Intraoperative magnetic resonance imaging for skull base surgery

Intraoperative Magnetic Resonance Imaging Assisted Endoscopic Endonasal Resection of Clival Chordomas

BACKGROUND

Cranial base chordomas are typically indolent and usually appear as encapsulated tumors. They slowly grow by infiltrating the bone, along with the lines of least resistance. Due to its relationship with important neurovascular structures, skull base chordoma surgery is challenging.

OBJECTIVE

The usefulness of intraoperative magnetic resonance imaging (IO-MRI) in achieving the goal of surgery, is evaluated in this study.

METHODS

Between March 2018 and March 2020, 42 patients were operated on for resection of skull base chordomas in our institution. All of them were operated on under IO-MRI. Patients were analyzed retrospectively for identifying common residue locations, complications and early post-operative outcomes.

RESULTS

In 22 patients (52,4%) gross total resection was achieved according to the final IO-MRI. In 20 patients (47,6%) complete tumor removal was not possible because of extension to the petrous bone (8 patients), pontocerebellar angle (6 patients), prepontine cistern (4 patients), temporobasal (1 patient), cervical axis (1 patient). In 13 patients, the surgery was continued after the first IO-MRI control was performed, which showed a resectable residual tumor. 7 of these patients achieved total resection according to the second IO-MRI, in the other 6 patients all efforts were made to ensure maximal resection of the tumor as much as possible without morbidity. Repeated IO-MRI helped achieve gross total resection in 7 patients (53.8%).

CONCLUSIONS

Our study proves that the use of IO-MRI is a safe method that provides the opportunity to show the degree of resection in skull base chordomas and to evaluate the volume and location of the residual tumor intraoperatively. Hence IO-MRI can improve the life expectancy of patients because it provides an opportunity for both gross total resection and maximal safe resection in cases where total resection is not possible.

Impact of intraoperative magnetic resonance imaging on gross total resection, extent of resection, and residual tumor volume in pituitary surgery: systematic review and meta-analysis

BACKGROUND

Residual tumor tissue after pituitary adenoma surgery, is linked with additional morbidity and mortality. Intraoperative magnetic resonance imaging (ioMRI) could improve resection. We aim to assess the improvement in gross total resection (GTR), extent of resection (EOR), and residual tumor volume (RV) achieved using ioMRI.

METHODS

A systematic review was carried out on PubMed/MEDLINE to identify any studies reporting intra- and postoperative (1) GTR, (2) EOR, or (3) RV in patients who underwent resection of pituitary adenomas with ioMRI. Random effects meta-analysis of the rate of improvement after ioMRI for these three surgical outcomes was intended.

RESULTS

Among 34 included studies (2130 patients), the proportion of patients with conversion to GTR (∆GTR) after ioMRI was 0.19 (95% CI 0.15-0.23). Mean ∆EOR was + 9.07% after ioMRI. Mean ∆RV was 0.784 cm3. For endoscopically treated patients, ∆GTR was 0.17 (95% CI 0.09-0.25), while microscopic ∆GTR was 0.19 (95% CI 0.15-0.23). Low-field ioMRI studies demonstrated a ∆GTR of 0.19 (95% CI 0.11-0.28), while high-field and ultra-high-field ioMRI demonstrated a ∆GTR of 0.19 (95% CI 0.15-0.24) and 0.20 (95% CI 0.13-0.28), respectively.

CONCLUSIONS

Our meta-analysis demonstrates that around one fifth of patients undergoing pituitary adenoma resection convert from non-GTR to GTR after the use of ioMRI. EOR and RV can also be improved to a certain extent using ioMRI. Endoscopic versus microscopic technique or field strength does not appear to alter the impact of ioMRI. Statistical heterogeneity was high, indicating that the improvement in surgical results due to ioMRI varies considerably by center.

