Classifications

• • 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

Definitions

• This invention generally relates to a method and apparatus for dental imaging and more particularly to an improved method for early detection of caries using fluorescence and scattering of light.
• QLF quantitative light-induced fluorescence
• U.S. Patent No. 4,515,476 discloses use of a laser for providing excitation energy that generates fluorescence at some other wavelength for locating carious areas.
• U.S. Patent No. 6,231 ,338 discloses an imaging apparatus for identifying dental caries using fluorescence detection.
• U.S. Patent Application Publication No. 2004/0240716 discloses methods for improved image analysis for images obtained from fluorescing tissue.
• U.S. Patent No. 4,479,499 describes a method for using transillumination to detect caries based on the translucent properties of tooth structure.
• U.S. Patent Application Publication No. 2004/0202356 (Stookey et al.) describes mathematical processing of spectral changes in fluorescence in order to detect caries in different stages with improved accuracy. Acknowledging the difficulty of early detection when using spectral fluorescence measurements, the '2356 Stookey et al. disclosure describes approaches for enhancing the spectral values obtained, effecting a transformation of the spectral data that is adapted to the spectral response of the camera that obtains the fluorescent image. While the disclosed methods and apparatus show promise in providing non-invasive, non-ionizing imaging methods for caries detection, there is still room for improvement.
• One recognized drawback with existing techniques that employ fluorescence imaging relates to image contrast.
• the present invention provides a method for forming an enhanced image of a tooth comprising: a) obtaining fluorescence image data from a tooth by: (i) directing incident light toward the tooth;
• Figure 1 is a schematic block diagram of an imaging apparatus for caries detection according to one embodiment
• Figure 2 is a schematic block diagram of an imaging apparatus for caries detection according to an alternate embodiment
• Figure 3 is a schematic block diagram of an imaging apparatus for caries detection according to an alternate embodiment
• Figure 4A is a schematic block diagram of an imaging apparatus for caries detection according to an alternate embodiment using polarized light
• Figure 4B is a schematic block diagram of an imaging apparatus for caries detection according to an alternate embodiment using a polarizing beamsplitter to provide polarized light and to minimize specular reflection;
• Figure 5 is a view showing the process for combining dental image data to generate a fluorescence image with reflectance enhancement according to the present invention
• Figure 6 is a composite view showing the contrast improvement of the present invention in a side-by-side comparison with conventional visual and fluorescence methods
• Figure 7 is a block diagram showing a sequence of image processing for generating an enhanced threshold image according to one embodiment
• Figure 8 is a schematic block diagram of an imaging apparatus for caries detection according to an alternate embodiment using multiple light sources
• Figure 9 is a plan view comparing results from scalar multiplication and asymmetric illuminance methods of the present invention
• Figure 10 shows graphs for input/output pixel mapping of code values without any image modification and for pixel mapping with an applied offset as used in one embodiment
• Figure 11 is a plan view comparing results from scalar multiplication and downshifting methods of the present invention.
• Figure 12 is a plan view showing an example display with white light and enhanced images displayed for a tooth according to one embodiment.
• fluorescence can be used to detect dental caries using either of two characteristic responses: First, excitation by a blue light source causes healthy tooth tissue to fluoresce in the green spectrum. Secondly, excitation by a red light source can cause bacterial by-products, such as those indicating caries, to fluoresce in the red spectrum.
• reflectance In order for an understanding of how light is used in the present invention, it is important to give more precise definition to the terms “reflectance” and “back-scattering” as they are used in biomedical applications in general and, more particularly, in the method and apparatus of the present invention.
• reflectance generally denotes the sum total of both specular reflectance and scattered reflectance. (Specular reflection is that component of the excitation light that is reflected by the tooth surface at the same angle as the incident angle.)
• specular component of reflectance is of no interest and is, instead, generally detrimental to obtaining an image or measurement from a sample.
• the component of reflectance that is of interest for the present application is from back-scattered light only. Specular reflectance must be blocked or otherwise removed from the imaging path.
