The Experts below are selected from a list of 10572 Experts worldwide ranked by ideXlab platform
Andreas Rennings - One of the best experts on this subject based on the ideXlab platform.
-
Investigating the influence of dielectric pads in 7T magnetic resonance imaging – simulated and experimental assessment
Current Directions in Biomedical Engineering, 2020Co-Authors: Maíra M. Garcia, Andreas Rennings, Daniel Erni, Khallil T. Chaim, Maria Concepcion Garcia Otaduy, Maryam Vatanchi, Waldemar ZylkaAbstract:AbstractDipole radiofrequency (RF) Elements have been successfully used to compose multi-channel RF Coils for ultrahigh fields (UHF) magnetic resonance imaging (MRI). As magnetic components of RF fields (B1) can be very inhomogeneous at UHF (B0≥7T), dielectric pads with high dielectric constants were proposed to improve the B1 efficiency and homogeneity [1]. Dielectric pads can be used as a passive B1 shimmimg technique thanks to inducing a strong secondary magnetic field in their vicinity. The use of such dielectric pads affect not only the B1 field but also the electric field. This in turn affects the specific absorption rate (SAR) and consequently the temperature distribution inside the patient’s body. To study these effects, a 29 cm-long transmission dipole RF Coil Element terminated by two meander was used for 7T MRI [2]. Using a cylindrical agarose-gel phantom, numerical and experimental results were analyzed with respect to homogeneity and amplitude of the magnetic and electric fields generated by the RF Element in various configurations with and without dielectric pads. Calculated and measured B1 results were cross-checked and found to be in good agreement. When using dielectric pads B1 homogeneity and magnitude increase in regions where it was previously weak or insufficient. Calculations suggest that SAR distribution will change when using the pads.
-
Electromagnetic Field Analysis of a Dipole Coil Element With Surface Impedance Characterized Shielding Plate for 7-T MRI
IEEE Transactions on Microwave Theory and Techniques, 2016Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper, we systematically investigate the electromagnetic (EM) field of a stripline dipole Coil Element backed by various shielding plates, which are characterized by surface impedance. The initial analysis is based on a 2-D finite-Element-method model, where the considered surface impedance was categorized in terms of magnitude and phase. It has been demonstrated that the shielding plate can be approximately modeled by the magnitude of a complex surface impedance if the absolute EM field distribution is considered. Additionally, as the magnitude of the surface impedance increases, the magnetic and electric fields excited by the stripline tend to distribute in a broader manner. Thus, the transversal homogeneity of the $B_{1}$ field of a stripline Coil can be improved by a shielding plate with a high surface impedance, which has been verified by 3-D models based on single- and multi-Coil Elements. For the experimental validation, two shielding plates—a copper-plated substrate and a high-impedance surface, which exhibits a small and large surface impedance, respectively—are considered. An excellent agreement of field distributions between numerical simulation and measurement has been observed.
-
Coupling investigation between RF Coil array Elements backed by surface impedance characterized shields for 7 Tesla MRI
2016 German Microwave Conference (GeMiC), 2016Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper we present a coupling investigation between RF Coil array Elements which are backed by surface impedance characterized RF shields for 7 Tesla magnetic resonance imaging (MRI). Two simulation models for the RF Coil Elements are considered here: an ideal impressed current model for an initial 2-D investigation, and a symmetrically fed dipole with meander terminals for the 3-D investigation. The RF shield, which is placed behind the Coil Element, is characterized by a surface impedance boundary condition (SIBC), where different surface impedances are defined. An optimal surface impedance of the RF shield can be found to achieve minimum coupling between neighboring Coil Elements. Different spatial arrangements (e.g. the shape of the phantom, the edge-to-edge separation between coupled Coil Elements, the separation from Coil Element to RF shield) are considered. In general, a large surface impedance provides a higher coupling level in comparison to a small surface impedance. As the separation distance from the Coil Element to the RF shield increases, a reduced surface impedance selectivity of the coupling behavior is observed. The proposed fundamental investigation reveals a new approach to modify the coupling characteristics of the dipole Coil Elements for MRI.
