<i>B<sub>1</sub></i> estimation using adiabatic refocusing: BEAR

Jordanova, Kalina V.; Nishimura, Dwight G.; Kerr, Adam B.

doi:10.1002/mrm.25049

Cited by 7 publications

(22 citation statements)

References 25 publications

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“…It is possible to decrease the adiabatic threshold by using higher n or δ , at the cost of potentially increased errors within the bandwidth of the pulses, or lower phase sensitivity to B 1 . An analysis of the effect of decreased B 1 values below the adiabatic threshold conducted in the original BEAR paper [15] illustrated that the largest effect on the acquired signal comes from the decrease in the magnitude of the signal below the adiabatic threshold. For the HSn pulses used in this study, a decrease of 50% in the expected refocused magnitude occurs for B 1 ~ 56% of the adiabatic threshold.…”

Section: Discussionmentioning

confidence: 99%

“…This study uses the BEAR method [15, 20], a phase-based B 1 mapping method, to encode B 1 variations into an MR signal. Figure 1a shows the BEAR sequence diagram with two adiabatic full passage HSn pulses [21] that have different relative peak magnitudes, and different magnitude and frequency sweep shapes, which are determined by the parameters n 1 and n 2 .…”

Section: Theorymentioning

confidence: 99%

“…We propose a B 1 distribution measurement method that uses an existing signal phase-based B 1 mapping method, BEAR [15]. When acquiring a spatial B 1 map, BEAR is insensitive to T 1 , T 2 , repetition time (TR) and off-resonance frequency variations, and allows for direct mapping between signal phase and B 1 .…”

Section: Introductionmentioning

confidence: 99%

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Measuring distributions by phase encoding

Jordanova

Nishimura

Kerr

2016

Magnetic Resonance in Med

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Purpose We propose a method to acquire B1 distribution plots by encoding in B1 instead of image space. Using this method, B1 data is acquired in a different way from traditional spatial B1 mapping, and allows for quick measurement of high dynamic range B1 data. Methods To encode in B1, we acquire multiple projections of a slice, each along the same direction, but using a different phase sensitivity to B1. Using a convex optimization formulation, we reconstruct histograms of the B1 distribution estimates of the slice. Results We verify in vivo B1 distribution measurements by comparing measured distributions to distributions calculated from reference spatial B1 maps using the Earth Mover’s Distance (EMD). Phantom measurements using a surface coil show that for increased spatial B1 variations, measured B1 distributions using the proposed method more accurately estimate the distribution than a low-resolution spatial B1 map, resulting in a 37% EMD decrease while using fewer measurements. Conclusion We propose and validate the performance of a method to acquire B1 distribution information directly without acquiring a spatial B1 map. The method may provide faster estimates of a B1 field for applications that do not require spatial B1 localization, such as the transmit gain calibration of the scanner, particularly for high dynamic B1 ranges.

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Section: Discussionmentioning

confidence: 99%

Section: Theorymentioning

confidence: 99%

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Measuring distributions by phase encoding

Jordanova

Nishimura

Kerr

2016

Magnetic Resonance in Med

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“…Using HS1 pulses, the second echo's phase depends approximately linearly on B 1 while maintaining very little variation over the pulse bandwidth . This relationship is only valid for B 1 greater than the sequence adiabatic threshold (

\sim 1 / δ

times the individual pulse adiabatic threshold), limiting the method's dynamic B 1 range .…”

Section: Theorymentioning

confidence: 99%

“…The B 1 estimation using adiabatic refocusing (BEAR) method is a recently proposed signal phase‐based B 1 mapping method that uses two unequal adiabatic refocusing pulses in a twice‐refocused spin‐echo sequence, resulting in signal phase that has linear dependence on B 1 , and is insensitive to frequency variation, TR, T 1 and T 2 effects. The method can be localized to a volume, acquire fast projection measurements, and be tuned to a B 1 range by varying the ratio of the adiabatic pulse magnitudes, but has a limited dynamic B 1 range due to the adiabatic pulses' adiabatic thresholds.…”

