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. current imaging | EEG | magnetoencephalography I n some cases, including living neural tissue, electric charges moving within the sample produce oscillating magnetic fields that can be visualized by MRI methods. The imaging of current distributions by MRI has developed significantly over the last 20 years, with early applications being directed toward the imaging of current density and conductivity in model systems (1, 2) and later in vivo (3-5). However, the primary focus in the development of current imaging is the possibility of directly imaging neuronal currents.While the currents generated by a single neuron are far too small to measure, detectable magnetic field changes on the order of 0.1-1 nT (6) may result from synchronized postsynaptic currents in a large number of neurons. The frequency of oscillatory neural activity is also extremely significant. In addition to the previously demonstrated importance of alpha wave (∼10 Hz) processes (7), a body of recent work has identified the importance of brain activity in the gamma and high gamma frequency ranges (25-250 Hz) (8-10) to the synchronization of anatomically distant centers. To date, most successful approaches to the mapping of these frequencies have involved the implantation of electrodes in direct contact with the brain, usually during a surgical procedure. A noninvasive measurement of oscillating c...

Bloch simulations towards direct detection of oscillating magnetic fields using MRI with spin-lock sequence

“…Based on this effect, by focusing on neural magnetic fields oscillating at specific frequenciessuch as α and γ waves-it is possible to conduct fMRI studies with the spin-lock sequence [7], [8].…”

“…Spin-lock imaging is another reported method for achieving fMRI focusing on neural magnetic fields [7], [8]. As *This work was partially supported by a Grant-in-Aid for Challenging Exploration Research (24650221) , a Graint-in-Aid for Scientific Research (A) (24240081) and a Grant-in-Aid for JSPS Fellows Spin-lock prepared spin echo pulse sequence.…”

Magnetic Resonance Current Density Imaging: Applications to Electrochemotherapy and Irreversible Electroporation

“…Soon after, the method was also implemented in 3D [2]. This was followed by a modification of CDI to image radiofrequency currents at the Larmor frequency (RF-CDI) [3] and alternating electric currents (AC-CDI) [4,5].…”

Realistic models for detection of neuronal currents with magnetic resonance imaging

“…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. The SIRS method has been demonstrated to be an effective method to image audio-frequency magnetic fields in a water phantom (Halpern-Manners et al, 2010). However, the SIRS method suffers from low sensitivity, as the longitudinal relaxation time of the magnetization in the rotating-frame (T1ρ) is very short (approximately 100 msec), limiting the spin lock time ( sl  ) used to modulate NMFs.…”

“…The spinlock state can be tuned to match the frequency of oscillating NMFs (Halpern-Manners et al, 2010). However, the grey and white matter in the brain have short spin-lattice relaxation time in the rotating frame (known as 1 T  ) (less than 100 msec at 1.5T) (Borthakur et al, 2006), which imposes a primary limitation on the spin-lock time and consequently reduced the sensitivity of the technique.…”