NL2016273B1

NL2016273B1 - Method, apparatus and software for detection and localization of hidden defects in optically diffuse media.

Info

Publication number: NL2016273B1
Application number: NL2016273A
Authority: NL
Inventors: Jacques Marie Ruers Theodoor; Kho Esther; Josephus Cornelus Maria Sterenborg Henricus
Original assignee: Stichting Het Nederlands Kanker Instituut-Antoni van Leeuwenhoek Ziekenhuis
Priority date: 2016-02-16
Filing date: 2016-02-16
Publication date: 2017-08-22

Abstract

In a method and apparatus, a property of an optically diffuse medium comprising a first optical absorber having a first concentration and a second optical absorber having a second concentration is determined. A surface area of the medium is imaged at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber. A reflectance spectrum of the medium at the surface area at the multiple wavelengths is determined. A derivative of the determined reflectance spectrum around the isosbestic wavelength is determined. From the derivative, a concentration ratio of the first concentration and the second concentration is estimated.

Description

Method, apparatus and software for detection and localization of hidden defects in optically diffuse media

FIELD OF THE INVENTION

The invention relates to optical imaging of highly scattering media. In particular, this invention relates to optical imaging of optically diffuse biological materials, or media, for detecting and locating damage, disease and/or (defects in) anatomic structure. This damage, disease and (defects in) anatomic structure may be hidden, i.e. poorly visible or invisible to the human eye.

BACKGROUND OF THE INVENTION

Visual inspection is the oldest technique available to mankind to assess the quality of biological materials such as fruits, vegetables, meat, fish and live human tissue. Moreover, until recently, visual inspection has been the only technique available to mankind to assess the health status of said biological materials from the outer surface thereof.

Recently, camera based optical imaging has made its entry into the medical field. The power of optical imaging lies in the phenomenon that light penetrates up to centimeters into many biological materials. In most of these materials light is scattered strongly and becomes diffuse. In addition, very specific wavelengths of light are absorbed by specific tissue components.

Together, absorption and scattering determine the amount and spectral distribution of the light that is reflected. Hence, diffuse optical images do not only give information on the shape and the surface of the material imaged, but more importantly on what is below the surface.

Depending on the wavelength of the light, sampling depths in the order of, or up to, a centimeter, cm, are feasible.

In multispectral or hyperspectral imaging, optical images are acquired using dedicated camera systems that detect light in different wavelength bands. The images taken by these cameras are often processed by various image processing methods to obtain information on the status of biochemical composition of the imaged object. Examples of materials subjected to diffuse optical imaging range from raw foods to assess their freshness, crops to assess their status, to live human tissues to detect and locate remaining cancer cells during and after surgery.

Although light scattering is the very phenomenon that enables diffuse optical imaging, it also complicates quantitatively accurate imaging, and it limits the sharpness of the images taken.

When imaging a homogenous medium, the diffuse light captured in the form of diffuse reflection gives an excellent representation of the optical properties of the tissue. If images are taken at sufficient wavelengths, then there are ample mathematical models available that allow spectral unmixing, i.e. the extraction of the different concentrations of the tissue components.

These methods vary from complicated diffusion theory based spectral analyses based on a lot of prior knowledge on light transport and the tissue components to purely mathematical techniques such as spectral unmixing, SU, or machine learning, ML. Often, more simple approaches are used such as ratioing the diffuse reflectance at two strategically chosen wavelengths.

Preferentially, however, diffuse optical imaging is used in cases where a defect is not spread over the tissue evenly, and its location is not known. In fact, the purpose of imaging usually is to detect and locate a defect. In such a case the diffuse nature of the reflected light limits the sharpness of the image, and thereby decreases the accuracy of detection of small defects.

In diffuse media, the sampling depth and imaging resolution are inversely related: in the case of a decreased scattering coefficient the diffuse light detected has sampled deeper into the tissue, generating an image with a lower spatial resolution, while a sample with a larger scattering coefficient samples more superficially and may have a higher spatial resolution. A case of inhomogeneously distributed optical properties is not rare, since most biological materials are very inhomogeneous. A possible defect can be considered as an additional inhomogeneity. For diffuse light this has serious consequences as the diffuse light will encounter different optical properties at different locations. Because these optical properties are wavelength dependent and this wavelength dependence is different for different tissue compositions, the diffuse light will have different distributions at different wavelengths.

As a consequence, different wavelengths will sample different volumes. This will compromise any spectral unmixing approach. This phenomenon is very well known in the field of diffuse optical spectroscopy.

In many cases, simple ratios of images taken at strategically chosen wavelengths can give excellent information on the ratio between different absorbing components. These numbers can then be used to characterize the tissue.

As an example, a fat-to-water ratio image has been proposed to be of particular interest, in particular for a diagnosis of breast cancer. Such a ratio would be accurate in homogenous tissue. Due to the inhomogeneous nature of the distributions of these tissue components, the ratios calculated have been documented to be in error by as much as 50%.

For a medium, the diffuse reflection ^dcan be related to the absorption coefficient by the relation:

(1.1) where Aa stands for the absorption coefficient and ii) for the average path length of the detected photons.

When taking two wavelengths and 'h, a ratio X can be calculated from the natural logs of the diffuse reflections:

(1.2)

Assuming the presence of two absorbers at molar concentrations Q and ^2, then:

(1.3) where i^3.m(A)stands for the molar absorption coefficient of absorber with index m.

