Effects of pulse duration on magnetostimulation thresholds

Area of Science:

• Biomedical Engineering
• Medical Imaging Physics

Background:

• Medical imaging techniques like magnetic resonance imaging and magnetic particle imaging (MPI) use time-varying magnetic fields.
• Magnetostimulation limits, influenced by field amplitude, frequency, and pulse characteristics, can restrict imaging speed.
• Previous studies suggested pulse cycle number affects thresholds, but lacked clarity on pulse duration's role and applicability to homogeneous fields.

Purpose of the Study:

• To investigate the dependence of magnetostimulation thresholds on pulse duration using homogeneous magnetic fields.
• To determine if this effect varies across a 20-fold range of frequencies relevant to MPI.
• To clarify the relationship between pulse duration and safety limits in homogeneous magnetic fields.

Main Methods:

• Magnetostimulation thresholds were measured in six healthy subjects' arms.
• Experiments involved testing eight pulse durations (2-125 ms) at three frequencies (1.2, 5.7, 25.5 kHz) using a solenoid coil.
• Subjects reported perceived stimulation, and data were analyzed using sigmoid function fitting to determine thresholds.

Main Results:

• Magnetostimulation thresholds significantly decreased with increasing pulse duration (T(pulse)).
• Thresholds were independent of the tested frequencies, showing consistent trends across all.
• Thresholds were ~24% higher at short durations (~2 ms) and approached asymptotic values for T(pulse) > 20 ms, characterized by a stretched exponential function.

Conclusions:

• Magnetostimulation thresholds decrease with increasing pulse duration, irrespective of operating frequency.
• The observed trends are consistent across a 20-fold frequency range, with thresholds stabilizing for pulse durations over 20 ms.
• Understanding pulse duration dependence is critical for aligning human-subject safety experiments with specific imaging system conditions and enhancing imaging efficiency.