Magneto-stimulation limits in medical imaging applications with rapid field dynamics

Area of Science:

• Biomedical Engineering
• Medical Imaging Physics
• Neuroscience

Background:

• Peripheral nerve stimulation (PNS) is crucial for understanding the safety and performance of medical imaging systems like MRI and MPI.
• Existing PNS research often focuses on lower frequencies, limiting applicability to emerging high-frequency imaging techniques.

Purpose of the Study:

• To investigate PNS thresholds under conditions relevant to high-frequency Magnetic Particle Imaging (MPI) and low-field Magnetic Resonance Imaging (MRI).
• To characterize the impact of magnetic field dynamics, pulse characteristics, and spatial distribution on PNS.
• To bridge the gap in PNS literature for fast-regime MRI and broad-band stimulation in MPI.

Main Methods:

• Developed a specialized apparatus for determining PNS thresholds on human limbs.
• Employed narrow and broad-band magnetic stimulation with pulse characteristic times as low as 40 μs.
• Conducted measurements on 51 volunteers, analyzing responses to varying pulse frequencies, rise-times, and field spatial distributions.

Main Results:

• PNS frequency/rise-time dependence aligns with established models (rheobase and chronaxie).
• PNS thresholds significantly increase as pulse trains decrease from tens to a few pulses at fast timescales.
• Higher PNS thresholds were observed in approximately linearly inhomogeneous fields (MRI-relevant) compared to homogeneous fields (MPI-relevant).

Conclusions:

• PNS characteristics at kilohertz frequencies are consistent with traditional neurostimulation models.
• The spatial distribution of magnetic fields significantly influences PNS thresholds, with inhomogeneous fields posing a greater stimulation risk.
• These findings provide essential data for optimizing the clinical performance and safety of advanced MRI and MPI systems.