WO2003067229A1

WO2003067229A1 - Examination of superficial regions of a body

Info

Publication number: WO2003067229A1
Application number: PCT/GB2003/000463
Authority: WO
Inventors: Stephen Peter Morgan
Original assignee: The University Of Nottingham
Priority date: 2002-02-06
Filing date: 2003-02-05
Publication date: 2003-08-14
Also published as: AU2003205848A1; GB0202654D0

Abstract

Polarized light is used to illuminate a body (34), and the polarization of the scattered light is analysed using an analyzer (38) and camera (36). Analysis is made of the cross-polarized component of scattered incident linearly polarized light, and the polarization maintaining component of scattered incident circularly polarized light. These components, which do not contain surface reflected light, are processed to remove the components of multiply scattered light, thereby enabling analysis of only the weakly scattered light from superficial regions of the body.

Description

EXAMINATION OF SUPERFICIAL REGIONS OF A BODY

The present invention relates to examination of superficial regions of a body using polarized light. It has application in the examination of biological tissue, as well as the examination of other bodies such as architectural and vehicular structures.

It is well known, for example from US 6, 177,984, to use polarized light to form images of superficial tissue. This imaging process makes use of the fact that when polarized light is scattered in a tissue, the effect on the polarization depends on the number and type of scattering events that the light has been subject to in the tissue, and therefore on the depth that it has travelled into the tissue.

It can be a problem with this type of imaging that surface reflected light, that has been reflected from the surface of the body, can swamp the images formed from light which has passed into the body. US 6,177,984 discloses a system which overcomes this problem by positioning the detector so that surface reflected light will not be picked up. This system requires the use of an optical element in contact with the tissue which ensures that surface reflected light is all specularly reflected, there being no diffuse surface reflection, so surface reflected light is reflected at a consistent angle, so that the detector can then be positioned away from its path.

The present invention provides apparatus for examining a superficial region below the surface of a body, the apparatus comprising illumination means arranged to direct linearly polarized and circularly polarized illuminating radiation onto the body, and detection means arranged to detect said radiation when it has been scattered by the body, and to measure a linearly polarized component and a circularly polarized component of the scattered radiation, each of which is substantially free of surface reflected radiation, and to combine said components to provide a measure of radiation scattered within the superficial region.

The radiation may be visible light, but could be electromagnetic radiation of any suitable frequency.

Preferably one of the components includes weakly scattered radiation from the superficial region and multiply scattered radiation from a deeper region within the body, and the other component includes multiply scattered radiation from the deeper region.

The linear component may be cross-polarized with respect to the linearly polarized illuminating radiation, and the circularly polarized component may be polarized with the same helicity as the circularly polarized illuminating radiation.

Preferably the detection means is arranged to subtract the linearly polarized component from the circularly polarized component to determine a part of the circularly polarized component which has been weakly scattered within the superficial region.

Where the body has substantially uniform scattering properties throughout, the selection of weakly scattered radiation will select a superficial region just below the surface. On the other hand, where the body has non-uniform scattering properties, the selection of weakly scattered radiation can be used to examine only those parts of the body that produce significant weak scattering. In particular, if the body is of a relatively highly scattering material but has a layer of less highly scattering material on it, the selection of weakly scattered radiation can be used to examine the main body below the covering layer. Preferably the illuminating means and the detection means are arranged such that, over at least a part of their paths, the direction of propagation of the scattered radiation is substantially parallel to the direction of propagation of the illuminating radiation. This makes use of the ability of the method of the invention to avoid interference from surface reflected light, and enables the apparatus to be compactly designed. It is of use, for example, in endoscopes.

More preferably, therefore, the illuminating means and the detection means are arranged such that, over at least a part of their paths, the scattered radiation will travel along the same path as the illuminating light, but in the opposite direction.

The detection means may include a beam splitter arranged to separate the scattered radiation from the illuminating radiation.

The apparatus may include at least one radiation transmitting fibre arranged to transmit the illuminating radiation and the scattered radiation in opposite directions.

The detection means may be arranged to detect radiation scattered from a plurality of areas of the superficial region, and to form an image of the superficial region from the scattered light. This can be useful, for example, in the analysis of skin lesions.

For example the detection means may be arranged to form an image using the linearly polarized component and an image using the circularly polarized component and to process the images to form an image of the superficial region. Alternatively, or in addition, the detection means may be arranged to measure the frequencies of the two components and to combine the results to analyse the frequencies of the light scattered from the superficial region.