Introducing the concentration ratio:

(1.4) so that (1.3) can be written as:

(1.5)

Now expression (1.2) can be rewritten into:

(1.6)

From expression (1.6) it is clear that, when the path lengths Ui) and (½) are identical and the absorption properties of both absorbers are known, the concentration ratio Ψ can be derived from the ratio of the natural logarithms of the reflectances. The strongest sensitivity is found when wavelengths and ^2 are chosen to be at wavelengths where the two absorbers have maximum differences. This ratio is often used in hyper or multi spectral imaging to quantify concentration ratios of chromophores.

The main problem in practical application of expression (1.6), lies in the assumption that {li}=Uz). In practice the path lengths, ih) and (½), are dependent on the optical properties.

There are three main reasons why the assumption (41)=(^2) is problematic in practical use. A first reason is that the path iength is dependent on both the absorption and the scattering properties. When aiming at a maximum sensitivity, the wavelengths and h are chosen to be at wavelengths where the two absorbers have maximum differences. This results in very different path lengths and sampling volumes, even in homogeneous tissue. A second reason is that the two tissue types with different concentrations of the two absorbers considered may be biologically very different. Hence it is likely that these tissues will also have very different scattering properties. This too causes large differences in path lengths and hence differences in sampling volumes. A third reason is that the need for imaging implies inhomogeneous tissue, where different types of tissue with very different optical properties coexist. This results in very different path lengths and very different sampling volumes in the different tissue areas. Also it can generate artefacts in the boundary region between the two tissues. Detected photons show a preference for ‘paths of lowest absorption’: Light detected at the wavelength of maximum absorption of a first absorber will have a stronger contribution from photons that have travelled preferentially through tissue containing a higher concentration of a second absorber, and vice versa. So, even if (k) would be of the same length as (h), the two paths would have sampled very different tissue volumes. This problem leads to an overestimation of the total amount of tissue components.

SUMMARY OF THE INVENTION

It would be desirable to provide a method and apparatus for optically imaging a spatial distribution of the ratio of concentrations of optical absorbers comprised in a medium. It would also be desirable to more accurately perform said optical imaging.

It would further be desirable to provide a method and device to optimize the spatial resolution in said imaging method.

To better address one or more of these concerns, in a first aspect of the invention a method of determining a property of an optically diffuse medium is provided. The medium comprises a first optical absorber having a first concentration and a second optical absorber having a second concentration. The method comprises the steps of; (a) imaging a surface area of a volume of the medium at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber; (b) determining a reflectance spectrum of the medium at the surface area at the multiple wavelengths; (c) determining a derivative of the determined reflectance spectrum around the isosbestic wavelength; and (c) estimating a concentration ratio of the first concentration and the second concentration from the derivative.

The invention is based on the approach that if there are problems due to differences in optical properties, then wavelengths should be used where the optical properties, in particular the optical path lengths are identical. Hereby, variations in sampling volume caused by differences in optical properties at the imaging wavelengths can be prevented. A meaningful image can still be created when using such wavelengths, as explained below.

As no expression is available for the optical path length, the optical penetration depth,is looked into, defined as: (1.7)

wherein /j s is a reduced scattering coefficient. Expression (1.6) can be rewritten as:

(1.8) A path length corrected absorption ratio can now be calculated with:

(1.9)

This clearly illustrates a part of the problem: to obtain Ψ from the ratio of the scattering coefficients

needs to be known.

Now a wavelength, is looked for, for which the two tissues, first and second optical absorbers, have identical path lengths at the two wavelengths. This ensures that photons at both wavelengths do not see any differences between the two tissues and cannot do anything else than sampling the same volume.

The question, however, is that if the photons cannot ‘see’ differences between the tissues, how any differences can be imaged, i.e., the consequence of such a choice would be that at every concentration ratio the ratio X would be equal to 1.

Therefore, concentration ratios are obtained from the differences in shapes of the diffuse reflection spectra, by looking at the first or higher derivatives of the diffuse reflectance with respect to the wavelength measured closely around wavelengths that generate identical path lengths.

An imaging ratio DX is defined by subtracting two images taken ΔΑ apart from a central wavelength and dividing by the sum of the two images:

(1.10)

Because Δ1 is taken small with respect to h, around ks can be linearized, so that: (1.11)

This can be worked into

(1.12)

Using a linear expansion of around :

(1.13)

Rearranging:

(1.14) leads to:

(1.15)

Approximating (½) according to expression (1.7) yields:

(1.16) which leads to:

(1.17)

Thus: (1.18)

Differentiating expression (1.5) versus ^‘yields:

(1.19)

Before Ψ can be solved from expression (1.19) some strategic choices are made. First, wavelengths for which the absorption spectra of the two absorbers have an isosbestic point are considered. From expression (1.5) it is derived:

(1.20)

Where stands for the wavelength at which the two molar absorption coefficients ί-β ΐ'Λ) and are identical. Now expression (1.16) turns into:

(1.21)

The first term in expression (1.21) has a linear dependence on 'Ψ with constants that are related to the first derivative of the absorption spectra. Thus, the ratio image defined in expression (1.21) can be directly interpreted in terms of Ψ if the absorption spectra of the two absorbers are known and sufficiently different in shape. Obviously, the ratio increases with increasing Taking too large will induce deviations from the premise that βα is constant. The order of magnitude of the first term depends on the steepness of the absorption curves. A simple estimate can be by considering a Gaussian absorption profile with bandwidth

(1.22) where K and stand for the central wavelength and the half bandwidth, respectively.

