Introduction

Visual inspection of X-nuclear spectra and images sometimes suggests the presence of eddy currents not seen on ¹H-MRS, which is very sensitive to eddy current effects in general¹. We hypothesized that this was because the software-defined frequency pre-emphasis applied to correct for the spatially independent but time-varying eddy current field Δf₀(t) needs to be adjusted for the gyromagnetic ratio (γ) of the nucleus being observed. Here we present a method to characterize the severity of eddy current artifacts on different nuclei, axes, and scanners, and we demonstrate a solution on a GE scanner which greatly improves quality of X-nuclear images and spectra.

Methods

Experiments were performed on 3 T clinical systems at three sites: System A (MR750, GE Healthcare, Waukesha WI); System B (Skyra-fit, Siemens Healthcare, Erlangen, Germany); and System C (Achieva, Philips Healthcare, Best, Holland). All sites used similar quadrature birdcage head coils dually resonant for ¹H and either ²³Na, ¹³C or ³¹P (Rapid, Rimpar, Germany). Spectral distortions were assessed using spherical phantoms, and a resolution phantom filled with 80 mM saline was used for 3D ²³Na imaging. Eddy current distortion is maximized by minimizing the delay between the large spoiler gradient pulse at the end of one TR period and the acquisition in the next. Therefore, pulse-acquire spectra with matched parameters were collected for ¹H and X-nuclei (²³Na, ³¹P, and/or ¹³C) using the minimum allowed TR. Data were collected with the spoiler pulse applied along each of the three orthogonal axes in turn. Modifications to eddy current compensation system calibration files on System A optimized for each X-nucleus were developed and applied, by multiplying the correction terms applied for Δf₀(t) by the ratio of the gyromagnetic constants γ_X-nucleus /γ_¹H. Data were compared with and without these corrections using slice-selective MRS, 2D spirals², and 3D cones³.

Results and Discussion

On System A (GE), small distortions due to eddy currents were seen in water ¹H-MRS in axial and coronal orientations (Fig 1d and g), but not sagittal (Fig 1a). In ²³Na-MRS, there was still no evidence of eddy currents in the sagittal orientation, but the distortion was greater than seen for ¹H in coronal orientations, and very much greater than ¹H in axial orientations (Fig 1b, e ,h). This mirrored the relative amplitudes of coefficients for ‘very long time constant’ f₀ correction terms within the eddy current correction calibration file, which were 0, 0.6 and 4.4 respectively in the sagittal, coronal and axial orientations. Spectra were re-acquired following rescaling the eddy current f₀ correction terms for sodium as described above (Fig 1c, f, i), resulting in distortion free ²³Na spectra.
With the default eddy current compensation, low frequency, non-Cartesian X-nuclear coronal images were consistently displaced superiorly relative to the ¹H image. When the rescaled f₀ correction terms were used for compensation, no displacement was observed (Fig 2). Similar displacement effects were observed on System A when imaging ²³Na using 3D non-Cartesian sequences such as cones. Additionally, when using a short TR there was considerable blurring and distortion: rescaling the eddy current f₀ correction terms for sodium resulted in markedly improved image quality (Fig 3).
A similar effect was seen on System B (Siemens), where the ¹H MRS appeared clear of eddy current distortion but there was marked evidence of eddy currents on the X-nucleus (Fig 4). On System C (Philips), there were some distortions, but they were similar between nuclei, whereas Systems A and B showed much worse effects on X-nuclei than ¹H. It seems likely that Philips either follow a different method or do not correct for eddy-current induced frequency errors at all.
Within sites, the severity of distortion was similar between axes on System B (Siemens), and differed markedly between axes on System A (GE). The presence of residual minor ¹H eddy currents on System A in the coronal and axial orientations suggests that a recalibration of the correction terms is required. However, if these residual eddy currents were the source of the X-nuclear distortions, they would be expected to be of similar magnitude along the coronal and axial directions as seen for ¹H, instead of the large differences between these axes which were seen for ²³Na (Fig 1). These differences between axes must therefore be related not to residual eddy currents but to the applied correction itself.
Rescaling the eddy current frequency pre-emphasis was shown to improve the quality of not only spectroscopy but also fast imaging of X-nuclei using spiral and 3D cones sequences. These techniques are widely used in clinical research, and overcompensation of eddy current B₀ terms may have been having quite widespread effects for many years.

Conclusion

Spectroscopy using a spoiler gradient and the minimum allowed TR presents a simple, quick and effective method to evaluate the relative performance of eddy current compensation for different nuclei, axes and manufacturers. We demonstrated greater distortions due to eddy currents on X-nuclei than on ¹H, and presented a modification to markedly improve X-nuclear image quality.

Acknowledgements

Collection of the data presented in this study was funded by the Cancer Research UK Cambridge Centre, National Institute of Health Research-Cambridge Biomedical Research Centre, and Addenbrooke’s Charitable Trust. Thanks to Damian McHugh and Sha Zhao (University of Manchester) and Adrian Carpenter (Cambridge) for data collection.

References

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