-
Improved B 1 distribution of an MRI RF Coil Element using a high-impedance-surface shield
2015 German Microwave Conference, 2015Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper we propose an approach to improve the B 1 distribution in terms of homogeneity and penetration depth of a Coil Element by utilizing a high impedance surface (HIS) as the RF shield for 7 T magnetic resonance imaging (MRI). The transverse magnetic field distribution in the case of a HIS and a perfect electrical conductor (PEC) being the shielding plate are compared for different separation distances from the dipole Coil to the shielding plate. As the PEC shield is adjacent to the dipole Coil, an undesired surface current is induced on the PEC shielding plate by the dipole Coil, whereas the induced surface current on the HIS shield is sufficiently suppressed due to the high surface impedance. As a result, the dipole Coil with a HIS shield exhibits a broader and stronger field distribution, and thus achieves an improvement on the transverse B 1 homogeneity as well as the penetration depth. As the separation distance increases, the impact of the induced current is weakened and thus variations on the field distribution with different shielding scenarios (HIS and PEC) are reduced. The proposed approach has been validated by numerical simulations and experimental measurements, which show a good agreement.
-
Coupling investigation of different RF Coil Elements for 7-tesla magnetic resonance imaging based on characteristic mode analysis
2014 IEEE MTT-S International Microwave Symposium (IMS2014), 2014Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:Here in this paper we propose an approach to investigate the coupling mechanism of different RF Coil Elements based on characteristic mode analysis. The Coil Element with lumped-Element-connection to the shielding plate focuses the magnetic field below the strip-line due to a loop current from the dominant mode. Denoted by a negative characteristic mode eigenvalue, the Coil Element without direct connection to the shielding plate behaves more like an electric dipole and the field is distributed mainly above the strip-line. A compromise between these two scenarios which suffers neither from the field below nor above the strip-line achieves an optimal decoupling. FDTD simulation and experimental measurement confirmed the conclusion from characteristic mode analysis.
Daniel Erni - One of the best experts on this subject based on the ideXlab platform.
-
Investigating the influence of dielectric pads in 7T magnetic resonance imaging – simulated and experimental assessment
Current Directions in Biomedical Engineering, 2020Co-Authors: Maíra M. Garcia, Andreas Rennings, Daniel Erni, Khallil T. Chaim, Maria Concepcion Garcia Otaduy, Maryam Vatanchi, Waldemar ZylkaAbstract:AbstractDipole radiofrequency (RF) Elements have been successfully used to compose multi-channel RF Coils for ultrahigh fields (UHF) magnetic resonance imaging (MRI). As magnetic components of RF fields (B1) can be very inhomogeneous at UHF (B0≥7T), dielectric pads with high dielectric constants were proposed to improve the B1 efficiency and homogeneity [1]. Dielectric pads can be used as a passive B1 shimmimg technique thanks to inducing a strong secondary magnetic field in their vicinity. The use of such dielectric pads affect not only the B1 field but also the electric field. This in turn affects the specific absorption rate (SAR) and consequently the temperature distribution inside the patient’s body. To study these effects, a 29 cm-long transmission dipole RF Coil Element terminated by two meander was used for 7T MRI [2]. Using a cylindrical agarose-gel phantom, numerical and experimental results were analyzed with respect to homogeneity and amplitude of the magnetic and electric fields generated by the RF Element in various configurations with and without dielectric pads. Calculated and measured B1 results were cross-checked and found to be in good agreement. When using dielectric pads B1 homogeneity and magnitude increase in regions where it was previously weak or insufficient. Calculations suggest that SAR distribution will change when using the pads.
