Eddy Current Compensation Research Articles

Unwrapping eddy current compensation: Improved compensation of eddy current induced baseline shifts in high-resolution phase-contrast MRI at 9.4 tesla

Phase-contrast MRI (PC-MRI) is a versatile tool allowing evaluation of in vivo motion, but is sensitive to eddy current induced phase offsets, causing errors in the measured velocities. In high-resolution PC-MRI, these offsets can be sufficiently large to cause wrapping in the baseline phase, rendering conventional eddy current compensation (ECC) inadequate. The purpose of this study was to develop an improved ECC technique (unwrapping ECC) able to handle baseline phase discontinuities. Baseline phase discontinuities are unwrapped by minimizing the spatiotemporal standard deviation of the static-tissue phase. Computer simulations were used for demonstrating the theoretical foundation of the proposed technique. The presence of baseline wrapping was confirmed in high-resolution myocardial PC-MRI of a normal rat heart at 9.4 Tesla (T), and the performance of unwrapping ECC was compared with conventional ECC. Areas of phase wrapping in static regions were clearly evident in high-resolution PC-MRI. The proposed technique successfully eliminated discontinuities in the baseline, and resulted in significantly better ECC than the conventional approach. We report the occurrence of baseline phase wrapping in PC-MRI, and provide an improved ECC technique capable of handling its presence. Unwrapping ECC offers improved correction of eddy current induced baseline shifts in high-resolution PC-MRI.

Image‐based method to measure and characterize shim‐induced eddy current fields

ABSTRACTDynamic magnetic field shimming is gaining interest for field sensitive MRI acquisitions. Using slice based or real‐time shim updating, significant improvements in static field (B0) uniformity can be obtained. While the ability to rapidly switch shim fields can improve overall B0 homogeneity, it induces eddy current fields that must be characterized and compensated for. Methods used to achieve this have thus far been based on linear projection spin echo sequences or field probe assemblies. Here, a novel image‐based method is presented to measure and characterize eddy current fields without the need for field probes or projection based measurements. This technique can be extended to characterize very high order spherical harmonic fields, making it a useful tool to calibrate next‐generation shim systems implementing dynamic field steering with greater than third order shim terms. Results are used to calibrate a Dynamic Shim Updating unit for pre‐emphasis and eddy current compensation. Three‐dimensional datasets are acquired at multiple MR facilities containing complete spatiotemporal field information to compensate eddy current field self‐ and cross‐terms for up to third order. Furthermore, simulation studies are performed to investigate the effect of scan resolution and phantom size with respect to accurate eddy current field characterization. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part A 42A: 245–260, 2013.

93P - Assessment of Response to Neoadjuvant Chemotherapy with Diffusion Weighted Imaging at 3T

Abstract Purpose To assess radiological tumor response to neoadjuvant chemotherapy (NAC) with Diffusion weighted imaging (DWI) at 3Tesla. Materials and methods 30 patients (mean age 51.4 years) with a histopatholgical verified breast cancer (BIRADS 6) who underwent NAC were included in this prospective IRB approved study. Additionally to routine staging with dynamic contrast enhanced magnetic resonance imaging (CE-MRI) protocol a diffusion weighted sequence with the following parameters was included in the imaging protocol: single-shot, echo-planar imaging sequence with eddy current compensation, a three direction trace diffusion scheme, inversion recovery fat suppression and two b-values of 50 and 850 s/mm2 (TR/TE/TI 13700/83/220ms; FOV 340x117 mm; 40 axial slices; SI 3.5 mm (no gap); matrix 192x64 two averages; spatial resolution, 1.8x1.8x3.5 mm; bandwidth 1446 Hz/pixel; 3:19 min. Patients were examined before the beginning and after 5 or 6 cycles of NAC. In DWI manually drawn ROIs of suspicious areas were assessed for restricted diffusion and apparent diffusion coefficient (ADC) values were calculated. An ADC threshold 1.25x10-3mm2/s was defined as a marker of malignancy. Data were dichotomized and complete response (cr) and partial response (pr) were considered as response and stable disease (sd) and progressive disease (pd) were considered as non-response to NAC. The histopathological assessment of tumor response was used as the standard of reference. Results There were 23 invasive ductal cancers (IDC) and 6 invasive lobular cancers (ILC). Average tumor size was 43.4mm. In the baseline scan one IDC (3%) had an ADC value above the cut-off for malignancy and was excluded. There were 4 (14%) non-responders and 25 (86%) responders. Responders demonstrated an average increase in ADC values of 0.58x10-3mm2/s whereas non-responders failed to show a significant increase in ADC values (average increase 0.09x10-3mm2/s). The classification of DWI as responders and non-responders correlated with histopathological diagnoses of treatment response. Conclusion ADC as an adjunct to conventional CE-MRI is an additional valuable imaging biomarker for the assessment of treatment response to NAC in breast cancer patients. Disclosure All authors have declared no conflicts of interest.

