gms | German Medical Science

Objective: We report our experience with intra-operative magnetic resonance imaging (iMRI) using the low field PoleStar N20 scanner.

Methods: The PoleStar N20 (0.15 Tesla) magnet was used for iMRI in 58 cases from January 2007 to November 2008, for intra-cerebral tumors (mainly gliomas) and pituitary lesions. At least two MRI sessions were performed during surgery, one before and at least one after resection. Standard MRI (1, 1.5 or 3 Tesla) was obtained post-operatively. IMRI was T1 weighted without and with gadolinium, or T2 weighted (T2w) including FLAIR. Navigation was performed simultaneously on pre- and intra-operative MRI. T2 and FLAIR parameters were mainly used in non-enhancing lesions, found in 6 of the patients (3 female, 3 male; mean age 24.5, range 17–33): 3 diffuse WHO grade II astrocytomas, 1 oligoastrocytoma, 1 oligodendroglioma, and one lesion finally diagnosed to be gliotic without any evidence for a tumor. All lesions were in the right frontal lobe, except for one cerebellar astrocytoma.

Results: The quality of iMRI was high and the obtained images allowed precise neuronavigation. They showed the complete region of surgical interest in all 58 cases. Initially, positioning of the patient could be challenging, but the learning curve was steep and rapidly allowed integration of iMRI into regular surgical activity. In the 6 cases with non-enhancing lesions, neuronavigation was based on T2w and FLAIR iMRI to delineate the excision zone, and, depending on the relative quality of the initial intraoperative images, either T2w or FLAIR images were chosen for resection control. Overall, FLAIR settings provided best contrast and were used in 5 of the cases. In 2 cases, resection was completed on the basis of iMRI and a third acquisition obtained at the end of the operation for final resection control.

Conclusions: In our experience, 0.15 Tesla iMRI is highly useful. It allows for precise localization of normal and abnormal structures, and the gain in spatial reliability largely compensates the loss of resolution compared to the preoperative standard diagnostic MRI. In addition to the most commonly cited advantage of iMRI, i.e. the possibility to account for brain shift, we confirm its usefulness in the assessment of the extent of tumor resection, not only for gadolinium-enhancing tumors, but also for low grade glioma (and similar non-enhancing lesions) which are particularly well demonstrated by FLAIR imaging.