Now an estimate for the first term in expression (1.18) can be derived:

(1.23)

At the steepest point of the absorption curve, i.e. ^ “ *ic equals ±Λί.·. In case the isosbestic point -½ lies anywhere near the absorption band this turns into:

(1.24)

The second term has a more complicated behaviour. It describes the relative change in scattering coefficient with wavelength. In the case of Mie scatter;

(1.26)

The second term in expression (1.18) simply becomes:

(1.27)

It is important to note that this is independent of the amount of scattering. It can be seen that the first term is much larger than the second term:

(1.28)

While values for b for most tissues are in the range from 0.5 to 2.0 and typical valuea for and of 40 nm and 1000 nm it is clear that the requirement of expressions (1.28) is fulfilled and that the first term in expression (1.18) is much bigger than the second term. An estimation of the concentration ratio Ψ can now with sufficiently high accuracy be calculated from:

(1.29) where the constants fandgare the relative derivatives of the two absorbers defined by expressions (1.30):

and

(1.30) and where: - λ indicates a wavelength; - λο indicates an isosbestic wavelength; - Αλ indicates a wavelength difference from the isosbestic wavelength; - DX indicates an imaging ratio, defined by subtracting two images taken Αλ apart from an isosbestic wavelength λο and dividing by the sum of the two images; - indicates a molar absorption coefficient of absorber m; - ga,o indicates a molar absorption coefficient at isosbestic wavelength.

Thus, the concentration ratio ( Ψ) may be determined using the partial derivative of a first molar absorption coefficient of the first absorber as a function of the wavelength, and the partial derivative of a second molar absorption coefficient of the second absorber as a function of the wavelength, both taken at a wavelength at which the first molar absorption coefficient and the second molar absorption coefficient are equal.

Instead of, or in addition to, the first derivative used here, similar considerations as the ones above may be applied to estimate the concentration ratio Ψ, based on the use of second or higher derivatives. However, it is noted that the signal to noise ratio decreases substantially for the second or higher derivatives, which makes it at least less effective to use the second or higher derivatives.

In an embodiment, the method of the invention further comprises the steps of comparing the concentration ratio Ψ to a predetermined concentration ratio range, and indicating a defect for the surface area when the concentration ratio is outside the concentration ratio range.

For various media, in particular media comprised of, or comprising, living tissue, a concentration ratio of a first absorber and a second absorber in healthy tissue differs, often substantially, from a concentration ratio of unhealthy tissue. As an example, in mammal breast tissue, normal tissue has a lower fat to water ratio than cancer tissue. Now, by defining a concentration ratio range applicable to normal tissue, cancer tissue may be detected from its higher fat to water ratio, outside the concentration ratio range.

Accordingly, for one or more surface areas the concentration ratio may be determined, and when a defect is indicated for at least one surface area, an indication of a defect may be provided for the totality of surface areas. For example, an indication may be provided in an image of a plurality of surface areas, by colouring any surface area in which the concentration ratio is outside the concentration ratio range differently than other surface areas. From the image, a location and extension of cancer tissue may be recognized.

In an embodiment, the method of the invention further comprises the steps of repeating steps (a) to (d) for different isosbestic wavelengths of the first absorber and the second absorber in different wavelength regions to obtain estimated concentration ratios for each one of the isosbestic wavelengths.

Over a large range of wavelengths, there will often be several wavelength regions containing one or more isosbestic wavelengths where the optical penetration depth is independent of the ratio of the absorbers. The optical penetration depths in different wavelength regions can vary from a few centimetre to less than a millimetre. By performing the first and higher derivative imaging around isosbestic wavelengths in different wavelength regions, the optical penetration depth (sampling depth), and thereby the sampling volume of the imaging method can be changed.

In an embodiment, the method of the invention with repeating performing steps (a) to (d) for different isosbestic wavelengths further comprises comparing each one of the estimated concentration ratios to a predetermined concentration ratio range, and indicating a defect for the surface area when at least one of the concentration ratios is outside the concentration ratio range.

In an embodiment, the method of the invention with repeating performing steps (a) to (d) for different isosbestic wavelengths further comprises associating each isosbestic wavelength with a respective optical penetration depth, and indicating a defect and a depth thereof for the surface area if at least one of the concentration ratios is outside the concentration ratio range.

Thus, the method allows for providing not only indications of defects for surface areas, i.e. in two dimensions, but also for providing indications of the defects for sampling volumes, i.e. in three dimensions including a volume located beneath the imaged surface area.

In an embodiment, the method of the invention further comprises associating each isosbestic wavelength with a respective optical penetration depth, and, if small defects are to be located. selecting an isosbestic wavelength among the different isosbestic wavelengths with a low optical penetration depth.

By changing isosbestic wavelengths, not only the optical penetration depth is changed, but also the spatial resolution. A relatively high optical penetration depth is associated with a relatively low spatial resolution, and a relatively low optical penetration depth is associated with a relatively high spatial resolution. Thus, if particular sensitivity to small defects is required, a wavelength region with relatively low optical penetration depth is selected, to obtain a higher spatial resolution and hence a lower threshold for detection.