Early postoperative MRI and detection of residual adenoma after transsphenoidal pituitary surgery

OBJECTIVE

Current practice guidelines recommend delayed (≥ 3 months after operation) postoperative MRI after transsphenoidal surgery for pituitary adenomas, although this practice defers obtaining important information, such as the presence of a residual adenoma, that might influence patient management during the perioperative period. In this study, the authors compared detection of residual adenomas by means of early postoperative (EPO) MRI (< 48 hours postsurgery) with both surgeon intraoperative assessment and late postoperative (LPO) MRI at 3 months.

METHODS

Adult patients who underwent microscopic transsphenoidal surgery for pituitary adenomas with MRI preoperatively, < 48 hours after the operation, 3 months postoperatively, and yearly for 4 years were included. The presence or absence of residual tumor was assessed intraoperatively by a single surgeon and postoperatively by 2 neuroradiologists blinded to the intraoperative assessment and other postoperative imaging studies. The presence of residual tumor was confirmed by reresection, tumor growth on imaging, or hormonal evidence. Interreader reliability was calculated at each imaging time point. Specificity, sensitivity, positive predictive value, and negative predictive value for EPO and LPO imaging and intraoperative assessment were determined.

RESULTS

In total, 102 consecutive patients who underwent microscopic transsphenoidal resection of a pituitary adenoma were included. Eighteen patients (18%) had confirmed residual tumors (12 confirmed by tumor growth, 5 by surgery, and 1 by biochemical evidence of persistent disease). Interreader reliability for detecting residual tumor on EPO MRI was almost perfect (κ = 0.88) and significantly higher than that for LPO MRI (κ = 0.69, p = 0.03). EPO MRI was highly specific for residual tumor (98%), a finding similar to that for intraoperative assessment (99%, p = 0.60) and significantly higher than that for LPO MRI (81%, p < 0.001). Notably, EPO MRI was significantly more sensitive for residual tumor (100%) than both intraoperative assessment (78%, p = 0.04) and LPO MRI (78%, p = 0.04). EPO MRI had a 100% negative predictive value and was used to find 4 residual tumors that were not identified intraoperatively. Residual tumors found on EPO MRI allowed for reresection during the same hospitalization for 3 patients.

CONCLUSIONS

EPO MRI after transsphenoidal pituitary surgery can be reliably interpreted and has greater sensitivity for detecting residual tumor than intraoperative assessment and LPO MRI. This result challenges current guidelines stating that delayed postoperative imaging is preferable to early imaging. Pituitary surgeons should consider performing EPO MRI either in addition to or instead of delayed imaging.

10 Years' Experience of Using Low-Field Intraoperative MRI in Transsphenoidal Surgery for Pituitary Adenoma: Results of the Swiss Pituitary Registry (SwissPit)

BACKGROUND

For 20 years, several studies have reported intraoperative magnetic resonance imaging (iMRI) utility to achieve gross total resections in transsphenoidal pituitary adenoma surgery. Although few studies on low-field iMRI included >100 patients, data on hormonally active tumors remain scarce and follow-up times are <3 years. This is not sufficient to judge the long-term efficiency of the use of low-field iMRI. The aim of this retrospective study is to report the detailed outcome of iMRI-controlled transsphenoidal surgery in >200 patients during a follow-up exceeding 5 years.

METHODS

Patients undergoing surgery for pituitary adenoma by iMRI-controlled, endoscopically assisted transsphenoidal surgery at the authors' institution between 2006 and 2016 were eligible for inclusion. Data were collected in the Swiss Pituitary Registry. A Polestar 0.15T-scanner was used.

