Search Results (3)

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. Full article

This paper proposes the use of a triple-line microstrip array for transmitting a magnetic field (|B₁⁺|) into the whole body for magnetic resonance applications at ultra-high field strength, such as 7 T. We explored some technologies that can potentially be applied for whole-body 7 T magnetic resonance imaging, as there is ongoing research on this topic. The triple-line microstrip transmission line (t-MTL) array consists of 32 channels. Each channel has a t-MTL, comprising a main conductor line and two adjacent coupled lines. The adjacent lines are not connected directly to the source. This configuration resulted in increased intensity and a centered |B₁⁺|-field. We compared the proposed structure and some reference radiofrequency (RF) transmitters, such as a patch antenna, using a magnet bore as a waveguide and a whole-body birdcage coil. We evaluated the performance of the t-MTL using cylindrical phantoms. We computed the |B₁⁺|-field from each RF transmitter inside a 3D human model containing more than 200 tissues. We compared their uniformity and field intensity and proposed a t-MTL array that yielded better performance. The proposed design also showed a lower specific absorption rate compared with a patch antenna. Full article

A novel analytical solution for the designing of the birdcage RF coil has been demonstrated in this paper. A new concept of dominant resonance path has been introduced in this paper which is used to identify the specific closed current loop in the birdcage RF coil which is responsible for the dominant resonance frequency mode. This concept is used to determine the precise numerical values of the lumped capacitance deployed in the legs and/or end-rings of the birdcage RF coil for its proper operation at the desired resonance frequency. The analytical solution presented in this paper has been established by performing the two-port network based equivalent circuit modeling of the birdcage RF coil. The proposed analytical solution uses T-matrix theory and develops a relationship between the input impedance of the birdcage coil and the impedances of its leg and end-ring segments. The proposed analytical solution provides the information about the resonance frequency spectrum of the birdcage RF coil and solves the issue of its interfacing with external circuits without affecting its resonance characteristics. Based upon the proposed analysis and designing strategy presented in this paper, the low pass, high pass and band pass configurations of the birdcage RF coil were successfully implemented with FPCB (Flexible Printed Circuit board) technique for small volume NMR imaging applications at 1.5 T and 3.0 T MRI system. The results obtained for the implemented birdcage coils using the proposed analysis and designing technique are in closed agreement with already established methods. Full article