Application and Progress of Real-time Interleaved Transcranial Magnetic Stimulation Functional Magnetic Resonance Imaging

Transcranial magnetic stimulation (TMS), as a non-invasive neuromodulation technique, has achieved certain results in the study of brain function localization, treatment of nervous and mental diseases, but its mechanism of action and physiological effects are difficult to be clarified. The signals in blood oxygenation level dependent functional magnetic resonance imaging (BOLD-fMRI) is capable of reflecting the activities of brain tissue neurons. TMS-fMRI combines the advantages of two techniques to monitor changes in excitability of the TMS stimulation site and the distal brain region with which it is functionally linked. In this review, we elaborates the advances, limitations and future expectations of real-time interleaved TMS-fMRI. From the perspective of application progress, in the field of brain functional connectivity network research, TMS-fMRI can observe the dynamic connection between brain networks with a resolution of 100 ms, which is an important progress in the exploration of time-specific functional connectivity of brain regions. However, the TMS-fMRI on spatial specificity of functional connectivity of brain regions is still unclear, and future studies can focus on this aspect. In addition, TMS-fMRI can assess the effect of TMS on the cerebral cortex and the interaction between brain regions, help us understand the neural mechanism behind attention control, and study the brain's processing of somatosensory sensation. Nevertheless, TMS-fMRI can only observe the correlation between excitability of different brain regions under the stimulation of TMS, but the mechanism of this phenomenon and whether the correlation between brain regions is specific needs more research. Futhermore, TMS-fMRI can also be used to study the pathogenesis and therapeutic effect of neurological and psychiatric diseases, and the effects of psychoactive compounds on brain regions. Nonetheless, currently TMS-fMRI is still difficult to be widely used in clinical practice, so more efforts are needed in the study of clinical indications of TMS-fMRI. There are two major technical problems of TMS-fMRI. One major problem is that it is difficult for TMS coils to accurately position specific areas of cerebral cortex in MRI scanner. Another major problem is that TMS coils affect the static magnetic field (B0), resulting in image artifacts, spatial distortion and local signal loss of echo-Planar (EP) images. Nowadays, researchers have solved the two major problems through continuous technical updates, but TMS-fMRI still has problems in parameter setting, user experience, simplicity and universality of application and other aspects, which is the direction of future technological progress.