Simple method for RF pulse measurement using gradient reversal

Landes, Vanessa; Nayak, Krishna S.

doi:10.1002/mrm.26920

Cited by 5 publications

(11 citation statements)

References 10 publications

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“…A limitation of this study is that GRATER‐based predistortion was not compared with an alternative technique such as the pick‐up coil approach . RF measurements made with GRATER and the pick‐up coil method proved comparable, however, equivalence in the context of pre‐distortion remains future work.…”

Section: Discussionmentioning

confidence: 88%

“…The main advantage of using GRATER‐based predistortion is that it does not require any extra hardware to measure RF envelopes. However, GRATER comes with several disadvantages, including potential biases because of object non‐uniformity, nonlinearity of the excited signal with higher flip angles, and sensitivity to noise . Despite these disadvantages, these experiments demonstrated significant improvement in RF excitation after predistortion and <2.7% NRMSE.…”

Section: Discussionmentioning

confidence: 92%

“…Scaling and time‐shift were calculated and updated each iteration. Computation time for an in‐house MATLAB implementation was reduced to <2 s (compared to <7 s in Landes and Nayak) by setting the initial guess for initial phase angle as the difference between the mean of the phase angle of the desired waveform and the raw GRATER data.…”

Section: Methodsmentioning

confidence: 99%

“…These methods require extra hardware, synchronization, and processing to measure transmitted RF waveforms. Recently, the gradient‐reversal approach to evaluate RF (GRATER) was introduced as a fast and simple way to measure RF pulses without extra hardware or synchronization . GRATER involves (1) excitation in the presence of a gradient, and (2) immediate signal acquisition in the presence of an inverted gradient along the same axis.…”

Section: Introductionmentioning

confidence: 99%

“…Recently, the gradient-reversal approach to evaluate RF (GRATER) was introduced as a fast and simple way to measure RF pulses without extra hardware or synchronization. 8 GRATER involves (1) excitation in the presence of a gradient, and (2) immediate signal acquisition in the presence of an inverted gradient along the same axis. GRATER was combined with an outer-volume suppression (OVS) pre-pulse 9 to ensure equal excited signal along the slice-select direction and produce measurements with <2.4% error in a variety of small FA multiband pulses.…”

Section: Introductionmentioning

confidence: 99%

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Iterative correction of RF envelope distortion with GRATER‐measured waveforms

Landes

Nayak

2019

Magnetic Resonance in Med

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Purpose To develop and evaluate a method for RF envelope correction without extra hardware or synchronization. Methods Transmitted RF waveforms are measured through a simple pulse sequence called the gradient reversal approach to evaluate RF (GRATER). The measured RF waveforms are used to compute predistorted RF waveforms. This process is repeated until a stopping criterion is met, for example, based on pulse performance or a maximum number of iterations. Excitation profiles and simultaneous multi‐slice (SMS) image quality are compared before and after RF predistortion. Results The proposed method improved the accuracy of multiband RF pulses, reducing normalized root mean squared error (NRMSE) by >12‐fold, and reducing spurious side‐lobe excitation by >6‐fold. The reduction in unwanted side‐lobe signal is demonstrated using SMS bSSFP imaging at 3T in phantoms and in the heart. Conclusion Iterative GRATER‐based predistortion is a practical, hardware‐free way to boost performance of short duration, low flip angle RF pulses, such as those used in SMS bSSFP imaging. Because of its efficiency, this technique could be included as part of an initial scan setup or for use with subsequent scans.

