A computer simulation of the static magnetic field distribution in the human head

Li, Shizhe; Williams, Gerald D.; Frisk, T. A.; Arnold, Blake W.; Smith, Michael B.

doi:10.1002/mrm.1910340219

Cited by 47 publications

(44 citation statements)

References 30 publications

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“…The calculated result was in good agreement with the field map obtained by using the DANTE tagging image sequence (25). The field distribution in localized regions was evaluated by using the magnetic field histogram technique, which provides the frequency shift and line broadening of NMR signals at specific regions of interest.…”

Section: Introductionsupporting

confidence: 76%

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Three‐dimensional mapping of the static magnetic field inside the human head

Dardzinski

Collins

et al. 1996

Magnetic Resonance in Med

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Finite element analysis was used to calculate the static magnetic field within the three-dimensional head model. Localized field distributions were evaluated by using the magnetic field histogram technique. Experimental field maps and histograms of the human head were also obtained to validate the simulation results. Field deviations and gradients inside the human head cause NMR signal frequency shifts and line broadening, respectively. Voxels 2 x 2 x 0.5 cm may have frequency differences of more than 2.0 ppm. The linewidth of a single voxel may be broadened by more than 0.5 ppm. Calculated and experimental field maps are in excellent agreement. The global field distortion in the human head is primarily due to the susceptibility difference between air and tissues and their corresponding geometrical shapes.

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Section: Introductionsupporting

confidence: 76%

“…We have previously applied the finite element analysis method to calculate the static magnetic field distribution for a two-dimensional (2D) human head model in the sagittal plane (25). The calculated result was in good agreement with the field map obtained by using the DANTE tagging image sequence (25).…”

Section: Introductionsupporting

confidence: 53%

Three‐dimensional mapping of the static magnetic field inside the human head

Dardzinski

Collins

et al. 1996

Magnetic Resonance in Med

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“…For this study, the magnetic field homogeneity was optimal in the superior prefrontal cortex. A two-dimensional calculation of magnetic field mapping of the human head (20) showed that coronal planes at this location of the brain suffer from large magnetic field deviations in the inferior regions (because of air-filled cavities and sinuses), whereas the magnetic field is very homogeneous in the superior parts. Magnetic field mapping with optimal shimming of the human brain at 2.1 T has confirmed these theoretical findings (21).…”

Section: Resultsmentioning

confidence: 98%

“Willed action”: A functional MRI study of the human prefrontal cortex during a sensorimotor task

Hyder

Phelps

Wiggins

et al. 1997

Proc. Natl. Acad. Sci. U.S.A.

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“…For B 0 mapping, typically a single, relatively short phase evolution time is used because of the complexities associated with unwrapping in two or three dimensions (10 -12). In the temporal lobe, strong susceptibility effects arising from the air-filled sinuses and ear canals result in strong high-order gradients (1-2 ppm/cm) (13,14). Coupled with the anatomical variability, this often results in initial frequency distributions with ranges of several hundred hertz.…”

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confidence: 99%

Robust fully automated shimming of the human brain for high‐field ¹H spectroscopic imaging

Hetherington

Chu

Gonen

et al. 2006

Magnetic Resonance in Med

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Although a variety of methods have been proposed to provide automated adjustment of shim homogeneity, these methods typically fail or require large numbers of iterations in vivo when applied to regions with poor homogeneity, such as the temporal lobe. These limitations are largely due to 1) the limited accuracy of single evolution time measurements when full B 0 mapping studies are used, and 2) inaccuracies arising from projectionbased methods when the projections pass through regions where the inhomogeneity exceeds the order of the fitted parameters. To overcome these limitations we developed a novel B 0 mapping method using multiple evolution times with a novel unwrapping scheme in combination with a user-defined ROI selection tool. We used these methods at 4T on 10 control subjects to obtain high-resolution spectroscopic images of glutamate from the bilateral hippocampi. A variety of rapid automated shim mapping methods for the human brain based on acquisition of a limited number of columnar projections using phase mapping have been described for small localized volumes (1,2). Shen and colleagues (3,4) demonstrated the need for alternate projection geometries when shimming planar volumes. Although these methods have been successfully applied in regions of good homogeneity, such as the occipital lobe for single volume measurements (1,2) and for slices above the corpus callosum (3), they can fail or require large numbers of iterations in vivo when applied to regions with very poor B 0 homogeneity adjacent to air-filled sinuses, such as the temporal lobes. Intrinsic to these maps is the assumption that the B 0 inhomogeneity present in the entire sample or region of interest (ROI) can be accurately represented by the limited number of columns acquired, and that the columns are sufficiently narrow for there to be negligible inhomogeneity across them. However, this assumption fails when the spatial order of the inhomogeneity present (i.e., the order of the spherical harmonic terms that characterize the spatial symmetry of the inhomogeneity) exceeds the spatial order that is used or measurable in the columnar analysis model, or when the inhomogeneity changes very rapidly in relationship to the cross-sectional width of the column (5). In this case lower-order corrections in localized regions are then used to attempt to compensate for higher-order corrections that do not have the same spatial symmetry. The calculated shim corrections can either over-or underestimate the size of the adjustments required to provide "optimal" homogeneity over the entire target volume given the available currents.Alternatively, shim methods that utilize complete maps (5-9) as opposed to columnar projections can provide accurate maps over large regions without assumptions regarding the order of the inhomogeneity present. In these cases the residual inhomogeneity is minimized by using information from sampling a much greater extent of the target volume. For B 0 mapping, typically a single, relatively short phase evolution time is used becau...

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A computer simulation of the static magnetic field distribution in the human head

Cited by 47 publications

References 30 publications

Three‐dimensional mapping of the static magnetic field inside the human head

Three‐dimensional mapping of the static magnetic field inside the human head

“Willed action”: A functional MRI study of the human prefrontal cortex during a sensorimotor task

Robust fully automated shimming of the human brain for high‐field ¹H spectroscopic imaging

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A computer simulation of the static magnetic field distribution in the human head

Cited by 47 publications

References 30 publications

Three‐dimensional mapping of the static magnetic field inside the human head

Three‐dimensional mapping of the static magnetic field inside the human head

“Willed action”: A functional MRI study of the human prefrontal cortex during a sensorimotor task

Robust fully automated shimming of the human brain for high‐field 1H spectroscopic imaging

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Robust fully automated shimming of the human brain for high‐field ¹H spectroscopic imaging