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

Large improvement in RF magnetic fields and imaging SNR with whole-head high-permittivity slurry helmet for human-brain MRI applications at 7 T

PURPOSE

To optimize the design and demonstrate the integration of a helmet-shaped container filled with a high-permittivity material (HPM) slurry with RF head coil arrays to improve RF coil sensitivity and SNR for human-brain proton MRI.

METHODS

RF reception magnetic fields (B 1 - $$ {\mathrm{B}}_1^{-} $$ ) of a 32-channel receive-only coil array with various geometries and permittivity values of HPM slurry helmet are calculated with electromagnetic simulation at 7 T. A 16-channel transmit-only coil array, a 32-channel receive-only coil array, and a 2-piece HPM slurry helmet were constructed and assembled. RF transmission magnetic field (B 1 + $$ {\mathrm{B}}_1^{+} $$ ),B 1 - $$ {\mathrm{B}}_1^{-} $$ , and MRI SNR maps from the entire human brain were measured and compared.

RESULTS

Simulations showed that averagedB 1 - $$ {\mathrm{B}}_1^{-} $$ improvement with the HPM slurry helmet increased from 57% to 87% as the relative permittivity (εr) of HPM slurry increased from 110 to 210. In vivo experiments showed that the averageB 1 + $$ {\mathrm{B}}_1^{+} $$ improvement over the human brain was 14.5% with the two-piece HPM slurry (εr ≈ 170) helmet, and the averageB 1 - $$ {\mathrm{B}}_1^{-} $$ and SNR were improved 63% and 34%, respectively, because the MRI noise level was increased by the lossy HPM.

CONCLUSION

The RF coil sensitivity and MRI SNR were largely improved with the two-piece HPM slurry helmet demonstrated by both electromagnetic simulations and in vivo human head experiments at 7 T. The findings demonstrate that incorporating an easily producible HPM slurry helmet into the RF coil array significantly enhances human-brain MRI SNR homogeneity and quality at ultrahigh field. Greater SNR improvement is anticipated using the less lossy HPM and optimal design.

Evaluation of the RF depositions at 3T in routine clinical scans with respect to the SAR safety to improve efficiency of MRI utilization

OBJECTIVES

This study explores the potential for improving of 3T MRI utilization by assessing and tailoring RF exposure in routine clinical scans while complying to standard safety limit.

METHODS

Using two generic human body models, we evaluated pbSAR10g values at four landmark positions (knee, pelvis, thoracic spine, head) at different wbSAR levels. Specifically, we analyzed local SAR10g in different operating modes and computed the maximum safety wbSAR, ensuring compliance with IEC limits.

RESULTS

In normal operating mode, the RF power deposition reached wbSAR limit before the pbSAR10g limit. In the first level controlled operating mode, pbSAR10g limit is reached before the wbSAR limit in the knee, thoracic spine scanning scenarios, while the wbSAR limit is reached first in the pelvis scanning scenarios, making it the most potential-releasing (up to 33.33 %) scanning scenario. For head exposure, the head SAR10g limit is reached before the wbSAR limit, highlighting the necessity for strict SAR control. Moreover, we calculated the minimum allowable TR for common imaging sequences for reference.

CONCLUSIONS

Different RF exposure setups are necessary to meet safety standards in various scenarios. By implementing careful RF exposure setups in routine clinical scans, the high potential capacity of 3T MRI can be fully released.

Improving Image Quality and Decreasing SAR With High Dielectric Constant Pads in 3 T Fetal MRI

BACKGROUND

At high magnetic fields, degraded image quality due to dielectric artifacts and elevated specific absorption rate (SAR) are two technical challenges in fetal MRI.

PURPOSE

To assess the potential of high dielectric constant (HDC) pad in increasing image quality and decreasing SAR for 3 T fetal MRI.

STUDY TYPE

Prospective.

FIELD STRENGTH/SEQUENCE

3 T. Balanced steady-state free precession (bSSFP) and single-shot fast spin-echo (SSFSE).

POPULATION

One hundred twenty-eight participants (maternal-age 29.0 ± 3.6, range 20-40; gestational-age 30.3 ± 3.5 weeks, range 22-37 weeks) undertook bSSFP and 40 participants (maternal-age 29.5 ± 3.8, range 19-40; gestational-age 30.4 ± 3.5 weeks, range 23-37 weeks) undertook SSFSE.

