GB2589071A - Dental imaging - Google Patents
Dental imaging Download PDFInfo
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- GB2589071A GB2589071A GB1915897.1A GB201915897A GB2589071A GB 2589071 A GB2589071 A GB 2589071A GB 201915897 A GB201915897 A GB 201915897A GB 2589071 A GB2589071 A GB 2589071A
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Classifications
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/0059—Measuring for diagnostic purposes; Identification of persons using light, e.g. diagnosis by transillumination, diascopy, fluorescence
- A61B5/0082—Measuring for diagnostic purposes; Identification of persons using light, e.g. diagnosis by transillumination, diascopy, fluorescence adapted for particular medical purposes
- A61B5/0088—Measuring for diagnostic purposes; Identification of persons using light, e.g. diagnosis by transillumination, diascopy, fluorescence adapted for particular medical purposes for oral or dental tissue
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/0059—Measuring for diagnostic purposes; Identification of persons using light, e.g. diagnosis by transillumination, diascopy, fluorescence
- A61B5/0075—Measuring for diagnostic purposes; Identification of persons using light, e.g. diagnosis by transillumination, diascopy, fluorescence by spectroscopy, i.e. measuring spectra, e.g. Raman spectroscopy, infrared absorption spectroscopy
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/45—For evaluating or diagnosing the musculoskeletal system or teeth
- A61B5/4538—Evaluating a particular part of the muscoloskeletal system or a particular medical condition
- A61B5/4542—Evaluating the mouth, e.g. the jaw
- A61B5/4547—Evaluating teeth
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
- G01N21/3563—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing solids; Preparation of samples therefor
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/47—Scattering, i.e. diffuse reflection
- G01N2021/4704—Angular selective
- G01N2021/4709—Backscatter
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/47—Scattering, i.e. diffuse reflection
- G01N2021/4735—Solid samples, e.g. paper, glass
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/47—Scattering, i.e. diffuse reflection
- G01N2021/4792—Polarisation of scatter light
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2201/00—Features of devices classified in G01N21/00
- G01N2201/06—Illumination; Optics
- G01N2201/067—Electro-optic, magneto-optic, acousto-optic elements
- G01N2201/0675—SLM
Abstract
Apparatus 100 determines a scattering coefficient of tooth 300, possibly in generating a 2D map (figure 12). Patterned infrared light, typically sinusoidal, from e.g. digital light projector 10 is delivered to the tooth along a light delivery axis. Backscattered light is detected e.g. by camera 60. Polarising beam filter 30, e.g. a polarising beam splitter cube, is configured so that a backscattered light detection axis is co-aligned with the light delivery axis, which may supress aberrations. The apparatus may quantify early dental caries as a function of the tooth’s depth. Also disclosed are a system for detecting healthy or disrupted material in a tooth, and a method including a patterned light delivery with a systematic variation of spatial intensity as a function of spatial frequency or phase, with alternating component AC images selected to generate a stack of images from which a scattering coefficient can be determined.
Description
DENTAL IMAGING
FIELD OF THE INVENTION
This invention relates generally to the field of dental imaging, in particular, to a teeth imaging apparatus and a method. More particularly, the invention relates to an apparatus and a method for determining at least one scattering coefficient of at least one section of at least one tooth. Furthermore, the invention relates to a system and a method for detecting healthy and/or disrupted material in at least one section of at least one tooth. More particularly, the invention relates to a system and a method comprising the apparatus and the method for determining at least one scattering coefficient of at least one section of at least one tooth.
BACKGROUND OF THE INVENTION
Medical optical imaging consists of the use of light (or electromagnetic radiation) of various wavelengths as an investigational imaging technique for medical applications, such as optical imaging of teeth or optical imaging of various parts of the body. Examples of medical optical imaging include optical microscopy, fluorescence imaging, near infrared imaging, spectroscopic or hyperspectral imaging, endoscopy, scanning laser ophthalmology, optical coherence tomography (OCT) and spatial frequency domain imaging (SFDI).
The wavelengths of the investigational light depend on the part of the body to be investigated and the imaging modality selected. For example, optical imaging of hard (such as teeth) or soft (such as the gums) tissues (or materials) in the oral cavity could employ infrared light of wavelengths ranging from 700 nm to 10 pm (near IR ranging from 700 nm to about 3 pm). However, optical imaging of hard tooth tissues (such as enamel, dentin, cementum, dental caries) could also employ x-ray light of wavelengths ranging from 0.01 nm to 10 nm. The information gained using light of different wavelengths to investigate the tooth's hard tissues could provide insights into early-stage caries (i.e., early-stage demineralisation of teeth) when using infrared light or into advanced caries when using x-ray light.
SFDI operates by projecting a spatially modulated (i.e., the spatial frequency and/or spatial phase of the pattern are modulated) pattern of light (for example a sinusoidal wave or a square wave pattern of light) onto a sample of tissue (for example tooth hard tissue or soft dental tissue). In a simple visualisation, the spatially frequency modulated pattern of light may consist of a series of dark bars (no light present) interspaced with a series of white bars (light is present), the spacing between the bars corresponding to the modulation frequency of the pattern of light. In yet another simple visualisation, for a chosen/fixed spatial frequency, the spatially phase modulated pattern of light may consist of physically shifting the bars such that all sections of a tooth are exposed to the patterned light (this avoids having tooth sections only exposed to the dark bars).
As the light propagates through the tooth's tissue, optical scattering causes the pattern of light to be blurred and optical absorption by the tissue causes the pattern of light to become weaker in intensity (although optical absorption in teeth is negligible from around 500 nm upwards, barring some water absorption peaks). There is therefore a loss of contrast in any pattern of light projected through the tooth.
The skilled person would make a distinction between optical scattering and optical reflection and would understand scattering' to mean the process of a wave (i.e., a packet of photons) of incident light inducing a dipole moment in particles of a sample (e.g., a tooth tissue) which then re-radiate to produce a second wave. This process is described by Maxwell's equations. In some cases, the induced waves destructively interfere with each other in the forward direction and/or constructively interfere with each other in the 'epi' (backwards) direction to form a 'reflection' (i.e., a reflection is a specific case of scattering and is described by the laws of reflection). Reflection may thus be considered to be a specific case of scattering.
However, typically 'reflections' occur at changes in the refractive index of the sample. VVhere the reflection is at a smooth sample surface, the reflection is referred to as specular reflection and corresponds to the reflection observed in mirrors. When light is transmitted past the sample surface it then may be reflected at changes in the refractive index within the sample, for instance due to small variations in the sample's density. Multiple reflections can also occur, resulting in an induced wave comprising photons of random directions and polarisafions. This is referred to as 'diffuse reflectance' and it may also occur at rough surfaces.
The skilled person would appreciate that the presence of specular reflection may not prohibit the presence of diffuse reflectance. For instance, a marble sample may be very smooth, and a small fraction of the incident light wave may experience specular reflection, but the light wave that penetrates the surface of the marble will then experience diffuse reflectance. Hence, in the context of the present invention, the skilled person should understand 'reflection' to refer purely to specular reflection and 'scattering to refer to all scattering events other than specular reflection.
Commonly, a scattering coefficient can be used to express the rate at which scattering occurs, which may in turn be dependent on the angle between the direction of propagation of light before and after a scattering event. This is often referred to as 'scattering angle'. In some cases, a reduced scattering coefficient may be used to express an "average" scattering coefficient that is independent of the scattering angle.
Typically, only tissue near the surface of the tooth is detected before the pattern of light is blurred due to optical scattering. Tissue deeper in the tooth experiences uniform illumination due to the optical scattering removing the pattern of the light.
Varying the pattern's spatial frequency (i.e. the width of the bars) causes the depth at which the pattern of light is blurred out to be varied, providing a parameter which varies with depth in the tissue.