Eddy current compensation for delta relaxation enhanced MR by dynamic reference phase modulation

Eddy current compensation by dynamic reference phase modulation (eDREAM) is a compensation method for eddy current fields induced by B 0 field-cycling which occur in delta relaxation enhanced MR (dreMR) imaging. The presented method is based on a dynamic frequency adjustment and prevents eddy current related artifacts. It is easy to implement and can be completely realized in software for any imaging sequence. In this paper, the theory of eDREAM is derived and two applications are demonstrated. The theory describes how to model the behavior of the eddy currents and how to implement the compensation. Phantom and in vivo measurements are carried out and demonstrate the benefits of eDREAM. A comparison of images acquired with and without eDREAM shows a significant improvement in dreMR image quality. Images without eDREAM suffer from severe artifacts and do not allow proper interpretation while images with eDREAM are artifact free. In vivo experiments demonstrate that dreMR imaging without eDREAM is not feasible as artifacts completely change the image contrast. eDREAM is a flexible eddy current compensation for dreMR. It is capable of completely removing the influence of eddy currents such that the dreMR images do not suffer from artifacts.

MO‐G‐BRA‐05: High Fidelity, Low Distortion Tetrahedral Diffusion‐Weighted MRI for Radiation Treatment Planning

Purpose: Diffusion‐weighted (DW) MR imaging provides supplemental information that may aid in target delineation during radiation treatment planning. However, geometric distortion artifacts and the inability to acquire higher b‐value DW images in tissues with short T2s have encumbered the utilization of DW MRI in radiation treatment planning. To rectify these issues, we present here a novel DW imaging approach that combines tetrahedral diffusion encoding and a multi‐shot, variable‐density spiral (MS‐VDS) acquisition. Methods: A custom MS‐VDS pulse sequence with Stejkal‐Tanner (ST) and twice refocused spin echo (TRSE) excitation schemes was implemented on a Siemens 3T scanner. For comparison to current state‐of‐the‐art, orthogonal and tetrahedral DW images were acquired in the brains of three healthy volunteers (FOV: 220mm2, matrix: 1282, interleaves: 16, TR: 3000ms, TH: 3mm, b‐values: 0/1000 s/mm2). Self‐navigated phase correction was applied during reconstruction to correct each spiral interleaf for eddy current and motion‐induced phase errors. Results: The MS‐VDS acquisition was effective at diminishing geometric distortion artifacts. The minimum echo times for the orthogonal ST, tetrahedral ST, and tetrahedral TRSE acquisitions were 79 ms, 56 ms, and 71 ms, respectively. Although the inherent self‐navigating capability of VDS was effective at diminishing eddy current and motion‐induced phase errors in the orthogonal ST images, the tetrahedral ST images were contaminated by residual eddy current‐induced blurring. However, the tetrahedral TRSE images exhibited quality similar to the orthogonal ST images, indicating that the TRSE sequence provided effective compensation of residual eddy currents. Conclusions: Tetrahedral encoding results in a reduction in minimum echo time for a given b‐value, facilitating acquisition of DW images in tissues with shorter T2s. A TRSE excitation is required to provide eddy current compensation in tetrahedral DW acquisitions. The combined MS‐VDS TRSE tetrahedral DW imaging method permits acquisition of high fidelity, low distortion DW images suitable for radiation treatment planning.

