HK1134230B - Method for detection of caries - Google Patents

Description

Method for examining dental caries

Technical Field

The present invention relates generally to dental imaging methods and apparatus, and more particularly to an improved method for early detection of dental caries using fluorescence and light scattering.

Background

Despite improvements in examination, treatment, and prevention techniques, caries symptoms are prevalent in a wide range, affecting people of all age groups. If not treated properly and promptly, dental caries can lead to permanent tooth damage and even to loss of teeth.

Conventional caries detection methods include visual inspection and tactile probing with sharp dental probing equipment, often aided by radiographic (X-ray) imaging. Inspection using the above methods can always be somewhat subjective, with accuracy varying with many factors including operator experience, infected site, extent of infection, viewing conditions, accuracy of X-ray equipment and treatment, and the like. There are also hazards associated with conventional examination techniques, including the risk of damaging delicate teeth, transmission of infection by tactile methods, and exposure to X-ray radiation. By the time caries becomes evident under visual and tactile examination, the condition is usually in the stage of onset, requiring tooth replacement, and if not treated in a timely manner, can result in tooth loss.

In order to meet the need for improved caries detection methods, considerable attention has been directed to improved imaging techniques that do not employ X-rays. One method that has been commercially available uses fluorescence caused by the illumination of the tooth with high intensity blue light. The technique known as quantitative obscuration fluorescence analysis (QLF) operates on the principle that sound, healthy tooth enamel produces fluorescence at intensities higher than that of demineralized enamel resulting from caries lesions under excitation at certain wavelengths. The strong correlation between mineral loss and loss of blue-excited fluorescence is then used to identify and evaluate carious areas of the tooth. A different relationship has been found for red light excitation, with the spectral regions absorbed and the spectral regions emitted by bacteria and bacterial by-products in carious areas being more distinct than in healthy areas.

The proposed caries light examination solution includes the following:

U.S. patent No.4,515,476 (to Ingmar) discloses the use of a laser for providing excitation energy that produces fluorescence at some other wavelength for the localization of carious regions.

U.S. patent No.6,231,338 (to de Josselin de Jong et al) discloses an imaging device for identifying caries using fluorescence detection.

U.S. patent application publication 2004/0240716(de Josselin de Jong et al) describes a method for improved image analysis of images obtained from fluorescing tissue.

U.S. patent No.4,479,499 (to Alfano) discloses a method for using transverse illumination to detect caries based on the translucent nature of tooth structure.

Among the commercial products used for dental imaging using fluorescence behavior are the QLF medical system from inspector Research Systems BV of Amsterdam, the Netherlands. Using a different approach, a diagnostic laser caries detection aid from KaVo dental Inc. of lake Zurich, Illinois detects caries conditions by monitoring the fluorescence intensity of bacterial by-products under red illumination.

U.S. patent application publication 2004/0202356(Stookey et al) describes mathematical manipulation of the spectral changes of fluorescence to examine caries at various stages with improved accuracy. The disclosure of' 2356Stookey et al is aware of the difficulty of early detection using fluorescence spectroscopy, and thus illustrates a method for enhancing the resulting spectral values in which spectral data appropriate to the spectral response of the camera from which the fluorescence image was obtained is transformed.

While the disclosed methods and apparatus show promise for providing non-invasive, non-ionizing imaging methods for caries detection, there is still room for improvement. One of the recognized deficiencies of the prior art using fluorescence imaging relates to image contrast. Fluorescence generation techniques such as QLF may provide images that are difficult to assess due to relatively poor contrast between healthy and infected areas. As explained in the disclosure of' 2356Stookey et al, the spectral and intensity variations of incipient caries can be subtle, making it difficult to distinguish asperities from incipient caries on non-diseased tooth surfaces.

In general, it is generally accepted that the image contrast obtained by each fluorescence technique corresponds to the severity of the symptoms. Proper identification of caries using the above techniques often requires that the symptoms be at a relative stage of onset, far from incipient or early caries, because the difference in fluorescence between carious and sound tooth structure is small for caries at an early stage. In this case, the accuracy of examination using fluorescence techniques may not show a significant improvement over conventional methods. Because of this deficiency, the use of fluorescence effects obviously has some practical limitations that prevent proper diagnosis of incipient caries. Thus, the symptoms of caries will continue undetected until they are more severe, such as when a filling is required.

Very early stage caries detection is of particular interest for preventive dental medicine. As previously explained, conventional techniques often fail to detect caries at reversible stages of the condition. According to common experience, incipient caries is a lesion that has not penetrated substantially into the enamel of the teeth. In cases where such carious lesions are identified before they threaten the dentinal region of the tooth, remineralization is often achieved, reversing the early lesions, and avoiding the need for filling. But the difficulty of treatment is greatly increased relative to onset caries, more frequently requiring some type of filling or other type of intervention.