• the term "back-scattered reflectance” is used in the present application to denote the component of reflectance that is of interest.
• “Back- scattered reflectance” is defined as that component of the excitation light that is elastically back-scattered over a wide range of angles by the illuminated tooth structure.
• Reflectance image” data refers to image data obtained from back-scattered reflectance only, since specular reflectance is blocked or kept to a minimum.
• back- scattered reflectance may also be referred to as back-reflectance or simply as back-scattering. Back-scattered reflectance is at the same wavelength as the excitation light.
• the inventors have found, however, that this back-scattered reflectance change can be used in conjunction with the fluorescent effects to more clearly and more accurately pinpoint a carious location. Moreover, the inventors have observed that the change in light scattering activity, while it can generally be detected wherever a caries condition exists, is more pronounced in areas of incipient caries. This back-scattered reflectance change is evident at early stages of caries, even when fluorescent effects are least pronounced.
• the present invention takes advantage of the observed back- scattering behavior for incipient caries and uses this effect, in combination with fluorescence effects described previously in the background section, to provide an improved capability for dental imaging to detect caries.
• FIRE fluorescence imaging with reflectance enhancement
• a light source 12 directs an incident light, at a blue wavelength range or other suitable wavelength range, toward tooth 20 through an optional lens 14 or other light beam conditioning component.
• the tooth 20 may be illuminated at a proximal surface (as shown) or at an occlusal surface (not shown).
• Two components of light are then detected by a monochrome camera 30 through a lens 22: a back-scattered light component having the same wavelength as the incident light and having measurable reflectance; and a fluorescent light that has been excited due to the incident light.
• specular reflection causes false positives and is undesirable.
• the camera 30 is positioned at a suitable angle with respect to the light source 12. This allows imaging of back-scattered light without the confounding influence of a specularly reflected component.
• monochrome camera 30 has color filters 26 and 28.
• One of color filters 26 and 28 is used during reflectance imaging, the other is used during fluorescence imaging.
• a processing apparatus 38 obtains and processes the reflectance and fluorescence image data and forms a FIRE image 60.
• FIRE image 60 is an enhanced diagnostic image that can be printed or can appear on a display 40.
• FIRE image 60 data can also be transmitted to storage or transmitted to another site for display.
• FIG. 2 there is shown an alternate embodiment using a color camera 32.
• auxiliary filters would not generally be needed, since color camera 32 would be able to obtain the reflectance and fluorescence images from the color separations (also called color planes) of the full color image of tooth 20.
• Light source 12 is typically centered around a blue wavelength, such as about 405 nm in one embodiment. In practice, light source 12 could emit light ranging in wavelength from an upper ultraviolet range to a deeper blue, between about 300 and 500nm.
• Light source 12 can be a laser or could be fabricated using one or more light emitting diodes (LEDs). Alternately, a broadband source, such as a xenon lamp, having a supporting color filter for passing the desired wavelengths could be used.
• Lens 14 or other optical element may serve to condition the incident light, such as by controlling the uniformity and size of the illumination area. For example, a diffuser 13, shown as a dotted line in Figure 2, might be used before or after lens 14 to smooth out the hot spots of an LED beam.
• the path of illumination light might include light guiding or light distributing structures such as optical fibers or a liquid light guide, for example (not shown).
• Light level is typically a few milliwatts in intensity, but can be more or less, depending on the light conditioning and sensing components used.
• the illumination arrangement could alternately direct light at normal incidence, turned through a beamsplitter 34. Camera 32 would then be disposed to obtain the image light that is transmitted through beamsplitter 34.
• Other options for illumination include multiple light sources directed at the tooth with angular incidence from one or more sides. Alternately, the illumination might use an annular ring or an arrangement of LED sources distributed about a center such as in a circular array to provide light uniformly from multiple angles. Illumination could also be provided through an optical fiber or fiber array.
• the imaging optics, represented as lens 22 in Figures 1-3 could include any suitable arrangement of optical components, with possible configurations ranging from a single lens component to a multi-element lens.
