Development of a unique phantom to assess the geometric accuracy of magnetic resonance imaging for stereotactic localization

OBJECTIVE: To test the spatial accuracy of coordinates generated from magne tic resonance imaging (MRI) scans, using the Brown-Roberts-Wells head frame and localizer system (Radionics, Inc., Burlington, MA). METHODS: An anthropomorphic head phantom, consisting of a two-dimensional l attice of acrylic spheres (4-mm diameter) spaced 10 mm apart and embedded i n a brain tissue-mimicking gelatin-agar gel, was constructed. The intersphe re distances for the target lattice positions in MRI and computed tomograph ic scan sets were compared. The data sets were fused, and differences in fi ducial marker and intraphantom target positions were measured. RESULTS: Intersphere distances were identical for the MRI and computed tomo graphic scan sets (10 +/- 0.1 mm). Differences in fiducial marker positions [maximal lateral difference, 0.97 mm; mean absolute lateral difference, 0. 69 +/- 0.22 mm; maximal anteroposterior (AP) difference, 1.99 mm; mean abso lute AP difference, 1.29 +/- 0.67 mm] were correlated with differences in i ntraphantom target positions (maximal lateral difference, 0.83 mm; mean abs olute lateral difference, 0.28 +/- 0.24 mm; maximal AP difference, -1.97 mm ; mean absolute AP difference, 1.63 +/- 25 mm; maximal vertical difference, -0.73 mm; mean absolute vertical difference, 0.34 +/- 0.21 mm). This sugge sted that improper fiducial rod identification and the subsequent transform ation to stereotactic coordinate space were the greatest sources of spatial uncertainty. CONCLUSION: With computed tomographic data as the standard, these differenc es resulted in maximal and minimal composite uncertainties of 2.06 and 1.17 mm, respectively. The measured uncertainties exceed recommended standards for radiosurgery but allow the possible use of MRI-based stereotactic treat ment planning for certain intracranial lesions, if the errors are corrected using appropriate software. Clinicians must recognize that error magnitude s vary for different systems, and they should perform systematic, scheduled , institutional error analyses as part of their ongoing quality assurance p rocesses. This phantom provides one tool For measuring such variances.