Constructive Interference in Steady State Imaging in the Central Nervous System
There are three main types of fast GRE sequence: spoiled, partially refocused, and fully refocused. When the pre- and post-excitation signals are lost and wasted, the resulting image is T1-weighted. When only the post-excitation signal is acquired, the images obtained are very sensitive to magnetic field inhomogeneities, with a resulting T2* (star) effect. A very similar image to CISS can be obtained by acquiring the pre-excitation signal only. This sequence has a relatively long echo time (TE) and therefore suffers from more flow artifacts and a lower signal-to-noise ratio. CISS is a fully refocused fast GRE sequence in which both pre- and post-excitation signals are sampled, with the subsequent advantage of having a high signal-to-noise ratio and high spatial resolution.1
Advantages of Constructive Interference in Steady State Imaging
One of the most important advantages of steady-state imaging is the short acquisition time thanks to the short TR and TE. With short TR and TE, tissues with long T2 relaxation will demonstrate wider range and additional signals owing to various refocused echo paths. Other advantages are the high signal-to-noise ratio and better contrast-to-noise ratio. Additionally, CISS images do not present significant susceptibility-, motion-, or flow-related artifacts.2
Uses of Constructive Interference in Steady State
CISS is widely used to assess the cranial nerves (CNs). The CNs were first studied by CISS at the beginning of the 1990s, and the first CNs studied were the facial (CNVII) and vestibulocochlear (CNVIII) nerves.3
Owing to its
cisternographic effect, CISS provides finely detailed images of the CNS, especially of their cisternal and canalicular portions (see Figures 1 and 2).
In cases of pulsatile tinnitus or facial spasms, CISS plays a major role in the assessment of vascular loops compressing the nerves in the cerebellopontine angle cistern (CPA) or inside the internal auditory canal (IAC) (see Figure 3).4
CPA cistern, especially in cases of hearing impairment.5 The Cranial Nerves The Olfactory and Optic Nerves
CISS can be used in the evaluation of the olfactory nerve (CNI). It provides better results than 2D turbo spin echo, although not significantly better in comparison with ultrafast spoiled gradient echo 3D imaging (multiplanar gradient-recalled [MPGR]/magnetization-prepared rapid acquisition gradient echo [MPRAGE]).6
CISS provides similar results to
MPRAGE in the assessment of the optic nerve (CNII), although MPRAGE is superior to CISS for studying the optic chiasm and tract.7
The Oculomotor Nerve
The oculomotor nerve (CNIII) is readily seen from the interpeduncular fossa to the cavernous sinuses (see Figure 4).8
The CNIII runs between the
posterior cerebral artery (PCA) and the superior cerebellar artery (SCA) and innervates the extraocular muscles, including the elevator muscle of the upper eyelid, except for the superior oblique and the lateral rectus muscles. CISS can potentially be used to rule out aneurysms arising from the posterior circulation or from the posterior communicating artery that impinge on the CNIII.9
US NEUROLOGY Arrow showing the right oculomotor nerve (CNIII). The Trochlear Nerve
The trochlear nerve (CNIV) is the longest intracranial nerve, although it is the smallest in terms of number of axons. It originates in the dorsal
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There is an anterior inferior cerebellar artery loop inside the right internal auditory canal (arrow) in close association with and likely compressing the seventh and eighth nerves.
Figure 4: Axial Constructive Interference in Steady State Image at Level of Interpeduncular Fossa Demonstrating the Cisternal Portions of the Oculomotor Nerves
Figure 1: Axial Constructive Interference in Steady State Image of the Posterior Fossa at the Level of the Inner Ear Demonstrating the Normal Anatomy of the Cisternal and Canalicular Portions of the Seventh and Eighth Nerves
Cochlear nerve Facial nerve
Vestibulocochlear nerve
Figure 2: Sagittal Constructive Interference in Steady State Image Through the Internal Auditory Canal, Showing the Facial, Cochlear, and the Superior and Inferior Vestibular Nerves
Facial nerve
Superior vestibular nerve
Cochlear nerve
Inferior vestibular nerve
Figure 3: Axial Constructive Interference in Steady State Images of the Posterior Fossa, Centered at the Right Inner Ear, in a Patient with Pulsatile Tinnitus
This sequence is also useful to study masses in the
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