• Imaging optics Preferably, for optimal resolution, the imaging optics provide an image size that substantially fills the sensor element of the camera. Telecentric optics are advantaged for lens 22, providing image-bearing light that is not highly dependent on ray angle. Image capture can be performed by either monochrome camera 30
• spectral filter 26 for capturing reflectance image data would transmit predominately blue light.
• Spectral filter 28 for capturing fluorescence image data would transmit light at a different wavelength, such as predominately green light.
• spectral filters 26 and 28 are automatically switched into place to allow capture of both reflectance and fluorescence images in very close succession. Both images are obtained from the same position to allow accurate registration of the image data.
• Spectral filter 28 would be optimized with a pass-band that captures fluorescence data over a range of suitable wavelengths.
• the fluorescent effect that has been obtained from tooth 20 can have a relative broad spectral distribution in the visible range, with light emitted that is outside the wavelength range of the light used for excitation.
• the fluorescent emission is typically between about 450 ran and 650 nm, while generally peaking in the green region, roughly from around 500 nm to about 600 nm.
• a green light filter is generally preferred for spectral filter 28 in order to obtain this fluorescence image at its highest energy levels.
• other ranges of the visible spectrum could also be used in other embodiments.
• spectral filter 26 would be optimized with a pass-band that captures reflectance data over a wavelength range covering at least a significant portion of the spectral energy of the light source 12 used.
• a blue light filter is generally used for spectral filter 26 in order to obtain the reflectance image at its highest energy level.
• Camera controls are suitably adjusted for obtaining each type of image. For example, when capturing the fluorescence image, it is necessary to make appropriate exposure adjustments for gain, shutter speed, and aperture, since this image may not be intense.
• color camera 32 Figure 2
• color filtering is performed by the color filter arrays on the camera image sensor. The reflectance image is captured in the blue color plane; simultaneously, the fluorescence image is captured in the green color plane. That is, a single exposure captures both back-scattered reflectance and fluorescence images.
• Processing apparatus 38 is typically a computer workstation but may, in its broadest application, be any type of control logic processing component or system that is capable of obtaining image data from camera 30 or 32 and executing image processing algorithms upon that data to generate the FIRE image 60 data. Processing apparatus 38 may be local or may connect to image sensing components over a networked interface.
• FIG. 5 there is shown, in schematic form, how the FIRE image 60 is formed according to the present invention.
• Two images of tooth 20 are obtained, a green fluorescence image 50 and a blue reflectance image 52.
• the reflectance light used for reflectance image 52 and its data is from back-scattered reflectance, with specular reflectance blocked or kept as low as possible.
• the carious region 58 may be imperceptible or barely perceptible in either fluorescence image 50 or reflectance image 52, taken individually.
• Processing apparatus 38 operates upon the image data using an image processing algorithm as discussed below for both images 50 and 52 and provides FIRE image 60 as a result.
• the contrast between carious region 58 and sound tooth structure is heightened, so that a caries condition is made more visible in FIRE image 60.
• Figure 6 shows the contrast improvement of the present invention in a side-by-side comparison with a visual white-light image 54 and conventional fluorescence methods.
• the carious region 58 may look indistinct from the surrounding healthy tooth structure in white-light image 54, either as perceived directly by eye or as captured by an intraoral camera.
• the carious region 58 may show up as a very faint, hardly noticeable shadow.
• the FIRE image 60 generated by the present invention, the same carious region 58 shows up as a darker, more detectable spot.
• the FIRE image 60 offers greater diagnostic value.
• processing of the image data uses both the reflectance and fluorescence image data to generate a final image that can be used to identify carious areas of the tooth.
• Back-scattered reflectance is higher (brighter) for image pixels in the carious region, yielding a higher reflectance value R va ⁇ ue for these pixels than for surrounding pixels.
• the fluorescence meanwhile, is lower (darker) for image pixels in the carious region, yielding a lower fluorescence value F va r U e for these pixels than for surrounding pixels.
