Dynamic B 0 shimming of the human brain at 9.4 T with a 16‐channel multi‐coil shim setup
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Purpose A 16‐channel multi‐coil shimming setup was developed to mitigate severe B 0 field perturbations at ultrahigh field and improve data quality for human brain imaging and spectroscopy. Methods The shimming setup consisted of 16 circular B 0 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 B 0 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.Keywords:
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Flip angle
Purpose The purpose of this study is to introduce a novel design method of a shim coil array specifically optimized for whole brain shimming and to compare the performance of the resulting coils to conventional spherical harmonic shimming. Methods The proposed design approach is based on the stream function method and singular value decomposition. Eighty‐four field maps from 12 volunteers measured in seven different head positions were used during the design process. The cross validation technique was applied to find an optimal number of coil elements in the array. Additional 42 field maps from 6 further volunteers were used for an independent validation. A bootstrapping technique was used to estimate the required population size to achieve a stable coil design. Results Shimming using 12 and 24 coil elements outperforms fourth‐ and fifth‐order spherical harmonic shimming for all measured field maps, respectively. Coil elements show novel coil layouts compared to the conventional spherical harmonic coils and existing multi‐coils. Both leave‐one‐out and independent validation demonstrate the generalization ability of the designed arrays. The bootstrapping analysis predicts that field maps from approximately 140 subjects need to be acquired to arrive at a stable design. Conclusions The results demonstrate the validity of the proposed method to design a shim coil array matched to the human brain anatomy, which naturally satisfies the laws of electrodynamics. The design method may also be applied to develop new shim coil arrays matched to other human organs.
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Abstract Shim coils used in magnetic resonance imaging and NMR to produce magnetic fields are commonly designed to generate spherical harmonics, and thus achieve accuracy over a spherical region. Herein a cylindrical basis set is presented as an alternative to spherical harmonics, so as to better suit the cylindrical coil geometry and, in cases where it more closely matches a cylinder, the imaging region of interest. Example coil winding patterns for a selection of functions in this new set are derived using the target field method. A computational approach is taken to determine their accuracy, and it is found the basis functions are generated with high accuracy over cylindrical imaging regions within the coil. The basis set is also applied to a double‐coil configuration with active shielding, which results in slightly reduced accuracy within the coil but highly effective field nullification beyond the coil radius.
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A new passive shim design method is presented which is based on a magnetization mapping approach. Well defined regions with similar magnetization values define the optimal number of passive shims, their shape and position. The new design method is applied in a shimming process without prior-axial shim localization; this reduces the possibility of introducing new errors. The new shim design methodology reduces the number of iterations and the quantity of material required to shim a magnet. Only a few iterations (1-5) are required to shim a whole body horizontal bore magnet with a manufacturing error tolerance larger than 0.1 mm and smaller than 0.5 mm. One numerical example is presented
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In this paper, we have studied about design and fabrication of the actively shielded superconducting MRI magnet. Nonlinear optimization methods are usually used to find optimum coil configurations. However the selection of initial coil configurations is very difficult. In case bad initial data are used, it is even impossible to find optimum coil configurations which satisfy predefined constraints. We have developed computer optimization program which consists of two steps. Initial coil configurations are easily selected through linear optimization in the first step and optimum coil configurations are found through nonlinear optimization in the second step. We have also studied about superconducting shim coils to cancel error fields caused by coil fabrication errors. Many researchers published design concepts of shim coil. However all these studies are for shim coil design using filamentary coils with single turn. Shim coils with multi-turns should be used to produce enough field strength to cancel error fields. We have developed computer program for the design of shim coils which have proper thickness and length. An actively shielded superconducting MRI magnet with a small warm bore was fabricated and four sets of superconducting shim coils were equipped. The magnetic field distributions were measured and field correction was carried out using shim coils.
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Purpose To evaluate the use of the recently proposed ultrafast B 1 + mapping approach DREAM (Dual Refocusing Echo Acquisition Mode) for a refinement of patient adaptive radiofrequency (RF) shimming. Materials and Methods Volumetric DREAM B 1 + calibration scans centered in the upper abdomen were acquired in 20 patients and three volunteers with written informed consent at a clinical dual source 3 Tesla (T) MR system. Based on these data, RF transmit settings were optimized by central‐slice based RF‐shimming (CS‐RF shim) and by a refined, multi‐slice adaptive approach (MS‐RF shim). Simulations were performed to compare flip angle accuracy and B 1 + homogeneity (cv = stddev/mean) achieved by CS‐RF shim versus MS‐RF shim for transversal and coronal slices, and for volume shimming on the spine. Results By MS‐RF shim, mean deviation from nominal flip angle was reduced to less than 11% in all slices, all targets, and all subjects. Relative improvements in B 1 + cv (MS‐RF shim versus CS‐RF) were up to 14%/39%/47% in transversal slices/coronal slices/ spine area. Conclusion Volumetric information about B 1 + can be used to further improve the accuracy and homogeneity of the B 1 + field yielding higher diagnostic confidence, and will also be of value for various quantitative methods which are sensitive to flip angle imperfections. J. Magn. Reson. Imaging 2014;40:857–863 . © 2013 Wiley Periodicals, Inc .
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Abstract Room temperature (RT) shims are used routinely in MRI to remove global and local B 0 field inhomogeneity introduced by the subject. Most clinical scanners use only second‐order spherical harmonic terms, but with the increasing availability of very high field systems, third‐ and fourth‐order terms are a serious consideration. However, choosing appropriate coil strengths is of critical importance in shim coil design since overspecification of the shim strengths can lead to a variety of problems, including shim coil self‐resonance. In this study B 0 field map data collected over a period of 6 months (over 400 brain volumes) were analyzed to find the characteristic B ‐fields required to shim these brains. These data can be used to specify the coil requirements to effectively shim the human brain. Magn Reson Med, 2006. © 2005 Wiley‐Liss, Inc.
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Abstract In a magnetic resonance imaging equipment, gradient and shim coils are needed to produce a spatially varying magnetic field throughout the sample being imaged. Such coils consist of turns of wire wound on the surface of a cylindrical tube. Shim coils in particular, must sometimes be designed to produce complicated magnetic fields to correct for impurities. Streamline patterns for shim coils are much more complicated than those for gradient coils. In this work we present a detailed analysis of streamline methods and their application to shim coil design. A method is presented for determining the winding patterns to generate these complicated fields. © 2002 John Wiley & Sons, Inc. Concepts Magn Reson 14: 9–18, 2002
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