Reduction and removal of artifacts from a three-dimensional dental X-ray data set using surface scan information

A system for removing artifacts caused by x-ray reflective materials from an x-ray image of a patient's teeth. The system includes an x-ray source, an x-ray detector that captures several x-ray images, and a surface scanner that captures a surface scan of the patient's teeth. An image processor generates three-dimensional models from the optical surface data and the CT volumetric data. The models are resized and oriented to be of the same scale and orientation and then overlaid to create a combined data set. Data points that extend beyond the surface of the patient's teeth in the surface model are identified and may be removed if it is determined that they are artifacts. An artifact-reduced CT model is then displayed.

BACKGROUND

The present invention relates to dental imaging technology. More specifically, the present invention relates to reconstructing a three-dimensional image of a patient's teeth using both x-ray and surface scanning technology.

X-ray technology can be used to generate a three-dimensional, digital representation of a subject using computed tomography (CT). However, metal and other objects can reflect x-rays that would otherwise penetrate through human tissue and be detected by the x-ray detector. This reflection can cause unwanted artifacts to appear in the captured data set. This effect is particularly prevalent in dental imaging where foreign substances such as metal fillings or braces are often installed in the patient's mouth.

There are some prior systems in which artifacts are removed from x-ray data by, simply stated, “combining” x-ray and non-x-ray data. However, as best known by the inventors, in addition to removing or reducing artifacts, such systems also remove significant amounts of desired image data.

U.S. Publication No. 2010/0124367 has suggested that artifacts can be removed from x-ray data by the “fusion of the x-ray data set with an optical image of the jaw, which is completely free of metal artifacts . . . .” However, details regarding how the artifacts would be removed are not provided and the “fusion” disclosed in the '367 publication uses a pre-positioning technique that requires identifying registration points on a screen or other manual means prior to combining the data. While this pre-positioning makes the task of combining the two data sets substantially easier than a completely automatic method, the method requires manual intervention. That is, the x-ray technician, dentist, or other dental professional must manually manipulate the images on a screen.

U.S. Pat. No. 6,671,529 describes a method of creating a composite skull model by combining three-dimensional CT data and laser surface scans of a patient's teeth. In the '529 patent, the teeth are completely removed from the CT model and replaced with only the surface scan data of the patient's teeth.

U.S. Pat. No. 7,574,025 describes a method of removing artifacts from a three-dimensional model (such as CT or MRI) by a negative impression template of the patient's teeth. In the '025 patent, a negative impression template is cast of the patient's teeth. A first model is generated while the negative impression template is placed in the patient's mouth. A second model is generated of only the negative impression template using the same imaging technology as the first. Voxels from the first digital image are substituted for corresponding voxels from the second digital image to create a model of the patient's teeth without artifacts.

SUMMARY

It would be useful to have an improved method and system of removing artifacts from x-ray data that did not remove significant portions of desired CT image data, substitute data from multiple x-rays, or require manual pre-positioning of the data sets.

In one embodiment, the invention provides a system for generating a three-dimensional, digital representation including a patient's teeth using both CT and surface scanning data. The system includes an x-ray source and an x-ray detector that are used to capture several x-ray images. The images are transmitted to an image processing system where they are used to construct a three-dimensional CT model of the patient's teeth. The system also includes a surface scanner (such as a laser or structured light scanning system) that captures data representing the shape and texture of the surface of the patient's teeth. The surface data is also transmitted to the image processing system where it is used to construct a three-dimensional model of the surface of the patient's teeth. The image processing system then resizes and orients the surface model and the CT model so that the two models are of the same scale and orientation.

In some embodiments, the surface model is then overlaid onto the CT model. This is achieved without requiring manual intervention. The system of this embodiment then detects artifacts in the CT model by detecting any data points in the CT model that extend beyond the overlaid surface model. Data points extending beyond the surface model are considered to be artifacts and the image processing system removes the artifact data points from the CT model. In other embodiments, any data points in the CT model that extend beyond the surface model are processed to determine whether they are artifacts. Processed data points that are identified as artifacts are then removed from the CT model. In some embodiments, after the artifact data points are identified and removed from the CT model, the overlaid surface data is then removed leaving only the three-dimensional CT model.

In some embodiments, the surface model is forward projected to create projection data in the same two-dimensional (2D) format as the CT projection data. The forward projected data is combined with the CT projection data to identify regions of metal and teeth and allow the CT reconstruction to remove the effects of metal from the reconstructed CT images. Again, this is achieved without requiring manual pre-positioning of the two sets of data with respect to one another.