In order to previously arrest caries by taking advantage of the opportunity of non-invasive dental techniques, it is necessary to examine caries at the initial stage of onset. In many cases, as is known from the disclosure of' 2356Stookey et al, this level of examination has been found to be difficult to achieve using existing fluorescence imaging techniques such as QLF. Thus, early caries may not continue to be detected and thus may lose time to be reversed using low-cost preventative measures before a positive detection is obtained.

Thus, it can be appreciated that there is a need for non-invasive, non-ionizing imaging methods for caries detection that provide improved accuracy for caries detection, particularly at relatively early stages thereof.

Disclosure of Invention

The invention provides a method for forming an enhanced image of a tooth, comprising the steps of:

a) fluorescence image data of the tooth is obtained by:

(i) directing incident light toward the tooth;

(ii) detecting fluorescence emission from the tooth;

(iii) storing a fluorescence image data value for each pixel location in the fluorescence image;

b) obtaining reflectance image data of the tooth by:

(i) directing incident light toward the tooth;

(ii) detecting backscattered reflectance light from the tooth;

(iii) storing a reflected image data value for each pixel location in the reflected image;

c) combining each pixel in the fluorescence image data with its corresponding pixel in the reflectance image data by:

(i) subtracting a compensation value from the reflectance image data value to generate a compensated reflectance image data value;

(ii) calculating an enhanced image data value based on a difference between the fluorescence image data value and the compensated reflectance image data value,

thereby forming an enhanced image from the generated array of pixels of enhanced image data values.

The invention is characterized in that both fluorescence image data and reflectance image data are used for dental imaging.

The present invention advantageously provides enhancements over existing fluorescence imaging techniques for the examination of caries at its infancy stage.

The above and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the accompanying drawings wherein there is illustrated an exemplary embodiment of the invention.

Drawings

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be apparent from the following description when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic block diagram of an imaging apparatus for caries detection in accordance with one embodiment;

FIG. 2 is a schematic block diagram of an imaging apparatus for caries detection of an alternate embodiment;

FIG. 3 is a schematic block diagram of an imaging apparatus for caries detection of an alternate embodiment;

FIG. 4A is a schematic block diagram of an imaging apparatus for caries detection using polarized light according to an alternative embodiment;

FIG. 4B is a schematic block diagram of an imaging apparatus for caries detection of an alternate embodiment using a polarizing beam splitter to provide polarized light and to reduce specular reflection;

FIG. 5 is a schematic diagram of a process for combining dental image data to generate a fluorescence image with reflectance enhancement according to the present invention;

FIG. 6 is a composite graph showing the contrast improvement of the present invention compared side-by-side with a conventional visual fluorescence method;

FIG. 7 is a block diagram showing image processing steps for generating an enhanced threshold image in one embodiment;

FIG. 8 is a schematic block diagram of an imaging apparatus for caries detection of an alternate embodiment using multiple light sources;

FIG. 9 is a plan view of a comparison of the scaled multiplication and asymmetric illumination methods of the present invention;

FIG. 10 shows a graph of input/output pixel transforms for code values without any image modification and pixel transforms with compensation values added as used in one embodiment;

FIG. 11 is a plan view of the comparative results of the scale multiplication and downshifting method of the present invention; and

FIG. 12 is a plan view showing a display example in which one embodiment displays white light and an enhanced image to a tooth.

Detailed Description

The present description is directed in particular to elements of the apparatus or parts thereof in more direct cooperation with the apparatus. It is to be understood that elements not specifically illustrated may take various forms well known to those skilled in the art.

As explained in the background section above, it is known to use fluorescence for medical examination of dental caries using either of two characteristic responses: first, excitation by a blue light source causes healthy dental tissue to fluoresce in the green spectrum. The second is that excitation by a red light source causes bacterial by-products, such as those that are cariogenic, to fluoresce in the red spectrum.

To understand how light is used in the present invention, it is important to give a more precise definition of the terms "reflectance" and "backscatter" as used in biomedical applications in general, and in particular as used in the methods and apparatus of the present invention. In broadest optical terms, reflection generally refers to the sum of both specular and diffuse forms of reflection. (specular reflection is that portion of the excitation light that is reflected by the tooth surface at the same angle of incidence.) in many biomedical applications, as is the case in the dental applications of the present invention, the reflected specular portion is not of interest, but is generally the portion that is compromised by the image or measurement taken of the sample. The only part of the reflection that is of interest for this application is from the back-scattered light. Specular reflections must be blocked or otherwise removed from the imaging path. In light of this difference in opinion, the term "back-scattered reflectance" is used in this application to refer to reflectance of a portion of interest therein. "back-scattered reflectance" is defined as that portion of the excitation light that is elastically back-scattered over a wide range of angles by the illuminated tooth structure. "reflectance image" as that term is used in the present invention refers to image data obtained from back-scattered reflectance only, since specular reflectance is blocked or kept to a minimum. In scientific terms, the form of back-scattering can also be referred to as back-reflection, or simply back-scattering. The back-scattered reflection is in the same wavelength range as the excitation light.

