vessel wall is at least semipermeable for the analyte and the method comprises determination of a position of the first volume and determination of a second volume with respect to the position of the first volume. The second volume does substantially not overlap with the first volume. Hence, first and second volumes are therefore separated by a predefined distance. After having specified the second volume, which is typically in close proximity to the first volume and therefore in close proximity to a capillary vessel, the inventive method provides application of excitation radiation into the second volume by means of a radiation source and a radiation guiding arrangement.

In response to impingement of excitation radiation in the second volume, various scattering processes of either elastic or inelastic type may occur, the latter of which typically features a wavelength shift being allowing to identify those molecules that are located inside the second volume. Detection of scattered radiation and in particular of inelastically scattered radiation emanating from the second volume allows to perform a spectral analysis for determining the concentration of a specific analyte.

In a preferred embodiment the inventive method further provides determining of at least a third volume that does substantially not overlap with the first volume. This at least third volume is determined with respect to the position and/or geometry and size of the first volume and/or with respect to the position and/or geometrical structure of the second volume. In an additional successive step the excitation radiation is then also directed into the at least third volume by means of the radiation guiding arrangement. Typically, the excitation radiation is focused into the at least third volume by means of a focusing arrangement of the radiation guiding arrangement. Accordingly, return radiation emanating from the third volume is detected and exploited for spectral analysis. In this embodiment spectroscopic investigation of the second and the at least third volume is typically performed successively. Further, the second and the at least third volumes may at least partially overlap.

In another aspect the invention provides a computer program product for a spectroscopic apparatus for determining a concentration of an analyte of a bodily fluid, which is located in a capillary vessel that is confined by a capillary vessel wall of a biological sample. The capillary vessel wall is at least semipermeable for the analyte, thus providing diffusion of an analyte of interest into the vicinity of the capillary vessel. The computer program product is operable by the spectroscopic apparatus and comprises computer program means for processing of an output of an imaging system for obtaining position information of the capillary vessel, for determining a volume by making use of the position information, wherein the volume is substantially not overlapping with the capillary vessel. The computer program means further provide control of a radiation guiding arrangement for directing excitation radiation into the volume. The program means are further adapted to process an output signal of a detector of the spectroscopic apparatus for spectral analysis of return radiation that is detectable by the detector.

Further, the computer program means of the computer program product provide determination of the concentration of the analyte by making use of the position information and the spectral analysis of the detected return radiation. The determined concentration of the analyte may either refer to the analyte concentration in the volume or an analyte concentration within the capillary vessel.

Further, it is to be noted that any reference signs in the claims are not to be construed as limiting the scope of the present invention

FIG. 1shows a schematic block diagram of the spectroscopic apparatus and its major components. The spectroscopic apparatus100has a radiation source118, a light coupling arrangement110, a focusing lens112, an imaging system114, a spectroscopic analysis unit116and a control unit120. In the illustrated embodiment, the spectroscopic apparatus100is applicable to skin tissue of e.g. a human patient. The tissue or body part of the human patient comprises a blood vessel104underneath of the surface of the skin102. The blood vessel104features a blood vessel wall108that is highly permeable for the analyte that shall become subject to concentration determination, like e.g. blood glucose.

The shape and dimensions of the blood vessel104specify a first volume, whose location and structure is determined by means of the imaging system114. Preferably, imaging as well as spectroscopic analysis is performed by making use of the common objective lens112. Even though imaging as well as spectroscopic analysis may both make use of a common radiation source118, also an additional light source for image acquisition might be implemented, e.g. operating in a different spectral range than the radiation source118which typically provides excitation radiation in the near infrared or infrared spectral range.

The spectroscopic apparatus100makes effective use of the fact, that e.g. blood glucose is capable of penetrating through the vessel wall108of a blood vessel104. Therefore, in order to minimize scattering effects of red blood cells as well as to reduce an impact of interference of spectroscopic signals, the excitation radiation is preferably focused into an inspection volume122, which is located at a predefined distance106with respect to the position of the blood vessel104. Hence, the inventive spectroscopic apparatus makes effective use of biological transport processes, such as e.g. diffusion, therefore principally allowing to detect a blood glucose concentration not inside a blood vessel but in close proximity outside a blood vessel.

The distance106between the blood vessel104and the spectroscopic inspection volume122is governed by the permeability of the vessel wall108as well as by the underlying analyte transport properties of the tissue surrounding the blood vessel104. Typically, with increasing distance106, the concentration level of the analyte decreases. Therefore, the distance106shall not exceed a predefined maximum distance for which the analyte concentration gradient may drop below a minimum value.

