Characterization and correction of diffusion gradient‐induced eddy currents in second‐order motion‐compensated echo‐planar and spiral cardiac DTI

Abstract

Purpose: Very high gradient amplitudes played out over extended time intervals as required for second-order motion-compensated cardiac DTI may violate the assumption of a linear time-invariant gradient system model. The aim of this work was to characterize diffusion gradient-related system nonlinearity and propose a correction approach for echo-planar and spiral spin-echo motion-compensated cardiac DTI. Methods: Diffusion gradient-induced eddy currents of 9 diffusion directions were characterized at b values of 150 s/mm 2 and 450 s/mm 2 for a 1.5 Tesla system and used to correct phantom, ex vivo, and in vivo motion-compensated cardiac DTI data acquired with echo-planar and spiral trajectories. Predicted trajectories were calculated using gradient impulse response function and diffusion gradient strength-and direction-dependent zeroth-and first-order eddy current responses. A reconstruction method was implemented using the predicted k-space trajectories to additionally include off-resonances and concomitant fields. Resulting images were compared to a reference reconstruction omitting diffusion gradient-induced eddy current correction.Results: Diffusion gradient-induced eddy currents exhibited nonlinear effects when scaling up the gradient amplitude and could not be described by a 3D basis alone. This indicates that a gradient impulse response function does not suffice to describe diffusion gradient-induced eddy currents. Zeroth-and first-order diffusion gradient-induced eddy current effects of up to −1.7 rad and −16 to +12 rad/m, respectively, were identified. Zeroth-and first-order diffusion gradient-induced eddy current correction yielded improved image quality upon image reconstruction. Conclusion:The proposed approach offers correction of diffusion gradient-induced zeroth-and first-order eddy currents, reducing image distortions to promote improvements of second-order motion-compensated spin-echo cardiac DTI.

Beat phenomena of oscillating readouts

“…If an oscillatory gradient waveform is used, the output will also reflect the beat phenomenon, given that the GMTF resolves the mechanical resonance. However, given that the LTI assumption may not hold for strong gradients as used in diffusion MRI, 11 the validity of the gradient correction may be limited as well. In addition, the time horizon for the impulse response (and therefore for the validity of the convolution) is limited by the time duration of the GMTF acquisition.…”

“…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.…”

Maximizing SNR per unit time in diffusion MRI with multiband T‐Hex spirals

“…In the setup used for the current work, due to the nature of the field camera in use, measurements of the field dynamics had to be performed after the actual scanning, thereby effectively doubling the required scan time. This, however, can be tackled readily with cameras designed for concurrent field monitoring 62 or one‐time calibration‐based gradient impulse response modeling 63–65 …”

“…This, however, can be tackled readily with cameras designed for concurrent field monitoring 62 or one-time calibration-based gradient impulse response modeling. [63][64][65] The key aim of T-Hex is to minimize the number of shots needed, by maximizing k-space coverage per shot. This is particularly relevant for dMRI with high b-values and long diffusion or mixing times, 66 which incur a large time overhead per shot.…”

Cardiac DTI using short‐axis PROPELLER: A feasibility study

“…This technique holds promise for enhancing our understanding of cardiac diseases and their impact on myocardial microstructure. [1][2][3][4] By measuring key parameters such as the trace apparent diffusion coefficient (trADC), fractional anisotropy (FA), and myofiber orientation, cDTI enables the characterization of healthy and pathological myocardial tissue. Studies in healthy volunteers have demonstrated the effectiveness of cDTI in assessing myocardial microstructure.…”