In a second aspect of the invention, an apparatus for determining a property of an optically diffuse medium is provided. The medium comprises a first optical absorber having a first concentration and a second optical absorber having a second concentration. The apparatus comprises: a light source configured to illuminate a surface area of a volume of the medium; a filtering device configured to receive reflected light from the surface area of the medium and to transmit filtered light to an optical imaging device, wherein the filtering device is configured to filter multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber; a twodimensional, 2D, optical imaging device configured to receive the filtered light from the filtering device; an image processing component configured to determine a reflectance spectrum of the medium at the surface area at the multiple wavelengths; a calculating component configured to determine a derivative of the determined reflectance spectrum around the isosbestic wavelength; and an estimating component configured to estimate a concentration ratio of the first concentration and the second concentration from the derivative.

In a third aspect of the invention, an apparatus for determining a property of an optically diffuse medium is provided. The medium comprises a first optical absorber having a first concentration and a second optical absorber having a second concentration. The apparatus comprises: a light source configured to illuminate a surface area of a volume of the medium; a onedimensional, 1D, or twodimensional, 2D, hyperspectral optical imaging device configured to receive reflected light from the surface area of the medium; an image processing component configured to determine the reflectance spectrum of the medium at the surface area at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber; a calculating component configured to determine a derivative of the determined reflectance spectrum around the isosbestic wavelength; and an estimating component configured to estimate a concentration ratio of the first concentration and the second concentration from the derivative.

In an embodiment of the apparatus, wherein the optical imaging device is a 2D hyperspectral optical imaging device comprising an image sensor having a plurality of pixels, the image processing component is further configured to: assign a cluster of pixels to the surface area of the medium; determine the reflectance spectrum for each pixel of the image sensor; and determine the reflectance spectrum of the medium at the surface area from the reflectance spectra of the pixels of the cluster.

In a fourth aspect of the invention, an apparatus for determining a property of an optically diffuse medium is provided. The medium comprises a first optical absorber having a first concentration and a second optical absorber having a second concentration. The apparatus comprises: a light source configured to illuminate a surface area of a volume of the medium; a spectrograph configured to receive reflected light from the surface area of the medium; a light processing component configured to determine the reflectance spectrum of the medium at the surface area at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber; a calculating component configured to determine a derivative of the determined reflectance spectrum around the isosbestic wavelength; and an estimating component configured to estimate a concentration ratio of the first concentration and the second concentration from the derivative.

In a fifth aspect of the invention, a computer program of computer program product is provided. The computer program comprises computer instructions which, when loaded in a processor, enable the processor to carry out the method of the invention.

In a sixth aspect of the invention, a non-volatile storage medium is provided. The storage medium stores computer instructions which, when loaded in a processor, enable the processor to carry out the method of the invention.

Briefly stated, the present invention provides a method, apparatus and software for accurate diffuse optical imaging. The apparatus comprises a light source emitting a plurality of wavelengths and an optical imaging device capable of producing images at a number of prescribed wavelength bands. Images taken at strategically chosen wavelength bands may be combined to produce an image representing a twodimensional spatial distribution of the ratio of two absorbing substances.

These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a diagram showing graphs of an effective penetration depth [cm] at different wavelengths [nm] of a medium containing different ratios of fat and water.

Figure 2 depicts different diagrams showing graphs of effective penetration depth versus wavelength, diffuse reflectance versus wavelength, first derivative of diffuse reflectance versus wavelength, and second derivative of diffuse reflectance versus wavelength, all at three different wavelength ranges.

Figure 3 depicts a flow diagram illustrating steps of an embodiment of a method according to the invention.

Figure 4 schematically illustrates components of a first embodiment of an apparatus according to the invention.

Figure 5 schematically illustrates components of a second embodiment of an apparatus according to the invention.

Figure 6 schematically illustrates components of a third embodiment of an apparatus according to the invention.

Figure 7 schematically illustrates components of a fourth embodiment of an apparatus according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 depicts a diagram showing graphs of an effective penetration depth [cm] at different wavelengths [nm] of a medium containing different ratios of fat (a first optical absorber having a first concentration) and water (a second optical absorber having a second concentration). The optical penetration depths have been calculated with diffusion theory.

In a first (low) graph, marked A, the effective penetration depth versus wavelength for a medium containing 10% by volume of fat and 90% by volume of water is shown. In a second (middle) graph, marked B, the effective penetration depth versus wavelength for a medium containing 50% by volume of fat and 50% by volume of water is shown. In a third (high) graph, marked C, the effective penetration depth versus wavelength for a medium containing 90% by voiume of fat and 10% by volume of water is shown.

As indicated in Figure 1 by upwards pointing arrows, different isosbestic points (wavelengths) can be identified, at about 930 nm, about 1217 nm, about 1700 nm, and about 1736 nm, respectively. At these wavelengths, the effective (optical) penetration depth is the same for the different ratios of fat and water according to graphs A, B and C, and for other ratios of fat and water. Aiso, at these wavelengths, inhomogeneous distributions of fat and water in the medium do not infiuence the optical penetration depth.

In case the optical properties of the two absorbers, i.e. the first optical absorber fat and the second optical absorber water, do not differ much, a concentration ratio cannot be determined; at an isosbestic wavelength.

Hence, Figure 2 depicts different diagrams showing graphs of effective penetration depth versus wavelength, diffuse reflectance versus wavelength, first derivative of diffuse reflectance versus wavelength, and second derivative of diffuse reflectance versus wavelength, all at three different wavelength ranges around isosbestic wavelengths.