By using the rate of loss of the pattern of light in the backscattered image of the tooth, one can determine the local scattering properties of the tooth and hence infer the tooth's structural integrity as a function of depth. This effect can be seen in Figure 1, whereby: the up-pointing arrow indicates the direction of propagation of the patterned light, - at the bottom of Figure 1(a) is shown a square wave pattern of light of a low-frequency (i.e., widely spaced bars), - at the bottom of Figure 1(b) is shown a square wave pattern of light of a high-frequency (i.e., narrowly spaced bars), - the upper two lines indicate two of the tooth's boundaries of interest -the rough lines indicate the tooth's surface, whereas the wavy lines indicate the depth of the pattern penetration.
The skilled person can see from Figure 1(a) that, as the low-frequency patterned light propagates through the tooth, the pattern is largely preserved, although it does spread out. In the case of Figure 1(b), the high-frequency pattern is lost a short way into the tooth as the scattered light soon "fills in" the dark areas in the illumination, therefore resulting in uniform illumination at a relatively short distance away from the tooth's surface. It is therefore evident that the depth at which the pattern of light is lost is dependent on the pattern's frequency, with the pattern of a higher frequency being lost at shallower depths within the tooth compared to lower frequencies where the patterns will continue to a greater depth.
There is a need for an apparatus and a method able to detect and quantify dental caries and/or other dental defects (often referred to as 'disrupted dental materials') as a function of the tooth's depth, with a defined focus on early caries and/or early dental defects. These early stage caries and/or defects may be suitable for re-mineralisation, thus restoration to the tooth's original, un-affected state.
Optical imaging can be performed using transmitted light and/or backscattered light. These two techniques will have different sensitivities to different structures within the tissue sample. It should be obvious that, whilst Figure 1 illustrates only the effect of optical scattering as the light propagates in the forward direction through the tooth, scattering will continue once light has been backscattered within the depth of the tooth. Consequently, in practice, the detected scattered pattern of light will experience more blurring than that shown in Figure 1 before re-emerging from the tooth and arriving at a light detector.
Although the transmitted pattern of light is unlikely to be recorded in the scenario presented in Figure 1, in another practical imaging instrument a full illumination transmission image (i.e., a conventional full frame image with uniform illumination) may be taken as this will be affected by local changes in scattering, hence providing a quick X-ray-like image of the tooth in 2D. In clinical practice this might be used even from an instrument capable of imaging in 3D using the pattern illumination method as it provides the dentist with an early indication of potentially compromised teeth.
The skilled person would be guided by the following glossary of terms which, within the context of the present invention, should be taken to mean: - exposure -a single data file from the camera, taken at one spatial frequency and spatial phase and typically showing the backscattered light.
-image -multi-dimensional representation of data, rather than purely a 2D digitized representation of detected radiation.
- AC image -an image constructed from the differences between multiple exposures taken at different spatial phases, but the same spatial frequency, to select the alternating component AC of exposures. Contains information to a certain level of scattering and depth.
- DC image -an image constructed from the sum of multiple exposures taken at different spatial phases, but the same spatial frequency, to select the direct component DC of exposures. Contains information to the maximum achievable depth.
-image stack -a set of images taken at different spatial frequencies.
- image analysis -the construction of an image from a set of exposures taken at the same frequency, but different spatial phases.
- image stack analysis -the construction of one or multiple scattering profiles, scattering coefficients and/or 2D or 3D maps of localised scattering coefficients from an image stack - scattering profile -a plot of AC (alternating component) images as a function of illumination pattern frequency.
- scattering coefficient -a quantification of the rate at which scattering occurs in a region of tooth tissue, which may or may not be dependent on scattering angle and may encompass reduced scattering coefficients.
- reduced scattering coefficient -a quantification of the rate at which scattering occurs in a region of tissue that is modified to reflect the distribution of scattering angles that may occur.
- scattering angle -the angle between the direction of propagation of light before and after a scattering event.
- depth profile -a 3D representation of the tooth formed from the image stack and analysis logic.
- surface -the 2D interface of the tooth with the environment.
- pattern penetration depth -the depth at which the illumination pattern is significantly blurred due to scattering.
- reflection -specular reflection (i.e. the throwing back of light at a surface that follows the laws of reflection and that does not randomise the polarisation of a photon of light).
- scattering -Rayleigh scattering, Mie scattering, Raman scattering, diffuse reflection, etc that randomises the direction and polarisation of a photon of light.
- backscattering -(single or multiple) scattering that causes the direction of a photon of light to be changed sufficiently to exit the tooth from the same surface that it entered.
-absorption -a photon being taken up by an atom and causing a change in the state of an electron in the atom, typically followed by radiative decay (fluorescence, phosphorescence etc) or thermal relaxation.
- mean free path -the length over which the photon direction is randomised.
PROBLEM TO BE SOLVED BY THE INVENTION
There is a need for an apparatus and method able to determine scattering coefficients of tooth sections, particularly as a function of the tooth's depth. The so-determined scattering coefficients could then be employed to detect and quantify dental caries and/or other dental defects or pathological dental features as a function of the tooth's depth.
Therefore, it is an object of the present invention to provide an apparatus and a method for determining at least one scattering coefficient of at least one section of the least one tooth, the apparatus and the method having a defined focus on the detection and quantification of early stage caries and/or dental defects. These early stage dental abnormalities (i.e., disrupted materials in a tooth) may be suitable for re-mineralisation, thus for the restoration to the tooth's original, unaffected state.
It is a further object of this invention to provide a system and a method for detecting healthy and/or disrupted material in a tooth as a function of the tooth's depth. The system and the method may comprise the apparatus and the method for determining at least one scattering coefficient in at least one section of the tooth. Determining the healthy and/or the disrupted material of a tooth would allow the dentist to compare the evolution of dental caries and/or defects over time and as a function of the tooth's depth, hence allowing the dentist to focus on the early stage dental caries and/or defects.
SUMMARY OF THE INVENTION The invention is defined by the claims.
In accordance with a first aspect of the invention, there is provided an apparatus for determining at least one scattering coefficient of at least one section of at least one tooth, the apparatus comprising: a patterned light generator system configured to generate at least one pattern of infrared light, a patterned light delivery system configured to deliver the at least one pattern of infrared light to the at least one section of the at least one tooth along a patterned light delivery axis, a polarising beam filtering system and a detection system configured to detect the at least one pattern of infrared light backscattered from the at least one section of the at least one tooth along a backscattered light detection axis, wherein the polarising beam filtering system is configured such that the patterned light delivery axis and the backscattered light detection axis are co-aligned onto the at least one section of the at least one tooth.
The advantage of having a so-configured polarising beam filtering system allows the backscattered light detection axis (i.e., the imaging axis or the detection axis) to accurately match (or co-align with) the patterned light delivery axis (i.e., the projection axis or the delivery axis), therefore minimising optical losses. The two axes are then said to be co-aligned in the 'epi' (backscattered or backwards) configuration. This configuration also ensures that the patterned light delivery axis and the backscattered light detection axis are substantially the same, thus allowing for a compact apparatus.
Furthermore, typically a small angle between the imaging and projection planes either requires small optics or long distances of the imaging and projection optics from the tooth sample to fit in a compact apparatus. Both of these requirements reduce the detection efficiency of the apparatus, as fewer scattered photons will be incident on the detector if the detector optics are small or far away. The apparatus of the present invention allows the polarising beam filtering system to be located very close to the tooth sample, allowing imaging optics to be placed close to the sample without obstructing the projection pattern. This in turn increases the detection efficiency, reducing the level of incident light (and therefore reducing photodamage to the tooth tissue) required to illuminate the sample, or reducing the imaging time necessary to obtain high contrast images.
Preferably, whilst co-aligned, the backscattered light detection axis and the patterned light delivery axis may also both be at approximately normal incidence (i.e., at approximately 90 degrees) to the surface of the section of the tooth under investigation. The normal incidence requirement needs to only be approximately satisfied since the surface of the tooth is not flat. Delivering and detecting light at normal incidence to the tooth's surface is generally preferable in imaging systems, as mapping between two non-parallel planes (either a projection plane to a sample plane or a sample plane to a detector plane) may require complicated optics or induce aberrations.