-
Electromagnetic Field Analysis of a Dipole Coil Element With Surface Impedance Characterized Shielding Plate for 7-T MRI
IEEE Transactions on Microwave Theory and Techniques, 2016Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper, we systematically investigate the electromagnetic (EM) field of a stripline dipole Coil Element backed by various shielding plates, which are characterized by surface impedance. The initial analysis is based on a 2-D finite-Element-method model, where the considered surface impedance was categorized in terms of magnitude and phase. It has been demonstrated that the shielding plate can be approximately modeled by the magnitude of a complex surface impedance if the absolute EM field distribution is considered. Additionally, as the magnitude of the surface impedance increases, the magnetic and electric fields excited by the stripline tend to distribute in a broader manner. Thus, the transversal homogeneity of the $B_{1}$ field of a stripline Coil can be improved by a shielding plate with a high surface impedance, which has been verified by 3-D models based on single- and multi-Coil Elements. For the experimental validation, two shielding plates—a copper-plated substrate and a high-impedance surface, which exhibits a small and large surface impedance, respectively—are considered. An excellent agreement of field distributions between numerical simulation and measurement has been observed.
-
Coupling investigation between RF Coil array Elements backed by surface impedance characterized shields for 7 Tesla MRI
2016 German Microwave Conference (GeMiC), 2016Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper we present a coupling investigation between RF Coil array Elements which are backed by surface impedance characterized RF shields for 7 Tesla magnetic resonance imaging (MRI). Two simulation models for the RF Coil Elements are considered here: an ideal impressed current model for an initial 2-D investigation, and a symmetrically fed dipole with meander terminals for the 3-D investigation. The RF shield, which is placed behind the Coil Element, is characterized by a surface impedance boundary condition (SIBC), where different surface impedances are defined. An optimal surface impedance of the RF shield can be found to achieve minimum coupling between neighboring Coil Elements. Different spatial arrangements (e.g. the shape of the phantom, the edge-to-edge separation between coupled Coil Elements, the separation from Coil Element to RF shield) are considered. In general, a large surface impedance provides a higher coupling level in comparison to a small surface impedance. As the separation distance from the Coil Element to the RF shield increases, a reduced surface impedance selectivity of the coupling behavior is observed. The proposed fundamental investigation reveals a new approach to modify the coupling characteristics of the dipole Coil Elements for MRI.
-
Improved B 1 distribution of an MRI RF Coil Element using a high-impedance-surface shield
2015 German Microwave Conference, 2015Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper we propose an approach to improve the B 1 distribution in terms of homogeneity and penetration depth of a Coil Element by utilizing a high impedance surface (HIS) as the RF shield for 7 T magnetic resonance imaging (MRI). The transverse magnetic field distribution in the case of a HIS and a perfect electrical conductor (PEC) being the shielding plate are compared for different separation distances from the dipole Coil to the shielding plate. As the PEC shield is adjacent to the dipole Coil, an undesired surface current is induced on the PEC shielding plate by the dipole Coil, whereas the induced surface current on the HIS shield is sufficiently suppressed due to the high surface impedance. As a result, the dipole Coil with a HIS shield exhibits a broader and stronger field distribution, and thus achieves an improvement on the transverse B 1 homogeneity as well as the penetration depth. As the separation distance increases, the impact of the induced current is weakened and thus variations on the field distribution with different shielding scenarios (HIS and PEC) are reduced. The proposed approach has been validated by numerical simulations and experimental measurements, which show a good agreement.
-
Coupling investigation of different RF Coil Elements for 7-tesla magnetic resonance imaging based on characteristic mode analysis
2014 IEEE MTT-S International Microwave Symposium (IMS2014), 2014Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:Here in this paper we propose an approach to investigate the coupling mechanism of different RF Coil Elements based on characteristic mode analysis. The Coil Element with lumped-Element-connection to the shielding plate focuses the magnetic field below the strip-line due to a loop current from the dominant mode. Denoted by a negative characteristic mode eigenvalue, the Coil Element without direct connection to the shielding plate behaves more like an electric dipole and the field is distributed mainly above the strip-line. A compromise between these two scenarios which suffers neither from the field below nor above the strip-line achieves an optimal decoupling. FDTD simulation and experimental measurement confirmed the conclusion from characteristic mode analysis.
Klaus Solbach - One of the best experts on this subject based on the ideXlab platform.
