Z-shim with Parallel-Transmit Methods (pTX) in MR Neuro Applications
Lukas Mario Gottwald1, Rainer Schneider1, and Josef Pfeuffer1

1MR Application Development, Siemens Healthcare, Erlangen, Germany

Synopsis

In clinical 3T imaging, gradient-echo-based sequences often suffer from signal loss induced by patient-specific susceptibility artifacts. To tackle this problem, a proposed local signal recovery z-shim method using parallel transmission was implemented for a clinical setup. The approach was further extended by an automated slice-specific delay-time calculation to ensure user-friendly operation as well as maximal signal gain in artifact regions. Studies in humans were carried out using commonly used GRE and EPI sequences. Signals could be nearly fully recovered in artifact regions; tSNR gain maps for EPI time series showed an increase up to 148%.

Purpose

At higher field strengths (3T and above), T2*-weighted images acquired with gradient-echo (GRE)-based sequences, e.g. FLASH, SWI and BOLD EPI, are often impaired by local signal loss induced by susceptibility effects. In the human brain, these artifacts typically appear near the orbital frontal and inferior temporal cortex and affect the imaging of brain activity with fMRI or the diagnosis of diseases, e.g. stroke or hemorrhage. The effect worsens with increasing echo time (TE) and slice thickness, and is often dominated by the through-plane signal loss. Proposed methods (1, 2) mitigate through-plane dephasing using parallel transmission without increasing scan time or changing the slice coverage.

In this work, we implemented the pTX z-shim including B1 inhomogeneity correction (2) on a clinical 3T scanner equipped with two whole-body transmit channels. Furthermore, slice-specific delay-times were calculated automatically inline to ensure user-friendly operation as well as maximal signal gain in artifact regions. Finally, human studies were performed to demonstrate the potential benefit for clinical GRE and fMRI imaging.

Methods

To summarize the methods (1, 2), the excitation pulse is applied on separate transmit channels with a time shift to impose a linear “prewinding” phase to compensate the phase cancellation induced by the through-plane susceptibility. Our prototype sequences included an inline pTX pulse optimization (2) to mitigate the B1 inhomogeneity.

Five human subjects were studied on a 3T MAGNETOM Skyra (Siemens Healthcare, Erlangen, Germany) using prototype gradient-echo (FLASH) and EPI-based sequences with image resolutions of 0.7x0.7x6 mm3 and 1.7x1.7x6 mm3, respectively. Studies with standard RF excitation pulses (reference) were compared to slice-specific pTX z-shim pulses, which were calculated fully automatically from inline B0 and B1 maps and designed for maximal signal gain in artifact regions.

For both gradient-echo and EPI images, difference images (corrected vs. reference) were calculated in % of the reference mean intensity. Temporal-signal-to-noise ratio (tSNR) maps were generated from EPI time series of 100 repetitions (ratio of the mean signal intensity to the standard deviation of the noise: $$$ tSNR =\overline{S}_{d}/\sigma_{N} $$$). tSNR provides information on the detection ability for fMRI and is particularly suitable to highlight the benefit for fMRI applications. In all series, noise regions were masked and time series were detrended voxelwise by a second-order polynomial. To evaluate the actual signal or tSNR gain, respectively, artifact regions-of-interest (ROI) were defined in affected slices, and mean or maximum values were calculated.

Results and Discussion

Exemplary signal gain in FLASH studies is shown in Fig. 1. Some image slices acquired with the standard excitation pulse suffered from strong subject-induced susceptibility artifacts (left) in the frontal orbital and temporal cortex. Commonly, only few slices were affected (e.g. slices 7-11 of 20) and corrected by the pTX z-shim, and the rest remained untouched. In contrast, images with a slice-specific correction showed significantly signal gain in artifact regions at the cost of global SNR penalty. Difference images (right) revealed that a gain relative to the reference mean could be achieved up to 102% in the areas suffering from through-plane susceptibility. Fig. 2 summarizes the signal gain in GRE studies for all subjects. On average over all subjects, the frontal (temporal) orbital cortex had a maximal signal gain of 88+-13% (74+-21%) and a mean signal gain of 31+-11% (31+-12%).

Exemplary signal gain in gradient-echo EPI time series is shown in Fig. 3. A similar signal recovery in artifact regions compared to GRE could be achieved. In addition, tSNR maps from EPI time series (Fig. 4) show a remarkable increase with the pTX z-shim. A mean tSNR increase up to 121% in artifact regions can be seen from difference maps. Fig. 4 summarizes the tSNR gain for all subjects. On average, the frontal (temporal) orbital cortex had a mean tSNR gain of 88+-38% (86+-43%).

In the slices with signal and tSNR gains in artifact regions (ROI), a reduced SNR and fMRI detection power is visible outside the ROI (unaffected areas). In order to be able to adapt gains/losses for specific application protocols, an additional user factor to scale the pTX z-shim correction was implemented (data not shown).

Conclusion

The proposed method and implementation on a clinical scanner enables an easy-to-use and automated compensation for slices with local signal loss induced by susceptibility effects. Moreover, significant tSNR gain could be achieved in susceptibility-induced artifact regions in fMRI image series. Hence, this application is of high interest to enhance the visualization of brain activity and the diagnosis of diseases in former dark artifact regions.

Acknowledgements

N/A

References

[1] Deng, W., Yang, C., Alagappan, V., Wald, L. L., Boada, F. E., & Stenger, V. A. (2009). Simultaneous z-shim method for reducing susceptibility artifacts with multiple transmitters. Magnetic Resonance in Medicine, 61(2), 255-259.

[2] Schneider, R., Ritter, D., Haueisen, J., & Pfeuffer, J. (2014). B0-informed variable density trajectory design for enhanced correction of off-resonance effects in parallel transmission. Magnetic Resonance in Medicine, 71(4), 1381-1393.

Figures

Left: reference image; middle: corrected image, scaled by 2; right: difference image in % to the reference mean intensity; first (second) row: images of slice 10 (8).

FOV 220x220 mm2, matrix size 320x320, slices 20, slice thickness 6 mm, TE/TR 20/700 ms, GRAPPA 2.


Table of GRE signal gains.

Left: reference image; middle: corrected image, scaled by 1.5; right: difference image in % to the reference mean intensity; first (second) row: images of slice 10 (8).

FOV 220x220 mm2, matrix size 128x128, interpolation factor 2, slices 20, slice thickness 6 mm, TE/TR 30/2000 ms, GRAPPA 2.


tSNR map of EPI time series with correction showing up to 121% mean increase in tSNR in ROI.

Left: tSNR map of reference time series; middle: tSNR map of corrected time series; right: difference map in % to the reference mean intensity; first (second) row: images of slice 10 (8).


Table of tSNR gains.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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