Automatic shimming for localized spectroscopy

Holz, Dietrich; Jensen, David; Proksa, Roland; Tochtrop, M.; Vollmann, W.

doi:10.1118/1.596173

Cited by 12 publications

(9 citation statements)

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“…The scaling (Eq. [4]) was applied after the polynomial regression to minimize computational efforts. Second-order polynomials of the form…”

Section: Pulse Sequencementioning

confidence: 99%

“…The scaling (Eq. [4]) was applied after the polynomial regression to minimize computational efforts. Second-order polynomials of the form ao(i) + al(j)r + a2(i)p [51 were used as the model function for AviG) utilizing explicit formulas that are given in the Appendix.…”

Section: Pulse Sequencementioning

confidence: 99%

“…For in vivo spectroscopy, a homogeneity on the order of 0.01-0.1 ppm is required, which introduces the additional time consuming shim adjustment procedure into the setup. Shimming can be a very time-consuming process, especially when performed iteratively, which may depend on operator skills as well as the starting conditions (4). Clearly, such a situation is undesirable in the context of minimizing time requirements for human applications.…”

Section: Introductionmentioning

confidence: 99%

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Automatic, localized in Vivo adjustment of all first‐and second‐order shim coils

Gruetter

1993

Magnetic Resonance in Med

848

733

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The implementation of a "fast, automatic shimming technique by mapping along projections" (FASTMAP) on a 2.1-Tesla whole-body system is described. The method provides a localized adjustment of all first- (X, Y, Z) and second-order (Z2, ZX, ZY, X2-Y2, 2XY) coils in 2 min. The time savings are achieved by mapping the magnetic field along six projections rather than from whole-imaging data sets. An analysis of noise error propagation suggests that in 64-ml volumes the residual linewidths obtained with the method are negligible when the signal-to-rms-noise ratio is above 30. The initial application of the method to localized 13C, 31P, and 1H spectroscopy of the human brain resulted in linewidths of approximately 2 Hz for 13C (144-ml volume), approximately 2.5 Hz for 31P (36-ml volume) and approximately 4 Hz for 1H (36-ml volume) with symmetric and reproducible lineshapes.

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“…The scaling (Eq. [4]) was applied after the polynomial regression to minimize computational efforts. Second-order polynomials of the form…”

Section: Pulse Sequencementioning

confidence: 99%

Section: Pulse Sequencementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Automatic, localized in Vivo adjustment of all first‐and second‐order shim coils

Gruetter

1993

Magnetic Resonance in Med

848

733

View full text Add to dashboard Cite

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“…However, the practical optimization of numerous higher‐order resistive shim currents is only possible with automated techniques. Manual shimming by monitoring integrated signal magnitude and line shape during repeated adjustment of individual shim currents according to operator judgment or some mathematical algorithm (1) requires prohibitive amounts of time. Iterative readjustment of shim currents is required due to shim coil cross‐terms resulting from either imperfect coils or off‐center localization.…”

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confidence: 99%

Robust automated shimming technique using arbitrary mapping acquisition parameters (RASTAMAP)

Klassen

Menon

2004

Magnetic Resonance in Med

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Quantitative MRI techniques as well as methods such as blood oxygen level-dependent (BOLD) imaging and in vivo spectroscopy require stringent optimization of magnetic field homogeneity, particularly when using high main magnetic fields. Automated shimming approaches require a method of measuring the main magnetic field, B 0 , followed by adjusting the currents in resistive shim coils to maximize homogeneity. A robust automated shimming technique using arbitrary mapping acquisition parameters (RASTAMAP) using a 3D multiecho gradient echo sequence that measures B 0 with high precision was developed. Inherent compensation and postprocessing methods enable removal of artifacts due to hardware timing errors, gradient propagation delays, gradient amplifier asymmetry, and eddy currents. This allows field maps to be generated for any field of view, bandwidth, resolution, or acquisition orientation without custom tuning of sequence parameters. The homogeneity of the static magnetic field, B 0 , is critical for many fast, quantitative, and spectroscopic imaging techniques. The trend towards higher static magnetic fields, both clinically and experimentally, is tempered by the increased magnetic field distortions caused by susceptibility differences. One method to compensate for these distortions is using higher-order resistive shim coils. However, the practical optimization of numerous higher-order resistive shim currents is only possible with automated techniques. Manual shimming by monitoring integrated signal magnitude and line shape during repeated adjustment of individual shim currents according to operator judgment or some mathematical algorithm (1) requires prohibitive amounts of time. Iterative readjustment of shim currents is required due to shim coil cross-terms resulting from either imperfect coils or off-center localization. In addition, signal weighting due to variations in longitudinal and transverse relaxation rates can cause inappropriate spatial biasing of these FID envelope methods.Automated methods of improving homogeneity developed to alleviate these inherent difficulties fall into two classes, projection mapping (2,3) and volume mapping (4,5). Projection mapping is advantageous for its short acquisition time. However, it relies on the incorrect assumption that shim coil fields are always fully characterized by a minimal set of spherical harmonics. In general, projection mapping also involves localization techniques that make it ill-suited for disjoint regions such as in multivoxel spectroscopy. Volume field mapping eliminates the reliance on spherical harmonics by using full 3D maps of the magnetic field generated by the shim coils to determine the optimum current settings. Regions of signal void do not affect the shim current calculations, and arbitrary, potentially disjoint regions over which to shim can be specified. This additional flexibility comes at the expense of increased time, since chemical shift imaging (6,7) and phase mapping (8,9) techniques that have been proposed to create volume B 0 ma...

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“…With expert knowledge of how each of the spherical harmonic B 0 component manifests itself in the free induction decay (FID), currents in the shim coils can be manually adjusted to minimise the rate of signal decay [6,7]. Automatic FID shimming [8,9] involves finding the shim currents that maximise the time integral of the magnitude of the complex FID signal using search algorithms, such as the simplex algorithm (e.g., [10,Sect. 10.4]).…”

Section: Introductionmentioning

confidence: 99%