Pros and cons of ultra-high-field MRI/MRS for human application.

In ultrahigh-field MRI, such as 7 T, the signal-to-noise ratio (SNR) increases while transmit (Tx) field (B1+) can be degraded due to inhomogeneity and elevated specific absorption rate (SAR). By applying new array coil concepts to both Tx and receive (Rx) coils, the B1+ homogeneity and SNR can be improved. In this study, we developed and tested in vivo a new RF coil system for 7 T breast MRI.

An RF coil system composed of an eight-channel Tx-only array based on a tic-tac-toe design (can be combined to operate in single-Tx mode) in conjunction with an eight-channel Rx-only insert was developed. Characterizations of the B1+ field and associated SAR generated by the developed RF coil system were numerically calculated and empirically measured using an anatomically detailed breast model, phantom and human breasts. In vivo comparisons between 3 T (using standard commercial solutions) and 7 T (using the newly developed coil system) breast imaging were made.

At 7 T, about 20% B1+ inhomogeneity (standard deviation over the mean) was measured within the breast tissue for both the RF simulations and 7 T experiments. The addition of the Rx-only array enhances the SNR by a factor of about three. High-quality MR images of human breast were acquired in vivo at 7 T. For the in vivo comparisons between 3 T and 7 T, an approximately fourfold increase of SNR was measured with 7 T imaging.

The B1+ field distributions in the breast model, phantom and in vivo were in reasonable agreement. High-quality 7 T in vivo breast MRI was successfully acquired at 0.6 mm isotropic resolution using the newly developed RF coil system.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/nbm.3664

Development of a 7 T RF coil system for breast imaging

To explore the use of five meandering dipole antennas in a multi-transmit setup, combined with a high density receive array for breast imaging at 7 T for improved penetration depth and more homogeneous B1 field. Five meandering dipole antennas and 30 receiver loops were positioned on two cups around the breasts. Finite difference time domain simulations were performed to evaluate RF safety limits of the transmit setup. Scattering parameters of the transmit setup and coupling between the antennas and the detuned loops were measured. In vivo parallel imaging performance was investigated for various acceleration factors. After RF shimming, a B1 map, a T1-weighted image, and a T2-weighted image were acquired to assess B1 efficiency, uniformity in contrast weighting, and imaging performance in clinical applications. The maximum achievable local SAR10g value was 7.0 W/kg for 5 × 1 W accepted power. The dipoles were tuned and matched to a maximum reflection of −11.8 dB, and a maximum inter-element coupling of −14.2 dB. The maximum coupling between the antennas and the receive loops was −18.2 dB and the mean noise correlation for the 30 receive loops 7.83 ± 8.69%. In vivo measurements showed an increased field of view, which reached to the axilla, and a high transmit efficiency. This coil enabled the acquisition of T1-weighted images with a high spatial resolution of 0.7 mm3 isotropic and T2-weighted spin echo images with uniformly weighted contrast.

https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/pdf/10.1002/nbm.4039

Homogeneous B1 + for bilateral breast imaging at 7 T using a five dipole transmit array merged with a high density receive loop array

We propose a flexible form-fittingMRI receiver coil array assembledby individualcoilmodules. This design targetsMRI applications requiring a receiver array conforming to the anatomy of various shapes or sizes. Coil modules in our proposed array were arranged with gaps between them. Each coil module had a circumferential shielding structure stacked on top of the coil. Together they achieve robust decoupling when the array was bent differently. Two types of the circumferential shielding structure were investigatedby using full-wave electromagnetic simulations and imaging experiments. Results showed that our flexible coil array had good decoupling between coils whether they were on a flat or curved surface with the S21 magnitude ranged between −18.1 dB and −19.9 dB in simulations, and with the average of off-diagonal entries of the noise correlationmatrix less than 0.047 in experimentalmeasurements. Anatomical images of human brain, calf, and knee were acquired by our seven-channel prototype on a 3T MRI system. The maximal and the average SNR within 50 mm from our array surpassed those from the commercial 32-channel head and 4-channel flexible coil arrays by 2.63/1.35-fold and 3.89/1.50-fold, respectively.

A Flexible and Modular Receiver Coil Array for Magnetic Resonance Imaging

Purpose To facilitate assessment of RF power deposition and temperature rise within the breast, we present a method to seamlessly join heterogeneous breast models with standard whole-body models and demonstrate simulations at 7 T. Methods Finite-difference time-domain electromagnetic and bioheat simulations are performed to analyze the specific absorption rate (SAR) and temperature rise distributions in 36 Breast Imaging Reporting and Data System (BI-RADS) categorized breast models fused to 2 female whole-body models while transmitting from a 7T breast volume coil. The breast models are uncompressed in the prone position and feature heterogeneous tissue contents; fusion with human models uses affine transformation and the level-set method. Results The fusion method produces a continuous transient from the chest region to the posterior portion of breast models while preserving the original volume and shape of breast models. Simulation results of both Ella and Hanako models indicate that the maximum local SAR, partial body SAR, and local tissue temperature rise are positively correlated with both breast density and the highest BI-RADS density classification. Additionally, maximum local tissue temperature rise is positively correlated with maximum 10-g SAR values. Conclusion Fibroglandular tissue content plays an important role in the distribution of SAR and temperature rise within breast tissue. The combined body-breast models preserve the integrity of breast models while concurrently exhibiting the loading of whole-body human models. The procedures presented in this simulation study facilitate safety assessments for breast MRI across the population at both clinical and ultrahigh field strengths.