-
Electromagnetic Field Analysis of a Dipole Coil Element With Surface Impedance Characterized Shielding Plate for 7-T MRI
IEEE Transactions on Microwave Theory and Techniques, 2016Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper, we systematically investigate the electromagnetic (EM) field of a stripline dipole Coil Element backed by various shielding plates, which are characterized by surface impedance. The initial analysis is based on a 2-D finite-Element-method model, where the considered surface impedance was categorized in terms of magnitude and phase. It has been demonstrated that the shielding plate can be approximately modeled by the magnitude of a complex surface impedance if the absolute EM field distribution is considered. Additionally, as the magnitude of the surface impedance increases, the magnetic and electric fields excited by the stripline tend to distribute in a broader manner. Thus, the transversal homogeneity of the $B_{1}$ field of a stripline Coil can be improved by a shielding plate with a high surface impedance, which has been verified by 3-D models based on single- and multi-Coil Elements. For the experimental validation, two shielding plates—a copper-plated substrate and a high-impedance surface, which exhibits a small and large surface impedance, respectively—are considered. An excellent agreement of field distributions between numerical simulation and measurement has been observed.
-
Coupling investigation between RF Coil array Elements backed by surface impedance characterized shields for 7 Tesla MRI
2016 German Microwave Conference (GeMiC), 2016Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper we present a coupling investigation between RF Coil array Elements which are backed by surface impedance characterized RF shields for 7 Tesla magnetic resonance imaging (MRI). Two simulation models for the RF Coil Elements are considered here: an ideal impressed current model for an initial 2-D investigation, and a symmetrically fed dipole with meander terminals for the 3-D investigation. The RF shield, which is placed behind the Coil Element, is characterized by a surface impedance boundary condition (SIBC), where different surface impedances are defined. An optimal surface impedance of the RF shield can be found to achieve minimum coupling between neighboring Coil Elements. Different spatial arrangements (e.g. the shape of the phantom, the edge-to-edge separation between coupled Coil Elements, the separation from Coil Element to RF shield) are considered. In general, a large surface impedance provides a higher coupling level in comparison to a small surface impedance. As the separation distance from the Coil Element to the RF shield increases, a reduced surface impedance selectivity of the coupling behavior is observed. The proposed fundamental investigation reveals a new approach to modify the coupling characteristics of the dipole Coil Elements for MRI.
-
Improved B 1 distribution of an MRI RF Coil Element using a high-impedance-surface shield
2015 German Microwave Conference, 2015Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper we propose an approach to improve the B 1 distribution in terms of homogeneity and penetration depth of a Coil Element by utilizing a high impedance surface (HIS) as the RF shield for 7 T magnetic resonance imaging (MRI). The transverse magnetic field distribution in the case of a HIS and a perfect electrical conductor (PEC) being the shielding plate are compared for different separation distances from the dipole Coil to the shielding plate. As the PEC shield is adjacent to the dipole Coil, an undesired surface current is induced on the PEC shielding plate by the dipole Coil, whereas the induced surface current on the HIS shield is sufficiently suppressed due to the high surface impedance. As a result, the dipole Coil with a HIS shield exhibits a broader and stronger field distribution, and thus achieves an improvement on the transverse B 1 homogeneity as well as the penetration depth. As the separation distance increases, the impact of the induced current is weakened and thus variations on the field distribution with different shielding scenarios (HIS and PEC) are reduced. The proposed approach has been validated by numerical simulations and experimental measurements, which show a good agreement.
-
Coupling investigation of different RF Coil Elements for 7-tesla magnetic resonance imaging based on characteristic mode analysis
2014 IEEE MTT-S International Microwave Symposium (IMS2014), 2014Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:Here in this paper we propose an approach to investigate the coupling mechanism of different RF Coil Elements based on characteristic mode analysis. The Coil Element with lumped-Element-connection to the shielding plate focuses the magnetic field below the strip-line due to a loop current from the dominant mode. Denoted by a negative characteristic mode eigenvalue, the Coil Element without direct connection to the shielding plate behaves more like an electric dipole and the field is distributed mainly above the strip-line. A compromise between these two scenarios which suffers neither from the field below nor above the strip-line achieves an optimal decoupling. FDTD simulation and experimental measurement confirmed the conclusion from characteristic mode analysis.
