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.
Key points
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Low- and high-field iMRI is used for resection control of pituitary macroadenomas.
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Expert interpretation of iMRI images is required to achieve best results.
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iMRI can improve outcomes of nonfunctioning and functioning pituitary macroadenomas.
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iMRI is not useful to detect functioning pituitary microadenomas.
Introduction
Transsphenoidal surgery (TSS) has an important role in the management of pituitary tumors. For many tumors, including nonfunctioning pituitary adenomas, corticotrophin-secreting adenomas causing Cushing disease, and growth hormone–secreting adenomas causing acromegaly, TSS remains the treatment of first choice. Medical management has replaced TSS as the treatment of first choice for one type of pituitary tumor: prolactinoma. For the rest of the tumor types, patients are first advised to undergo TSS.
First described in 1907 by Schloffer, TSS was later refined and popularized by Harvey Cushing. Despite rapid refinements in the technique that allowed for reduction of mortality rates to 5.3% by 1925, the procedure was abandoned by Dr Cushing. With improved visualization through the operating microscope, Jules Hardy, reintroduced TSS in the modern era, setting the stage for a later development of techniques necessary for selective removal of microadenomas (tumors smaller than 1 cm) and macroadenomas. Today, the procedure is widely used, and is the technique of choice for resection of pituitary tumors. For patients undergoing TSS for pituitary tumors, remission rates vary. In a recent meta-analysis, the mean remission rates (ranges) were 68.8% (27–100) for prolactinomas, 47.3% (3–92) for Non functioning adenoma (NFA), 61.2% (37–88) for growth hormone–secreting adenomas, and 71.3% (41–98) for corticotrophin-secreting adenoma tumors. Remission rates and incidence of recurrence have improved modestly over the past three decades.
Currently, overcoming the following challenges could improve remission rates after pituitary surgery. (1) Visibility of small tumors: remission is dependent of the ability to detect the adenomas. (2) Visualization of true extent of large tumors: for larger tumors, cure rates are reduced by tumor remnants. (3) Visualization of tumor invasion: extension of tumor into the structures surrounding the sella. Technologies introduced to take on these challenges include endoscopy, frameless stereotaxy, color Doppler ultrasonography, and real-time intraoperative MRI (iMRI).
Introduction
Transsphenoidal surgery (TSS) has an important role in the management of pituitary tumors. For many tumors, including nonfunctioning pituitary adenomas, corticotrophin-secreting adenomas causing Cushing disease, and growth hormone–secreting adenomas causing acromegaly, TSS remains the treatment of first choice. Medical management has replaced TSS as the treatment of first choice for one type of pituitary tumor: prolactinoma. For the rest of the tumor types, patients are first advised to undergo TSS.
First described in 1907 by Schloffer, TSS was later refined and popularized by Harvey Cushing. Despite rapid refinements in the technique that allowed for reduction of mortality rates to 5.3% by 1925, the procedure was abandoned by Dr Cushing. With improved visualization through the operating microscope, Jules Hardy, reintroduced TSS in the modern era, setting the stage for a later development of techniques necessary for selective removal of microadenomas (tumors smaller than 1 cm) and macroadenomas. Today, the procedure is widely used, and is the technique of choice for resection of pituitary tumors. For patients undergoing TSS for pituitary tumors, remission rates vary. In a recent meta-analysis, the mean remission rates (ranges) were 68.8% (27–100) for prolactinomas, 47.3% (3–92) for Non functioning adenoma (NFA), 61.2% (37–88) for growth hormone–secreting adenomas, and 71.3% (41–98) for corticotrophin-secreting adenoma tumors. Remission rates and incidence of recurrence have improved modestly over the past three decades.
Currently, overcoming the following challenges could improve remission rates after pituitary surgery. (1) Visibility of small tumors: remission is dependent of the ability to detect the adenomas. (2) Visualization of true extent of large tumors: for larger tumors, cure rates are reduced by tumor remnants. (3) Visualization of tumor invasion: extension of tumor into the structures surrounding the sella. Technologies introduced to take on these challenges include endoscopy, frameless stereotaxy, color Doppler ultrasonography, and real-time intraoperative MRI (iMRI).
