Stimulus-induced Rotary Saturation (SIRS): A potential method for the detection of neuronal currents with MRI

Abstract

Neuronal currents produce local transient and oscillatory magnetic fields that can be readily detected by MEG. Previous work attempting to detect these magnetic fields with MR focused on detecting local phase shifts and dephasing in T2 or T2* weighted images. For temporally biphasic and multiphasic local currents the sensitivity of these methods can be reduced through the cancellation of the accrued phase induced by positive and negative episodes of the neuronal current. The magnitude of the phase shift is also dependent on the distribution of the current within the voxel. Since spins on one side of a current source develop an opposite phase shift relative to those on the other side, there is likely to be significant cancellation within the voxel.We introduce a potential method for detecting neuronal currents though their resonant T1ρ saturation during a spin-lock preparation period. The method is insensitive to the temporal and spatial cancellation effects since it utilizes the multi-phasic nature of the neuronal currents and thus is not sensitive to the sign of the local field. To produce a T1ρ reduction, the Larmor frequency in the rotating frame, which is set by γB 1lock (typically 20 Hz -5 kHz), must match the major frequency components of the stimulus-induced neuronal currents. We validate the method in MRI phantom studies. The rotary saturation spectra showed a sharp resonance when a current dipole within the phantom was driven at the Larmor frequency in the rotating frame. A 7 minute block design experiment was found to be sensitive to a current dipole strength of 56 nAm, an approximate magnetic field of 1 nT at 1.5 mm from the dipole. This dipole moment is similar to that seen using the phase shift method in a similar experimental setup by Konn et.al. (2003), but is potentially less encumbered by temporal and spatial cancellation effects.

Magnetic resonance imaging of oscillating electrical currents

“…4B is calculated to be approximately 0.46 nT, based on the level of attenuation and the field map in Fig. 1E, which agrees well with previous sensitivity characterizations (39).…”

“…As has been shown previously (39), many issues related to cancellation of accumulated phase in phase contrast techniques can be resolved by using a rotating-frame resonant mechanism, termed stimulus-induced resonant saturation (SIRS), to generate current-induced contrast. SIRS builds upon earlier work (38), in which a prepolarized sample is simultaneously placed in an ultralow field (ω H ¼ 40-4; 000 Hz) and an orthogonal magnetic field designed to mimic the oscillating fields of interest.…”

Relationship altered between functional T1ρ and BOLD signals in bipolar disorder

“…This chemical exchange is influenced by pH and metabolite concentration (e.g., glucose and glutamate) (Jin et al., 2011; Kettunen, Gröhn, Silvennoinen, Penttonen, & Kauppinen, 2002), which have been shown to change in response to neural activation prior to changes in blood flow (Belanger, Allaman, & Magistretti, 2011). T1ρ relaxation is also sensitive to stimulus‐induced rotary saturation (SIRS), which may directly measure neuronal currents (Witzel, Lin, Rosen, & Wald, 2008). Thus, quantification of T1ρ relaxation temporally is hypothesized to provide a means to detect an early, localized, and nonhemodynamic tissue response to brain activation.…”

Realistic models for detection of neuronal currents with magnetic resonance imaging

“…One challenge of detecting the neuronal current using conventional SE-EPI and GE-EPI sequences is that these sequences do not specifically target neuronal current signals; they also detect signals from other sources, for example physiological noise, the susceptibility effect and changes due to diffusion. One approach to avoid these interfering signals is to image neuronal currents using the stimulus-induced resonance saturation (SIRS) method (Witzel et al, 2008), which is a resonance mechanism in a rotating-frame. SIRS targets NMFs that oscillate at a specific frequency range.…”

“…SIRS uses a spin-lock method to shift the NMFs to a longitudinal magnetization before a conventional SE or GE sequence is used to measure it (see Figure 3-8). In the spin-lock module, the oscillatory NMFs are used as a "RF" pulse to rotate the magnetization away from a spin-lock magnetic field ( sl B ) in a frame rotating at the Larmor frequency around the imaging field (see Figure 3-8 and also (Witzel et al, 2008)). This mechanism is similar to the application of an RF pulse to rotate the magnetization away from the imaging field, except that it occurs in a rotating frame.…”