The backscattered detected light may also be patterned. The advantage of capturing a pattern in the detected light is that it is the loss of the pattern of the delivered light which provides information on the scattering properties of the section of tooth under investigation. Therefore, the detection system can detect both backscattered patterned light and light that has lost the pattern, but then the data processing method separates out the useful (i.6., the patterned) light from the not-required, blurred (i.e., the un-patterned) light. This separation is possible due to the presence in the detected backscattered light of a direct component (DC) (no pattern) and of an alternating component (AC) (patterned). Therefore, the backscattered un-patterned light is mathematically removed, but it is still detected by the detection system.
Alternatively, or additionally, the apparatus may be employed in determining the scattering coefficients of several sections of the same tooth or several sections of several teeth. The illumination of several sections at a time may be useful for a fast, low-resolution scan of the tooth/teeth and would simply require the use of fewer spatial frequencies for the patterned light.
The apparatus may further comprise at least one imaging optics system. In one arrangement, the imaging optics system may be part of the patterned light delivery system. In another, complementary or alternative, arrangement the imaging optics system may be positioned between the patterned light delivery system and the tooth/teeth to be imaged. In yet another, complementary or alternative, arrangement the imaging optics system may be positioned between the polarising beam filtering system and the detection system. In yet a further, complementary or alternative, arrangement a first imaging optics system is positioned between the patterned light delivery system and the tooth/teeth to be imaged and a second imaging optics system is positioned between the polarising beam filtering system and the detection system.
The polarising beam filtering system may be further configured to separate the delivered and/or the reflected patterned infrared light from the backscattered detected infrared light such that the delivered and/or the reflected patterned infrared light and the backscattered detected infrared light have different polarisations. Preferably, the polarising beam filtering system may be configured such that the delivered and/or the reflected patterned infrared light and the backscattered detected infrared light are polarised orthogonal to each other.
Alternatively, the polarising beam filtering system may be configured such that the delivered and/or the reflected patterned infrared light and the backscattered detected infrared light have different circular polarisations. Preferably, the reflected patterned infrared light may be a specularly reflected patterned infrared light. These configurations support the primary functioning of the polarising beam filtering system to separate light of different polarisations.
The polarising beam filtering system may comprise a single polarising beam filter. Having a single polarising beam filter simplifies the design of the apparatus, makes it more compact and reduces costs. Furthermore, this configuration eliminates the need to manually cross (linear) polarisers and eliminates the (very small) risk of position drift of the polarisers.
The single polarising beam filter may comprise a polarising beam splitter, preferably comprising a beam splitter cube or a beam splitter sheet. Alternatively, the single polarising beam filter may comprise a combination of wave plates (e.g., a Faraday rotator) and polarising optical elements. Alternatively, the polarising beam splitter may comprise Wollaston prisms and/or Rochon prisms (which split un-polarised beams of light into two orthogonally polarised beams with an angle between them, typically a few degrees to a few tens of degrees) or displacement crystals of, for example, calcite.
Alternatively, the polarising beam filtering system may comprise a first polarising element, preferably a linear polarising element or a % waveplate polarising element, and a beam splitter, preferably a polarising beam splitter. A polarising beam filtering system having multiple components helps reduce background and stray light. For example, one delivered (or incoming) light polarisation onto a polarising beam filtering system comprising a beam splitter cube is reflected upwards, but then this can be scattered back downwards towards the detector using additional scattering elements. Therefore, having additional elements in combination with a % waveplate/s helps to perform more effective filtering.
More preferably, the polarising beam filtering system may further comprise a second polarising element, wherein the second polarising element may be configured to be aligned substantially orthogonal to the first polarising element.
The second polarising element may preferably be a linear polarising element. The configuration of two orthogonal polarising elements enables an effective separation of the delivered and the reflected light from the backscattered detected light, at the same time as maintaining minimal optical loses and eliminating specular reflections. Even more preferably, the first polarising element may additionally have to be aligned / configured such that the delivered light is also effectively transmitted (or reflected, depending on the desired orientation) through the polarising beam splitter.
The patterned light generator system may comprise a narrow-band (e.g., a laser) light source or a wide-band (e.g., a light bulb) light source. A wide-band light source may be 'spectrally narrowed' using coloured filters, dispersive elements, dielectric films and other such methods. Preferably, the wavelengths of these light sources fall in these categories: lasers -mm, LEDs -50nm, thermal light sources ->100nm.
The patterned light generator system may comprise a digital light projector (DLP), the digital light projector preferably comprising a spatial light modulator. The advantage of a DLP is that it may incorporate both a spatial light modulator for generating a pattern and a light source for generating the light.
The DLP may include a megapixel array of mirrors which can be switched on and off independently, therefore forming a 1-bit pattern. By varying the length of time a megapixel mirror array is "on" during an exposure, the light intensity at that point is encoded. An 8-bit pattern is formed by rapidly cycling between 256 1-bit patterns, although it is noted that other different bit-number patterns are possible.
The array may comprise 100k elements to megapixel mirror arrays of up to 10M pixels, the chosen number depending, for instance, on the desired system resolution.
Additionally or alternatively, the patterned light generator system may comprise any one of or a combination of any one of a light source (such as a micro-LED (light emitting diode) array or an individual or assembly of collimated or uncollimated LEDs, lasers or spectrally-filtered bulbs) coupled with a pattern generator (such as a digital light projector, a spatial light modulator, grating, slits, deformable mirror devices, transparent sheets with patterns printed on or a focused point that is rapidly scanned in two dimensions). The spatial light modulator may comprise a digital mirror element, a holographic (liquid crystal) modulator, another spatial profiler, a digital mirror spatial light modulator or variable grid arrangement. The combination of a light source with a pattern generator is advantageous since it reduces the cost and number of components of the patterned light generator system.
Preferably, the patterned light generator system may comprise a combination of any one of a collimated or un-collimated light source and a spatial light modulator.
The at least one pattern of infrared light may be generated with at least one systematic variation of a spatial intensity of the generated patterned infrared light, the at least one systematic variation of the spatial intensity being a function of at least one spatial frequency and/or at least one spatial phase. More preferably, the patterned light generator system may be configured to generate patterns of several spatial frequencies and several spatial phases. Such configuration of the patterned light generator system permits determining scattering coefficients of tooth sections at various depths within the tooth.
The at least one systematic variation of the spatial intensity may comprise any one or a combination of any one of a sinusoidal, a square, a triangular (or sawtooth) or a Hadamard variation of the spatial frequency. Preferably, the systematic variation of the spatial intensity comprises a sinusoidal variation of the spatial frequency since sinusoidal waves offer the most spatially uniform sensitivity to scattering (whereas square waves are most sensitive to scattering near the edges of the wave and less at the centre of the intensity profile.
Preferably, for each at least one systematic variation of the spatial intensity as a function of at least one spatial frequency, the patterned light generator system may be configured to generate n systematic variations of the at least one spatial phase of the delivered patterned infrared light. One systematic variation of the spatial phase may be used if/when undertaking Fourier filtering and when having a varying sensitivity across the tooth's image. Two systematic variations of the spatial phase may be used to avoid Fourier filtering, but this still gives varying sensitivity across the tooth's image. Three systematic variations of the spatial phase represent the minimum number which may be used for an un-ambiguous tooth imaging and to obtain uniform sensitivity.
The n systematic variations of the at least one spatial phase comprises a shift of the delivered patterned infrared light by at least an nth of a period of the at least one spatial phase. For a preset spatial frequency, the shift of the spatial phase enables a complete and uniform illumination of the section of the tooth under investigation.
Preferably, the nth of the period of the at least one spatial phase is at least 1. As a rule of thumb, if the patterned light generator system were to generate 3 spatial phases, then the pattern would shift by a 3"d (i.e., 1/3 of a period) of the period of the spatial phase; for 5 spatial phases, the pattern would shift by a 51h (i.e., 1/5 of a period) of the period, etc. The backscattered detected infrared light comprises backscattered infrared light detected by the detection system at the at least one systematic variation of the spatial intensity as a function of at least one spatial frequency and/or at least one spatial phase of the delivered patterned infrared light. The information obtained from a stand-alone exposure may be incomplete, but, by repeating the exposure at different phases, a single image with useful information on a frequency-and scattering-dependent depth can be obtained. And, by repeating the exposure at multiple spatial frequencies, then information at different depths can be obtained. Therefore, the advantage of these types of exposures is that they enable determining the loss of pattern as the patterned light propagates through the tooth.