RESULTS

A total of 231 patients had surgery for 160 nonfunctioning adenomas; 28 hGH-, 27 PRL-, 10 ACTH-secreting and 6 mixed adenomas and were followed for 62 months (9-178). Additional iMRI-guided resections were possible in 54% and increased the gross total resection rate by 4% (P = 0.004). Remission rates were as follows: nonfunctioning adenoma, 53%; acromegaly, 61%; prolactinoma, 50%; Cushing disease, 90%. Tumor regrowth and recurrence was detected at a mean time of 24 and 63 months, respectively. Recovery of deficient hormone axes was detected in 22% to 27%. The risk for new postoperative hormonal deficiencies was 15%. Postoperative relieve of visual field and visual acuity deficiencies was seen in 94 (86%) and 73 (81%) patients, respectively.

CONCLUSION

Judged by long-term follow-ups of >200 nonfunctioning/functioning pituitary adenomas, the use of low-field iMRI in transsphenoidal surgery increases resection rates and sustainably influences outcomes.

Identification of tumor residuals in pituitary adenoma surgery with intraoperative MRI: do we need gadolinium?

OBJECTIVE

To evaluate the diagnostic accuracy of high-resolution T2w intraoperative magnetic resonance imaging (iMRI) for detecting pituitary adenoma remnants compared to contrast-enhanced T1-weighted images.

METHODS

42 patients underwent iMRI-guided resection of large pituitary macroadenomas and fulfilled the inclusion criteria for this retrospective analysis. Intraoperative and postoperative imaging evaluation of tumor residuals and localization were assessed by two experienced neuroradiologists in a blinded fashion. The diagnostic accuracy of T2w and contrast-enhanced T1w images were evaluated.

RESULTS

The diagnostic accuracy for detecting tumor residuals of high-resolution T2w images showed highly significant association to contrast-enhanced T1w images (p < 0.0001). Furthermore, identification rate of tumor remnants in different compartments, e.g., cavernous sinus, was comparable. In total, coronal T2w images provided a diagnostic sensitivity of 97.7% and specificity of 100% compared to the gold standard of contrast-enhanced T1w images. The postoperatively expected extent of resection proved to be true in 97.6% according to MRI 3 months after resection.

CONCLUSIONS

High-resolution T2w intraoperative MR images provide excellent diagnostic accuracy for detecting tumor remnants in macroadenoma surgery with highly significant association compared to T1w images with gadolinium. The routine-use and need of gadolinium in these patients should be questioned critically in each case in the future.

Can Intraoperative Magnetic Resonance Imaging Be Helpful in the Surgical Resection of Parasellar Meningiomas? A Case Series

OBJECTIVE

The surgery of parasellar meningiomas is crucial. There are only a few reports of the use of intraoperative magnetic resonance imaging (iMRI) for resection of these lesions. We discuss the safety and usefulness of this technique in achieving the planned surgical goal and analyze patients' outcomes.

METHODS

Nineteen cases of parasellar meningioma were treated in our institution using iMRI. We classified the tumors according to their primary location: tuberculum sellae, clinoidal, and cavernous sinus meningiomas. We evaluated the history of previous surgery, outcome, residual (if present) tumor volume, degree of resection, achievement of the surgical goal, and number of iMRI scans.

RESULTS

The preoperative surgical goal was achieved in all patients. In 7 of 19 patients, (37%) further tumor resection was performed after the first iMRI scan. Regarding the cavernous sinus group, the surgical resection was continued after the first iMRI in 56% of patients, obtaining substantial additional volume reduction. No complications were found related to the use of iMRI scan.

CONCLUSIONS

iMRI has been effective in safely increasing the extent of parasellar meningioma resection mainly for recurrent and invasive tumors. Its usefulness has been seen mostly in cavernous sinus lesions, in which it allowed the further safe resection in 56% of cases. Moreover, this tool was particularly useful in recurrent or residual meningiomas with extension in extracranial compartments.

iMRI During Transsphenoidal Surgery

A variety of intraoperative MRI (iMRI) systems are in use during transsphenoidal surgery (TSS). The variations in iMRI systems include field strengths, magnet configurations, and room configurations. Most studies report that the primary utility of iMRI during TSS lies in detecting resectable tumor residuals following maximal resection with conventional technique. Stereotaxis, neuronavigation, and complication avoidance/detection are enhanced by iMRI use during TSS. The use of iMRI during TSS can lead to increased extent of resection for large tumors. Improved remission rates from hormone-secreting tumors have also been reported with iMRI use. This article discusses the history, indications, and future directions for iMRI during TSS.

