Endosphenoidal CoilFeasibility of Endosphenoidal Coil Placement for Imaging of the Sella During Transsphenoidal Surgery

Tumors of the pituitary gland comprise up to 20% of all brain tumors. The central location and the small size of the pituitary gland make the management of tumors particularly challenging. Transsphenoidal surgery (TSS) to resect pituitary tumors is highly successful at achieving complete cure for functional pituitary adenomas. It is most successful when such adenomas can be localized by preoperative MRI of the pituitary. However, in some instances, small functional tumors cannot be visualized. In the case of Cushing s disease (CD), such non-visualization may be as high as 50%. The success of transsphenoidal surgery is substantially reduced in patients with negative MRI, as some of the adenomas that cause CD are so small that they are difficult to find during surgical exploration of the pituitary. Surgical success is also diminished when tumors invade the walls of the cavernous sinus. MRI of the pituitary lacks imaging resolution to detect such invasion and so the surgeon cannot perform a complete resection with surgery based on the preoperative MRI.

Signal to noise ratio (SNR) is the primary constraint on achieving high quality high resolution MRI images. SNR can be improved by longer scan times or by increasing the field strength of the MRI magnet. SNR is proportional to the square of imaging time, however, long imaging times are not clinically feasible. SNR is linearly proportional to field strength, however, replacing MRI magnets is cost prohibitive.

Another strong determinant of SNR is the proximity of the MRI receiver coil to the tissue being imaged. Placement of a coil in close proximity to the structure of interest dramatically increases SNR, often as much as 10-fold. Clinically this is routinely put into practice for superficial body parts, such as the temporomandibular joints, in which small coils are placed directly over the joints to achieve rapid high-resolution imaging. For deep structures, the use of superficial coils is of no benefit. This has led to the development of endocavitary coils, such as the endorectal coil used to image the prostate gland. Such coils are now in routine clinical use here at the NIH and elsewhere.

During routine TSS, the surgical approach to the pituitary provides a route for placement of imaging tools, such as handheld ultrasound and Doppler probes in close proximity to the gland. Extending this model to MRI imaging, we realized that an endocavitary surface coil within the sphenoid sinus will allow for a marked improvement in SNR for imaging the sella. To this end, we have developed an endosphenoidal coil (ESC), demonstrated its MRI safety, and performed preliminary studies in cadaver heads to determine that the ESC can be placed through the transsphenoidal approach. Placement of ESC needs no modification in the surgical TSS approach to the pituitary gland. The goal of this protocol is to examine the safety and feasibility of ESC placement and imaging during TSS.