Nearly 14% of thyroid and parathyroid surgeries result in temporary recurrent laryngeal nerve (RLN) palsies. Diagnostic imaging of the RLN and vagus nerve (VN) could help in surgical planning and in minimizing the risk of damage to the nerves. However, imaging these nerves is technically challenging due to their size, location (close to the trachea and esophagus), and physiological motion such as breathing and swallowing. The purpose of this work is to develop an MR imaging technique to visualize RLN and VN while overcoming the challenges associated with motion and susceptibility artifacts resulting from the trachea and esophagus.Various MR imaging sequences (VIBE, T1 & T2 TSE, MPRAGE, CISS and PSIF) were optimized and tested on 15 volunteers, of which CISS and T2 TSE showed the most promise. The volunteers were placed with their neck extended, mimicking the surgical position in a Magnetom Skyra wide bore 3T scanner (Siemens Healthcare, Germany). Optimal imaging was achieved with a 20 channel head-neck coil. The CISS sequence (3D acquisition, 0.8mm isotropic at a FOV of 240mm, TE/TR = 2.2/5, pixel BW = 505, FA = 42deg) was obtained un-navigated while free breathing. A novel phase navigated [1] T2 TSE sequence (2D acquisition, resolution of 0.5x0.5x2 mm at a FOV of 200mm, TE/TR = 80/2800, ETL = 20, pixel BW = 195, flip angle = 160deg) was optimized without fat saturation to accentuate the nerves. The novelty of the phase navigator lies in the ability to navigate on the larynx. An expert radiologist and surgeon evaluated the MRI to delineate the RLN. The left and right RLNs were confirmed by tracing the path from the vagus nerve, around the aortic arch and subclavian artery, respectively. For comparison the T2 TSE sequence was also acquired un-navigated and with the conventional liver dome scout in a subset of four volunteers.

Fig 1(a) shows the placement of the navigator on the larynx. Fig 1(b) shows the RLN (indicated by a green arrow) and the VN (indicated by a red arrow) on the T2 TSE. The RLN was identified by tracing the VN around the aortic arch. Fig 1(c,d) are the sagittal views of T2 TSE and CISS and Fig 1(e,f) are their corresponding axial views. As seen from these images the VN can be visualized on both T2 TSE and CISS (indicated by an arrow), however it is better delineated on the T2 TSE. Fig 2 shows the respiratory motion as for (a) regular and (b) irregular breathing/swallowing respectively. FIg 3 compares (a) un-navigated, (b) phase navigated and (c) liver dome navigated TSE acquisitions fused with the MIP acquired from Fig 4. The MIP from Fig 4. registered well with the images from Fig 3 with rigid registration. The visualization of the vessels overlaid over the TSE images helped locate the tiny nerves that could be easily missed. We observed that the phase navigator was less sensitive to the placement of the navigator and more sensitive to fine motion when compared to the liver dome navigator.

The RLN and VN can be visualized on the CISS and T2 TSE. Due to the phase navigator, the nerves are better delineated on the motion-compensated T2 TSE. The phase navigator is less sensitive to the placement and more sensitive to fine motion. With this technique we plan to implement preoperative imaging of the RLN and VN to help minimize nerve damage during minimally invasive parathyroidectomy or thyroidectomy.