Abstract
Improvements in transmission and reception sensitivities of radiofrequency (RF) coils used in ultra-high field (UHF) magnetic resonance imaging (MRI) are needed to reduce specific absorption rates (SAR) and RF power deposition, albeit without applying high-power RF. Here, we propose a method to simultaneously improve transmission efficiency and reception sensitivity of a band-pass birdcage RF coil (BP-BC RF coil) by combining a multi-channel wireless RF element (MCWE) with a high permittivity material (HPM) in a 7.0 T MRI. Electromagnetic field (EM-field) simulations, performed using two types of phantoms, viz., a cylindrical phantom filled with oil and a human head model, were used to compare the effects of MCWE and HPM on BP-BC RF coils. EM-fields were calculated using the finite difference time-domain (FDTD) method and analyzed using Matlab software. Next, to improve RF transmission efficiency, we compared two HPM structures, namely, a hollow cylinder shape HPM (hcHPM) and segmented cylinder shape HPM (scHPM). The scHPM and MCWE model comprised 16 elements (16-rad BP-BC RF coil) and this coil configuration demonstrated superior RF transmission efficiency and reception sensitivity along with an acceptable SAR. We expect wider clinical application of this combination in 7.0 T MRIs, which were recently approved by the United States Food and Drug Administration.
Highlights
Received: 4 January 2022The recent steady increase in the main magnetic field (B0 -field) strength in magnetic resonance imaging (MRI) systems has been able to improve the signal-to-noise ratio (SNR), among other advantages
We have previously described a set of electromagnetic field (EM-field) simulations that combine various methods to improve B1+ -field efficiency and B1− -field sensitivity and have proposed the use of an optimal combination of a band-pass type bird cage coil (BP-BC RF coil) with a multi-channel wireless RF element (MCWE) [43]
To evaluate the impact of simultaneous use of an MCWE and high permittivity material (HPM), we compared the above configurations of BPBC with an MCWE (w-MCWE) and without an MCWE
Summary
Received: 4 January 2022The recent steady increase in the main magnetic field (B0 -field) strength in magnetic resonance imaging (MRI) systems has been able to improve the signal-to-noise ratio (SNR), among other advantages. As UHF MRIs began to be used in clinical trials and the B0 -field strength increased, magnetic flux field (B1 -field) sensitivity and the uniformity of RF coils became crucial design factors of RF coils [4–6]. Even though the 7.0 T MRI has recently provided limited approval by the US FDA for use in head and extremities (arms and legs) imaging, whole body imaging is not permitted due to safety issues, including tissue heating, which occurs because UHF MRI requires a large amount of RF energy for human imaging [7]. RF safety studies have evaluated higher specific absorption rates (SAR), which indirectly measure tissue heating upon RF exposure [8–15], and research has focused on signal strength enhancement and SAR safety for RF coils used in 7.0 T MRI approved for clinical use
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