The dual‐mode dipole: A new array element for 7T body imaging with reduced SAR

Solomakha, Georgiy; Leeuwen, C. van; Raaijmakers, Alexander J. E.; Simovski, Constantin; Popugaev, Alexander; Abdeddaïm, Redha; Melchakova, Irina V.; Glybovski, Stanislav

doi:10.1002/mrm.27485

Cited by 20 publications

(23 citation statements)

References 32 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Dipoles have recently been introduced as a potential building block for UHF transmit coils for body imaging 24–26 and head imaging, 12,27–30 due to being a high‐field radiator that can improve efficiency in deep tissues while simultaneously reducing SAR 31,32 . Dipoles were implemented as the transmit topology in this study due in large part to their geometric characteristics.…”

Section: Introductionmentioning

confidence: 99%

“…22,23 Dipoles have recently been introduced as a potential building block for UHF transmit coils for body imaging [24][25][26] and head imaging, 12,[27][28][29][30] due to being a high-field radiator that can improve efficiency in deep tissues while simultaneously reducing SAR. 31,32 Dipoles were implemented as the transmit topology in this study due in large part to their geometric characteristics. Bent, 33 meandered 29 and curved 34 dipole coils have all been developed for the head; however, straight dipoles 35 require less angular space and therefore allow considerable flexibility in their placement on a multi-piece housing.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Radiofrequency coil for routine ultra‐high‐field imaging with an unobstructed visual field

et al. 2020

View full text Add to dashboard Cite

Many neuroscience applications have adopted functional MRI as a tool to investigate the healthy and diseased brain during the completion of a task. While ultra‐high‐field MRI has allowed for improved contrast and signal‐to‐noise ratios during functional MRI studies, it remains a challenge to create local radiofrequency coils that can accommodate an unobstructed visual field and be suitable for routine use, while at the same time not compromise performance. Performance (both during transmission and reception) can be improved by using close‐fitting coils; however, maintaining sensitivity over the whole brain often requires the introduction of coil elements proximal to the eyes, thereby partially occluding the subject's visual field. This study presents a 7 T head coil, with eight transmit dipoles and 32 receive loops, that is designed to remove visual obstructions from the subject's line of sight, allowing for an unencumbered view of visual stimuli, the reduction of anxiety induced from small enclosures, and the potential for eye‐tracking measurements. The coil provides a practical solution for routine imaging, including a split design (anterior and posterior halves) that facilitates subject positioning, including those with impaired mobility, and the placement of devices required for patient comfort and motion reduction. The transmit and receive coils displayed no degradation of performance due to adaptions to the design topology (both mechanical and electrical) required to create an unobstructed visual field. All computer‐aided design files, electromagnetic simulation models, transmit field maps and local specific absorption rate matrices are provided to promote reproduction.

show abstract

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

Radiofrequency coil for routine ultra‐high‐field imaging with an unobstructed visual field

et al. 2020

View full text Add to dashboard Cite

show abstract

“…For example, we have performed simulations on the SAR efficiency of a passively fed straight dipole and found similar results to those presented here in terms of the improvement in SAR efficiency. We anticipate that similar behaviour would be found for other types of dipole such as folded, snake, or the recently introduced dual mode . We also anticipate that a passive‐feeding network would increase the SAR efficiency of electrically short dipoles, which are known to have high transmit efficiency but relatively low SAR efficiency, and are suitable, for example, for cardiac or pancreatic imaging.…”

Section: Discussionmentioning

confidence: 54%

“…Dipoles can be used as transmit‐only or transceiver elements, either alone or in combination with loop antennas as additional receive elements . Fractionated dipoles can be combined in pairs to give independent excitation of the even and odd modes of the dipole pair …”

Section: Introductionmentioning

confidence: 99%

Design and characterization of an eight‐element passively fed meander‐dipole array with improved specific absorption rate efficiency for 7 T body imaging

