Modulatory effects of magnetic vestibular stimulation on resting-state networks can be explained by subject-specific orientation of inner-ear anatomy in the MR static magnetic field

Abstract

Strong static magnetic fields, as used in magnetic resonance imaging (MRI), stimulate the vestibular inner ear leading to a state of imbalance within the vestibular system that causes nystagmus. This magnetic vestibular stimulation (MVS) also modulates fluctuations of resting-state functional MRI (RS-fMRI) networks. MVS can be explained by a Lorentz force model, indicating that MVS is the result of the interaction of the static magnetic field strength and direction (called “B0 magnetic field” in MRI) with the inner ear’s continuous endolymphatic ionic current. However, the high variability between subjects receiving MVS (measured as nystagmus slow-phase velocity and RS-fMRI amplitude modulations) despite matching head position, remains to be explained. Furthermore, within the imaging community, an “easy-to-acquire-and-use” proxy accounting for modulatory MVS effects in RS-fMRI fluctuations is needed. The present study uses MRI data of 60 healthy volunteers to examine the relationship between RS-fMRI fluctuations and the individual orientation of inner-ear anatomy within the static magnetic field of the MRI. The individual inner-ear anatomy and orientation were assessed via high-resolution anatomical CISS images and related to fluctuations of RS-fMRI networks previously associated with MVS. More specifically, we used a subject-specific proxy for MVS (pMVS) that corresponds to the orientation of the individual inner-ear anatomy within the static magnetic field direction (also called “z-direction” in MR imaging). We found that pMVS explained a considerable fraction of the total variance in RS-fMRI fluctuations (for instance, from 11% in the right cerebellum up to 36% in the cerebellar vermis). In addition to pMVS, we examined the angle of Reid’s plane, as determined from anatomical imaging as an alternative and found that this angle (with the same sinus transformation as for pMVS) explained considerably less variance, e.g., from 2 to 16%. In our opinion, an excess variability due to MVS should generally be addressed in fMRI research analogous to nuisance regression for movement, pulsation, and respiration effects. We suggest using the pMVS parameter to deal with modulations of RS-fMRI fluctuations due to MVS. MVS-induced variance can easily be accounted by using high-resolution anatomical imaging of the inner ear and including the proposed pMVS parameter in fMRI group-level analysis.

Comment On: Modulatory Effects of Magnetic Vestibular Stimulation on Resting-State Networks Can be Explained by Subject-Specific Orientation of Inner Ear Anatomy in the MR Static Magnetic Field

“…We have now demonstrated how these MVS-induced RSN modulations can be addressed in fMRI analysis on the group-level [12]. We propose a parameter, derived from high-resolution anatomical imaging of the inner ear, that can be used for regressing out significant parts of the variance in RS-fMRI fluctuations [12]. We acquired RS-fMRI, and high-resolution CISS anatomical images of the inner ear in a sample of N=60 healthy volunteers balanced for age, gender, and handedness.…”

“…We have now demonstrated how these MVS-induced RSN modulations can be addressed in fMRI analysis on the group-level [12]. We propose a parameter, derived from high-resolution anatomical imaging of the inner ear, that can be used for regressing out significant parts of the variance in RS-fMRI fluctuations [12].…”

“…The nystagmus was observed in healthy subjects but was absent for patients with bilateral vestibular loss, indicating a stimulation of the semicircular canals. done, either by repeating the same experiment at scanners with different field strengths, different field directions (south pole at foot end vs. south pole at the head end of the MRI, e.g., Siemens vs. Philips) or by varying the head positions across different sessions [1,2,12]. It is crucial to keep in mind that the effect of MVS is not like the constant acoustic noise stimulation during fMRI.…”

“…Our results suggest caution when interpreting such studies. In the case of patients with bilateral vestibular loss, it should be noted that the patients will not show an MVS influence, but the healthy control group will be under the influence of MVS [1][2][3]12,20]. This might then lead to changes in the comparison of differences between the two groups as examined with fMRI that are not expected to appear in imaging methods without the use of strong magnetic fields, e.g., PET (for a methods review see [19]).…”

“…This might then lead to changes in the comparison of differences between the two groups as examined with fMRI that are not expected to appear in imaging methods without the use of strong magnetic fields, e.g., PET (for a methods review see [19]). In this case, the healthy controls might be more akin to patients with acute unilateral vestibular neuritis, given that such patients also show a directional imbalance with horizontal nystagmus, not unlike that evoked by MVS [1- 3,5,12]. For studies of vestibular neuritis patients versus healthy controls (e.g.…”