gms | German Medical Science

Objective: Magnetic Resonance Imaging (MRI) is replacing Computer Tomography (CT) as the "gold standard" for stereotactic transformations. Superior tissue contrast and the lack of radiation exposure led to an increase in solely MRI-based stereotactic procedures. MR-Imaging techniques are prone to spatial distortion artefacts which can lead to inaccuracies in stereotactic procedures. Modern MRI systems provide built-in distortion correction algorithms that may ameliorate this problem, but the quality of these algorithms has not been established in practice. The present study investigates the effect of the stereotactic transformation workflow (MRI-only against CT/MRI image fusion) on the observed deviation from the gold standard using a 1.5T, 3T and an investigational 7T-scanner in a stereotactic scenario.

Method: A cylindrical phantom with seven preset points was mounted on a stereotactic frame with a combined MRI/CT localizer. We then performed a CT- and three MRI-scans (Siemens Espree (1.5T), Verio (3T) and investigational 7T Siemens MR-scanner) to acquire an isotropic (1 x 1 x 1 mm) dataset. Two commonly used 3D-sequences (VIBE, MPRAGE) were acquired and automatic distortion correction was performed by the scanner hardware. Using stereotactic planning software (Praezis Plus), global stereotactic transformation of all thirteen datasets was performed. Two stereotactic planning workflows used in clinical practice (MRI-only vs. CT/MR-image fusion) were analysed. Finally, coordinates of all seven preset points were independently identified and recorded by two investigators and compared to the gold standard (CT).

Results: In a direct comparison of the two workflows, the CT/MRI image fusion shows a slight advantage over the MRI-only workflow. By using CT/MRI image fusion, a minimum positional error of 0.80 ± 0.24 mm (1.5T VIBE) can be achieved. The comparable 1.5T MRI-only sequence revealed an error of 1.02 ± 0.39 mm. Using the image fusion for the 3T MRI (VIBE-sequence) an error of 1.03 ± 0.44 mm was found whereas the MRI-only sequence yielded errors of 1.78 ± 0.89 mm. The effect was more pronounced when using VIBE compared to MPRAGE sequences. In ultra-high-field (7T) MR imaging distortion correction algorithm introduced even higher positional errors.

Conclusions: Modern MRI built-in distortion correction algorithms are efficient in reducing spatial image distortion. For stereotactic procedures that require the highest level of precision, however, an additional CT fusion should be performed.