DETAILED DESCRIPTION

FIG. 1Ais a block diagram illustrating the components of a system for removing artifacts from a three-dimensional digital CT model of a patient's teeth. The system can also be used to create three-dimensional digital models of the patient's jaw and other facial bones and tissue. The system includes an x-ray source101and an x-ray detector103. The x-ray source101is positioned to project x-rays toward a patient's teeth. The x-ray detector103is positioned on the opposite side of the patient's teeth—either inside the patient's oral cavity or on the opposite side of the patient's head. The x-rays from the x-ray source101are attenuated differently by the patient's tissue and are detected by the x-ray detector103.

The x-ray detector103is connected to an image processing system105. The data captured by the x-ray detector103is used by the image processing system to generate a three-dimensional CT model of the patient's teeth. As such, in one embodiment, the x-ray source101and the x-ray detector103are part of a cone-beam, scanning CT system that rotates around the patient's head to collect x-ray image data as illustrated inFIG. 1B. An example of one such scanning system is described in U.S. application Ser. No. 12/700,028 filed on Feb. 4, 2010, the entire contents of which are incorporated herein by reference. The '028 application relates to motion correction, but the imaging components—sensor and source mounted on a rotatable C-arm, are applicable to the techniques described herein.

The system illustrated inFIG. 1Aalso includes a surface scanning imaging system107. The surface scanning system107captures data relating to the surface texture, size, and geometry of the patient's teeth. The captured surface data is then transmitted to the image processing system105where it is used to generate a three-dimensional surface model of the patient's teeth. In some embodiments, the surface scanning imaging system includes a laser transceiver system such as one including a laser source107A and a laser sensor or digital camera107B. The laser source scans a laser line across the surface of the patient's tooth. The sensor or camera captures images of the projected line. The image processing system105analyzes how the shape of the laser line from the perspective of the sensor or camera changes as it is scanned across the patient's tooth. This data is then used to generate the three-dimensional surface model of the patient's teeth.

The image processing system105inFIG. 1Aincludes a processor109for executing computer instructions and a memory111for storing the instructions and data transmitted from the x-ray detector103and the surface scanning system107. In some embodiments, the image processing system includes one or more desktop computers running image processing software. In other embodiments, the image processing system105is a device designed specifically for processing image data received from the x-ray detector103and the surface scanning system107. Data captured by the x-ray detector103and the surface scanning imaging system107as well as processed volumetric data is displayed on a display113.

FIG. 2is a flowchart illustrating one method for how the system ofFIG. 1Acan be used to generate to a CT model of a patient's teeth and remove artifacts from a CT model of the patient's teeth. The system begins by capturing cone-beam CT (CBCT) image data of the patient's teeth (step201). This is done by rotating the x-ray source101and the x-ray detector103around the patient's head to collect several x-ray images of the patient's teeth. The captured data is then used to generate a three-dimensional CT model of the patient's teeth (step203). The surface scanning system107is used to capture optical surface data (step205), which the image processing system105then uses to generate a three-dimensional surface model of the patient's teeth (step207). In various embodiments, the surface data can be captured before or after the CT image is captured. Similarly, the CT data can be processed by the image processing system (step203) before or after the surface data is captured by the surface scanning system107(step205).

After both the CT model and the surface model have been generated, the image processing system105correlates the three-dimensional CT volume model and the three-dimensional optical surface model to determines a proper scale and orientation of the two models. The surface model is overlaid onto the CT model to generate a combined data set (step209). In some embodiments, the system is calibrated such that the captured data includes registration information indicating the location and perspective from which the data was captured. In such embodiments, the proper scale and orientation of the two models is determined by matching the registration information from the CT model to the corresponding registration information from the surface model.

In other embodiments, the image processing system105uses surface matching algorithms to identify corresponding physical structures in both of the models. The identification of corresponding structures can be achieved, for example, by the identification of three or more anatomical landmarks that appear in both of the two models and then rotating, translating, and scaling one model until the differences between these landmarks is minimized within a predetermined tolerance. Alternatively, the entire surface in the two models can be matched by scaling, rotation, and translation through various well-known optimization techniques such as simulated annealing. A number of features in the two models can be characterized in each and correlated to determine the best match of the surfaces. The image processing system105sizes and orients the models according to the matching structures.

