DENTAL SURFACE IMAGING USING POLARIZED FRINGE PROJECTION
An intra-oral imaging apparatus having a fringe pattern generator energizable to emit a fringe pattern illumination having a predetermined spatial frequency, with light in the approximate 350-500 nm range. A polarizer in the path of the fringe pattern illumination has a first polarization transmission axis. A projection lens is disposed to direct the polarized fringe pattern illumination as incident illumination toward a tooth surface. An imaging lens is disposed to direct light reflected and scattered at the tooth surface along a detection path. An analyzer is disposed along the detection path, having a second polarization transmission axis. A detector disposed along the detection path obtains image data from the light provided through the analyzer. A control logic processor is responsive to programmed instructions and actuable to adjust the intensity over one or more portions of the fringe pattern illumination according to the image data obtained from the detector.
The invention relates generally to the field of diagnostic imaging using structured light and more particularly relates to a method for three-dimensional imaging of the surface of teeth and other structures using fringe projection.
BACKGROUND OF THE INVENTIONFringe projection imaging uses patterned or structured light to obtain surface contour information for structures of various types. In fringe projection imaging, a pattern of lines of an interference fringe or grating is projected toward the surface of an object from a given direction. The projected pattern from the surface is then viewed from another direction as a contour image, taking advantage of triangulation in order to analyze surface information based on the appearance of contour lines. Phase shifting, in which the projected pattern is incrementally spatially shifted for obtaining additional measurements at the new locations, is typically applied as part of fringe projection imaging, used in order to complete the contour mapping of the surface and to increase overall resolution in the contour image.
Fringe projection imaging has been used effectively for surface contour imaging of solid, highly opaque objects and has been used for imaging the surface contours for some portions of the human body and for obtaining detailed data about skin structure. However, a number of technical obstacles have prevented effective use of fringe projection imaging of the tooth. One particular challenge with dental surface imaging relates to tooth translucency. Translucent or semi-translucent materials in general are known to be particularly troublesome for fringe projection imaging. Subsurface scattering in translucent structures can reduce the overall signal-to-noise (S/N) ratio and shift the light intensity, causing inaccurate height data. Another problem relates to high levels of reflection for various tooth surfaces. Highly reflective materials, particularly hollowed reflective structures, can effectively reduce the dynamic range of this type of imaging.
In fringe projection imaging overall, contrast is typically poor, with noise as a significant factor. To improve contrast, many fringe projection imaging systems take measures to reduce the amount of noise in the contour image. In general, for accurate surface geometry measurement using fringe imaging techniques, it is important to obtain the light that is directly reflected from the surface of a structure under test and to reject light that is reflected from material or structures that lie beneath the surface. This is the approach that is generally recommended for 3D surface scanning of translucent objects. A similar approach must be used for intra-oral imaging.
From an optics perspective, the structure of the tooth itself presents a number of additional challenges for fringe projection imaging. As noted earlier, light penetrating beneath the surface of the tooth tends to undergo significant scattering within the translucent tooth material. Moreover, reflection from opaque features beneath the tooth surface can also occur, adding noise that degrades the sensed signal and thus further complicating the task of tooth surface analysis.
One corrective measure that has been attempted to make fringe projection workable for contour imaging of the tooth is application of a coating that changes the reflective characteristics of the tooth surface itself. Here, to compensate for problems caused by the relative translucence of the tooth, a number of conventional tooth contour imaging systems apply a paint or reflective powder to the tooth surface prior to surface contour imaging. For the purposes of fringe projection imaging, this added step enhances the opacity of the tooth and eliminates or reduces the scattered light effects noted earlier. However, there are drawbacks to this type of approach. The step of applying a coating powder or liquid adds cost and time to the tooth contour imaging process. Because the thickness of the coating layer is often non-uniform over the entire tooth surface, measurement errors readily result. More importantly, the applied coating, while it facilitates contour imaging, can tend to mask other problems with the tooth and can thus reduce the overall amount of information that can be obtained.
Even where a coating or other type of surface conditioning of the tooth is used, however, results can be disappointing due to the pronounced contours of the tooth surface. It can be difficult to provide sufficient amounts of light onto, and sense light reflected back from, all of the tooth surfaces. The different surfaces of the tooth can be oriented at 90 degrees relative to each other, making it difficult to direct enough light for accurately imaging all parts of the tooth.
