Assessment and correction of macroscopic field variations in 2D spoiled gradient‐echo sequences

Soellradl, Martin; Lesch, Andreas; Strasser, Johannes; Pirpamer, Lukas; Stollberger, Rudolf; Ropele, Stefan; Langkammer, Christian

doi:10.1002/mrm.28139

Cited by 1 publication

(5 citation statements)

References 56 publications

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“…We have recently introduced a signal modeling approach for an arbitrary excitation pulse and

G_{z}

, 23 which has been adapted in the current work to describe signal dephasing

F_{z ‐ s h i m}

due to

G_{z}

and the compensation gradient

{\bar{G}}_{c}

. Because

R_{2}^{*}

is estimated from the measured data by nonlinear fitting of Equation (), any modeling error in

F_{z ‐ s h i m}

will propagate into the

R_{2}^{*}

estimate.…”

Section: Discussionmentioning

confidence: 99%

“…In the case of small flip angles, the resulting signal decay is described by the pulse envelope of the RF excitation pulse 20 . Otherwise, the integral in Equation can be solved numerically, where

{\underset{M}{true}}_{italicxy} (z)

is obtained by the numerical solution of the Bloch equations 22,23 …”

Section: Methodsmentioning

confidence: 99%

“…For all measurements, the complex‐valued raw data were first corrected with the phase of the navigator echo as described by Wen et al, 34 followed by a coil combination using the method proposed by Luo et al 35 Then,

F_{z ‐ s h i m} (T E_{i})

was calculated as described in Soellradl et al 23 for the model

F_{4} (t)

. In this model,

{\underset{M}{true}}_{italicxy} (z)

is estimated for a certain RF pulse shape and

G_{slice}

with a numerical Bloch solver 36 .…”

Section: Methodsmentioning

confidence: 99%

“…Additionally, a more effective z‐shim pattern is introduced, in which six

G_{z}

values are successively compensated between echo acquisitions. By adapting a signal modeling approach for 2D spoiled mGRE sequences, 23 we compare this novel approach, in terms of

R_{2}^{*}

mapping, with a global z‐shim approach with linearly increasing moments 31,32 and a conventional mGRE sequence without z‐shim gradients. Furthermore, to highlight the importance of adequate signal modeling in the presence of

G_{z}

R_{2}^{*}

is also estimated from the conventional mGRE data with a more widespread used mono‐exponential signal model.…”

Section: Introductionmentioning

confidence: 99%

“…Consequently, an effective approach to reduce signal dephasing is to decrease the slice thickness, which is accompanied by reduced SNR and prolonged acquisition time. 17 Various postprocessing methods for reducing these dephasing effects by considering the slice profile and macroscopic field variations have been proposed, [18][19][20][21][22][23] but at very strong field gradients the correction of the fast signal decay might not be feasible.…”

mentioning

confidence: 99%

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Adaptive slice‐specific z‐shimming for 2D spoiled gradient‐echo sequences

Soellradl

Strasser

Lesch

et al. 2020

Magnetic Resonance in Med

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Purpose To reduce the misbalance between compensation gradients and macroscopic field gradients, we introduce an adaptive slice‐specific z‐shimming approach for 2D spoiled multi‐echo gradient‐echoe sequences in combination with modeling of the signal decay. Methods Macroscopic field gradients were estimated for each slice from a fast prescan (15 seconds) and then used to calculate slice‐specific compensation moments along the echo train. The coverage of the compensated field gradients was increased by applying three positive and three negative moments. With a forward model, which considered the effect of the slice profile, the z‐shim moment, and the field gradient, R2∗ maps were estimated. The method was evaluated in phantom and in vivo measurements at 3 T and compared with a spoiled multi‐echo gradient‐echo and a global z‐shimming approach without slice‐specific compensation. Results The proposed method yielded higher SNR in R2∗ maps due to a broader range of compensated macroscopic field gradients compared with global z‐shimming. In global white matter, the mean interquartile range, proxy for SNR, could be decreased to 3.06 s−1 with the proposed approach, compared with 3.37 s−1 for global z‐shimming and 3.52 s−1 for uncompensated multi‐echo gradient‐echo. Conclusion Adaptive slice‐specific compensation gradients between echoes substantially improved the SNR of R2∗ maps, and the signal could also be rephased in anatomical areas, where it has already been completely dephased.

show abstract

“…We have recently introduced a signal modeling approach for an arbitrary excitation pulse and

G_{z}

, 23 which has been adapted in the current work to describe signal dephasing

F_{z ‐ s h i m}

due to

G_{z}

and the compensation gradient

{\bar{G}}_{c}

. Because

R_{2}^{*}

is estimated from the measured data by nonlinear fitting of Equation (), any modeling error in

F_{z ‐ s h i m}

will propagate into the

R_{2}^{*}

estimate.…”

Section: Discussionmentioning

confidence: 99%

{\underset{M}{true}}_{italicxy} (z)

is obtained by the numerical solution of the Bloch equations 22,23 …”

Section: Methodsmentioning

confidence: 99%

F_{z ‐ s h i m} (T E_{i})

was calculated as described in Soellradl et al 23 for the model

F_{4} (t)

. In this model,

{\underset{M}{true}}_{italicxy} (z)

is estimated for a certain RF pulse shape and

G_{slice}

with a numerical Bloch solver 36 .…”

Section: Methodsmentioning

confidence: 99%

“…Additionally, a more effective z‐shim pattern is introduced, in which six

G_{z}

values are successively compensated between echo acquisitions. By adapting a signal modeling approach for 2D spoiled mGRE sequences, 23 we compare this novel approach, in terms of