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Improved Efficacy of Cerebellar fMRI at 7T with Dielectric Pads Extending the Imaging Region of a Commercial Head Coil
Manushka V. Vaidya1,2,3, Mariana Lazar2, Cem M. Deniz1,2, Gillian G. Haemer1,2,3, Gang Chen1,2,3, Mary Bruno2, Daniel K. Sodickson1,2,3, Riccardo Lattanzi1,2,3, and Christopher M. Collins1,2,3

1Center for Advanced Imaging Innovation and Research (CAI2R), Department of Radiology, New York University School of Medicine, New York, NY, United States, 2Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY, United States, 3Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY, United States

Synopsis

To improve cerebellar fMRI using a commercial head coil at 7T, we arranged High Permittivity Material (HPM) pads against the inferior, posterior region of the head. We first simulated the change in Specific energy Absorption Rate (SAR) with HPM and then examined the effects on experimental Signal to Noise Ratio (SNR), flip angle maps, and measured fMRI activation throughout the brain (including the cerebellum) during a finger-tapping task in eight subjects. Both SNR and fMRI BOLD contrast were improved in the cerebellum with no potential safety detriments.

PURPOSE:

Functional magnetic resonance imaging (fMRI) has facilitated non-invasive in vivo studies of brain function and connectivity. There is growing interest in studying the functional role of cerebro-cerebellar circuits (1-4), which requires robust sensitivity to blood oxygenation level dependent (BOLD) contrast throughout the whole brain. Ultra-high field MRI (>4 Tesla) is promising for fMRI, because of improved BOLD contrast (5). However, whole brain fMRI remains challenging, because commonly used head coils at 7 Tesla (T) have poor coverage in inferior regions of the brain and particularly in the cerebellum. Previous work has shown that appropriately placed HPM pads can extend the coil sensitivity into inferior regions of the brain and neck (6). In this work, we tested whether this extension in coil sensitivity can translate to an improvement in the detection of functional activation in the cerebellum for a finger tapping motor task in eight subjects.

METHODS:

Simulations: Numerical simulations were performed to assess the Specific energy Absorption Rate (SAR) with and without HPM pads (εr=110 and σ=0.08 S/m) (Fig 1). A head-sized quadrature-driven birdcage coil, similar to the commercial transmit head coil used in the experiments (Fig. 1), was modeled using CST Microwave Studio 2015 (Computer Simulation Technology, Darmstadt, Germany), and loaded with a human model (“Duke,” Virtual Population, IT’IS Foundation, Zurich, Switzerland) (7) with a voxel resolution of 5x5x5mm3. Each port was tuned to 297.2 MHz, and matched to 50 Ohms in the absence of HPM. Simulations were performed with an accuracy of -30 dB to ensure convergence, and approximately 14 million mesh cells were used. The simulated 10 g local maximum SAR values were then scaled (8) to estimate SAR for the gradient-echo EPI sequence used in experiments. Experiments: Two identical HPM pads (12x14x2cm3) constructed from calcium titanate and deuterium oxide (9) were positioned inside a commercial head coil (Nova Medical 1Transmit/24Receive, Wilmington, MA) (Fig. 1). All studies involving human subjects were performed in accordance with our institution’s IRB. Eight healthy volunteers were enrolled in a prospective motor fMRI study, performed both with and without HPM. Volunteers were scanned on a whole-body 7 T scanner (Magnetom, Siemens Healthineers, Erlangen, Germany) using the commercial head coil. For each volunteer, the transmit reference voltage was set, with no HPM in the coil, to achieve a 90o flip angle at the center of the brain, using a turbo-FLASH based technique (10), and kept constant in the presence of HPM pads. To separate the receive-only coil contribution to SNR, SNR maps (11) were normalized by the sine of the flip angle maps (10), which were separately obtained. Functional activation maps were obtained using a gradient echo EPI sequence (101 measurements, slices=55, voxel size=2x2x2.5mm3, TR=3s, TE=23ms, bandwidth = 1930 Hz/pixel, flip angle = 15deg) for a right hand finger tapping task. fMRI data was analyzed using FEAT FSL (fMRI Expert Analysis Tool, version 6.0, FMRIB’s software Library, version 5.0.10) for each subject, followed by a group level analysis using paired t-test of acquisitions with and without HPM.