Light scattering properties have been shown to differ between healthy and carious areas. In particular, the light reflectance from the illuminated area may be at a different measured level for normal, non-carious areas. This transformation of the reflection, taken alone, may not be sufficient to be unequivocally a diagnostic value to be considered in itself, since this effect is detectable, but subtle. For example, for caries at a more advanced stage, its back-scattered reflectance may be a less effective indicator than at an earlier stage.

In conventional fluorescence measurements, such as those obtained using QLF techniques, reflectance itself is an effect that is avoided rather than exploited. A filter is typically used to block all of the excitation light from reaching the detection device. For this reason, small but perceptible changes in the back-scattered form of the excitation light are of little concern for caries diagnosis.

The inventors have discovered that this back-scattered reflectance variation can be used in conjunction with fluorescence effects to more specifically and accurately identify caries sites. Furthermore, the inventors have observed that the change in light scattering activity, while detectable wherever caries symptoms are present, is more pronounced in the incipient carious region. This back-scattered form of reflectance change is well-defined at caries sites at early stages, even if the fluorescence effect is highly ambiguous.

For incipient caries, the present invention utilizes the observed backscattering behavior and combines this effect with the fluorescence effect previously described in the background section to provide improved dental imaging capabilities for caries detection. The technical invention, hereinafter referred to as fluorescence imaging with reflectance enhancement (FIRE), not only helps to improve the contrast of the image relative to earlier methods, but also enables the examination of incipient caries at a stage where preventive measures can be taken to remineralize, repair lesions formed by caries infection, at an earlier stage before more sophisticated rehabilitation measures are required. Advantageously, FIRE examination can lock in on caries infection at an earlier stage than has been demonstrated using prior fluorescence methods that measure fluorescence alone.

Image forming apparatus with a plurality of image forming units

Referring to FIG. 1, one embodiment illustrates an imaging apparatus 10 for caries detection using the FIRE method. Light source 12 directs incident light in a suitable wavelength range, such as the blue wavelength range, toward tooth 20 through a beam conditioning element, such as optical lens 14. The teeth 20 may be illuminated at the proximal surface (as shown) or at the occlusal surface (not shown). Two of these portions are then detected by the monochrome camera 30 via the lens 22: a back-scattered light component having the same wavelength as the incident light and having a measurable reflection; and fluorescence excited by incident light. For FIRE imaging, specular reflection is a negative factor and is therefore undesirable. To reduce the detection of specular reflections, the camera 30 is positioned at a suitable angle relative to the light source 12. This allows imaging of the back-scattered light without confounding effects on the specular component.

In the embodiment of fig. 1, monochrome camera 30 has color filters 26 and 28. One of the color filters 26 and 28 is used during reflectance imaging and the other is used during fluorescence imaging. Processing device 38 obtains and processes the reflectance and fluorescence image data and forms FIRE image 60. FIRE image 60 is an enhanced diagnostic image that may be printed or otherwise visualized on display 40. FIRE image 60 data can also be transmitted to a memory device or to another location for display.

Referring to fig. 2, an alternative embodiment employing a color camera 32 is shown. In this configuration, auxiliary filters would typically not be needed because color camera 32 would be able to obtain both reflectance and fluorescence images from the color separations (also referred to as color planes) of the full color image of tooth 20.

The light source 12 typically has a center wavelength in one of the embodiments in the blue wavelength range, such as about 405 nanometers. In practice, the light source 12 may emit light in a wavelength range between the upper ultraviolet range to deep blue, approximately 300 nanometers and 500 nanometers. Light source 12 may be a laser or may be fabricated using one or more Light Emitting Diodes (LEDs). Alternatively, a broadband light source such as a xenon lamp having a primary color filter for passing light of a desired wavelength may be employed. Optical elements such as lens 14 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 in dashed lines in fig. 2, may be used before or after the lens 14 to smooth out the hot spots of the LED beam. The optical path of the illumination light may comprise a light guide or light distribution structure (not shown), such as an optical fiber or a liquid light guide, for example. The light intensity level is typically a small milliwatt level, but may be larger or smaller depending on the respective light conditioning detection components used.

Referring to fig. 3, the illumination configuration may alternatively orient the light at a normal angle of incidence and turn through the beam splitter 34. Camera 32 would then be positioned to obtain the image light transmitted through beam splitter 34. Other illumination options include multiple light sources directed at the tooth at incident angles from one or more sides. Alternatively, the illumination may use annular LED light sources or LED light source configurations distributed around its center, such as in a circular array, to provide uniform light at multiple angles. Illumination may also be provided via an optical fiber or an array of optical fibers.

