Distortion of three-dimensional computed tomography. Effect of the reconstruction function

Three-dimensional computed tomography is distorted in the Z-direction because of the partial volume effect. This distortion depends on slice width, helical scanning pitch, interpolation, reconstruction function, and reconstruction interval. We studied the effect of the reconstruction function on distortion by scanning high- and low-contrast phantoms in which an acrylic ball was set on expandable polystyrene in Japanese isinglass. We found that, if the acrylic ball was larger than the slice width, a three-dimensional image could be reconstructed without distortion by setting the threshold between the acrylic ball's CT numbers and the CT numbers of the acrylic ball's surroundings (air or Japanese isinglass). The image was not dependent on the reconstruction function. However, the three-dimensional size of the acrylic ball was smaller than the ball's actual size. Further, with the reconstruction function in which the edge was emphasized, distortion and the change caused by the threshold were larger than with the normal reconstruction function. This was because the style of the profile curve on the axial-transverse direction (X-Y direction) was affected by emphasizing the edge. (author)