Human PNS and SAR study in the frequency range from 24 to 162 kHz

Schmale, Ingo; Gleich, Bernhard; Schmidt, J.; Rahmer, Jürgen; Bontus, Claas; Eckart, R.; David, B.; Heinrich, M.; Mende, Oliver; Woywode, O.; Jokram, J.; Borgert, J.

doi:10.1109/iwmpi.2013.6528346

Cited by 35 publications

(36 citation statements)

References 5 publications

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“…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.…”

Section: A Resolution Improvement With Lower Drive Field Strengthsmentioning

confidence: 99%

“…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.…”

Section: A Resolution Improvement With Lower Drive Field Strengthsmentioning

confidence: 99%

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

et al. 2015

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Purpose: Magnetic particle imaging (MPI) is a new imaging technology that directly detects superparamagnetic iron oxide nanoparticles. The technique has potential medical applications in angiography, cell tracking, and cancer detection. In this paper, the authors explore how nanoparticle relaxation affects image resolution. Historically, researchers have analyzed nanoparticle behavior by studying the time constant of the nanoparticle physical rotation. In contrast, in this paper, the authors focus instead on how the time constant of nanoparticle rotation affects the final image resolution, and this reveals nonobvious conclusions for tailoring MPI imaging parameters for optimal spatial resolution. Methods: The authors first extend x-space systems theory to include nanoparticle relaxation. The authors then measure the spatial resolution and relative signal levels in an MPI relaxometer and a 3D MPI imager at multiple drive field amplitudes and frequencies. Finally, these image measurements are used to estimate relaxation times and nanoparticle phase lags.Results: The authors demonstrate that spatial resolution, as measured by full-width at half-maximum, improves at lower drive field amplitudes. The authors further determine that relaxation in MPI can be approximated as a frequency-independent phase lag. These results enable the authors to accurately predict MPI resolution and sensitivity across a wide range of drive field amplitudes and frequencies. Conclusions: To balance resolution, signal-to-noise ratio, specific absorption rate, and magnetostimulation requirements, the drive field can be a low amplitude and high frequency. Continued research into how the MPI drive field affects relaxation and its adverse effects will be crucial for developing new nanoparticles tailored to the unique physics of MPI. Moreover, this theory informs researchers how to design scanning sequences to minimize relaxation-induced blurring for better spatial resolution or to exploit relaxation-induced blurring for MPI with molecular contrast. C 2016 American Association of Physicists in Medicine. [http://dx

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Section: A Resolution Improvement With Lower Drive Field Strengthsmentioning

confidence: 99%

Section: A Resolution Improvement With Lower Drive Field Strengthsmentioning

confidence: 99%

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

et al. 2015

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“…9 Later, a direct study on the torso (i.e., for whole body imaging) determined the magnetostimulation limits to lie within the range 8.8-15.2 mT-pp for the torso (for axial fields with frequencies between 24 and 162 kHz). 10 Hence, accurately controlled experiments as the ones in this work shed light onto the magnetostimulation limits in general, whichever part of the body the application may target. In addition, as discussed above, the chronaxie time did not show a significant variation with body part size.…”

Section: Discussionmentioning

confidence: 90%

“…8,9 Specifically, for the drive field in MPI, magnetostimulation was shown to be the dominant safety concern for frequencies up to 50 kHz, 1,9 potentially extending up to 150 kHz. 10,11 Knowing the safety limits and their dependence on all contributing factors is crucial for fast imaging. In MPI, both the scanning rate and the signal-to-noise ratio (SNR) scale with the amplitude and the frequency of the drive field.…”

