- Studies of the use of intraoperative MRI (iMRI) for pituitary adenomas have shown that images are highly sensitive and specific for detecting residual tumor
- Detection of residual tumor with iMRI has been found to increase the likelihood of gross total resection
- The safety of iMRI in transsphenoidal surgery for pituitary adenomas is well established
- iMRI shows promise for remission from functioning adenomas and may have as longstanding effects on tumor growth control rates
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For more than 20 years, intraoperative magnetic resonance imaging (iMRI) has been used to help with intraoperative orientation and determine the adequacy of resection. In Neurosurgery Clinics of North America, Neurosurgeons Pamela S. Jones, MD, MS, MPH, and Brooke Swearingen, MD, of the Department of Neurosurgery at Massachusetts General Hospital, review the literature on the use of iMRI during surgical management of pituitary adenomas.
Types of iMRI Systems
The earliest iMRI systems incorporated 0.5T scanners that allowed for realtime stereotaxy. However, the image resolution was poor.
Mass General has used a 3T IMRIS system for all iMRI cases since 2012. The scanner is stored in a room between two operating rooms and is moved into each operating room on a ceiling-mounted rail system. The operating suite is equipped with radiofrequency-shielded doors and includes an MRI-compatible operating room table with head fixation devices specifically designed to fit with the IMRIS radiofrequency coils. Surgical workflow is typically interrupted only for 30 minutes to obtain images.
Portable MRI units allow iMRI to be performed without significant modifications to the operating suite. However, reports are mixed about whether these low-field units improve rates of resection.
iMRI and Pituitary Tumor Resection
The major value of iMRI is to find an unexpected actionable residual tumor. Studies have consistently shown that ≥0.5T iMRI offers high sensitivity and specificity for detecting a residual tumor. Detection may improve with greater magnet strength, although improved imaging detail increases the need to interpret subtle findings.
Studies also show that, because of its ability to detect residual tumor, ≥0.5T iMRI improves the likelihood of gross tumor resection, typically within the sellar and suprasellar regions. According to a recent systematic review, when an actionable residual tumor is identified through iMRI, further resection is possible in 10% to 83% of cases.
iMRI and Endoscopy
Most studies indicate that iMRI has greater sensitivity and specificity than endoscopy for identifying the residual tumor, even in the hands of experienced endoscopic surgeons. However, endoscopy may be a good substitute for surgeons who lack access to iMRI.
It is worth considering whether some patients would benefit from a combination of endoscopy and iMRI. In a large retrospective series of transsphenoidal surgeries for pituitary adenomas, the combination was associated with a higher extent of resection when compared with conventional transsphenoidal microsurgery, endoscopy without iMRI or microscopy with iMRI.
iMRI and Functioning Adenomas
Most studies of iMRI have focused on nonfunctioning adenomas, but some research has demonstrated that 1.5T iMRI is associated with improved remission rates in patients with acromegaly or functioning microadenomas.
The retrospective series mentioned above included 66 cases in which functioning tumors were resected after iMRI. The extent of resection was significantly lower in functioning than in nonfunctioning tumors, which may reflect the more aggressive, more invasive nature of functioning tumors.
Safety of iMRI
The safety of iMRI in transsphenoidal surgery for pituitary adenomas is well established. No studies have noted significant adverse events or accidents caused by the magnet. At Mass General, more than 500 transsphenoidal pituitary surgeries have been performed with iMRI.
Multiple studies have indicated that finding a residual tumor on intraoperative imaging does not result in overzealous exploration that leads to increased postoperative morbidity.
- Visual outcomes: A common reason for surgery on nonfunctioning pituitary adenomas is that the tumor is compromising vision because of pressure on the optic nerves or chiasm. Extensive literature supports the role of iMRI in aiding visual recovery in these cases
- Endocrine outcomes: Multiple retrospective studies have analyzed whether the use of iMRI influences postoperative hormonal outcomes. Most have found no significant risk to endocrine function or recovery, including no higher risk of diabetes insipidus or hypopituitarism
- Long-term outcomes: iMRI was introduced relatively recently, and pituitary tumors grow slowly, so there are few long-term data. However, studies with the longest follow-up show that increased rates of gross tumor resection with iMRI translate into higher rates of progression-free survival and less need for subsequent radiosurgery or resection
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