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

Abstract

To demonstrate that strategic use of materials with high electric permittivity along with integrated head-sized coil arrays can improve SNR in the entire brain. Methods: Numerical simulations were used to design a high-permittivity material (HPM) helmet for enhancing SNR throughout the brain in receive arrays of 8 and 28 channels. Then, two 30-channel head coils of identical geometry were constructed: one fitted with a prototype helmet-shaped ceramic HPM helmet, and the second with a helmet-shaped low-permittivity shell, each 8-mm thick. An eight-channel dipole array was used for excitation. In vivo maps of excitation flip angle and SNR were acquired. Results: Simulation results showed improvement in transmit efficiency by up to 65% and in receive-side SNR by up to 47% on average through the head with use of an HPM helmet. Experimental results showed that experimental transmit efficiency was improved by approximately 56% at the center of brain, and experimental receive-side SNR (SNR normalized to flip angle) was improved by approximately 21% on average through orthogonal planes through the cerebrum, including at the center of the brain, with the HPM. Conclusion:Although HPM is used increasingly to improve transmit efficiency locally in situations in which the transmit coil and imaging volume are much larger than the HPM, here we demonstrate that HPM can also be used to improve transmit efficiency and receive-side SNR throughout the brain by improving performance of a head-sized receive array. This includes the center of the brain, where it is difficult to improve SNR by other means.

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

“…In a recent study, improved transmit efficiency using a helmet of HDC ceramic has been demonstrated at 7 T with computer modeling and in vivo human experiment. Interestingly, this study demonstrated improvements not only in transmit efficiency using an eight‐channel dipole transmit array but also in SNR with a 28‐channel receive array throughout the head, including at the center of the brain 23,25 . The observed SNR improvement in the center of the brain, where it is difficult to improve SNR by other means, could likely relate to such a propagating wave behavior over an HDC helmet structure.…”

“…Interestingly, this study demonstrated improvements not only in transmit efficiency using an eight-channel dipole transmit array but also in SNR with a 28-channel receive array throughout the head, including at the center of the brain. 23,25 The observed SNR improvement in the center of the brain, where it is difficult to improve SNR by other means, could likely relate to such a propagating wave behavior over an HDC helmet structure. Thus, for optimization of the design of HDC material for high-field applications, it is necessary to further investigate propagating wave behavior of displacement current in a large continuous structure, such as the helmets presented here, and its influences on the RF field distribution in the human body.…”

“…HDC materials have previously been used to enhance performances of standard commercial RF coils for lower magnetic fields [16][17][18][19][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 B + 1 field and SNR in the human brain at 3 T and 7 T. [23][24][25] The translation of this technology to 10.5 T is not straightforward because, at such a high magnetic field, the wavelength of the RF field used at the proton resonance frequency becomes much shorter in the human head (with an average dielectric constant of ε r = 47 considering the brain white-matter and gray-matter dielectric constant values at 447 MHz are 41.5 and 56.6, respectively) and even shorter in HDC materials. Under such a condition, it is uncertain how the induced displacement currents in a HDC conformal helmet would form in response to the RF fields generated by the transmission coils, and how HDC materials, acting as a secondary field source, would impact the resultant electric and magnetic field distributions in the human head, and related safety issues.…”

Novel materials in magnetic resonance imaging: high permittivity ceramics, metamaterials, metasurfaces and artificial dielectrics

“…They obtained images which showed similar signal to noise ratio (SNR) maps using a 20-channel receive coil with the HPM helmet to those obtained using a 64-channel coil without the HPM insert, as shown in Fig. 5 (this helmet approach has subsequently been used for neuroimaging at 7 T and 10.5 T with lower permittivity materials [ 35 , 36 ]). One of the questions which often arises is how can the SNR be increased when typically body loss dominates over coil loss at clinical field strengths?…”

Repeatability of B1+ inhomogeneity correction of volumetric (3D) glutamate CEST via High‐permittivity dielectric padding at 7T

“…It can be seen the conduction current is dependent on the conductivity ($$$ \sigma $$$) value of the material, while the displacement current depends on the relative permittivity value ($$$ {\varepsilon}_r $$$), frequency $$$ \left(\omega \right) $$$, and permittivity of free space $$$ \left({\varepsilon}_o\right) $$$. This implies that at a sufficiently high $$$ \omega $$$ or with sufficiently high $$$ {\varepsilon}_r $$$, the displacement current can become a significant contributor to the main magnetic field created by the RF coil 25 $$$$ {J}_T={J}_C+{J}_D=\sigma E+ i\omega {\varepsilon}_o{\varepsilon}_rE $$$$ These high permittivity materials are typically manufactured as a powder, with the internal grain size being an important factor in the macroscopic field altering properties.…”