Intraoperative magnetic resonance imaging assessment of non-functioning pituitary adenomas during transsphenoidal surgery

PURPOSE

To review the utility of intraoperative imaging in facilitating maximal resection of non-functioning pituitary adenomas (NFAs).

METHODS

We performed an exhaustive MEDLINE search, which yielded 5598 articles. Upon careful review of these studies, 31 were pertinent to the issue of interest.

RESULTS

Nine studies examined whether intraoperative MRI (iMRI) findings correlated with the presence of residual tumor on MRI taken 3 months after surgical resection. All studies using iMRI of >0.15T showed a ≥90% concordance between iMRI and 3-month post-operative MRI findings. 24 studies (22 iMRI and 2 intraoperative CT) examined whether intraoperative imaging improved the surgeon's ability to achieve a more complete resection. The resections were carried out under microscopic magnification in 17 studies and under endoscopic visualization in 7 studies. All studies support the value of intraoperative imaging in this regard, with improved resection in 15-83% of patients. Two studies examined whether iMRI (≥0.3T) improved visualization of residual NFA when compared to endoscopic visualization. Both studies demonstrated the value of iMRI in this regard, particularly when the tumor is located lateral of the sella, in the cavernous sinus, and in the suprasellar space.

CONCLUSION

The currently available literature supports the utility of intraoperative imaging in facilitating increased NFA resection, without compromising safety.

Pituitary surgery and volumetric assessment of extent of resection: a paradigm shift in the use of intraoperative magnetic resonance imaging

OBJECTIVE

The aim of this study was to quantitatively assess the role of intraoperative high-field 3-T MRI (3T-iMRI) in improving the gross-total resection (GTR) rate and the extent of resection (EOR) in endoscopic transsphenoidal surgery (TSS) for pituitary adenomas.

METHODS

Radiological and clinical data from a prospective database were retrospectively analyzed. Volumetric measurements of adenoma volumes pre-, intraoperatively, and 3 months postoperatively were performed in a consecutive series of patients who had undergone endoscopic TSS. The quantitative contribution of 3T-iMRI was measured as a percentage of the additional rate of GTR and of the EOR achieved after 3T-iMRI.

RESULTS

The cohort consisted of 50 patients (51 operations) harboring 33 nonfunctioning and 18 functioning pituitary adenomas. Mean adenoma diameter and volume were 21.1 mm (range 5–47 mm) and 5.23 cm3 (range 0.09–22.14 cm3), respectively. According to Knosp's classification, 10 cases were Grade 0; 8, Grade 1; 17, Grade 2; 12, Grade 3; and 4, Grade 4. Gross-total resection was the surgical goal (targeted [t]GTR) in 34 of 51 operations and was initially achieved in 16 (47%) of 34 at 3T-iMRI and in 30 (88%) of 34 cases after further resection. In this subgroup, the EOR increased from 91% at 3T-iMRI to 99% at the 3-month MRI (p < 0.05). In the 17 cases in which subtotal resection (STR) had been planned (tSTR), the EOR increased from 79% to 86% (p < 0.05) and GTR could be achieved in 1 case. Intrasellar remnants were present in 20 of 51 procedures at 3T-iMRI and in only 5 (10%) of 51 procedures after further resection (median volume 0.15 cm3). Overall, the use of 3T-iMRI led to further resection in 27 (53%) of 51 procedures and permitted GTR in 15 (56%) of these 27 procedures; thus, the GTR rate in the entire cohort increased from 31% (16 of 51) to 61% (31 of 51) and the EOR increased from 87% to 95% (p < 0.05).

