The present invention relates to three-dimensional image producing methods and apparatus which produce an image by projection of a three-dimensional shape onto a two-dimensional plane on the basis of three-dimensional data which defines the three-dimensional shape, and display the image.
Conventionally, three-dimensional image producing techniques have been used widely which produce a two-dimensional image by projection of a three-dimensional shape determined on the basis of the values of three-dimensional data or volume data onto a two-dimensional projection plane. Such image is, of course, a two-dimensional image, but generally called a three-dimensional image. This term is used also in the description of this specification to denote a similar image.
One of conventional techniques which produce such a three-dimensional image is the volume rendering described in M.Levoy, "Display of Surface from Volume Data", IEEE CG & A, vol. 8, No. 5, pp. 29-37 (1988)".
In this technique, it is assumed that rays of light emitted from a two-dimensional projection plane are transmits through voxels each as a unit of volume data while undergoing attenuation. A quantity of attenuation which the rays of light from the respective voxels undergo before they arrive at the projection plane is calculated on the side of the projection plane to thereby obtain the brightness of the voxels projected to the projection plane.
In the technique described in Terry S. Yoo et al., "Direct Visualization of Volume Data", IEEE CG & A, vol. 12, pp. 63-71 (July 1992), an opacity is set in each voxel in dependence on its value. Respective parts of a three-dimensional shape are superposed as translucent to produce a two-dimensional image.
In the technique described in "Surface Rendering", IEEE CG & A, vol. 10, pp. 41-53 (March, 1990), volume data is divided along a plane. Two-dimensional images of different ones selected from among parts of each of the two volume data halves are formed, and combined into a complete image, which is then displayed. For example, a two-dimensional image is displayed which includes a combination of a right-hand half of a regular body surface (skin) and a left-hand half composed of bones.
Generally, in order to produce a three-dimensional image, much processing time is required because much data is required to be processed. Conventionally, various techniques have been proposed to speed up the production of a three-dimensional image.
For example, in the technique described in M. Levoy, "Efficient Ray Tracing of Volume Data", ACM Trans. on Graphics, vol. 9, No. 3, pp. 245-261 (1990), transparent voxels, i.e., having an opacity of 0, which do not influence the production of a two-dimensional image are put in a hierarchical structure. Those voxels are not subjected to processing to thereby speed up the production of a three-dimensional image. Processing is also speeded up by enhancing the resolution of highly changing voxels and decreasing the resolution of less changing ones.
In the technique described in J.K.Uduepa et al., "Shell Rendering: Fast Volume Rendering and Analysis of Fussy Surface", SPIE, vol. 1653, Image Capture, Formatting and Display, pp. 35-43 (1992), new volume data on a surface alone is produced from the original volume data and processed to speed up the whole processing.
The techniques for producing a three-dimensional image in a medical field are used to observe the inside of a human body from various angles and view points mainly on the basis of three-dimensional measured data on the human body obtained with an X-ray CT and/or an MRI device.
In those applications, the observation of the inside of a human body is performed, using that measured human body data values obtained with an X-ray CT and/an MRI device differ in value in terms of bones and internal organs. The observation of the inside of a human body is performed, for example, by displaying voxels alone which have measured data of more than a predetermined threshold, or by displaying the respective voxels on the basis of the respective opacities of the voxels set and changed in accordance with the measured data values, using the techniques described above in "Display of Surface from Volume Data", IEEE CG & A, vol. 8, No. 5, pp. 29-37 (1988) and "Direct Visualization of Volume Data", IEEE CG & A, vol. 12, pp. 63-71 (July, 1992).
The technique using the threshold, however, cannot extract and display only a bone whose measured data value is large and a blood vessel whose measured data value is smaller to know their positional relationship even when it is desired to do so. The technique for setting and changing the opacity is very difficult to set an appropriate opacity. When necessary and unnecessary parts of an object to be examined are displayed in a superimposed manner, a desired part is difficult to view.
Since the technique described in "Surface Rendering", IEEE CG & A, vol. 10, pp. 41-53 (March, 1990) displays, for example, a bone and a blood vessel as different parts of a body, their positional relationship cannot be gripped.
Since the difference in measured data value between a blood vessel and its neighboring tissues is small, extraction and display of data on the blood vessel is difficult. In order to avoid this problem, injection of a contrast medium into a blood vessel and then data measurement have been widely performed when an X-ray CT is used. This allows extraction of the image of a blood vessel. When the wall of the blood vessel is to be displayed in terms of its inside and outside, however, troublesome and complicated operations would be required, inclusive of a change of the relationship between a measured data value and an opacity.
On the other hand, the conventional techniques which speed up the production of a three-dimensional image have the following problems:
In the technique described in M.Levory, "Efficient Ray Tracing of Volume Data", ACM Trans. on Graphics", vol. 1, No. 3, pp. 245-261 (1990), transparent voxels are required to be put in a hierarchical structure before production of a three-dimensional image. Thus, when display is desired while changing the opacity sequentially, the effect of speeding up the processing cannot be greatly expected. The technique of switching the resolution cannot prevent a deterioration in the quality of the whole image.
In the technique described in "Shell Rendering: Fast Volume Rendering and Analysis of Fuzzy Surface", SPIE, vol. 1653, Image Capture, Formatting and Display, pp. 35-43 (1992), volume data is required to be created for each of all the objects which are each handled as a display unit (the surfaces of a body, a bone, etc.). Thus, each time an object to be displayed is changed, much time is taken for its pre-processing.
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