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Shan S, Li M, Tang F, Ma H, Liu F, Crozier S. Gradient Field Deviation (GFD) Correction Using a Hybrid-Norm Approach With Wavelet Sub-Band Dependent Regularization: Implementation for Radial MRI at 9.4 T. IEEE Trans Biomed Eng 2019; 66:2693-2701. [DOI: 10.1109/tbme.2019.2895091] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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Wang Y, Liu F, Zhou X, Li Y, Crozier S. A numerical study of the acoustic radiation due to eddy current-cryostat interactions. Med Phys 2017; 44:2196-2206. [PMID: 28380260 DOI: 10.1002/mp.12261] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2016] [Revised: 03/23/2017] [Accepted: 03/25/2017] [Indexed: 11/08/2022] Open
Abstract
PURPOSE To investigate the acoustic radiation due to eddy current-cryostat interactions and perform a qualitative analysis on noise reduction methods. METHODS In order to evaluate the sound pressure level (SPL) of the eddy current induced warm bore wall vibration, a Finite Element (FE) model was created to simulate the noises from both the warm bore wall vibration and the gradient coil assembly. For the SPL reduction of the warm bore wall vibration, we first improved the active shielding of the gradient coil, thus reducing the eddy current on the warm bore wall. A damping treatment was then applied to the warm bore wall to control the acoustic radiation. RESULTS Initial simulations show that the SPL of the warm bore wall is higher than that of the gradient assembly with typical design shielding ratios at many frequencies. Subsequent simulation results of eddy current control and damping treatment application show that the average SPL reduction of the warm bore wall can be as high as 9.6 dB, and even higher in some frequency bands. CONCLUSIONS Combining eddy current control and suggested damping scheme, the noise level in a MRI system can be effectively reduced.
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Affiliation(s)
- Yaohui Wang
- School of Information Technology and Electrical Engineering, The University of Queensland, St Lucia, Brisbane, QLD, 4072, Australia
| | - Feng Liu
- School of Information Technology and Electrical Engineering, The University of Queensland, St Lucia, Brisbane, QLD, 4072, Australia
| | - Xiaorong Zhou
- College of Mechanical Engineering, Guangxi University, Daxue Road 100, Nanning, Guangxi, China
| | - Yu Li
- School of Information Technology and Electrical Engineering, The University of Queensland, St Lucia, Brisbane, QLD, 4072, Australia
| | - Stuart Crozier
- School of Information Technology and Electrical Engineering, The University of Queensland, St Lucia, Brisbane, QLD, 4072, Australia
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Tang F, Liu F, Freschi F, Li Y, Repetto M, Giaccone L, Wang Y, Crozier S. An improved asymmetric gradient coil design for high-resolution MRI head imaging. Phys Med Biol 2016; 61:8875-8889. [PMID: 27910827 DOI: 10.1088/1361-6560/61/24/8875] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
For head magnetic resonance imaging, local gradient coils are often used to achieve high solution images. To accommodate the human head and shoulder, the head gradient coils are usually designed in an asymmetric configuration, allowing the region-of-uniformity (ROU) close to the coil's patient end. However, the asymmetric configuration leads to technical difficulties in maintaining a high gradient performance for the insertable head coil with very limited space. In this work, we present a practical design configuration of an asymmetric insertable gradient head coil offering an improved performance. In the proposed design, at the patient end, the primary and secondary coils are connected using an additional radial surface, thus allowing the coil conductors distributed on the flange to ensure an improvement in the coil performance. At the service end, the primary and shielding coils are not connected, to permit access to shim trays, cooling system piping, cabling, and so on. The new designs are compared with conventional coil configurations and the simulation results show that, with a similar field quality in the ROU, the proposed coil pattern has improved construction characteristics (open service end, well-distributed wire pattern) and offers a better coil performance (lower inductance, higher efficiency, etc) than conventional head coil configurations.