ASSESSMENT

Patient clinical characteristics were recorded, such as gestational-age, amniotic-fluid-index, abdominal-circumference, body-mass-index, and fetal-presentation. Quantitative Image-quality analysis included signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). Qualitative analysis was performed by three radiologists with four-point scale to evaluate overall image quality, dielectric artifact, and diagnostic confidence. Whole-body total SAR was obtained from the vendor workstation.

STATISTICAL TESTING

Paired rank sum test was used to analyze the differences in SNR, CNR, overall image quality, dielectric artifact, diagnostic confidence, and SAR with and without HDC pad. Spearman correlation test was used to detect correlations between image quality variable changes and patient clinical characteristics. P values <0.05 were set as statistical significance.

RESULTS

With HDC pad, SNR and CNR was significantly higher (41.45% increase in SNR, 54.05% increase in CNR on bSSFP; 258.76% increase in SNR, 459.55% increase in CNR on SSFSE). Overall qualitative image quality, dielectric artifact and diagnostic confidence improved significantly. Adding HDC pad significantly reduced Whole-body total SAR (32.60% on bSSFP; 15.40% on SSFSE). There was no significant correlation between image quality variable changes and participant clinical characteristics (P-values ranging from 0.072 to 0.992).

DATA CONCLUSION

In the clinical setting, adding a HDC pad might increase image quality while reducing dielectric artifact and SAR.

PLAN LANGUAGE SUMMARY

Dielectric artifacts and elevated SAR are two technical problems in 3T fetal MRI. In a prospective analysis of 168 pregnant participants undertaking 3.0T fetal MRI scanning, high dielectric constant (HDC) pad increased SNR by 41.45%, CNR by 54.05% on bSSFP, and SNR by 258.76%, CNR by 459.55% on SSFSE. Overall image quality, dielectric artifact reduction, and diagnostic confidence assessed by three radiologists was improved. Whole-body total SAR decreased by 32.60% on bSSFP and by 15.40% on SSFSE. These findings suggested that the HDC pad can enhance fetal MRI safety and quality, making it a promising tool for clinical practice.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 5.

In vivo B 1 + enhancement of calf MRI at 7 T via optimized flexible metasurfaces

PURPOSE

Ultrahigh field (≥7 T) MRI is at the cutting edge of medical imaging, enabling enhanced spatial and spectral resolution as well as enhanced susceptibility contrast. However, transmit (B 1 + $$ {\mathrm{B}}_1^{+} $$ ) field inhomogeneity due to standing wave effects caused by the shortened RF wavelengths at 7 T is still a challenge to overcome. Novel hardware methods such as dielectric pads have been shown to improve theB 1 + $$ {\mathrm{B}}_1^{+} $$ field inhomogeneity but are currently limited in their corrective effect by the range of high-permittivity materials available and have a fixed shelf life. In this work, an optimized metasurface design is presented that demonstrates in vivo enhancement of theB 1 + $$ {\mathrm{B}}_1^{+} $$ field.

METHODS

A prototype metasurface was optimized by an empirical capacitor sweep and by varying the period size. Phantom temperature experiments were performed to evaluate potential metasurface heating effects during scanning. Lastly, in vivo gradient echo images andB 1 + $$ {\mathrm{B}}_1^{+} $$ maps were acquired on five healthy subjects on a 7 T system. Dielectric pads were also used as a comparison throughout the work as a standard comparison.

RESULTS

The metasurfaces presented here enhanced the average relative SNR of the gradient echo images by a factor of 2.26 compared to the dielectric pads factor of 1.61. AverageB 1 + $$ {\mathrm{B}}_1^{+} $$ values reflected a similar enhancement of 27.6% with the metasurfaces present versus 8.9% with the dielectric pads.

CONCLUSION

The results demonstrate that metasurfaces provide superior performance to dielectric padding as shown byB 1 + $$ {\mathrm{B}}_1^{+} $$ maps reflecting their direct effects and resulting enhancements in image SNR at 7 T.