Intraoperative MRI for transsphenoidal surgery
History of Intraoperative Imaging During Transsphenoidal Surgery
Surgeons routinely use adjunct imaging tools during TSS. Popularized by Jules Hardy, intraoperative fluoroscopic imaging is widely used by surgeons to define the superior and inferior limits of the sella turcica. Frameless stereotactic fluoroscopic guidance registers preoperative computed tomography or MRI images with intraoperative fluoroscopy. This technique uses accurate stereotaxy to ensure that the surgical approach avoids injury to critical structures, such as the internal carotid arteries. These stereotaxic techniques, however, are not useful for monitoring extent of resection (EOR; resection control) of pituitary macroadenomas. Intraoperative ultrasonography (iUSG), either by transcranial or transsellar routes, provides imaging of sellar/suprasellar contents in real time. Investigators have used iUSG to detect tumor residuals and critical structures, such as the carotid arteries. In addition, iUSG has had some success in detecting microadenomas. In patients with Cushing disease with negative preoperative imaging, iUSG detected up to 69% of microadenomas. Despite significant advantages including ease of use, real-time imaging, low cost, and lack of radiation, iUSG remains infrequently used during TSS because of poor image quality.
History of Intraoperative MRI for Transsphenoidal Surgery
Interventions in the head and neck region within the MRI suite were initially limited to needle biopsies and aspirations. The limitations were the product of conventional horizontal bore design of the MRI machines. Long acquisition times compared with other guidance methods including computed tomography or fluoroscopy made interventions in the MRI suite complicated and difficult to perform. Another MRI configuration was needed to ensure ease of manipulation and surgical access. A midfield MRI system (Signa SP, General Electric, Boston, MA) was conceptualized and installed at the Brigham and Women’s Hospital in Boston in 1994 to address issues of surgical access. Its “double doughnut” configuration allowed real-time monitoring and complete access to the surgical site and an iMRI tracking system for real-time stereotaxy and neuronavigation. The surgeon accessed the patient’s head and neck region between two large doughnuts containing superconducting magnets. Although the surgical access was unparalleled among the iMRI system, the design was not widely replicated in other centers introducing iMRI systems. Another popular early design was the open MRI configuration that allowed improved lateral access, but with restricted vertical access. The examples include the Toshiba Access (Toshiba America Medical Systems, Tustin, CA) in a temple format, and the Magnetom Open (Siemens AG, Erlangen, Germany) in the C-arm format in Erlangen, Germany. These and the subsequent iterations of iMRI invoked the single room, moveable table format that allowed surgery to be performed outside the 5-G line. Surgical procedures outside the 5-G line could be performed using the standard surgical instruments including the operating microscope, and patients could be moved quickly into the scanner for intraoperative imaging. Initial reports of iMRI use for neurosurgical indications included TSS procedures. Surgeons recognized the potential of iMRI for resection control of macroadenomas and for detection of intraoperative hematomas during TSS. Other systems including the retractable ultra-low-field strength (PoleStar N-10 and N-20, Odin Medical Technologies, Newton, MA), low-field-strength moveable magnet (Hitachi AIRIS II, Hitachi Medical, Twinsburg, OH), high-field moveable magnet (IMRIS, Marconi Medical System, Winnipeg, Ontario, Canada), and the 3-T machines have all been designed for optimal use of existing surgical tools and microscopes outside the 5-G line. The ability to use conventional tools likely reduces the barrier to introduction and adoption of iMRI procedures. Similarly, shared-resource strategies for using the iMRI machine for intraoperative imaging and routine diagnostic imaging are increasingly being adopted to offset initial investment costs. Recently, most centers are reintroducing conventional horizontal bore machines with high (1.5 T) or ultra-high (3 T) field strength. Higher field strength improves image resolution and reduces image acquisition time, at the expense of surgical access during this period.
