Development of a three-dimensional CT simulator and its clinical application

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In 1996, a new helical CT scanner was added to our CT simulator system. This CT scanner facilitates not only rapid CT scanning by the conventional parallel scanning method but also helical and continuous scanning. However, the accuracy of the reconstructed helical CT image for radiotherapy treatment planning (RTP) is not established. The usefulness and limitations of reconstructed helical CT images were demonstrated using an acryl phantom.A conical phantom measuring 100-mm in diameter was specially constructed using acryl. For identification of marginal distortion in the reconstructed CT image, the conical phantom was scanned using 5 mm and 10 mm collimation with 5 or 10mm/sec table speed for 1 sec/rotation with scanning parameters of 120 kVp and 50 mA.Image reconstruction was performed with both the 180-degree compensation algorithm (SHI algorithm) and the 360-degree compensation algorithm (SFI algorithm). The CT window and level was fixed at 200 Hounsfield units (HU), and 120 HU,resp … More ectively. A column phantom was also used for measuring the gradient in the CT number at the marginal surface of different materials. In the study of a conical phantom, the distorsion (D) was measured as a difference netween a larger radius (R) and a smaller radius (r) scanned at the level of 100 mm in radius. In the study of a column phantom, the CT number of each CT slice was measured at every 1 mm intervals.As a result, the reconstructed shape of the margin was competely round when the conical phantom was scanned by the routine conventional CT method. However, distortion appeared when it was reconstructed from helical CT scans. The maximal distortion at a eadius of 100 mm was 2 mm when the phantom was scanned at 5 mm or 10 mm collimation thickness and 10 mm/sec table speed and reconstructed with the SHI algorithm. However, the distortion was below the limits of detection when the phantom was scanned at 5 mm or 10 mm collimation thickness and 5 mm/sec table speed and reconstructed with the SFI algorithm. The gradient of the change in the CT number was larger at the boundary between the acryl and air than at the boundary between the acryl and water. The gradient was also larger at the 5 mm thickness than 10 mm thickness, and at 5 mm/sec table speed than at 10 mm/sec table speed.In conclusion, helical CT parameters should be set up very cautiously for radiotherapy treatment planning because they are subject to image distortion and CT number changes.On the other hand, a network was developed netween the CT simulator and a treatment planning machine. The three-dimensional treatment planning can be possible for clinical application. The clinical significance of the helical CT simulator will be evaluated. Less