JP3814932B2 - 3D image generator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a three-dimensional image generation apparatus that performs rendering from three-dimensional volume data obtained by an X-ray CT apparatus, an MRI apparatus (nuclear magnetic resonance imaging apparatus), or the like to create a three-dimensional image.
[0002]
[Prior art]
In the medical field, an image for diagnosing a human being is taken using an X-ray CT apparatus or an MRI apparatus. Although this image is planar, digital data arranged in three dimensions can be obtained by obtaining a number of images in a direction perpendicular to the plane. Represents.
[0003]
As a technique for generating and displaying a stereoscopic image from such volume data, surface rendering for generating and displaying a surface has been conventionally known. In this method, regions are extracted by dividing voxel values according to threshold values, and the boundary of the region is shaded. Furthermore, by performing semi-transparent display according to the voxel material content rate from volume data without performing binarization by threshold value, the unnaturalness due to aliasing and quantization seen in surface rendering is eliminated. Volume rendering is also known (R. A. Drebin L. Hanrahan, “Volume Rendering”, Computer Graphics (Proc. SIGGRAPH), Vol. 22, No. 3 (Aug. 1988) pp. 65-74). .
[0004]
[Problems to be solved by the invention]
However, in surface rendering, the threshold value is divided into 100% and 0% where the voxel is located, so if you want to express details in small shades of objects, grain noise will appear. There is a problem that it comes out or becomes a dotted line in a thin blood vessel.
[0005]
In addition, volume rendering eliminates the inconvenience of surface rendering, but there are many parameters that should be set, such as opacity, and is complicated. What value should be given as a parameter? There is a problem that it is unclear whether it can be obtained. Therefore, in practice, many cuts and tries are required, and experience and intuition are required.
[0006]
In view of the above, an object of the present invention is to provide a three-dimensional image generation apparatus which is easy to set parameters and is improved so that an intended image can be easily obtained.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, in the three-dimensional image generating apparatus according to the present invention, the threshold value and the translucent width relating to the voxel value are input, and the input threshold value and the translucent width are represented Means to convert voxel values to opacity using straight line as conversion characteristics, normal vector from volume data, boundary surface strength that is absolute value of normal vector, unit normal vector, and above input translucent Means for obtaining a normalized boundary surface strength obtained by normalizing the above-mentioned boundary surface strength by width, and volume rendering means for performing volume rendering processing using the normalized boundary surface strength for each voxel and the above opacity It is characterized by being.
[0008]
The threshold value and translucency width related to the voxel value are input, and the straight line represented by these is used as a conversion characteristic from the voxel value to the opacity, and the conversion from the voxel value to the opacity is performed according to the conversion characteristic. . On the other hand, the normal vector, the boundary surface strength, which is the absolute value of the normal vector, and the unit normal vector are obtained from the volume data, and further normalized by normalizing the boundary surface strength with the above-mentioned translucent width. The interface strength is required. Then, volume rendering processing is performed using the normalized boundary surface strength obtained for each voxel and the opacity. In this case, the gradient of the voxel value (boundary surface strength) is normalized by the translucent width, so when the voxel value range that should be made translucent is changed by changing the translucent width, the change in the image is extracted And the threshold does not change near the middle of its translucent range. Therefore, an image that correctly reflects the set threshold value is obtained, and appropriate opacity can be set only by setting the threshold value and the translucent width. This means that parameter setting for volume rendering can be performed as easily as surface rendering.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described in detail with reference to the drawings. As shown in FIG. 1, the three-dimensional image generation apparatus according to the present invention includes a volume rendering device 1, an input device 2, and an image display device 3. The volume rendering apparatus 1 is a computer-based image processing apparatus that performs image processing on volume data received from an X-ray CT apparatus or MRI apparatus online or offline, and can be configured as dedicated hardware or software. It is. The input device 2 includes a keyboard and a mouse. The image display device 3 displays the input assist image and the generated image. These can be configured as an apparatus integrated with an X-ray CT apparatus or an MRI apparatus, or as a separate apparatus.
[0010]
