The current standard for stereotactic imaging of intracranial targets is magnetic resonance imaging (MRI). Despite MRI being superior to computed tomography (CT) in terms of better imaging of anatomical brain structures the question remains regarding to geometric uncertainties (distortions) of the final MRI image. The precision of geometric uncertainty of stereotactic localization and volume determination with the use of Leksell’s stereotactic frame and Leksell’s MRI indicator box and Siemens 1T MAGNETOM Expert scanner was tested in this study. Measurements performed with the use of a special phantom simulating the patient’s head with different target volumes assessed distortions in each spatial stereotactic coordinate and volume determination errors. Average values of geometrical distortion determined by these measurements demonstrated satisfactory precision for stereotactic MRI localization (average distortion 0.5 mm). There was no significant dependence of the magnitude of these distortions on: MRI investigating sequence, MRI slice orientation, spatial position of measured point in the investigated volume. The average percentage error in volume due to technical capacities of imaging and the treatment planning system was 10 % for smaller volumes (volumes smaller than 2000 mm 3 ) and 5 % for larger volumes (volumes larger than 10 000 mm 3 ). Subjective errors of volume determination were in this study expressed as a one standard deviation of average volume calculated, based on contours recorded by six independent observers (physicians) for each of five selected volumes (meningioma, metastases, acoustic neurinoma, pituitary adenoma and arteriovenous malformation). Minimal standard deviation for volume recorded by six independent physicians was 5 % (for metastases) and maximal 40 % (for arteriovenous malformation).

        Key words: magnetic resonance imaging - stereotactic methods - quality assurance - phantom measurements