Imaging system and method

Abstract: In an imaging system and method a volume image memory (24) stores an electronic image representation of a volumetric region, such as a volumetric region examined by a medical diagnostic scanner (10). A sub-region of interest of the volume is selected (30) and the surface of the selected region or object of interest is divided into triangular surface regions with a triangular surface processor (38). A vertex merging processor (40) examines the triangles to locate vertices spaced by less than a preselected minimum. Vertices closer than the preselected minimum are replaced with a vertex at a median position merging the triangle into adjacent triangles. A vertex removal processor (50) identifies groups of triangles having a common vertex (V c ) that form a pyramid. The surface normals (N) of the triangles which define a pyramid are examined to determine whether they are within a preselected deviation of parallel. The altitude of the pyramid, i.e., a distance between the common vertex and an average plane (P avg ) of the triangles is compared with a preselected maximum height to see if they are below it. Triangle groups which pass the surface normal and maximum height tests have their common vertex removed (72) and are retriangulated (74) to reduce the number of triangles by two. An image processor (84) converts the angles of viewing and illumination direction relative to the surface normal of each visible triangle into a corresponding image value, e.g. gray scale value, for display on a video monitor (34).

Description: This invention relates to imaging systems and methods. It finds especial application in imaging systems and methods which are suitable for use in medical diagnostic imaging and which provide three-dimensional surface renderings, and will be described with particular reference thereto. Heretofore, three-dimensional surface renderings have commonly been provided using a constant triangular mesh over the surface of interest. That is, a surface of interest of a computer generated three-dimensional image representation is selected. For example, one might use a nuclear, CT scanner, or other diagnostic imaging device to generate a three-dimensional image representation of a portion of a patient's torso surrounding the liver. Using conventional techniques, such as comparing the gray scale or CT number of each pixel value in the image with a characteristic gray scale or CT number for the liver, one can identify the liver and the surface of the liver. Because all of the liver surface tissue has substantially the same gray scale or CT number, if a perspective view of the liver were recreated and displayed on a video monitor, the resultant display would have the appearance of a silhouette. There would be substantially no surface texture information conveyed. In order to convey contour information, the surface of the liver or other structure which is visible from a viewing direction is overlaid with a constant triangular mesh. That is, the surface of the liver or other object of interest is approximated by a mesh of triangles of as uniform a size as permitted by the surface texture. In a typical 512x512 medical diagnostic image, one might expect to find about 20,000-50,000 triangles on the surface of interest. A surface normal is projected from a centroid or other characteristic location on each triangle of the mesh. The vector surface normals are each compared with an illumination direction vector and a viewing direction vector. Based on the vector comparison, a gray scale (or color or hue) for each triangle is selected. Typically, the gray scales are selected to approximate the lighting and shading conditions that an object of the selected configuration would have at the corresponding point on the surface when viewed from the viewing direction and illuminated from the illumination direction. The display may represent more than just the surface of the object. In particular, one can commonly define a cut plane on a medical diagnostic image and remove the portion of the object to one side of the cut plane. The portions of the image corresponding to the interior of the liver or other object of interest are displayed with appropriate gray scales or colors which correspond to the CT numbers or pixel values of each internal pixel of the liver or other structure that is intercepted by the slice. The remaining surface portion continues to be displayed with three-dimensional surface rendering. Typically, the operator has the ability to rotate the displayed object. Each time the object rotates an incremental distance, the surface normals are again computed and compared with the illumination and viewing direction vectors and reassigned gray scale values to each triangle in accordance with the comparison. The large number of calculations necessary to reassign the gray scales are commonly slow relative to the rotation rate of the object. That is, the operator rotates the object faster than the new gray scales can be computed causing the object to appear to rotate in jumps rather than smoothly. For example, if an object rotates about 5° in the time necessary to recalculate the gray scale values, then the object appears to rotate in 5° steps. "Decimation of Triangle Meshes" by Schroeder, et al., Computer Graphics, Vol. 26, No. 22, pgs. 65-70, July 1992, describes an algorithm which reduces the number of triangles required to model a physical or abstract object. In the Schroeder technique, multiple passes are made over an existing triangular mesh using local geometry and topology to remove vertices that pass a distance or angle criterion. Holes left by the vertex removal are patched using a local triangulation process. According to the invention there is provided an imaging system which includes a memory for storing a volumetric image representation, a triangular surface processor for replacing a surface of a selected volumetric region of the volumetric image representation in the image memory with a triangular mesh, an image processor