Magnetic resonance imaging of oscillating electrical currents

Abstract

Functional MRI has become an important tool of researchers and clinicians who seek to understand patterns of neuronal activation that accompany sensory and cognitive processes. However, the interpretation of fMRI images rests on assumptions about the relationship between neuronal firing and hemodynamic response that are not firmly grounded in rigorous theory or experimental evidence. Further, the blood-oxygen-level-dependent effect, which correlates an MRI observable to neuronal firing, evolves over a period that is 2 orders of magnitude longer than the underlying processes that are thought to cause it. Here, we instead demonstrate experiments to directly image oscillating currents by MRI. The approach rests on a resonant interaction between an applied rf field and an oscillating magnetic field in the sample and, as such, permits quantitative, frequency-selective measurements of current density without spatial or temporal cancellation. We apply this method in a current loop phantom, mapping its magnetic field and achieving a detection sensitivity near the threshold required for the detection of neuronal currents. Because the contrast mechanism is under spectroscopic control, we are able to demonstrate how ramped and phase-modulated spin-lock radiation can enhance the sensitivity and robustness of the experiment. We further demonstrate the combination of these methods with remote detection, a technique in which the encoding and detection of an MRI experiment are separated by sample flow or translation. We illustrate that remotely detected MRI permits the measurement of currents in small volumes of flowing water with high sensitivity and spatial resolution.

Fig. 1.

Data from current-imaging experiments using a single loop phantom. (A) Integrated peak area during a sweep of the spin-lock power across resonance with an audio-frequency magnetic field (τ_sl = 100 ms, ν_current = 100 Hz, and V_current = 800 μV). (B) Two-dimensional images with ν_current = 100 Hz, and V_current = 1.5 mV, at τ_sl = 20, 40, 60, and 80 ms. (C–E) Data from a τ_sl-incremented series of images with ν_current = 100 Hz and V_current = 1.5 mV. The magnitude of a voxel in the center of the current loop is shown in C with an overlaid fit of a cos²-modulated exponential decay that would simulate the projection of the y component of an oscillating transverse magnetization. Fourier transformation of these data (after correcting for the relaxation decay) (D) reveals an average oscillation frequency of ∼12.8 Hz in the center of the loop, corresponding to a field strength of ∼300 nT at this voltage. Plotting the magnetic field given by the average frequency in each voxel yields a map of the field strength throughout the slice (E).

Fig. 2.

Two-dimensional images taken with multitone current signals (V_current = 1.5 mV). Two illustrative situations are shown: the top row shows images taken with a current pulse containing 70- and 130-Hz frequencies, both with (A) the standard constant-amplitude spin lock at resonance and with (B) a ramped spin lock that increases from 70% to 130% of the resonant power (τ_sl = 100 ms). The bottom row shows images taken with a current pulse containing frequencies from 70 to 130 Hz, in 10-Hz increments, again (D) with and (C) without a ramped spin lock.

Fig. 3.

Three-dimensional images of the saturation effect at V_current = 500 μV. In A, the single-loop phantom is imaged with τ_sl = 50 ms and ν_current = 100 Hz. In the double-loop phantom of B, current (τ_sl = 160 ms) is applied simultaneously in each loop at different frequencies, while the spin-lock power is switched between resonance conditions to selectively image only one loop. The top loop (red) has ν_current = 100 Hz, while the bottom loop (blue) has ν_current = 250 Hz. Contours are shown at 30%, 50%, 70%, and 90% saturation with respect to controls.

Fig. 4.

Images of a single loop at very low-driving voltages. The voltage of the audio-frequency currents are approximately (A) 4.74 μV and (B) 2.38 μV, with estimated field strengths at the center of the loop of (A) 0.92 nT and (B) 0.46 nT. Images were taken with ν_current = 100 Hz, τ_sl = 160 ms, and eight averages. Voxel magnitude is displayed as percentage saturation with respect to a control.

Fig. 5.

Remotely detected current-imaging experiments in (A) a serpentine flow phantom. Water flows in 150-μm PEEK tubing laid out in s-shaped curves, travels through a solenoid in which audio-frequency current encoding takes place and then flows into an optimized microsolenoid NMR detector. (B) A single time-of-flight image from a control experiment without current excitation. Images illustrating the phase accrued during current excitation and due to a resonant mechanism, relative to a control, are shown for (C) a nonselective experiment with FOV_y = 2.41 cm and FOV_z = 3.62 cm and for (D) a zoomed-in experiment that isolates a slice containing the coil, giving FOV_y = 0.48 cm and FOV_z = 1.45 cm. Images were taken with ν_current = 400 Hz, τ_sl = 20 ms, and V_current = 1.6 mV. All images have resolution 90 × 90 after zero filling by a factor of 2 and have comparable signal to noise.