The present invention further provides a method of examining a superficial region below the surface of a body, comprising the steps of directing linearly polarized and circularly polarized illuminating radiation onto the body, detecting said radiation when it has been scattered by the body, measuring a linearly polarized component and a circularly polarized component of the scattered radiation, each of which is substantially free of surface reflected radiation, and combining said components to provide a measure of radiation scattered within the superficial region.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a diagram showing the different types of scattering that can occur when radiation is incident on a body,

Figure 2 is a diagram showing the effects on linearly polarized light of different types of scattering within a body of tissue;

Figure 3 is a diagram showing the effects on circularly polarized light of different types of scattering within a body of tissue;

Figure 4 is a schematic representation of a non-contact imaging camera according to the invention; Figure 5 is a schematic representation of an endoscope according to the invention;

Figure 6 is a section through a layered body which can be examined using the camera of Figure 4; and

Figure 7 is a schematic representation of a blood flow measuring device according to the invention.

Referring to Figure 1, when light 10 is incident on a body of tissue 12 it is scattered by the tissue. Some of it is then absorbed, and some of it re- emerges from the tissue and can be detected and measured. Generally the detected light will have undergone one of three types of scattering. Some photons 14 will have undergone surface reflection from the surface 15 of the tissue. This can either be specular reflection or, as the tissue has a rough surface, diffuse reflection. Some photons 16 will have undergone weak forward scattering from superficial tissue 18 in the region just below the surface 15. Some photons 20 will have undergone multiple scattering from deeper tissue 22 below the superficial tissue 18. If the incident light is polarized, then the polarisation of the detected light depends on the type of scattering that it has undergone in the tissue.

Referring to Figure 2, if the incident light 10 is linearly polarized, then both the surface reflected light 14 and the weakly scattered light 16 have the same polarization as the incident light 10. Multiply scattered light 20 has random polarization and therefore includes equal components of all polarizations. Therefore two regions of the tissue can be distinguished, an upper layer A which includes the surface and a superficial layer of tissue, and from which scattered light maintains its polarization, and deeper tissue B from which scattered light is randomly polarized. Referring to Figure 3, if the incident light 10 is circularly polarized, then the surface reflected light 14 is polarized with the opposite helicity to the incident light 10, weakly scattered light 16 has the same polarization helicity as the incident light 10, and multiply scattered light 20 has random polarization and therefore includes equal components of all polarizations. Therefore there are three regions of the tissue from which different types of light will emerge: a surface layer C, from which scattered light has its polarisation helicity reversed, a superficial layer D from which scattered light maintains its polarization, and deeper tissue E from which scattered light is randomly polarized.

Referring to Figure 4, an imaging device comprises a light source 30, a polarizer 32 arranged to control in known manner the polarization of the light from the source which is incident on the body of tissue 34, a detection device in the form of a camera 36, and an analyzer 38, arranged to select in known manner the polarization of the components of the light from the tissue 34 which reach the camera 36. The optical elements, that is the polarizer 32 and analyzer 38, in this case comprise a quarter-wave plate and linear polarizer, but other elements such as spatial light modulators can be used.

The camera 36 is a CCD camera and is connected to a computer 39 having a memory 40 for storing images captured by the camera, a processor 42 for processing the stored images, and a monitor 44 for displaying the processed images.

The system is arranged to measure the scattered light, and form images, on four channels, two with linearly polarized incident light and two with circularly polarized incident light. The first channel is for linearly polarized scattered light which is co-polarized with the incident light. The second channel is linearly polarized scattered light which is cross- polarized, i.e. polarized at an angle of 90° with respect to the incident light. The third channel is for circularly polarized light which is polarized in the same sense as the incident circularly polarized light. The fourth channel is for circularly polarized light which is polarized with the opposite helicity to the incident circularly polarized light. The channel on which an image is to be recorded is selected by controlling the optical elements 32, 38 so as to obtain the required polarizations of the incident and detected light, illuminating the tissue and capturing an the image.

From Figures 2 and 3 it can be seen that the channel 1 will contain substantially all surface reflected and weakly scattered light, and a component of the multiply scattered light. Channel 2 will contain only a component of the multiply scattered light. Channel 3 will contain substantially all the weakly scattered light and a component of the multiply scattered light. Channel 4 will contain the surface reflected light and a component of the multiply scattered light. These relationships are summarized in the table below.

Where:

S denotes surface scattered, or reflected light, W denotes weakly scattered light, M denotes multiply scattered light. In order to form an image of the superficial tissue within the body, a first image is captured on channel 2, and a second image is captured on channel 3. Channels 1 and 4 are not used. Assuming that the channels have been normalized so the component of the first image from the multiply scattered light is the same as the component of the second image from the multiply scattered light, the first image can be simply subtracted from the second image by the processor 42 so as to produce an image of just the weakly scattered component of the circularly polarized light. This image will therefore include substantially no components of the surface scattered light, and substantially no components of the multiply scattered light.

