Browsing by Subject "Peripheral nerve stimulation"

Now showing 1 - 5 of 5

Open Access
Effects of duty cycle on magnetostimulation thresholds in MPI
(Infinite Science Publishing, 2017) Demirel, Omer Burak; Sarıtaş, Emine Ülkü
Magnetic Particle Imaging (MPI) relies on time-varying magnetic fields to generate an image of the spatial distribution of superparamagnetic iron oxide nanoparticles. However, these oscillating magnetic fields form electric field patterns within the body, which in turn can cause peripheral nerve stimulations (PNS), also known as magnetostimulation. To prevent potential safety hazards and to optimize the scanning parameters such as field-of-view (FOV) and scanning speed in MPI, the factors that affect drive field magnetostimulation limits need to be determined accurately. In this work, we investigate the effects of the duty cycle on magnetostimulation thresholds in MPI. We performed human subject experiments by using a highly homogenous solenoidal coil on the upper arm of six subjects. Six different duty cycles ranging between 5 % and 100 % were applied at 25 kHz. Accordingly, magnetostimulation limits first decrease and then increase with increasing duty cycle, reaching a maximum at 100 % duty cycle. Since high duty cycles would be the preferred operating mode for rapid imaging with MPI, these results have promising implications for future human-sized MPI systems.
Open Access
Effects of pulse duration on magnetostimulation thresholds
(Wiley-Blackwell Publishing, Inc., 2015-06) Saritas, E. U.; Goodwill, P. W.; Conolly, S. M.
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 magneto stimulation 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 3040 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 (Tpulse). For Tpulse < 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 (BNorm) 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 (Tpulse ≈ 2 ms), the thresholds were approximately 24% higher than at the asymptotes. The thresholds decreased to within 4% of their asymptotic values for Tpulse > 20 ms. These trends were well characterized (R2 = 0.78) by a stretched exponential function given by BNorm = 1+αe?(Tpulse/β)γ, 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 identical for a 20-fold range of frequencies: the thresholds are significantly higher at short pulse durations and settle to within 4% of their asymptotic values for durations longer than 20 ms. These results emphasize the importance of matching the human-subject experiments to the imaging conditions of a particular setup. Knowing the dependence of the safety limits to all contributing factors is critical for increasing the time-efficiency of imaging systems that utilize time-varying magnetic fields. © 2015 American Association of Physicists in Medicine.
Open Access
Minimizing electric fields and increasing peripheral nervestimulation thresholds using a body gradient array coil
(John Wiley & Sons, Inc., 2024-04-16) Babaloo, Reza; Atalar, Ergin
**Purpose:** To demonstrate the performance of gradient array coils in minimizing switched-gradient-induced electric fields (E-fields) and improving peripheral nerve stimulation (PNS) thresholds while generating gradient fields with adjustable linearity across customizable regions of linearity (ROLs). **Methods:** A body gradient array coil is used to reduce the induced E-fields on the surface of a body model by modulating applied currents. This is achieved by performing an optimization problem with the peak E-field as the objective function and current amplitudes as unknown variables. Coil dimensions and winding patterns are fixed throughout the optimization, whereas other engineering metrics remain adjustable. Various scenarios are explored by manipulating adjustable parameters. **Results:** The array design consistently yields lower E-fields and higher PNS thresholds across all scenarios compared with a conventional coil. When the gradient array coil generates target gradient fields within a 44-cm-diameter spherical ROL, the maximum E-field is reduced by 10%, 18%, and 61% for the X, Y, and Z gradients, respectively. Transitioning to a smaller ROL (24 cm) and relaxing the gradient linearity error results in further E-field reductions. In oblique gradients, the array coil demonstrates the most substantial reduction of 40% in the Z–Y direction. Among the investigated scenarios, the most significant increase of 4.3-fold is observed in the PNS thresholds. **Conclusion:** Our study demonstrated that gradient array coils offer a promising pathway toward achieving high-performance gradient coils regarding gradient strength, slew rate, and PNS thresholds, especially in scenarios in which linear magnetic fields are required within specific target regions.
Open Access
Safety limits & rapid scanning methods in magnetic particle imaging
(2017-07) Demirel, Ömer Burak
Magnetic Particle Imaging (MPI) is a new imaging modality that utilizes nonlinear magnetization of superparamagnetic tracers, with high sensitivity and zeroionizing radiation advantages. Since the introduction of MPI in 2005, there have been substantial contributions to pre-clinical applications such as cancer imaging, cell tracking, and angiography. These studies have promising implications for future clinical human-sized MPI systems. However, the time-varying magnetic fields that are used during image acquisition are subject to human safety concerns, especially in applications that require rapid imaging. By forming electric field patterns in the body, these fields may result in peripheral nerve stimulation, also known as magnetostimulation. To prevent potential stimulations; the effects of frequency, duration, and direction of the fields, as well as body part size were previously investigated. This thesis investigates the effects of duty cycle and fat/water tissue ratio on magnetostimulation thresholds for the drive field in MPI. Human subject experiments with in-house magnetostimulation setup were conducted at 25 kHz, followed by anatomical Magnetic Resonance Imaging (MRI) of the subjects. Accordingly, magnetostimulation thresholds first decrease then increase with increasing duty cycle and reach a maximum at 100% duty cycle. The results also show that the thresholds are strongly correlated with fat/water tissue ratio. Finally, this thesis also demonstrates that MPI image quality can be preserved for rapid scanning scenarios within the human safety limits.
Open Access
Technical innovations in gradient array systems for MRI applications
(2023-02) Babaloo, Reza
In Magnetic Resonance Imaging, gradient array coils have lately been employed in a variety of applications, such as field profiling. This capability of array technology can be used to minimize electric fields induced by gradient waveforms. For this purpose, a whole-body gradient array with all three gradients is being investigated. Gradient current amplitudes are optimized to produce a target magnetic field within a desired region of linearity (ROL) while minimizing induced electric fields. By reducing the diameter of ROL, generating a target gradient within a slice, and relaxing the linearity error, array coil electric fields are significantly reduced compared to a conventional coil. When a linear gradient is required in a small region, higher gradient strengths and slew rates can be achieved without exceeding peripheral nerve stimulation thresholds. Because of a high number of channels in the array design, feedback controllers significantly raise the system cost due to the expensive current sensors used for gradient current measurements. Thus, a nonlinear second-order feed-forward controller is introduced for the gradient array chain. The feed-forward controller is then modified to update the controller coefficients based on thermal behavior prediction to deal with time-varying parameters caused by temperature-dependent resistances. Gradient current measurements and MRI experiments are conducted to show the effectiveness of the proposed method. In the scope of this thesis, novel applications and hardware solutions are proposed to make array technology valuable and feasible.