The imaging system114of the spectroscopic apparatus100is adapted to acquire images of the area in the vicinity of the blood vessel104and might be provided with image processing means allowing to identify or to recognize the position, geometry and course of the blood vessel104underneath the surface of the skin102. The imaging system can for instance be implemented by making use of e.g. Orthogonal Polarized Spectral Imaging (OPSI), Confocal Video Microscopy (CVM), Optical Coherence Tomography (OCT), Confocal Laser Scanning Microscopy (CLSM), Doppler based imaging, photoacoustic and ultrasound based imaging.

Based on the image acquisition performed by the imaging system114and subsequent image processing, the control unit120is adapted to autonomously determine the location and size of the inspection volume or excitation volume122. Determination of the inspection or excitation volume122may further be performed with respect to parameters classifying the tissue surrounding the blood vessel104. For instance, determination of the position and size of the inspection volume may be performed with respect to the blood glucose diffusion speed of the tissue and/or with respect to the tissue specific spatial blood glucose concentration gradient in the tissue. Alternatively, also the detection volume might be determined irrespectively of the size of the excitation or inspection volume, i.e. based on the image processing, the control unit may autonomously specify the size and/or location within the excitation volume from which emanating return radiation is detected for spectroscopic analysis.

Once the inspection volume122has been determined by the control unit120, excitation radiation124provided by the radiation source118is focused into the inspection volume122. In particular that portion of the return radiation126that has been subject to inelastic scattering in the inspection volume122and therefore provides a wavelength shift compared to the wavelength of the excitation radiation124can be effectively exploited for spectral analysis and principally allows to determine the concentration of a distinct analyte being located inside the inspection volume122.

Generally, there exist various constellations of how to make use of inspection volume, excitation volume and detection volume. First, excitation and detection volume might entirely coincide and may be used to successively scan the inspection volume with a relatively small detection and excitation volume. Second, excitation and detection volume might be as large as the entire inspection volume and third, the excitation volume may completely coincide with the inspection volume whereas the relatively small detection volume is used to successively scan the area of the inspection volume.

Separation of elastically and inelastically scattered return radiation126is effectively performed by the light coupling arrangement102, which typically comprises various beam splitters and dichroic elements providing a wavelength specific deflection of the spectral components of the return radiation126.

If appropriately calibrated, the spectroscopic apparatus100not only provides concentration determination of the analyte inside the inspection volume122but also provides analyte concentration determination inside the blood vessel104by making use of a correlation between analyte concentration levels inside the inspection volume122and inside the blood vessel104. Having knowledge of e.g. a correlation of the blood glucose concentration level inside and outside the blood vessel104and having further knowledge of a typical spatial blood glucose concentration gradient in the surrounding tissue, by determining the blood glucose concentration inside the inspection volume122and by determining the distance106between inspection volume122and blood vessel104, also the blood glucose concentration level inside the blood vessel104can be precisely derived.

FIG. 2schematically shows a diagram200exemplary illustrating blood glucose concentration204versus distance202from a blood vessel104. It can clearly be seen that with increasing distance from the blood vessel, the blood glucose concentration decreases monotonously. By means of a calibration procedure such spatial blood glucose concentration gradients can be recorded and stored and may serve as a reliable means for correlating a blood glucose concentration measure outside a capillary vessel to a blood glucose concentration inside the capillary vessel. Since the blood glucose concentration level constantly drops for increasing distance from the blood capillary, it is advantageous to specify a maximum distance106between the inspection volume122and the capillary vessel104.

FIG. 3schematically illustrates a graph300displaying a lag time of the analyte concentration304versus distance302from the blood vessel104. The lag time specifies a time interval after which a change of the analyte concentration in the blood vessel104can be measured in the inspection volume122that is located at a given distance302. The lag time increases constantly with increasing distance and is further governed by the underlying biological transport mechanism of the surrounding tissue. For instance, the lag time reflects the diffusion speed of the analyte in the tissue surrounding the blood vessel. It therefore represents a temporal delay between analyte concentration changes that occur in the blood vessel104and in the inspection volume122.

Since the lag time increases with increasing distance from the blood vessel, it is advantageous to specify an upper limit for the distance106in order to guarantee that a change in the analyte concentration within the blood vessel can be detected by the inventive method within a predetermined time interval. This aspect is extremely relevant in emergency situations, where the blood glucose concentration may drop below a critical value thus causing a clinical shock state of the patient.

