Abstract
Objective
Despite the increasing utilization of image-guided surgery, no radiology protocols for obtaining magnetic resonance (MR) imaging of adequate quality are available in the current literature. At our institution, more than 300 endonasal cranial base procedures including pituitary, extended pituitary, and other anterior skullbase procedures have been performed in the past 3 years. To facilitate and optimize preoperative evaluation and assessment, there was a need to develop a magnetic resonance protocol.
Methods
Retrospective Technical Assessment was performed.
Discussion
Through a collaborative effort between the otolaryngology, neurosurgery, and neuroradiology departments at our institution, a skull base MR image–guided (IGS) protocol was developed with several ends in mind. First, it was necessary to generate diagnostic images useful for the more frequently seen pathologies to improve work flow and limit the expense and inefficiency of case specific MR studies. Second, it was necessary to generate sequences useful for IGS, preferably using sequences that best highlight that lesion. Currently, at our institution, all MR images used for IGS are obtained using this protocol as part of preoperative planning. The protocol that has been developed allows for thin cut precontrast and postcontrast axial cuts that can be used to plan intraoperative image guidance. It also obtains a thin cut T2 axial series that can be compiled separately for intraoperative imaging, or may be fused with computed tomographic images for combined modality. The outlined protocol obtains image sequences effective for diagnostic and operative purposes for image–guided surgery using both T1 and T2 sequences.
Advances in endonasal surgery, computed tomography (CT), and magnetic resonance (MR) imaging currently allow for entirely endoscopic approaches to many lesions of the cranial base. Principle to this development has been the incorporation of frameless stereotactics for intraoperative image guidance that allows for the transfer of patient images onto the operative field. Coupled with the prerequisite detailed knowledge of the surgical anatomy, the use of intraoperative image guidance techniques using CT, MR, and CT-MR fusion imaging may shorten the length of procedures, reduce complications and surgical morbidity, and improve the extent of resections . Consequently, high-quality, reliable preoperative imaging in endoscopic skull base surgery is fundamental.
Whereas CT has been widely available for image-guided surgery in endoscopic sinus surgery and endoscopic skull base surgery for some time, CT-MR fusion for skull base procedures has not been as widely used even though it has been available for more than 10 years. Much of this lack of clinical acceptance and utilization may have been driven by the often complicated and lengthy registration and fusion process . MR offers several significant advantages over CT, including enhanced evaluation of soft tissue lesions and evaluation key features such as dural extension or loculation of cystic lesions. It also allows determination of proximity or involvement of critical structures such as the carotid or basilar arterial systems . Furthermore, MR images can be fused with CT to allow simultaneous detailed evaluation of bony anatomy of the skull base and orbits with the benefit of soft tissue detail from MR imaging, thus using the best aspects of these complimentary imaging modalities.
Despite the increasing utilization of image-guided surgery, no radiology protocols for obtaining MR imaging of adequate quality are available in the current literature. At our institution, more than 300 endonasal cranial base procedures including pituitary, extended pituitary, and other anterior skull base procedures have been performed in the past 3 years. Through a collaborative effort between the otolaryngology, neurosurgery, and radiology departments of our institution, a skull base image-guided surgery MR protocol was developed with several goals in mind. First, it was necessary to generate diagnostic images useful for a wide variety of the more frequently seen pathologies to improve work flow and limit the expense and inefficiency of case-specific MR studies. Second, it was necessary to generate sequences useful for image-guided surgery, preferably using sequences that best highlight the lesion in question. Currently, at our institution, all MR images used for image-guided surgery are obtained using this protocol as part of preoperative planning ( Table 1 ).
| Series | Plane | TE | TR | TI | ETL | BW | FOV, cm | Slice, mm | Gap, mm | Matrix | Nex | Comments |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Sagittal T1 | 12 | 350 | NA | 16 | 22 | 5 | 2.5 | 256 × 192 | 1 | Routine localizer whole brain | |
| 2 | Axial T2 | 102 | 4000 | 8 | 16 | 24 | 2 | Interleave | 256 × 192 | 2 | Scan hard palate to midfrontal midsinus, “A” shift | |
| 3 | Axial T1 | 12 | 500 | NA | 16 | 16 | 4 | 1 | 256 × 192 | 2 | T1 whole brain coverage | |
| 4 | Flair | 120 | 6000 | 2000 | 23 | 16 | 24 | 5 | 1 | 256 × 1920 | 2 | Whole brain coverage |
| 5 | DWI | 74 | 4301 | 51 | 62 | 24 | 5 | None | 128 × 128 | 2 | Whole brain coverage | |
| Inject | Gadolinium | |||||||||||
| 6 | Axial T1 | 12 | 675 | NA | 16 | 16 | 4 | 1 | 256 × 192 | 2 | Same as series 3, postgad fatsat | |
| 7 | Coronal T1 | 12 | 650 | NA | 16 | 16 | 4 | 1 | 256 × 192 | 2 | Scan post sella thru frontal nose, postgad fatsat | |
| 8 | Axial 3D T1 SPGR | 1.7 | 37 | NA | 16 | 24 | 1.5 | None | 256 × 192 | 1 | 3D T1 scan covering entire brain |
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