61O Assessment of Response to Neoadjuvant Chemotherapy with Diffusion Weighted Imaging at 3T

PurposeTo assess radiological tumor response to neoadjuvant chemotherapy (NAC) with Diffusion weighted imaging (DWI) at 3Tesla. Materials and methods30 patients (mean age 51.4 years) with a histopatholgical verified breast cancer (BIRADS 6) who underwent NAC were included in this prospective IRB approved study. Additionally to routine staging with dynamic contrast enhanced magnetic resonance imaging (CE-MRI) protocol a diffusion weighted sequence with the following parameters was included in the imaging protocol: single-shot, echo-planar imaging sequence with eddy current compensation, a three direction trace diffusion scheme, inversion recovery fat suppression and two b-values of 50 and 850 s/mm2 (TR/TE/TI 13700/83/220ms; FOV 340x117 mm; 40 axial slices; SI 3.5mm (no gap); matrix 192x64 two averages; spatial resolution, 1.8x1.8x3.5 mm; bandwidth 1446 Hz/pixel; 3:19 min. Patients were examined before the beginning and after 5 or 6 cycles of NAC. In DWI manually drawn ROIs of suspicious areas were assessed for restricted diffusion and apparent diffusion coefficient (ADC) values were calculated. An ADC threshold 1.25x10-3mm2/s was defined as a marker of malignancy. Data were dichotomized and complete response (cr) and partial response (pr) were considered as response and stable disease (sd) and progressive disease (pd) were considered as non-response to NAC. The histopathological assessment of tumor response was used as the standard of reference. ResultsThere were 23 invasive ductal cancers (IDC) and 6 invasive lobular cancers (ILC). Average tumor size was 43.4mm. In the baseline scan one IDC (3%) had an ADC value above the cut-off for malignancy and was excluded. There were 4 (14%) non-responders and 25 (86%) responders. Responders demonstrated an average increase in ADC values of 0.58x10-3mm2/s whereas non-responders failed to show a significant increase in ADC values (average increase 0.09x10-3mm2/s). The classification of DWI as responders and non-responders correlated with histopathological diagnoses of treatment response. ConclusionADC as an adjunct to conventional CE-MRI is an additional valuable imaging biomarker for the assessment of treatment response to NAC in breast cancer patients. DisclosureAll authors have declared no conflicts of interest.

Software compensation of Eddy current fields in multislice high order dynamic shimming

Dynamic B 0 shimming (DS) can produce better field homogeneity than static global shimming by dynamically updating slicewise shim values in a multislice acquisition. The performance of DS however is limited by eddy current fields produced by the switching of 2nd and 3rd order unshielded shims. In this work, we present a novel method of eddy field compensation (EFC) applied to higher order shim induced eddy current fields in multislice DS. This method does not require shim shielding, extra hardware for eddy current compensation or subject specific prescanning. The interactions between shim harmonics are modeled assuming steady state of the medium and long time constant, cross and self term eddy fields in a DS experiment and ‘correction factors’ characterizing the entire set of shim interactions are derived. The correction factors for a given time between shim switches are shown to be invariable with object scanned, shim switching pattern and actual shim values, allowing for their generalized prospective use. Phantom and human head, 2nd and 3rd order DS experiments performed without any hardware eddy current compensation using the technique show large reductions in field gradients and offsets leading to significant improvements in image quality. This method holds promise as an alternative to expensive hardware based eddy current compensation required in 2nd and 3rd order DS.