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Affiliation(s)
- Fangfang Tang
- School of Information Technology and Electrical Engineering, the University of Queensland, Brisbane, QLD 4072, Australia
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Wang G, Kalra M, Murugan V, Xi Y, Gjesteby L, Getzin M, Yang Q, Cong W, Vannier M. Vision 20/20: Simultaneous CT-MRI--Next chapter of multimodality imaging. Med Phys 2016; 42:5879-89. [PMID: 26429262 DOI: 10.1118/1.4929559] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
Multimodality imaging systems such as positron emission tomography-computed tomography (PET-CT) and MRI-PET are widely available, but a simultaneous CT-MRI instrument has not been developed. Synergies between independent modalities, e.g., CT, MRI, and PET/SPECT can be realized with image registration, but such postprocessing suffers from registration errors that can be avoided with synchronized data acquisition. The clinical potential of simultaneous CT-MRI is significant, especially in cardiovascular and oncologic applications where studies of the vulnerable plaque, response to cancer therapy, and kinetic and dynamic mechanisms of targeted agents are limited by current imaging technologies. The rationale, feasibility, and realization of simultaneous CT-MRI are described in this perspective paper. The enabling technologies include interior tomography, unique gantry designs, open magnet and RF sequences, and source and detector adaptation. Based on the experience with PET-CT, PET-MRI, and MRI-LINAC instrumentation where hardware innovation and performance optimization were instrumental to construct commercial systems, the authors provide top-level concepts for simultaneous CT-MRI to meet clinical requirements and new challenges. Simultaneous CT-MRI fills a major gap of modality coupling and represents a key step toward the so-called "omnitomography" defined as the integration of all relevant imaging modalities for systems biology and precision medicine.
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Affiliation(s)
- Ge Wang
- Biomedical Imaging Center/Cluster, Rensselaer Polytechnic Institute, Troy, New York 12180
| | - Mannudeep Kalra
- Department of Imaging, Massachusetts General Hospital, 55 Fruit Street, Boston, Massachusetts 02114
| | - Venkatesh Murugan
- Department of Imaging, Massachusetts General Hospital, 55 Fruit Street, Boston, Massachusetts 02114
| | - Yan Xi
- Biomedical Imaging Center/Cluster, Rensselaer Polytechnic Institute, Troy, New York 12180
| | - Lars Gjesteby
- Biomedical Imaging Center/Cluster, Rensselaer Polytechnic Institute, Troy, New York 12180
| | - Matthew Getzin
- Biomedical Imaging Center/Cluster, Rensselaer Polytechnic Institute, Troy, New York 12180
| | - Qingsong Yang
- Biomedical Imaging Center/Cluster, Rensselaer Polytechnic Institute, Troy, New York 12180
| | - Wenxiang Cong
- Biomedical Imaging Center/Cluster, Rensselaer Polytechnic Institute, Troy, New York 12180
| | - Michael Vannier
- Department of Radiology, University of Chicago, Chicago, Illinois 60637
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Tang F, Freschi F, Repetto M, Li Y, Liu F, Crozier S. Mitigation of Intra-coil Eddy Currents in Split Gradient Coils in a Hybrid MRI-LINAC System. IEEE Trans Biomed Eng 2016; 64:725-732. [PMID: 27249823 DOI: 10.1109/tbme.2016.2573316] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
OBJECTIVE The aim of this study is to mitigate intra-gradient coil eddy currents in a hybrid MRI-LINAC system. METHODS The tracks of the gradient coils are modified by cutting slits along the current flow direction. The electromagnetic model developed was first experimentally validated and then used to study the impacts of the slit conductors on the energized and surrounding coils. In this study, gradient coils were slit with different numbers of sub-tracks and driven by a current with frequencies ranging from 100 Hz to 10 kHz. The proposed configuration was assessed by evaluating a number of system parameters, such as the gradient magnetic field, the power loss generated in the surrounding unenergized coil (hereafter referred to as passive coils), and the performance of the energized coil. RESULTS It was found that at a typical operating frequency of 1 kHz and compared with a conventional (no cut) split coil structure, the new coil pattern (with four slits) offered improved coil parameters. 1) The average power loss dissipated in the surrounding passive coil was significantly reduced by 85%, 2) the cuts largely reduced the secondary field generated by the eddy currents in the passive coil, which was reduced to about 4% of that produced by the uncut coil and, 3) the performance of the energized coil with slit tracks was significantly improved. Some typical gradient coil parameters, such as the figure of merit, efficiency (η), and η2/R (where η is the efficiency and R is the resistance), were improved by 8.0%, 11.9%, and 45.7%, respectively. CONCLUSION AND SIGNIFICANCE The new slit coil structure is effective in mitigating intra-coil eddy current effects, which is an important issue in the MRI-LINAC system.
