Exploring Advanced Brain Stimulation Techniques: A Dive into the Use of Transcranial Magnetic Stimulation (TMS) and Transcranial Ultrasound Stimulation (TUS).

The integration of cutting-edge technologies in neuroscience research continues to push the boundaries of our understanding of brain function and plasticity. A recent study showcased the innovative use of Transcranial Magnetic Stimulation (TMS) and Transcranial Ultrasound Stimulation (TUS) to investigate neural excitability in the motor cortex (M1). These tools were instrumental for Bao et al. (2024) in achieving precise and effective brain stimulation, ultimately leading to significant findings in the field. 

 

Transcranial Ultrasound Stimulation (TUS) 

TUS was administered using a four-element spherical ultrasound transducer, driven at a frequency of 500 kHz. The transducer was positioned over the hand representation area of the left M1, identified using MRI scans and the Brainsight Neuronavigation system. Acoustic coupling was ensured with a water-filled cone and ultrasound gel to maximise the efficiency of ultrasound energy transmission. 

Key parameters for TUS included: 

  • Focal Depths: Approximately 5 mm and 16 mm beneath the cortical surface. 
  • Intensity Levels: ISPPA of 2.38 W/cm² and ISPTA of 0.24 W/cm², well within FDA safety guidelines. 

 

Transcranial Magnetic Stimulation (TMS)  

To assess changes in M1 excitability post-TUS, single pulses of TMS were applied using the DuoMAG MP-Dual system. This system featured a figure-8 butterfly coil, positioned over the hand representation area of M1. The MEPs (motor-evoked potentials) recorded from the first dorsal interosseous muscle served as a measure of corticospinal excitability, providing insights into neuroplastic changes induced by TUS. 

 

TUS Simulation with k-Plan Software 

The k-Plan software was employed to simulate the depth and acoustic intensity of TUS within the brain. Using the pseudo-CT images, the software allowed researchers to tailor the TUS parameters to each participant’s unique skull morphology. This ensured precise delivery of ultrasound energy to the targeted brain regions while maintaining safe thermal and acoustic conditions. 

 

Insights into M1 Excitability 

The simulations produced precise models of skull acoustic properties, enabling accurate targeting of M1 at both superficial (∼5 mm) and deep (∼16 mm) levels. Acoustic pressure maps revealed that individualised simulations minimised interactions between superficial and deep stimulation, ensuring targeted effects. Temperature increases due to TUS were minimal, highlighting the technique's safety. 

 

Continuous and Intermittent Theta-Burst TUS 

  • Continuous Theta-Burst TUS: This protocol induced long-term depression (LTD)-like plasticity in both superficial and deep layers of M1, as evidenced by significant reductions in MEP amplitudes at both 5- and 30-minutes post-stimulation. These findings align with previous studies on continuous theta-burst TMS, demonstrating TUS's efficacy in modulating M1 excitability. 
  • Intermittent Theta-Burst TUS: Contrary to continuous stimulation, intermittent theta-burst TUS did not significantly alter MEP amplitudes, suggesting that this protocol does not induce long-term potentiation (LTP)-like plasticity in M1. 

 

The use of TMS and TUS in this study underscores the potential of combining advanced technologies to achieve precise and effective brain stimulation. The integration of personalised simulations with TUS and the robust assessment capabilities of TMS allowed for a detailed exploration of M1 plasticity, paving the way for future research and therapeutic applications. As these technologies continue to evolve, their combined use promises to enhance our understanding of brain function and facilitate the development of novel interventions for neurological disorders.