Types of Intraoperative MRI Systems
A variety of iMRI systems have been used during TSS ( Table 1 ). The iMRI systems vary in field strengths (0.15–3 T), magnet configurations (eg, open, retractable, double doughnut), and room configurations. Most studies report that the primary benefit of iMRI during TSS lies in intraoperative detection of tumor residuals following maximal resection with conventional technique. Few studies compare the iMRI systems head-to-head to evaluate their comparative effectiveness in detecting tumor residuals.
| Study, References | TSS Type | Manufacturer and Room Design | Magnet Field Strength and Field Type | Number of Tumors/NFPA/Macroadenoma/Microadenoma | Unexpected Residual (%) | Cases Re-explored (%) | Further Resection Possible (%) |
|---|---|---|---|---|---|---|---|
| Low-field iMRI | |||||||
| Steinmeier et al, 1998, Erlangen | Microscope | Siemens Magnetom Open, twin room, moving table/single room, moving table | 0.2 T, vertical field open | 18/15/18/0 | 6 (33) | 5 (28) | 3 (17) |
| Martin et al, 1999, Boston | Microscope | GE Signa SP, single room | 0.5 T, double doughnut | 5/2/5/0 | 3 (60) | 3 (60) | 3 (60) |
| Schwartz et al, 1999, Boston | Microscope | GE Signa SP, single room | 0.5 T, double doughnut | 5/-/-/- | — | — | — |
| Hlavin et al, 2000, Cleveland | Microscope | Siemens Magnetom Open, single room, moveable table | 0.2 T, vertical field open | 1/1/1/0 | — | — | — |
| Bohinski et al, 2001, Cincinnati | Microscope | Hitachi AIRIS II, twin room, moving table/single room, moving table | 0.3 T, vertical field open | 30/22/30/0 | 19 (63) | 19 (63) | 19 (63) |
| Fahlbusch et al, 2001, Erlangen | Microscope | Siemens Magnetom Open, twin room, moving table/single room, moving table | 0.2 T, vertical field open | 44/39/44/0 | — | — | — |
| Pergolizzi et al, 2001, Boston | Microscope | GE Signa SP, single room | 0.5 T, double doughnut | 17/-/-/- | — | — | — |
| Walker & Black, 2002, Boston | Microscope | GE Signa SP, single room | 0.5 T, double doughnut | 23/-/19/4 | 13 (57) | 7 (30) | 7 (30) |
| Kanner et al, 2002, Cleveland | Microscope | Odin PoleStar N-10, retractable magnet | 0.12 T, horizontal field open | 9/8/0/- | — | — | — |
| Nimsky et al, 2003, Erlangen | Microscope | Siemens Magnetom Open, twin room, moving table/single room, moving table | 0.2 T, vertical field open | 6/0/6/0 | 1 (17) | 1 (17) | 1 (17) |
| McPherson et al, 2003, Cincinnati | Microscope | Hitachi AIRIS II, twin room, moving table/single room, moving table | 0.3 T, vertical field open | 30/22/30/0 | 19 (63) | 19 (63) | 19 (63) |
| Nimsky et al, 2005, Erlangen a | Microscope | Siemens Magnetom Open, twin room, moving table/single room, moving table | 0.2 T, vertical field open | 59/-/59/0 | — | 30 (51) | 17 (29) |
| Anand et al, 2006, New York | Endoscope | Odin PoleStar N-10, retractable magnet | 0.12 T, horizontal field open | 10/2/10/0 | 2 (20) | — | — |
| Schwartz et al, 2006, New York | Endoscope | Odin PoleStar N-10, retractable magnet | 0.12 T, horizontal field open | 15/12/15/0 | 7 (47) | 7 (47) | 3 (20) |
| Ahn et al, 2008, Seoul | Microscope | Odin Polestar N-20, retractable magnet | 0.15 T, horizontal field open | 63/-/-/0 | 19 (30) | 19 (30) | 19 (30) |
| Gerlach et al, 2008, Frankfurt am Main | Microscope | Odin Polestar N-20, retractable magnet | 0.15 T, horizontal field open | 40/28/40/0 | 7 (18) | 7 (18) | — |
| Wu et al, 2009, Shanghai | Microscope | Odin Polestar N-20, retractable magnet | 0.15 T, horizontal field open | 55/-/55/0 | 23 (42) | 17 (31) | 9 (16) |