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Smith E, Freschi F, Repetto M, Crozier S. A Method for Reducing Secondary Field Effects in Asymmetric MRI Gradient Coil Design. IEEE Trans Biomed Eng 2016; 63:924-932. [DOI: 10.1109/tbme.2015.2479230] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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Tang F, Freschi F, Sanchez Lopez H, Repetto M, Liu F, Crozier S. Intra-coil interactions in split gradient coils in a hybrid MRI-LINAC system. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2016; 265:52-58. [PMID: 26852418 DOI: 10.1016/j.jmr.2016.01.013] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/05/2015] [Revised: 01/15/2016] [Accepted: 01/19/2016] [Indexed: 06/05/2023]
Abstract
An MRI-LINAC system combines a magnetic resonance imaging (MRI) system with a medical linear accelerator (LINAC) to provide image-guided radiotherapy for targeting tumors in real-time. In an MRI-LINAC system, a set of split gradient coils is employed to produce orthogonal gradient fields for spatial signal encoding. Owing to this unconventional gradient configuration, eddy currents induced by switching gradient coils on and off may be of particular concern. It is expected that strong intra-coil interactions in the set will be present due to the constrained return paths, leading to potential degradation of the gradient field linearity and image distortion. In this study, a series of gradient coils with different track widths have been designed and analyzed to investigate the electromagnetic interactions between coils in a split gradient set. A driving current, with frequencies from 100 Hz to 10 kHz, was applied to study the inductive coupling effects with respect to conductor geometry and operating frequency. It was found that the eddy currents induced in the un-energized coils (hereby-referred to as passive coils) positively correlated with track width and frequency. The magnetic field induced by the eddy currents in the passive coils with wide tracks was several times larger than that induced by eddy currents in the cold shield of cryostat. The power loss in the passive coils increased with the track width. Therefore, intra-coil interactions should be included in the coil design and analysis process.
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Affiliation(s)
- Fangfang Tang
- School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia
| | - Fabio Freschi
- School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia; Department of Energy, Politecnico di Torino, Corso Duca Degli Abruzzi, 24, 10129 Torino, Italy
| | - Hector Sanchez Lopez
- Department of Electrical Engineering, Universitas Dian Nuswantoro, Jl. Imam Bonjol No. 207, Semarang, Indonesia
| | - Maurizio Repetto
- School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia; Department of Energy, Politecnico di Torino, Corso Duca Degli Abruzzi, 24, 10129 Torino, Italy
| | - Feng Liu
- School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia.
| | - Stuart Crozier
- School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia
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Zhao Y, Zhao T, Raval SB, Krishnamurthy N, Zheng H, Harris CT, Handler WB, Chronik BA, Ibrahim TS. Dual optimization method of radiofrequency and quasistatic field simulations for reduction of eddy currents generated on 7T radiofrequency coil shielding. Magn Reson Med 2014; 74:1461-9. [PMID: 25367703 DOI: 10.1002/mrm.25424] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2014] [Revised: 06/26/2014] [Accepted: 08/04/2014] [Indexed: 01/20/2023]
Abstract
PURPOSE To optimize the design of radiofrequency (RF) shielding of transmit coils at 7T and reduce eddy currents generated on the RF shielding when imaging with rapid gradient waveforms. METHODS One set of a four-element, 2 × 2 Tic-Tac-Toe head coil structure was selected and constructed to study eddy currents on the RF coil shielding. The generated eddy currents were quantitatively studied in the time and frequency domains. The RF characteristics were studied using the finite difference time domain method. Five different kinds of RF shielding were tested on a 7T MRI scanner with phantoms and in vivo human subjects. RESULTS The eddy current simulation method was verified by the measurement results. Eddy currents induced by solid/intact and simple-structured slotted RF shielding significantly distorted the gradient fields. Echo-planar images, B1+ maps, and S matrix measurements verified that the proposed slot pattern suppressed the eddy currents while maintaining the RF characteristics of the transmit coil. CONCLUSION The presented dual-optimization method could be used to design RF shielding and reduce the gradient field-induced eddy currents while maintaining the RF characteristics of the transmit coil.
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Affiliation(s)
- Yujuan Zhao
- Department of Bioengineering, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Tiejun Zhao
- Siemens Medical Solutions, Pittsburgh, Pennsylvania, USA
| | - Shailesh B Raval
- Department of Bioengineering, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.,Department of Radiology, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | | | - Hai Zheng
- Department of Bioengineering, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Chad T Harris
- Department of Physics and Astronomy, University of Western Ontario, London, Canada
| | - William B Handler
- Department of Physics and Astronomy, University of Western Ontario, London, Canada
| | - Blaine A Chronik
- Department of Physics and Astronomy, University of Western Ontario, London, Canada
| | - Tamer S Ibrahim
- Department of Bioengineering, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.,Department of Radiology, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
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