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A Henning - One of the best experts on this subject based on the ideXlab platform.
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dynamic b0 Shimming of the human brain at 9 4 t with a 16 channel multi coil Shim setup
Magnetic Resonance in Medicine, 2018Co-Authors: A Aghaeifar, A Henning, C Mirkes, J Bause, Theodor Steffen, N Avdievitch, K SchefflerAbstract:PURPOSE A 16-channel multi-coil Shimming setup was developed to mitigate severe B0 field perturbations at ultrahigh field and improve data quality for human brain imaging and spectroscopy. METHODS The Shimming setup consisted of 16 circular B0 coils that were positioned symmetrically on a cylinder with a diameter of 370 mm. The latter was large enough to house a shielded 18/32-channel RF transceiver array. The Shim performance was assessed via simulations and phantom as well as in vivo measurements at 9.4 T. The global and dynamic Shimming performance of the multi-coil setup was compared with the built-in scanner Shim system for EPI and single voxel spectroscopy. RESULTS The presence of the multi-coil Shim did not influence the performance of the RF coil. The performance of the proposed setup was similar to a full third-order spherical harmonic Shim system in the case of global static and dynamic slice-wise Shimming. Dynamic slice-wise Shimming with the multi-coil setup outperformed global static Shimming with the scanner's second-order spherical-harmonic Shim. The multi-coil setup allowed mitigating geometric distortions for EPI. The combination of the multi-coil Shim setup with the zeroth and first-order Shim of the scanner further reduced the standard deviation of the B0 field in the brain by 12% compared with the case in which multi-coil was used exclusively. CONCLUSION The combination of a multi-coil setup and the linear Shim channels of the scanner provides a straightforward solution for implementing dynamic slice-wise Shimming without requiring an additional pre-emphasis setup.
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modeling real Shim fields for very high degree and order b0 Shimming of the human brain at 9 4 t
Magnetic Resonance in Medicine, 2018Co-Authors: P Chang, S Nassirpour, A HenningAbstract:Purpose To describe the process of calibrating a B0 Shim system using high-degree (or high order) spherical harmonic models of the measured Shim fields, to provide a method that considers amplitude dependency of these models, and to show the advantage of very high-degree B0 Shimming for whole-brain and single-slice applications at 9.4 Tesla (T). Methods An insert Shim with up to fourth and partial fifth/sixth degree (order) spherical harmonics was used with a Siemens 9.4T scanner. Each Shim field was measured and modeled as input for the Shimming algorithm. Optimal Shim currents can therefore be calculated in a single iteration. A range of Shim currents was used in the modeling to account for possible amplitude nonlinearities. The modeled Shim fields were used to compare different degrees of whole-brain B0 Shimming on healthy subjects. Results The ideal Shim fields did not correctly Shim the subject brains. However, using the modeled Shim fields improved the B0 homogeneity from 55.1 (second degree) to 44.68 Hz (partial fifth/sixth degree) on the whole brains of 9 healthy volunteers, with a total applied current of 0.77 and 6.8 A, respectively. Conclusions The necessity of calibrating the Shim system was shown. Better B0 homogeneity drastically reduces signal dropout and distortions for echo-planar imaging, and significantly improves the linewidths of MR spectroscopy imaging. Magn Reson Med 79:529–540, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
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fast iterative pre emphasis calibration method enabling third order dynamic Shim updated fmri
Magnetic Resonance in Medicine, 2016Co-Authors: A Fillmer, Matteo Pavan, Klaas P Pruessmann, S J Vannesjo, Milan Scheidegger, A HenningAbstract:PURPOSE: To calibrate a pre-emphasis to sufficiently compensate eddy currents for application of dynamic Shim updating to fMRI without extension of scan times. METHODS: Eddy current effects induced into all Shim terms up to third-order were characterized by spatiotemporal field monitoring, using a third-order field camera. Pre-emphasis settings were derived from the measurements and iteratively evaluated and refined. The calibrated pre-emphasis was applied to slice-wise dynamic Shim updating in combination with a dynamic excitation frequency (F0) determination and a slice-wise B0 optimization routine for in vivo echo planar imaging