A TECHNIQUE OF MEASURING THE PRECISION OF AN MR-GUIDED STEREOTAXIC INSTALLATION USING ANATOMIC SPECIMENS

PURPOSE: To develop a method for direct measurement, using anatomic sp ecimens, of the precision of MR-guided stereotaxic location and to des cribe its application to a 1.5-T MR unit with a Leksell stereotaxic fr ame. METHODS: Small pieces of gelfoam (1X1X1 mm), soaked in gadopentet ate dimeglumine, were stereotaxically introduced into formalin-fixed h uman heads using a Leksell D (three experiments) or G (nine experiment s) stereotaxic frame. The head and the frame were then introduced into a 1.5-T MR unit. The target coordinates (as set on the stereotaxic fr ame by one investigator) were then compared with the MR-determined ste reotaxic coordinates (calculated independently by another investigator ). The imprecisions Ex, Ey, and Ez in each direction were defined as t he differences between the calculated and. the chosen coordinates. RES ULTS: Regarding the three targets studied with the D frame, mean impre cision Ex was 1.08 +/- 0.50 mm (mean +/- SEM), Ey 0.83 +/- 0.58 mm, an d Ez 0.75 +/- 0.25 mm. For the nine targets studied with the G frame, Ex was 0.48 +/- 0.17 mm, Ey 0.69 +/- 0.14 mm, and Ez 0.82 +/- 0.13 mm. Statistical analysis of the results showed no significant difference between Ex or Ey and half the size of a pixel, indicating that, in the axial plane, stereotaxic MR precision is limited only by pixel size. A statistically significant difference was observed in the coronal pla ne between Ez and half the size of a pixel, but it must be stressed th at Ez remained smaller than 1 mm. CONCLUSION: MR-guided stereotaxic lo cation can be used with confidence for most diagnostic, functional, an d therapeutic procedures.