Toward whole‐cortex enhancement with an ultrahigh dielectric constant helmet at 3T

Abstract

Purpose:To present a 3T brain imaging study using a conformal prototype helmet constructed with an ultra-high dielectric constant (uHDC; ε r ~ 1000) materials that can be inserted into standard receive head-coils. Methods: A helmet conformal to a standard human head constructed with uHDC materials was characterized through electromagnetic simulations and experimental work. The signal-to-noise ratio (SNR), transmit efficiency, and power deposition with the uHDC helmet inserted within a 20-channel head coil were measured in vivo and compared with a 64-channel head coil and the 20-channel coil without the helmet. Seven healthy volunteers were analyzed. Results: Simulation and in vivo experimental results showed that transmit efficiency was improved by nearly 3 times within localized regions for a quadrature excitation, with a measured global increase of 58.21 ± 6.54% over 7 volunteers. The use of a parallel transmit spokes pulse compensated for severe degradation of B + 1 homogeneity, at the expense of higher global and local specific absorption rate levels. A SNR histogram analysis with statistical testing demonstrated that the uHDC helmet enhanced a 20-channel head coil to the level of the 64-channel head coil, with the improvements mainly within the cortical brain regions. Conclusion: A prototype uHDC helmet enhanced the SNR of a standard head coil to the level of a high density 64-channel coil, although transmit homogeneity was compromised. Further improvements in SNR may be achievable with optimization of this technology, and could be a low-cost approach for future radiofrequency engineering work in the brain at 3T.

Improved whole‐brain SNR with an integrated high‐permittivity material in a head array at 7T

“…A recent experimental study at 3 T showed an HPM cap having ε r of 1000 or greater within a commercial receive array provided great improvement in receive SNR selectively in the cortex, but with strong adverse distortion of the excitation field, 11 similar to results of simulations for a ε r of 200 shown here (Figure 3). From quasistatic approximations, achieving a similar effect at different frequencies with a given HPM and coil design should require an approximate relationship of ε r ∝ 1/ ω 2 .…”

“…28,29 Extrapolation of results shown here to 128 MHz (3 T) indicate ε r for optimal whole-brain improvement of transmit efficiency, and receive-array SNR may be closer to 500 or 600, and use of ε r of 1000 would result in effects like those reported. 11 The results presented here show the potential benefit for incorporating HPM into coil design for improving both transmit efficiency and receive array SNR for the entire brain at 7 T. The choices of coil and HPM geometry, and ε r of the HPM, all depend on the intended use of the array. Although this study includes a first experimental demonstration of advantages of incorporating HPM into a custom head receive array, we expect that further dedicated work in this area will result in further improvements.…”

“…10 The SNR was improved selectively in the cortex of brain for an HPM cap within a commercial receive array at 3 T, with adverse effects on the transmit field. 11 Here we report numerical simulations and experimental measurements of the effects of HPM encompassing the head on SNR throughout the brain for head-sized receive arrays, as well as on transmit efficiency for different excitation coils, at 7 T. Preliminary reports of much of this work have been shown at scientific meetings. 12-16…”

“…6 Some studies have shown potential advantages of HPM with one or more surface coils. [7][8][9][10][11] These include simulations and experiments for a wide variety of coil, sample, and HPM combinations, and in many cases show improvement in SNR at a desired region (sometimes at the expense of SNR elsewhere) with placement of an HPM, even if that placement causes the coil to be slightly further from the sample. 7,8 Improvement has been shown for cervical spine imaging using a receive array at 3 T, where the region of interest and HPM are small compared with the array and its individual elements, 9 and for some regions in a cylindrical sample (at the expense of others) within an HPM shell and cylindrical transceiver array.…”

Displacement current distribution on a high dielectric constant helmet and its effect on RF field at 10.5 T (447 MHz)

“…HDC materials have previously been used to enhance performances of standard commercial RF coils for lower magnetic fields 16–20 and 7 T with various geometrical shapes (flexible pads and rigid blocks) with different HDC materials (pure water, slurries, and ceramics) 21,22 . In recent efforts toward whole‐cortex enhancement, HDC helmets have been developed and demonstrated an overall increase in field and SNR in the human brain at 3 T and 7 T 23–25 .…”