A single-board NMR spectrometer based on a software defined radio architecture

A single-board software defined radio (SDR) spectrometer for nuclear magnetic resonance (NMR) is presented. The SDR-based architecture, realized by combining a single field programmable gate array (FPGA) and a digital signal processor (DSP) with peripheral radio frequency (RF) front-end circuits, makes the spectrometer compact and reconfigurable. The DSP, working as a pulse programmer, communicates with a personal computer via a USB interface and controls the FPGA through a parallel port. The FPGA accomplishes digital processing tasks such as a numerically controlled oscillator (NCO), digital down converter (DDC) and gradient waveform generator. The NCO, with agile control of phase, frequency and amplitude, is part of a direct digital synthesizer that is used to generate an RF pulse. The DDC performs quadrature demodulation, multistage low-pass filtering and gain adjustment to produce a bandpass signal (receiver bandwidth from 3.9 kHz to 10 MHz). The gradient waveform generator is capable of outputting shaped gradient pulse waveforms and supports eddy-current compensation. The spectrometer directly acquires an NMR signal up to 30 MHz in the case of baseband sampling and is suitable for low-field (<0.7 T) application. Due to the featured SDR architecture, this prototype has flexible add-on ability and is expected to be suitable for portable NMR systems.

Evaluating passively shielded gradient coil configurations for optimal eddy current compensation

In magnetic resonance imaging, rapidly switching magnetic fields are used to spatially encode the signal. The temporal change of these fields induces eddy currents in nearby conducting structures of the scanner. These eddy currents, in turn, generate a secondary magnetic field that opposes and distorts the desired gradient field. Eddy current compensation methods are generally applied assuming that the primary and secondary magnetic field gradients possess similar spatial characteristics in the imaging volume (field matching). In this work an optimization method is used to deform the shape of the coil support and/or a highly conductive passive shield in order to improve the field matching and reduce the inductive coupling between the gradient coil and the passive shield. Using the residual field after eddy current compensation as the objective function, the coil support and/or conducting surfaces were deformed to obtain passively shielded x- and z-gradient coils with improved field matching and eddy current compensation. Assuming a single frequency, quasi-static simulation, it was demonstrated that the residual field was reduced up to 24 times by reshaping the coil and passive shield surfaces due to the improved field matching. However, using transient analyses we showed that in the case of the passively shielded x-gradient coil the residual field may only be reduced by five times from a cylindrical coil configuration. A bulge shape is created in the conducting surface as a mechanism of matching the field and at the same time reducing the mutual inductive coupling between the coil and the passive shield. An actively shielded coil with control over the magnetic field produced by the induced current was used as a reference coil that produces the minimal residual field. The actively shielded gradient coil produces minimal residual field for short and long pulses in the transient analyses.

MO‐A‐303A‐01: MR Acceptance Testing and Quality Control: Report of AAPM MR Subcommittee TG1

The MR Subcommittee of the AAPM Imaging Physics Committee (IPC) has recently completed a revision of the previous AAPM TG1 and TG6 reports on MR acceptance testing and quality assurance. The report of the MR Subcommittee has been approved by the IPC and will be available electronically (http://www.aapm.org/pubs/reports/). In this session of the MR continuing education course, the contents of the report will be presented. Recommendations regarding testing of magnetic and radiofrequency (RF) shielding will be followed by recommendations on specific acceptance tests of magnetic field homogeneity, the RF subsystem (calibration, stability), the gradient subsystem (calibration, eddy current compensation), combined subsystems (slice thickness and spacing), global system performance (SNR, uniformity, high and low contrast resolution), and advanced tests (MR spectroscopy, echo planar imaging).Objectives:1. Understand the requirements for MR site and system acceptance tests.2. Review recommended MR site and system acceptance testing procedures, including phantom selection, data acquisition, data analysis, and reporting.3. Review recommended action criteria for both standard imaging options and advanced imaging and spectroscopy options.