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Section: Discussionmentioning

confidence: 88%

Section: Discussionmentioning

confidence: 92%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Iterative correction of RF envelope distortion with GRATER‐measured waveforms

Landes

Nayak

2019

Magnetic Resonance in Med

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Time optimal control‐based RF pulse design under gradient imperfections

Aigner

Rund

Seada

et al. 2019

Magnetic Resonance in Med

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Purpose: This study incorporates a gradient system imperfection model into an optimal control framework for radio frequency (RF) pulse design. Theory and Methods: The joint design of minimum-time RF and slice selective gradient shapes is posed as an optimal control problem. Hardware limitations such as maximal amplitudes for RF and slice selective gradient or its slew rate are included as hard constraints to assure practical applicability of the optimized waveforms. In order to guarantee the performance of the optimized waveform with possible gradient system disturbances such as limited system bandwidth and eddy currents, a measured gradient impulse response function (GIRF) for a specific system is integrated into the optimization. Results: The method generates optimized RF and pre-distorted slice selective gradient shapes for refocusing that are able to fully compensate the modeled imperfections of the gradient system under investigation. The results nearly regenerate the optimal results of an idealized gradient system. The numerical Bloch simulations are validated by phantom and in-vivo experiments on 2 3T scanners. Conclusions: The presented design approach demonstrates the successful correction of gradient system imperfections within an optimal control framework for RF pulse design. K E Y W O R D Sgradient imperfections, gradient impulse response function, pulse design, simultaneous multi-slice excitation, time optimal control 562 | AIGNER Et Al.

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Simultaneous multi-slice T1 mapping using MOLLI with blipped CAIPIRINHA bSSFP

Bentatou¹,

Troalen²,

Bernard³

et al. 2023

Magnetic Resonance Imaging

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BackgroundThis study evaluates the possibility for replacing conventional 3 slices, 3 breath-holds MOLLI cardiac T1 mapping with single breath-hold 3 simultaneous multi-slice (SMS3) T1 mapping using blipped-CAIPIRINHA SMS-bSSFP MOLLI sequence. As a major drawback, SMS-bSSFP presents unique artefacts arising from side-lobe slice excitations that are explained by imperfect RF modulation and bSSFP low flip angle enhancement. Amplitude-only RF modulation (AM) is proposed to reduce these artefacts in SMS-MOLLI compared to conventional Wong multi-band RF modulation (WM). Materials and methodsPhantoms and ten healthy volunteers were imaged at 1.5T using a modified blipped-CAIPIRINHA SMS-bSSFP MOLLI sequence with 3 simultaneous slices. WM-SMS3 and AM-SMS3 were compared to conventional single-slice (SMS1) MOLLI. First, SNR degradation and T1 accuracy were measured in phantoms. Second, artefacts from side-lobe excitations were obtained from conventional MOLLI and AM-SMS3-MOLLI were equivalent in LV myocardium (SMS1-T1=935.5±36.1ms; AM-SMS3-T1=933.8±50.2ms; P=0.436) and LV blood pool (SMS1-T1=1475.4±35.9ms; AM-SMS3-T1=1452.5±70.3ms; P=0.515). Identically, no differences were found between SMS1 and SMS3 postcontrast T1 values in the myocardium (SMS1-T1=556.0±19.7ms; SMS3-T1=521.3±28.1ms; P=0.626) and the blood (SMS1-T1= 478±65.1ms; AM-SMS3-T1=447.8±81.5; P=0.085). ConclusionsCompared to WM RF modulation, AM SMS-bSSFP MOLLI was able to reduce side-lobe artefacts considerably, providing promising results to image the three levels of the heart in a single breath hold. However, few artefacts remained even using AM-SMS-bSSFP due to residual RF imperfections. The proposed blipped-CAIPIRINHA MOLLI T1 mapping sequence provides accurate in vivo T1 quantification in line with those obtained with a single slice acquisition.

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Simple method for RF pulse measurement using gradient reversal

Cited by 5 publications

References 10 publications

Iterative correction of RF envelope distortion with GRATER‐measured waveforms

Iterative correction of RF envelope distortion with GRATER‐measured waveforms

Time optimal control‐based RF pulse design under gradient imperfections

Simultaneous multi-slice T1 mapping using MOLLI with blipped CAIPIRINHA bSSFP

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