A 31‐channel integrated “AC/DC” B0 shim and radiofrequency receive array coil for improved 7T MRI
Jason StockmannNicolas ArangoThomas WitzelAzma MareyamCharlotte R. SappoJiazheng ZhouLucas J. JenkinsLincoln Craven‐BrightmanEugene MilshteynMathias DavidsW. Scott HogeMonika ŚliwiakShahin NasrBoris KeilElfar AdalsteinssonBastien GuérinJacob WhiteKawin SetsompopJon̈athan R. PolimeniLawrence L. Wald
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Purpose To test an integrated “AC/DC” array approach at 7T, where B 0 inhomogeneity poses an obstacle for functional imaging, diffusion‐weighted MRI, MR spectroscopy, and other applications. Methods A close‐fitting 7T 31‐channel (31‐ch) brain array was constructed and tested using combined Rx and ΔB 0 shim channels driven by a set of rapidly switchable current amplifiers. The coil was compared to a shape‐matched 31‐ch reference receive‐only array for RF safety, signal‐to‐noise ratio (SNR), and inter‐element noise correlation. We characterize the coil array’s ability to provide global and dynamic (slice‐optimized) shimming using ΔB 0 field maps and echo planar imaging (EPI) acquisitions. Results The SNR and average noise correlation were similar to the 31‐ch reference array. Global and slice‐optimized shimming provide 11% and 40% improvements respectively compared to baseline second‐order spherical harmonic shimming. Birdcage transmit coil efficiency was similar for the reference and AC/DC array setups. Conclusion Adding ΔB 0 shim capability to a 31‐ch 7T receive array can significantly boost 7T brain B 0 homogeneity without sacrificing the array’s rdiofrequency performance, potentially improving ultra‐high field neuroimaging applications that are vulnerable to off‐resonance effects.Keywords:
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The growth in the use of magnetic resonance imaging (MRI) data for radiation therapy (RT) treatment planning has been facilitated by scanner hardware and software advances that have enabled RT patients to be imaged in treatment position while providing morphologic and functional assessment of tumor volumes and surrounding normal tissues. Despite these advances, manufacturers have been slow to develop radiofrequency (RF) coils that closely follow the contour of a RT patient undergoing MR imaging. Instead, relatively large form surface coil arrays have been adapted from diagnostic imaging. These arrays can be challenging to place on, and in general do not conform to the patient's body habitus, resulting in sub optimal image quality. The purpose of this study is to report on the characterization of a new flexible and highly decoupled RF coil for use in MR imaging of RT patients. Coil performance was evaluated by performing signal-to-noise ratio (SNR) and noise correlation measurements using two coil (SNR) and four coil (noise correlation) element combinations as a function of coil overlap distance and comparing these values to those obtained using conventional coil elements. In vivo testing was performed in both normal volunteers and patients using a four and 16 element RF coil. Phantom experiments demonstrate the highly decoupled nature of the new coil elements when compared to conventional RF coils, while in vivo testing demonstrate that these coils can be integrated into extremely flexible and form fitting substrates that follow the exact contour of the patient. The new coil design addresses limitations imposed by traditional surface coil arrays and have the potential to significantly impact MR imaging for both diagnostic and RT applications.
