A phantom study of the geometric accuracy of computed tomographic and magnetic resonance imaging stereotactic localization with the Leksell stereotactic system

OBJECTIVE: To assess the spatial accuracy of magnetic resonance imaging (MR I) and computed tomographic stereotactic localization with the Leksell ster eotactic system. METHODS: The phantom was constructed in the shape of a box, 164 mm in each dimension, with three perpendicular arrays of solid acrylic rod, 5 mm in di ameter and spaced 30 mm apart within the phantom. In this study, images fro m two different MRI scanners and a computed tomographic scanner were obtain ed using the same Leksell (Elekta Instruments, Stockholm, Sweden) head fram e placement. The coordinates of the rod images in the three principal plane s were measured by using a tool provided with Leksell GammaPlan software (E lekta Instruments, Norcross, CA) and were compared with the physical phanto m measurements. RESULTS: The greatest distortion was found around the periphery, and the le ast distortion (<1.5 mm) was present in the middle and most other areas of the phantom. In the phantom study using computed tomography, the mean value s of the maximum errors for the x, y, and z axes were 1.0 mm (range, 0.2-1. 3 mm), 0.4 mm (range, 0.1-0.8 mm), and 3.8 mm (range, 1.9-5.1 mm), respecti vely. The mean values of the maximum errors when using the Philips MRI scan ner (Philips Medical Systems, Shelton, CT) were 0.9 mm (range, 0.4-1.7 mm), 0.2 mm (range, 0.0-0.7 mm), and 1.9 mm (range, 1.3-2.3 mm), respectively. Using the Siemens MRI scanner (Siemens Medical Systems, New York, NY), thes e values were 0.4 mm (range, 0.0-0.7 mm), 0.6 mm (range, 0.0-1.0 mm), and 1 .6 mm (range, 0.8-2.0 mm), respectively. The geometric accuracy of the MRI scans when using the Siemens scanner was greatly improved after the impleme ntation of a new software patch provided by the manufacturer. The accuracy also varied with the direction of phase encoding. CONCLUSION: The accuracy of target localization for most intracranial lesio ns during stereotactic radiosurgery can be achieved within the size of a vo xel, especially by using the Siemens MRI scanner at current specifications and with a new software patch. However, caution is warranted when imaging p eripheral lesions, where the distortion is greatest.