Hardware considerations for functional magnetic resonance imaging

Silva, Afonso C.; Merkle, Hellmut

doi:10.1002/cmr.a.10052

Cited by 19 publications

(14 citation statements)

References 68 publications

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“…It may be considered as an assembly of two crossed surface coil pairs. Previously, this concept was suggested to maximize the filling factor (i.e., the SNR) in the superior brain areas, such as the location of the motor cortex (7)(8)(9). For this purpose, the helmet design was short along its longitudinal direction (i.e., the z-axis) to avoid loading from more caudal parts.…”

Section: Introductionmentioning

confidence: 99%

Reengineered helmet coil for human brain studies at 3 Tesla

Driesel

Merkle

Hetzer

et al. 2005

Concepts Magn. Reson.

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ABSTRACT:We describe the further development of a circularly polarized helmet coil for magnetic resonance imaging (MRI) of the human brain at 3 T. The coil is used in transmit and receive (transceive) mode, a useful alternative over commercially available quadrature headcoils with scanners where phased arrays are unavailable. The coil is simple to build. Its structure is based on an assembly of two coplanar dual-loop coils of the split-circle design, which are arranged in crossed fashion. Both coils circumscribe dome-like the human head. Because of the symmetry of the assembly, a high degree of circularly polarized radiofrequency (RF) is obtained within the brain. Bench and in situ measurements of the RF magnetic field, B 1 , indicated good axial homogeneity and a moderate gradient along the symmetry axis. Compared to a commercial birdcage coil, the signal-to-noise ratio was increased up to a factor of 1.7 as a result of its superior filling factor. Initial applications in healthy volunteers included anatomical MRI and functional imaging with a cognitive paradigm.

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Section: Introductionmentioning

confidence: 99%

Reengineered helmet coil for human brain studies at 3 Tesla

Driesel

Merkle

Hetzer

et al. 2005

Concepts Magn. Reson.

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“…It also reduces the physiological stimulation dB/dt due to a smaller gradient region size and has a smaller heat generation (see Ref. 8 and references therein). As a result, it provides a better performance for high-speed MRI than coils of a whole body imaging system.…”

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confidence: 99%

Concomitant field terms for asymmetric gradient coils: Consequences for diffusion, flow, and echo‐planar imaging

Meier

Zwanger

Feiweier

et al. 2008

Magnetic Resonance in Med

121

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As a consequence of the Maxwell equations, linear field gradients are accompanied by additional spatially dependent field components. A description of the Maxwell field terms is presented which explicitly takes into account the asymmetry of the gradient coil. It is shown both theoretically and experimentally that, in contrast to symmetric coils, an asymmetric coil generates concomitant field terms of zeroth and first order in space. Artifacts induced by concomitant fields can be much more pronounced for asymmetric coil designs than for symmetric ones. For the strong gradient amplitudes available on modern MR systems the effect of these concomitant magnetic fields on the evolution of magnetization has to be taken into consideration in a variety of NMR acquisition techniques. The formalism is used experimentally to compensate for artifacts ob- In MRI, linear magnetic field gradients are used to encode spatial information with the requirement that the gradient coils produce a linear field over the volume of interest. However, this requirement cannot be completely fulfilled as it contradicts the general principle of electromagnetism expressed by Maxwell's equations. For a source-free region they require a zero divergence (div B ϭ 0) and a zero rotation (rot B ϭ 0) of the magnetic flux. Consequently, the linear magnetic fields are accompanied by secondary field terms, which are typically referred to as Maxwell or concomitant fields.Concomitant fields and their effects on NMR images were first described in 1985 (1), but remained mostly unrecognized in routine imaging. Since these early observations, improvement in gradient coil technology has made it possible to significantly increase the amplitude of the applied magnetic field gradients used in NMR imaging studies. This has amplified the influence of the secondary fields and has lead to a new appreciation of their effects. They have subsequently been studied extensively in a series of publications (2-7), which have provided a mathematical description of the concomitant fields and an understanding of some of the associated image artifacts. These studies have focused on the case of cylindrical symmetry, which is appropriate for the symmetrical gradient coils used by most MR systems.However, some systems use an asymmetric coil design, which can be advantageous in some cases. The MRI system used in this work is a human head-only scanner equipped with a short-axis gradient coil of a reduced inner diameter of 36 cm. This compact gradient design increases efficiency and minimizes coil inductance. It reduces the noise level due to a smaller number of coil turns and a shortened length. It also reduces the physiological stimulation dB/dt due to a smaller gradient region size and has a smaller heat generation (see Ref. 8 and references therein). As a result, it provides a better performance for high-speed MRI than coils of a whole body imaging system. Because the patient's head access into the coil is limited by the shoulders, the MRI system uses an asymmetric transverse coil w...

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“…Passive shimming utilizes small ferromagnetic materials distributed cylindrically in a grid between the gradients and the magnet's bore, as magnetic dipoles to correct main magnetic field inhomogeneity. Switching the MRI system's gradients induces eddy currents in the resistive iron shims, which heats them [18]. Because the magnetic susceptibility of the material is both temperature dependent and has a high thermal expansion coefficient, temperature variations can affect the spatial distribution of the passive correction field.…”

Section: Discussionmentioning

confidence: 99%

Monitoring and correcting spatio-temporal variations of the MR scanner’s static magnetic field

El-Sharkawy

Schär

Bottomley

et al. 2006

Magn Reson Mater Phy

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The homogeneity and stability of the static magnetic field are of paramount importance to the accuracy of MR procedures that are sensitive to phase errors and magnetic field inhomogeneity. It is shown that intense gradient utilization in clinical horizontal-bore superconducting MR scanners of three different vendors results in main magnetic fields that vary on a long time scale both spatially and temporally by amounts of order 0.8-2.5 ppm. The observed spatial changes have linear and quadratic variations that are strongest along the z direction. It is shown that the effect of such variations is of sufficient magnitude to completely obfuscate thermal phase shifts measured by proton-resonance frequency-shift MR thermometry and certainly affect accuracy. In addition, field variations cause signal loss and line-broadening in MR spectroscopy, as exemplified by a fourfold line-broadening of metabolites over the course of a 45 min human brain study. The field variations are consistent with resistive heating of the magnet structures. It is concluded that correction strategies are required to compensate for these spatial and temporal field drifts for phase-sensitive MR protocols. It is demonstrated that serial field mapping and phased difference imaging correction protocols can substantially compensate for the drift effects observed in the MR thermometry and spectroscopy experiments.

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Hardware considerations for functional magnetic resonance imaging

Cited by 19 publications

References 68 publications

Reengineered helmet coil for human brain studies at 3 Tesla

Reengineered helmet coil for human brain studies at 3 Tesla

Concomitant field terms for asymmetric gradient coils: Consequences for diffusion, flow, and echo‐planar imaging

Monitoring and correcting spatio-temporal variations of the MR scanner’s static magnetic field

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