A self-matched leaky-wave antenna for ultrahigh-field magnetic resonance imaging with low specific absorption rate

Abstract

The technology of magnetic resonance imaging is developing towards higher magnetic fields to improve resolution and contrast. However, whole-body imaging at 7 T or even higher flux densities remains challenging due to wave interference, tissue inhomogeneities, and high RF power deposition. Nowadays, proper RF excitation of a human body in prostate and cardiac MRI is only possible to achieve by using phased arrays of antennas attached to the body (so-called surface coils). Due to safety concerns, the design of such coils aims at minimization of the local specific absorption rate (SAR), keeping the highest possible RF signal in the region of interest. Most previously demonstrated approaches were based on resonant structures such as e.g. dipoles, capacitively-loaded loops, TEM-line sections. In this study, we show that there is a better compromise between the transmit signal $${{\bf{B}}}_{{\bf{1}}}^{{\boldsymbol{+}}}$$ B 1 + and the local SAR using non-resonant surface coils generating a low electric field in the proximity of their conductors. With this aim, we propose and experimentally demonstrate a leaky-wave antenna implemented as a periodically-slotted microstrip transmission line. Due to its non-resonant radiation, it induces only slightly over half the peak local SAR compared to a state-of-the-art dipole antenna but has the same transmit efficiency in prostate imaging at 7 T. Unlike other antennas for MRI, the leaky-wave antenna does not require to be tuned and matched when placed on a body, which makes it easy-to-use in prostate imaging at 7 T MRI.

32‐Channel self‐grounded bow‐tie transceiver array for cardiac MR at 7.0T

“…The footprint of the SGBT building block (89.3 × 48.0 × 25.8 mm 3 ) is reduced by 64% versus a very well established fractionated dipole (300 × 40 × 20 mm 3 ), 11 59% versus a bow‐tie building block (150 × 70 × 40 mm 3 ), 17 43% versus a single‐side adapted dipole (143 × 70 × 42 mm 3 ), 16 and 87% versus a self‐matched leaky‐wave antenna (384 × 85 × 18 mm 3 ) 5 . The weight of the proposed building block is 156 g which is 56 g heavier compared to a 32‐channel cardiac loop element configuration with approximately 100 g (per channel), but 264 g lighter compared to a 16‐channel cardiac bow‐tie building block configuration exhibiting a weight of 420 g per building block (size 40 × 150 × 70 mm 3 filled with D 2 O) 6,17 .…”

“…47,48 The footprint of the SGBT building block (89.3 × 48.0 × 25.8 mm 3 ) is reduced by 64% versus a very well established fractionated dipole (300 × 40 × 20 mm 3 ), 11 59% versus a bow-tie building block (150 × 70 × 40 mm 3 ), 17 43% versus a single-side adapted dipole (143 × 70 × 42 mm 3 ), 16 and 87% versus a self-matched leaky-wave antenna (384 × 85 × 18 mm 3 ). 5 The weight of the proposed building block is 156 g which is 56 g heavier compared to a 32-channel cardiac loop 7 This translates into an at least 55% higher standard deviation and 38% higher coefficient of variation (ratio of standard deviation to mean), resulting in a reduced B + 1 homogeneity. The maximum SAR 10g obtained for both human voxel models does not exceed 0.3 W/kg per Watt of input power using the proposed optimized phase set.…”

“…3 Research directions for B + 1 inhomogeneity compensation include RF pulse design and enabling multi-channel RF coil technology. Pioneering RF array developments for CMR at 7.0T include striplineelements, 4 stripline waveguide like elements, 5 loop elements, [6][7][8][9][10] dipoles 11,12 , slot-antennas, 13 loop-dipoles, 14,15 and dipole building block elements. 16,17 Dipole antenna configurations have gained increased attention for UHF-CMR.…”

An Antenna Based on Three Coupled Dipoles With Minimized E-Field for Ultra-High-Field MRI

“…Different types of antennas have been proposed as on-body (surface) array elements for ultra-high-field body imaging such as loop coils [9], TEM transmission-line resonators [11], slot [12], dipole [13]- [15], and leaky-wave antennas [16], as well as their combinations [17]. A way of combining the antenna array with excitation by a traveling-wave coil has also been proposed [18].…”

A nine‐channel transmit/receive array for spine imaging at 10.5 T: Introduction to a nonuniform dielectric substrate antenna

“…Raaijmakers et al 39 introduced the fractionated dipole-an inductively shortened dipole (~30 cm)-without sacrificing its transmit performance, for body imaging at 7 T. Later, several studies were performed to improve the B + 1 -SAR efficiency of the dipole by altering its geometry (e.g., snake antenna 40,46 ), spatial positioning, 41,47 and resonant nature. 48 Duan et al 29 used 2 dipoles (i.e., arranged in transversal direction), along with 4 loops (arranged in z-direction), as transmitters for spine imaging at 7 T. The dipoles were not stacked in z-direction due to their length (~25 cm). Ertürk et al combined this structure with loops for 7 T body imaging, 49 re-designed the fractionated dipole to have a physical length of ~20 cm, and arranged a 10-channel single-row TxArray for 10.5 T torso imaging.…”