The volume data is a three-dimensional array of voxel values as shown in FIG. The input device 2 inputs a threshold value T and a translucent width W as shown in FIG. That is, the translucent width W is a range of voxel values that are desired to be displayed in a translucent (opacity range of 0% to 100%), and the threshold value T represents a voxel value having an opacity of 50%. A conversion table from voxel values to opacity as shown in FIG. 2A is formed by the inputted T and W. Normally, the conversion characteristics can be freely set as shown in FIG. 2B, but doing so makes the result complicated and unclear, so the straight lines defined by T and W as described above. Conversion characteristics.
[0011]
In addition to normal volume rendering processing, the volume rendering apparatus 1 performs conversion from voxel values to opacity according to the linear conversion characteristics described above. Further, for a voxel with an opacity greater than 0, a normal vector N, a boundary surface strength | N |, a unit normal vector N / | N |, and a boundary surface strength | N | / W normalized by W calculate.
[0012]
If the values of voxels A, B, C, D,... Arranged along the line of sight as shown in FIG. 3 are 0, 50, 100, 150,..., T = 150 and W = 200. In the setting, the opacity is greater than 0 on the right side from voxel B. The normal vector N of each voxel is obtained by calculating the gradient of the voxel value and its inclination direction from the neighboring voxel values. The boundary strength | N | is the absolute value of the normal vector N, that is, the absolute value of the gradient. The unit normal vector is obtained by N / | N |. In the example of FIG. 3, the boundary strength of any voxel is 50 per distance 1 (sampling interval).
[0013]
By dividing the boundary strength | N | by the set semi-transparent width W, the boundary strength | N | / W normalized by the semi-transparent width is obtained. Thus, normal volume rendering is performed using the normalized boundary strength and opacity obtained for each voxel. At this time, since the normalized boundary surface strength is multiplied by the opacity data, the normalized boundary surface strength corresponds to the existence probability of the voxel (the material content expressed by opacity). For the shading, the above-mentioned unit normal vector is used, and the method of Phong can be used (Phong, Buitong, “Illumination for Computer Generated Images”, CACM 18 (6) (June 1975) pp. 311-317).
[0014]
In this case, if the threshold value T is fixed and the translucent width W is increased or decreased, the number of voxels integrated along the line of sight increases or decreases, but the normalized boundary surface strength decreases in inverse proportion to the increase and decrease. It grows and grows. Therefore, the change in the image due to the operation of W is only the ambiguity of the region to be extracted, and the fact that the threshold value T is near the center of the translucent width W does not change (strictly speaking, along the line of sight in the light source model). Since the integration involves attenuation of light corresponding to the opacity at each voxel, the threshold T does not come at exactly 50% opacity). Therefore, an image that correctly reflects the set threshold value T is obtained, and appropriate opacity can be set only by setting the threshold value T and the translucent width W.
[0015]
Actually, first, the threshold value T is set while displaying the original image on the image display device 3 and observing it. The threshold value T in this case can be specified with the same feeling as in the surface rendering. Next, the translucent width W is adjusted according to the contrast, S / N ratio, etc. of the object to be extracted contained in the original image. When the S / N ratio of the portion to be extracted is poor, the setting / adjustment is performed so that W is larger than the noise amplitude. In this way, by observing the original image, the appropriate threshold value T and translucent width W can be found, and the opacity can be easily set. Moreover, since the boundary strength is normalized by the translucent width W, the obtained image will correctly reflect the set threshold value T even if W is changed, so it is necessary to repeat cut and try. Disappears.
[0016]
【The invention's effect】
As described above, according to the three-dimensional image generation apparatus of the present invention, it is possible to set parameters for volume rendering as easily as surface rendering, and to easily obtain a desired three-dimensional image. Can do.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an embodiment of the present invention.
FIG. 2 is a graph showing conversion characteristics from voxel values to opacity.
FIG. 3 is a diagram schematically showing the arrangement of voxels.
[Explanation of symbols]
1 Volume rendering device 2 Input device 3 Image display device

Claims (1)

  Means for inputting a threshold value and translucency width relating to the voxel value, means for converting voxel values to opacity using the straight line represented by the input threshold value and translucency width as a conversion characteristic, and volume Means for obtaining normalized boundary strength obtained by normalizing the boundary strength from the normal vector, the boundary strength that is the absolute value of the normal vector, the unit normal vector, and the input semitransparent width from the data. And a volume rendering means for performing a volume rendering process using the normalized boundary surface strength for each voxel and the opacity described above.

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