and resting-state functional MRI. RESULTS: The described method for pre-emphasis calibration led to settling times of remaining eddy current effects below 2 ms, allowing for the application of dynamic Shim updating to fMRI without extension of scan times or induction of eddy current related artifacts. A dynamic F0 determination compensates frequency shifts induced by the superposition of different Shim fields, and therefore, prevents an image shift within the field of view. Hardware limitations necessitate the reduction of the maximum applicable B0 Shim field amplitudes and restrict the Shim performance. CONCLUSION: The proposed method enables accurate pre-emphasis calibration, and therefore, the application of dynamic Shim updating to fMRI. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
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system identification and signal processing for pid control of b0 Shim systems in ultra high field magnetic resonance applications
IFAC Proceedings Volumes, 2014Co-Authors: Yuchun Chang, N I Avdievich, A HenningAbstract:Abstract Magnetic resonance (MR) scanners are important tools in medical diagnostics and in many areas of neuroscience. MR technology is moving towards ultra-high field (UHF) 7T and 9.4T scanners which provide more signal intensity. However they also suffer from inhomogeneity of the static (B0) magnetic field which can lead to artifacts and uninterpretable data. B0 Shimming is a technique used to reduce inhomogeneities but most MR scanners use static Shim settings for the duration of the experiment. Dynamic Shim updating (DSU) updates the Shim in real-time while the scan is in process and can hence reduce any fluctuations in B0 field which may arise due to patient breathing, mechanical vibrations and soforth. However DSU is currently very slow and if we intend to increase the update rate then control theory needs to be applied. This paper presents an application of basic system identification and signal processing in the context of MR systems for DSU. Although system identification of these systems has been done before, they are non-parametric frequency domain approaches. These systems can be modelled as linear multivariable systems.
Juan Bascunan - One of the best experts on this subject based on the ideXlab platform.
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prototype rebco z1 and z2 Shim coils for ultra high field
Scientific Reports, 2020Co-Authors: Dongkeun Park, Jiho Lee, Juan Bascunan, Y IwasaAbstract:We present promising results of novel high-temperature superconducting (HTS) Shim coil prototypes that circumvent the size and strength limitation of our earlier innovative HTS Shim concept based on 46-mm wide REBCO tape. The HTS Shim coil is placed inside the HTS magnet, mainly for ultra-high-field (> 1 GHz or 23.5 T) NMR magnets, and thus unaffected from the windings' diamagnetic wall effects. One full-scale version will be applied to clean up Z1 and Z2 harmonic errors in the MIT 1.3-GHz high-resolution NMR magnet composed of an 835-MHz HTS insert, while another version for an MIT 1-GHz microcoil NMR magnet whose small-scale model we are currently building. The prototype sets were wound with a 2-pile, 1.03-mm wide, 0.30-mm thick REBCO conductor. Operated at 77 K, the Z1 Shim set generated a 1st harmonic field strength of 179 kHz/cm at 70 A, while the Z2 Shim set, composed of two pairs, Z21 and Z22, generated the 2nd harmonic field of 141 kHz/cm2 at 50 A. Together with discussion on technical challenges for this REBCO Shim coil concept, we demonstrate its feasibility for the next generation of ultra-high-field (UHF) HTS NMR magnets.
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hts Shim coils energized by a flux pump for the mit 1 3 ghz lts hts nmr magnet design construction and results of a proof of concept prototype
IEEE Transactions on Applied Superconductivity, 2018Co-Authors: Dongkeun Park, Jiho Lee, Juan Bascunan, Philip C Michael, Y IwasaAbstract:In this paper we present design, construction, and preliminary results of a proof-of-concept prototype of high-temperature superconductor (HTS) Shim coils operated at 77 K and energized, for the first time among all Shim coils, by a flux pump, here called digital flux injector (DFI). Although the prototype Shims were wound with 2-mm wide REBCO tape, and DFI with Bi2223 and REBCO tapes, the HTS Z1 and Z2 Shims to be installed in the MIT 1.3-GHz LTS/HTS NMR magnet (1.3G) currently under construction and operated at 4.2 K will be wound with reinforced Bi2212 wire and DFI with Nb $_3$ Sn tape. The paper concludes with two sets of Bi2212 Z1 and Z2 Shims for 1.3G.