-
A 4-channel RF Coil with large longitudinal field-of-view for 7-T-MRI
Magnetic Resonance Materials in Physics Biology and Medicine, 2013Co-Authors: Zhichao Chen, Andreas Rennings, Klaus Solbach, Daniel ErniAbstract:Recently multi-channel RF Coils based on several longitudinally oriented strip-line Elements have been successfully applied in ultra-high field MRI. Among the different utlized dipole approaches [1], [2], the symmetrically fed RF Coil Element which was terminated by two meanders [2] seems to be one of the most promising candidates. Compared to this well established 25 cm-long Element, a novel 41 cm-long Coil Element with an optimized SAR distribution and a better H-field homogeneity has been presented in [3]. In this study we establish a preliminary 4-channel RF Coil for whole-body 7-T MRI with the aforementioned two kinds of Elements. We show a qualitative comparison based on numerical simulation and experimental measurement.
Zhichao Chen - One of the best experts on this subject based on the ideXlab platform.
-
Electromagnetic Field Analysis of a Dipole Coil Element With Surface Impedance Characterized Shielding Plate for 7-T MRI
IEEE Transactions on Microwave Theory and Techniques, 2016Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper, we systematically investigate the electromagnetic (EM) field of a stripline dipole Coil Element backed by various shielding plates, which are characterized by surface impedance. The initial analysis is based on a 2-D finite-Element-method model, where the considered surface impedance was categorized in terms of magnitude and phase. It has been demonstrated that the shielding plate can be approximately modeled by the magnitude of a complex surface impedance if the absolute EM field distribution is considered. Additionally, as the magnitude of the surface impedance increases, the magnetic and electric fields excited by the stripline tend to distribute in a broader manner. Thus, the transversal homogeneity of the $B_{1}$ field of a stripline Coil can be improved by a shielding plate with a high surface impedance, which has been verified by 3-D models based on single- and multi-Coil Elements. For the experimental validation, two shielding plates—a copper-plated substrate and a high-impedance surface, which exhibits a small and large surface impedance, respectively—are considered. An excellent agreement of field distributions between numerical simulation and measurement has been observed.
-
Coupling investigation between RF Coil array Elements backed by surface impedance characterized shields for 7 Tesla MRI
2016 German Microwave Conference (GeMiC), 2016Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper we present a coupling investigation between RF Coil array Elements which are backed by surface impedance characterized RF shields for 7 Tesla magnetic resonance imaging (MRI). Two simulation models for the RF Coil Elements are considered here: an ideal impressed current model for an initial 2-D investigation, and a symmetrically fed dipole with meander terminals for the 3-D investigation. The RF shield, which is placed behind the Coil Element, is characterized by a surface impedance boundary condition (SIBC), where different surface impedances are defined. An optimal surface impedance of the RF shield can be found to achieve minimum coupling between neighboring Coil Elements. Different spatial arrangements (e.g. the shape of the phantom, the edge-to-edge separation between coupled Coil Elements, the separation from Coil Element to RF shield) are considered. In general, a large surface impedance provides a higher coupling level in comparison to a small surface impedance. As the separation distance from the Coil Element to the RF shield increases, a reduced surface impedance selectivity of the coupling behavior is observed. The proposed fundamental investigation reveals a new approach to modify the coupling characteristics of the dipole Coil Elements for MRI.
-
Improved B 1 distribution of an MRI RF Coil Element using a high-impedance-surface shield
2015 German Microwave Conference, 2015Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:In this paper we propose an approach to improve the B 1 distribution in terms of homogeneity and penetration depth of a Coil Element by utilizing a high impedance surface (HIS) as the RF shield for 7 T magnetic resonance imaging (MRI). The transverse magnetic field distribution in the case of a HIS and a perfect electrical conductor (PEC) being the shielding plate are compared for different separation distances from the dipole Coil to the shielding plate. As the PEC shield is adjacent to the dipole Coil, an undesired surface current is induced on the PEC shielding plate by the dipole Coil, whereas the induced surface current on the HIS shield is sufficiently suppressed due to the high surface impedance. As a result, the dipole Coil with a HIS shield exhibits a broader and stronger field distribution, and thus achieves an improvement on the transverse B 1 homogeneity as well as the penetration depth. As the separation distance increases, the impact of the induced current is weakened and thus variations on the field distribution with different shielding scenarios (HIS and PEC) are reduced. The proposed approach has been validated by numerical simulations and experimental measurements, which show a good agreement.