The detection system may comprise a wide-field detector (as opposed to a point detector), such as a wide-field camera or a CMOS camera, or an array of detectors, such as an array of point detectors or a CCD array. Alternatively, the detection system may comprise a bolometer array. Wide-field detectors enable imaging over larger sections of tooth or over several teeth in a single imaging scan.
The patterned light delivery system may comprise any one or a combination of any one of a free-space delivery system, an articulated arm or a fibre system. The free-space delivery system would offer the most compact apparatus as the beams of light would simply be passing through air. However, using a fibre system, for example in a hand-held light delivery component of the apparatus, has the advantage of being able to provide a more compact apparatus compared to using an articulated arm. Preferably the fibre system may comprise a coherent fibre bundle or a single multimode fibre.
The patterned light delivery system may further comprise pattern delivery optics means. Preferably, the pattern delivery optics means may comprise re-imaging optics means for re-imaging the backscattered light back into, for example, the articulated arm or the coherent fibre bundle.
In accordance with a second aspect of the invention, there is provided a system for detecting healthy and/or disrupted material in at least one section of at least one tooth, the system comprising an apparatus for determining at least one scattering coefficient of the healthy and/or disrupted material in the at least one section of the at least one tooth. Preferably, the apparatus comprises the apparatus of the first aspect of the invention.
The disrupted material of the at least one section of the at least one tooth may comprise dental caries, such as carious de-mineralisation or carious re-mineralisation, non-carious disorders of the dental hard tissues, such as hyper-fluorosis, de-mineralisation, re-mineralisation or hyper-mineralisation, or erosive tooth wear.
Healthy sections of teeth also scatter the patterned incident light, the scattering being different than that produced by disrupted material in teeth. Detecting healthy sections of teeth would be useful as a reference point and to track changes in the tooth's structure over time. For example, erosive tooth wear could be closely monitored and quantified using the system of the second aspect of the invention.
In accordance with a third aspect of the invention, there is provided a method for determining at least one scattering coefficient of at least one section of at least one tooth, the method comprising: providing an apparatus for determining the at least one scattering coefficient of the at least one section of the at least one tooth, exposing the at least one section of the at least one tooth to at least one pattern of infrared light, the at least one pattern of infrared light being configured to be generated by a patterned light delivery system with at least one systematic variation of a spatial intensity as a function of at least one spatial frequency and/or at least one spatial phase of the generated patterned infrared light, separating, for each of the at least one systematic variation of the spatial intensity, backscattered infrared light from the delivered and/or the reflected patterned infrared light by means of a polarising beam filtering system such that the delivered and/or the reflected patterned infrared light and the backscattered infrared light have different polarisations, detecting the separated backscattered infrared light for each of the at least one systematic variation of the spatial intensity by means of a detection system, selecting, for each of the at least one systematic variation of the spatial intensity, the alternating component AC images from the detected separated backscattered infrared light, generating a stack of images for the at least one section of the at least one tooth from the selected alternating component AC images, and determining the at least one scattering coefficient from the generated image stack.
Preferably, the separating of the backscattered infrared light from the delivered and/or the reflected patterned infrared light for each of the at least one systematic variation of the spatial intensity is such that the delivered and/or the reflected patterned infrared light and the backscattered infrared light are polarised orthogonal to each other. Alternatively, the separating of the backscattered infrared light from the delivered and/or the reflected patterned infrared light for each of the at least one systematic variation of the spatial intensity is such that the delivered and/or the reflected patterned infrared light and the backscattered infrared light have different circular polarisations.
Preferably, the provided apparatus comprises the apparatus of the first aspect of the invention.
Preferably, the separating polarising beam filtering system may be configured such that an axis of the delivered patterned infrared light and an axis of the backscattered infrared light are co-aligned onto the at least one section of the at least one tooth.
The skilled person would understand alternating component AC image to mean the component of the exposures that are dependent on the pattern's spatial frequency, rather than the direct component DC of the exposures which are independent of the pattern's spatial frequency.
The image stack is taken from alternating component AC images and not from raw exposures. Alternating component AC images are mathematically produced as the normalized square root of the sum of the square of the differences of exposures detected at different pattern phases and the same spatial frequency.
In accordance with a fourth aspect of the invention, there is provided a method for detecting healthy and/or disrupted material in at least one section of at least one tooth, the method comprising determining at least one scattering coefficient for the healthy and/or disrupted material, and generating a 2D or a 3D map of localised scattering coefficients for the healthy and/or disrupted material using the determined at least one scattering coefficient.
Preferably, the determining of the at least one scattering coefficient of the healthy and/or disrupted material may comprise using the method of the third aspect of the invention.
The disrupted material of the at least one section of the at least one tooth may comprise dental caries, such as carious de-mineralisation or carious re-mineralisation, non-carious disorders of the dental hard tissues, such as hyperfluorosis, de-mineralisation, re-mineralisation or hyper-mineralisation, or erosive tooth wear.
BRIEF DESCRIPTION OF THE DRAWINGS
Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, of which: Figure 1 illustrates representations of the rate of loss of a square wave pattern of light in a backscattered image of a tooth as a function of the tooth's depth (Figure 1(a) shows a low-frequency square wave pattern, whereas Figure 1(b) shows a high-frequency square wave pattern of light); Figure 2 shows a configuration of the apparatus of the first aspect of the invention; Figure 3 shows an embodiment of the patterned light generator system of the apparatus of the first aspect of the invention; Figure 4 shows a specific configuration of the apparatus of the first aspect of the invention comprising a first specific embodiment of the patterned light delivery system; Figures 5 shows another specific configuration of the apparatus of the first aspect of the invention comprising a second specific embodiment of the patterned light delivery system Figures 6 shows another specific configuration of the apparatus of the first aspect of the invention comprising a third specific embodiment of the patterned light delivery system and a specific embodiment of the second (relay) imaging optics system; Figure 7 shows another specific configuration of the apparatus of the first aspect of the invention comprising a fourth specific embodiment of the patterned light delivery system and a specific embodiment of the polarising beam filtering system; Figure 8 shows another specific configuration of the apparatus of the first aspect of the invention comprising another specific embodiment of the polarising beam filtering system; Figure 9 illustrates the effect of scattering sites (indicated by filled black circles) on the transport mean free path of photons and the corresponding effect on the scattering profile as a function of the spatial frequency; Figure 10 shows a scattering profile in the case of (i) scattering only (dotted line) and (ii) scattering with absorption (solid line); Figure 11(a) shows illustrative scattering profiles for enamel (solid line), for dentin (dotted line) and for a surface defect (dashed line).
Figures 11(b) -(d) show alternating component AC images taken at different spatial frequencies (0.5 mm-1, 1.9 mm-1, 5 mm-1), highlighting the different optical properties of enamel, dentin and surface defects.
Figure 12 shows a 2D map of scattering coefficients of a tooth (left image), alongside a direct component DC image of the tooth (top right image) and an assessment of the scattering coefficients' fit quality (bottom right image).
DETAILED DESCRIPTION OF THE INVENTION
Figure 2 shows a configuration of the apparatus of the first aspect of the invention, the apparatus being used for determining at least one scattering coefficient of at least one section of at least one tooth. The core configuration of the apparatus comprises a light generator system to project a pattern of light onto the tooth sample, an optical filtering system to select the backscattered, rather than reflected light, and a detection system. The optical filtering system serves both to eliminate surface reflections and also to allow imaging on a co-aligned axis to the projection axis, whilst ensuring only light of the desired polarisation is incident on the tooth sample, therefore reducing unnecessary exposure.
In the configuration of Figure 2, the apparatus (100) is shown to comprise a patterned light generator system (10), a patterned light delivery system (20), a polarising beam filtering system (30), a first (delivery) imaging optics system (40), a second (relay) imaging optics system (50) and a detection system (60).