In a left column comprising four diagrams, from top to bottom an effective penetration depth, an associated diffuse reflectance, and associated first derivative of the diffuse reflectance, and an associated second derivative of the diffuse reflectance, respectively, are shown for three different concentration ratios of fat and water (10/90 (graph A), 50/50 (graph B), and 90/10 (graph C)), in a first wavelength range from 910 nm to 950 nm around an isosbestic wavelength of about 930 nm, as indicated by a vertical dashed line.

In a middle column comprising four diagrams, from top to bottom an effective penetration depth, an associated diffuse reflectance, and associated first derivative of the diffuse reflectance, and an associated second derivative of the diffuse reflectance, respectively, are shown for three different concentration ratios of fat and water (10/90 (graph A), 50/50 (graph B), and 90/10 (graph C)), in a second wavelength range from 1170 nm to 1240 nm around an isosbestic wavelength of about 1217 nm, as indicated by a vertical dashed line.

In a right column comprising four diagrams, from top to bottom an effective penetration depth, an associated diffuse reflectance, and associated first derivative of the diffuse reflectance, and an associated second derivative of the diffuse reflectance, respectively, are shown for three different concentration ratios of fat and water (10/90 (graph A), 50/50 (graph B), and 90/10 (graph C)), in a third wavelength range from 1680 nm to 1750 nm around isosbestic wavelengths of about 1700 and 1736 nm, as indicated by vertical dashed lines.

From the third and fourth rows, each comprising three diagrams showing the first derivative and the second derivative, respectively, it can be seen these first and second derivatives of the diffuse reflection spectra clearly are different at the isosbestic wavelengths for different concentration ratios of the first optical absorber fat and the second optical absorber water.

Based on this property of the diffuse reflection spectrum of a medium containing essentially two main optical absorbers, the following method is proposed, as illustrated in the flow diagram of Figure 3.

With the method, a property of an optically diffuse medium, in particular a concentration ratio of a first concentration of a first optical absorber and a second concentration of a second optical absorber, can be determined, i.e. estimated.

In a step 301, a surface area of the medium is imaged at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber. The surface area is selected to be in conformity with the spatial resolution to be expected for the medium. The imaging may be performed using an appropriate optical imaging device, as will be explained below by reference to Figures 4 to 7.

In Figure 3, in a subsequent step 302, a reflectance spectrum of the medium is determined at the surface area at the multiple wavelengths.

In a subsequent step 303, a derivative of the determined reflectance spectrum around the isosbestic wavelength is determined.

In a subsequent step 304, a concentration ratio of the first concentration and the second concentration is estimated from the derivative. In particular, a concentration ratio Ψ may be determined using the following expression:

where: fand gare defined by and - λ indicates a wavelength; - λο indicates an isosbestic wavelength; - Αλ indicates a wavelength difference from the isosbestic wavelength; - DX indicates an imaging ratio, defined by subtracting two images taken Αλ apart from an isosbestic wavelength λο and dividing by the sum of the two images; - ga m indicates a molar absorption coefficient of absorber m; - ga,o indicates a molar absorption coefficient at isosbestic wavelength.

Once the concentration ratio Ψ has been determined, then according to step 305, it may be compared to a predetermined concentration ratio range. This predetermined concentration ratio range may have been established based on evidence that certain concentration ratios within the range are normal for the medium. Other concentration ratios outside the concentration ratio range are abnormal for the medium, and therefore may be qualified as a defect. According to step 306, if the concentration ratio is outside the concentration ratio range, a defect is indicated for the surface area of the medium, in an appropriate way.

The steps 302 to 306 may be performed by components of one or more (data) processing devices and associated input and output devices.

Figure 4 schematically illustrates components of a first embodiment of an apparatus 400 according to the invention.

The apparatus 400 comprises a light source 402 configured to illuminate (as indicated by arrow 404) a surface area 406 of a volume of an optically diffuse medium. The medium comprises a first optical absorber having a first concentration and a second optical absorber having a second concentration. Light is reflected (as indicated by arrow 408) from the surface area 406 of the medium to a filtering device 410 configured to receive such reflected light from the surface area of the medium, and to transmit filtered light (as indicated by arrow 420) to a twodimensional, 2D, optical imaging device 422 configured to receive the filtered light 420 from the filtering device 410.

The filtering device 410 is configured to filter multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber. The filtering device 410 comprises a mechanical device 412 (also referred to as a filter wheel) comprising a filter support 414 with multiple different optical filters 416 each filtering a different wavelength from the reflected light 408. In the mechanical device 412, the filter support 414 which can be rotated in either one of directions indicated by double arrow 417 by an actuator 418 coupled to the filter support 414, whereby each one of the multiple optical filters 416 can be positioned in the optical path of the reflected light 408 to provide light 420 filtered at different wavelengths.

The optical imaging device 422 may e.g. comprise a CCD camera, an InGaAs camera or a CMOS camera, and provides image data which are transferred to an image processing component 424 configured to determine a reflectance spectrum of the medium at the surface area 406 at the multiple wavelengths. Data relating to the reflectance spectrum are transferred from the image processing component 424 to a calculating component 426 configured to determine a derivative of the determined reflectance spectrum around the isosbestic wavelength. Data relating to the derivative of the determined reflectance spectrum around the isosbestic wavelength are transferred from the calculating component 426 to an estimating component 428 configured to estimate a concentration ratio Ψ of the first concentration and the second concentration from the derivative. Data relating to an estimated concentration ratio Φ are transferred to a comparator 430 configured to compare the concentration ratio Φ to a predetermined concentration ratio range. The comparator 430 controls an output component 432, such as an image output component, configured to indicate a defect for the surface area 406 when the concentration ratio Ψ is outside the concentration ratio range. For example, if a defect is to be indicated for the surface area 406, the output component 432 may display the surface area 406 having a color or any other indication to differentiate the surface area 406 from other surface areas for which a defect is not to be indicated.