Estimation of k‐space trajectories in spiral MRI

For non-Cartesian data acquisition in MRI, k-space trajectory infidelity due to eddy current effects and other hardware imperfections will blur and distort the reconstructed images. Even with the shielded gradients and eddy current compensation techniques of current scanners, the deviation between the actual k-space trajectory and the requested trajectory remains a major reason for image artifacts in non-Cartesian MRI. It is often not practical to measure the k-space trajectory for each imaging slice. It has been reported that better image quality is achieved in radial scanning by correcting anisotropic delays on different physical gradient axes. In this article the delay model is applied in spiral k-space trajectory estimation to reduce image artifacts. Then a novel estimation method combining the anisotropic delay model and a simple convolution eddy current model further reduces the artifact level in spiral image reconstruction. The root mean square error and peak error in both phantom and in vivo images reconstructed using the estimated trajectories are reduced substantially compared to the results achieved by only tuning delays. After a one-time calibration, it is thus possible to get an accurate estimate of the spiral trajectory and a high-quality image reconstruction for an arbitrary scan plane.

Improved slice selection for R2* mapping during cryoablation with eddy current compensation

To improve the slice profile and image quality of R2* mapping in the iceball during cryoablation with ultrashort echo time (UTE) imaging by compensating for eddy currents induced by the selective gradient when half-pulse radiofrequency (RF) excitation is employed to achieve UTEs. A method to measure both B0 and linear eddy currents simultaneously is first presented. This is done with a least-square fitting process on calibration data collected on a phantom. Eddy currents during excitation are compensated by redesigning the RF pulse and the selective gradient accordingly, while that resultant from the readout gradient are compensated for during image reconstruction. In vivo data were obtained continuously during the cryoablation experiments to calculate the R2* values in the iceball and to correlate them with the freezing process. Image quality degradation due to eddy currents is significantly reduced with the proposed approaches. R2* maps of iceball throughout the cryoablation experiments were achieved with improved quality. The proposed approaches are effective for compensating eddy currents during half-pulse RF excitation as well as readout. TEs as short as 100 microsec were obtained, allowing R2* maps to be obtained from frozen tissues with improved quality.

The effect of impermeable boundaries of arbitrary geometry on the apparent diffusion coefficient

The apparent diffusion coefficient (ADC) obtained from NMR measurements is modelled for diffusion in a compartment restricted by an impermeable boundary. For a given pulse sequence, the ADC can be determined from the connected velocity autocorrelation function (the second-order velocity cumulant), which we show can be expressed as a double surface integral over the boundary, involving the probability for molecules to diffuse from one boundary point to another. There is no restriction on the geometry of the boundary. This result allows a fast calculation of the ADC for an arbitrary time course of the diffusion-sensitizing gradient. Explicit examples are given for diffusion within three basic geometries for different pulse sequences. The ADCs measured with the Stejskal–Tanner pulse sequence and a more realistic pulse sequence with slice selection gradient and eddy current compensation are found to yield almost identical results. The application of the results are discussed in relation to determination of the microscopic structure of brain white matter.

A New Eddy-current Compensation Method in MRI

We present here a new eddy-current compensation method. A mathematical model of eddy-current fleld is introduced to calculate flrst order eddy-current distorted non-uniform sampling trajectory in k-space. Based on the computed k-space trajectory, compensated image is reconstructed using the gridding method. In addition, the model of eddy-current fleld can also be used to calculate zero order eddy-current induced additional phase to correct the phase of MRI signal before gridding reconstruction, as long as such phase correction is able to further improve qualities of reconstructed image. Initial experiment on a 0.3T permanent MRI system demonstrates that the proposed method is able to reconstruct FSE (fast spin-echo) images of similar qualities as those produced by conventional gradient pre-emphasis method. DOI: 10.2529/PIERS061006031452

Fast eddy current compensation by feedback linearization neural networks: Applications in diffusion-weighted echo planar imaging

This study describes the application of the method of feedback linearization neural networks, known from neural network computing, to the problem of gradient preemphasis. This approach of preemphasis adjustment does not require an iterative procedure between measurement and adjustment, therefore is essentially instantaneous in its execution. Based on our study, gradient compensation determined by our procedure effectively suppressed eddy current-induced geometric distortion and spatial shift of diffusion-weighted EPI images. © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 29B: 1–8, 2006

