System and SAR characterization in parallel RF transmission

Zhu, Yangying; Alon, Leeor; Deniz, Cem M.; Brown, Ryan; Sodickson, Daniel K.

doi:10.1002/mrm.23126

Cited by 56 publications

(80 citation statements)

References 13 publications

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“…Our results may inspire further research to gain a better insight into the effect of RF pulse sequences on temperature elevation for a given time-average SAR [55] together with system and SAR characterization of parallel RF transmission [56]. Our work also suggests further innovations for directly measuring and monitoring E-fields [57]–[59], temperature changes induced by the radiofrequency fields in interventional MRI [60] as well as developments of B 1 + phase mapping techniques at ultrahigh fields and its application for in vivo electrical conductivity and permittivity mapping [61].…”

Section: Discussionmentioning

confidence: 85%

Design and Evaluation of a Hybrid Radiofrequency Applicator for Magnetic Resonance Imaging and RF Induced Hyperthermia: Electromagnetic Field Simulations up to 14.0 Tesla and Proof-of-Concept at 7.0 Tesla

et al. 2013

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This work demonstrates the feasibility of a hybrid radiofrequency (RF) applicator that supports magnetic resonance (MR) imaging and MR controlled targeted RF heating at ultrahigh magnetic fields (B0≥7.0T). For this purpose a virtual and an experimental configuration of an 8-channel transmit/receive (TX/RX) hybrid RF applicator was designed. For TX/RX bow tie antenna electric dipoles were employed. Electromagnetic field simulations (EMF) were performed to study RF heating versus RF wavelength (frequency range: 64 MHz (1.5T) to 600 MHz (14.0T)). The experimental version of the applicator was implemented at B0 = 7.0T. The applicators feasibility for targeted RF heating was evaluated in EMF simulations and in phantom studies. Temperature co-simulations were conducted in phantoms and in a human voxel model. Our results demonstrate that higher frequencies afford a reduction in the size of specific absorption rate (SAR) hotspots. At 7T (298 MHz) the hybrid applicator yielded a 50% iso-contour SAR (iso-SAR-50%) hotspot with a diameter of 43 mm. At 600 MHz an iso-SAR-50% hotspot of 26 mm in diameter was observed. RF power deposition per RF input power was found to increase with B0 which makes targeted RF heating more efficient at higher frequencies. The applicator was capable of generating deep-seated temperature hotspots in phantoms. The feasibility of 2D steering of a SAR/temperature hotspot to a target location was demonstrated by the induction of a focal temperature increase (ΔT = 8.1 K) in an off-center region of the phantom. Temperature simulations in the human brain performed at 298 MHz showed a maximum temperature increase to 48.6C for a deep-seated hotspot in the brain with a size of (19×23×32)mm3 iso-temperature-90%. The hybrid applicator provided imaging capabilities that facilitate high spatial resolution brain MRI. To conclude, this study outlines the technical underpinnings and demonstrates the basic feasibility of an 8-channel hybrid TX/RX applicator that supports MR imaging, MR thermometry and targeted RF heating in one device.

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Section: Discussionmentioning

confidence: 85%

Design and Evaluation of a Hybrid Radiofrequency Applicator for Magnetic Resonance Imaging and RF Induced Hyperthermia: Electromagnetic Field Simulations up to 14.0 Tesla and Proof-of-Concept at 7.0 Tesla

et al. 2013

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“…In practice, local averaged SAR often reaches maximum upper limits before global SAR at UHF [11]. While global SAR estimation can be estimated by real-time measurement of forward and reflected RF power at the coil ports [13], the estimation of local SAR for human MR experiments typically relies on highly time-consuming computational electromagnetic simulations. Such extensive computations have been conducted using a variety of commercial software, based on the numerical models of EM properties of the human body [14–19].…”

Section: Introductionmentioning

confidence: 99%

From Complex <formula formulatype="inline"><tex Notation="TeX">${\rm B}_{1}$</tex></formula> Mapping to Local SAR Estimation for Human Brain MR Imaging Using Multi-Channel Transceiver Coil at 7T

Zhang

Schmitter

Moortele

et al. 2013

IEEE Trans. Med. Imaging

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Elevated Specific Absorption Rate (SAR) associated with increased main magnetic field strength remains as 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 radiofrequency (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.

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“…For situations with larger fields of view and/or higher B 0 field strengths, this would no longer be appropriate. In those circumstances, predictions from SAR models could be used both to assess safety and also to penalize/constrain solutions by suitably modifying the cost function; such methods are actively under development in the wider field [e.g., ] and could readily be adopted into the proposed methodology when necessary.…”

Section: Discussionmentioning

confidence: 99%

Direct signal control of the steady-state response of 3D-FSE sequences

et al. 2014

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Citing this paper Please note that where the full-text provided on King's Research Portal is the Author Accepted Manuscript or Post-Print version this may differ from the final Published version. If citing, it is advised that you check and use the publisher's definitive version for pagination, volume/issue, and date of publication details. And where the final published version is provided on the Research Portal, if citing you are again advised to check the publisher's website for any subsequent corrections. General rightsCopyright and moral rights for the publications made accessible in the Research Portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognize and abide by the legal requirements associated with these rights.•Users may download and print one copy of any publication from the Research Portal for the purpose of private study or research.•You may not further distribute the material or use it for any profit-making activity or commercial gain •You may freely distribute the URL identifying the publication in the Research Portal Take down policy If you believe that this document breaches copyright please contact librarypure@kcl.ac.uk providing details, and we will remove access to the work immediately and investigate your claim. Parallel transmission (PTx) offers spatial control of radiofrequency (RF) fields that can be used to mitigate non-uniformity effects in high-field MRI. In practice the ability to achieve uniform RF fields by static shimming is limited by the typically small number of channels, so tailored RF pulses that mix gradient with RF encoding have been proposed. A complementary approach termed "Direct Signal Control" (DSC) is to dynamically update RF shims throughout a sequence, exploiting interactions between each pulse and the spin system to achieve uniform signal properties from potentially non-uniform fields. This work applied DSC to T2 weighted driven-equilibrium 3D-FSE brain imaging at 3T. Theory & Methods:The DSC concept requires an accurate signal model, provided by extending the Spatially-Resolved Extended Phase Graph framework to include the steady-state response of driven-equilibrium sequences. An 8-channel PTx body coil was used for experiments. Results:Phantom experiments showed the model to be accurate to within 0.3% (RMS). In-vivo imaging showed over two-fold improvement in signal homogeneity compared with quadrature excitation. Although the non-linear optimization cannot guarantee a global optimum, significantly improved local solutions were found. Conclusion:DSC has been demonstrated for 3D-FSE brain imaging at 3T. The concept is generally applicable to higher field strengths and other anatomies.

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System and SAR characterization in parallel RF transmission

Cited by 56 publications

References 13 publications

Design and Evaluation of a Hybrid Radiofrequency Applicator for Magnetic Resonance Imaging and RF Induced Hyperthermia: Electromagnetic Field Simulations up to 14.0 Tesla and Proof-of-Concept at 7.0 Tesla

Design and Evaluation of a Hybrid Radiofrequency Applicator for Magnetic Resonance Imaging and RF Induced Hyperthermia: Electromagnetic Field Simulations up to 14.0 Tesla and Proof-of-Concept at 7.0 Tesla

From Complex <formula formulatype="inline"><tex Notation="TeX">${\rm B}_{1}$</tex></formula> Mapping to Local SAR Estimation for Human Brain MR Imaging Using Multi-Channel Transceiver Coil at 7T

Direct signal control of the steady-state response of 3D-FSE sequences

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