Cross-encoded magnetic resonance imaging in inhomogeneous fields.
ABSTRACT In magnetic resonance imaging (MRI), it is possible to cancel the effects of severe inhomogeneities of the magnetic field even if the field profile is unknown. The new 'cross-encoded' method is based on adiabatic frequency-modulated pulses combined with two orthogonal gradients that are applied simultaneously during encoding and decoding. Undistorted two- and three-dimensional images can be obtained in inhomogeneous fields where the breadth of the water resonance extends over several kHz.
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Article: Cross-encoded magnetic resonance imaging in inhomogeneous fields.
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ABSTRACT: Using a RF pulse with linear frequency sweep and a simultaneous encoding gradient, magnetization is sequentially excited accompanied by a quadratic phase profile. This quadratic dependence of magnetization phase on position dephases magnetization away from its vertices, allowing direct spatial encoding and image formation in the time domain. In this work, we show that Fourier decoding or least square fitting in combination with frequency sweep spatial encoding schemes can generate high fidelity images and we also extend spatial encoding to include nonlinear frequency sweep. Application to in vivo multiscan susceptibility-weighted imaging is demonstrated. Our results show that Fourier-decoded, spatially encoded images compare favorably with conventional high resolution images while preserving the unique features of sequential excitation.Journal of Magnetic Resonance 02/2010; 204(2):200-7. DOI:10.1016/j.jmr.2010.02.014 · 2.32 Impact Factor
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ABSTRACT: A half-century quest for improving resolution in Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) has enabled the study of molecular structures, biological interactions, and fine details of anatomy. This progress largely relied on the advent of sophisticated superconducting magnets that can provide stable and homogeneous fields with temporal and spatial variations below ΔB0/B0<0.01 ppm. In many cases however, inherent properties of the objects under investigation, pulsating arteries, breathing lungs, tissue-air interfaces, surgical implants, etc., lead to fluctuations and losses of local homogeneity. A new method dubbed “long-lived-coherence correlation spectroscopy” (LLC-COSY) opens the way to overcome both inhomogeneous and homogeneous broadening, which arise from local variations in static fields and fluctuating dipole-dipole interactions, respectively. LLC-COSY makes it possible to obtain ultrahigh resolution two-dimensional spectra, with linewidths on the order of Δν=0.1 to 1 Hz, even in very inhomogeneous fields (ΔB0/B0>10 ppm or 5000 Hz at 9.7 T), and can improve resolution by a factor up to 9 when the homogeneous linewidths are determined by dipole-dipole interactions. The resulting LLC-COSY spectra display chemical shift differences and scalar couplings in two orthogonal dimensions, like in “J spectroscopy.” LLC-COSY does not require any sophisticated gradient switching or frequency-modulated pulses. Applications to in-cell NMR and to magnetic resonance spectroscopy (MRS) of selected volume elements in MRI appear promising, particularly when susceptibility variations tend to preclude high resolution.Physical Review Letters 07/2012; 109(4):047602. DOI:10.1103/PhysRevLett.109.047602 · 7.73 Impact Factor
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ABSTRACT: Intermolecular zero-quantum coherences (iZQCs) have been utilized to achieve high-resolution nuclear magnetic resonance (NMR) proton spectra under inhomogeneous and/or unstable fields. In this paper, we demonstrated that despite the insensitivity of iZQCs to B(0) variations, the influence of unstable fields on the observable single-quantum coherence signals causes strong t(1) noises in the high-resolution iZQC spectra. Short-time acquisition (STA) and phase spectrum schemes were proposed for noise suppression in in vivo iZQC magnetic resonance spectroscopy (MRS) under temporal B(0) variations. The feasibility of these schemes were verified by localized spectroscopic studies under B(0) variations generated by the Z0 coil current oscillations and by voxel position variations in the presence of field gradients, which simulate the field conditions of MRS in the presence of physiological motions. The phase scheme not only improves the signal-to-noise ratio but also further reduces the linewidth by half.Physical Chemistry Chemical Physics 06/2010; 12(23):6014-20. DOI:10.1039/b920180g · 4.20 Impact Factor