The imaging optics represented as lens 22 in fig. 1-3 may include any suitable configuration of optical components, with a range of possible configurations being single lens components to multi-element lenses. Clear imaging of tooth surfaces that are not flat but may have areas with both smooth contours and high ridges requires sufficient depth of focus for each imaging optic. Preferably, the imaging optics provide an image size that substantially fills the sensor elements of the camera for optimal resolution. Telecentric optics facilitates lens 22 to provide image-generating light that is not highly dependent on ray angle.

Image capture may be performed by a monochrome camera 30 (fig. 1) or a color camera 32 (fig. 2). Typically, the cameras 30 or 32 employ CMOS or CCD image sensors. Monochromatic versions will typically employ retractable spectral filters 26, 28 appropriate to the wavelength of interest. For a light source 12 having a blue wavelength, the spectral filter 26 used to capture the reflected image data will transmit primarily blue light. The spectral filter 28 used to capture the fluorescence image data transmits light at a different wavelength, such as primarily green light. Preferably, spectral filters 26 and 28 are automatically switched into position to allow both reflectance and fluorescence images to be captured in closer order. Both images are taken from the same location to allow accurate positioning of the image data.

Spectral filter 28 will be optimized for the pass band in which fluorescence data is acquired over the entire range of suitable wavelengths. The fluorescence effect obtained from tooth 20 has a relatively broad spectral distribution in the visible range, where the emitted light is outside the wavelength range of the light used for excitation. The fluorescence emission is typically between about 450 nm and 650 nm, while the peak is typically in the green region, typically around 500 nm to 600 nm. Thus, to obtain such a fluorescence image at its highest energy level, a green filter is generally preferred for spectral filter 28. Other ranges of the visible spectrum may be used in other embodiments.

Likewise, the spectral filter 26 will be optimized for the pass band in which the reflection data is collected over a range of wavelengths covering at least a significant portion of the spectral energy of the light source 12 used. For the reasons discussed above, a blue filter is typically employed for spectral filter 26 in order to obtain a reflectance image at its highest energy level.

The camera control is suitably adjusted for obtaining each kind of image. For example, when taking a fluorescence image, corresponding exposure adjustments for gain, shutter speed, and aperture are required, since the image may not be very strong. With the color camera 32 (fig. 2), color filtering is performed on the camera image sensor by a color filter array. The reflected image is taken in the blue color plane while the fluorescence image is taken in the green color plane. That is, a single exposure takes both a reflectance image and a fluorescence image in the form of back scattering.

Processing device 38 is typically a computer workstation, but in its broadest application may be any type of control logic processing portion or system capable of obtaining image data from cameras 30 or 32 and performing image processing algorithms on the data to generate FIRE image 60 data. The processing device 38 may be local or may be connected to the image detection section through a network interface.

Referring to fig. 5, there is shown in schematic form how FIRE image 60 is formed in accordance with the present invention. Two images of tooth 20 are obtained, a green fluorescence image 50 and a blue reflectance image 52. As previously explained, it must be emphasized that the reflected light used to reflect image 52 and its data comes from back-scattered reflections, whereas specular reflections are blocked or kept as low as possible. In the example of FIG. 5, carious regions 58, represented by dashed outlines, are present in each of images 50, 52, and 60, which cause a slight decrease in fluorescence and a slight increase in reflectance. The carious region 58 may be undetectable or barely perceptible in the fluorescence image 50 or reflectance image 52 taken alone. Processing device 38 operates on the image data using an image processing algorithm, as described below, for both images 50 and 52, and provides FIRE image 60 as a result. Contrast between carious region 58 and sound tooth structure is enhanced so that the carious condition is more visible in FIRE image 60.

FIG. 6 shows the contrast improvement of the present invention compared edge-to-edge with visible white light images and conventional fluorescence methods. For caries at a very early stage, carious region 58, whether directly perceived by the eye or captured by an intra-oral camera, will appear indistinguishable from the surrounding healthy tooth structure in white-light image 54. The carious region 58 appears as a very faint, inconspicuous shadow in the green fluorescence image 52 taken by the prior art fluorescence method. The same carious region 58 appears as a darker, more easily detected spot in the FIRE image 60 generated by the present invention. Clearly, FIRE image 60 with its contrast enhancement provides a greater diagnostic value.

