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An 8/15-Channel Tx/Rx Head Neck RF Coil Combination with Semi-Dynamic B1 Shimming for Improved fMRI of the Cerebellum at 7 T
Viktor Pfaffenrot1,2, Sascha Brunheim1,2, Stefan H.G. Rietsch1,2, Thomas M. Ernst1,3, Oliver Kraff1, Stephan Orzada1, and Harald H. Quick1,2

1Erwin L. Hahn Institute for Magnetic Resonance Imaging, University Duisburg-Essen, Essen, Germany, 2High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany, 3Department of Neurology, University Hospital Essen, Essen, Germany

Synopsis

Functional MRI of the human cerebellum is challenging at ultrahigh fields, since conventional RF head coils hardly cover the cerebellum with sufficient signal-to-noise ratio and B1+-inhomogeneities introduce challenges. In order to overcome these problems, a coil combination consisting of an 8ch transceiver head coil and a 7ch receive only array are combined to improve imaging of the whole brain with special focus on the cerebellum. A ‘semi-dynamic’ B1+-shimming technique is introduced which provides a tSNR-gain of 29 % and voxels with higher significance in a finger tapping fMRI experiment when comparing the coil combination to a 32ch receive head coil.

Purpose:

Functional magnetic resonance imaging (fMRI) at 7 T benefits from higher signal-to-noise ratio (SNR) as well as increased functional contrast based on blood oxygenation level. To fully exploit these advantages, radiofrequency (RF) coils need to provide sufficient signal at the region of interest (ROI). Furthermore, B1+-inhomogeneities need to be addressed to ensure a homogeneous excitation. In this work, the main ROI is the human cerebellum. Its high anatomical diversity and location pose challenges for fMRI1. In order to overcome these, we combine an 8ch transceiver (Tx/Rx) head coil2 with a 7ch receive only array3 including modified detuning to provide high local SNR at the cerebellum. ‘Semi-dynamic’ B­1+-shimming was employed in combination with Variable Slice-Thickness (VAST) EPI4, to perform fMRI focusing on the human cerebellum, while maintaining whole brain coverage. The RF-shimming method’s performance was evaluated in the coil combination setup and compared with a conventional 1chTx/32chRx coil (Nova Medical Inc., Wilmington, USA) in terms of B­1+-homogeneity and temporal signal-to-noise ratio (tSNR) during a basic finger tapping fMRI experiment.

Methods:

The coil combination setup as depicted in fig. 1A & C consists of the 8ch Tx/Rx coil in which the 7ch receive array is placed occipital. A detuning network (fig. 1B yellow box) consisting of seven circuits with PIN diodes was implemented to improve detuning. The coil was additionally equipped with floating cable traps (fig. 1B red arrow) to decrease coupling between the receive array and the transceiver coil. In an in vivo acquisition (healthy male volunteer: 35 y, 75 kg, 175 cm) with the coil combination, flip angle (FA) maps for four slices with a thickness of 24 mm (fig. 2A) were obtained using B1TIAMO5. Based on this information, three different RF-shims were calculated (fig. 2A). The VAST-EPI sequence was modified by introducing an additional trigger which control the switching between the individual shim settings via a custom modulator system6. In fig. 3A, the red and blue slice groups represent an acquisition with 1.5 mm slice thickness, where the red block is shimmed with one setting (fig. 2B) and the blue block with another (fig. 2C). The yellow slice block is acquired with 3 mm slice thickness and shimmed with the third setting (fig. 2D). The same participant was scanned with the Nova coil, whereas dielectric bags where used to passively shim the B­1+-field for the cerebellum. With these shimming procedures, finger tapping experiments with a random tapping pattern of the right hand (TE/TR = 22/2800 ms, TA = 9:41 min, 28 s tapping vs. 28 s rest in ten blocks resulting in 202 volumes) were conducted. Functional t-maps in the cerebellum and neocortex where obtained using SPM12 and the SUIT7 toolbox for cerebellum only. As a measure of quality, tSNR-maps in the cerebellum were calculated using the first hundred realigned functional scans. Therefore, the voxel-wise mean and standard deviation along the time course where calculated and divided by each other.