-
Coupling investigation of different RF Coil Elements for 7-tesla magnetic resonance imaging based on characteristic mode analysis
2014 IEEE MTT-S International Microwave Symposium (IMS2014), 2014Co-Authors: Zhichao Chen, Klaus Solbach, Daniel Erni, Andreas RenningsAbstract:Here in this paper we propose an approach to investigate the coupling mechanism of different RF Coil Elements based on characteristic mode analysis. The Coil Element with lumped-Element-connection to the shielding plate focuses the magnetic field below the strip-line due to a loop current from the dominant mode. Denoted by a negative characteristic mode eigenvalue, the Coil Element without direct connection to the shielding plate behaves more like an electric dipole and the field is distributed mainly above the strip-line. A compromise between these two scenarios which suffers neither from the field below nor above the strip-line achieves an optimal decoupling. FDTD simulation and experimental measurement confirmed the conclusion from characteristic mode analysis.
-
A 4-channel RF Coil with large longitudinal field-of-view for 7-T-MRI
Magnetic Resonance Materials in Physics Biology and Medicine, 2013Co-Authors: Zhichao Chen, Andreas Rennings, Klaus Solbach, Daniel ErniAbstract:Recently multi-channel RF Coils based on several longitudinally oriented strip-line Elements have been successfully applied in ultra-high field MRI. Among the different utlized dipole approaches [1], [2], the symmetrically fed RF Coil Element which was terminated by two meanders [2] seems to be one of the most promising candidates. Compared to this well established 25 cm-long Element, a novel 41 cm-long Coil Element with an optimized SAR distribution and a better H-field homogeneity has been presented in [3]. In this study we establish a preliminary 4-channel RF Coil for whole-body 7-T MRI with the aforementioned two kinds of Elements. We show a qualitative comparison based on numerical simulation and experimental measurement.
Xiaotong Zhang - One of the best experts on this subject based on the ideXlab platform.
-
Investigating the Influence of Spatial Constraints on Ultimate Receive Coil Performance for Monkey Brain MRI at 7 T
IEEE transactions on medical imaging, 2018Co-Authors: Yang Gao, Weidao Chen, Xiaotong ZhangAbstract:The RF receive Coil array has become increasingly vital in current MR imaging practice due to its extended spatial coverage, maintained high SNR, and improved capability of accelerating data acquisition. The performance of a Coil array is intrinsically determined by the current patterns generated in Coil Elements as well as by the induced electromagnetic fields inside the object. Investigations of the ultimate performance constrained by a specific Coil space, which defines all possible current patterns flowing within, offer the opportunity to evaluate Coil-space parameters (i.e., coverage, Coil-to-object distance, layer thickness, and Coil Element type) without the necessity of considering the realistic Coil Element geometry, Coil Elements layout, and number of receive channels in modeling. In this paper, to mimic 7-T monkey RF head Coil design, seven hypothetical ultimate Coil arrays with different Coil-space configurations were mounted over a numerical macaque head model; by using Huygens’s surface approximation method, the influences of Coil-space design parameters were systematically investigated through evaluating the spatial constrained ultimate intrinsic SNR and ultimate g-factor. Moreover, simulations were also conducted by using four Coil arrays with limited number of loop-only Elements, in order to explore to what extent the ultimate Coil performance can be achieved by using practical Coil designs, and hence several guidelines in RF Coil design for monkey brain imaging at 7 T have been tentatively concluded. It is believed that the present analysis will offer important implications in novel receive array design for monkey brain MR imaging at ultra-high field.
-
Gradient-based electrical properties tomography (gEPT): A robust method for mapping electrical properties of biological tissues in vivo using magnetic resonance imaging.