Referring to Figure 5, in a second embodiment of the invention a light source 100, polarizer 102, camera 104 and analyzer 106 are provided which are the same as in the first embodiment. However, rather than illuminating the tissue directly the illuminating light is passed through a beam splitter 108 and into one end 110 of a fibre bundle 112 which comprises a large number of parallel optical fibres 114 bound together. The light therefore passes down the fibre bundle 112 and out of its remote end 116 from where it is emitted as an illuminating beam which is incident on, and illuminates, the tissue 118 to be imaged. Scattered light from the tissue 118 which has been scattered through approximately 180° and is therefore travelling parallel to the incident beam but in the opposite direction, passes back into the fibre bundle 112 from which it emerges onto the beam splitter 108 which separates it from the illuminating light and directs it to the camera 104.

It will be appreciated that the ability to use light which has been scattered through substantially 180°, which is possible because the specularly reflected component of the scattered light has been removed, means that the same optical fibres can be used for the illuminating light as for the scattered and detected light. This allows the minimum amount of optical fibre to be used, which is advantageous in the design of endoscopes where the overall width of the fibre bundle is preferably kept to a minimum.

While the embodiments described above are arranged to produce still images of the superficial tissue, it will be appreciated that, for example with spatial light modulators or Pockels cells for the polarizer and analyzer, the polarization of the illuminating light can be switched fast enough, and the images to be captured fast enough, to produce a real time video image of the region of interest.

Referring to Figure 6 in a third embodiment of the invention, the apparatus of Figure 4 is used to form an image of a structural body 200 having one set of scattering characteristics which has a layer 202 of another material having different scattering characteristics over its surface. The layer 202 is much less scattering than the body, and therefore the amount of weakly scattered light from the surface layer is negligible. The apparatus is therefore used, as described above with reference to Figure 4, to form an image using only weakly scattered light. This image will therefore be of the superficial layer of the structural body 200, with substantially no components from either the overlying layer, surface reflection, or deeper regions of the body. The body in this example could be, for example, the wing of an aircraft, and the overlying layer a layer of ice. The invention therefore allows the detection of features, such as cracks, in the main body without the need to remove the overlying layer of ice.

Referring to Figure 7, according to a fourth embodiment of the invention, a laser Doppler bloodflow measuring device comprises a laser light source 300 which produces an illuminating beam of laser light, a polarizer 302, a detection unit 304 and an analyzer 306, and a computer 308 connected to the detection unit 304. The detection unit 304 is arranged to carry out a spectral analysis of the detected light scattered from the tissue 310 so as to determine the frequency characteristics of the scattered light. The spectral analysis is used in known manner to determine the Doppler shift of the incident light, and hence the flux or concentration of blood in the tissue 310. The polarizer 302 and analyzer 306 are controlled so as to allow analysis of light detected on the second and third channels described above with reference to Figures 2 to 4, and the spectral analysis of light on these two channels compared to extract the spectral components which have undergone weak scattering and have therefore been scattered in the superficial region of the tissue 310. This enables the blood flow measurement to be restricted to the superficial region, without interference from deeper regions.

While the above embodiments use visible light, it may be preferable in some circumstances to use electromagnetic radiation of other frequencies, for example to control the types of scattering that will occur in a particular body. Because the wavelength of the radiation used affects the scattering processes that will occur, the depth of the superficial layer being examined can be controlled to a certain extent by the selection of wavelength used.

It will be appreciated that the present invention can be used in a wide variety of applications. As well as those described above, a further example is in imaging parts of the eye, and in particular the retina. This can be useful in monitoring various diseases, such as diabetic retinopathy, glaucoma, macular holes and macular oedema. The instrument for imaging the eye can be configured in the same way as that described above for the skin. Also the polarising, detection and imaging method of the invention can easily be incorporated in existing imagers such as fundus cameras and scanning laser opthalmoscopes.

Claims

1. Apparatus for examining a superficial region (D) below the surface of a body, the apparatus comprising illumination means (30, 32) arranged to direct linearly polarized and circularly polarized illuminating radiation onto the body (34), and detection means (38, 36, 39) arranged to detect said radiation when it has been scattered by the body (34), characterized in that the detection means is further arranged to measure a linearly polarized component and a circularly polarized component of the scattered radiation (14, 16, 20) , each of which is substantially free of surface reflected radiation (14), and to combine said components to provide a measure of radiation (16) scattered within the superficial region.

2. Apparatus according to claim 1 wherein one of the components includes weakly scattered radiation (16) from the superficial region (D) and multiply scattered radiation (20) from a deeper region (E) within the body, and the other component includes multiply scattered radiation (20) from the deeper region (E) .