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persistent mode high temperature superconductor Shim coils a design concept and experimental results of a prototype z1 high temperature superconductor Shim
Applied Physics Letters, 2013Co-Authors: Yukikazu Iwasa, Dongkeun Park, Seungyong Hahn, John P Voccio, Youngjae Kim, Juan BascunanAbstract:Design, fabrication, and test results of a type persistent-mode high-temperature superconductor (HTS) Shim coil are presented. A prototype Z1 rectangle-loop Shim, cut from 46-mm wide Y-Ba-Cu-O tape manufactured by AMSC, was fabricated and tested at 77 K. The HTS Shim, much thinner than the conventional NbTi Shim, is placed inside the main magnet and immune to its diamagnetic wall effects. Combined with the >12-T and >10-K operation capability, the HTS Shim offers a versatile design option for nuclear magnetic resonance (NMR) magnets, liquid-helium-free as well as conventional, and is particularly attractive in the next generation NMR magnets.
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nonlinear behavior of a Shim coil in an lts hts nmr magnet with an hts insert comprising double pancake hts tape coils
IEEE Transactions on Applied Superconductivity, 2009Co-Authors: Seungyong Hahn, Juan Bascunan, Min Cheol Ahn, Weijun Yao, Y IwasaAbstract:This paper reports results, experimental and analytical, of the nonlinear behavior of a Shim coil in the presence of an HTS coil assembled with double-pancake (DP) HTS-tape coils. The experimental results are from: 1) operation of a 700 MHz LTS/HTS NMR magnet (LH700) consisting of a 600 MHz LTS NMR magnet (L600) equipped with superconducting Shim coils and a 100 MHz DP-assembled HTS insert (H100) and; 2) an experiment with a room-temperature (RT) Z1 Shim coil coupled to a 50 MHz DP-assembled HTS insert (H50). A field mapping theory with a harmonic analysis is applied to interpret both results. Based on experimental results and analyses, we conclude that the screening-current-induced field (SCF) generated by a DP-assembled HTS insert is primarily responsible for the nonlinear behavior, including strength degradation, of a Shim coil.
Ernesta M Meintjes - One of the best experts on this subject based on the ideXlab platform.
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repeatability and reproducibility of prospective motion and Shim corrected 2d glycocest mri
Quantitative imaging in medicine and surgery, 2019Co-Authors: Gizeaddis Lamesgin Simegn, Andre Van Der Kouwe, Ernesta M Meintjes, Ali Alhamud, Frances RobertsonAbstract:Background: Repeated glycoCEST MRI measurements on the same subject should produce similar results under the same environmental and experimental conditions. However, fluctuations in the static B 0 field, which may occur between and within measurements due to heating of the Shim iron or subject motion, may alter results and affect reproducibility. Here we investigate the repeatability and reproducibility of glycoCEST measurements and examine the effectiveness of a real-time Shim- and motion navigated chemical exchange saturation transfer (CEST) sequence to improve reproducibility. Methods: In nine subjects, double volumetric navigated (DvNav)-CEST acquisitions in the calf muscle were repeated five times in each of two sessions—the first without correction, and the second with real-time Shim- and motion correction applied. In both sessions a dynamically changing field was introduced by running a 5-minute gradient intensive diffusion sequence. We evaluated the effect of the introduced B 0 inhomogeneity on the reproducibility of glycoCEST, where the small chemical shift difference between the hydroxyl and bulk water protons at 3 T makes CEST quantification extremely sensitive to magnetic field inhomogeneities. Results: With real-time Shim- and motion correction, glycoCEST results were relatively consistent with mean coefficient of variation (CoV) 2.7%±1.4% across all subjects, whereas without correction the results were less consistent with CoV 84%±71%. Conclusions: Our results demonstrate that real-time Shim- and motion correction can mitigate effects of B 0 field fluctuations and improve reproducibility of glycoCEST data. This is important when conducting longitudinal studies or when using glycoCEST MRI to assess treatment or physiological responses over time.
