Patient safety concept for multichannel transmit coils

Seifert, F.; Wübbeler, Gerd; Junge, Sven; Ittermann, Bernd; Rinneberg, H.

doi:10.1002/jmri.21149

Cited by 63 publications

(76 citation statements)

References 21 publications

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“…T½ C ¼ 200:2 À 105:9 Á Df gB 0 [7] with B 0 the amplitude of the static field and g the gyromagnetic ratio of hydrogen nuclei in MHz=T. This result is within the range of the other relationships described in literature.To quantify background gradients caused by the susceptibility distribution of the setup, a B 0 map was constructed out of the scans without flyback.…”

Section: Methodssupporting

confidence: 84%

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Compensating for magnetic field inhomogeneity in multigradient-echo-based MR thermometry

et al. 2014

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Purpose: MR thermometry (MRT) is a noninvasive method for measuring temperature that can potentially be used for radio frequency (RF) safety monitoring. This application requires measuring absolute temperature. In this study, a multigradientecho (mGE) MRT sequence was used for that purpose. A drawback of this sequence, however, is that its accuracy is affected by background gradients. In this article, we present a method to minimize this effect and to improve absolute temperature measurements using MRI. Theory: By determining background gradients using a B 0 map or by combining data acquired with two opposing readout directions, the error can be removed in a homogenous phantom, thus improving temperature maps. Methods: All scans were performed on a 3T system using ethylene glycol-filled phantoms. Background gradients were varied, and one phantom was uniformly heated to validate both compensation approaches. Independent temperature recordings were made with optical probes. Results: Errors correlated closely to the background gradients in all experiments. Temperature distributions showed a much smaller standard deviation when the corrections were applied (0.21 C vs. 0.45 C) and correlated well with thermo-optical probes. Conclusion: The corrections offer the possibility to measure RF heating in phantoms more precisely. This allows mGE MRT to become a valuable tool in RF safety assessment. Magn

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Section: Methodssupporting

confidence: 84%

“…MRT can also be useful for RF safety assessments in MR (4)(5)(6)(7). Simulation studies have shown that RF deposition quantified by specific absorption rate (SAR) can spatially be very inhomogeneous, leading to hotspots that can exceed temperature limits set in regulations (8,9).…”

Section: Introductionmentioning

confidence: 99%

Compensating for magnetic field inhomogeneity in multigradient-echo-based MR thermometry

et al. 2014

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“…[27]). In a practical implementation, there are various options how to define the channel weights w. One approach is to use the voltages supplied to the RF amplifiers or the RF power at the feeding ports of the RF coil [10]. The multi-channel body coil used in this work is equipped with pick-up coils to monitor the currents in each Tx element [28].…”

Section: Rf Shimmingmentioning

confidence: 99%

“…Instead, local SAR estimates are usually obtained based on numerical simulations of the RF field propagation in human body models. First approaches concentrated on estimates of the worst-case SAR that could occur for any expected input signal [10,11]. However, this restricts the allowed RF duty-cycle significantly, prohibiting even the use of sequences that are allowed with conventional quadrature excitation [12,13].…”

Section: Introductionmentioning

confidence: 99%

Local SAR management by RF Shimming: a simulation study with multiple human body models

Homann

Graesslin

Eggers

et al. 2011

Magn Reson Mater Phy

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“…Especially for highly accelerated RF pulses, the specific absorption rate (SAR) tends to increase rapidly and can easily exceed the SAR guidelines (16)(17)(18)(19). On the other hand, when integrated in the RF pulse design, pTx offers additional degrees of freedom to manipulate the SAR deposition for a given target excitation pattern (7).…”

mentioning

confidence: 99%

Fast design of local N‐gram‐specific absorption rate–optimized radiofrequency pulses for parallel transmit systems

Sbrizzi

Hoogduin

Lagendijk

et al. 2011

Magnetic Resonance in Med

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Designing multidimensional radiofrequency pulses for clinical application must take into account the local specific absorption rate (SAR) as controlling the global SAR does not guarantee suppression of hot spots. The maximum peak SAR, averaged over an N grams cube (local NgSAR), must be kept under certain safety limits. Computing the SAR over a three-dimensional domain can require several minutes and implementing this computation in a radiofrequency pulse design algorithm could slow down prohibitively the numerical process. In this article, a fast optimization algorithm is designed acting on a limited number of control points, which are strategically selected locations from the entire domain. The selection is performed by comparing the largest eigenvalues and the corresponding eigenvectors of the matrices which locally describe the tissue's amount of heating. The computation complexity is dramatically reduced. An additional critical step to accelerate the computations is to apply a multi shift conjugate gradient algorithm. Two transmit array setups are studied: a two channel 3 T birdcage body coil and a 12-channel 7 T transverse electromagnetic (TEM) head coil. In comparison with minimum power radiofrequency pulses, it is shown that a reduction of 36.5% and 35%, respectively, in the local NgSAR can be achieved within short, clinically feasible, computation times. Magn Reson Med 67:824-834, 2012. © 2011 Wiley Periodicals, Inc.Key words: parallel transmission; multidimensional RF pulse design; transmit array; ultra-high-field MRI; local SAR reduction High-field MRI faces problems with respect to B + 1 field inhomogeneity (1). These problems can be addressed by using parallel transmit systems. In radiofrequency (RF) shimming, the interference of the B + 1 fields of the individual channels is optimized for excitation homogeneity (2-5). Parallel transmit systems using spatially tailored RF pulses facilitate even more control of the local spin excitation within sufficiently short RF pulse durations and over a sufficient spectral range (6-9). Experimental demonstrations of these techniques have been shown in recent years (10-13). Their potential goes beyond the compensation for B + 1 inhomogeneity. Other applications include compensation for B 0 distortions (12) and reduced field-of-view imaging by spatially reduced excited region (inner volume excitation) (14,15). A major concern in spatially tailored parallel excitation (pTx) is the heating of the tissue being scanned. Especially for highly accelerated RF pulses, the specific absorption rate (SAR) tends to increase rapidly and can easily exceed the SAR guidelines (16-19). On the other hand, when integrated in the RF pulse design, pTx offers additional degrees of freedom to manipulate the SAR deposition for a given target excitation pattern (7).In vivo, local SAR guidelines are more likely to be exceeded than global SAR when using pTx excitation. Zelinski et al. (16) showed for 7 T head imaging how intricate the local SAR behavior can be for various target patterns an...

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Patient safety concept for multichannel transmit coils

Cited by 63 publications

References 21 publications

Compensating for magnetic field inhomogeneity in multigradient-echo-based MR thermometry

Compensating for magnetic field inhomogeneity in multigradient-echo-based MR thermometry

Local SAR management by RF Shimming: a simulation study with multiple human body models

Fast design of local N‐gram‐specific absorption rate–optimized radiofrequency pulses for parallel transmit systems

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