Field camera versus phantom-based measurement of the gradient system transfer function (GSTF) with dwell time compensation

Non-Cartesian imaging sequences involve sampling during rapid variation of the encoding field gradients. The quality of the reconstructed images often suffers from insufficient knowledge of the exact dynamics of the actual fields applied during sampling. Methods: We propose determination of the accurate field dynamics by measuring the currents at the gradient amplifier outputs using the amplifiers' internal sensors concurrently with imaging. The actual dynamic field evolution is then determined by convolution with the measured current-to-field impulse response function of the gradient coil. Integration of the gradient field evolution allows derivation of the k-space trajectory for reconstruction. Results: The current-based approach is investigated in spiral and ultrashort TE phantom imaging. In comparison with the model-based product reconstruction as well as a correction approach based on the conventional input waveform-to-field impulse response function, it provides slightly improved image quality. The improvement is ascribed to a better representation of eddy current and amplifier nonlinearity effects. Conclusion: Trajectory calculation based on measured amplifier output currents offers a robust, purely measurement-based alternative to conventional model-based approaches. The implementation can mitigate gradient amplifier imperfections with no or little additional hardware effort.

Section: Discussionsupporting

confidence: 70%

Section: Discussionmentioning

confidence: 99%

Non‐Cartesian k‐space trajectory calculation based on concurrent reading of the gradient amplifiers’ output currents

Rahmer

Schmale

Mazurkewitz

et al. 2021

“…On the other hand, it proved sufficient to use large triangular pulses to probe the long‐term response of the system, as narrow peaks in the GSTF are expected only to occur at low frequencies. Additionally, we noticed that a TR of 500 ms instead of 1 s 23 or more 12,19 is long enough to allow for field settling between repetitions on our scanner. The appropriate choice of TR may, however, depend on the calibration of the system's ECC.…”

Section: Discussionmentioning

confidence: 99%

“…Two different regularization approaches were used: a generic one, minimizing the norm of the solution, 22 and a physics‐informed one, taking advantage of prior knowledge about the gradient response. The GSTF was finally obtained by Fourier transform of the GIRF, and dwell time compensation was performed as described by Stich et al 23 …”

Section: Theorymentioning

confidence: 99%

Fast measurement of the gradient system transfer function at 7 T

Scholten

Lohr

Wech

et al. 2022

Purpose In this work, a new method to determine the gradient system transfer function (GSTF) with high frequency resolution and high SNR is presented, using fast and simple phantom measurements. The GSTF is an effective instrument for hardware characterization and calibration, which can be used to correct for gradient distortions, or enhance gradient fidelity. Methods The thin‐slice approach for phantom‐based measurements of the GSTF is expanded by adding excitations that are shifted after the application of the probing gradient, to capture long‐lasting field fluctuations with high SNR. A physics‐informed regularization procedure is implemented to derive high‐quality transfer functions from a small number of measurements. The resulting GSTFs are evaluated by means of gradient time‐course estimation and pre‐emphasis of a trapezoidal test gradient on a 7T scanner. Results The GSTFs determined with the proposed method capture sharp mechanical resonances with a high level of detail. The measured trapezoidal gradient progressions are authentically reproduced by the GSTF estimations on all three axes. The GSTF‐based pre‐emphasis considerably improves the gradient fidelity in the plateau phase of the test gradient and almost completely eliminates lingering field oscillations. Conclusion The presented approach allows fast and simple characterization of gradient field fluctuations caused by long‐living eddy current and vibration effects, which become more pronounced at ultrahigh field strengths.

“…3 Assuming a linear time-invariant (LTI) system, the gradient modulation transfer function (GMTF) describes the relationship between actual and desired gradient waveforms. To this end, a GMTF measurement permits, for example, the correction of k-space trajectories of echo planar, 4,5 spiral, [6][7][8][9] or radial imaging 10 sequences. However, the validity of the LTI assumption can be challenged if nonlinearity effects (e.g., due to strong diffusion gradients 11 ) or temperature changes 12 in the gradient chain are no longer negligible.…”

Section: Introductionmentioning

confidence: 99%

Beat phenomena of oscillating readouts

Dillinger,

Peereboom,

Kozerke

2024

PurposeTo demonstrate slowly varying, erroneous magnetic field gradients for oscillating readouts due to the mechanically resonant behavior of gradient systems.MethodsProjections of a static phantom were acquired using a one‐dimensional (1D) EPI sequence with varying EPI frequencies ranging from 1121 to 1580 Hz on clinical 3T systems (30 mT/m, 200 T/m/s). Phase due to static B0 inhomogeneities was eliminated by a complex division of two separate scans with different polarities of the EPI readout. The temporal evolution of phase was evaluated and related to the mechanical resonances of the gradient systems derived from the gradient modulation transfer function. Additionally, the impact of temporally varying mechanical resonance effects on EPI was evaluated using an echo‐planar spectroscopic imaging sequence.ResultsA beat phenomenon resulting in a slowly varying phase was observed. Its temporal frequency was given by the difference between the EPI frequency and the mechanical resonance frequency of the activated gradient axis. The maximum erroneous, oscillating phase during phase encoding was ±0.5 rad for an EPI frequency of 1281 Hz. Echo‐planar spectroscopic imaging images showed the resulting time‐dependent stretching/compression of the FOV.ConclusionOscillating readouts such as those used in EPI can result in low‐frequency, erroneous phase contributions, which are explained by the beat phenomenon. Therefore, EPI phase‐correction approaches may need to include beat effects for accurate image reconstruction.