Results and Discussion:

In fig. 2B-D, the resulting FA maps after RF-shimming are shown. From left to right, they correspond to the three slice positions from bottom to top shown in fig. 2A, whereas fig. 2E-G shows maps of the Nova birdcage mode. This result indicates a FA increase and improved homogeneity, especially at the cerebellar position. Since the experiment is driven by a hypothesis of the location of activation, the shimming ROIs (black ROIs in fig. 2) are chosen a priori to homogenize B­1+ for this particular area. This shimming process translates into higher significance when performing fMRI experiments (fig. 3B-D). However, significant caudal activation is less for the semi-dynamic shimming case in this particular experiment. One possible reason could be that the Nova coil fits the shape of the participant’s head better. The tSNR-maps for the cerebellum as shown in fig. 4 indicate higher tSNR with the coil combination (B) and a further increase when applying semi-dynamic shimming (C). Computing the mean tSNR in every slice and through all slices from caudal (most left) to cranial (most right) reveals a tSNR-gain of 16 % when applying semi-dynamic shimming in comparison to birdcage-mode of the coil combination and a gain of 29 % in comparison with the Nova coil.

Conclusion:

A coil combination and method has been presented which improves fMRI of the human cerebellum in comparison with a conventional RF head coil, while maintaining whole brain coverage. An overall tSNR-gain in the cerebellum can be seen, although some parts seem to be covered less good with the receive elements. In future work, this experiment will be repeated in more participants applying group analysis.

Acknowledgements

No acknowledgement found.

References

1. J. Diedrichsen et al.: Advances in functional imaging of the human cerebellum. Curr. Opin. Neurol. 23(4). 382-7 (2010).

2. S. Orzada et al.: 8-channel transmit/receive head coil for 7 T human imaging using intrinsically decoupled strip line elements with meanders. Proc. 17th Annual Meeting ISMRM. #3010 (2009).

3. S. Rietsch et al.: Cost-Efficient 7ch Rx Shoulder Array for 7T UHF MRI Featuring External Switchbox Detuning. Proc. 24th Annual Meeting ISMRM. #2142 (2016).

4. S. Brunheim et al.: Variable Slice-Thickness (VAST) EPI for the reduction of signal voids in GE-EPI at 7 Tesla Proc. Org. Hum. Brain Map. 20. #2089 (2014).

5. S. Brunheim et al.: Combining B1 mapping with TIAMO for fast and accurate multi-channel RF shimming in 7 Tesla body MRI. Proc. 24th Annual Meeting ISMRM. #936 (2016).

6. A.K. Bitz, I. Brote, S. Orzada et al.: An 8-channel add-on RF shimming system for whole-body 7 Tesla MRI including real-time SAR monitoring. Proc. 17th Sci. Meet. Int. Soc. Magn. Reson. Med. #4767 (2009).

7. J. Diedrichsen et al.: A spatially unbiased atlas template of the human cerebellum. Neuroimage 33(1). 127-138 (2006).

Figures

Figure 1: The setup of the RF coil combination (A), consisting of the 8ch octagonal microstrip line transceiver head coil with meanders and the 7ch receive only array (B). The receive array is equipped with a box using PIN diodes (yellow box). Additional cable traps (red arrow) reduce coupling between receive array and transceiver coil. The receive array was placed slightly off-centered between two transceiver elements to cover the cerebellum with its sensitive volume (C).

Figure 2: FA-maps acquired with the 8/15ch coil combination (upper row) and the 1/32ch Nova coil (lower row) for the caudal part of the cerebellum (B and E, red box in A), the cranial part (C and F, blue box in A) and the neocortex (D and G, yellow box in A), respectively. The shimming ROIs are shown in B-D in case of the 8/15ch coil combination. Higher FA, especially at the cerebellum are visible as well as a more homogeneous field distribution.

Figure 3: Finger tapping results of VAST-EPI-acquisitions with a 3-slice-group FOV as depicted in A. The upper three rows of B, C and D show unsmoothed t-statistics of the cerebellum (p = 0.01 FDR) overlaid on a structural image (0.7 mm isotropic T1-weighted MPRAGE). Results are shown at x=16, y=-55 and z=-53 in non-normalized SUIT space. The lower row depicts unsmoothed t-values for the neocortex (z=61 non-normalized MNI-space). An increase in significance is visible when comparing the Nova coil (B) with the birdcage-mode of the coil-combination (C). This can be further improved with semi-dynamic shimming, as shown in D

Figure 4: tSNR-maps calculated from the first 100 functional scans (50 task-scans, 50 rest-scans) in the cerebellum for the Nova coil (A), the coil-combination with birdcage-mode (B) and the semi-dynamic shimming method (C). Shown is the cerebellum from down (most left part of each fig.) to up. The tSNR-gain from A-B and A-C is 16% and 29%, respectively. A homogeneous tSNR is visible, although tSNR is less in the caudal parts in C compared with A. This is likely to be due to a better fit of the Nova coil on the participant’s head. However, the general increase is obvious.

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)
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