Image processing

As previously described with reference to fig. 5 and 6, the processing of the image data employs both the reflectance and fluorescence image data to generate a final image that can be used to identify carious regions of the tooth. There are several alternative processing methods for combining reflectance and fluorescence image data to form a FIRE image 60 for diagnosis. Co-pending U.S. patent application serial No. 11/262,869, previously cited, illustrates a scale-multiplication method for combining fluorescence and reflectance data. In the scale-multiplication embodiment, the image processing performs the following operations for each pixel:

(m＊F_value)-(n＊R_value) (1)

where m and n are suitable multipliers (positive coefficients), F_valueAnd R_valueRespectively, code values derived from the fluorescence image data and the reflectance image data.

The back-scattered reflectance is higher (brighter) for each image pixel in the carious region, which produces a higher reflectance value R than the surrounding pixels_value. At the same time, the fluorescence is lower (darker) in the carious region for each image pixel, which yields a lower fluorescence value F than the surrounding pixels_value. For pixels in carious regions, fluorescence is considerably less intense than reflectance. After multiplying the fluorescence and reflectance by appropriate scaling multipliers m and n, respectively, where m > n, the scaled fluorescence values of all pixels exceed or equal to the corresponding scaled reflectance values:

(m＊F_value) Greater than or (n < x > R_value) (2)

The scaled fluorescence value for each pixel is then subtracted from the scaled back-scattered reflectance value to produce a processed image in which the contrast between the intensity values of the carious region pixels and the healthy region pixels is enhanced, resulting in a contrast enhancement that can be readily displayed and appreciated. In one embodiment, the reflection value R_valueIs 1.

After the initial combination of fluorescence and reflectance values as described above with reference to the example of expression (1), additional beneficial image processing may also be performed. Thresholding operations performed using image processing techniques familiar to those skilled in the imaging arts, or some other suitable adjustment to the combined image data used for FIRE image 60, can be used to further enhance the contrast between carious regions and sound tooth structure. Referring to FIG. 7, image processing steps for generating an enhanced threshold FIRE image 64 in one embodiment are shown in block diagram form. The fluorescence image 50 and reflectance image 52 are first combined to form a FIRE image 60 as previously described. Following a thresholding operation, the threshold image 62 is provided to more clearly define the region of interest, i.e., carious region 58. Next, threshold image 62 is combined with original FIRE image 60 to generate enhanced threshold FIRE image 64. Similarly, the results of threshold detection can also be superimposed on the white light image 54 (FIG. 6) to give a well-defined outline of the caries site of infection.

The selection of the respective coefficients m and n depends on the spectral composition of the light source and the spectral response of the image capture system. For example, there is adjustability in the center wavelength and spectral bandwidth of one LED to the next. Also, there is adjustability in the spectral response of the color filters and image sensors of different image capture systems. This variation affects the relative magnitude of the measured reflectance and fluorescence values. Thus, different values of m and n may need to be determined for each imaging device 10 as part of an initial calibration process. The calibration process employed during the manufacture of the imaging apparatus 10 may then optimize the m and n values to provide as good a contrast enhancement as possible in the formed FIRE image.

In one of the calibration steps, a spectral measurement of the light source 12 used for reflectance imaging is obtained. Subsequently, the fluorescence emitted by the tooth is measured spectroscopically. This data provides a profile of the relative amount of light energy available in each wavelength range of interest. The spectral response of the camera 30 (with corresponding filters) or 32 is then quantified relative to a known reference. The data described above is then used, for example, to generate an optimized set of multiplier m and n values to be used by the processing device of a particular imaging device 10 to form FIRE image 60.

While the scale-multiplication method provides improved results over conventional fluorescence imaging, there is still some room for improvement, particularly with respect to edge definition and overall image quality. One of the inherent problems with the proportional multiplication method is that the noise floor is also improved by the multiplication of the weaker fluorescence signal. This causes more noise, with some loss defined by edges in the FIRE image.

In an alternative embodiment of the proportional-multiplication method, a different method, referred to below as an asymmetrical illumination method, may be employed. In this method, fluorescence and reflectance are obtained as separate acquisitions, and fluorescence imaging allows more light to reach the tooth than reflectance imaging. The significant increase in excitation light used for fluorescence imaging results in higher light levels of the formed fluorescence with significantly improved S/N ratios for fluorescence image data. By increasing the fluorescence to a sufficiently high level, the fluorescence response can bring a level comparable to or slightly greater than the reflectance, allowing direct subtraction to be used to obtain the difference between the fluorescence and reflectance images used for FIRE imaging. It is emphasized that this method does not involve an increase in the fluorescence signal and thus there is no amplification of the noise floor.

In practice, there are limitations on the amount of light that can be provided by a source, particularly a source of a relatively small size, such as would be used in an imaging device. By also using reduced illumination during reflectance acquisition, comparable fluorescence and reflectance levels can be obtained without requiring a substantial increase in illumination for fluorescence acquisition.