Figure 5 schematically illustrates components of a second embodiment of an apparatus 500 according to the invention.

The apparatus 500 comprises a light source 502 configured to illuminate (as indicated by arrow 504) a surface area 506 of a volume of an optically diffuse medium. The medium comprises a first optical absorber having a first concentration and a second optical absorber having a second concentration. Light is reflected (as indicated by arrow 508) from the surface area 506 of the medium. The apparatus 500 further comprises a onedimensional, 1D, hyperspectral optical imaging device 510 configured to receive reflected light 508 from a scanned line 512 of the surface area 506 of the medium. For each pixel of the hyperspectral optical imaging device 510, an entire spectrum is acquired.

The medium is supported on a stage 520 coupled to an actuator 522 configured to drive the stage 520 to move the medium relative to the optical imaging device 510 in directions as indicated by double arrow 524. For example, the actuator 522 is coupled to the stage 520 through a rotatable spindle 526 to move the stage 520. By incrementally moving the stage 520, different lines of the surface area 506 are imaged.

The 1D optical imaging device 510 comprises, for example, a CCD , InGaAs or CMOS hyperspectral camera, and acquires image data in the form of a (x,y,A) (x-coordinate, y-coordinate, wavelength) data cube, from which imaging wavelengths are chosen to select images. The optical imaging device 510 provides the selected image data which are transferred to an image processing component 530 configured to determine the reflectance spectrum of the medium at the surface area 506 at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber. Data relating to the reflectance spectrum are transferred from the image processing component 530 to a calculating component 532 configured to determine a derivative of the determined reflectance spectrum around the isosbestic wavelength. Data relating to the derivative of the determined reflectance spectrum around the isosbestic wavelength are transferred from the calculating component 532 to an estimating component 534 configured to estimate a concentration ratio Ψ of the first concentration and the second concentration from the derivative. Data relating to an estimated concentration ratio Ψ are transferred to a comparator 536 configured to compare the concentration ratio Ψ to a predetermined concentration ratio range. The comparator 536 controls an output component 538, such as an image output component, configured to indicate a defect for the surface area 506 when the concentration ratio Ψ is outside the concentration ratio range. For example, if a defect is to be indicated for the surface area 506, the output component 538 may display the surface area 506 having a color or any other indication to differentiate the surface area 506 from other surface areas for which a defect is not to be indicated.

Figure 6 schematically illustrates components of a third embodiment of an apparatus 600 according to the invention.

The apparatus 600 comprises a light source 602 configured to illuminate (as indicated by arrow 604) a surface area 606 of a volume of an optically diffuse medium. The medium comprises a first optical absorber having a first concentration and a second optical absorber having a second concentration. Light is reflected (as indicated by arrow 608) from the surface area 606 of the medium. The apparatus 600 further comprises a twodimensional, 2D, hyperspectral optical imaging device 610 configured to receive reflected light 608 from a scanned surface sub-area 612 of the surface area 606 of the medium. For each pixel of the hyperspectral optical imaging device 610, an entire spectrum is acquired.

The medium is supported on a stage 620 coupled to an actuator 622 configured to drive the stage 620 to move the medium relative to the optical imaging device 610 in directions as indicated by double arrows 624, 625. By incrementally moving the stage 620, different surface sub-areas 612 of the surface area 606 are imaged.

In another embodiment lacking a stage, different surface sub-areas 612 of the surface area 606 may be scanned by use of movable scanning mirrors (not shown) directing the reflected light 608 to the 2D optical imaging device 610.

The 2D optical imaging device 610 comprises, for example, a CCD , InGaAs or CMOS hyperspectral camera, and acquires image data in the form of a (x,y,A) (x-coordinate, y-coordinate, wavelength) data cube, from which imaging wavelengths are chosen to select images. The optical imaging device 610 provides the selected image data which are transferred to an image processing component 630 configured to determine the reflectance spectrum of the medium at the surface area 606 at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber.

The 2D hyperspectral optical imaging device 610 comprises an image sensor having a plurality of pixels. The image processing component 630 is configured to assign a cluster of pixels of the image sensor to the surface sub-area 612 of the medium, to determine the reflectance spectrum for each pixel of the image sensor, and to determine the reflectance spectrum of the medium at the surface area 606 from the reflectance spectra of the pixels of the cluster.

Data relating to the reflectance spectrum are transferred from the image processing component 630 to a calculating component 632 configured to determine a derivative of the determined reflectance spectrum around the isosbestic wavelength. Data relating to the derivative of the determined reflectance spectrum around the isosbestic wavelength are transferred from the calculating component 632 to an estimating component 634 configured to estimate a concentration ratio Ψ of the first concentration and the second concentration from the derivative. Data relating to an estimated concentration ratio Ψ are transferred to a comparator 636 configured to compare the concentration ratio # to a predetermined concentration ratio range. The comparator 636 controls an output component 638, such as an image output component, configured to indicate a defect for the surface area 606 when the concentration ratio Ψ is outside the concentration ratio range. For example, if a defect is to be indicated for the surface area 606, the output component 638 may display the surface area 606 having a color or any other indication to differentiate the surface area 606 from other surface areas for which a defect is not to be indicated.