| Bellut et al, 2010, Zurich | Microscope | Odin Polestar N-20, retractable magnet | 0.15 T, horizontal field open | 39/0/49/10 | 8 (21) | 8 (21) | 8 (21) |
| Baumann et al, 2010, Zurich | Microscope | Odin Polestar N-20, retractable magnet | 0.15 T, horizontal field open | 6/5/6/0 | 5 (83) | 5 (83) | 5 (83) |
| Theodosopoulos et al, 2010, Cincinnati | Endoscope | Hitachi AIRIS II, twin room, moving table/single room, moving table | 0.3 T, horizontal bore | 27/10/27/0 | 4 (15) | 3 (11) | 3 (11) |
| Vitaz et al, 2011, Louisville | Both | GE Signa SP, double donut | 0.5 T, double doughnut | 100/-/81/9 | — | 41 (41) | 41 (41) |
| Berkmann et al, 2011, Aarau | Microscope | Odin Polestar N-20, retractable magnet | 0.15 T, horizontal field open | 32/26/32/0 | 15 (47) | 9 (28) | 9 (28) |
| Berkmann et al, 2012, Aarau | Microscope | Odin Polestar N-20, retractable magnet | 0.15 T, horizontal field open | 60/60/60/0 | 23 (38) | 20 (33) | 20 (33) |
| Hlavica et al, 2013, Zurich | Microscope | Odin Polestar N-20, retractable | 0.15 T, horizontal field open | 104/104/-/- | 48 (46) | 43 (41) | 43 (41) |
| Jiménez et al, 2016, Palma de Mallorca | Endoscope | Odin Polestar N-20, single room | 0.15 T, horizontal field open | 18/10/-/- | 10 (56) | 8 (44) | 8 (44) |
| High-field iMRI | |||||||
| Dort & Sutherland, 2001, Calgary | Microscope | Marconi IMRIS, single room, moveable magnet | 1.5 T, horizontal bore | 15/-/-/- | 9 (60) | 8 (53) | 8 (53) |
| Nimsky et al, 2005, Erlangen | Microscope | Siemens Magneton Sonata, single room, moveable table | 1.5 T, horizontal bore | 129/-/129/0 | — | 30 (23) | 28 (22) |
| Fahlbusch et al, 2005, Erlangen | Microscope | Siemens Magneton Sonata, single room, moveable table | 1.5 T, horizontal bore | 23/0/23/0 | 13 (57) | 5 (22) | 5 (22) |
| Nimsky et al, 2006, Erlangen a | Microscope | Siemens Magneton Sonata, single room | 1.5 T, horizontal bore | 85/85/85/0 | 36 (42) | 29 (34) | 29 (34) |
| Szerlip et al, 2011, New York | Microscope | Siemens Espree, single room | 1.5 T, horizontal bore | 59/-/-/- | 30 (51) | 29 (49) | — |
| Tanei et al, 2013, Nagoya | Endoscope | Siemens Magnetom Symphony, | 1.5 T, horizontal bore | 14/0/7/7 | 7 (50) | 5 (36) | 5 (36) |
| Kuge et al, 2013, Yamagata | Endoscope | GE Signa HDx, twin operating room | 1.5 T, horizontal bore | 35/27/-/- | 12 (34) | 3 (9) | 3 (9) |
| Coburger et al, 2014, Gunzburg | Microscope | Siemens Espree, single room | 1.5 T, horizontal bore | 76/52/-/- | — | — | — |
| Berkmann et al, 2014, Erlangen | Microscope | Siemens Magneton Sonata, | 1.5 T, horizontal bore | 85/-/-/- | — | — | — |
| Sylvester et al, 2015, St. Louis | Both | Siemens Espree, twin operating room, moveable magnet | 1.5 T, horizontal bore | 156/90/-/- | — | 56 (36) | 56 (36) |
| Zaidi et al, 2016, Boston | Endoscope | Siemens Verio, single room, moveable magnet | 3 T, horizontal bore | 20/11/-/- | 6 (30) | 6 (30) | 6 (30) |
| Ultra-high-field iMRI | |||||||
| Netuka et al, 2011, Prague | Microscope | GE Signa HDx, twin operating room | 3 T, horizontal bore | 85/-/75/10 | 31 (36) | 31 (36) | — |
| Pamir, 2011, Istanbul | Microscope | Siemens Trio, twin operating room, shared resource | 3 T, horizontal bore | 42/42/-/- | — | — | — |
| Fomekong et al, 2014, Brussels | Microscope | Phillips Intera, twin operating room | 3 T, horizontal bore | 73/-/-/- | 11 (15) | 8 (11) | 8 (11) |
| Serra et al, 2016, Zurich | Endoscope | Siemens Skyra, twin operating room | 3 T, horizontal bore | 50/33/-/- | — | — | — |
| Totals | 1763/706/906/40 | (42) | (36) | (33) | |||
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