B₁ field, SAR, and SNR comparisons for birdcage, TEM, and microstrip coils at 7T

Abstract

Purpose:To compare the performance of birdcage, transverse electromagnetic (TEM) and microstrip volume coils at 7T under the same geometric conditions. Materials and Methods: Birdcage, TEM, and microstrip coils are modeled with the same dimensions. The finite difference time domain (FDTD) method is adopted to calculate the electromagnetic fields of the coils. Further, B 1 field, specific absorption rate (SAR) and signal-to-noise ratio (SNR) are calculated for these coils. Results:In the unloaded case, within the central axial plane, the variation of B 1 field magnitude over 18-cm distance is about 15% for the birdcage coil, 23% for the TEM coil, and 38% for the microstrip coil. In the loaded case, the percentages of the samples on the central axial plane, which have B 1 field magnitude within Ϯ20% of the average B 1 field magnitude, are about 57% for the birdcage, 72% for the TEM, and 59% for the microstrip coil. Average SAR levels are 11.4% and 42.9% higher in the birdcage than those in the TEM and microstrip coils, respectively. The average relative SNR on the central axial plane for the shielded birdcage, TEM, and microstrip coils are 1, 1.07, and 1.48, respectively. Conclusion:The birdcage coil has the best unloaded B 1 field homogeneity, and the TEM coil has the best loaded B 1 field homogeneity and the lowest radiation loss; while the microstrip coil is better in SAR and SNR at 7T than the birdcage and TEM coils.

Comparison of three commercially available radio frequency coils for human brain imaging at 3 Tesla

“…While comparisons of the TEM coil with other coil types have been carried out using simulations [12][13][14], benchmark tests [15], and research dedicated high field scanners [4,16], the authors are not aware of any published experimental coil comparison studies at the clinically relevant field strength of B 0 = 3 T. Moreover, in large MRI centers multiple commercial RF-coils designed for human brain imaging might be available. Therefore, the aim of this study was to compare a TEM, a circularly polarized (CP) (birdcage), and a 12-channel phased array head coil on a clinical 3 T MRI system and to provide experimental data that aid in coil selection.…”

Extended RF shimming: Sequence‐level parallel transmission optimization applied to steady‐state free precession MRI of the heart

“…a circularly polarized birdcage‐like mode, as described above) in which phase offsets between each element are fixed. Quadrature operation produces similar results to more standard birdcage RF coils . A bank of eight Analogic (Peabody, MA, USA) AN8134 RF amplifiers is used with the coil.…”

Evaluation of a new 1H/31P dual‐tuned birdcage coil for 31P spectroscopy

“…Dual-tuned (e.g., 1 H/ 31 P) birdcage coils have become a well known and widely used tool for magnetic resonance spectroscopy (MRS) (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). They provide a uniform magnetic field inside the coil and can be made to fit most anatomies (1,(16)(17)(18)(19)(20)(21)(22)(23). MRS can be used for the study of an assortment of different metabolites in the human body.…”

“…The 31 P MRS signal sensitivity is usually low due to low concentrations of the nucleus, a relatively small gyromagnetic ratio compared with 1 H, and magnetic field inhomogeneity from currently available commercial surface coils. Birdcage coils, on the other hand, give homogeneous magnetic fields and high efficiency at low specific absorption rates (SARs) (16,(18)(19)(20)(21)(22)(23)34). This point has been demonstrated in the past by simulating B 1 magnetic field maps to analyze homogeneity (18,19,21,23) along with SAR calculations with a simulated model of the human body loaded in the coil (17,20,22).…”