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The three-dimensional (3D) object data obtained from a CT scanner usually have unequal sampling frequencies in the x-, y- and z-directions. Generally, the 3D data are first interpolated between slices to obtain isotropic resolution, reconstructed, then operated on using object extraction and display algorithms. The traditional grey-level interpolation introduces a layer of intermediate substance and is not suitable for objects that are very different from the opposite background. The shape-based interpolation method transfers a pixel location to a parameter related to the object shape and the interpolation is performed on that parameter. This process is able to achieve a better interpolation but its application is limited to binary images only. In this paper, we present an improved shape-based interpolation method for grey-level images. The new method uses a polygon to approximate the object shape and performs the interpolation using polygon vertices as references. The binary images representing the shape of the object were first generated via image segmentation on the source images. The target object binary image was then created using regular shape-based interpolation. The polygon enclosing the object for each slice can be generated from the shape of that slice. We determined the relative location in the source slices of each pixel inside the target polygon using the vertices of a polygon as the reference. The target slice grey-level was interpolated from the corresponding source image pixels. The image quality of this interpolation method is better and the mean squared difference is smaller than with traditional grey-level interpolation.

Effects of neutron irradiation and subsequent thermal annealing on positron-lifetime spectra in p-type silicon have been studied. In unirradiated samples, two annihilation rates (4.13 X 10 sec ' with an intensity of 98% and 7.14 )& 10 sec ' with an intensity of 2%) are observed. The annihilation rate of the dominant component becomes smaller upon irradiation. The effect saturates at high integrated neutron flux,~1 X 10" n/cm', and disappears completely upon thermal annealing at 400-500'C. The annealing behavior is dependent on the integrated neutron flux. These results show that some neutron-induced defects act as positron traps at room temperature. Our study also gives evidence that high-order vacancy defects are formed during annealing near 300 C, The mean electron density of the positron-sensitive defects (positron traps) is estimated from the saturation value of the annihilation rate to be about 35%%uo less than that in the perfect crystal. The diffusion coefficient and the mobility of thermalized positrons in silicon are estimated to be about 0.16 cm'/sec and 6.4 cm'/secV, respectively.