Parallel transmission 2D RARE imaging at 7T with transmit field inhomogeneity mitigation and local SAR control

Yetisir, Filiz; Poser, Benedikt A.; Ellen, Grant, P.; Adalsteinsson, Elfar; Wald, Lawrence L.; Guérin, Bastien

doi:10.1016/j.mri.2022.08.006

Cited by 6 publications

(6 citation statements)

References 70 publications

(100 reference statements)

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“…For this strategy, a set of numerical pregnant body models is needed to determine the scaling factor necessary to decrease the wbSAR limit . Although very practical, this strategy is indirect, conservative, and can be detrimental to pTx performance 10,17–19 Strategy B: Explicitly constrain maternal and fetal pSAR10g and fetal aveSAR by using subject‐specific numerical body models while keeping wbSAR limit at the regulatory limit.…”

Section: Methodsmentioning

confidence: 99%

“…Although very practical, this strategy is indirect, conservative, and can be detrimental to pTx performance. 10,[17][18][19] • Strategy B: Explicitly constrain maternal and fetal pSAR10g and fetal aveSAR by using subject-specific numerical body models while keeping wbSAR limit at the regulatory limit. For this strategy, each subject's numerical body model is needed to estimate their SAR values.…”

Section: Local Sar Management Strategiesmentioning

confidence: 99%

“…A common strategy to limit local SAR is to limit the RF power 13–16 or similarly decrease the whole‐body average SAR (wbSAR), beyond that of the hardware and the regulatory limits. Although very practical, this strategy is indirect, conservative, and can be detrimental to pTx performance—especially for SAR intensive imaging 10,17–19 . A less conservative approach to limit local SAR is to use subject‐specific numerical body models to directly estimate and constrain peak local SAR, while constraining the wbSAR at the regulatory limit and the RF power at the hardware limit.…”

Section: Introductionmentioning

confidence: 99%

“…Although very practical, this strategy is indirect, conservative, and can be detrimental to pTx performance-especially for SAR intensive imaging. 10,[17][18][19] A less conservative approach to limit local SAR is to use subject-specific numerical body models to directly estimate and constrain peak local SAR, while constraining the wbSAR at the regulatory limit and the RF power at the hardware limit. Although very efficient, this approach is more difficult to implement, as numerical body models need to be generated and electromagnetic (EM) simulations need to be carried out for each subject, which are both time-consuming tasks.…”

Section: Introductionmentioning

confidence: 99%

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Local SAR management strategies to use two‐channel RF shimming for fetal MRI at 3 T

Yetisir,

Abaci Turk,

Adalsteinsson

et al. 2023

Magnetic Resonance in Med

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PurposeThis study evaluates the imaging performance of two‐channel RF‐shimming for fetal MRI at 3 T using four different local specific absorption rate (SAR) management strategies.MethodsDue to the ambiguity of safe local SAR levels for fetal MRI, local SAR limits for RF shimming were determined based on either each individual's own SAR levels in standard imaging mode (CP mode) or the maximum SAR level observed across seven pregnant body models in CP mode. Local SAR was constrained either indirectly by further constraining the whole‐body SAR (wbSAR) or directly by using subject‐specific local SAR models. Each strategy was evaluated by the improvement of the transmit field efficiency (average |B1+|) and nonuniformity (|B1+| variation) inside the fetus compared with CP mode for the same wbSAR.ResultsConstraining wbSAR when using RF shimming decreases B1+ efficiency inside the fetus compared with CP mode (by 12%–30% on average), making it inefficient for SAR management. Using subject‐specific models with SAR limits based on each individual's own CP mode SAR value, B1+ efficiency and nonuniformity are improved on average by 6% and 13% across seven pregnant models. In contrast, using SAR limits based on maximum CP mode SAR values across seven models, B1+ efficiency and nonuniformity are improved by 13% and 25%, compared with the best achievable improvement without SAR constraints: 15% and 26%.ConclusionTwo‐channel RF‐shimming can safely and significantly improve the transmit field inside the fetus when subject‐specific models are used with local SAR limits based on maximum CP mode SAR levels in the pregnant population.