FIG. 4schematically illustrates various inspection volumes122and128to which excitation radiation can be applied. For example, inspection volume122represents the focal spot size of the excitation radiation124covering an area which is of a similar size than the diameter of the blood vessel108. This rather focused spot provides a rather large radiation density in the inspection area122leading to a corresponding large intensity of the scattered radiation126. However, scattered radiation obtained from rather small focal spots is also quite sensitive to variations of the morphology or biological structure of the irradiated tissue. Hence, the spectrum of scattered radiation obtained from radiation focused inside a cell may drastically vary from the spectrum that is obtained when the focal spot is directed into interstitial fluid between the cells.

Therefore, the light guiding arrangement and its focusing arrangement of the spectroscopic apparatus100provide variation of the focal spot size of the excitation radiation inside the second volume. Hence, the focal spot described by the inspection volume122can for instance be enlarged to the inspection volume128. In this case the radiation intensity typically decreases but scattering processes occur in a variety of different biological structures, thus leading to a spatial averaging of the spectrum of the return radiation126.

FIG. 5schematically shows a lateral displacement of the second volume underneath of the surface of the skin Here, the various positions of the inspection volume122are indicated by positions130,132,134and136. As can be seen, the inspection volume122has been displaced along a horizontal inspection path138as indicated by the arrow. In this way the concentration of the analyte can be determined at various different distances from the capillary vessel104. Typically, the inspection volume122, hence the focal spot of the excitation radiation, is moved along the inspection path138during detection of return radiation from the respective focus spots. In this way the inventive method even allows to determine a spatial analyte concentration gradient, which in turn can be exploited as an indicator for diseases, such as diabetics. For instance, blood vessels of diabetic patients typically feature a different permeability with respect to blood glucose compared to blood vessels of healthy persons. In this way by measuring a spatial glucose concentration gradient an indication of a disease might be directly obtained.

The inspection path138does by no means have to be substantially perpendicular to the elongation of the blood vessel108. For instance, the inspection path138may also specify numerous inspection volumes, each of which featuring the same distance to the blood vessel108. As an example the various inspection volumes130, . . . ,136may be arranged in a vertical direction mimicking the course of the blood vessel108. In this constellation, the successively obtained spectra can be mutually combined for an averaging procedure allowing for effective elimination of measurement artefacts.

FIG. 6shows an alternative embodiment, where various inspection volumes140,142,144and146are arranged in a rectangular like way in close vicinity to the blood vessel108. In this constellation each of the inspection volumes140, . . . ,146essentially features a comparable distance to the blood vessel. Therefore, spectra that may be obtained from these inspection volumes should all represent a similar analyte concentration level. Hence, combining of the spectra that correspond to the indicated inspection volumes provides an effective means of averaging and error elimination.

FIG. 7illustrates a flowchart of performing the inventive method of determining of the analyte concentration. In a first step702, the position and/or geometry as well as the course of a blood vessel is determined by making use of the imaging system. Based on the obtained image of the blood vessel and successive image processing, in a following step704the position of the second volume, i.e. the inspection volume is determined. Determination of the position and/or size of the second volume is typically performed with respect to the position of the blood vessel as well as with respect to the transport properties of the tissue surrounding the blood vessel.

After determination of the inspection volume, in a successive step706spectroscopic excitation radiation generated by the radiation source is focused into the determined second volume, which typically leads to a variety of elastic and inelastic scattering processes. In a further, not illustrated step, the detection volume of the spectroscopic apparatus may be adapted and adjusted with respect to the size and/or position of the excitation or inspection volume.

During irradiation of excitation radiation into the second volume, in the following step708scattered return radiation is detected by means of a detector whose output is processed and analyzed in step710. Here, a spectral analysis of the return radiation is performed by making use of a spectrometer. Based on the analyzed spectrum in a final step712, the concentration of the analyte can be determined. Additionally, by making use of a distance parameter between the second volume and the position of the blood vessel as well as by making use of classified diffusion properties of the surrounding tissue, also the analyte concentration within the blood vessel can be precisely derived.

In essence, the invention provides non-invasive determination of blood glucose concentration by making use of spectral analysis of tissue in close vicinity of a blood vessel. Since the spectroscopic inspection volume does substantially not overlap with the blood vessel, a disadvantageous signal degradation due to scattering from red blood cells as well as temporal variations of an obtained spectroscopic signal that are due to the blood flow can be effectively reduced to a minimum. Also, effects of interference of spectroscopic signals arising from various non-relevant analytes of the blood can be effectively eliminated.

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