There have been a number of attempts to adapt structured light surface-profiling techniques to the problems of tooth structure imaging. For example, U.S. Pat. No. 5,372,502 entitled “Optical Probe and Method for the Three-Dimensional Surveying of Teeth” to Massen et al. describes the use of an LCD matrix to form patterns of stripes for projection onto the tooth surface. A similar approach is described in U.S. Patent Application Publication 2007/0086762 entitled “Front End for 3-D Imaging Camera” by O'Keefe et al. U.S. Pat. No. 7,312,924 entitled “Polarizing Multiplexer and Methods for Intra-Oral Scanning” to Trissel describes a method for profiling the tooth surface using triangularization and polarized light, but needing application of a fluorescent coating for operation. Similarly, U.S. Pat. No. 6,885,464 entitled “3-D Camera for Recording Surface Structures, In Particular for Dental Purposes” to Pfeiffer et al. discloses a dental imaging apparatus using triangularization but also requiring the application of an opaque powder to the tooth surface for imaging.
It can be appreciated that an apparatus and method that provides accurate surface contour imaging of the tooth, without the need for applying an added coating or other conditioning of the tooth surface for this purpose, would help to speed reconstructive dentistry and could help to lower the inherent costs and inconvenience of conventional methods, such as those for obtaining a cast or other surface profile for a crown, implant, or other restorative structure.
SUMMARY OF THE INVENTIONIt is an object of the present invention to advance the art of diagnostic imaging, particularly for intra-oral imaging applications. With this object in mind, the present invention provides an intra-oral imaging apparatus comprising: a fringe pattern generator energizable to emit a fringe pattern illumination having a predetermined spatial frequency, with light in the 350-500 nm range; a polarizer in the path of the fringe pattern illumination emitted from the fringe pattern generator and having a first polarization transmission axis; a projection lens disposed to direct the polarized fringe pattern illumination as incident illumination toward a tooth surface; an imaging lens disposed to direct at least a portion of the light reflected and scattered from the incident illumination at the tooth surface along a detection path; an analyzer disposed along the detection path and having a second polarization transmission axis; a detector disposed along the detection path for obtaining image data from the light provided through the analyzer; and a control logic processor responsive to programmed instructions and actuable to obtain image data from the detector and to adjust the intensity over one or more portions of the fringe pattern illumination that is emitted from the fringe pattern generator according to the obtained image data.
It is a feature of the present invention that it applies light of suitable polarization and wavelength along with fringe projection patterning of varying brightness to the task of tooth contour imaging.
An advantage offered by the apparatus and method of the present invention relates to improved imaging of tooth surfaces and at lower cost over conventional contour imaging methods. Unlike conventional methods, no powder or other opaque substance must be applied to the tooth as a preparatory step for contour imaging.
These objects are given only by way of illustrative example, and such objects may be exemplary of one or more embodiments of the invention. Other desirable objectives and advantages inherently achieved by the disclosed invention may occur or become apparent to those skilled in the art. The invention is defined by the appended claims.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the embodiments of the invention, as illustrated in the accompanying drawings. The elements of the drawings are not necessarily to scale relative to each other.
The figures provided herein are given in order to illustrate key principles of operation and component relationships along their respective optical paths according to the present invention and are not drawn with intent to show actual size or scale. Some exaggeration may be necessary in order to emphasize basic structural relationships or principles of operation. Some conventional components that would be needed for implementation of the described embodiments, such as support components used for providing power, for packaging, and for mounting and protecting system optics, for example, are not shown in the drawings in order to simplify description of the invention itself. In the drawings and text that follow, like components are designated with like reference numerals, and similar descriptions concerning components and arrangement or interaction of components already described are omitted.
In the context of the present disclosure, the term “fringe pattern illumination” is used to describe the type of structured illumination that is used for fringe projection imaging or “contour” imaging. The fringe pattern itself can include, as pattern features, multiple lines, circles, curves, or other geometric shapes that are distributed over the area that is illuminated and that have a predetermined spatial frequency, recurring at a given period.
Two portions of a line of light or other feature in a pattern of structured illumination can be considered to be substantially “dimensionally uniform” when their line width is the same over the length of the line to within no more than +/−15 percent. As is described in more detail subsequently, dimensional uniformity of the pattern of structured illumination is needed to maintain a uniform spatial frequency.