RESULTS AND DISCUSSION:

SAR values (Table) were well below the corresponding limits recommended by the FDA and IEC (12,13). Normalized experimental SNR maps demonstrated that the SNR improvement in the cerebellum (+26.79%) observed in the presence of HPM pads was primarily due to a larger achieved flip angle (+38.67%). The flip angle averaged across all subjects showed a mean improvement of 33% in the cerebellum and a decrease of 3.4% at the center, while the average flip angle in the whole brain remained approximately the same (Fig. 2). Functional MRI showed contralateral somatosensory motor cortical activation and activation of the superior and inferior axial slices of the ipsilateral cerebellar cortex (Fig. 3). In the presence of HPM pads a statistically significant improvement in the signal as well as the functional activation was observed for the inferior axial slices of the cerebellum (Fig. 3). This confirmed that an improvement in the flip angle (Fig. 2) can translate to an improvement in the detection of BOLD fMRI in the cerebellum. Group analysis results showed a statistically significant increase in activation in the cerebellar cortex with the HPM (Fig. 4), adjacent to the HPM pads (Fig. 1), and with no considerable loss elsewhere in the brain (Fig. 4).

CONCLUSIONS:

HPM pads can improve cerebellar fMRI at 7T with a commonly used commercial head coil without compromising RF safety.

Acknowledgements

This work was supported by the Center for Advanced Imaging Innovation and Research (www.cai2r.net), a NIBIB Biomedical Technology Resource Center (NIH P41 EB017183) and NIH R01 EB021277.

References

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6. Vaidya MV, Haemer G, Collins CM, Chen G, Carluccio G, Bruno M, Wiggins GC, Sodickson DK, Lattanzi R. Extending the Sensitivity of a Head Coil towards Simultaneous Head and Neck Imaging Using High Permittivity Materials at 7 T. In: Proceedings of the 24th Annual Meeting of the International Society for Magnetic Resonance in Medicine, Singapore, May 2016, p. 3544.

7. Andreas C, Wolfgang K, Eckhart GH, Katharina H, Marcel Z, Esra N, Wolfgang R, Rolf J, Werner B, Ji C, Berthold K, Peter S, Hans-Peter H, Jianxiang S, Michael O, Dominik S, Anthony K, Joshua WG, Niels K. The Virtual Family—development of surface-based anatomical models of two adults and two children for dosimetric simulations. Physics in medicine and biology 2010;55(2):N23.

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Figures

Figure 1: Simulation and experimental setup. The dimensions of the birdcage model in simulation (A) approximated the dimensions of the transmit coil in experiment (B). HPM pads are shown in pink in the simulation setup and are white in the experimental setup. Experimental setup is shown before positioning the volunteer in the head coil.

Table: Peak local 10g SAR and Head average SAR values obtained from simulations for the experimental EPI sequence are below the prescribed limits by the FDA and IEC (10 W/kg for maximum local SAR and 3.2 W/kg for head average SAR).

Figure 2: Experimental flip angle maps, showing improved coverage with HPM pads in the inferior regions of the brain. The mean flip angle across all subjects (with/without HPM: ROI 1: 67.70/67.86, ROI 2: 86.37/89.45, ROI 3: 68/51.06) showed 33% improvement in the cerebellum.

Figure 3: Functional activation maps (shown for Z >=4) overlaid on echo-planar anatomical images for one volunteer. Both signal and functional activation improved in the inferior axial slices of the cerebellum with HPM pads. Functional activation for right hand finger tapping shows contralateral activation in the somatosensory motor cortex and an ipsilateral activation in the cerebellar cortex. A gradient-echo based EPI sequence was used for all subjects.

Figure 4: Higher-level statistical comparison of activation patterns (shown for Z>=2.3) in the presence versus the absence of HPM pads across eight subjects. The use of HPM pads resulted in notable increase in activation in the cerebellar cortex. Higher level modeling was carried out using mixed effects with FLAME 1 (FMRIB’s Local Analysis of mixed effects) in FEAT (fMRI Expert Analysis Tool, version 6.0).

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)
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