for comparing a viewing direction, an illumination direction and a surface normal of each visible triangle of the triangular mesh to generate a corresponding image value, and an image display device for converting the image values into a corresponding human readable image; a vertex merging means for merging areas of triangles of the triangular mesh that have a dimension less than a preselected minimum dimension into areas of adjoining triangles, a vertex removal means for removing a vertex shared by a plurality of triangles which are within a preselected tolerance of coplanar and retriangulating a polygon defined by remaining peripheral vertices of the triangles with a smaller number of triangles; means for identifying a vertex common to a group of triangles which define a pyramid; means for determining an average plane of the vertices of the pyramid; means for comparing deviation among surface normals of the triangles of the pyramid for a preselected maximum deviation; means for determining a distance between the avergae plane and the common vertex to determine a height of the pyramid; means for comparing the pyramid height with a preselected maximum pyramid height; means for removing the common vertex of the group of triangles which define each pyramid whose surface normals diverge less than the preselected divergence and whose height is less than the preselected maximum pyramid height, characterised in that said vertex removal means includes: means for projecting the peripheral vertices onto the average plane to define a planar polygon; means for triangulating the polygon on the average plane; and means for connecting corresponding peripheral vertices in the image memory in accordance with the retriangulating on the average plane. Further according to the invention there is provided an imaging method according to claim 3. One imaging system and its method of operation as a medical diagnostic imaging apparatus will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic illustration of the system; Figure 2 is a top view of a plurality of triangles with a common vertex illustrating a feature of the system; Figure 3 is a side view of the triangles of Figure 2; Figure 4 illustrates a polygon formed by removal of the common vertex of Figure 2; and Figure 5 illustrates an exemplary retriangulation of the polygon of Figure 4. Referring to Figure 1, the system includes a medical diagnostic imaging device 10, such as a nuclear camera, CT scanner, magnetic resonance imager, or the like, arranged to perform a non-invasive examination of a subject disposed in an examination region 12. In the nuclear camera embodiment, one or more detector heads 14 detect emitted radiation from a radio isotope injected into a subject as well as transmitted radiation from a radiation source 16. Each of the detector heads 14 includes a scintillation crystal (not shown) which is elongated in both an axial and circumferential direction. Photomultiplier tubes (not shown) monitor radiation events on the scintillation crystal and produce electrical signals indicative of a ray along which each radiation event was received and the energy of each radiation event. The energy can be used to distinguish between emitted and transmitted radiation, between two different radio isotopes, or the like. Each detector head 14 has a two-dimensional radiation receiving area so that the output data from the head as it rotates is indicative of a three-dimensional volume, e.g. a plurality of parallel slices transverse to an axis about which the detector heads are rotated. Other diagnostic imaging devices such as magnetic resonance imagers, CT scanners, spiral trajectories CT scanners, and the like which generate data indicative of a volumetric region of a subject can be used instead of a nuclear camera. Lines or views of data from the diagnostic imaging device 10 are stored in a data buffer 20. A reconstruction processor 22, reconstructs the data into a volumetric image representation. In the case of the illustrated SPECT nuclear camera, in the processor 22 a filtered backprojector backprojects and filters each line of data from the data buffer 20 into a volume image memory 24. Other techniques for reconstructing diagnostic data from other imaging modalities as are known in the art may be used. Although the volume image data of the preferred embodiment is medical diagnostic image data, it is to be appreciated that the present invention is applicable to a volumetric data from other sources. Such other sources might include models such as from a CAD-CAM system, terrain data from satellite or aerial surveillance, and other techniques which convert a three-dimensional object or its surface into an electronic image representation. A sub-region selection processor 30 under control of an operator at an operator terminal 32 selects a sub-region of the three-dimensional diagnostic image for display. For example, in medical diagnostic imaging, each type of tissue commonly has a common value, CT number, gray scale, count density, or the like. By examining the entire volumetric region for tissue with a characteristic value, a sub-region of interest is identified. For a more accurate representation, it is common to "grow" the sub-region of interest. That is, a pixel or group of pixels of the characteristic value are displayed on a video monitor 34 for the operator to review. The operator indicates the position of the organ of interest in the volume. Thereafter, the sub-region selection processor examines pixels of the characteristic value which are contiguous to the operator selected coordinate and grows the region of contiguous pixels from the selected coordinate outward. Discontinuous pixels of the same value are discarded to remove noise. For convenience of explanation, the selected sub-region is illustrated as transferred to a selected sub-region memory 36. In the selected sub-region memory 36, pixels of the volumetric object of interest are given a common value and the remaining pixels are given the value indicative of