As noted above in the background section, conventional approaches for fringe projection imaging fall short of providing good results for tooth tissue for a number of reasons. Apparatus and methods of the present invention address the problems of obtaining images of the tooth when using fringe projection imaging with fringe pattern illumination by selection of favorable light properties and by techniques that improve light delivery to the highly contoured tooth surface.
Referring to the schematic block diagram of
One function of control logic processor 34 for fringe projection imaging is to incrementally shift the position of the fringe and trigger the detector to take images that are then used to calculate three-dimensional information of tooth surface. For the phase shifting fringe projection method, at least three images are typically needed in order to provide enough information for calculating the three-dimensional information of the object. The relative positions of the fringes for these three projected images are typically shifted by one-third of the fringe period. Control logic processor 34 can be a computer, microprocessor, or other dedicated logic processing apparatus that executes programmed instructions.
Intra-oral imaging apparatus 10 of
-
- (a) Same polarization transmission axis as polarizer 14. In this “co-polarization” position, detector 30 obtains the specular light reflected from the surface of tooth 20, and most of the light scattered and reflected from the superficial layer of enamel surface of tooth 20, as well as some of the light scattered back from sub-surface portions of the tooth. The co-polarization orientation of the analyzer 28 axis is shown in
FIG. 2A . Parallel or co-polarization provides improved contrast over other configurations. - (b) Orthogonal polarization transmission axis relative to polarizer 14. Using the orthogonal polarization, or cross-polarization, helps to reduce the specular component from the tooth surface and obtain more of the scattered light from inner portions of the tooth. The cross-polarization orientation of the analyzer 28 axis is shown in
FIG. 2B .
- (a) Same polarization transmission axis as polarizer 14. In this “co-polarization” position, detector 30 obtains the specular light reflected from the surface of tooth 20, and most of the light scattered and reflected from the superficial layer of enamel surface of tooth 20, as well as some of the light scattered back from sub-surface portions of the tooth. The co-polarization orientation of the analyzer 28 axis is shown in
When the tooth is imaged with an imaging system and sensor, the light that is available to the sensor can be (i) light reflected from the tooth top surface; (ii) light scattered or reflected from the near surface volume or portion of the tooth; and (iii) light scattered inside the tooth. In the context of the present disclosure, the “near-surface volume” of the tooth is that portion of the tooth structure that lies within no more than a few hundred μm of the surface.
It is known that the light reflected from the tooth surface (i), the specular light, maintains the polarization state of the incident light. As the incident light propagates further into the tooth, the light is increasingly depolarized.
Disadvantageously, some portion of the specular light (i) for a contour pattern may be incident on more highly reflective portions of the tooth surface, even causing some amount of saturation that degrades light detection. In contrast to conventional approaches that use all the light from the tooth, methods of the invention use at least portions of both the specular light (i) and the near-surface reflected light (ii), and avoid the light scattered deep inside the tooth (iii). Applicants have found that the near-surface light (ii), particularly for blue light and shorter wavelengths, is still substantially polarized. Thus, for example, a large portion of the light scattered and reflected from the superficial layer of the tooth enamel also has the same polarization state as the incident light and as the specular light (i).
Of particular interest, the spatial “footprint” of the scattered light P2, which relates to the dimensions of pattern features of the structured light, such as line thicknesses, shows an increase over the corresponding spatial footprint of reflected light P1. For example, where the structured light pattern consists of parallel lines of light of a given thickness, the reflected light P1 from these pattern features has lines of substantially the same thickness as the projected pattern. However, the scattered light P2 is detected as lines of slightly increased thickness. That is, since light P2 has been scattered inside the tooth, the projected footprint on the tooth surface is broader than that of the specular reflected light, which is the same size as the illumination beam. The graph of
The group of contour images shown in
In addition to taking advantage of favorable properties of polarized light, embodiments of the present invention also take advantage of different amounts of reflection that correspond to the wavelength of light directed toward the tooth.
For the embodiment of
Because the co-polarized and cross-polarized light provide different types of information about the surface and near-surface of the tooth, imaging apparatus 40 of
Detectors 30, 30a, or 30b in the embodiments described herein can be any of a number of types of image sensing array, such as a CCD device, for example. Polarizers and analyzers can be wire-grid or other polarizer types.