In some embodiments, the overlay process can be executed by overlaying the entire surface model onto the entire CT model. The image processing system105can also include various functions that segment the CT model into sub-volumes. The sub-volume functions can be used to isolate a single tooth from the CT model. InFIGS. 5-8, artifacts caused by x-ray reflective materials (such as metal filings) are removed from the CT model by overlaying data from the surface model onto sub-volumes of the CT model one tooth at a time.

FIG. 5shows a single horizontal slice from the CT model of the patient's teeth. Artifacts301can be seen reflecting from structures in the patient's teeth303.FIG. 6shows the fully constructed CT model. Again, artifacts301are clearly visible extending from the patient's teeth303. These artifacts301hide some of the surface details of the patient's teeth303in the CT model.FIG. 7shows a portion of the surface model generated by the image processing system105based on the surface information captured by the surface scanning system107. The image processing system isolates a horizontal cut plane501in the surface model. This cut plane501corresponds to the slice from the CT model illustrated inFIG. 5. Because the surface model does not contain any data points from within the patient's teeth, the cut plane data identifies a silhouette shape503that corresponds to the outer surface of the patient's tooth on a given horizontal plane. InFIG. 8, the silhouette shape503from the cut plane501is overlaid onto the same tooth in the slice from the CT model.

After the silhouette data503from the surface model is overlaid onto the corresponding tooth in a slice of the CT model, the image processing system105identifies data points in the CT model that extend beyond the silhouette503(step211). If data is detected outside of the silhouette shape503, the system determines whether this data is artifact data. In some embodiments, all data in the CT model that extends beyond the silhouette shape503is assumed to be or is identified as artifact data and is removed from the CT model. In other embodiments, the data outside of the silhouette503is processed by a filtering or interpolation algorithm. The interpolation algorithm detects picture elements in the data just outside of the silhouette shape503that have densities that are above a threshold. The algorithm then interpolates data for these identified, artifact-associated pixels (or data points) with data from adjacent pixels not associated with artifact.FIG. 8illustrates an area601just outside of the silhouette shape503that is to be analyzed by such a filtering algorithm.

After the CT data outside of the silhouette shape503has been interpolated, adjusted, or removed, the image processing system105moves onto another tooth in the same slice of CT data. After the necessary corrections have been made for each tooth, the image processing system105moves to another slice of CT data. This process repeats until all of the teeth in each of the CT data slices have been analyzed and the artifact data has been removed.

After the CT data has been analyzed and the artifacts have been identified and removed, the image processing system105removes the overlaid surface model and any silhouette shapes from the CT data (step213) and generates an artifact-reduced CT model. As pictured inFIG. 9, the artifacts have been removed from the artifact-reduced CT model and the surface of each tooth is visible.

An alternative approach for combining the CT and optically derived surface data as presented inFIG. 3. As in the method ofFIG. 2above, the CT and optical data are both captured (steps221,225) and are both processed to produce 3D volumetric data (steps223,227). The data models are again correlated and overlaid (step229). Once correlated, the two volumetric datasets in the combined data set are then both forward-projected in order to create a set of two-dimensional projection images (step231). Optically-derived data is then used to identify the teeth surface and filter or compensate for the artifact-causing elements (created by scatter and beam hardening associated with metal in the image volume) in the images or frames that lead to artifacts in the reconstructed images (step233). The filtering process removes artifacts before a three-dimensional model is reconstructed from the two-dimensional projection images (step235).

A third approach to combining the CT and optically-derived surface data is presented inFIG. 4. In this case, the CT and optical data are both captured as described above (steps241,243). However, instead of generating two three-dimensional models, the optical surface data is forward projected to generate two-dimensional projection images (step245). The two dimensional images from the CT data and the two-dimensional projection images are correlated and overlaid (step247). Artifact-causing elements that lead to artifacts in the reconstructed images are filtered from the raw CT data using the anatomy defined by the two-dimensional optical frames (step249). A three-dimensional CT model is then constructed from the filtered and corrected CT data frames.

Thus, the invention provides, among other things, a system for capturing CT data, generating a CT model, and removing artifacts from the generated CT model by capturing surface scan data of the patient's teeth, overlaying the surface scan data onto the CT model, and identifying, reducing, and removing artifacts that are outside of the surface scan data. Various features and advantages are set forth in the following claims.