Section: Introductionmentioning

confidence: 99%

Effects of pulse duration on magnetostimulation thresholds

2015

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Purpose: Medical imaging techniques such as magnetic resonance imaging and magnetic particle imaging (MPI) utilize time-varying magnetic fields that are subject to magnetostimulation limits, which often limit the speed of the imaging process. Various human-subject experiments have studied the amplitude and frequency dependence of these thresholds for gradient or homogeneous magnetic fields. Another contributing factor was shown to be number of cycles in a magnetic pulse, where the thresholds decreased with longer pulses. The latter result was demonstrated on two subjects only, at a single frequency of 1.27 kHz. Hence, whether the observed effect was due to the number of cycles or due to the pulse duration was not specified. In addition, a gradient-type field was utilized; hence, whether the same phenomenon applies to homogeneous magnetic fields remained unknown. Here, the authors investigate the pulse duration dependence of magnetostimulation limits for a 20-fold range of frequencies using homogeneous magnetic fields, such as the ones used for the drive field in MPI. Methods: Magnetostimulation thresholds were measured in the arms of six healthy subjects (age: 27 ± 5 yr). Each experiment comprised testing the thresholds at eight different pulse durations between 2 and 125 ms at a single frequency, which took approximately 30-40 min/subject. A total of 34 experiments were performed at three different frequencies: 1.2, 5.7, and 25.5 kHz. A solenoid coil providing homogeneous magnetic field was used to induce stimulation, and the field amplitude was measured in real time. A pre-emphasis based pulse shaping method was employed to accurately control the pulse durations. Subjects reported stimulation via a mouse click whenever they felt a twitching/tingling sensation. A sigmoid function was fitted to the subject responses to find the threshold at a specific frequency and duration, and the whole procedure was repeated at all relevant frequencies and pulse durations. Results: The magnetostimulation limits decreased with increasing pulse duration (T pulse ). For T pulse < 18 ms, the thresholds were significantly higher than at the longest pulse durations (p < 0.01, paired Wilcoxon signed-rank test). The normalized magnetostimulation threshold (B Norm ) vs duration curve at all three frequencies agreed almost identically, indicating that the observed effect is independent of the operating frequency. At the shortest pulse duration (T pulse ≈ 2 ms), the thresholds were approximately 24% higher than at the asymptotes. The thresholds decreased to within 4% of their asymptotic values for T pulse > 20 ms. These trends were well characterized (R 2 = 0.78) by a stretched exponential function given by B Norm = 1 + αeγ , where the fitted parameters were α = 0.44, β = 4.32, and γ = 0.60. Conclusions: This work shows for the first time that the magnetostimulation thresholds decrease with increasing pulse duration, and that this effect is independent of the operating frequency. Normalized threshold vs duration trends are almost identi...

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“…Once the technical challenges of upscaling will have been overcome, the first clinical demonstrator will help to actually evaluate the performance of MPI at a whole-body scale. Then, different acquisition schemes will have to be studied on a clinically relevant scenario, taking into account the different trade-offs to be made, particularly regarding PNS and energy absorption (specific absorption rate [SAR]) 63,71,100…”

Section: Discussionmentioning

confidence: 99%

Magnetic particle imaging: current developments and future directions

Panagiotopoulos¹,

Duschka²,

Ahlborg³

et al. 2015

IJN

234

160

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Magnetic particle imaging (MPI) is a novel imaging method that was first proposed by Gleich and Weizenecker in 2005. Applying static and dynamic magnetic fields, MPI exploits the unique characteristics of superparamagnetic iron oxide nanoparticles (SPIONs). The SPIONs’ response allows a three-dimensional visualization of their distribution in space with a superb contrast, a very high temporal and good spatial resolution. Essentially, it is the SPIONs’ superparamagnetic characteristics, the fact that they are magnetically saturable, and the harmonic composition of the SPIONs’ response that make MPI possible at all. As SPIONs are the essential element of MPI, the development of customized nanoparticles is pursued with the greatest effort by many groups. Their objective is the creation of a SPION or a conglomerate of particles that will feature a much higher MPI performance than nanoparticles currently available commercially. A particle’s MPI performance and suitability is characterized by parameters such as the strength of its MPI signal, its biocompatibility, or its pharmacokinetics. Some of the most important adjuster bolts to tune them are the particles’ iron core and hydrodynamic diameter, their anisotropy, the composition of the particles’ suspension, and their coating. As a three-dimensional, real-time imaging modality that is free of ionizing radiation, MPI appears ideally suited for applications such as vascular imaging and interventions as well as cellular and targeted imaging. A number of different theories and technical approaches on the way to the actual implementation of the basic concept of MPI have been seen in the last few years. Research groups around the world are working on different scanner geometries, from closed bore systems to single-sided scanners, and use reconstruction methods that are either based on actual calibration measurements or on theoretical models. This review aims at giving an overview of current developments and future directions in MPI about a decade after its first appearance.

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Human PNS and SAR study in the frequency range from 24 to 162 kHz

Cited by 35 publications

References 5 publications

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

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

Effects of pulse duration on magnetostimulation thresholds

Magnetic particle imaging: current developments and future directions

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