Improved illumination can be obtained by increasing the drive current of a light source such as an LED for exciting fluorescence. In certain embodiments (fig. 1-4B), the same light source 12 is used for both fluorescence imaging and reflectance imaging. In other embodiments, a separate light source 16a is used to excite the fluorescence (FIG. 8). Whether the same light source 12 is used for both reflectance and fluorescence imaging, or separate light sources 16a and 16b are used, each imaging action may require separate illumination levels, making it necessary to take separate fluorescence and reflectance images at different times. In one embodiment, the images are captured at one-half second intervals. Separate filters may be required, perhaps by switching into place quickly depending on the image being taken.

The results of asymmetric illumination imaging show an improvement over the scale multiplication method of equation (1). FIG. 9 shows two example FIRE images generated from the same tooth. To the left is an image 70 obtained by a scale-multiplication method, the image structure, particularly the edge features, being significantly darker. Moreover, caries lesions 86a and 86b are too dark to show a distinct stage of caries onset in both lesions. The right image was taken using the asymmetric illumination imaging method described in relation to this second embodiment, with significant improvements in dynamic range and contrast, and with improved edge definition.

Another alternative embodiment for combining fluorescence and reflectance images is to use a method that differs from the scale-multiplication and asymmetric illumination imaging methods just described. This "downshifting" or "compensation" method does not risk image data distortion such as may be caused by scaling, nor does it require the drive current to reach a higher level. Downshifting imaging methods may be characterized by maintaining image values at a certain brightness range and maintaining the input/output ratio of those image values during image processing. In effect, the method preserves the input/output relationships and the complete structure of the original data.

The downshifting imaging method works as follows:

1. reflectance image data and fluorescence image data of the tooth are obtained at appropriate illumination levels, respectively.

2. In the combination of the two image data values, a compensation is subtracted from the reflection image data (or in other words a negative compensation is added thereto), wherein the compensation approximates the intensity difference between the two image data distributions.

In summary, the downshifting imaging method obtains each image value using the following equation:

(F_value)-(R_value-offset)(3)

for example

(F_value)-(R_value-110)(4)

The implicit function of equation (3) is performed as a clipping (clipping) operation, where any negative result of the subtraction operation is set to zero. Thus, equation (3) can be more specifically given by:

Clip{(F_value)-Clip[(R_value-offset)]}(5)

here, F_valueAvailable from the green channel, R_valueAvailable from the blue channel of the same color camera. Or, F_valueAnd R_valueAn alternative embodiment as described above results from two separate shots.

FIG. 10 is a graph exemplarily illustrating downshifting imaging method versus reflectance value R_valueThe function is played. This increase in compensation effectively causes an offset within the effective range of reflected data values. The horizontal axis (abscissa) represents the input data code value. The vertical axis (ordinate) represents the output data code value. As shown in the left graph, the input/output transform 74 has a slope of 1, transforming each input to an output with the same code value, without any image modification. The right graph applies negative compensation 78 to the input/output transitions 74 in the darker regions, resulting in unused portions 76 of the input data. Each output value is attenuated within the portion of the input/output transform 74 used therein; but maintain the same overall relationship (with the same slope 1); only the overall intensity level is reduced for the reflected data.

The downshifting imaging method shows a clear improvement over the scale multiplication method used to combine fluorescence image data and reflectance image data. FIG. 11 shows two example FIRE images generated from the same tooth using the same illumination level. On the left is an image 70 obtained using the scale multiplication method. The right image 80 provided using the downshifting imaging method with compensation described for the third embodiment shows a significant improvement in dynamic range and contrast and improved edge definition. Using the downshifting imaging method, the measure of contrast (i.e., the difference in intensity) between the carious lesions 86a and 86b and the surrounding sound structures can be adjusted by adjusting the compensation values used.

It is noted that the three different embodiments described for combining fluorescence data and reflectance data can themselves be combined in part to obtain a FIRE image. For example, the drive current to the light source 12 or 16a/16b may be adjusted for various settings to obtain fluorescence and reflectance images with a specified range. Then, the above values can be adjusted using some scaling multiplication method in combination with some amount of downshifting using the global adjustment equation:

(m＊F_value)-(n＊R_value-offset)(6)

it is emphasized that the image contrast enhancement achieved in the present invention is superior to conventional methods using only fluorescence image data because it employs both reflectance and fluorescence data. Conventionally, in the case where only fluorescence data is obtained, image processing is employed to optimize the data, such as fluorescence data conversion according to appropriate characteristics such as spectral response of a camera or a camera filter. For example, the method disclosed in' 2356Stookey et al, cited above, performs such optimization with a fluorescence image data transformation based on camera response. The conventional approach described above overestimates the additional image information obtained from the back-scattered reflectance data to its additional advantages.