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Section: Methodsmentioning

confidence: 99%

Section: Local Sar Management Strategiesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Local SAR management strategies to use two‐channel RF shimming for fetal MRI at 3 T

Yetisir,

Abaci Turk,

Adalsteinsson

et al. 2023

Magnetic Resonance in Med

Self Cite

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show abstract

“…Many researchers have conducted comparative studies on the injection dose at 1.5T versus 3.0 T, and 3.0 T versus 7.0T, and showed the feasibility for injecting less-dose contrast agents at higher field MRI for brain tumours [11,12]. However, whole body imaging at 7.0 T has not been approved by the FDA and is not available in the clinic, due to safety issues such as increased specific absorption rate (SAR) distributions at the body and technical issues such as B1 field inhomogeneity [13,14]. Recently, a 5.0 T clinical MRI scanner was invented, and can be used to scan the whole body with good image homogeneity and contrast uniformity, meanwhile avoid issues such as high SAR [15].…”

Section: Introductionmentioning

confidence: 99%

The Feasibility of Half-Dose Contrast-Enhanced Scanning of Brain Tumours at 5.0 T: a preliminary study

Jiang

Sun

et al. 2023

Preprint

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Purpose This study investigated the Gd-based enhanced effects on brain tumours with half-dose at 5.0 T and with full-dose at 3.0 T. Both quantitative and subjective evaluation parameters suggested that half-dose enhanced scanning at 5.0 T MRI might be feasible to meet the clinical diagnostic requirements. Methods Twelve subjects diagnosed with brain tumours were included in this study, and scanned after contrast agent injection at 3.0 T (full-dose) and 5.0 T (half-dose) with a 3D T1-weighted gradient echo sequence, respectively. The post-contrast images were compared between 5.0 T and 3.0 T, in terms of the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and subjective image quality score with a ten-point Likert scale. Comparing quantitative indices and subjective picture quality ratings with a paired Student's t-test and assessing inter-reader agreement with the intra-class correlation coefficient (ICC). Results A total of 16 enhanced tumours lesions were detected. The SNR was significantly higher at 5.0 T than 3.0 T in gray matter, white matter and enhanced lesions (p < 0.001). The CNR was also significantly higher at 5.0 T than 3.0 T MRI in gray matter-tumor lesion, white matter-tumor lesion, and gray matter-white matter (p < 0.001). Subjective evaluation showed that the internal structure and outline of the tumor lesions were more clearly displayed with half-does at 5.0 T, and the enhanced effects of the lesions was comparable to that with full-dose at 3.0 T. All subjective scores were good to excellent at both 5.0 T and 3.0 T. Conclusion 5.0 T half-dose intravenous contrast agents enhanced scanning might be feasible to meet the clinical diagnostic requirements of brain tumours.

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Design of calibration‐free RF pulses for T 2$$ {}_2 $$‐weighted single‐slab 3D turbo‐spin‐echo sequences at 7T utilizing parallel transmission

Lowen,

Pracht,

Gras

et al. 2024

Magnetic Resonance in Med

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PurposeT‐weighted turbo‐spin‐echo (TSE) sequences are a fundamental technique in brain imaging but suffer from field inhomogeneities at ultra‐high fields. Several methods have been proposed to mitigate the problem, but were limited so far to nonselective three‐dimensional (3D) measurements, making short acquisitions difficult to achieve when targeting very high resolution images, or needed additional calibration procedures, thus complicating their application.MethodsSlab‐selective excitation pulses were designed for flexible placement utilizing the concept of k‐spokes. Phase‐coherent refocusing universal pulses were subsequently optimized with the Gradient Ascent Pulse Engineering algorithm and tested in vivo for improved signal homogeneity.ResultsImplemented within a 3D variable flip angle TSE sequence, these pulses led to a signal‐to‐noise ratio (SNR) improvement ranging from 10% to 30% compared to a two‐dimensional (2D) T2w TSE sequence employing ‐shimmed pulses. field inhomogeneities could be mitigated and artifacts from deviations reduced. The concept of universal pulses was successfully applied.ConclusionWe present a pulse design method which provides a set of calibration‐free universal pulses (UPs) for slab‐selective excitation and phase‐coherent refocusing in slab‐selective TSE sequences.

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Parallel transmission 2D RARE imaging at 7T with transmit field inhomogeneity mitigation and local SAR control

Cited by 6 publications

References 70 publications

Local SAR management strategies to use two‐channel RF shimming for fetal MRI at 3 T

Local SAR management strategies to use two‐channel RF shimming for fetal MRI at 3 T

The Feasibility of Half-Dose Contrast-Enhanced Scanning of Brain Tumours at 5.0 T: a preliminary study

Design of calibration‐free RF pulses for T 2$$ {}_2 $$‐weighted single‐slab 3D turbo‐spin‐echo sequences at 7T utilizing parallel transmission

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