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real time simultaneous Shim and motion measurement and correction in glycocest mri using double volumetric navigators dvnavs
Magnetic Resonance in Medicine, 2019Co-Authors: Gizeaddis Lamesgin Simegn, Andre Van Der Kouwe, Frances Robertson, Ernesta M Meintjes, Ali AlhamudAbstract:Purpose CEST MRI allows for indirect detection of molecules with exchangeable protons, measured as a reduction in water signal because of continuous transfer of saturated protons. CEST requires saturation pulses on the order of a second, as well as repeated acquisitions at different offset frequencies. The resulting extended scan time makes CEST susceptible to subject motion, which introduces field inhomogeneity, shifting offset frequencies and causing distortions in CEST spectra that resemble true CEST effects. This is a particular problem for molecules that resonate close to water, such as hydroxyl group in glycogen. To address this, a technique for real-time measurement and correction of motion and field inhomogeneity is proposed. Methods A CEST sequence was modified to include double volumetric navigators (DvNavs) for real-time simultaneous motion and Shim correction. Phantom tests were conducted to investigate the effects of motion and Shim changes on CEST quantification and to validate the accuracy of DvNav motion and Shim estimates. To evaluate DvNav Shim and motion correction in vivo, acquisitions including 5 experimental conditions were performed in the calf muscle of 2 volunteers. Results Phantom data show that DvNav-CEST accurately estimates frequency and linear gradient changes because of motion and corrects resulting image distortions. In addition, DvNav-CEST improves CEST quantification in vivo in the presence of motion. Conclusion The proposed technique allows for real-time simultaneous motion and Shim correction with no additional scanning time, enabling accurate CEST quantification even in the presence of motion and field inhomogeneity.
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reproducibility of macromolecule suppressed gaba measurement using motion and Shim navigated mega special with lcmodel jmrui and gannet
Magnetic Resonance Materials in Physics Biology and Medicine, 2016Co-Authors: Andre Van Der Kouwe, Frances Robertson, A Alhamud, Muhammad G Saleh, Jamie Near, Ernesta M MeintjesAbstract:Measuring the pure form of GABA has become increasingly important because of its association with behaviour and certain pathologies. The aim of this study was to assess the reproducibility of GABA measurements using a Shim and motion navigated MEGA-SPECIAL sequence with LCModel, jMRUI and GANNET software. Motion and Shim navigated MEGA-SPECIAL scans were acquired in 20 healthy subjects. Two acquisitions were performed for each of two regions: the anterior cingulate (ACC) and medial-parietal (PAR) cortices. Absolute GABA concentration ( $${\text{GABA}}_{{{\text{H}}_{2} {\text{O}}}}$$ ) and GABA-to-Creatine ratio (GABA/Cr) were quantified using the three software packages. Using the within-subject coefficient of variation (CVws) as an index, reproducibility for both GABAH20 and GABA/Cr ranged from 13 to 22 % in the ACC and 13 to 18 % in PAR using the three software packages. Based on CVws, GABA concentrations in both the ACC and PAR are reproducible using a Shim and motion navigated MEGA-SPECIAL sequence with any of the three software packages, thus demonstrating the ability to quantify the pure form of GABA using these software in studies relating GABA to pathology and healthy behaviour.