3. Apparatus according to claim 1 or claim 2 wherein the linear component is cross-polarized with respect to the linearly polarized illuminating radiation.

4. Apparatus according to any foregoing claim wherein the circularly polarized component is polarized with the same helicity as the circularly polarized illuminating radiation.

5. Apparatus according to any foregoing claim wherein the detection means (36, 38, 39) is arranged to subtract the linearly polarized component from the circularly polarized component to determine a part of the circularly polarized component which has been weakly scattered within the superficial region (D) .

6. Apparatus according to any foregoing claim wherein the illuminating means (30, 32) and the detection means (36, 38, 39) are arranged such that, over at least a part of their paths, the direction of propagation of the scattered radiation is substantially parallel to the direction of propagation of the illuminating radiation.

7. Apparatus according to claim 6 wherein the illuminating means (30, 32) and the detection means (36, 38, 39) are arranged such that, over at least a part of their paths, the scattered radiation will travel along the same path as the illuminating radiation, but in the opposite direction.

8. Apparatus according to claim 7 wherein the detection means (108, 106, 104) includes a beam splitter (108) arranged to separate the scattered radiation from the illuminating radiation.

9. Apparatus according to any of claims 6 to 8 including at least one radiation transmitting fibre (112) arranged to transmit the illuminating radiation and the scattered radiation in opposite directions.

10. Apparatus according to any foregoing claim wherein the detection means (36, 38, 39) is arranged to detect radiation scattered from a plurality of areas of the superficial region (D) , and to form an image of the superficial region (D) from the scattered radiation.

11. Apparatus according to any foregoing claim wherein the detection means (36, 38, 39) is arranged to form an image using the linearly polarized component and an image using the circularly polarized component and to process the images to form an image of the superficial region (D) .

12. Apparatus according to any foregoing claim wherein the detection means (36, 38, 39) is arranged to measure the frequencies of the two components and to combine the results to analyse the frequencies of the radiation scattered from the superficial region (D) .

13. A method of examining a superficial region (D) below the surface (15) of a body (12), comprising the steps of directing linearly polarized and circularly polarized illuminating radiation (10) onto the body, detecting said radiation when it has been scattered by the body, measuring a linearly polarized component and a circularly polarized component of the scattered radiation (14, 16, 20) , each of which is substantially free of surface reflected radiation (14), and combining said components to provide a measure of radiation scattered within the superficial region (D) .

14. A method according to claim 13 wherein one of the components includes weakly scattered radiation (16) from the superficial region (D) and multiply scattered radiation (20) from a deeper region (E) within the body, and the other component includes multiply scattered radiation (20) from the deeper region (E) .

15. A method according to claim 13 or claim 14 wherein the linear component is cross-polarized with respect to the linearly polarized illuminating radiation.

16. A method according to any of claims 13 to 15 wherein the circularly polarized component is polarized with the same helicity as the circularly polarized illuminating radiation.

17. A method according to any of claims 13 to 16 including subtracting the linearly polarized component from the circularly polarized component to determine a part (16) of the circularly polarized component which has been weakly scattered within the superficial region (D) .

18. A method according to any of claims 13 to 17 over at least a part of their paths, the scattered radiation (14, 16, 20) is propagated in a direction substantially parallel to the direction of propagation of the illuminating radiation (10) .

19. A method according to claim 18 wherein over at least a part of their paths, the scattered radiation (14, 16, 20) is arranged to travel along the same path as the illuminating radiation (10), but in the opposite direction.

20. A method according to claim 19 wherein a beam splitter is arranged to separate the scattered radiation (14, 16, 20) from the illuminating radiation (10) .

21. A method according to any of claims 18 to 20 wherein at least one radiation transmitting fibre (112) is arranged to transmit the illuminating radiation (10) and the scattered radiation (14, 16, 20) in opposite directions.

22. A method according to any of claims 13 to 21 wherein radiation scattered from a plurality of areas of the superficial region (D) is detected, and an image of the superficial region (D) is formed from the scattered radiation.

23. A method according to any of claims 18 to 22 wherein an image is formed using the linearly polarized component, an image is formed using the circularly polarized component and the images are processed to form an image of the superficial region (D) .

24. A method according to any of claims 18 to 23 including measuring the frequencies of the two components and combining the results to analyse the frequencies of the radiation scattered from the superficial region (D) .

25. Apparatus for examining a superficial region below the surface of a body substantially as described herein with reference to Figures 1 to 4,

Figure 5, Figure 6 or Figure 7 of the accompanying drawings.

26. A method of examining a superficial region below the surface of a body substantially as described herein with reference to Figures 1 to 4, Figure 5, Figure 6 or Figure 7 of the accompanying drawings.