A review of recent developments and applications of high-permittivity dielectric shimming in magnetic resonance

We present a review outlining the basic mechanism, background, recent technical developments, and clinical applications of aqueous dielectric padding in the field of MRI. Originally meant to be a temporary solution, it has gained traction as an effective method for correcting B1 + inhomogeneities due to the unique properties of the calcium titanate and barium titanate perovskites used. Aqueous dielectric pads have used a variety of high-permittivity materials over the years to improve the quality of MRI acquisitions at 1.5 and 3 T and more recently for 7 T neuroimaging applications. The technical development and assessment of these pads have been advanced by an increased use of mathematical modeling and electromagnetic simulations. These tools have allowed for a more complete understanding of the physical interactions between dielectric pads and the RF coil, making testing and safety assessments more accurate. The ease of use and effectiveness that dielectric pads offer have allowed them to become more commonplace in tackling imaging challenges in more clinically focused environments. More recently, they have seen usage not only in anatomical imaging methods but also in specialized metabolic imaging sequences such as GluCEST and NOEMTR . New colossally high-permittivity materials have been proposed; however, practical utilization has been a continued challenge due to unfavorable frequency dependences as well as safety limitations. A new class of metasurfaces has been under development to address the shortcomings of conventional dielectric padding while also providing increased performance in enhancing MRI images.

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

BACKGROUND

The 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.

PURPOSE

The 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.

METHODS

Numerical 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).

RESULTS

Experimental 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.

CONCLUSION

Addition 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.

B 1 + $$ {\mathrm{B}}_1^{+} $$ inhomogeneity correction of volumetric brain NOEMTR via high permittivity dielectric padding at 7 T

PURPOSE

Nuclear Overhauser effect magnetization transfer ratio (NOEMTR ) is a technique used to investigate brain lipids and macromolecules in greater detail than other techniques and benefits from increased contrast at 7 T. However, this contrast can become degraded because ofB 1 + $$ {\mathrm{B}}_1^{+} $$ inhomogeneities present at ultra-high field strengths. High-permittivity dielectric pads (DP) have been used to correct for these inhomogeneities via displacement currents generating secondary magnetic fields. The purpose of this work is to demonstrate that dielectric pads can be used to mitigateB 1 + $$ {\mathrm{B}}_1^{+} $$ inhomogeneities and improve NOEMTR contrast in the temporal lobes at 7 T.

METHODS

Partial 3D NOEMTR contrast images and whole brainB 1 + $$ {\mathrm{B}}_1^{+} $$ field maps were acquired on a 7 T MRI across six healthy subjects. Calcium titanate DP, having a relative permittivity of 110, was placed next to the subject's head near the temporal lobes. Pad corrected NOEMTR images had a separate postprocessing linear correction applied.

RESULTS

DP provided supplementalB 1 + $$ {\mathrm{B}}_1^{+} $$ to the temporal lobes while also reducing theB 1 + $$ {\mathrm{B}}_1^{+} $$ magnitude across the posterior and superior regions of the brain. This resulted in a statistically significant increase in NOEMTR contrast in substructures of the temporal lobes both with and without linear correction. The padding also produced a convergence in NOEMTR contrast toward approximately equal mean values.

CONCLUSION

NOEMTR images showed significant improvement in temporal lobe contrast when DP were used, which resulted from an increase inB 1 + $$ {\mathrm{B}}_1^{+} $$ homogeneity across the entire brain slab. DP-derived improvements in NOEMTR are expected to increase the robustness of the brain substructural measures both in healthy and pathological conditions.

Quadrature transceive wireless coil: Design concept and application for bilateral breast MRI at 1.5 T

PURPOSE

Development of a novel quadrature inductively driven transceive wireless coil for breast MRI at 1.5 T.

METHODS

A quadrature wireless coil (HHMM-coil) design has been developed as a combination of two linearly polarized coils: a pair of 'metasolenoid' coils (MM-coil) and a pair of Helmholtz-type coils (HH-coil). The MM-coil consisted of an array of split-loop resonators. The HH-coil design included two electrically connected flat spirals. All the wireless coils were coupled to a whole-body birdcage coil. The HHMM-coil was studied and compared to the linear coils in terms of transmit and SAR efficiencies via numerical simulations. A prototype of HHMM-coil was built and tested on a 1.5 T scanner in a phantom and healthy volunteer. We also proposed an extended design of the HHMM-coil and compared its performance to a dedicated breast array.