It is noted that, instead of a 2D hyperspectral optical imaging device 610, a spectrograph can be applied, as explained by reference to Figure 7.

Figure 7 schematically illustrates components of a fourth embodiment of an apparatus 700 according to the invention.

The apparatus 700 comprises a light source 702 configured to illuminate (as indicated by arrow 704) a surface area 706 of a volume of an optically diffuse medium. The medium comprises a first optical absorber having a first concentration and a second optical absorber having a second concentration. Light is reflected (as indicated by arrow 708) from the surface area 706 of the medium. The apparatus 700 further comprises a spectrograph 710 configured to receive reflected light 708 from a scanned surface sub-area 712 of the surface area 706 of the medium. For each pixel of the spectrograph 710, an entire spectrum is acquired.

The medium is supported on a stage 720 coupled to an actuator 722 configured to drive the stage 720 to move the medium relative to spectrograph 710 in directions as indicated by double arrows 724, 725. By incrementally moving the stage 720, different surface sub-areas 712 of the surface area 706 are imaged.

In another embodiment lacking a stage, different surface sub-areas 712 of the surface area 706 may be scanned by use of a scanning device comprising movable scanning mirrors (not shown) directing the reflected light 708 to the spectrograph 710.

The spectrograph 710 acquires image data in the form of a (x,y,A) (x-coordinate, y-coordinate, wavelength) data cube, from which imaging wavelengths are chosen to select images. The spectrograph 710 provides the selected image data which are transferred to an image processing component 730 configured to determine the reflectance spectrum of the medium at the surface area 706 at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber.

Data relating to the reflectance spectrum are transferred from the image processing component 730 to a calculating component 732 configured to determine a derivative of the determined reflectance spectrum around the isosbestic wavelength. Data relating to the derivative of the determined reflectance spectrum around the isosbestic wavelength are transferred from the calculating component 732 to an estimating component 734 configured to estimate a concentration ratio 4> of the first concentration and the second concentration from the derivative. Data relating to an estimated concentration ratio Ψ are transferred to a comparator 736 configured to compare the concentration ratio Ψ to a predetermined concentration ratio range. The comparator 736 controls an output component 738, such as an image output component, configured to indicate a defect for the surface area 706 when the concentration ratio # is outside the concentration ratio range. For example, if a defect is to be indicated for the surface area 706, the output component 738 may display the surface area 706 having a color or any other indication to differentiate the surface area 706 from other surface areas for which a defect is not to be indicated.

Products, such as software or apparatus, for imaging relative concentrations of light absorbing components/absorbers in (samples of) optically diffuse materials, e.g. find applications in the following fields.

Healthcare: • Diagnosis of disease by in vivo imaging of tissue components in suspect areas. • Assessment of resection margins by ex vivo imaging the ratio between fat and water concentrations in resection margins in the resection specimen. • Assessment of resection margins in vivo during surgery imaging the ratio between fat and water concentrations. • Evaluation of stool samples for colon cancer risk stratification.

Agro/Food: • Estimating freshness of fruit, meat or fish by quantitative imaging of essential components. • Estimating nutritional value of food by quantitative imaging of nutritional components. • Estimating a vase life of flowers.

Environmental: • Quality determination and monitoring of surface water.

Forensic: • Child abuse: Determination of the age of a bruise by accurate imaging of the spatial distribution of biochemical components occurring in haemoglobin breakdown. • Crime scene evaluation: determination of the age of blood stains or other traces of other bodily fluids. • Forensic pathology: Determination of the time of death by accurate imaging of the distribution of biochemical components occurring in livor mortis.

As explained in detail above, in a method and apparatus, a property of an optically diffuse medium comprising a first optical absorber having a first concentration and a second optical absorber having a second concentration is determined. A surface area of the medium is imaged at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber. A reflectance spectrum of the medium at the surface area at the multiple wavelengths is determined. A derivative of the determined reflectance spectrum around the isosbestic wavelength is determined. From the derivative, a concentration ratio of the first concentration and the second concentration is estimated.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms "aV'an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

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.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. A single processor or other unit may fulfil the functions of several items recited in the claims.

The terms software, program, software application, and the like as used herein, are defined as a sequence of instructions designed for execution in a processor of a computer system. A program, computer program, or software application may include a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system. A computer program may be stored and/or distributed on a suitable non-volatile medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

A method for determining a property of an optically diffused medium, wherein the medium comprises a first optical absorbent with a first concentration and a second optical absorbent with a second concentration, the method comprising the steps of: (a) imaging a surface area of the medium at multiple wavelengths around an isosbestic wavelength of the first absorbent and the second absorbent; (b) determining a reflection spectrum of the medium at the surface area at the multiple wavelengths; (c) determining a derivative of the determined reflection spectrum around the isosbestic wavelength; and (d) estimating a concentration ratio (Ψ) of the first concentration and the second concentration based on the derivative.

The method of claim 1, wherein the concentration ratio (Ψ) is determined using the partial derivative of a first molar absorption coefficient of the first absorbent as a function of the wavelength, and the partial derivative of a second molar absorption coefficient of the second absorbent as a function of the wavelength, both taken at a wavelength where the first molar absorption coefficient and the second molar absorption coefficient are equal.