Figure 2 further shows the tooth sample (300) under investigation, a source of clean compressed air (700) and associated air delivery tube (720) and computing means (400). The air source (700) is present near all dental chairs. The air delivery tube (720) may consist of a component (not shown) which can be kept clean and another component (not shown) which may be supplied biologically clean and disposable.
The patterned light generator system (10) is configured to generate at least one pattern of infrared light and comprises a combination of a collimated LED light source (typically operating with a central wavelength around 850 nm, although other wavelengths are viable) incident on a digital light projector (DLP). The DLP is an array of -1.5 megapixel mirror arrays. Telescopes (not shown) may be used to control the beam size between the collimated LED light source and the DLP, and between the DLP and the tooth sample.
Figure 3 shows an embodiment of the patterned light generator system (10) of the apparatus of the first aspect of the invention. The system consists of a light source (11) (e.g., a laser, a LED or a spectrally filtered light bulb) which is initially collected and collimated using optical element (12) (e.g., a single lens or a lens combination) and may then be spatially filtered using spatial filter (13) (e.g., a lens, a mirror, a combination of lenses and pinhole combination) to improve the beam homogeneity. The resulting light is then sent to an optical element (14) which adjusts the spatial distribution of the light to produce the required pattern.
The optical element (14), generally known as a spatial light modulator, may comprise a digital light projector DLP (such as a digital micro-mirror array), a liquid crystal modulator, another spatial profiler, a digital mirror spatial light modulator or variable grid arrangement. The resulting pattern of light is then passed through to the remaining elements of the generator system but may pass through a further spatial filter (15) (e.g., a lens, or mirror, and pinhole combination) to improve the quality of the light pattern. As an alternative, the light pattern may be directly produced using a micro-LED or organic LED array with subsequent imaging optics.
The light from the patterned light generator system then propagates to the patterned light delivery system (20) which may adopt several specific embodiments.
Figure 4 shows a first specific embodiment of the patterned light delivery system (20) wherein the output from the patterned light generator system (10) is directed into an articulated beam delivery arm (210). The arm (210) then may direct the light pattern to the tooth sample (300) through a handpiece (not shown) suitable for use in the oral cavity. The light backscattered by the tooth (300) passes back into the core configuration of the apparatus (100) for subsequent detection by the detection system (60).
Figure 5 shows a second specific embodiment of the patterned light delivery system (20) wherein the light from the patterned light generator system (10) is directed into a coherent, single mode fibre bundle (222) via imaging lens (220). The output from the bundle is initially collimated using an optical system (224) and is then directed onto the polarising beam filtering system (30) with one polarisation being subsequently directed onto the tooth sample (300) using the first imaging optics system (40). The returned, backscattered and reflected, light from the tooth sample (300) is then collected by the first imaging optics system (40) and passes back into the polarising beam filtering system (30). Here a proportion of the backscattered light, with its changed polarisation, is directed down through the second imaging optics system (50) and on to a 20 array detection system (60).
In a preferred configuration, the polarising beam filtering system (30) both polarises and separates the returned, altered polarisation of light. An optional optical element (not shown) may be introduced between the polarising beam filtering system (30) and the optical system (224) such that only one polarisation enters the polarising beam filtering system (30), the polarisation that will be transmitted. This additional optical element has the advantage of minimising the risk of light of the other polarisation entering the polarising beam filtering system (30) and some of this light being reflected via instrument casing or dust into the detection system (60). The detected electronic image is then sent via an electrical cable to the control computer (400).
Figure 6 shows a third specific embodiment of the patterned light delivery system (20) wherein the light from the patterned light generator system (10) is directed into a coherent, single mode fibre bundle (232) via imaging lens (230). The output from the bundle (232) is initially collimated using an optical system (234) and is then directed onto the polarising beam filtering system (30) with one polarisation being subsequently directed onto the tooth sample (300) using the first imaging optics system (40). The returned, backscattered and reflected, light from the tooth sample (300) is then collected by the first imaging optics system (40) and passes back into the polarising beam filtering system (30). Here a proportion of the backscattered light, with its changed polarisation, is directed down through the second optics imaging system (50), which in this embodiment comprise a second coherent, single mode fibre bundle (514) and optics (512) and (516).
In a preferred configuration, the polarising beam filtering system (30) both polarises and separates the returned, altered polarisation of light. An optional optical element (not shown) may be introduced between the polarising beam filtering system (30) and the optical system (234) such that only one polarisation enters the polarising beam filtering system (30), the polarisation that will be transmitted. This additional element has the advantage of minimising the risk of light of the other polarisation entering the polarising beam filtering system (30) and some of this light being reflected via instrument casing or dust into the detector fibre (514). The returned signal from the second coherent fibre bundle (514) is then re-imaged using re-imaging optics (516) and transmitted onto the detection system (60) for subsequent processing.
Figure 7 shows a fourth specific embodiment of the patterned light delivery system (20) wherein the light from the patterned light generator system (10) is directed into a coherent, single mode fibre bundle (242) via a coupling lens (240) and a beam splitter (34) (e.g., a beam splitter cube) of the polarising beam filtering system (30). The output from the bundle (242) is initially collimated using an optical system (244) and is then directed onto polarising element (36) of the polarising beam filtering system (30) with one polarization being subsequently directed through a 1/4 wave plate (or Faraday rotator) (38) onto the tooth sample (300) using the first imaging optics system (40). The transmitted polarisation is then subsequently directed through a % waveplate (or a Faraday rotator) (38) onto the tooth sample (300) using the first imaging optics system (40). The returned, backscattered and reflected, light from the tooth sample (300) is then collected by the first imaging optics system (40) and passes back through the 1/4 waveplate (or Faraday rotator) (38) into the polarising element (36). Here the reflected returned light has had its polarisation rotated by 90 degrees, and is absorbed or reflected, rather than transmitted, by the polarising beam filtering system (30). A proportion of the backscattered light, with its randomised polarisation, is transmitted through the polarising beam filtering system (30) and back into the coherent fibre bundle (242) via the optics (244). The returned signal from the coherent fibre bundle (242) is then re-imaged via coupling lens (240) onto the beam splitter (34) and a proportion of the light is subsequently directed through the second imaging optics system (50) and onto the detection system (60) for processing.
Figure 8 shows a specific embodiment of the polarising beam filtering system (30) which may comprise either a single element (e.g., a polarising beam splitter cube or sheet) or multiple elements. Light from the patterned light generator system (10) is sent through a linear polarising element (32) and then transmitted through the beam splitter (31) and onto the tooth sample (300). The returned light is then sent to the beam splitter (31) and a proportion is directed through a second linear polarising element (33) and onto the detection system (60) via the second imaging optics (50). The two polarisers (32, 33) are "crossed" such that their planes of polarisation are orthogonal and are orientated such that the first polariser (32) transmits light that is also transmitted by the beam splitter (31), whilst the second polariser (33) transmits light that is reflected by the beam splitter (31). This means that only light with a changed polarisation will reach the detection system (60).
When light is incident on the tooth sample (300), both reflection and backscattering can occur. Reflection occurs from a surface (i.e. the interface of the tooth sample and the environment) and it is often considered to be analogous to bouncing' off the surface of a sample. Consequently, if the incident light is linearly polarised, the reflected light will have the same polarisation as the incident light and, if the incident light is circularly polarised, the reflected light will have its polarisation inverted (i.e. left-hand circularly polarised light will become right-hand circularly polarised light after reflection). This is referred to as specular reflection' as opposed to diffuse reflection and references to reflection are to 'specular reflection'. Scattering' is the randomisation of direction and polarisation of light by particles, often through multiple scattering events. It should be noted that preferential scattering directions or polarisations may still be present in scattering events, despite the random element of scattering. Technically, reflection is also a form of scattering.
The critical property of these two processes is that reflection occurs at the surface of the tooth and does not randomise the polarisation of the light, whereas scattering occurs at all depths of the tooth and may randomise the polarisation of the light. The apparatus (100) is used to detect light photons that have penetrated the tooth's tissue, rather than those that have only been reflected at the surface of the tooth.
