Artifacts in CSI-measurements caused by the drift of the static magnetic field

Prospective frequency correction using outer volume suppression-localized navigator for MR spectroscopy and spectroscopic imaging

PURPOSE

New frequency correction methods are required to achieve the accurate measurement of frequency drifts in MRS and MRSI. We present a prospective frequency correction method with outer volume suppression (OVS)-based localization and selective water excitation for effective frequency correction with better SNR improvement compared to other techniques.

METHODS

An OVS-localized navigator was developed to prospectively correct frequency drifts during MRS and MRSI measurements. The performance of the navigator was tested on the human brain and a solution phantom for frequency drifts induced by head motion or gradient heating by a preceding DWI experiment at 3T.

RESULTS

The OVS-localized navigator could accurately track motion-induced frequency drifts with an RMS error of 0.5 Hz. The SNR of MRS signals was not affected by use of the OVS-localized navigator when compared with and without the navigator (P > 0.05). The frequency drifts induced by DWI experiments were 5.1 ± 0.3 Hz/min during MRSI measurements on humans, resulting in increased spectral linewidth, significant bias in metabolite concentrations, and significantly increased Cramér-Rao lower bounds (P < 0.05). After prospective frequency corrections, the quality of MRSI was recovered to the level of those without any DWI-induced frequency drifts, judged by the spectral linewidth, metabolite concentrations, and Cramér-Rao lower bounds.

CONCLUSION

The OVS-localized navigator demonstrated effective prospective frequency corrections for large frequency drifts (5 Hz/min) without introducing any saturation-induced SNR loss. These benefits can be particularly beneficial for the acquisition of MRS signals with long T₁ and/or short TR, and spectral editing.

Localization errors in MR spectroscopic imaging due to the drift of the main magnetic field and their correction

PURPOSE

To analyze the effect of B0 field drift on multivoxel MR spectroscopic imaging and to propose an approach for its correction.

THEORY AND METHODS

It is shown, both theoretically and in a phantom, that for ∼30 min acquisitions a linear B0 drift (∼0.1 ppm/h) will cause localization errors that can reach several voxels (centimeters) in the slower varying phase encoding directions. An efficient and unbiased estimator is proposed for tracking the drift by interleaving short (∼ T2*), nonlocalized acquisitions on the nonsuppressed water each pulse repetition time, as shown in 10 volunteers at 1.5 and 3 T.

RESULTS

The drift is shown to be predominantly linear in both the phantom and volunteers at both fields. The localization errors are observed and quantified in both phantom and volunteers. The unbiased estimator is shown to reliably track the instantaneous frequency in vivo despite only using a small portion of the FID.

CONCLUSION

Contrary to single-voxel MR spectroscopy, where it leads to line broadening, field drift can lead to localization errors in the longer chemical shift imaging experiments. Fortunately, this drift can be obtained at a negligible cost to sequence timing, and corrected for in post processing.

Frequency stabilization using infinite impulse response filtering for SSFP fMRI at 3T

The steady-state free precession (SSFP) method has been shown to exhibit strong potential for distortion-free functional magnetic resonance imaging (fMRI). One major challenge of SSFP fMRI is that the frequency band corresponding to the highest functional sensitivity is extremely narrow, leading to substantial loss of functional contrast in the presence of magnetic field drifts. In this study we propose a frequency stabilization scheme whereby an RF pulse with small flip angle is applied before each image scan, and the initial phase of the free induction decay (FID) signals is extracted to reflect temporal field drifts. A simple infinite impulse response (IIR) filter is further employed to obtain a low-pass-filtered estimate of the central reference frequency for the upcoming scan. Experimental results suggest that the proposed scheme can stabilize the frequency settings in accordance with field drifts, with oscillation amplitudes of <0.5 Hz. Phantom studies showed that both slow drifts and fast fluctuations were prominently reduced, resulting in less than 5% signal variations. Visual fMRI at submillimeter in-plane resolution further demonstrated 15% activation signals that were nicely registered in the microvessels within the sulci. It is concluded that the IIR-filtered frequency stabilization is an effective technique for achieving reliable SSFP fMR images at high field strengths.