Magnetic resonance in medicine, 2014Co-Authors: Jiaen Liu, Xiaotong Zhang, Sebastian Schmitter, P. F. Van De MoorteleAbstract:Purpose To develop high-resolution electrical properties tomography (EPT) methods and investigate a gradient-based EPT (gEPT) approach that aims to reconstruct the electrical properties (EP), including conductivity and permittivity, of an imaged sample from experimentally measured B1 maps with improved boundary reconstruction and robustness against measurement noise. Theory and Methods Using a multichannel transmit/receive stripline head Coil with acquired B1 maps for each Coil Element, and by assuming negligible Bz component compared to transverse B1 components, a theory describing the relationship between B1 field, EP value, and their spatial gradient has been proposed. The final EP images were obtained through spatial integration over the reconstructed EP gradient. Numerical simulation, physical phantom, and in vivo human experiments at 7 T have been conducted to evaluate the performance of the proposed method. Results Reconstruction results were compared with target EP values in both simulations and phantom experiments. Human experimental results were compared with EP values in literature. Satisfactory agreement was observed with improved boundary reconstruction. Importantly, the proposed gEPT method proved to be more robust against noise when compared to previously described nongradient-based EPT approaches. Conclusion The proposed gEPT approach holds promises to improve EP mapping quality by recovering the boundary information and enhancing robustness against noise. Magn Reson Med 74:634–646, 2015. © 2014 Wiley Periodicals, Inc.
-
From Complex ${\rm B}_{1}$ Mapping to Local SAR Estimation for Human Brain MR Imaging Using Multi-Channel Transceiver Coil at 7T
IEEE transactions on medical imaging, 2013Co-Authors: Xiaotong Zhang, Sebastian Schmitter, P. F. Van De Moortele, Jiaen LiuAbstract:Elevated specific absorption rate (SAR) associated with increased main magnetic field strength remains a major safety concern in ultra-high-field (UHF) magnetic resonance imaging (MRI) applications. The calculation of local SAR requires the knowledge of the electric field induced by radio-frequency (RF) excitation, and the local electrical properties of tissues. Since electric field distribution cannot be directly mapped in conventional MR measurements, SAR estimation is usually performed using numerical model-based electromagnetic simulations which, however, are highly time consuming and cannot account for the specific anatomy and tissue properties of the subject undergoing a scan. In the present study, starting from the measurable RF magnetic fields (B1) in MRI, we conducted a series of mathematical deduction to estimate the local, voxel-wise and subject-specific SAR for each single Coil Element using a multi-channel transceiver array Coil. We first evaluated the feasibility of this approach in numerical simulations including two different human head models. We further conducted experimental study in a physical phantom and in two human subjects at 7T using a multi-channel transceiver head Coil. Accuracy of the results is discussed in the context of predicting local SAR in the human brain at UHF MRI using multi-channel RF transmission.
-
Complex B1 mapping and electrical properties imaging of the human brain using a 16-channel transceiver Coil at 7T.
Magnetic resonance in medicine, 2012Co-Authors: Xiaotong Zhang, P. F. Van De Moortele, Sebastian SchmitterAbstract:The electric properties of biological tissue provide important diagnostic information within radio and microwave frequencies, and also play an important role in specific absorption rate calculation which is a major safety concern at ultrahigh field. The recently proposed electrical properties tomography (EPT) technique aims to reconstruct electric properties in biological tissues based on B1 measurement. However, for individual Coil Element in multichannel transceiver Coil which is increasingly utilized at ultrahigh field, current B1-mapping techniques could not provide adequate information (magnitude and absolute phase) of complex transmit and receive B1 which are essential for electrical properties tomography, electric field, and quantitative specific absorption rate assessment. In this study, using a 16-channel transceiver Coil at 7T, based on hybrid B1-mapping techniques within the human brain, a complex B1-mapping method has been developed, and in vivo electric properties imaging of the human brain has been demonstrated by applying a logarithm-based inverse algorithm. Computer simulation studies as well as phantom and human experiments have been conducted at 7T. The average bias and standard deviation for reconstructed conductivity in vivo were 28% and 67%, and 10% and 43% for relative permittivity, respectively. The present results suggest the feasibility and reliability of proposed complex B1-mapping technique and electric properties reconstruction method. Magn Reson Med 69:1285–1296, 2013. V C 2012 Wiley Periodicals, Inc.