Additionally, the apparatus (100) is configured such that both the illumination axis and the imaging axis are co-aligned and as close to normal incidence to the imaged tooth section as possible. This configuration suppresses aberrations or complicated optics systems that occur when mapping non-parallel planes to each other. This configuration is made possible by the polarising beam filtering system (30), which allows the imaging axis to exactly match the projection axis with minimal optical losses.
The detection system (60) employed in the apparatus (100) of Figures 2, 4-8 is a camera used to image the tooth sample (300) whilst it is illuminated. The light backscattered (and which has had its polarisation changed) by the tooth (300) is focused through imaging optics onto the camera. It is necessary to use a wide-field detector to image the sample, as SFDI is a wide-field imaging technique (i.e., it can image a 2D plane, rather than offering a point-by-point detection such as confocal microscopy or OCT). In principle "single pixel" or "ghost" imaging methods could be applied using this technique but would in general be slower. The camera must image a large area of the tooth (300) (approximately 1cm x 1cm), with sufficient resolution to resolve dental caries.
In use, light from the patterned light generator system (10) is directed onto the polarising beam filtering system (30) with one polarisation being subsequently directed onto the tooth sample (300) using the polarising beam filtering system (30). The returned, backscattered and reflected, light from the tooth sample (300) is then collected by the first imaging optics system (40) and passes back into the polarising beam filtering system (30). Here the backscattered light, with its changed polarisation, is directed down through the second optics imaging system (50) and onto the detection system (60). The preferred configuration is for the polarising beam filtering system (30) to both polarise and separate the returned, altered polarisation of light. This configuration also ensures that the patterned light delivery axis and the backscattered light detection axis are co-aligned onto the surface of the tooth (300).
Before reaching the polarising beam filtering system (30), the generated light propagates through the patterned light delivery system (20) which may be air, an articulated arm or a coherent fibre bundle. The generated light is spatially structured and, for this purpose, it is necessary to change both the spatial phase and the spatial frequency of the generated pattern of light. Typically, the pattern is a sinusoidal wave with an offset: I ma, iLlumination(X) = (1+ sin(2n-fx + (p)) Here'macis the maximum intensity, f is the pattern frequency, x is the spatial position and cp is the phase. However, other patterns of illumination are possible including square or triangular waves.
The apparatus (100) is employed in determining the scattering coefficient of at least once section of at least one tooth (300). The resulting scattering coefficient data is then analysed to generate useful information (for example, the existence of early stage dental caries) for the tooth's condition at different spatial positions along the tooth's depth.
The flow diagram below illustrates the sequence of steps used to image tooth sections or sections of several adjacent teeth.
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Sy,lect ACwtg& 6etipyry" If lilgfi I (;e1 in'ait iiI trbi ellil+.'ilt.
1"1?,I ti,f* rnagt eflec all El tr di I. OE pattern TaU el Fria-q! cLy[ak.ally,3 Limes) Itepea, ---^ ------ ---mg( As it can be seen from the flow diagram, the imaging method comprises the following steps: -exposing at least one section of the at least one tooth (300) to at least one pattern of infrared light, the at least one pattern of infrared light being configured to be generated by a patterned light delivery system (10) with at least one systematic variation of a spatial intensity as a function of at least one spatial frequency and/or at least one spatial phase of the generated patterned infrared light, - separating backscattered infrared light from the delivered patterned infrared light for each of the at least one systematic variation of the spatial intensity by means of the polarising beam filtering system (30) such that the delivered patterned infrared light and the backscattered infrared light have different polarisations to each other, - detecting the separated backscattered infrared light for each of the at least one systematic variation of the spatial intensity by means of a detection system (60), - selecting, for each spatial frequency (from tooth exposures taken at multiple spatial phases), the alternating component AC images from the separated backscattered detected infrared light, - generating a stack of images for the at least one section of the at least one tooth (300) from the calculated alternating component AC images, and - determining the at least one scattering coefficient from the generated image stack.
Typically, the at least one section of the at least one tooth is exposed to a pattern of infrared light employing 3 spatial phases and multiple spatial frequencies. However, other useful combinations of variations of spatial phases and spatial frequencies can be used.
Data taking / acquisition for determining scattering coefficients of teeth Considerations on focal planes If the illumination pattern on the tooth sample is blurred, residual pattern may occur on the processed images. It is therefore necessary to ensure both that the projection plane and the imaging plane are in the same position, and that the tooth sample is at this position too. In some implementations no optics are used along the projection axis, simplifying this process, as the projection is always in focus. This effect is dependent on the focal length of the projection and imaging lenses. Stronger lenses (i.e. with shorter focal lengths), require better alignment, but also improve the efficiency with which scattered light is detected, enabling faster imaging or lower incident intensity light.
By using a single polariser the inventors have ensured that both the imaging axis and detection axis project and detect different polarisations of light, thus detecting only scattered light, achieving both of these steps with a single optical component. This also reduces the apparatus' sensitivity to mis-alignment of typically used crossed polarisers (these would be aligned during manufacturing, so the end user would not experience a difference). This technique also allows pattern projection and imaging at a normal angle to the surface of the tissue. A further advantage of this technique is that the inventors could place the imaging lens close to the imaged tooth surface (thus improving detection efficiency) without obstructing the projection axis.
Considerations on rejection port light The use of a polarising beam splitting (PBS) cube in the polarising beam filtering system (30) results in light of the 'wrong' polarisation from the LED light source being rejected. If this light is incident on a surface that scatters or reflects, or if the exit surface of the PBS cube has some reflection, and if the PBS does not have perfect extinction, then some of the rejected light can reach the detecting camera. This effect is made more severe if the projected pattern of light is in focus on the surface. However, this effect can easily be suppressed, either through choice of an appropriate beam dump in the rejection port beam path, and/or through taking a 'background' exposure and subtracting this from the final image, and/or through use of anti-reflection coated optics, or use of a plate polarising beam splitter to minimise reflections. Alternatively, light of the 'wrong' polarisation could be filtered out by another polarising filter in the projection system prior to the filtering polarising beam splitter.
Considerations on the use of a sinusoidal wave It is preferable to have uniform illumination when summed across all exposures (typically 3 exposures), which is desirable when performing a range of imaging techniques to avoid artefacts associated with illumination intensity. It is also preferable not to have regions in which illumination is uniform for all patterns since when the illumination is uniform there is no sensitivity to scattering.
Finally, it is preferable not to have sharp changes in illumination profile as regions near the sharp edges of the pattern are more sensitive to scattering than those far from the edges. This requires at least three spatial phases of patterned light to be used. The inventors are therefore illuminating the tooth sample (300) three times at each spatial frequency with a spatial phase difference of 2 r/3 between each pattern.
A sinusoidal pattern meets all of the above conditions, so is the preferred choice. However, other patterns may also be used, such as square or triangular wave patterns.
Considerations on the clinical use of the dental imaging method In some investigation/measurement circumstances it is possible that the dentist may wish to take a transmission image, with uniform illumination, to do a quick inspection of the tooth and then to switch to the pattern projection method if a potential problem is detected.
It may also be possible that initially only a few patterns may be used to take a "crude" 3D data set and then a higher resolution method, with a larger number of spatial frequencies, if there is an area of interest (such as a dental caries). Potentially, the reflected pattern could be of interest to map out the overall surface morphology of the tooth.
The inventors may choose to bin' images on the camera or during analysis or image processing. 'Binning' is the process of combining the counts of neighboring pixels, e.g. turning each set of 4 x 4 pixels into one effective pixel with the count of all 16 pixels. This may improve the signal to noise ratio or reduce the image processing time. If this is done on the camera (i.e. during camera read-out) this may reduce noise levels and/or allow reduced light exposure (less light on the sample). It may also increase the speed of data-taking (faster read-out). This binning may be dependent on the illumination pattern frequency i.e. at low pattern frequencies, the inventors may use larger bin sizes.
