<i>B</i><sub>1</sub> destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil

Moortele, Pierre-François Van de; Akgün, Can; Adriany, Gregor; Moeller, Steen; Ritter, Johannes; Collins, Christopher M.; Smith, Michael B.; Vaughan, J. Thomas; Uğurbil, Kâmil

doi:10.1002/mrm.20708

Cited by 421 publications

(470 citation statements)

References 33 publications

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“…[2] emphasizes that the classical method implicitly assumes that both ⌽ 0 and ⌬B 0 are constant at times t 1 and t 2 . The phase image ⌽ 0 contains phase values that are independent of both the echo time and the spatial magnetic field inhomogeneity (36,37). It is related to the spatial heterogeneity of the transverse magnetic field B 1 in the imaged object, which in turn depends on the coil and the Larmor frequency.…”

Section: Theorymentioning

confidence: 99%

A new EPI‐based dynamic field mapping method: Application to retrospective geometrical distortion corrections

Lamberton¹,

Delcroix²,

Grenier³

et al. 2007

Magnetic Resonance Imaging

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Purpose: To retrospectively correct for geometrical distortions, a new dynamic field mapping method suitable for dynamic single-shot gradient-echo type echo-planar imaging (GRE-EPI) is proposed. Materials and Methods:The method requires a single volume additional acquisition and allows the extraction of a field map from each phase volume, assuming invariance across time of the echo time-independent phase component. Performances of the method are assessed using three sets of experiments: the first tests the prerequisite and the modeling; the second tests the method with time-dependent geometrical distortions; and the third presents a comparison with two other methods. Results:Our results legitimize the modeling procedure and demonstrate that the dynamic method is less sensitive to noise than the other methods. A theoretical explanation for this is proposed in the discussion section. Conclusion:Given the minor increase in the acquisition time, this method is well suited for functional magnetic resonance imaging; prospective direction.

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Section: Theorymentioning

confidence: 99%

A new EPI‐based dynamic field mapping method: Application to retrospective geometrical distortion corrections

Lamberton¹,

Delcroix²,

Grenier³

et al. 2007

Magnetic Resonance Imaging

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“…To fully benefit from the improved signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio at UHF 2, 3, 4, 5, the well‐known problem of radiofrequency (RF) transmit (

{normalB}_{1}^{+}

) inhomogeneity 6 must be overcome first. Solutions that are currently available include novel RF coil design 7, dielectric pads 8, 9, RF pulse design 10, parallel transmission (pTx) with RF shimming 11, 12, 13, 14, and transmit sensitivity encoding (SENSE) 15, 16, 17.…”

Section: Introductionmentioning

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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“…However, at high frequencies the radiofrequency (RF) wavelength approaches the dimension of the head and complex wave behavior occurs within the heterogeneous brain tissue (1). Destructive and constructive interferences of the RF field can cause transmit field (B þ 1 ) inhomogeneities (2)(3)(4), as well as local regions of increased power deposition (5). Transmit arrays have been developed to mitigate sample-dependent RF inhomogeneities (6,7).…”

mentioning

confidence: 99%

A conformal transceive array for 7 T neuroimaging

Gilbert

Belliveau

Curtis

et al. 2011

Magnetic Resonance in Med

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The first 16-channel transceive surface-coil array that conforms to the human head and operates at 298 MHz (7 T) is described. Individual coil elements were decoupled using circumferential shields around each element that extended orthogonally from the former. This decoupling method allowed elements to be constructed with arbitrary shape, size, and location to create a three-dimensional array. Radiofrequency shimming achieved a transmit-field uniformity of 20% over the whole brain and 14% over a single axial slice. During radiofrequency transmission, coil elements couple tightly to the head and reduce the amount of power necessary to achieve a mean 90°flip angle (660-ms and 480-ms pulse lengths were required for a 1-kW hard pulse when shimming over the whole brain and a single axial slice, respectively). During reception, the close proximity of coil elements to the head increases the signal-to-noise ratio in the periphery of the brain, most notably at the superior aspect of the head. The sensitivity profile of each element is localized beneath the respective shield. When combined with the achieved isolation between elements, this results in the capacity for low geometry factors during both transmit and receive: 1.04/1.06 (mean) and 1.25/1.54 (maximum) for 3-by-3 acceleration in the axial/sagittal plane. High cortical signal-tonoise ratio and parallel imaging performance make the conformal coil ideal for the study of high temporal and/or spatial cortical architecture and function.

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B₁ destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil

Cited by 421 publications

References 33 publications

A new EPI‐based dynamic field mapping method: Application to retrospective geometrical distortion corrections

A new EPI‐based dynamic field mapping method: Application to retrospective geometrical distortion corrections

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

A conformal transceive array for 7 T neuroimaging

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B1 destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil

Cited by 421 publications

References 33 publications

A new EPI‐based dynamic field mapping method: Application to retrospective geometrical distortion corrections

A new EPI‐based dynamic field mapping method: Application to retrospective geometrical distortion corrections

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

A conformal transceive array for 7 T neuroimaging

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B₁ destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil