Visualization of the Trochlear Nerve in the Cistern with Use of High-Resolution Turbo Spin-Echo Multisection Motion-Sensitized Driven Equilibrium

Discussion

Our study demonstrates that we can almost always observe the cisternal portion of the trochlear nerve by using HR-MSDE. The trochlear nerve, the diameter of which ranges from approximately 0.3 to 0.9 mm, is the thinnest of all cranial nerves.¹³ Moreover, it has a complex 3D course around the midbrain, and many arteries and veins, as well as the cerebellar tentorium, are in close proximity to the nerve.^4⇓⇓–7 Thus, it has been difficult to identify the trochlear nerve by using a variety of even HR MR imaging sequences. We demonstrated that HR-MSDE was significantly superior to both HR-MRC and BS-MRC in visualizing the trochlear nerve, with high concordance between 2 neuroradiologists.

The perimesencephalic segment of the trochlear nerve was generally well identified on all MR images used. This is probably because of the course of the nerve around the midbrain. The trochlear nerve originates from the root exit zone in the dorsal pontomesencephalic junction and runs a curved course that differs from the course of the vessels in the perimesencephalic cistern. In contrast, the tentorial segment of the trochlear nerve tended to be obscured on the MRC images, as it runs along the tentorial edge and is parallel to the vessels in its tentorial segment. Therefore, the trochlear nerve may be difficult to differentiate from the tentorial edge and/or vessels on MRC images because both nerves and vessels are rendered with low intensity.

TSE-MSDE provides 3D T1 contrast images that can nearly entirely suppress flow signals by using a preparation pulse. Previous reports^{8⇓⇓⇓–12} have discussed TSE-MSDE regarding the atherosclerotic change in vessels, the existence of brain metastasis, and focal deformation of nerves by vessels in neurovascular compression syndrome. In our study, we obtained HR images with TSE-MSDE (ie, HR-MSDE) to visualize the trochlear nerve. The flow signals from vessels and CSF were well suppressed, and only the cranial nerves were clearly visualized in the cistern. In addition, the signal of the cerebellar tentorium was also suppressed because its low T1 contrast was the same as that of CSF. Thus, the trochlear nerve was well visualized not only in its perimesencephalic segment but also in its tentorial segment. Because the oculomotor nerve and the trigeminal nerve are much thicker than the trochlear nerve, these cranial nerves were clearly visualized in the same scan range. As MR imaging contrast media can be used in conjunction with HR-MSDE, we may in the future be able to visualize actual lesions causing trochlear nerve paralysis by using this method and applying it to clinical cases.

MRC is generally an adequate method to delineate cranial nerves and related vessels.¹⁴ The trochlear nerve is so thin, however, that much thinner sections are required to visualize it than are routinely acquired. If very thin sections are acquired, then prominent flow-related artifacts will occur as a result of CSF, and this mechanism creates a significant problem in the observation of the nerves and vessels. Indeed, the trochlear nerve was poorly identified on HR-MRC images, especially in its tentorial segment.

In general, 3D constructive interference in steady state and fast imaging employing steady-state acquisition visualize the cranial nerves well, such as the oculomotor, trigeminal, abducens, facial, auditory, and lower cranial nerves, but the trochlear nerve is an exception.^{15⇓⇓⇓⇓⇓–21} Cranial nerves that penetrate the cavernous sinus, such as the oculomotor, trochlear, trigeminal, and abducens nerve, can be visualized by 3D constructive interference in steady state.²¹ However, it has been difficult to visualize the trochlear nerve in its cisternal segment. Choi et al¹³ reported that the trochlear nerve can be clearly visualized with HR 3D balanced turbo-field echo. We applied this method in our MR imaging unit and obtained BS-MRC. It is easier to identify the trochlear nerve with BS-MRC compared with HR-MRC because there is decreased influence of flow-related artifacts from CSF.²² However, band artifacts often occur in the peripheral area of the cistern on BS-MRC images, and there is a scan range limit. Thus, we can only observe the central area in the area of interest, and the observable area is narrow.

The major limitation of our study was the long scan time required. It took approximately 30 minutes for HR-MSDE; therefore, the scan time is so long that this sequence is not yet of practical use. In our study, we acquired 120 sections in HR-MSDE to visualize a thin nerve, the trochlear nerve, with HR and high signal-to-noise ratio. If the number of sections is reduced, then signal-to-noise ratio decreases, and it may become impossible to identify the trochlear nerve. A much shorter scan time for this sequence for practical use should be achievable through the improvement of sequence parameters or the use of a 7T MR imaging unit in the future. Decreased scan time would also allow application of this method in pediatric cases (eg, congenital palsy of the superior oblique muscle). In our study, however, the scan time was not able to be reduced anymore; therefore, it is impossible to apply this method to pediatric cases at present. We also acknowledge that our BS-MRC was a slightly different sequence from the originally reported one, as we had to adjust parameters for use on a different MR imaging machine.