Magnetic Resonance in Medicine · 2015 · 442 citations · 36 references
MedicineBiomedical EngineeringRadiologyDiagnostic ImagingDiagnostic RadiologyNeuroradiologyBrain ImagingMagnetic Resonance Image ReconstructionMagnetic Resonance ImagingMagnetic Resonance Imaging PhysicsMedical ImagingNeuroimaging PhysicsField Strength DependenceGradient Echo SequencesHigh-field Magnetic Resonance ImagingMagnetic Field StrengthParallel ImagingSignal-to-noise RatioSignal-to-noise Ratio AnalysisStructural Brain Imaging
Higher field strengths improve SNR but worsen B1‑homogeneity and reduce T2*, limiting their benefit. The study measured relaxation times, transmit homogeneity, SNR, and g‑factors in the same subjects at 3 T, 7 T, and 9.4 T using identical sequences and standard coil arrays, correcting SNR with flip‑angle, T2*, and T1 maps and deriving g‑factors from GRAPPA‑accelerated 3D gradient‑echo images. SNR increased supralinearly with field strength (≈ 3.1× from 3 T to 7 T and 1.8× from 7 T to 9.4 T), while T1 rose, T2* fell, B1‑homogeneity degraded, and g‑factors did not decrease as expected, indicating coil design dominates.
Relaxation times, transmit homogeneity, signal-to-noise ratio (SNR) and parallel imaging g-factor were determined in the human brain at 3T, 7T, and 9.4T, using standard, tight-fitting coil arrays.The same human subjects were scanned at all three field strengths, using identical sequence parameters and similar 31- or 32-channel receive coil arrays. The SNR of three-dimensional (3D) gradient echo images was determined using a multiple replica approach and corrected with measured flip angle and T2 (*) distributions and the T1 of white matter to obtain the intrinsic SNR. The g-factor maps were derived from 3D gradient echo images with several GRAPPA accelerations.As expected, T1 values increased, T2 (*) decreased and the B1 -homogeneity deteriorated with increasing field. The SNR showed a distinctly supralinear increase with field strength by a factor of 3.10 ± 0.20 from 3T to 7T, and 1.76 ± 0.13 from 7T to 9.4T over the entire cerebrum. The g-factors did not show the expected decrease, indicating a dominating role of coil design.In standard experimental conditions, SNR increased supralinearly with field strength (SNR ∼ B0 (1.65) ). To take full advantage of this gain, the deteriorating B1 -homogeneity and the decreasing T2 (*) have to be overcome.
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Generalized autocalibrating partially parallel acquisitions (GRAPPA)
Mark A. Griswold, Peter M. Jakob, Robin M. Heidemann et al. · Magnetic Resonance in Medicine · 2002 · 5.3K citations · Full text
P. B. Roemer, W. A. Edelstein, Cecil E. Hayes et al. · Magnetic Resonance in Medicine · 1990 · 2.3K citations