Method according to one of the preceding claims, wherein the concentration ratio {Ψ) is determined by:

at which:

ƒ and Qf are defined by

and - λ indicates a wavelength; - λο indicates an isosbestic wavelength; - Δλ indicates a wavelength difference from the isosbestic wavelength; DX indicates an image ratio defined by subtracting two images taken Δλ from each other from an isosbestic wavelength λο and dividing by the sum of the two images; j ua.m denotes a molar absorption coefficient of absorbant m; -) Ua, o indicates a molar absorption coefficient at isosbestic wavelength.

The method of any one of the preceding claims, wherein the derivative is any of a first and higher derivatives.

The method of any one of claims 1-4, further comprising: comparing the concentration ratio with a predetermined concentration ratio range; and indicating a defect for the surface area if the concentration ratio is outside the concentration ratio range.

The method of any one of claims 1-4, further comprising: repeating steps (a) to (d) for different isosbestic wavelengths of the first absorbant and the second absorbant in different wavelength ranges to obtain estimated concentration ratios for each of the isosbestic wavelengths.

The method of claim 6, further comprising: comparing each of the estimated concentration ratios with a predetermined concentration ratio range; and indicating a defect for the surface area when at least one of the concentration ratios is outside the concentration ratio range.

The method of claim 6 or 7, further comprising: associating each isosbestic wavelength with a respective optical penetration depth; indicating a defect and a depth thereof for the surface area when at least one of the concentration ratios is outside the concentration ratio range.

The method of any preceding claim, further comprising: associating each isosbestic wavelength with a respective optical penetration depth; and if small defects are to be located, selecting an isosbestic wavelength under different isosbestic wavelengths with a low optical penetration depth.

A device for determining a property of an optically diffused medium, the medium comprising a first optical absorbent with a first concentration and a second optical absorbent with a second concentration, the device comprising: a light source configured to illuminate from a surface area of a volume of the medium; a filtering device configured to receive reflected light from the surface area of the medium and to transfer filtered light to an optical imaging device, the filtering device configured to filter multiple wavelengths around an isosbestic wavelength of the first absorbent and the second absorbent; a two-dimensional, 2D, optical imaging device configured to receive the filtered light from the filtering device; an image processing component configured to determine a reflection spectrum of the medium at the surface area at the multiple wavelengths; a calculation component configured to determine a derivative of the determined reflection spectrum around the isosbestic wavelength; and an estimation component configured to estimate a concentration ratio (Ψ) of the first concentration and the second concentration based on the derivative.

The apparatus of claim 10, further comprising: a comparator configured to compare the concentration ratio (Ψ) with a predetermined concentration ratio range; and an output component configured to indicate a defect for the surface area when the concentration ratio is outside the concentration ratio range.

The device of claim 10 or 11, wherein the filter device comprises: a filter carrier comprising a plurality of different optical filters; and an actuator for positioning each of the optical filters in an optical path from the surface area of the medium to the optical imaging device.

The device of any one of claims 10 to 12, wherein the optical imaging device comprises a CCD camera, an InGasAS camera or a CMOS camera.

A device for determining a property of an optically diffused medium, the medium comprising a first optical absorbent with a first concentration and a second optical absorbent with a second concentration, the device comprising: a light source configured to illuminate from a surface area of a volume of the medium; a one-dimensional, 1D, or two-dimensional, 2D, hyperspectral optical imaging device configured to receive reflected light from the surface area of the medium; an image processing component configured to determine the reflection spectrum of the medium at the surface region at a plurality of wavelengths around an isosbestic wavelength of the first absorbent and the second absorbent; a calculation component configured to determine a derivative of the determined reflection spectrum around the isosbestic wavelength; and an estimation component configured to estimate a concentration ratio (Ψ) of the first concentration and the second concentration based on the derivative.

The device of claim 14, further comprising: a table configured to carry the medium; and an actuator configured to drive the table for moving the medium relative to the optical imaging device.

The device of claim 14 or 15, wherein the optical imaging device is a 2D hyperspectral optical imaging device comprising an image sensor with a plurality of pixels, and wherein the image processing component is further configured to: assign a cluster of pixels to the surface area of the medium; determining the reflection spectrum for each pixel of the image sensor; and determining the reflection spectrum of the medium at the surface area based on the reflection spectra of the pixels of the cluster.

17. Device for determining a property of an optically diffuse medium. wherein the medium comprises a first optical absorbent with a first concentration and a second optical absorbent with a second concentration, the device comprising: a light source configured to illuminate a surface area of a volume of the medium; a spectrograph configured to receive reflected light from the surface area of the medium; a light processing component configured to determine the reflection spectrum of the medium at the surface region at a plurality of wavelengths around an isosbestic wavelength of the first absorbent and the second absorbent; a calculation component configured to determine a derivative of the determined reflection spectrum around the isosbestic wavelength; and an estimation component configured to estimate a concentration ratio (Ψ) of the first concentration and the second concentration based on the derivative.

The device of claim 17, further comprising: a table configured to carry the medium; and an actuator configured to drive the table for moving the medium relative to the optical imaging device

The apparatus of claim 17 or 18, further comprising: a sensing device configured to transmit reflected light from different surface areas of the medium to the spectrograph.

A computer program comprising computer instructions which, when loaded in a processing unit, enable the processing unit to perform the method according to any of claims 1 to 9.

A non-volatile storage medium that stores computer instructions which, when loaded into a processing unit, enable the processing unit to perform the method of any one of claims 1 to 9.