air or another value which makes them readily identifiable and transparent for calculation purposes. Although indicated as a separate memory, it is to be appreciated that in many applications, the selected sub-region memory is incorporated in the volume image memory 24. A triangular surface processor 38 re-defines the surface of the selected sub-region as a constant triangle mesh, i.e, approximates the surface with triangles. When the triangular mesh is fitted to a contoured surface, some of the triangles become compressed forming thin elongated triangles. Some of the triangles are degenerate, i.e. they are so thin that they have zero area. A vertex merging processor or program in a general purpose processor 40 performs a vertex merging step on the triangles of the triangular mesh surface. The vertex merging step eliminates such degenerate and substantially degenerate triangles. More specifically, the vertex merging processor 40 performs a routine 42 which calculates a distance between each adjacent vertex. A comparator or comparing step 44 compares the distance between vertices with a preselected standard. Whenever a pair of adjoining vertices is less than the standard, a routine or step 46 replaces the pair of adjacent vertices with a single vertex at an intermediate, preferably the median, position. In this manner, triangles which are very thin as compared to the standard are removed and their area divided among two adjacent triangles. A vertex removal processor 50 identifies vertices which are common to a plurality of generally coplanar triangles, eliminates the common vertex, and retriangulates the remaining polygon with a smaller number of triangles. More specifically, a routine or step 52 identifies cycles of triangles that define pyramids, i.e. triangles with a common vertex V c such that each edge attached to V c forms the boundary between exactly two of the triangles. A top view of such a cycle of triangles is shown in FIGURE 2. A routine or step 54 calculates an average plane of the cycle of triangles. As illustrated in FIGURE 3, the peripheral vertices V p are not normally coplanar with each other. A plane P avg is calculated which is the average or closest plane to all of the triangles. A routine or step 56 calculates how flat the pyramid is. More specifically, the routine 56 includes a routine 58 which calculates surface normals N of each of the triangles of the cycle. A weighted average normal determining routine or step 60 weights each normal by the area of the corresponding triangle and determines a weighted average normal N avg . A dot product routine or step 62 determines a difference between each normal and the weighted average normal. A comparing routine or step 64 compares the deviation of the normals from the weighted average normal with a standard to determine whether the normals are all within a preselected range of parallel. If the normals are not sufficiently coplanar or sufficiently close to parallel, then the pyramid is accepted and not adjusted further. If the normals are sufficiently close to parallel, then a routine or step 66 calculates the pyramid height. That is, the distance from the average plane P avg to the central vertex V c is determined. A comparing routine or step 70 compares the pyramid height with a preselected height. If the pyramid is taller than the preselected height it is accepted as is and no vertices are removed. If the pyramid is below the preselected height, then a routine or step 72 removes the central vertex V c . See FIGURE 4. A retriangulation routine or step 74 retriangulates the polygon defined by the peripheral vertices V c . See FIGURE 5. By comparing FIGURES 3 and 5, it will be seen that for a polygon with five peripheral vertices, the number of triangles is reduced from five to three. The number of triangles is always reduced by two with this procedure. More specific to the preferred embodiment, the retriangulation routine 74 includes a routine or step 76 which projects the peripheral vertices V p onto the average plane P avg . A routine or step 78 retriangulates the coplanar vertices using a conventional delaunay triangulation algorithm. The delaunay triangulation algorithm is preferred because it avoids long sliver triangles by minimizing their maximum angles to avoid obtuse angles and long skinny triangles. Once the connections are decided by the delaunay triangulation algorithm on the coplanar projected peripheral vertices, a step or routine 80 makes the determined connections 78 on the original grid, i.e. between the peripheral vertices in their actual position. The vertex removal process is preferably repeated at least once. The vertex removal process may be iteratively repeated until it fails to remove a substantial number of vertices. In this manner, the surface in memory 36 is retriangulated to have a lesser number of triangles. The retriangulated surface can be moved to a different memory 82 to free the memory 36 to triangulate another surface. An image processor 84 is controlled by an operator through the work station 32 to select a viewing direction. The image processor 84 then identifies the triangles visible from the viewing direction. The image processor 84 further defines a direction of illumination and a vector normal for each visible triangle. The image processor 84 then calculates or, preferably, retrieves from a look-up table, a gray scale value for each triangular region in accordance with the relative directions of the surface normal, viewing direction, and illumination direction, as is conventional in the art. Each time the operator rotates the viewing direction, the differences between the surface normals, viewing direction, and illumination direction for each visible triangle are recalculated. When the number of triangles is reduced by the present technique, often by a factor of half or even two-thirds, the calculation time is reduced analogously, such as by half or even two-thirds. This enables the image displayed on the video monitor 34 to rotate smoothly to the viewer.

