[Three-dimensional CT angiography of the portal vein using multiple threshold display]

To evaluate the usefulness of three-dimensional (3D) CT angiography of the portal vein obtained by using a multiple threshold display, 3D reconstructions were performed in 15 patients. The CT scanner employed was the Toshiba Xvigor. A volume of 150 ml of lopamidol 300 mg1/ml was administered at 3.3 ml/sec intravenously. Portal venous phase helical scanning was initiated 80 seconds after the start of the injection. Helical CT data were acquired using up to 25 continuous 1.0-sec rotations during a single breath-hold with an X-ray beam width of 7 mm and a couchtop movement speed of 7 mm/sec. Axial images were reconstructed at a section interval of 3 mm. Both the shaded surface display (SSD) and multiple threshold display (MTD) were generated by using Xtension with Sparc20. MTD was performed as follows. First, voxels, with higher CT values than that of liver parenchyma, were selected. Then, selected voxels were divided into eight parts, which were each assigned gradations of white to grey. The highest part of selected voxels, with higher CT values than that of the second branch of the portal vein, were set to white and a transparency of 0%. The other seven parts were each assigned transparencies of more than 0%. MTD images were compared with SSD in 15 cases by two radiologists. MTD images were superior to SSD images in quality, because MTD diminished surrounding artifacts due to liver parenchyma and enabled small vessels to be depicted clearly. Based on the above results, it was considered that MTD was a useful method for 3D CT angiography using enhanced helical CT.