Abstract
Multi-element transmit arrays with low peak 10 g specific absorption rate (SAR) and high SAR efficiency (defined as are essential for ultra-high field (UHF) magnetic resonance imaging (MRI) applications. Recently, the adaptation of dipole antennas used as MRI coil elements in multi-channel arrays has provided the community with a technological solution capable of producing uniform images and low SAR efficiency at these high field strengths. However, human head-sized arrays consisting of dipole elements have a practical limitation to the number of channels that can be used due to radiofrequency (RF) coupling between the antenna elements, as well as, the coaxial cables necessary to connect them. Here we suggest an asymmetric sleeve antenna as an alternative to the dipole antenna. When used in an array as MRI coil elements, the asymmetric sleeve antenna can generate reduced peak 10 g SAR and improved SAR efficiency. To demonstrate the advantages of an array consisting of our suggested design, we compared various performance metrics produced by 16-channel arrays of asymmetric sleeve antennas and dipole antennas with the same dimensions. Comparison data were produced on a phantom in electromagnetic (EM) simulations and verified with experiments at 10.5 Tesla (T). The results produced by the 16-channel asymmetric sleeve antenna array demonstrated 28 % lower peak 10 g SAR and 18.6 % higher SAR efficiency when compared to the 16-channel dipole antenna array.
Highlights
M AGNETIC resonance imaging (MRI) at ultra-high fields (UHF, defined as ≥7 tesla (T)) are increasingly pursued for biomedical research due to gains in signal-tonoise [1]–[4] and, in some cases, contrast-to-noise ratios (SNR and CNR, respectively) (e.g. [5], [6])
UHF RF coil designs frequently migrate towards far field antenna concepts rather than the near field domain, like those used at current clinical MRI field strengths
We describe a 16-channel asymmetric sleeve antenna array design for 447 MHz (10.5 T human head 1H imaging), which, at the time of this publication, is the highest magnetic field available for human imaging
Summary
M AGNETIC resonance imaging (MRI) at ultra-high fields (UHF, defined as ≥7 tesla (T)) are increasingly pursued for biomedical research due to gains in signal-tonoise [1]–[4] and, in some cases, contrast-to-noise ratios (SNR and CNR, respectively) (e.g. [5], [6]). As MRI pushes into the UHF regime, electric (E) and magnetic (B) field amplitude and phase non-uniformities increase over the sample volume, and this leads to non-uniform power deposition and transmit efficiency [4], [9]–[12]. Radiative type antennas [8], [13]–[15], half wavelength (λ/2) dipole antennas, have been suggested as building blocks for such UHF transmit arrays and have recently shown promising performance initially for applications in the human torso [7], [14], [16], [17] and recently in the human head [8], [18], [19] enabling improved transmit B1 efficiency and minimized power deposition in the imaging target (i.e. specific absorption rate (SAR)). In order to minimize coupling for radiative antenna arrays, a number of innov-
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