Claims: An imaging system which includes a memory (24) for storing a volumetric image representation, a triangular surface processor (38) for replacing a surface of a selected volumetric region of the volumetric image representation in the image memory (24) with a triangular mesh, an image processor (84) for comparing a viewing direction, an illumination direction and a surface normal of each visible triangle of the triangular mesh to generate a corresponding image value, and an image display device (34) for converting the image values into a corresponding human readable image; a vertex merging means (40) for merging areas of triangles of the triangular mesh that have a dimension less than a preselected minimum dimension into areas of adjoining triangles, a vertex removal means (50) for removing a vertex shared by a plurality of triangles which are within a preselected tolerance of coplanar and retriangulating a polygon defined by remaining peripheral vertices of the triangles with a smaller number of triangles; means for identifying a vertex common to a group of triangles which define a pyramid; means (54) for determining an average plane of the vertices of the pyramid; means (56) for comparing deviation among surface normals of the triangles of the pyramid for a preselected maximum deviation; means (66) for determining a distance between the average plane and the common vertex to determine a height of the pyramid; means (70) for comparing the pyramid height with a preselected maximum pyramid height; means (72) for removing the common vertex of the group of triangles which define each pyramid whose surface normals diverge less than the preselected divergence and whose height is less than the preselected maximum pyramid height, characterised in that said vertex removal means (50) includes: means (76) for projecting the peripheral vertices onto the average plane to define a planar polygon; means (78) for triangulating the polygon on the average plane; and means (80) for connecting corresponding peripheral vertices in the image memory (24) in accordance with the retriangulating on the average plane. A system as claimed in claim 1 including a patient examination apparatus (10) for non-invasively examining an internal volumetric region of a patient and generating views of electronic data indicative thereof, a reconstruction processor (22) for reconstructing the views into a volumetric image representation which is stored in said image memory (24). An imaging method in which a surface of a volumetric image representation stored in memory (24) is replaced with a triangular mesh; a viewing direction, illumination direction and a surface normal of each visible triangle of the mesh are compared to generate a corresponding image value; and the image values are converted to a human-readable image; areas of triangles of the triangular mesh that have a dimension less than a preselected minimum dimension are merged into areas of adjoining triangles; vertices common to a group of triangles which define a pyramid are identified; an average plane of the group of triangles is determined; deviations among surface normals of the triangles of the pyramid are compared; the distance between the average plane and the common vertex is determined to determine the height of the pyramid; and the pyramid height is compared with a preselected maximum pyramid height; a vertex shared by a plurality of triangles which are within a preselected tolerance of coplanar is removed; and a polygon defined by remaining peripheral vertices of the triangles is retriangulated with a smaller number of triangles, characterized in that the retriangulating step includes: projecting the peripheral vertices onto the average plane to define a planar polygon on the average plane; triangulating the planar polygon on the average plane; and connecting the peripheral vertices which were projected onto the average plane in accordance with the retriangulation on the average plane. A method as claimed in Claim 3, wherein the merging step includes: comparing a distance between each pair of adjacent vertices of the triangles of the mesh with a preselected minimum distance; calculating a median position between the adjacent vertices which are separated by less than the minimum distance; placing a new vertex at the median position; and removing the pair of vertices. An imaging method as claimed in claim 3 or 4, wherein the volumetric image representation is obtained by reconstructing views of electrical data generated by non-invasively examining an internal volumetric region of a patient.

Inventors: ARATA LOUIS K, ARATA, LOUIS K.
Applicants: PICKER INT INC, PICKER INTERNATIONAL, INC.
Application year: 1995
Main classification: G06T17
IPC codes: G06T17, A61B6, A61B5, G06T15, G06T1