CONCLUSIONS

The use of high-definition 3T-iMRI allowed precise visualization and quantification of adenoma remnant volume. It helped to increase GTR and EOR rates in both tGTR and tSTR patient groups. Moreover, it helped to achieve low rates of intrasellar remnants. These data support the use of 3T-iMRI to achieve maximal, safe adenoma resection.

Follow-up and long-term outcome of nonfunctioning pituitary adenoma operated by transsphenoidal surgery with intraoperative high-field magnetic resonance imaging

BACKGROUND

Intraoperative MRI (iMRI) increases gross total resection (GTR) rates in transsphenoidal surgery; however, long-term follow-up data is lacking. The objective is to assess the outcome of patients with nonfunctioning pituitary adenomas (NFA) at a mean follow-up of > 5 years.

METHODS

Patients with NFA operated in a single institution with resection control by a 1.5 T intraoperative magnetic resonance imaging (iMRI) scanner and no previous pituitary surgery were included. Microscopical transsphenoidal approaches with optional endoscopy were used. The iMRI was chosen for spacious suprasellar or retrosellar and/or invasive tumours. IMRI-scans were made if GTR or if nonresectable remnants were presumed. The patients had a full neuroradiological, endocrinological and ophthalmological follow-up at the institution.

RESULTS

Eighty-five patients (67 % male;55 ± 14 years) with a follow-up of 5.6 ± 1.9 years were included. The initial GTR rate on iMRI was 44 %. In 83 %, further resections were possible, resulting in a final GTR rate of 66 %. In invasive tumours, the GTR rate was increased by 29 %. The detection of remnants by iMRI had high sensitivity and specificity (100 %), as opposed to endoscopy (21 %;78 %). During follow-up, four (7 %) tumours recurred and 14 (64 %) remnants grew. The recurrence and regrowth rate were 0.013 and 0.114 patients/years, respectively. Seventy-nine percent of the growing remnants were seen < 5 years postoperatively.

CONCLUSIONS

The use of iMRI for transsphenoidal resection leads to low recurrence rates. Even in case of invasive tumours, distinctly more patients show long tumour-free follow-ups. Tumour remnants detected by iMRI are at high risk to grow within 5 years after surgery.

High-Field iMRI in transsphenoidal pituitary adenoma surgery with special respect to typical localization of residual tumor

BACKGROUND

Intraoperative high-field magnetic resonance imaging (iMRI) is used as an immediate intraoperative quality control, evaluating the extent of tumor removal during the surgical procedure and allowing us to extend resections in those cases where tumor remnants are documented. The aim of the study was to analyze the typical localization of residual tumor remnants, detected by iMRI during transsphenoidal surgery of pituitary adenomas.

METHODS

We reviewed a series of 72 patients. All patients presented with macroadenomas with or without suprasellar extension. After high-field MRI investigation, we divided the series preoperatively into totally resectable (TR) and non-totally resectable (NTR) tumors. Tumor remnants were documented by iMRI, obtained directly after tumor removal, as well as by intraoperative surgical inspection of the sellar content.

RESULTS

In the TR group, we observed 23 cases suspicious for tumor remnants, located anteriorly, laterally, posteriorly, and suprasellar under descending folds of the diaphragm. Continuing surgery, upon a "second inspection", tumor resection could be completed in all cases.

CONCLUSIONS

Incomplete removal of resectable pituitary adenomas could be avoided in a higher number of cases with the knowledge of the location of the typical remnant tumors. In those cases where it is not possible to achieve a complete resection of adenoma, further treatment can be planned at an earlier stage, without any need to wait for the conventional postoperative MRI scan performed 2 to 3 months after surgery.

