Magnetostimulation Limits in Magnetic Particle Imaging

Abstract

For magnetic particle imaging (MPI), specific absorption rate (SAR) and more critically magnetostimulation (i.e., dB/dt) safety limits will determine the optimal scan parameters, such as the drive field strength and frequency. These parameters will impact the scanning speed, field-of-view (FOV) and signal-to-noise ratio in MPI. Understanding the potential safety hazards of the drive field is critical for scaling MPI for human use. In this work, we demonstrate that magnetostimulation is the primary magnetic safety consideration in MPI, and we describe the first human-subject magnetostimulation threshold experiments for MPI using homogeneous coils. Our experiments, performed on the arm and leg, indicate that magnetostimulation thresholds monotonically decrease with increasing frequency. Additionally, we show for the first time that a strong inverse correlation exists between the threshold and the body part size. The chronaxie time, on the other hand, did not vary with body part size. We conclude with an estimation of the magnetostimulation thresholds for a full-body MPI scanner: a mean asymptotic threshold of 14.3 mT-pp (peak-to-peak) with a mean chronaxie time of 289 μs, which correspond to a magnetostimulation threshold of about 15 mT-pp for frequencies between 25 and 50 kHz. These findings will have a great impact on the optimization of MPI parameters, especially in determining the number of partial FOVs required to cover a region of interest.

Low drive field amplitude for improved image resolution in magnetic particle imaging

“…57,58 As we develop scanners with larger diameter bores, peripheral nerve stimulation plays an increasing role and requires lower drive field amplitudes. It is estimated that the maximum drive field amplitude is 8 mT in the torso, 57,58 which is far lower than the 20 mT used in early preclinical imagers. The effect of this reduction in drive field amplitude on the image quality has not been thoroughly explored.…”

“…It remains to be seen if these trends continue to even higher frequencies as recent work indicates that drive fields of frequencies up to >100 kHz may be necessary in a human scanner to reach SAR limits. 57,58 Additionally, some degree of resolution loss at higher drive field amplitudes may be offset using deconvolution techniques that trade-off the improvement in received signal to recover resolution, as described in the previous works. Figure 4 shows that a typical measured PSF reconstructed using x-space theory is accurately modeled by nonadiabatic x-space theory when assuming Debye relaxation [Eq.…”

Slew-rate dependence of tracer magnetization response in magnetic particle imaging

“…Peripheral nerve stimulation (PNS) 3 and tissue heating (specific absorption rate, or SAR) 4 are the principle concerns during application of alternating magnetic fields. In a recent study, Saritas et al 5 reported the magnetostimulation threshold amplitude to be about 7.5 mT/l 0 for human torso MPI in the frequency range 25-50 kHz. This finding led us to investigate how MPI tracers would behave under varying field parameters.…”

Image reconstruction for Magnetic Particle Imaging using an Augmented Lagrangian Method

“…One practical alternative is to divide the FOV into smaller FOVs to lower the drive field amplitude, which was shown to increase resolution with small number of received harmonics [15]. In fact, such an approach may be necessary to avoid nerve stimulation or prevent over-heating of the tissue for human imaging applications [16].…”