Neural magnetic field dependent fMRI toward direct functional connectivity measurements: A phantom study

Abstract

Recently, the main issue in neuroscience has been the imaging of the functional connectivity in the brain. No modality that can measure functional connectivity directly, however, has been developed yet. Here, we show the novel MRI sequence, called the partial spinlock sequence toward direct measurements of functional connectivity. This study investigates a probable measurement of phase differences directly associated with functional connectivity. By employing partial spinlock imaging, the neural magnetic field might influence the magnetic resonance signals. Using simulation and phantom studies to model the neural magnetic fields, we showed that magnetic resonance signals vary depending on the phase of an externally applied oscillating magnetic field with non-right flip angles. These results suggest that the partial spinlock sequence is a promising modality for functional connectivity measurements. Comprehending brain activity is promising to conquer psychiatric and neurological disorders and develop emerging technologies, such as the brain-machine interface and neurocomputers. Many researchers, therefore, have been actively involved in unlocking the secret of brain activity, and developed many non-invasive techniques to measure brain activity. Non-invasive measurements of brain activity could be broadly divided into two categories. Electromagnetic methods detect the electric potentials and or magnetic fields generated by the electrical currents associated with local neuronal activity. The other group measures metabolic changes as a proxy for neuronal activities, such as oxygen and glucose concentrations. The former include electroencephalography (EEG) 1,2 and magnetoencephalography (MEG) 3,4 , having fine temporal resolution but coarse spatial resolution. The latter include functional magnetic resonance imaging (fMRI) 5 and functional near-infrared spectroscopy (fNIRS) 6. In particular, fMRI based on blood oxygenation level-dependent (BOLD) signals is widely used for functional brain mapping due to its fine spatial resolution 5. BOLD-fMRI, however, measures brain activity indirectly because it detects hemodynamic changes that follow the activation of neurons. Novel fMRI signals that measure neural magnetic fields directly, therefore, are desired. Although several attempts were made to detect neural magnetic fields, none have demonstrated human neural magnetic field detection with MRI 7-15. Such methods detect the phase shift or magnitude change of magnetisation correlating to the weak influence of neural magnetic fields on a static field. On the other hand, Witzel et al. 16 and Halpern-Manners et al. 17 reported neural magnetic field measurements with MRI instruments that exploit the spinlock sequence (Stimulus-Induced Rotary Saturation: SIRS). Based on the SIRS method, Jiang et al. proposed another spinlock sequence, called spinlock oscillatory excitation (SLOE) 18. Truong et al. proposed yet another spinlock sequence for fMRI 19. In this sequence, flip-back pulses are not applied in the SIRS sequence. These met...

Can MRI Be Used as a Sensor to Record Neural Activity?

“…Trong-Kha Truong and his colleagues, working in the laboratory of Allen Song at Duke University, developed a new MRI imaging method to record neural activity in humans [ 73 ]. The key to their method is to use the spin-lock technique [ 41 , 42 , 43 , 74 , 75 , 76 , 77 , 78 ].…”

Towards robust in vivo quantification of oscillating biomagnetic fields using Rotary Excitation based MRI

“…An essential part of the present study is the realization and validation of REX experiments without using external hardware such as loop-coils or dipole-electrodes for field generation 15 . Instead, we propose a new approach to study the interaction of ultra-weak magnetic field oscillations with spin-locking fields under in vivo conditions using exclusively the standardized hardware on a clinical MRI system.…”

“…Since the interaction of a SL pulse with a weak magnetic field oscillation has to be measured, phantom experiments have been performed so far in which the existing MRI setup had to be equipped with additional hardware. Examples include field-generating loop-coils or dipole electrodes 15 , which need precise calibration and RF shielding within the MRI room. Furthermore, accurate synchronization with the MRI sequence is required.…”

SIRS sequence as frequency filter: A simulation and phantom study of spin-lock preparations among composite signals

“…Several studies proved the capabilities of the method with simulations and phantom experiments [7][8][9][10][11] . Even more, the dependence of the contrast with the change of the excitation angle has been proposed as a new method for direct functional connectivity measurements [12] .…”