• F va r U e For a pixel in a carious region, the fluorescence is considerably weaker in intensity compared to the reflectance.
• scalar multiplier n for reflectance value Rvalue is one.
• FIG. 7 there is shown, in block diagram form, a sequence of image processing for generating an enhanced threshold FIRE image 64 according to one embodiment.
• Fluorescence image 50 and reflectance image 52 are first combined to form FIRE image 60, as described previously.
• a thresholding operation is next performed, providing threshold image 62 that defines more clearly the area of interest, carious region 58.
• threshold image 62 is combined with original FIRE image 60 to generate enhanced threshold FIRE image 64.
• the results of threshold detection can also be superimposed onto a white light image 54 ( Figure 6) in order to definitively outline the location of a carious infection.
• m and n are dependent on the spectral content of the light source and the spectral response of the image capture system. There is variability in the center wavelength and spectral bandwidth from one LED to the next, for example. Similarly, variability exits in the spectral responses of the color filters and image sensors of different image capture systems. Such variations affect the relative magnitudes of the measured reflectance and fluorescence values. Therefore, it may be necessary to determine a different m and n value for each imaging apparatus 10 as a part of an initial calibration process. A calibration procedure used during the manufacturing of imaging apparatus 10 can then optimize the m and n values to provide the best possible contrast enhancement in the FIRE image that is formed.
• a spectral measurement of the light source 12 used for reflectance imaging is obtained. Then, spectral measurement is made of the fluorescent emission that is excited from the tooth. This data provides a profile of the relative amount of light energy available over each wavelength range of interest. Then the spectral response of camera 30 (with appropriate filters) or 32 is quantified against a known reference. These data are then used, for example,, to generate a set of optimized multiplier m and n values to be used by processing apparatus 38 of the particular imaging apparatus 10 for forming FIRE image 60.
• scalar multiplication method provides improved results over conventional fluorescence imaging, however, there remains some room for improvement, particularly with respect to edge definition and overall image quality.
• One inherent problem with the scalar multiplication method is that multiplication of the weaker fluorescence signal also scales up the noise floor. This results in more noise and some loss of edge definition in the FIRE image.
• a different method hereafter called the asymmetric illuminance method.
• fluorescence and reflectance are obtained as separate captures, with more light delivered to the tooth for fluorescence imaging than for reflectance imaging.
• a significant increase in excitation light for fluorescence imaging results in a higher light level in the resulting fluorescence, with a significantly improved S/N ratio for the fluorescence image data.
• the fluorescent response can be brought to a level comparable to or slightly larger than the reflectance, allowing, straightforward subtraction to be used for obtaining the difference between the fluorescence and reflectance images used for FIRE imaging. It is emphasized that this method does not involve up-scaling of the fluorescence signal; thus there is no magnification of the noise floor.
• each imaging operation may require a separate illuminance level, making it necessary to capture separate fluorescence and reflectance images at different times. In one embodiment, these images are taken at a fraction of a second apart. Separate filters may be needed, possibly by switching rapidly into place according to the image that is being captured. Results from asymmetric illuminance imaging show improvement over the scalar multiplication method of Equation (1).
• Figure 9 shows two example FIRE images generated from the same tooth. At the left is an image 70 obtained using the scalar multiplication method. Image structure is noticeably darker, especially in the edge features.
• the downshifting imaging method operates as follows: 1. Obtain the reflectance image and fluorescence image data from the tooth, each with a suitable illumination level. 2. In combining the two image data values, subtract an offset
• the downshifting imaging method obtains each image value using:
• Equation (3) Implicit in the carrying out of Equation (3) is a clipping operation, where any negative result of the subtraction operation is set to zero.
• Equation (3) can be more clearly stated as:
• F va i U e could be obtained from the green color channel, and Rvalue from the blue color channel of the same color capture.
• F va ⁇ ue and R va ⁇ ue can be obtained from two separate captures, as in the alternative embodiments previously discussed.