Section: Introductionmentioning

confidence: 99%

Lowering the B₁ threshold for improved BEARB₁ mapping

Jordanova

Nishimura

Kerr

2015

Magnetic Resonance in Med

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Purpose Accurate measurement of the nonuniform transmit radiofrequency field is necessary for magnetic resonance imaging applications. The radiofrequency field excitation amplitude (B1) is often obtained by acquiring a B1 map. We modify the B1 Estimation Using Adiabatic Refocusing (BEAR) method to extend its range to lower B1 magnitudes. Theory and Methods The BEAR method is a phase-based B1 mapping method, wherein hyperbolic secant (HS1) pulses induce a phase sensitivity to B1. The measurable B1 range is limited due to the adiabatic threshold of the pulses. We redesign the method to use flattened hyperbolic secant (HSn) pulses, which have lower adiabatic thresholds. We optimize the HSn parameters to minimize phase sensitivity to frequency variations. Results We validate the performance of the new method via simulation and in vivo at 3T, and show that for n ≤ 8, accurate B1 maps can be acquired using reduced nominal peak B1 values. Conclusion The adiabatic threshold for BEAR is reduced with HSn pulses, which are optimized for accurate phase-to-B1 mapping over a frequency range, and allow for lower nominal B1 values. At 3T the nominal B1 is decreased by 52% and the sensitivity to B1 is increased by a factor of 3.8. This can improve the method’s applicability for measurement of low B1.

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Accelerated multi‐snapshot free‐breathing mapping based on the dual refocusing echo acquisition mode technique (DREAM): An alternative to measure RF nonuniformity for cardiac MRI

Rincón-Domínguez

Menini

Solana

et al. 2018

Magnetic Resonance Imaging

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Background: Field inhomogeneities in MRI caused by interactions between the radiofrequency field and the patient anatomy can lead to artifacts and contrast variations, consequently degrading the overall image quality and thereby compromising diagnostic value of the images. Purpose: To develop an efficient free-breathing and motion-robust B + 1 mapping method that allows for the investigation of spatial homogeneity of the transmitted radiofrequency field in the myocardium at 3.0T. Three joint approaches are used to adapt the dual refocusing echo acquisition mode (DREAM) sequence for cardiac applications: (1) electrocardiograph triggering; (2) a multi-snapshot undersampling scheme, which relies on the Golden Ratio, to accelerate the acquisition; and (3) motion-compensation based on low-resolution images acquired in each snapshot. Study type: Prospective. Phantom/subjects: Eurospin II T05 system, torso phantom, and five healthy volunteers. Field strength/sequence: 3.0T/DREAM. Assessment: The proposed method was compared with the Bloch-Siegert shift (BSS) method and validated against the standard DREAM sequence. Cardiac B + 1 maps were obtained in free-breathing and breath-hold as a proof of concept of the in vivo performance of the proposed method. Statistical tests: Mean and standard deviation (SD) values were analyzed for six standard regions of interest within the myocardium. Repeatability was assessed in terms of SD and coefficient of variation. Results: Phantom results indicated low deviation from the BSS method (mean difference = 3%). Equivalent B + 1 distributions for free-breathing and breath-hold in vivo experiments demonstrated the motion robustness of this method with good repeatability (SD < 0.05). The amount of B + 1 variations was found to be 26% over the myocardium within a short axis slice. Data conclusion: The feasibility of a cardiac B + 1 mapping method with high spatial resolution in a reduced scan time per trigger was demonstrated. The free-breathing characteristic could be beneficial to determine shim components for multichannel systems, currently limited to two for a single breath-hold.

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B₁ estimation using adiabatic refocusing: BEAR

Cited by 7 publications

References 25 publications

Measuring distributions by phase encoding

Measuring distributions by phase encoding

Lowering the B₁ threshold for improved BEARB₁ mapping

Accelerated multi‐snapshot free‐breathing mapping based on the dual refocusing echo acquisition mode technique (DREAM): An alternative to measure RF nonuniformity for cardiac MRI

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B1 estimation using adiabatic refocusing: BEAR

Cited by 7 publications

References 25 publications

Measuring distributions by phase encoding

Measuring distributions by phase encoding

Lowering the B1 threshold for improved BEARB1 mapping

Accelerated multi‐snapshot free‐breathing mapping based on the dual refocusing echo acquisition mode technique (DREAM): An alternative to measure RF nonuniformity for cardiac MRI

Contact Info

Product

Resources

About

B₁ estimation using adiabatic refocusing: BEAR

Lowering the B₁ threshold for improved BEARB₁ mapping