Monopole antenna array design for 3 T and 7 T magnetic resonance imaging

Abstract

Purpose Ultra-high field magnetic resonance imaging poses a number of challenges for robust radio frequency coil designs. A monopole antenna array can potentially overcome key limitations of birdcage coil designs and may provide a useful radio frequency coil for brain imaging. Methods Four, 8 and 12 element monopole antenna arrays were simulated using 3 T and 7T magnetic resonance imaging frequencies. For comparison, 4, 8 and 12 element birdcage coils were also simulated. Coil performance was evaluated and compared and the impact of shielding was assessed. A 4 element monopole antenna array was fabricated and bench tested. Results Comparison of the 4, 8 and 12 element designs suggest that the monopole antenna array leads to better field properties than the birdcage coil in all configurations studied: unloaded, loaded with saline and loaded using a head phantom. Improvements in field properties and homogeneity were evident at both field strengths, implying that the monopole antenna array has potential for head imaging. The monopole antenna array also appears to be more efficient than the comparable birdcage coil design. Additionally, the former is scalable via the addition of more elements whereas our results suggest that this is not the case for the latter. Bench testing results show that the monopole antenna array is well matched with the transmission line, and mutual coupling between elements is sufficiently low. Conclusion We found the monopole antenna array generated a larger field intensity than the birdcage coil design, whilst also producing a more useful magnetic resonance imaging field as measured by radio frequency field homogeneity. Our study suggests that magnetic resonance imaging of the brain can likely benefit from the use of radio frequency monopole antenna arrays.

A Review of Lunar Communications and Antennas: Assessing Performance in the Context of Propagation and Radiation

“…If the ground plane is large enough, the monopole acts precisely like a dipole, as though its reflection in the ground plane forms the dipole’s missing half. A typical monopole antenna feed is a coaxial wire with its inner conductor linked to the vertical monopole element via a hole in the ground plane and its outer conductor attached to the ground plane via a flange [ 72 , 73 , 74 ].…”

A pathway towards a two‐dimensional, bore‐mounted, volume body coil concept for ultra high‐field magnetic resonance imaging

“…As a result, current UHF imaging most commonly employs localized surface array solutions combining the fields from decoupled elements to create uniform B 1 + , often at the cost of higher specific absorption rate (SAR). Such arrays have been reported based on loops, [17][18][19][20] transverse electromagnetic (TEM) elements, [21][22][23] monopoles, 24,25 dipoles, [26][27][28][29] microstrip lines, [30][31][32] shielded coaxial cables, 33 slot antennas, 34 combinations of loops and dipoles, [35][36][37] multimodal antennas with coupled radiating structures, 38 array coils with surface elements, 39 and antennas that minimize rapid…”

“…As a result, current UHF imaging most commonly employs localized surface array solutions combining the fields from decoupled elements to create uniform B 1 + , often at the cost of higher specific absorption rate (SAR). Such arrays have been reported based on loops, 17–20 transverse electromagnetic (TEM) elements, 21–23 monopoles, 24,25 dipoles, 26–29 microstrip lines, 30–32 shielded coaxial cables, 33 slot antennas, 34 combinations of loops and dipoles, 35–37 multimodal antennas with coupled radiating structures, 38 array coils with surface elements, 39 and antennas that minimize rapid B 1 + fall off 40 . Parallel transmission (pTx) technology 41,42 often replaces the homogeneous body coil excitation concept in UHF applications by driving such individual array channels with independent signal shapes to control the field distribution for a specific field of view (FOV) or anatomy 43–45 …”

Effects of Non-symmetrical Current Distribution Controlled by Capacitor Placement in Loop Coils at 300 MHz MRI

“…Several RF transmissions and receptors [15][16][17] have been proposed to address the occurrence of eld non-uniformity. The use of high frequencies allows the application of antennas such as microstrips [18,19], dipoles [20,21], and monopoles [22], because the sizes of these devices, as determined by the wavelength, are acceptable and practical for use inside an MRI scanner within this frequency range.…”