Improvements of transmit efficiency and receive sensitivity with ultrahigh dielectric constant (uHDC) ceramics at 1.5 T and 3 T

Rupprecht, Sebastian; Sica, Christopher T.; Chen, Wei; Lanagan, Michael T.; Yang, Qing

doi:10.1002/mrm.26943

Cited by 28 publications

(27 citation statements)

References 32 publications

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“…Including losses in the HPM pads lowered array SNR, suggesting that aqueous suspensions of titanate powders, which are associated with a significant conductivity, should be avoided when using thicker HPMs surrounding the sample. Alternatively, HPM pads with very high relative permittivity and negligible conductivity, which would better match our simulations, could be obtained using ceramics sintered at high temperatures . However, such manufacturing techniques can be expensive.…”

Section: Discussionmentioning

confidence: 93%

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Disentangling the effects of high permittivity materials on signal optimization and sample noise reduction via ideal current patterns

Vaidya

Sodickson

Collins

et al. 2018

Magnetic Resonance in Med

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Purpose To investigate how high‐permittivity materials (HPMs) can improve SNR when placed between MR detectors and the imaged body. Methods We used a simulation framework based on dyadic Green’s functions to calculate the electromagnetic field inside a uniform dielectric sphere at 7 Tesla, with and without a surrounding layer of HPM. SNR‐optimizing (ideal) current patterns were expressed as the sum of signal‐optimizing (signal‐only) current patterns and dark mode current patterns that minimize sample noise while contributing nothing to signal. We investigated how HPM affects the shape and amplitude of these current patterns, sample noise, and array SNR. Results Ideal and signal‐only current patterns were identical for a central voxel. HPMs introduced a phase shift into these patterns, compensating for signal propagation delay in the HPMs. For an intermediate location within the sphere, dark mode current patterns were present and illustrated the mechanisms by which HPMs can reduce sample noise. High‐amplitude signal‐only current patterns were observed for HPM configurations that shield the electromagnetic field from the sample. For coil arrays, these configurations corresponded to poor SNR in deep regions but resulted in large SNR gains near the surface due to enhanced fields in the vicinity of the HPM. For very high relative permittivity values, HPM thicknesses corresponding to even multiples of λ/4 resulted in coil SNR gains throughout the sample. Conclusion HPMs affect both signal sensitivity and sample noise. Lower amplitude signal‐only optimal currents corresponded to higher array SNR performance and could guide the design of coils integrated with HPM.

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Section: Discussionmentioning

confidence: 93%

“…Alternatively, HPM pads with very high relative permittivity and negligible conductivity, which would better match our simulations, could be obtained using ceramics sintered at high temperatures. 33,34 However, such manufacturing techniques can be expensive.…”

mentioning

confidence: 99%

Disentangling the effects of high permittivity materials on signal optimization and sample noise reduction via ideal current patterns

Vaidya

Sodickson

Collins

et al. 2018

Magnetic Resonance in Med

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“…As described in detail previously, the uHDC enhancement mechanism can be illustrated through Ampere's law:

\nabla \times B = μ (σ + i ε_{normalr} ε_{0} ω) E .

…”

Section: Methodsmentioning

confidence: 99%

“…A solution to this problem was introduced with ultra‐HDC (uHDC) materials . uHDC materials are comprised of monolithic ceramics and capable of reaching permittivity values of several thousand in the corresponding frequency regime of 3T.…”

Section: Introductionmentioning

confidence: 99%

“…An important advantage gained with the transition to uHDC dielectric is the ability to achieve both higher relative electric permittivity with low loss. Blocks of this material have been applied to enhance both the brain and spine . However, enhancement of the brain cortex SNR requires a solution that encloses the head with uHDC material in a helmet‐type design.…”

Section: Introductionmentioning

confidence: 99%

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

Sica

Rupprecht

Hou

et al. 2019

Magnetic Resonance in Med

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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.

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Enhancing the brain MRI at ultra‐high field systems using a meta‐array structure

Alipour,

Seifert,

Delman

et al. 2023

Medical Physics

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BackgroundThe main advantage of ultra‐high field (UHF) magnetic resonance neuroimaging is theincreased signal‐to‐noise ratio (SNR) compared with lower field strength imaging. However, the wavelength effect associated with UHF MRI results in radiofrequency (RF) inhomogeneity, compromising whole brain coverage for many commercial coils. Approaches to resolving this issue of transmit field inhomogeneity include the design of parallel transmit systems (PTx), RF pulse design, and applying passive RF shimming such as high dielectric materials. However, these methods have some drawbacks such as unstable material parameters of dielectric pads, high‐cost, and complexity of PTx systems. Metasurfaces are artificial structures with a unique platform that can control the propagation of the electromagnetic (EM) waves, and they are very promising for engineering EM device. Implementation of meta‐arrays enhancing MRI has been explored previously in several studies.PurposeThe aim of this study was to assess the effect of new meta‐array technology on enhancing the brain MRI at 7T. A meta‐array based on a hybrid structure consisting of an array of broadside‐coupled split‐ring resonators and high‐permittivity materials was designed to work at the Larmor frequency of a 7 Tesla (7T) MRI scanner. When placed behind the head and neck, this construct improves the SNR in the region of the cerebellum,brainstem and the inferior aspect of the temporal lobes.MethodsNumerical electromagnetic simulations were performed to optimize the meta‐array design parameters and determine the RF circuit configuration. The resultant transmit‐efficiency and signal sensitivity improvements were experimentally analyzed in phantoms followed by healthy volunteers using a 7T whole‐body MRI scanner equipped with a standard one‐channel transmit, 32‐channel receive head coil. Efficacy was evaluated through acquisition with and without the meta‐array using two basic sequences: gradient‐recalled‐echo (GRE) and turbo‐spin‐echo (TSE).ResultsExperimental phantom analysis confirmed two‐fold improvement in the transmit efficiency and 1.4‐fold improvement in the signal sensitivity in the target region. In vivo GRE and TSE images with the meta‐array in place showed enhanced visualization in inferior regions of the brain, especially of the cerebellum, brainstem, and cervical spinal cord.ConclusionAddition of the meta‐array to commonly used MRI coils can enhance SNR to extend the anatomical coverage of the coil and improve overall MRI coil performance. This enhancement in SNR can be leveraged to obtain a higher resolution image over the same time slot or faster acquisition can be achieved with same resolution. Using this technique could improve the performance of existing commercial coils at 7T for whole brain and other applications.

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Improvements of transmit efficiency and receive sensitivity with ultrahigh dielectric constant (uHDC) ceramics at 1.5 T and 3 T

Cited by 28 publications

References 32 publications

Disentangling the effects of high permittivity materials on signal optimization and sample noise reduction via ideal current patterns

Disentangling the effects of high permittivity materials on signal optimization and sample noise reduction via ideal current patterns

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

Enhancing the brain MRI at ultra‐high field systems using a meta‐array structure

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