RESULTS

Numerical simulations of the HHMM-coil with a female voxel model have shown more than a 2.5-fold increase in transmit efficiency and a 1.7-fold enhancement of SAR efficiency compared to the linearly polarized coils. Phantom and in vivo imaging showed good agreement with the numerical simulations. Moreover, the HHMM-coil provided good image quality, visualizing all areas of interest similar to a multichannel breast array with a 32% reduction in signal-to-noise ratio.

CONCLUSION

The proposed quadrature HHMM-coil allows the B 1 + $$ {\mathrm{B}}_1^{+} $$ -field to be significantly better focused in the region-of-interest compared to the linearly polarized coils. Thus, the HHMM-coil provides high-quality breast imaging on a 1.5 T scanner using a whole-body birdcage coil for transmit and receive.

Improving detection of fMRI activation at 1.5 T using high permittivity ceramics

In this work, we propose an application of high permittivity materials (HPMs) to improve functional magnetic resonance imaging (fMRI) at 1.5 T, increasing the receive (Rx) sensitivity of a commercial multi-channel head coil. To evaluate the transmit efficiency, specific absorption rate (SAR), and the signal-to-noise ratio (SNR) changes introduced by the HPMs with relative permittivity of 4500, we considered the following configurations in simulation: a whole-body birdcage coil and an Rx-only multi-channel head coil with and without the HPM blocks in the presence of a homogeneous head phantom or a human body model. Experimental studies were also performed with a phantom and with volunteers. Seven healthy volunteers enrolled in a prospective study of fMRI activation in the motor cortex with and without HPMs. fMRI data were analyzed using group-level paired T-tests between acquisitions with and without HPM blocks. Both electromagnetic simulations and experimental measurements showed ∼25% improvement in the Rx sensitivity of a commercial head coil in the areas of interest when HPM blocks were placed in close proximity. It increased the detected motor cortex fMRI activation volume by an average of 56%, thus resulting in more sensitive functional imaging at 1.5 T.

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

This article reviews recent developments in designing and testing new types of materials which can be: (i) placed around the body for in vivo imaging, (ii) be integrated into a conventional RF coil, or (iii) form the resonator itself. These materials can improve the quality of MRI scans for both in vivo and magnetic resonance microscopy applications. The methodological section covers the basic operation and design of two different types of materials, namely high permittivity materials constructed from ceramics and artificial dielectrics/metasurfaces formed by coupled conductive subunits, either in air or surrounded by dielectric material. Applications of high permittivity materials and metasurfaces placed next to the body to neuroimaging and extremity imaging at 7 T, body and neuroimaging at 3 T, and extremity imaging at 1.5 T are shown. Results using ceramic resonators for both high field in vivo imaging and magnetic resonance microscopy are also shown. The development of new materials to improve MR image quality remains an active area of research, but has not yet found significant use in clinical applications. This is mainly due to practical issues such as specific absorption rate modelling, accurate and reproducible placement, and acceptable size/weight of such materials. The most successful area has been simple “dielectric pads” for neuroimaging at 7 T which were initially developed somewhat as a stop-gap while parallel transmit technology was being developed, but have continued to be used at many sites. Some of these issues can potentially be overcome using much lighter metasurfaces and artificial dielectrics, which are just beginning to be assessed.

Evaluation of high-dielectric pads for macaque brain imaging at 7 T

A non-human primate is a valuable model for investigating the structure and function of the brain. Different from the human brain imaging using radio frequency (RF) head coils, in the present study, on a human whole-body 7 T magnetic resonance imaging system, we used an RF knee coil for monkey brain imaging in vivo due to the smaller size of the macaque's brain compared to that of a human, and particularly, high-dielectric pads were also utilized in order to improve brain imaging performance. Our experimental results suggest that high-dielectric pads can effectively enhance the B₁ field strength and receive sensitivity, leading to a higher flip-angle magnitude, an image signal-to-noise ratio, and tissue contrast, and in the meantime, we did not observe elevated receive array element coupling and receive noise amplification nor apparent magnetic susceptibility-induced artifact or distortion, showing that the pads do not introduce adverse RF interferences in macaque brain imaging at 7 T.

Design of an Intraoral Dipole Antenna for Dental Applications

OBJECTIVE

In dental MRI, intraoral coils provide higher signal-to-noise ratio (SNR) than coils placed outside the mouth. This study aims to design an intraoral dipole antenna and demonstrates the feasibility of combining it with an extraoral coil.

