Interslice current change constrained B<sub>0</sub> shim optimization for accurate high‐order dynamic shim updating with strongly reduced eddy currents

Schwerter, Michael; Hetherington, Hoby P.; Moon, Chan Hong; Pan, Jullie W.; Felder, Jörg; Tellmann, Lutz; Shah, N. Jon

doi:10.1002/mrm.27720

Cited by 18 publications

(25 citation statements)

References 29 publications

(34 reference statements)

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“…However, resolution of the dephasing artifacts is desirable because there are several applications that can be applied to the real‐time cines that can benefit treatment including visual respiratory feedback and motion prediction. Dynamic shimming and eddy current methods are now available on commercial MRIs that may be adapted to minimizing gantry motion‐related artifacts …”

Section: Discussionmentioning

confidence: 99%

MRI quality control for low‐field MR‐IGRT systems: Lessons learned

Gach

Curcuru

Wittland

et al. 2019

J Applied Clin Med Phys

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Purpose: To present lessons learned from magnetic resonance imaging (MRI) quality control (QC) tests for low-field MRI-guided radiation therapy (MR-IGRT) systems.Methods: MRI QC programs were established for low-field MRI-60 Co and MRI-Linac systems. A retrospective analysis of MRI subsystem performance covered system commissioning, operations, maintenance, and quality control. Performance issues were classified into three groups: (a) Image noise and artifact; (b) Magnetic field homogeneity and linearity; and (c) System reliability and stability.Results: Image noise and artifacts were attributed to room noise sources, unsatisfactory system cabling, and broken RF receiver coils. Gantry angle-dependent magnetic field inhomogeneities were more prominent on the MRI-Linac due to the high volume of steel shielding in the gantry. B 0 inhomogeneities measured in a 24-cm spherical phantom were <5 ppm for both MR-IGRT systems after using MRI gradient offset (MRI-GO) compensation on the MRI-Linac. However, significant signal dephasing occurred on the MRI-Linac while the gantry was rotating. Spatial integrity measurements were sensitive to gradient calibration and vulnerable to shimming.The most common causes of MR-IGRT system interruptions were software disconnects between the MRI and radiation therapy delivery subsystems caused by patient table, gantry, and multi-leaf collimator (MLC) faults. The standard deviation (SD) of the receiver coil signal-to-noise ratio was 1.83 for the MRI-60 Co and 1.53 for the MRI-Linac. The SD of the deviation from the mean for the Larmor frequency was 1.41 ppm for the MRI-60 Co and 1.54 ppm for the MRI-Linac. The SD of the deviation from the mean for the transmitter reference amplitude was 0.90% for the MRI-60 Co and 1.68% for the MRI-Linac. High SDs in image stability data corresponded to reports of spike noise.Conclusions: There are significant technological challenges associated with implementing and maintaining MR-IGRT systems. Most of the performance issues were identified and resolved during commissioning.

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Section: Discussionmentioning

confidence: 99%

MRI quality control for low‐field MR‐IGRT systems: Lessons learned

Gach

Curcuru

Wittland

et al. 2019

J Applied Clin Med Phys

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“…Previously, it was demonstrated that nonorthogonal field profiles can compete with conventional orthogonal spherical harmonic fields for dynamic shimming purposes using a high number of shim array coils or RF coils as shim coils with dedicated hardware . Although the gradient field demands in terms of switching rates and field amplitudes are higher than in shim coils, mutual coupling might still affect the shimming performance of shim coil arrays in theory.…”

