| Research Abstract |
(Purpose and Methods) We investigated the geometric error of CT/MR images by means of water phantom measurement. First of all, the water phantom (Gammasonic Co.Ltd., Australia) was scanned with the Toshiba X-Vision CT under 512x512 matrix size, 2 mm slice thickness, and 2 mm slice interval. We compared the distances between some points measured by the software installed in the CT scanner with real distance measured by calipers. These measurement was undertaken three times using different points for each X and Y direction (parallel to the scanning plane). The distance for Z direction (parallel to the body axis) was calculated by reading the distance of the table slide in the serial CT scanning. Next, using similar manner, we also measured the distance on MR images between points of the same phantom. In contrast to CT images, MR images can be obtained at any direction. So phantom image was obtained for all of X-Y, Y-Z, and Z-X directions. GE Signa Horizon 1.5 T with 512x512 matrix size, 3 mm slice thickness, and 3 mm slice interval was the MR scanner we tested.
(Results) Geometrical errors in the CT images were within 1 mm in each direction without any unacceptable distortion in the X-Y plane. In contrast, geometric errors in MR image were as much as 5 mm, and with unacceptable distortion especially at the periphery.
(Conclusions) CT images are available without correction for the stereotactic irradiation which requires submillimeter accuracy. In contrast, there is unacceptable distortion in MR images, which requires some matching technique with CT to incorporate in the stereotactic irradiation system.
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