Cranial nerve impairments can significantly compromise functionality and quality of life. Knowledge of the anatomic course of each cranial nerve and its relationship with surrounding structures is important for diagnosing various cranial pathologies and preventing complications from surgical interventions.
Despite recent advances in MR imaging technology, visualization of the entire course of the cranial nerves, especially the extracranial segments, remains challenging in routine clinical practice. A 3D double-echo steady-state with water excitation (3D-DESS-WE) sequence, commonly used in the orthopedic field, was recently applied to visualize the intraparotid facial nerve with a 1.5T MR unit and was quite successful.1 We have applied this technique to the trigeminal nerve with a 3T MR unit to investigate the detectability of the extracranial peripheral branches of the mandibular nerve (V3) because the trigeminal nerve has the largest distribution of innervation among all the cranial nerves in the suprahyoid neck, and the V3 is the largest division of the trigeminal nerve.2
The 3D-DESS-WE sequence involves the acquisition of 2 different echoes during each TR based on the steady-state free precession technique. The first echo is the free induction decay gradient echo used in the FISP sequence and the second is the spin-echo used in the reversed fast imaging with steady-state free precession (PSIF) sequence. PSIF signal intensity has a dominant T2 contrast.3 The FISP signal intensity provides more anatomic details with tissue contrast dominated by the T1/T2 ratio.1 With the signal characteristics, the 3D-DESS-WE sequence shows the nerve itself as a high-signal-intensity structure.
There are 3 advantages to using the 3D-DESS-WE sequence for MR neurography: 1) it has a relatively short acquisition time, 2) it can be imaged using a standard, commercially available head/neck coil without a surface coil, and 3) it enables uniform detectability of the peripheral branches of the cranial nerve by readers at various training levels.2 These features make the 3D-DESS-WE sequence feasible in routine clinical practice. Currently, because the surgeons at our institution highly praise this novel MR neurography technique, we routinely run the 3D-DESS-WE sequence to demonstrate the relationship between cranial nerves and lesions and to help our head and neck/oral surgeons plan treatment.