Simulation and Verification of Magnetic Field Gradient Waveforms in the Presence of a Metallic Vessel in Magnetic Resonance Imaging

Goora, Frédéric G.; Han, Hua; Colpitts, Bruce G.; Balcom, Bruce J.

doi:10.1109/tmag.2012.2196051

Cited by 7 publications

(4 citation statements)

References 40 publications

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“…The measured step response data are split into two separate regions separated by an inflection point in the rising edge. The data prior to this inflection point are fitted with a high‐order polynomial in order to accurately model the instrument response which includes transient resistance effects . The data following the inflection point are fitted with 3–8 exponential terms.…”

Section: Theorymentioning

confidence: 99%

Accuracy of the Magnetic Field Gradient Waveform Monitor Technique and Consequent Accuracy of Pre‐Equalized Gradient Waveform

Goora

Balcom

2016

Concepts Magn Reson

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The magnetic field gradient waveform monitor (MFGM) technique permits characterization of the temporal evolution of magnetic field gradients in magnetic resonance (MR) instruments (MRIs). Knowledge of the gradient waveform performance permits the development of further techniques, such as gradient waveform pre‐equalization, that correct and optimize gradient waveform distortions due to eddy currents induced during the application of switched magnetic fields and other system limitations. The accuracy of the MFGM technique is important since the overall uncertainty of the gradient waveform measurement will propagate into an uncertainty in corrected gradient waveforms impacting the precision of the resulting MR/MRI measurements. The accuracy of MFGM is investigated through a treatment of the noise present in a MRI. A noisy receiver model provides the basis for characterization of the noise and permits examination of the overall impact of noise on the phase accumulated in a pure‐phase encoded MR signal. Ultimately, a relationship between the signal‐to‐noise ratio of a measurement and the corresponding MFGM uncertainty is developed. The theoretical development is supported through simulation in conjunction with experimental results. The propagation of uncertainties to gradient waveform pre‐equalization is also discussed.

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Section: Theorymentioning

confidence: 99%

Accuracy of the Magnetic Field Gradient Waveform Monitor Technique and Consequent Accuracy of Pre‐Equalized Gradient Waveform

Goora

Balcom

2016

Concepts Magn Reson

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“…Other methods, such as boundary element, can be used using professional simulation software, such as Maxwell, CST and COMSOL. Goora et al (2012) investigated the spatial distribution of eddy current induced by a switched gradient magnetic field in metallic vessels applied in high-pressure MRI system. CST EM Studio was used for simulation.…”

Section: Introductionmentioning

confidence: 99%

Fast analytical calculation method for eddy current induced by gradient fields in an MRI system

et al. 2017

COMPEL

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Purpose Eddy currents are inevitable in magnetic resonance imaging (MRI) systems. These currents are mainly induced by gradient fields. This study aims to propose a fast analytical method to calculate eddy currents induced by frequently switching gradient fields in a traditional C-shape MRI system. Design/methodology/approach Fourier decomposition and magnetic vector potentials were used to calculate the eddy currents. Calculations with the proposed analytical method revealed the spatial distribution and temporal evolution of eddy currents. Findings Calculation and Maxwell simulation results were consistent. The agreement between calculation and simulation results indicates that increasingly sophisticated structures could be developed. The calculated results could guide the design of improved gradient coils. Originality/value Eddy currents induced by gradient current are decomposed into currents induced by each time-harmonic component, and then adding them together to obtain complete contribution of the eddy current. The analytical method was used to characterize the properties of symmetric and asymmetric eddy currents induced by gradient coils in MRI systems. The analytical method can be used to improve the gradient shield during the design of the gradient coil in the MRI system.

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“…Switched magnetic field gradients and RF pulses can induce eddy currents in a metallic object, which may result in image distortion and image artifacts [20,[28][29][30][31][32][33]. In each case the varying field alters the magnetic flux through the conductive object and induces eddy currents due to Faraday's law.…”

Section: Introductionmentioning

confidence: 99%

“…In the electromagnetic simulation, the space was divided into many small mesh elements. The resulting mesh was generated such that regions of interest are meshed to a finer degree than those of less interest[28]. The number of mesh cells impacts the accuracy of the solution as well as the simulation run time.…”

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confidence: 99%

Mapping B1-induced eddy current effects near metallic structures in MR images: A comparison of simulation and experiment

Vashaee

Goora

Britton

et al. 2015

Journal of Magnetic Resonance

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Magnetic resonance imaging (MRI) in the presence of metallic structures is very common in medical and non-medical fields. Metallic structures cause MRI image distortions by three mechanisms: (1) static field distortion through magnetic susceptibility mismatch, (2) eddy currents induced by switched magnetic field gradients and (3) radio frequency (RF) induced eddy currents. Single point ramped imaging with T1 enhancement (SPRITE) MRI measurements are largely immune to susceptibility and gradient induced eddy current artifacts. As a result, one can isolate the effects of metal objects on the RF field. The RF field affects both the excitation and detection of the magnetic resonance (MR) signal. This is challenging with conventional MRI methods, which cannot readily separate the three effects. RF induced MRI artifacts were investigated experimentally at 2.4 T by analyzing image distortions surrounding two geometrically identical metallic strips of aluminum and lead. The strips were immersed in agar gel doped with contrast agent and imaged employing the conical SPRITE sequence. B1 mapping with pure phase encode SPRITE was employed to measure the B1 field around the strips of metal. The strip geometry was chosen to mimic metal electrodes employed in electrochemistry studies. Simulations are employed to investigate the RF field induced eddy currents in the two metallic strips. The RF simulation results are in good agreement with experimental results. Experimental and simulation results show that the metal has a pronounced effect on the B1 distribution and B1 amplitude in the surrounding space. The electrical conductivity of the metal has a minimal effect.

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Simulation and Verification of Magnetic Field Gradient Waveforms in the Presence of a Metallic Vessel in Magnetic Resonance Imaging

Cited by 7 publications

References 40 publications

Accuracy of the Magnetic Field Gradient Waveform Monitor Technique and Consequent Accuracy of Pre‐Equalized Gradient Waveform

Accuracy of the Magnetic Field Gradient Waveform Monitor Technique and Consequent Accuracy of Pre‐Equalized Gradient Waveform

Fast analytical calculation method for eddy current induced by gradient fields in an MRI system

Mapping B1-induced eddy current effects near metallic structures in MR images: A comparison of simulation and experiment

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