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Abstract Three quadrature transmit/receive radiofrequency magnetic resonance imaging breast coils were designed and built. The coils are geometrically equivalent but were scaled for different breast sizes. The coils were tested using a 4‐T Bruker/Siemens whole‐body scanner. We used different coil sizes to establish whether it is necessary to match coil size to load size. Moreover, our work investigated how the coil configuration can be changed to improve signal uniformity in the acquired images. We found that improvements can be gained by carefully matching the coil to the sample. In addition, we propose a coil layout that allows images quality to be improved, as measured by the signal‐to‐noise ratio and field uniformity of the coil. © 2012 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 41B: 50–56, 2012
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Objective To assess the value of radiofrequency coil in rat brain imaging with 3.0T MR system.Methods A kind of method that designed the coil structure was put forward.A saddle coil of cylinder high equal to the diameter was adopted,and the rat brain coil diameter was 5 cm.The capacitors and distributed capacitors were minimized and the coil bandwidth decreased,the coil quality factor(Q) raised as well.The coil with head coils and body coil were used to carry on scan to the self-made model respectively in same sequence,and the coils signal-to-noise ratio(SNR) of three groups images in same position slice were compared.The rat brain was carried on T1W FLAIR and T2W scan in three axes directions respectively to observe the imaging quality.Results The coil SNR was 5 times higher than that existing brain coil and could clearly distinguish grey matter from white matter,as well as the structure in the rat brain on T1WI and T2WI.Conclusion This coil has very high SNR and can obtain good results in the imaging researches of rat brain.
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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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Radiofrequency coils provide high-resolution magnetic resonance (MR) imaging of human tissues. A small RF coil produces MR images with a higher resolution compared to the commercial volume MR coils from mass limited samples. Signal to noise ratio (SNR) plays a key role in the optimal design of receiver radiofrequency coils. In this work, we present a three-loop saddle coil suitable for MR imaging of digits of the human body. The geometry of the introduced coil is optimized to achieve the highest SNR. The coil performance is evaluated through comparing the measured SNR maps of the optimal coil derived from MR images of a saline phantom with the corresponding measured SNR maps of a commercial head coil in axial and sagittal slices. Results verify that the image SNR of the introduced coil is 3.4 times higher than that of the head coil and 2 times higher than that of the similar saddle coils represented in the literature recently. To validate the measured results, SNR maps of the introduced saddle and head coils were simulated and their SNR difference was compared with the corresponding measured data of the two coils. Results show that the simulated and measured data are in agreement with less than 11.8% error.
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Abstract A general analytical framework is presented for the design of birdcage radiofrequency resonators on cylindrical formers having arbitrary cross‐sectional shape. The primary objective of such shapes would be to improve the sensitivity of the NMR experiment to noncircular regions of the human anatomy while maintaining field homogeneity and quadrature polarization comparable to those of standard circular birdcage coils. The shape of the corresponding radiofrequency screen, which is required to decouple the coil from the rest of the NMR system and which is key to the performance, is also provided by this methodology. The theory was tested by constructing a 3‐T, quadrature, proton coil on a shape conforming to the anthropomorphic mean of the human head, namely, the oval of Cassini. Both bench tests ( Q ) and in vivo spectral and imaging comparisons of the Cassinian coil with an equivalently dimensioned and constructed circular birdcage coil, respectively, predicted and demonstrated in vivo an improvement in SNR of ∼24% over the circular section coil. The experimental RF field homogeneity and quadrature performance were comparable for both coil geometries, with the circular coil being marginally superior. Magn Reson Med 53:201–211, 2005. © 2004 Wiley‐Liss, Inc.
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The performance of radio frequency (RF) coils, used in MRI units, is determined by the image uniformity and the signal-to-noise ratio (SNR). Birdcage and surface coils are commonly used. A birdcage coil provides a good image uniformity while a surface coil produces a high SNR. In this study, therefore, a staircase coil was designed from a standard version of a birdcage coil, with some structural changes to increase SNR while maintaining image uniformity. In phantom experiments, the improvement of the image to uniformity and the SNR increase of the staircase coil compared with the values for the birdcage coil were about 3.5% and 35%, respectively. In clinical experiment, the SNR increase of the staircase coil, compared with the value for the birdcage coil was about 40% in bone, muscle and blood-vessel tissues. These results show that the performance of the staircase coil was very improved over the standard birdcage coil in terms of SNR, and that image uniformity was maintained. Therefore, the staircase coil designed by this study should be useful in experimental and clinical l.5T MRI systems, and this coil offers an alternative method of quadrature detection.
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