METHODS

Dipole antenna design was chosen over loop design, as it is open toward the distal; therefore, it does not restrain tongue movement. The dipole design offers also an increased depth-of-sensitivity that allows for MRI of dental roots. Different dipole antenna designs were simulated using a finite-difference-time-domain approach. Ribbon, wire, and multi-wire arms were compared. The best design was improved further by covering the ends of the dipole arms with a high-permittivity material. Phantom and in vivo measurements were conducted on a 3T clinical MRI system.

RESULTS

The best transmit efficiency and homogeneity was achieved with a multi-wire curved dipole antenna with 7 wires for each arm. With an additional high-permittivity cap the transmit field inhomogeneity was further reduced from 20% to 5% along the dipole arm. When combined with extraoral flexible surface-coil, the coupling between the coils was less than -32dB and SNR was increased.

CONCLUSION

Using intraoral dipole design instead of loop improves patient comfort. We demonstrated feasibility of the intraoral dipole combined with an extraoral flexible coil-array for dental MRI. Dipole antenna enabled decreasing imaging field-of-view, and reduced the prevalent signal from tongue.

SIGNIFICANCE

This study highlights the advantages and the main challenges of the intraoral RF coils and describes a novel RF coil that addresses those challenges.

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

PURPOSE

Investigating the designs and effects of high dielectric constant (HDC) materials in the shape of a conformal helmet on the enhancement of RF field and reduction of specific absorption rate at 10.5 T for human brain studies.

METHODS

A continuous and a segmented four-piece HDC helmet fit to a human head inside an eight-channel fractionated-dipole array were constructed and studied with a phantom and a human head model using computer electromagnetic simulations. The simulated transmit efficiency and receive sensitivity were experimentally validated using a phantom with identical electric properties and helmet-coil configurations of the computer model. The temporal and spatial distributions of displacement currents on the HDC helmets were analyzed.

RESULTS

Using the continuous HDC helmet, simulation results in the human head model demonstrated an average transmit efficiency enhancement of 66%. A propagating displacement current was induced on the continuous helmet, leading to an inhomogeneous RF field enhancement in the brain. Using the segmented four-piece helmet design to reduce this effect, an average 55% and 57% enhancement in the transmit efficiency and SNR was achieved in human head, respectively, along with 8% and 28% reductions in average and maximum local specific absorption rate.

CONCLUSION

The HDC helmets enhanced the transmit efficiency and SNR of the dipole array coil in the human head at 10.5 T. The segmentation of the helmet to disrupt the continuity of circumscribing displacement currents in the helmet produced a more uniform distribution of the transmit field and lower specific absorption rate in the human head compared with the continuous helmet design.

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

PURPOSE

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.

Tunable Ultrahigh Dielectric Constant (tuHDC) Ceramic Technique to Largely Improve RF Coil Efficiency and MR Imaging Performance

This work introduces an innovative magnetic resonance (MR) imaging technology that incorporates radiofrequency (RF) coil(s) with permittivity-tunable ultrahigh dielectric constant (tuHDC) ceramics to significantly improve RF coil transmission and reception efficiencies, MR imaging sensitivity and signal-to-noise ratio (SNR). The tuHDC ceramics made of composite barium strontium titanate (BST) compounds (Ba_0.6 Sr_0.4 TiO₃) have low dielectric loss and very high permittivity tunability from 2,000 to 15000 by varying the ceramic temperature between 0°C and 40°C to achieve an optimal permittivity for MR imaging application. We demonstrated for the first time the proof of concept using the BST-based tuHDC-RF-coil technology to improve MR spectroscopic imaging performance of ¹⁷O nuclide at 10.5 Tesla (T) at a low ceramic temperature and ²³Na nuclide at 7T at room temperature. We discovered a large and spatially independent noise reduction under an optimal ceramic temperature, which synergistically resulted in an unprecedented SNR improvement. Large improvements were also demonstrated for ¹H MRI on a 1.5T clinical scanner using the same ceramics. The tuHDC-RF-coil technology is robust, flexible and cost-effective; it presents a technical breakthrough to significantly improve imaging sensitivity and resolution for broad MR imaging applications; which is critical for advancing biomedical and neuroscience research, and improving diagnostic imaging.