Intraoperative magnetic resonance imaging during surgery for pituitary adenomas: pros and cons

Surgery for pituitary adenomas still remains a mainstay in their treatment, despite all advances in sophisticated medical treatments and radiotherapy. Total tumor excision is often attempted, but there are limitations in the intraoperative assessment of the radicalism of tumor resection by the neurosurgeon. Standard postoperative imaging is usually performed with a few months delay from the surgical intervention. The purpose of this report is to review briefly the facilities and kinds of intraoperative magnetic resonance imaging for all physician and surgeons involved in the management of pituitary adenomas on the basis of current literature. To date, there are several low- and high-field magnetic resonance imaging systems available for intraoperative use and depiction of the extent of tumor removal during surgery. Recovery of vision and the morphological result of surgery can be largely predicted from the intraoperative images. A variety of studies document that depiction of residual tumor allows targeted attack of the remnant and extent the resection. Intraoperative magnetic resonance imaging offers an immediate feedback to the surgeon and is a perfect quality control for pituitary surgery. It is also used as a basis of datasets for intraoperative navigation which is particularly useful in any kind of anatomical variations and repeat operations in which primary surgery has distorted the normal anatomy. However, setting up the technology is expensive and some systems even require extensive remodeling of the operation theatre. Intraoperative imaging prolongs the operation, but may also depict evolving problems, such as hematomas in the tumor cavity. There are several artifacts in intraoperative MR images possible that must be considered. The procedures are not associated with an increased complication rate.

Dual-room 1.5-T intraoperative magnetic resonance imaging suite with a movable magnet: implementation and preliminary experience

We hereby report our initial clinical experience of a dual-room intraoperative magnetic resonance imaging (iMRI) suite with a movable 1.5-T magnet for both neurosurgical and independent diagnostic uses. The findings from the first 45 patients who underwent scheduled neurosurgical procedures with iMRI in this suite (mean age, 41.3 ± 12.0 years; intracranial tumors, 39 patients; cerebral vascular lesions, 5 patients; epilepsy surgery, 1 patient) were reported. The extent of resection depicted at intraoperative imaging, the surgical consequences of iMRI, and the clinical practicability of the suite were analyzed. Fourteen resections with a trans-sphenoidal/transoral approach and 31 craniotomies were performed. Eighty-two iMRI examinations were performed in the operating room, while during the same period of time, 430 diagnostic scans were finished in the diagnostic room. In 22 (48.9%) of 45 patients, iMRI revealed accessible residual tumors leading to further resection. No iMRI-related adverse event occurred. Complete lesion removal was achieved in 36 (80%) of all 45 cases. It is concluded that the dual-room 1.5-T iMRI suite can be successfully integrated into standard neurosurgical workflow. The layout of the dual-room suite can enable the maximum use of the system and save costs by sharing use of the 1.5-T magnet between neurosurgical and diagnostic use. Intraoperative MR imaging may provide valuable information that allows intraoperative modification of the surgical strategy.

Tumor-biology and current treatment of skull-base chordomas

Chordomas are rare, slow growing tumors of the axial skeleton, which derive from the remnants of the fetal notochord. They can be encountered anywhere along the axial skeleton, most commonly in the sacral area, skull base and less commonly in the spine. Chordomas have a benign histopathology but exhibit malignant clinical behavior with invasive, destructive and metastatic potential. Genetic and molecular pathology studies on oncogenesis of chordomas are very limited and there is little known on mechanisms governing the disease. Chordomas most commonly present with headaches and diplopia and can be readily diagnosed by current neuroradiological methods. There are 3 pathological subtypes of chordomas: classic, chondroid and dedifferentiated chordomas. Differential diagnosis from chondrosarcomas by radiology or pathology may at times be difficult. Skull base chordomas are very challenging to treat. Clinically there are at least two subsets of chordoma patients with distinct behaviors: some with a benign course and another group with an aggressive and rapidly progressive disease. There is no standard treatment for chordomas. Surgical resection and high dose radiation treatment are the mainstays of current treatment. Nevertheless, a significant percentage of skull base chordomas recur despite treatment. The outcome is dictated primarily by the intrinsic biology of the tumor and treatment seems only to have a secondary impact. To date we only have a limited understanding this biology; however better understanding is likely to improve treatment outcome. Hereby we present a review of the current knowledge and experience on the tumor biology, diagnosis and treatment of chordomas.