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real time measurement and correction of both b0 changes and subject motion in diffusion tensor imaging using a double volumetric navigated dvnav sequence
NeuroImage, 2016Co-Authors: A Alhamud, Paul A Taylor, Andre J W Van Der Kouwe, Ernesta M MeintjesAbstract:Diffusion tensor imaging (DTI) requires a set of diffusion weighted measurements in order to acquire enough information to characterize local structure. The MRI scanner automatically performs a Shimming process by acquiring a field map before the start of a DTI scan. Changes in B0, which can occur throughout the DTI acquisition due to several factors (including heating of the iron Shim coils or subject motion), cause significant signal distortions that result in warped diffusion tensor (DT) parameter estimates. In this work we introduce a novel technique to simultaneously measure, report and correct in real time subject motion and changes in B0 field homogeneity, both in and through the imaging plane. This is achieved using double volumetric navigators (DvNav), i.e. a pair of 3D EPI acquisitions, interleaved with the DTI pulse sequence. Changes in the B0 field are evaluated in terms of zero-order (frequency) and first-order (linear gradients) Shim. The ability of the DvNav to accurately estimate the Shim parameters was first validated in a water phantom. Two healthy subjects were scanned both in the presence and absence of motion using standard, motion corrected (single navigator, vNav), and DvNav DTI sequences. The difference in performance between the proposed 3D EPI field maps and the standard 3D gradient echo field maps of the MRI scanner was also evaluated in a phantom and two healthy subjects. The DvNav sequence was shown to accurately measure and correct changes in B0 following manual adjustments of the scanner's central frequency and the linear Shim gradients. Compared to other methods, the DvNav produced DTI results that showed greater spatial overlap with anatomical references, particularly in scans with subject motion. This is largely due to the ability of the DvNav system to correct Shim changes and subject motion between each volume acquisition, thus reducing shear distortion.
Lawrence L Wald - One of the best experts on this subject based on the ideXlab platform.
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a 32 channel combined rf and b0 Shim array for 3t brain imaging
Magnetic Resonance in Medicine, 2016Co-Authors: Jason P Stockmann, Azma Mareyam, Boris Keil, Lawrence L Wald, Thomas Witzel, Jonathan R Polimeni, Cristen Lapierre, Kawin SetsompopAbstract:Purpose We add user-controllable direct currents (DC) to the individual elements of a 32-channel radio-frequency (RF) receive array to provide B0 Shimming ability while preserving the array's reception sensitivity and parallel imaging performance. Methods Shim performance using constrained DC current (±2.5A) is simulated for brain arrays ranging from 8 to 128 elements. A 32-channel 3-tesla brain array is realized using inductive chokes to bridge the tuning capacitors on each RF loop. The RF and B0 Shimming performance is assessed in bench and imaging measurements. Results The addition of DC currents to the 32-channel RF array is achieved with minimal disruption of the RF performance and/or negative side effects such as conductor heating or mechanical torques. The Shimming results agree well with simulations and show performance superior to third-order spherical harmonic (SH) Shimming. Imaging tests show the ability to reduce the standard frontal lobe susceptibility-induced fields and improve echo planar imaging geometric distortion. The simulation of 64- and 128-channel brain arrays suggest that even further Shimming improvement is possible (equivalent to up to 6th-order SH Shim coils). Conclusion Including user-controlled Shim currents on the loops of a conventional highly parallel brain array coil is feasible with modest current levels and produces improved B0 Shimming performance over standard second-order SH Shimming. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
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simultaneous z Shim method for reducing susceptibility artifacts with multiple transmitters
Magnetic Resonance in Medicine, 2009Co-Authors: Weiran Deng, Cungeng Yang, Vijayanand Alagappan, Lawrence L Wald, Fernando E Boada, Andrew V StengerAbstract:The signal loss susceptibility artifact is a major limitation in gradient-echo MRI applications. Various methods, including z-Shim techniques and multidimensional tailored radio frequency (RF) pulses, have been proposed to mitigate the through-plane signal loss artifact, which is dominant in axial slices above the sinus region. Unfortunately, z-Shim techniques require multiple steps and multidimensional RF methods are complex, with long pulse lengths. Parallel transmission methods were recently shown to be promising for improving B1 inhomogeneity and reducing the specific absorption rate. In this work, a novel method using time-shifted slice-select RF pulses is presented for reducing the through-plane signal loss artifact in parallel transmission applications. A simultaneous z-Shim is obtained by concurrently applying unique time-shifted pulses on each transmitter. The method is shown to reduce the signal loss susceptibility artifact in gradient-echo images using a four-channel parallel transmission system at 3T.