• the graphs of Figure 10 show schematically what the downshifting imaging method does to the reflectance value R va ⁇ ue -
• the addition of the offset effectively causes a shift in the effective range of reflectance data values.
• the horizontal axis (abscissa) represents the input data code values.
• the vertical axis (ordinate) represents output data code values.
• the input/output mapping 74 has a slope of 1 , mapping each input to an output at the same code value.
• the graph at the right shows a negative offset 78 applied to input/output mapping 74, resulting in an unused portion 76 of the input data, over the darker region.
• Output values are attenuated over the portion of input/output mapping 74 that is used; however, the same overall relationship (having the same slope of 1) is maintained; only the overall intensity level is reduced for the reflectance data.
• the downshifting imaging method shows pronounced improvement over the multiplicative scaling method for combining fluorescence and reflectance image data.
• Figure 11 shows two example FIRE images generated from the same tooth using the same illumination level.
• Image 80 on the right provided using downshifting imaging with an offset, described with respect to this third embodiment, shows marked improvement in dynamic range and contrast and improved edge definition.
• the amount of contrast i.e., intensity difference
• the offset value used.
• portions of the three different embodiments described for combining fluorescence and reflectance data can themselves be combined to obtain a FIRE image.
• drive current to light source 12 or 16a/16b can be adjusted over various settings to obtain fluorescence and reflectance images that have predetermined ranges.
• some scalar multiplication can be used to adjust these values, combined with some amount of downshifting, using the general adjustment equation: (m * F mlue ) - (n * R valu ⁇ - offset) (6)
• the image contrast enhancement achieved in the present invention is advantaged over conventional methods that use fluorescent image data only.
• image processing has been employed to optimize the data, such as to transform fluorescence data based on spectral response of the camera or of camera filters or other suitable characteristics.
• the method of the '2356 Stookey et al. disclosure, cited above performs this type of optimization, transforming fluorescence image data based on camera response.
• these conventional approaches overlook the added advantage of additional image information that the back- scattered reflectance data obtains.
• each pixel in the fluorescence image data has a corresponding pixel in the reflectance image data.
• both fluorescence and reflectance images are captured with the imaging probe in the same position and with little or no time between image captures.
• the method of the present invention admits a number of alternate embodiments. For example, the contrast of either or both of the reflectance and fluorescence images may be improved by the use of a polarizing element.
• Polarization control can also be advantageously employed as a means to minimize specular reflection.
• Specular reflection tends to preserve the polarization state of the incident light.
• the specular reflected light is also S-polarized.
• Back-scattering tends to de-polarize or randomize the polarization of the incident light.
• incident light is S-polarized
• back-scattered light has both S- and P- polarization components. Using a polarizer and analyzer, this difference in polarization handling can be employed to help eliminate unwanted specular reflectance from the reflectance image, so that only back-scattered reflectance is obtained.
• Imaging apparatus 10 that employs a polarizer 42 in the path of illumination light.
• Polarizer 42 passes linearly polarized incident light.
• An analyzer 44 may be provided in the path of image-bearing light from tooth 20 as a means to minimize specular reflection component.
• reflectance light sensed by camera 30 or 32 is predominantly back-scattered light, that portion of the reflectance that is desirable for combination with the fluorescence image data according to the present invention.
• polarizer 42 In the case where the illumination light from light source 12 is already linearly polarized, such as from a laser, polarizer 42 is not needed; analyzer 44 would then be oriented with its polarization axis orthogonal to the polarization direction of the illumination light for rejecting specular reflection.
• FIG. 4B An alternate embodiment, shown in Figure 4B, employs a polarizing beamsplitter 18 (sometimes termed a polarization beamsplitter) as a polarizing element.
• polarizing beamsplitter 18 advantageously performs the functions of both the polarizer and the analyzer for image-bearing light, thus offering a more compact solution. Tracing the path of illumination and image-bearing light shows how polarizing beamsplitter 18 performs this function.