Intraoperative High-Field Magnetic Resonance Imaging in Transsphenoidal Surgery of Hormonally Inactivepituitary Macroadenomas

OBJECTIVE

The aim of the study was to evaluate the effect of intraoperative, high-field (1.5 T) magnetic resonance imaging (MRI) on the results of transsphenoidal surgery of hormonally inactive pituitary macroadenomas.

METHODS

One hundred six patients (tumor size, 29.9 +/- 10.1 mm; minimum, 11.3 mm; maximum, 57.2 mm) with hormonally inactive pituitary macroadenoma were investigated by intraoperative high-field MRI during transsphenoidal surgery. If intraoperative imaging depicted an accessible tumor remnant, resection was continued.

RESULTS

Among the 85 patients in whom complete tumor removal was intended preoperatively, intraoperative imaging revealed definite tumor remnants or suspicious findings in 36 (42%) patients. Imaging led to an extended resection in 29 (34%) patients of this group. Among them, resection could be completed in 21. This increased the rate of complete tumor removal from 58% (49 out of 85) to 82% (70 out of 85). In the group of patients with intended partial removal (n = 21), resection was extended in 38% (eight out of 21) because of intraoperative imaging. Comparison with scanning 3 months after surgery did not reveal any false-negative findings of intraoperative MRI; in six cases, intraoperative MRI was suspicious for some minor remnant that could not be reproduced in the postoperative control.

CONCLUSION

The extent of resection in transsphenoidal surgery can be reliably assessed using intraoperative high-field MRI. In addition to the suprasellar compartment, intra- and parasellar structures are also visualized in great detail. Intraoperative imaging acts as an immediate intraoperative quality control, allowing one to not only increase the extent of resection, but to also increase the percentage of complete removals.

Cranial surgery with an expanded compact intraoperative magnetic resonance imager

✓In this article the authors report the implementation of an expanded compact intraoperative magnetic resonance (iMR) imager that is designed to overcome significant limitations of an earlier unit.

The PoleStar N20 iMR imager has a stronger magnetic field than its predecessor (0.15 tesla compared with 0.12 tesla), a wider gap between magnet poles, and an ergonomically improved gantry design. The additional time needed in the operating room (OR) for use of iMR imaging and the number of sessions per patient were recorded. Stereotactic accuracy of the integrated navigational tool was assessed using a water-covered phantom.

Of the 55 patients who have undergone surgery in the PoleStar N20 device, diagnoses included glioma in 13, meningioma in 12, pituitary adenoma in nine, other skull base lesions in seven, and miscellaneous other diagnoses. The extra time required for use of the system averaged 1.1 hours (range 0.5–2 hours). Imaging sessions averaged 2.3 per surgery (range one–six sessions).

Measurement of stereotactic accuracy revealed that T1-weighted images were the most accurate. Thinner slices yielded measurably greater accuracy, although this was of questionable clinical significance (all sequences ≤ 4 mm had a mean error of ≤ 1.8 mm). The position of the phantom in the center compared with the periphery of the magnetic field did not affect accuracy (mean error 0.9 mm for each).

The PoleStar N20 appears to make intraoperative neuroimaging with a low-field-strength magnet much more practical than it was with the first-generation device. Greater ease of positioning resulted in a decrease in added time in the OR and encouraged a larger number of imaging sessions.

Transsphenoidal surgery in acromegaly investigated by intraoperative high-field magnetic resonance imaging

OBJECTIVE

The aim of the study was to evaluate the effect of intraoperative high-field (1.5 Tesla) magnetic resonance imaging (MRI) on the results of transsphenoidal surgery of GH-secreting pituitary macroadenomas.