In one embodiment of the present invention, the imaging apparatus is packaged in the form of a hand-held probe that can be easily positioned within the patient's mouth with little or no discomfort. Referring to
As noted in the background section, the pronounced contours of the tooth include surfaces that are steeply sloped with respect to each other, complicating the task of directing enough light onto each surface. As a result, some surfaces of the tooth may not provide 3-D information that is sufficient. Referring to
In order to compensate for this lack of brightness using conventional fringe projection patterning techniques, an embodiment of the present invention selectively increases the light intensity of the fringe pattern illumination over a given area. In
Maintaining dimensional uniformity and spatial frequency of the fringe pattern is advantageous for contour imaging because it provides a uniform resolution over the full image field. Other techniques have been proposed for changing the pattern dimensions itself, such as thickening the pattern lines over specific areas; however, because the spatial frequency of the fringe pattern changes when using such a technique, the resulting resolution of the contour image that is obtained is non-uniform. With respect to the example fringe pattern 50 given in
The schematic diagram of
In addition to increasing the light intensity over darker areas of the tooth surface relative to the position of imaging apparatus 10, it is also possible to reduce the light intensity over areas where there may be highly specular reflection that otherwise causes saturation of the detector. Again, it must be emphasized that what changes is the light intensity over one or more portions of the projected light pattern; line thickness and spacing, both related to the spatial frequency, remain the same for different intensities.
Referring again to
Referring again to
The logic flow diagram of
Still referring to
In the embodiment shown in
The schematic diagram of
Embodiments of the present invention provide improved contour imaging for teeth by taking advantage of properties of light and capabilities of spatial light modulators for forming an adaptive fringe projection pattern having suitable light intensity that is responsive to variability in tooth surface characteristics. The apparatus and methods of the present invention compensate for problems related to the translucence of the tooth by using short-wavelength light and by employing principles of polarized light. When light of suitable wavelength and polarization state is provided with an adaptable intensity arrangement, a more accurate indicator of the highly contoured tooth surface can be achieved.
The surface contour image that is obtained using the apparatus and methods of the present invention can be used in a number of ways. Contour data can be input into a system for processing and generating a restorative structure or can be used to verify the work of a lab technician or other fabricator of a dental appliance. This method can be used as part of a system or procedure that reduces or eliminates the need for obtaining impressions under some conditions, reducing the overall expense of dental care. Thus, the imaging performed using this method and apparatus can help to achieve superior fitting prosthetic devices that need little or no adjustment or fitting by the dentist. From another aspect, the apparatus and method of the present invention can be used for long-term tracking of tooth, support structure, and bite conditions, helping to diagnose and prevent more serious health problems. Overall, the data generated using this system can be used to help improve communication between patient and dentist and between the dentist, staff, and lab facilities.
Advantageously, the apparatus and method of the present invention provide an intra-oral imaging system for 3-D imaging of teeth and other dental features without requiring the use of a special powder or application of some other temporary coating for the tooth surface. The system offers high resolution, in the 25-50 μm range in one embodiment.
The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. For example, any of a number of different types of spatial light modulator could be used as part of the fringe pattern generator. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.
PARTS LIST
- 10. Imaging apparatus
- 12. Fringe pattern generator
- 14 Polarizer
- 16. Lens
- 18. Actuator
- 20. Tooth
- 22. Lens
- 26. Rear surface
- 28. Analyzer
- 30, 30a, 30b. Detector
- 34. Control logic processor
- 36. Polarization beam splitter
- 38. Display
- 40. Imaging apparatus
- 42. Intra-oral imaging system
- 44. Pattern
- 50. Fringe pattern
- 52, 54. Area
- 56. First intensity
- 58. Second intensity
- 60. Initial step
- 64. Analysis step
- 66. Third intensity
- 68. Fourth intensity
- 70. Map generation step
- 74. Image acquisition step
- 76. Looping step
- 80. Light source
- 82. Optical element
- 84. Spatial light modulator
- 88. Detection path
- 90. Filter
- B. Box
- P0, P1, P2. Polarized light
- λ1, λ2,λ3. Wavelength
Claims
1. An intra-oral imaging apparatus comprising:
- a fringe pattern generator energizable to emit a fringe pattern illumination having a predetermined spatial frequency, with light in the approximate 350-500 nm range;
- a polarizer disposed in the path of the fringe pattern illumination emitted from the fringe pattern generator and having a first polarization transmission axis;
- a projection lens disposed to direct the polarized fringe pattern illumination as incident illumination toward a tooth surface;
- an imaging lens disposed to direct at least a portion of the light reflected and scattered from the incident illumination at the tooth surface along a detection path;
- an analyzer disposed along the detection path and having a second polarization transmission axis;
- a detector disposed along the detection path for obtaining image data from the light provided through the analyzer; and
- a control logic processor responsive to programmed instructions and actuable to obtain image data from the detector and to adjust the intensity over one or more portions of the fringe pattern illumination that is emitted from the fringe pattern generator according to the obtained image data.