2019

View full text Add to dashboard Cite

Objective To evaluate the transmit efficiency and specific absorption rate (SAR) efficiency of a new eight‐element passively fed meander‐dipole antenna array designed for body MRI at 7 T, and to compare these values with a conventional directly fed meander‐dipole array. Methods The main radiating element of the passively fed dipole is printed on one side of a dielectric substrate and is capacitively coupled to a shorter feeding element (connected to the coaxial cable) printed on the opposite side of the substrate. The transmit (B1+) field and SAR were simulated on a phantom and on a human voxel model for both a passively fed and a directly fed single element. Two eight‐channel arrays containing, respectively, directly and passively fed meander dipoles were then simulated, and experimental B1+ maps and T2‐weighted spin echo images of the prostate were obtained in vivo for four healthy volunteers. Results In simulations, the mean transmit efficiency (B1+ per square root input power) value in the prostate was ~ 12.5% lower, and the maximum 10 g average SAR was 44% lower for the array containing passively fed dipoles, resulting in ~ 15% higher SAR efficiency for the passively fed array. In vivo RF‐shimmed turbo spin echo images were acquired from both arrays, and showed image SNRs within 5% of one another. Conclusion A passive‐feeding network for meander‐dipole antennas has been shown to be a simple method to increase the SAR efficiency of a multi‐element array used for body imaging at high fields. We hypothesize that the main reason for the increase in SAR efficiency is the storage of the strong conservative electric field in the dielectric between the feeding element and the radiating element of the dipole. The passive‐feeding approach can be generalized to other dipole geometries and configurations.

show abstract

“…The values of the quadrupole and magnetic dipole moments in this mode are balanced that makes the electric quadrupole (off-diagonal) and magnetic dipole not distinguishable in what concerns their electromagnetic fields the central transverse plane (H-plane of the array) [30]. Therefore, the currents of this mode create in the transverse plane the near magnetic field identical to that of a horizontal loop of the same length [31], [32]. This field is presented in the bottom panel of As we expected, when the mode order increases the corresponding electromagnetic field better concentrates around the MS.…”

Section: Decoupling By the Metasurface A Metasurface Decouplingmentioning

confidence: 99%

Decoupling of Closely Spaced Dipole Antennas for Ultrahigh Field MRI With Metasurfaces

Hurshkainen

Mollaei

Dubois

et al. 2021

IEEE Trans. Antennas Propagat.

Self Cite

View full text Add to dashboard Cite

Phased antenna arrays of dipoles are widely used in ultra-high field magnetic resonance imaging for creating the controllable radio-frequency magnetic field distributions in a human body. Due to safety and imaging quality reasons each individual channel of the array should be decoupled-electromagnetically isolated from the others. The required number of channels is large and in some techniques the dipole antennas should be located in the close proximity of the human body. Their ultimately dense arrangement leads to a strong mutual coupling and makes the conventional decoupling structures inefficient. This coupling needs to be suppressed without a significant distortion of radio-frequency fields in the imaged area. In this work, we propose and study a novel decoupling technique for two ultra-high field transceiver on-body dipole antennas. The decoupling is performed by a periodic structure of five parallel resonant wires referred to as a metasurface. In contrast to conventional decoupling with a single resonant wire, the metasurface decoupled by means of excitation of a higher-order coupled mode of the wires, which field is highly confined. The main advantage is a low distortion of the RF-field in the central region of the body.

show abstract

The dual‐mode dipole: A new array element for 7T body imaging with reduced SAR

Abstract: It was demonstrated that the array of dual-mode dipoles provided the same SNR and prostate imaging quality as the reference array, while demonstrating lower SAR. This is due to a smoother current distribution over a sample surface.

Cited by 20 publications

References 32 publications

Radiofrequency coil for routine ultra‐high‐field imaging with an unobstructed visual field

Radiofrequency coil for routine ultra‐high‐field imaging with an unobstructed visual field

Design and characterization of an eight‐element passively fed meander‐dipole array with improved specific absorption rate efficiency for 7 T body imaging

Decoupling of Closely Spaced Dipole Antennas for Ultrahigh Field MRI With Metasurfaces

Contact Info

Product

Resources

About