Method of determinina scattering coefficients of teeth -analysis and orocessina of the acquired data Data analysis techniques There are several stages of image analysis. Firstly, an alternating component AC image is constructed from the three exposures taken at different spatial phases, but the same spatial frequency. This is referred to as image analysis. Repeating to construct multiple images formed at different pattern frequencies, an image stack is produced. This can then be analysed to produce one or multiple scattering profiles, from which one or multiple scattering coefficients can be generated, and 2D or 3D maps of localised scattering coefficients produced from these scattering coefficients. These analyses are referred to as image stack analysis.
Image analysis The analysis necessary to produce an alternating component AC image from three exposures is described elsewhere in the literature (Cuccia et al, J. Biomed Opt. 14(2) 024012 (2009)). The alternating component AC image is the normalised root of the sum of the square of the differences between each pair of exposures: 21/2 AC = - 3 -/2)2 + 13)2 + -/011/2 Here the first term -3 is a normalisation factor, and /1, 12, and /3 represent exposures with the three different spatial phases. /AC is thus the alternate component AC image that contains information to a depth that is dependent on scattering and pattern frequency.
A direct component DC image can also be produced from the normalised sum of 20 exposures: 1.
DC = -3(4 + + 13) This is equivalent to taking an exposure of uniform illumination and has information for all scattering levels. This image may have use for some other data analysis techniques (e.g., for detecting changes in exposure intensity or for ensuring images are aligned correctly).
The 2D alternating component AC images can then be combined into an image stack ordered according to pattern's spatial frequency.
Scattering profile As the spatial frequency falls (i.e. the pattern becomes wider), greater scattering is required to blur out the pattern. Thus, for a constant level of scattering, a lower spatial frequency will return a stronger alternating component AC image signal compared to a higher spatial frequency. This is illustrated in Figure 9. The rate of change of the alternating component AC image signal with pattern frequency is dependent on the level of scattering of the tooth's tissue -the stronger the scattering, the shorter the transport mean free path (i.e., the length over which the photon direction is randomised) and the more likely a photon is to be backscattered and leave the tooth's tissue without significant migration or pattern blurring. Thus, stronger scattering and lower spatial frequency both result in stronger alternating component AC signal.
Figure 9 (a) illustrates the effect of scattering sites (indicated by filled black circles) on the mean free path. Stronger scattering (left-side image) leads to shorter mean free paths and lower penetration depths. The shorter mean free path also results in photons leaving the tooth's tissue closer to where they entered the tissue, resulting in less pattern blurring and stronger alternating component AC image signal. Weaker scattering (right-side image) leads to longer mean free paths and deeper penetration depths. The longer mean free path also results in more blurring of the pattern in the backscattered mode.
Figure 9 (b) illustrates the alternating component AC image signal plotted as a function of the spatial frequency (i.e. a scattering profile). The curve shapes reflect the level of scattering: stronger scattering results in slower drop-offs (dotted line), whereas weaker scattering results in steeper drop-offs (continuous line). In heterogeneous media, for example, sub-surface defects, different levels of scattering may result in different curve shapes (dashed line).
Tooth tissue properties (absorption and scattering coefficients) can be quantified from the scattering profile. Quantification techniques often stem from the radiative transfer equation or the diffusion equation (which describe changes of photon density in the tissue) or Monte Carlo techniques. In the case of dental tissue at the wavelengths used by the inventors, absorption is typically negligible, substantially simplifying these quantification techniques.
To effectively characterise tooth tissue it is useful to compare the scattering profile of the tissue to a standard. Several forms of standard may be used, such as a database of previously imaged teeth, a Monte Carlo model, a best fit curve, curves taken from other regions of the same tooth or data taken previously from the same tooth. Different standards have different merits -for example, a comparison to a database of previously imaged teeth or a Monte Carlo model may allow quantification of dental mineralisation. Comparison to other regions of the same tooth may allow identification of local defects. Comparison to previous datasets from the same tooth may allow identification of changes with time. Some or all of these standards may be used.
As a first step, a calibration stage may be required to modify the alternating component AC image signal to account for the instrument response function. This will be necessary to obtain quantified dental mineralisation data (i.e. converting the gradient of the curves in Figure 10 to a mineral content level).
A critical parameter is the ratio of the photon mean free path to the period of the illumination pattern. In the case of no absorption, this ratio alone dictates the behavior of the scattering profile. In the case of absorption, longer photon migration paths that are necessary to blur the illumination pattern at low spatial frequencies are preferentially suppressed due to absorption, relative to shorter photon migration paths. Thus, lower spatial frequencies are more sensitive to absorption than higher spatial frequencies. This is illustrated in Figure 10, with the dotted line representing a purely scattering medium and the solid line representing a scattering and absorbing medium. As absorption is typically negligible in dental tissue at the wavelengths used by inventors, the process of modelling a standard, fitting a best fit curve or quantifying mineral content may be substantially simpler. The absence of absorption is a significant deviation from previous experimental work. The metric for considering absorption to be negligible is that the absorption coefficient is much less than the smaller of the period of the lowest frequency illumination pattern that is used and the size of the tooth.
Depth profile The dentist's understanding of the data may be aided by a 3D representation of the tooth. Consequently, the data may be processed to produce such a representation. The simplest approach is to subtract high spatial frequency alternating component AC images (lower penetration depths) from low spatial frequency alternating component AC images (higher penetration depths) to obtain a 'slice' of tooth beneath the surface. However, this may not be suitable for heterogeneous tissue. A wide range of methods may be used to provide appropriate 3D image reconstruction (for example, Savitzky-Golay differentiation filtering or Monte-Carlo modelling). However, all of these methods rely on the generated underlying image stacks.
Whilst it is tempting to think of different pattern frequencies as corresponding to different imaging depths, the underlying thought process should be that different pattern frequencies have different levels of scattering required to blur out the signal. Consequently, whilst in a homogeneous medium, different pattern frequencies correspond to different imaging depths, in heterogeneous tissue (e.g. enamel and dentin or enamel with defects), the imaging depth will vary across the tissue even for a single pattern frequency.
Image representations of measured scattering data Figure 11(a) shows illustrative scattering profiles for enamel (solid line), for dentin (dotted line) and for a surface defect (dashed line). This offers a useful visualisation of different dental tissue types or conditions that have different scattering levels and therefore different scattering profiles. The weakly scattering enamel is represented by the solid line, the more strongly scattering dentin is represented by the dotted line and the very strongly scattering defect is represented by the dashed line.
Figures 11(b) -(d) show alternating component AC images taken at different spatial frequencies (0.5 mm-1, 1.9 mm-1, 5 mm-1), highlighting the different optical properties of enamel, dentin and surface defects.
In Figure 11 (b), the spatial frequency is low (0.5 mm-1) and the alternating component AC image signal is significant for all tissue types. In Figure 11 (c), the spatial frequency is higher (1.9 mm-1), causing the alternating component AC image signal to fall substantially for enamel, reducing the signal strength of enamel regions, whilst the dentin and defect regions still have strong signal. In Figure 11 (d), the spatial frequency is higher still (5 mm-1), causing only the defect to have strong signal and the enamel and dentin to both have weak signal. This technique relies on different tissue types or conditions that have different scattering levels. It was possible to combine these alternating component AC images with different gray-scale shadings to offer a representation of different tissue types encoded through the gray-scale in a single image.
As a general comment it should be noted that SFDI (spatial frequency domain imaging) can distinguish between tissue at different depths, but does not extend the optical imaging depth. In other words, if light can penetrate to a certain depth, SFDI does not increase this depth, it just provides information on what lies at different depths up to the maximum penetration depth.
Figure 12 shows a 2D map of scattering coefficients of a tooth (left image), alongside a direct component DC image of the tooth (top right image) and an assessment of the scattering coefficients' fit quality (bottom right image).
As it can be seen, a cavity (i.e., a defect) at the centre of the tooth and a region at the bottom right of the tooth both have high scattering coefficients. However, the integrity of the fit measurement is only high for regions around the cavity, suggesting that around the cavity there is heterogeneity, whilst at the bottom right of the tooth, the tissue is homogeneous.
Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.