METHODS

Twenty-three acromegalic patients (mean tumor size, 25 +/- 12 mm; untreated preoperative GH, 4.2-159 microg/l; IGF-I, 349-1111 microg/l) were investigated by intraoperative high-field MRI. If intraoperative imaging depicted an accessible tumor remnant, resection was continued.

RESULTS

In five patients intraoperative MRI led to further tumor removal, two of these met the consensus criteria for endocrine remission after 3 months. In two patients basal GH and oral glucose tolerance test (OGTT) were <2 microg/l, only IGF-I was slightly elevated, and in one patient GH was <5 microg/l and OGTT was 2 microg/l, with elevated IGF-I. Final intraoperative MRI showed no tumor remnants in 14 patients; eight of them met the consensus criteria for remission of acromegaly. In the patients with MRI showing incomplete removal (four suspect findings and five patients with intended partial removal) none was normalized.

CONCLUSION

With regard to the patients with a tumor configuration in whom complete tumor removal was considered (n = 18), intraoperative MRI increased the rate of endocrine normalization from 33 to 44% applying the consensus criteria, and improved endocrine outcome to 'nearly normalization' in another 17%. With regard to preoperative GH levels and tumor size, intraoperative MRI can help to achieve endocrine remission in patients who are normally considered not to be curable. However, taking GH as the tumor marker, even intraoperative high-field MRI was not able to detect tumor remnants in every case.

Intraoperative MR imaging

Intraoperative MR imaging has become a safe and effective technology that has revolutionized the way neurosurgery is performed. Benefits include the ability to update data sets for navigational systems, to monitor tumor resections, to adjust the approach to intracranial lesions, and to guide functional and drug or cell delivery procedures. Use of this technique can help avoid inadvertent injury of important anatomic and vascular structures. In addition, complications such as ischemia or hemorrhage can be detected early. Intraoperative MR imaging is particularly useful for ensuring that brain biopsies yield diagnostic tissue and for assessing the completeness of tumor resection. As is true for any new technology, the benefits of intraoperative MR imaging must be examined carefully to guarantee appropriate use. Many neurosurgical procedures do not require real-time image guidance and can be performed safely using current surgical techniques, including microsurgical methods and frameless and frame-based stereotaxy. Other tumor resections, tumor biopsies, and surgical and interventional procedures distinctly benefit from the sophisticated information provided by intraoperative imaging techniques. In surgery for low-grade gliomas, intraoperative MR imaging has found general acceptance, whereas its usefulness to monitor the resection of high-grade gliomas remains controversial. The economic issues related to intraoperative MR imaging cannot be overlooked. The acquisition of an intraoperative MR imaging system is associated with considerable expense, and its performance increases the cost of equipment and the operating time. Despite these additional expenses, intraoperative MR imaging can lead to a potential overall cost reduction in the treatment of certain patients if long-term cure can be achieved, repeat resection can be avoided, or procedure-associated morbidity can be reduced. Although intraoperative MR imaging techniques hold tremendous potential, the definition of their appropriate role in the delivery of successful and cost-effective medical care awaits further study.

Intraoperative MR at 1.5 Tesla--experience and future directions

The objective of this report is to present and contrast the development of the different intraoperative MR systems that are currently in use. The manuscript focuses on the design and clinical experience of a 1.5 Tesla MR system, based on a movable magnet. This configuration is similar to the operating microscope and other surgical adjuncts, with MR technology moved to and from the patient as needed. The system has been used to monitor 294 neurosurgical procedures. including CNS neoplasia. epilepsy, cervical spine disorders, arteriovenous malformations, cavernomas and aneurysms. In many cases the surgical procedure was significantly altered by intraoperatively acquired MRI. Future developments include the construction of a 3 Tesla intraoperative MR system and an ambidextrous MR-compatible robot. This seamless integration of robotic technology into an intraoperative MR environment may well revolutionize neurosurgery.