Toward 7T breast MRI clinical study: safety assessment using simulation of heterogeneous breast models in RF exposure

In high-field magnetic resonance imaging, the radio frequency wavelength within the human body is comparable to anatomical dimensions, resulting in B1 inhomogeneity and nonuniform sensitivity patterns. Thus, this relatively short wavelength presents engineering challenges for RF coil design. In this study, a bilateral breast coil for 1H imaging at 7 T was designed and constructed using forced-current excitation. By forcing equal current through the coil elements, we reduce the effects of coupling between the elements to simplify tuning and to ensure a uniform field across both breasts. To combine the benefits of the higher power efficiency of a unilateral coil with the bilateral coverage of a bilateral coil, a switching circuit was implemented to allow the coil to be reconfigured for imaging the left, right, or both breasts.

A Switched-Mode Breast Coil for 7 T MRI Using Forced-Current Excitation

Ultrahigh field imaging of the body and the spine is challenging due to the large field-of-view (FOV) required. It is especially difficult for RF transmission due to its requirement on both the length and the depth of the ${\rm{B}}_{1}^{{\rm + }}$ field. One solution is to use a long dipole to provide continuous current distribution. The drawback is the natural falloff of the ${\rm{B}}_{1}$ field toward the ends of the dipole, therefore the ${\rm{B}}_{1}^{{\rm + }}$ per unit square root of maximum specific absorption rate ${\rm{(B}}_{1}^{{\rm + }}{\rm{/ \surd SAR}}_{{\rm{max}}})$ performance is particularly poor toward the end of the dipole. In this study, a segmented element design using forced-current excitation and a switching circuit is presented. The design provides long FOV when desired and allows flexible FOV switching and power distribution without additional power amplifiers. Different element types and arrangements were explored and a segmented dipole design was chosen as the best design. The segmented dipole was implemented and tested on the bench and with a phantom on a 7T whole body scanner. The switchable mode dipole enabled a large FOV in the long mode and improved ${\rm{B}}_{1}^{{\rm + }}{\rm{/ \surd SAR}}_{{\rm{max}}}$ efficiency in a smaller FOV in the short mode.

An Adjustable-Length Dipole Using Forced-Current Excitation for 7T MR

PURPOSE This work describes the construction and evaluation of a bilateral 32-channel receive array for breast imaging at 7T. METHODS The receive array consisted of 32 receive coils, placed on two 3D-printed hemispherical formers. Each side of the receive array consisted of 16 receive loops, each loop having a corresponding detachable board with match/tune capacitors, active detuning circuitry, and a balun. Coil performance was evaluated on homogeneous canola oil phantoms using a Philips Achieva 7T system. Array coil performance was compared with a bilateral forced current excitation volume coil in transmit/receive mode and with a previously reported 16-channel unilateral coil with a similar design. RESULTS The 32-channel array had an increase in average SNR throughout both phantoms by a factor of five as compared with the volume coil, with SNR increases up to 10 times along the periphery and three times in the center. Noise measurements showed low interelement noise correlation (average: 5.4%; maximum: 16.8%). Geometry factor maps were acquired for various acceleration factors and showed mean geometry factors <1.2, for combined acceleration factors of up to six. CONCLUSIONS The improvements achieved demonstrate the clear potential for use in dynamic contrast-enhanced or diffusion-weighted MR studies, while maintaining diagnostically relevant spatial and temporal resolutions.

A 32-channel receive array coil for bilateral breast imaging and spectroscopy at 7T.

Purpose 
To enable high spatial and temporal breast imaging resolution via combined use of high field MRI, array coils, and forced current excitation (FCE) multi channel transmit.


Materials and Methods 
A unilateral 16-channel receive array insert was designed for use in a transmit volume coil optimized for quadrature operation with dual-transmit RF shimming at 7T. Signal-to-noise ratio (SNR) maps, g-factor maps, and high spatial and temporal resolution in vivo images were acquired to demonstrate the utility of the coil architecture.


Results 
The dual-transmit FCE coil provided homogeneous excitation and the array provided an increase in average SNR of 3.3 times (max 10.8, min 1.5) compared to the volume coil in transmit/receive mode. High resolution accelerated in vivo breast imaging demonstrated the ability to achieve isotropic spatial resolution of 0.5 mm within clinically relevant 90 s scan times, as well as the ability to perform 1.0 mm isotropic resolution imaging, 7 s per dynamics, with the use of bidirectional SENSE acceleration of up to R = 9.


Conclusion 
The FCE design of the transmit coil easily accommodates the addition of a sixteen channel array coil. The improved spatial and temporal resolution provided by the high-field array coil with FCE dual-channel transmit will ultimately be beneficial in lesion detection and characterization.

/pdf/a-16-channel-receive-forced-current-excitation-dual-transmit-4a58db7u56.pdf

A 16-Channel Receive, Forced Current Excitation Dual-Transmit Coil for Breast Imaging at 7T

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A Switched-Mode Breast Coil for 7 T MRI Using Forced-Current Excitation

An Adjustable-Length Dipole Using Forced-Current Excitation for 7T MR

A 32-channel receive array coil for bilateral breast imaging and spectroscopy at 7T.

A 16-Channel Receive, Forced Current Excitation Dual-Transmit Coil for Breast Imaging at 7T