gms | German Medical Science

Despite technical advances in pituitary surgery including intraoperative fluoroscopy, operating microscope and neuronavigation leading to improvements in visualisation and conceptualization of the transphenoidal approach to pituitary tumours, the visualisation of supra- and parasellar structures and the intraoperative assessment of the extent of tumour resection remains difficult. In particular for large suprasellar adenomas causing compression of the optic pathway the extent of resection is difficult to assess and is sometimes largely overestimated by the surgeon. This was demonstrated after introducing intraoperative magnet resonance imaging (iMRI) into the operating theatre. Various intraoperative MRI- systems are currently used having each specific advantages and disadvantages. In principle these systems differ in the magnetic field strength (high and low field MRI) and in the way of scanning the patient. Thus either the patient has to be moved into the magnet or the magnet is moving to the patient. The ability to assess the extent of resection immediately during the operation is very appreciated by the surgeon and based on iMRI either resection can be continued if residual tumour is still accessible to further resection or appropriate decompression can be documented and the operation can be terminated. Therefore it was the objective to review the utility, feasibility and the preliminary experience of the PoleStar N20 – a new generation of intraoperative low- field strength magnetic resonance imaging systems – during resection of pituitary adenomas.

To introduce the Pole Star N20 iMRI (Odin, Israel) into our standard neurosurgical theatre a complete shielding of the theatre had to be performed. The magnet is stored in a separated room and is moved outside immediately before surgery. The technical features of the Polestar N10, which resembles the basic features of the N20, are described elsewhere [Ref. 1]. Differences between the PoleStar N20 and N10 include an increased magnetic strength (0.15T vs. 0.12T), a larger gap between the magnets and optimized MRI sequences. The magnet is built in a vertical open construct to place the patients head between both magnet poles (Figure 1 [Fig. 1]). Therefore a specifically designed titanium 3 point pin head fixation clamp was used (Odin, Medical Technologies Ltd; Yokneam, Israel). Additionally a reference star for navigation was fixed at the head holder without restricting the movement of the magnet during the scanning procedure. For image quality it is crucial to place the head in the centre of the magnet to generate good images. After this precise head positioning the magnet is moved just under the operating table placed directly underneath the patients head.

The first intraoperative imaging was performed after sharp head fixation was finished and the magnet was moved upwards. This so called preoperative scan was performed in two steps. First for head positioning control a 8s scan was done using the e-steady sequence (mixed T1/ T2 contrast 8mm slices). Afterwards for navigation and tumour visualization a coronar T1- weighted gadolinium contrast enhanced scan was performed (0.4ml Gd/ kg body weight, 3-4mm slice thickness, 6-7 minute scan time). If the image was completed the magnet was moved under the table and kept there until further images are required. After disinfection and draping surgical procedure was started by use of the navigation system based on a 3 planar reconstruction of the previous scan. Surgery was performed via a standard transnasal transphenoidal approach to the tumour using a standard microscope and normal ferro magnetic instruments. After the surgeons impression of complete resection of the adenoma a T1 weighted Gd enhanced coronar scan was repeated. Therefore the magnet was moved upwards in the same position as for the first scan to confirm surgical resection. If tumour remnants were visible on the intraoperative MRI surgical resection was continued until further scan(s) documented complete resection or the operation had to be finished because of the inability to further remove tumour due to tumour adherence or infiltration of the cavernous sinus. Tumours extending into the cavernous sinus or with an extreme adherent tumour capsule were not considered for complete removal.

12 patients (8 male 4 female) mean age 55.5 ± 13 years have been operated for a pituitary adenoma in a standardized microsurgical procedure since September 2004. The adenoma was located purely intrasellar in 2 patients and showed suprasellar extension with or without compression of the optic chiasm in 10 patients. Endocrinological examination revealed a non secreting adenoma in 8 patients and 4 patients had hormone secreting tumours (3 GH secreting and 1 ACTH secreting tumour). All patients had preoperative 1.5T contrast enhanced MRI for diagnostic reasons.

Image quality was appropriate to visualize optic chiasm, pituitary stalk, carotid arteries, adenomas and tumour remnants in all patients. However, intraoperative MRI was not repeated in 1 patient because of a short, wide neck of a young male. In this particular case we did not want to apply to much pressure on his shoulders to place his head in the centre of the magnet. Complete tumour resection, as judged by preoperative 1.5T MRI was considered to be possible in 7 patients. In 6 patients resection was total as documented by the first intraoperative post-resection scan. In 1 patient resection could be continued to achieve complete resection as documented by a second scan. Therefore the operative aim was achieved in all 7 patients with complete accessible tumour removal. However subtotal resection was found in 4 of 11 patients (36.6%) patients and operation was continued in these 4 patients. However in only 1 patient a further resection could be accomplished. Therefore the goal of surgery was achieved in all patients and documented by iMRI. Neurosurgical work flow is not affected by the use of the iMRI including positioning of the patients. The surgical procedure can be performed using instant online navigation without registration and avoiding X-ray exposition to patients and operative staff.

11 of the 12 patients had 6 months follow up. 1.5T MRI confirmed the complete resection in 6 of 7 patients and demonstrated the intracavernous tumour rest in the 4 patients with residual tumour seen during iMRI scanning. All patients with visual disturbance (5 of 12) showed improvement of vision and visual field. Endocrinological follow up demonstrated a complete remission in all 4 patients with functioning tumours.

The 0,15T iMRI is a safe, helpful and feasible tool to navigate the surgeon using the transnasal, transphenoidal approach in pituitary surgery. The use of the iMRI PoleStar N20 provides immediate resection control and operation can be continued to further remove accessible tumour rests. Although obese patients with short neck are difficult to position (to have the tumour in the centre of the field of view) image quality is sufficient for these purposes. Clinical long term follow up and 3 month control MRI scans showed a good correlation with the intraoperative images. Further operations are necessary to proof the benefit of iMRT in small and hormonally secreting tumours.