Section: Discussionmentioning

confidence: 99%

Driving mutually coupled gradient array coils in magnetic resonance imaging

Ertan

Taraghinia

Atalar

2019

Magnetic Resonance in Med

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Purpose:In contrast to conventional linear gradients, gradient coil arrays with arbitrary spatial dependency might experience strong mutual coupling. Although conventional gradient power amplifiers with feedback loop might compensate the effect of coupling, required voltages for the compensation are generally unknown and has to be considered beforehand to ensure that amplifier voltage limits are not exceeded. A first-order circuit model is proposed to be used as a feedforward model which enables analytical formulas of required voltages to drive the mutually coupled gradient coil arrays. Theory and Methods: A first-order circuit model including the mutual couplings is provided to analytically calculate the input voltages and minimum achievable rise times for a given set of gradient array currents and amplifier limitations. Previously designed 9-channel Z-gradient coil array and home-built gradient amplifiers (50 V and 20 A) are used in the experiments. Three sets of currents optimized for linear Z-gradient, second-order Z2, and third-order Z3 fields are used in the bench-top experiments. The current weightings for the linear Z-gradient are also used as the readout gradient in the 3T MRI experiments. Results: Current measurements for the example magnetic field profiles with minimum rise times are demonstrated for the simultaneous use of 9-channel gradient coils and amplifiers. MRI experiments verify that a linear Z-gradient field with a desired time waveform can be generated using a mutually coupled array coils. Conclusion: Bench-top and MRI experiments demonstrate the feasibility of the proposed circuit model and analytical formulas to drive the mutually coupled gradient coils. K E Y W O R D Sfeedforward model, gradient arrays, gradient power amplifiers, MRI, mutual coupling, mutual inductance, mutually coupled gradient array coils

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“…set. 20 The RRI power supply values and the pulse sequence were synchronized by use of an external trigger pulse from the scanner console to the RRI power supply clocked 2 ms prior to the RF excitation pulse.…”

Section: General Hardware Configurationmentioning

confidence: 99%

“…For the human brain, optimizing the B 0 shim corrections on a slice‐by‐slice (SBS) basis using dynamic shim updating (DSU) as opposed to a single global solution over the majority or the entire brain, can significantly improve homogeneity. Because of this, several groups have developed DSU approaches, including strategies for optimization and pre‐emphasis mitigation 19‐22 …”

Section: Introductionmentioning

confidence: 99%

Dynamic B₀ shimming for multiband imaging using high order spherical harmonic shims

Hetherington

Moon

Schwerter

et al. 2020

Magnetic Resonance in Med

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Purpose: To describe and implement a strategy for dynamic slice-by-slice and multiband B 0 shimming using spherical harmonic shims in the human brain at 7T. Theory: For thin axial slices, spherical harmonic shims can be divided into pairs of shims (z-degenerate and non-z-degenerate) that are spatially degenerate, such that only ½ of the shims (non-z-degenerate) are required for single slice optimizations. However, when combined, the pairs of shims can be used to simultaneously generate the same in-plane symmetries but with different amplitudes as a function of their z location. This enables multiband shimming equivalent to that achievable by single slice-by-slice optimization. Methods: All data were acquired at 7T using a spherical harmonic shim insert enabling shimming up through 4th order with two additional 5th order shims (1st-4th+). Dynamic shim updating was achieved using a 10A shim power supply with 2 ms ramps and constrained optimizations to minimize eddy currents. Results: In groups of eight subjects, we demonstrated that: 1) dynamic updating using 1st-4th+ order shims reduced the SD of the B 0 field over the whole brain from 32.4 ± 2.6 and 24.9 ± 2 Hz with 1st-2nd and 1st-4th+ static global shimming to 15.1 ± 1.7 Hz; 2) near equivalent performance was achieved when dynamically updating only the non-z-degenerate shims (14.3 ± 1.5 Hz), or when a using multiband shim factor of 2, MB s = 2, and all shims (14.4 ± 2.0 Hz). Conclusion: High order spherical harmonics provide substantial improvements over static global shimming and enable dynamic multiband shimming with near equivalent performance to that of dynamic slice-by-slice shimming. This reduces distortion in echo planar imaging.

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Interslice current change constrained B₀ shim optimization for accurate high‐order dynamic shim updating with strongly reduced eddy currents

Cited by 18 publications

References 29 publications

MRI quality control for low‐field MR‐IGRT systems: Lessons learned

MRI quality control for low‐field MR‐IGRT systems: Lessons learned

Driving mutually coupled gradient array coils in magnetic resonance imaging

Dynamic B₀ shimming for multiband imaging using high order spherical harmonic shims

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Interslice current change constrained B0 shim optimization for accurate high‐order dynamic shim updating with strongly reduced eddy currents

Cited by 18 publications

References 29 publications

MRI quality control for low‐field MR‐IGRT systems: Lessons learned

MRI quality control for low‐field MR‐IGRT systems: Lessons learned

Driving mutually coupled gradient array coils in magnetic resonance imaging

Dynamic B0 shimming for multiband imaging using high order spherical harmonic shims

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Product

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Interslice current change constrained B₀ shim optimization for accurate high‐order dynamic shim updating with strongly reduced eddy currents

Dynamic B₀ shimming for multiband imaging using high order spherical harmonic shims