Dongkeun Park - One of the best experts on this subject based on the ideXlab platform.
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prototype rebco z1 and z2 Shim coils for ultra high field
Scientific Reports, 2020Co-Authors: Dongkeun Park, Jiho Lee, Juan Bascunan, Y IwasaAbstract:We present promising results of novel high-temperature superconducting (HTS) Shim coil prototypes that circumvent the size and strength limitation of our earlier innovative HTS Shim concept based on 46-mm wide REBCO tape. The HTS Shim coil is placed inside the HTS magnet, mainly for ultra-high-field (> 1 GHz or 23.5 T) NMR magnets, and thus unaffected from the windings' diamagnetic wall effects. One full-scale version will be applied to clean up Z1 and Z2 harmonic errors in the MIT 1.3-GHz high-resolution NMR magnet composed of an 835-MHz HTS insert, while another version for an MIT 1-GHz microcoil NMR magnet whose small-scale model we are currently building. The prototype sets were wound with a 2-pile, 1.03-mm wide, 0.30-mm thick REBCO conductor. Operated at 77 K, the Z1 Shim set generated a 1st harmonic field strength of 179 kHz/cm at 70 A, while the Z2 Shim set, composed of two pairs, Z21 and Z22, generated the 2nd harmonic field of 141 kHz/cm2 at 50 A. Together with discussion on technical challenges for this REBCO Shim coil concept, we demonstrate its feasibility for the next generation of ultra-high-field (UHF) HTS NMR magnets.
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hts Shim coils energized by a flux pump for the mit 1 3 ghz lts hts nmr magnet design construction and results of a proof of concept prototype
IEEE Transactions on Applied Superconductivity, 2018Co-Authors: Dongkeun Park, Jiho Lee, Juan Bascunan, Philip C Michael, Y IwasaAbstract:In this paper we present design, construction, and preliminary results of a proof-of-concept prototype of high-temperature superconductor (HTS) Shim coils operated at 77 K and energized, for the first time among all Shim coils, by a flux pump, here called digital flux injector (DFI). Although the prototype Shims were wound with 2-mm wide REBCO tape, and DFI with Bi2223 and REBCO tapes, the HTS Z1 and Z2 Shims to be installed in the MIT 1.3-GHz LTS/HTS NMR magnet (1.3G) currently under construction and operated at 4.2 K will be wound with reinforced Bi2212 wire and DFI with Nb $_3$ Sn tape. The paper concludes with two sets of Bi2212 Z1 and Z2 Shims for 1.3G.
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persistent mode high temperature superconductor Shim coils a design concept and experimental results of a prototype z1 high temperature superconductor Shim
Applied Physics Letters, 2013Co-Authors: Yukikazu Iwasa, Dongkeun Park, Seungyong Hahn, John P Voccio, Youngjae Kim, Juan BascunanAbstract:Design, fabrication, and test results of a type persistent-mode high-temperature superconductor (HTS) Shim coil are presented. A prototype Z1 rectangle-loop Shim, cut from 46-mm wide Y-Ba-Cu-O tape manufactured by AMSC, was fabricated and tested at 77 K. The HTS Shim, much thinner than the conventional NbTi Shim, is placed inside the main magnet and immune to its diamagnetic wall effects. Combined with the >12-T and >10-K operation capability, the HTS Shim offers a versatile design option for nuclear magnetic resonance (NMR) magnets, liquid-helium-free as well as conventional, and is particularly attractive in the next generation NMR magnets.
