Local specific absorption rate control for parallel transmission by virtual observation points

Eichfelder, Gabriele; Gebhardt, Matthias

doi:10.1002/mrm.22927

Cited by 226 publications

(309 citation statements)

References 11 publications

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“…Online local specific absorption rate (SAR) monitoring was achieved with a vendor‐provided system installed on the transmit chain 50. The SAR matrices used in the online monitoring were derived from an electromagnetic (EM) simulation 12 on an adult male model with a safety margin factor of 2, with limits set according to Hoffmann et al 51, and compressed according to the virtual observation points (VOPs) method 52. The same VOP‐compressed SAR matrices were used in local SAR estimation during the off‐line spokes pulses optimization.…”

Section: Methodsmentioning

confidence: 99%

High‐resolution gradient‐recalled echo imaging at 9.4T using 16‐channel parallel transmit simultaneous multislice spokes excitations with slice‐by‐slice flip angle homogenization

Tse

Wiggins²,

Poser

2016

Magnetic Resonance in Med

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PurposeIn order to fully benefit from the improved signal‐to‐noise and contrast‐to‐noise ratios at 9.4T, the challenges of normalB1+ inhomogeneity and the long acquisition time of high‐resolution 2D gradient‐recalled echo (GRE) imaging were addressed.Theory and MethodsFlip angle homogenized excitations were achieved by parallel transmission (pTx) of 3‐spoke pulses, designed by magnitude least‐squares optimization in a slice‐by‐slice fashion; the acquisition time reduction was achieved by simultaneous multislice (SMS) pulses. The slice‐specific spokes complex radiofrequency scaling factors were applied to sinc waveforms on a per‐channel basis and combined with the other pulses in an SMS slice group to form the final SMS‐pTX pulse. Optimal spokes locations were derived from simulations.ResultsFlip angle maps from presaturation TurboFLASH showed improvement of flip angle homogenization with 3‐spoke pulses over CP‐mode excitation (normalized root‐mean‐square error [NRMSE] 0.357) as well as comparable excitation homogeneity across the single‐band (NRMSE 0.119), SMS‐2 (NRMSE 0.137), and SMS‐3 (NRMSE 0.132) 3‐spoke pulses. The application of the 3‐spoke SMS‐3 pulses in a 48‐slice GRE protocol, which has an in‐plane resolution of 0.28 × 0.28 mm, resulted in a 50% reduction of scan duration (total acquisition time 6:52 min including reference scans).ConclusionTime‐efficient flip angle homogenized high‐resolution GRE imaging at 9.4T was accomplished by using slice‐specific SMS‐pTx spokes excitations. Magn Reson Med 78:1050–1058, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.

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Section: Methodsmentioning

confidence: 99%

High‐resolution gradient‐recalled echo imaging at 9.4T using 16‐channel parallel transmit simultaneous multislice spokes excitations with slice‐by‐slice flip angle homogenization

Tse

Wiggins²,

Poser

2016

Magnetic Resonance in Med

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“…The same formalism can be used to include constraints on global specific absorption rate and local specific absorption rate 35, 36. We expect that the computation time will increase by adding a large amount of local specific absorption rate constraints.…”

Section: Discussionmentioning

confidence: 99%

Optimal control design of turbo spin‐echo sequences with applications to parallel‐transmit systems

Sbrizzi

Hoogduin

Hajnal

et al. 2016

Magnetic Resonance in Med

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PurposeThe design of turbo spin‐echo sequences is modeled as a dynamic optimization problem which includes the case of inhomogeneous transmit radiofrequency fields. This problem is efficiently solved by optimal control techniques making it possible to design patient‐specific sequences online.Theory and MethodsThe extended phase graph formalism is employed to model the signal evolution. The design problem is cast as an optimal control problem and an efficient numerical procedure for its solution is given. The numerical and experimental tests address standard multiecho sequences and pTx configurations.ResultsStandard, analytically derived flip angle trains are recovered by the numerical optimal control approach. New sequences are designed where constraints on radiofrequency total and peak power are included. In the case of parallel transmit application, the method is able to calculate the optimal echo train for two‐dimensional and three‐dimensional turbo spin echo sequences in the order of 10 s with a single central processing unit (CPU) implementation. The image contrast is maintained through the whole field of view despite inhomogeneities of the radiofrequency fields.ConclusionThe optimal control design sheds new light on the sequence design process and makes it possible to design sequences in an online, patient‐specific fashion. Magn Reson Med 77:361–373, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine

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“…These building blocks demonstrated good RF heating and MR imaging performance at f=298MHz [3,12] For the proposed RF-applicator designs and tumor models, the E-fields of the transmit elements were combined [14][15][16][17][18] using an optimization algorithm [19] for the specific absorption rate (SAR)-distributions. Different constraints namely the maximum SAR 10g in healthy tissue and the maximum total power delivered to the head were set.…”

Section: Methodsmentioning

confidence: 99%

Radiofrequency applicator concepts for simultaneous MR imaging and hyperthermia treatment of glioblastoma multiforme

Oberacker

Kuehne²,

Nadobny

et al. 2017

Current Directions in Biomedical Engineering

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Glioblastoma multiforme is the most frequent and most aggressive malignant brain tumor with de facto no long term curation by the use of current multimodal therapeutic approaches. The efficacy of brachytherapy and enhancing interstitial hyperthermia has been demonstrated. RF heating at ultrahigh fields (B 0 =7.0T, f=298MHz) has the potential of delivering sufficiently large thermal dosage for hyperthermia of relatively large tumor areas. This work focuses on electromagnetic field (EMF) simulations and provides realistic applicator designs tailored for simultaneous RF heating and MRI. Our simulations took advantage of target volumes derived from patient data, and our preliminary results suggest that RF power can be focused to both a small tumor area and a large clinical target volume.

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Local specific absorption rate control for parallel transmission by virtual observation points

Cited by 226 publications

References 11 publications

High‐resolution gradient‐recalled echo imaging at 9.4T using 16‐channel parallel transmit simultaneous multislice spokes excitations with slice‐by‐slice flip angle homogenization

High‐resolution gradient‐recalled echo imaging at 9.4T using 16‐channel parallel transmit simultaneous multislice spokes excitations with slice‐by‐slice flip angle homogenization

Optimal control design of turbo spin‐echo sequences with applications to parallel‐transmit systems

Radiofrequency applicator concepts for simultaneous MR imaging and hyperthermia treatment of glioblastoma multiforme

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