Ceramic resonators for targeted clinical magnetic resonance imaging of the breast

Currently, human magnetic resonance (MR) examinations are becoming highly specialized with a pre-defined and often relatively small target in the body. Conventionally, clinical MR equipment is designed to be universal that compromises its efficiency for small targets. Here, we present a concept for targeted clinical magnetic resonance imaging (MRI), which can be directly integrated into the existing clinical MR systems, and demonstrate its feasibility for breast imaging. The concept comprises spatial redistribution and passive focusing of the radiofrequency magnetic flux with the aid of an artificial resonator to maximize the efficiency of a conventional MR system for the area of interest. The approach offers the prospect of a targeted MRI and brings novel opportunities for high quality specialized MR examinations within any existing MR system.

Here, the authors present a concept for targeted clinical magnetic resonance imaging for relatively small targets in the body. They use an artificial resonator for spatial redistribution and passive focusing of the radiofrequency magnetic flux and demonstrate feasibility for targeted breast imaging.

Design and characterization of receive-only surface coil arrays at 3T with integrated solid high permittivity materials

A receive-only surface coil array for 3 Tesla integrating a high-permittivity material (HPM) with a relative permittivity of 660 was designed and constructed and subsequently its performance was evaluated and compared in terms of transmit field efficiency and specific absorption ratio (SAR) during transmission, and signal-to-noise ratio during reception, with a conventional identically-sized surface coil array. Finite-difference time-domain simulations, bench measurements and in-vivo neck imaging on three healthy volunteers were performed using a three-element surface coil array with integrated HPMs placed around the larynx. Simulation results show an increase in local transmit efficiency of the body coil of ~10-15% arising from the presence of the HPM. The receiver efficiency also increased by approximately 15% close to the surface. Phantom experiments confirmed these results. In-vivo scans using identical transmit power resulted in SNR gains throughout the laryngeal area when compared with the conventional surface coil array. In particular specifically around the carotid arteries an average SNR gain of 52% was measured averaged over the three subjects, while in the spine an average of 20% SNR gain was obtained.

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

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.

High permittivity ceramics improve the transmit field and receive efficiency of a commercial extremity coil at 1.5 Tesla

OBJECTIVE

The purpose of this work is to investigate the use of ceramic materials (based on BaTiO₃ with ZrO₂ and CeO₂-additives) with very high relative permittivity (ε_r ∼ 4500) to increase the local transmit field and signal-to-noise ratio (SNR) for commercial extremity coils on a clinical 1.5 T MRI system.

METHODS

Electromagnetic simulations of transmit efficiency and specific absorption rate (SAR) were performed using four ferroelectric ceramic blocks placed around a cylindrical phantom, as well as placing these ceramics around the wrist of a human body model. Results were compared with experimental scans using the transmit body coil of the 1.5 T MRI system and an eight-element extremity receive array designed for the wrist. SNR measurements were also performed for both phantom and in vivo scans.

RESULTS

Electromagnetic simulations and phantom/in vivo experiments showed an increased in the local transmit efficiency from the body coil of ∼20-30%, resulting in an ∼50% lower transmit power level and a significant reduction in local and global SAR throughout the body. For in vivo wrist experiments, the SNR of a commercial eight-channel receive array, integrated over the entire volume, was improved by ∼45% with the ceramic.

CONCLUSION

The local transmit efficiency as well as the SNR can be increased for 1.5 T extremity MRI with commercial array coils by using materials with very high permittivity.

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

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.

Quantitative imaging of brain energy metabolisms and neuroenergetics using in vivo X-nuclear 2H, 17O and 31P MRS at ultra-high field

Brain energy metabolism relies predominantly on glucose and oxygen utilization to generate biochemical energy in the form of adenosine triphosphate (ATP). ATP is essential for maintaining basal electrophysiological activities in a resting brain and supporting evoked neuronal activity under an activated state. Studying complex neuroenergetic processes in the brain requires sophisticated neuroimaging techniques enabling noninvasive and quantitative assessment of cerebral energy metabolisms and quantification of metabolic rates. Recent state-of-the-art in vivo X-nuclear MRS techniques, including 2H, 17O and 31P MRS have shown promise, especially at ultra-high fields, in the quest for understanding neuroenergetics and brain function using preclinical models and in human subjects under healthy and diseased conditions.