2. The imaging apparatus of claim 1 wherein the second polarization transmission axis is parallel to the first polarization transmission axis.
3. The imaging apparatus of claim 1 wherein the second polarization transmission axis is orthogonal to the first polarization transmission axis.
4. The imaging apparatus of claim 1 further comprising an actuator coupled to either the polarizer or the analyzer and energizable to rotate the coupled polarizer or analyzer to one of two positions, substantially 90 degrees apart.
5. The imaging apparatus of claim 1 wherein the analyzer is a polarization beam splitter and wherein the detector is a first detector disposed to receive light transmitted through the polarization beam splitter and further comprising a second detector disposed to receive light reflected from the polarization beam splitter.
6. The imaging apparatus of claim 1 wherein the fringe pattern generator comprises a grating.
7. The imaging apparatus of claim 1 wherein the fringe pattern generator comprises a spatial light modulator.
8. The imaging apparatus of claim 7 wherein the spatial light modulator is taken from the group consisting of a digital micromirror device and a liquid crystal device.
9. The imaging apparatus of claim 1 wherein the fringe pattern generator comprises an emissive device that forms and emits the fringe pattern.
10. The imaging apparatus of claim 1 further comprising a filter disposed along the detection path for transmitting light in the 350-500 nm range.
11. A method for obtaining a surface image of a tooth comprising:
- obtaining a reference image of a portion of the tooth surface having at least a lighter area and a darker area;
- generating and projecting one or more fringe patterns onto the portion of the tooth surface, wherein the intensity of the fringe pattern illumination that is directed toward the darker area exceeds the intensity of the fringe pattern illumination that is directed toward the lighter area and wherein the fringe pattern has the same spatial frequency for pattern features over the lighter and darker areas; and
- obtaining one or more images from the one or more projected fringe patterns.
12. The method of claim 11 wherein obtaining the reference image further comprises directing a structured illumination onto the tooth surface.
13. The method of claim 11 wherein projecting the one or more fringe patterns comprises modulating light of a wavelength in the range from 350-500 nm.
14. The method of claim 11 wherein projecting the one or more fringe patterns comprises modulating polarized light.
15. The method of claim 11 wherein obtaining the reference image further comprises changing the transmission axis of an analyzer or a polarizer.
16. The method of claim 11 wherein generating the one or more fringe patterns comprises actuating a spatial light modulator.
17. The method of claim 11 wherein obtaining the reference image comprises obtaining light directed from a polarization beamsplitter.
18. The method of claim 11 wherein generating the one or more fringe patterns further comprises shifting the relative spatial position of two or more of the generated fringe patterns.
19. The method of claim 18 wherein shifting the relative spatial position of two or more of the generated fringe patterns comprises shifting by a fraction of the period of the fringe pattern.
20. An intra-oral imaging apparatus comprising:
- a fringe pattern generator comprising a spatial light modulator and energizable to emit a fringe pattern illumination having a plurality of pattern features with a predetermined spatial frequency;
- a polarizer disposed in the path of the fringe pattern illumination emitted from the fringe pattern generator and having a first polarization transmission axis;
- a projection lens disposed to direct the polarized fringe pattern illumination as incident illumination toward a tooth surface;
- an imaging lens disposed to direct at least a portion of the light reflected and scattered from the incident illumination at the tooth surface along a detection path;
- an analyzer disposed along the detection path and having a second polarization transmission axis;
- a detector disposed to form image data from light that is obtained from the tooth along the detection path;
- a filter disposed along the detection path for transmitting light in a range from approximately 350-500 nm and attenuating light outside the range; and
- a control logic processor responsive to programmed instructions and actuable to obtain image data from the detector and to adjust the intensity of light over one or more portions of the pattern features of the fringe pattern illumination that is emitted from the fringe pattern generator according to the obtained image data.
Type: Application
Filed: Apr 16, 2009
Publication Date: Oct 21, 2010
Inventor: Rongguang Liang (Penfield, NY)
Application Number: 12/424,562
International Classification: A61B 5/05 (20060101);