It is instructive to observe that spatial correlation of each pixel is required for combining fluorescence and reflectance values, whether using the proportional-multiplication method, the asymmetric illumination imaging method, or the downshifting method just described. That is, each pixel of the fluorescence image data has a corresponding pixel in the reflectance image data relative to the tooth surface. Thus, it is preferable to take both fluorescence and reflectance images with the same imaging head in the same position and with little to no time interval between the two shots.

Alternative embodiments

The method of the invention allows for several alternative embodiments. For example, the contrast of either or both of the reflectance and fluorescence images may be improved by using polarizing elements. It has been noted that enamel with a highly structured composition is sensitive to the polarization of incident light. For example, the "journal of biomedical optics" October 2002; 7 (4); in "caries lesion and lesion symptom imaging using polarization-sensitive optical coherence tomography" (Fried et al) at pages 618-627, polarized light has been used to improve the sensitivity of dental imaging techniques.

Polarization control may also be advantageously employed as a means of reducing specular reflection. Specular reflection tends to preserve the polarization state of incident light. For example, when the incident light is S-polarized, the light reflected by the mirror surface is also S-polarized. While backscattering tends to depolarize the incident light or randomize its polarization. In the case of S-polarized incident light, the backscattered form of the light has S and P polarization components. Using a polarizer and analyzer, this difference in polarization processing can be exploited to help eliminate unwanted specular reflections from the reflected image, so that only back-scattered forms of reflection are obtained.

Referring to FIG. 4A, an embodiment of the imaging device 10 is shown that employs a polarizer 42 in the path of the illumination light. Polarizer 42 passes linearly polarized incident light. An analyzer 44 may be provided in the optical path of the image-generating light from tooth 20 as a means to reduce the specular component. With this polarizer 42/analyzer 44 combination as the polarizing element, the reflected light detected by the camera 30 or 32 is primarily back-scattered light, i.e., reflects that portion of the light that is desired to be combined with the fluorescence image data of the present invention. Where the illumination light from light source 12, such as a laser, is readily linearly polarized, polarizer 42 is not required; the analyzer 44 would then be oriented with its polarization axis orthogonal to the polarization direction of the illumination light for eliminating specular reflection.

The alternative embodiment shown in fig. 4B employs a polarizing beam splitter 18 (sometimes referred to as a polarizing beam splitter) as the polarizing element. In this configuration, polarizing beam splitter 18 advantageously performs its function for both the polarizer and the analyzer for the image-generating light, thereby providing a more compact solution. The tracing of the illumination and image-generating light paths shows how polarizing beamsplitter 18 performs this function. As shown by the dashed arrows in FIG. 4B, polarizing beam splitter 18 transmits in the P polarization and reflects in the S polarization, directing the light to tooth 20. Back scattering of the tooth 20 structure depolarizes this light. The polarization beam splitter 18 processes the backscattered light in the same manner, transmitting in P polarization and reflecting in S polarization. The generated P-polarized light can then be detected by camera 30 (with appropriate filters as explained with reference to fig. 1) or color camera 32. Because the specularly reflected light is S-polarized, polarizing beam splitter 18 effectively eliminates the specularly reflected component from the light reaching cameras 30, 32.

Polarized illumination provides further improvement in image contrast, but at the expense of light level, as can be seen in the illustrations of fig. 4A and 4B. Therefore, when polarized light is used in this manner, it may be desirable to use a higher intensity light source 12. It is also beneficial to use a polarizing element that has a higher transmission in the wavelength range of interest.

One polarizer 42 having particular advantages for the present application is a wire grid polarizer, such as that available from U.S. patent u.pat. No.: 6,122,103(Perkins et al). Wire grid polarizers exhibit good angular and color response and relatively good transmission in the blue spectral range. One or both of the polarizer 42 and analyzer 44 in the configuration of fig. 4A may be a wire grid polarizer. Wire grid polarizing beamsplitters are also available and can be used in the configuration of FIG. 4B.

The method of the present invention utilizes the manner in which tooth tissue responds to incident light of sufficient intensity to visualize carious regions of a tooth with increased accuracy and clarity using a combination of fluorescence and light reflectance. In this manner, the present invention provides an improvement over existing non-invasive caries fluoroscopy techniques. As explained in the background section given above, images obtained using fluorescence alone may not show caries due to low contrast. The method of the present invention provides images with improved contrast and therefore has more potential benefits for diagnosticians used to identify caries.

In addition, unlike previous methods that employ fluorescence alone, the present method also provides images that can be used to detect caries at a very early, early stage. The increased ability to be possible due to the perceived back-scattering effects of very early carious lesions expands the usefulness of fluorescence techniques and facilitates caries detection in its reversible stages, thus obviating the need for restorative strategies such as dental filling.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as described above, and as defined in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention.

For example, various types of light sources 12 may be used in different embodiments employing cameras or other types of image sensors.