Double average parallel steady‐state free precession imaging: Optimized eddy current and transient oscillation compensation

Balanced steady-state free precession (SSFP) imaging is sensitive to off-resonance effects, which can lead to considerable artifacts during a transient phase following magnetization preparation or steady-state interruption. In addition, nonlinear k-space encoding is required if contrast-relevant k-space regions need to be acquired at specific delays following magnetization preparation or for transient artifact reduction in cardiac-gated k-space segmented CINE imaging. Such trajectories are problematic for balanced SSFP imaging due to nonconstant eddy current effects and resulting disruption of the steady state. In this work, a novel acquisition strategy for balanced SSFP imaging is presented that utilizes scan time reduction by parallel imaging for optimized "double average" eddy current compensation and artifact reduction during the transient phase following steady-state storage and magnetization preparation. Double average parallel SSFP imaging was applied to k-space segmented CINE SSFP tagging as well as nongated centrically encoded SSFP imaging. Phantom and human studies exhibit substantial reduction in steady-state storage and eddy current artifacts while maintaining spatial resolution, signal-to-noise ratio, and similar total scan time of a standard SSFP acquisition. The proposed technique can easily be extended to other acquisition schemes that would benefit from nonlinear reordering schemes and/or rely on interruption of the balanced SSFP steady state.

A programmable eddy-current compensation system for MRI and localized spectroscopy.

A programmable preemphasis system has been developed for eddy-current compensation in gradient sets with or without active screening. Preemphasis parameters are nonvolatile but can be changed rapidly when working with interchangeable gradient sets. The design allows easy retrofit to existing systems. A method for setting the preemphasis is presented, together with specimen results.

A novel eddy current compensation scheme for pulsed gradient systems

We describe a modified pre-emphasis network that uses a number of fixed time constants, only whose amplitudes are adjustable. In comparison with variable time constant pre-emphasis networks we have used, the fixed time constant network is more effective and easier to set on the fly. We have further developed a method to set the fixed time constant network mathematically using a linear least squares technique.

`NOWEL-2' Variable-Shaped Electron Beam Lithography System for 0.1 µm Patterns with Refocusing and Eddy Current Compensation

We have developed a variable-shaped electron beam (EB) lithography system which we call NOWEL-2. The system has the following features: (1) a short objective lens and a four-stage major deflection system; the landing angle at the corner of the 1.6 mm square major field is less than 2 mrad, and patterns from 0.1 to 3.0 µm are well resolved over the whole deflection field, (2) 20-bit digital-to-analog converter and high-precision current output amplifier, in which the least significant bit (LSB) corresponds to 0.0025 µm; linearity error is less than 1/4 LSB at 23±5°C, (3) refocusing and refocus-flyback; the edge sharpness of a 3 µm square beam was improved from 0.5 µm to 0.25 µm, (4) eddy current compensation; for a 100 µm electromagnetic jump, the waiting time is less than 50 µs, (5) continuously moving stage mode exposure; beam position accuracy is about 0.05 µm (3σ).

A MRI gradient waveform model for automated sequence calibration.

In order to set up magnetic resonance imaging (MRI) procedures of arbitrary voxel dimensions, slice orientation, and sequence timing in a reasonable time, some form of automatic gradient pulse calibration is required. One such method, involving simulation of gradient waveforms, is presented. Waveforms are modeled based on measurements of the step response. The model used divides each transition into three time regions: a "start" region in the first 0.3 ms, a "slew" region, and a "tail" region representing decay of the eddy current compensation error. In the "slew" region, the time derivative of the gradient, G' (t), is expressed as a function of G(t). The first two regions are nonlinear with respect to demand. The mean error in the simulated gradient is generally less than 0.04 mT m-1 in spin echo sequences. Image signal/noise ratios resulting from sequences calibrated using the model are within 5% of those of empirically calibrated sequences.