Claims (25)
- CLAIMS1 An apparatus (100) for determining at least one scattering coefficient of at least one section of at least one tooth (300), the apparatus comprising: a patterned light generator system (10) configured to generate at least one pattern of infrared light, a patterned light delivery system (20) configured to deliver the at least one pattern of infrared light to the at least one section of the at least one tooth (300) along a patterned light delivery axis, a polarising beam filtering system (30) and a detection system (60) configured to detect the at least one pattern of infrared light backscattered from the at least one section of the at least one tooth (300) along a backscattered light detection axis, wherein the polarising beam filtering system (30) is configured such that the patterned light delivery axis and the backscattered light detection axis are co-aligned onto the at least one section of the at least one tooth (300).
- 2. The apparatus of claim 1, wherein the polarising beam filtering system (30) is further configured to separate the delivered and/or the reflected patterned infrared light from the backscattered detected infrared light such that the delivered and/or the reflected patterned infrared light and the backscattered detected infrared light have different polarisafions.
- 3. The apparatus of any one of the preceding claims, wherein the polarising beam filtering system (30) comprises a single polarising beam filter.
- 4. The apparatus of claim 3, wherein the single polarising beam filter comprises a polarising beam splitter.
- 5 The apparatus of any one of claims 1 to 2, wherein the polarising beam filtering system (30) comprises a first polarising element (32,33), preferably a linear polarising element or a % wave plate polarising element, and a beam splitter (31), preferably a polarising beam splitter.
- 6 The apparatus of claim 5, wherein the polarising beam filtering system (30) further comprises a second polarising element (32,33), wherein the second polarising element (32,33) is configured to be aligned substantially orthogonal to the first polarising element (32,33).
- 7 The apparatus of any one of the preceding claims, wherein the patterned light generator system (10) comprises a digital light projector, the digital light projector preferably comprising a spatial light modulator.
- 8 The apparatus of any one of claims 1 to 6, wherein the patterned light generator system (10) comprises a combination of any one of a collimated or un-collimated light source and a spatial light modulator.
- 9 The apparatus of any one of the preceding claims, wherein the at least one pattern of infrared light is generated with at least one systematic variation of a spatial intensity of the generated patterned infrared light, the at least one systematic variation of the spatial intensity being a function of at least one spatial frequency and/or at least one spatial phase.
- 10. The apparatus of claim 9, wherein the at least one systematic variation of the spatial intensity comprises any one or a combination of any one of a sinusoidal, a square, a triangular or a Hadamard variation of the spatial frequency.
- 11. The apparatus of any one of claims 9 to 10, wherein, for each at least one systematic variation of the spatial intensity as a function of at least one spatial frequency, the patterned light generator system (10) is configured to generate n systematic variations of the at least one spatial phase of the delivered patterned infrared light.
- 12. The apparatus of claim 11, wherein the n systematic variations of the at least one spatial phase comprises a shift of the delivered patterned infrared light by at least an nth of a period of the at least one spatial phase.
- 13. The apparatus of claim 12, wherein the nth of the period of the at least one spatial phase is at least 1.
- 14 The apparatus of any one of claims 9 to 13, wherein the backscattered detected infrared light comprises backscattered infrared light detected by the detection system (60) at the at least one systematic variation of the spatial intensity as a function of at least one spatial frequency and/or at least one spatial phase of the delivered patterned infrared light.
- 15. The apparatus of any one of the preceding claims, wherein the detection system (60) comprises a wide-field detector, such as a wide-field camera or a CMOS camera, or an array of detectors, such as an array of point detectors or a CCD array.
- 16 The apparatus of any one of the preceding claims, wherein the patterned light delivery system (20) comprises any one or a combination of any one of a free-space delivery system, an articulated arm (210) or a fibre system (222, 232, 242).
- 17. The apparatus of claim 16, wherein the patterned light delivery system (20) further comprises pattern delivery optics means (210, 220, 224, 230, 234, 240, 244).
- 18. A system for detecting healthy and/or disrupted material in at least one section of at least one tooth (300), the system comprising an apparatus for determining at least one scattering coefficient of the healthy and/or disrupted material in the at least one section of the at least one tooth (300).
- 19. The system of claim 18, wherein the apparatus comprises the apparatus (100) of any one of claims 1 to 17.
- 20. The system of any one of claims 18 to 19, wherein the disrupted material of the at least one section of the at least one tooth (300) comprises dental caries, such as carious de-mineralisation or carious re-mineralisation, non-carious disorders of the dental hard tissues, such as hyper-fluorosis, de-mineralisation, re-mineralisation or hyper-mineralisation, or erosive tooth wear.
- 21. A method for determining at least one scattering coefficient of at least one section of at least one tooth, the method comprising: providing an apparatus for determining the at least one scattering coefficient of the at least one section of the at least one tooth, exposing the at least one section of the at least one tooth to at least one pattern of infrared light, the at least one pattern of infrared light being configured to be generated by a patterned light delivery system with at least one systematic variation of a spatial intensity as a function of at least one spatial frequency and/or at least one spatial phase of the generated patterned infrared light, separating, for each of the at least one systematic variation of the spatial intensity, backscattered infrared light from the delivered and/or the reflected patterned infrared light by means of a polarising beam filtering system such that the delivered and/or the reflected patterned infrared light and the backscattered infrared light have different polarisations, detecting the separated backscattered infrared light for each of the at least one systematic variation of the spatial intensity by means of a detection system, selecting, for each of the at least one systematic variation of the spatial intensity, the alternating component AC images from the detected separated backscattered infrared light, generating a stack of images for the at least one section of the at least one tooth from the selected alternating component AC images, and determining the at least one scattering coefficient from the generated image stack.
- 22. The method of claim 21, wherein the provided apparatus comprises the apparatus (100) of any one of claims 1 to 17.
- 23. The method of any one of claims 21 to 22, wherein the polarising beam filtering system is configured such that an axis of the delivered patterned infrared light and an axis of the backscattered infrared light are co-aligned onto the at least one section of the at least one tooth.
- 24. A method for detecting healthy and/or disrupted material in at least one section of at least one tooth, the method comprising determining at least one scattering coefficient for the healthy and/or disrupted material, and generating a 2D or a 3D map of localised scattering coefficients for the healthy and/or disrupted material using the determined at least one scattering coefficient.
- 25. The method of claim 24, wherein determining the at least one scattering coefficient of the healthy and/or disrupted material comprises using the method of any one of claims 21 to 24 26 The method any one of claims 24 to 25, wherein the disrupted material of the at least one section of the at least one tooth comprises dental caries, such as carious demineralisation or carious re-mineralisation, non-carious disorders of the dental hard tissues, such as hyper-fluorosis, de-mineralisation, re-mineralisation or hyper-mineralisation, or erosive tooth wear.
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| WO2010145669A1 (en) * | 2009-06-17 | 2010-12-23 | 3Shape A/S | Focus scanning apparatus |
| US20110287387A1 (en) * | 2009-04-16 | 2011-11-24 | Carestream Health, Inc. | System and method for detecting tooth cracks |
| US20160157723A1 (en) * | 2014-02-04 | 2016-06-09 | The Trustees Of Dartmouth College | Structured-light imaging systems and methods for determining sub-diffuse scattering parameters |
| US20160302669A1 (en) * | 2009-11-19 | 2016-10-20 | Modulated Imaging, Inc. | Method and apparatus for analysis of turbid media via single-element detection using structured illumination |
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| US9254103B2 (en) * | 2011-08-15 | 2016-02-09 | The Trustees Of Dartmouth College | Operative microscope having diffuse optical imaging system with tomographic image reconstruction and superposition in field of view |
| EP2979059B1 (en) * | 2013-03-27 | 2021-05-05 | Seikowave Health Solutions Inc. | Portable structured light measurement module with pattern shifting device incorporating a fixed-pattern optic for illuminating a subject-under-test |
| CA3075654A1 (en) * | 2017-09-12 | 2019-03-21 | Sonendo, Inc. | Optical systems and methods for examining a tooth |
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| US20160302669A1 (en) * | 2009-11-19 | 2016-10-20 | Modulated Imaging, Inc. | Method and apparatus for analysis of turbid media via single-element detection using structured illumination |
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