Although a single light source 12 may be used for fluorescence excitation, it may be beneficial to apply the light of multiple incident light sources 12 to obtain multiple images. Referring to the alternative embodiment of fig. 8, light source 12 may be a more complex assembly including one light source 16a for providing light of a suitable energy level and wavelength for exciting fluorescent emission and another light source 16b for providing illumination at different times. Additional light sources 16b may provide light at wavelengths and energy levels most suitable for back-scattered reflectance imaging. Alternatively, multi-color illumination, such as white light illumination, may be provided for capturing a white light image or multi-color image that, when displayed side-by-side in a FIRE image, may help identify features that may confound caries detection, such as stains or sub-calcifications.

In one embodiment, the white light image also provides back-scattered reflectance data that is used to generate a FIRE image using fluorescence data. To obtain a reflected image from the backlight image, a selected portion of the reflected light spectrum is transmitted using a suitable filter, blocking other portions of the reflected light. Alternatively, for the color sensor or camera 32, the reflectance data is derived from reflectance data from a color channel of the white light image (typically not the red channel). Although the blue portion of the spectrum may be most suitable for reflecting image data, the use of the green spectral range is advantageous, especially since the spectral response of the sensor or color camera tends to be advantageous for the green portion of the spectrum.

In one embodiment, as shown in FIG. 12, FIRE image 64 and white light image 54 are displayed side-by-side on display device monitor 82. FIRE image 64 is typically a grayscale image. Alternatively, FIRE image 64 can be tilted in green coloration. This has been found to be helpful to the dentist or technician operating the imaging apparatus because it suggests fluorescent content in the FIRE image 64.

Thus, provided are apparatus and methods for caries detection at early and late stages using the combined effect of back-scattered reflectance and fluorescence.

Parts list

10 image forming apparatus

12 light source

13 diffuser

14 lens

16a light source

16b light source

18 polarization beam splitter

20 teeth

22 lens

26 optical filter

28 optical filter

30 vidicon

32 vidicon

34 beam splitter

38 treatment device

40 display

42 polarizer

44 analyser

50 fluorescence image

52 reflection image

54 white light image

58 carious region

60FIRE image

62 threshold image

64 enhanced threshold FIRE image

70 image

72 image

74 input/output conversion

76 useless part

78 compensation

80 image

82 display device monitor

86a dental caries focus

86b dental caries focus

Claims (10)

1. A method for forming an enhanced image of a tooth comprising the steps of:

a) fluorescence image data of the tooth is obtained by:

(i) directing incident light toward the tooth;

(ii) detecting fluorescence emission from the tooth;

(iii) storing a fluorescence image data value for each pixel location in the fluorescence image;

b) obtaining reflectance image data of the tooth by:

(i) directing incident light toward the tooth;

(ii) detecting backscattered reflectance light from the tooth;

(iii) storing a reflected image data value for each pixel location in the reflected image;

c) combining each pixel in the fluorescence image data with its corresponding pixel in the reflectance image data by:

(i) subtracting a compensation value from the reflectance image data value to generate a compensated reflectance image data value;

(ii) calculating an enhanced image data value based on a difference between the fluorescence image data value and the compensated reflectance image data value,

thereby forming an enhanced image from the generated array of pixels of enhanced image data values.

2. The method of claim 1, further comprising the step of displaying an enhanced image of the tooth.

3. The method of claim 1, wherein the incident light comprises a wavelength between about 300 nanometers and 500 nanometers.

4. The method of claim 1, wherein the step of obtaining fluorescence image data comprises the step of using a green filter.

5. The method of claim 1, wherein the step of obtaining reflected image data comprises the step of using a blue filter.

6. The method of claim 1, wherein the step of obtaining reflected image data comprises using a camera.

7. The method of claim 6, wherein the camera is a color camera.

8. The method of claim 1, wherein the fluorescence image data and reflectance image data are obtained from different color planes of a single color, full color image capture.

9. The method of claim 1, wherein the fluorescence image data and reflectance image data are obtained from separate image captures.

10. A method for forming an enhanced image of dental tissue, comprising the steps of:

a) fluorescence image data of the tooth is obtained by:

(i) directing a first incident light toward the tooth;

(ii) detecting fluorescence emission from the tooth;

(iii) storing a fluorescence image data value for each pixel location in the fluorescence image;

b) obtaining reflectance image data of the tooth by:

(i) directing a second incident light toward the tooth;

(ii) detecting reflected light from the tooth;

(iii) storing a reflected image data value for each pixel location in the reflected image;

wherein the illuminance of the first incident light exceeds the illuminance of the second incident light; and

c) each pixel in the fluorescence image data is combined with its corresponding pixel in the reflectance image data to calculate an enhanced image data value from the difference between the fluorescence image data value and the reflectance image data value, thereby forming an enhanced image from the generated array of pixels of enhanced image data values.