• Polarization beamsplitter 18 transmits P-polarization, as shown by the dotted arrow in Figure 4B, and reflects S-polarization, directing this light to tooth 20. Back-scattering by the tooth 20 structure depolarizes this light.
• Polarization beamsplitter 18 treats the back-scattered light in the same manner, transmitting the P-polarization and reflecting the S-polarization. The resulting P- polarized light can then be detected at camera 30 (with suitable filter as was described with reference to Figure 1) or color camera 32. Because specularly reflected light is S-polarized, polarization beamsplitter 18 effectively removes this specular reflective component from the light that reaches camera 30, 32.
• Polarized illumination results in further improvement in image contrast, but at the expense of light level, as can be seen from the description of Figures 4A and 4B.
• polarizer 42 that has particular advantages for use in the present application is the wire grid polarizer, such as those available from Moxtek Inc. of Orem, Utah and described in U.S. Patent No. 6,122,103 (Perkins et al.).
• the wire grid polarizer exhibits good angular and color response, with relatively good transmission over the blue spectral range.
• Either or both polarizer 42 and analyzer 44 in the configuration of Figure 4A could be wire grid polarizers.
• Wire grid polarizing beamsplitters are also available, and can be used in the configuration of Figure 4B.
• the method of the present invention takes advantage of the way the tooth tissue responds to incident light of sufficient intensity, using the combination of fluorescence and light reflectance to indicate carious areas of the tooth with improved accuracy and clarity.
• the present invention offers an improvement upon existing non-invasive fluorescence detection techniques for caries.
• images that have been obtained using fluorescence only may not clearly show caries due to low contrast.
• the method of the present invention provides images having improved contrast and is, therefore, of more potential benefit to the diagnostician for identifying caries.
• the method of the present invention also provides images that can be used to detect caries in its very early incipient stages. This added capability, made possible because of the perceptible back-scattering effects for very early carious lesions, extends the usefulness of the fluorescence technique and helps in detecting caries during its reversible stages, so that fillings or other restorative strategies might not be needed.
• light sources 12 could be used, with various different embodiments employing a camera or other type of image sensor. While a single light source 12 could be used for fluorescence excitation, it maybe beneficial to apply light from multiple incident light sources 12 for obtaining multiple images.
• light source 12 might be a more complex assembly that includes one light source 16a for providing light of appropriate energy level and wavelength for exciting fluorescent emission and another light source 16b for providing illumination at different times.
• the additional light source 16b could provide light at wavelength and energy levels best suited for back-scattered reflectance imaging.
• white light illumination or other polychromatic illumination
• a white light image or polychromatic image which, when displayed side-by-side with a FIRE image, can help to identify features that might otherwise confound caries detection, such as stains or hypocalcification.
• a white light image also provides the back- scattered reflectance data that is used with the fluorescence data for generating the FIRE image.
• a suitable filter is used to transmit a selected portion of the spectrum of reflected light and to block other portions of reflected light.
• reflectance data is obtained from one color channel of the white light image, typically not from the red channel. While blue portions of the spectrum can be most favorably used for reflectance image data, there are advantages to using the green spectral range, particularly since the spectral response of sensors or a color camera is often advantaged for the green portion of the spectrum.
• FIRE image 64 and white-light image 54 display side-by-side on a display monitor 82.
• FIRE image 64 is generally a grayscale image.
• FIRE image 64 can be tinted with a greenish coloring. This has been found helpful for the dentist or technician operating the imaging apparatus, since it suggests fluorescence content in FIRE image 64.
• an apparatus and method for caries detection at early and at later stages using combined effects of back-scattered reflectance and fluorescence are examples of the light.
• PARTS LIST imaging apparatus light source diffuser lens a light source b light source polarizing beamsplitter tooth lens filter filter camera camera beamsplitter processing apparatus display polarizer analyzer fluorescence image reflectance image white-light image carious region
• FIRE image threshold image enhanced threshold FIRE image image image input/output mapping unused portion offset image 82 display monitor 86a carious lesions 86b carious lesions

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