Evaluation of Common RF Coil Setups for MR Imaging at Ultrahigh Magnetic Field: A Numerical Study

Abstract

This study is an evaluation of the ratio of electric field to magnetic field (E/B₁), specific absorption rate (SAR) and signal-to-noise ratio (SNR) generated by three different RF transceiver coil setups: surface coil, surface coil with shielding, and microstrip using a finite discrete time domain (FDTD) simulation in the presence of a head phantom. One of our main focuses in this study is to better understand coil designs that would improve patient safety at high fields by studying a coil type that may potentially minimize SAR while examining potential changes in SNR. In the presence of a human head load, the microstrip's E/B₁ ratio was on average smallest while its SAR was also on average smallest of the three setups, suggesting the microstrip may be a better RF coil choice for MRI concerning patient safety and parallel excitation applications than the other two coils. In addition, the study suggests that the microstrip also has a higher SNR compared with the other two coils demonstrating the possibility that the microstrip could lead to higher quality MRI images.

Keywords: High field; RF coil; electromagnetic calculation; microstrip; numerical modeling; signal-to-noise ratio (SNR); specific absorption rate (SAR).

Figure 1

(a) Surface Coil configuration. Capacitance value: 1.09 pF (b) Surface Coil and shielding configuration. Coil placed 5 centimeters from shielding. Capacitance value: 1.10 pF (c) Microstrip Configuration. Thickness= 5mm. Teflon width= 2.6 cm. Pair of capacitors each at: 3.35 pF. (d-f) Actual setup configuration including the human head phantom approximately 18 centimeters in diameter with a conductance value of 0.553 S/m, relative permittivity of 51.898, density of 1050 kg/m³, and its edge placed 1 cm away from the coil. The coils consist of a loop conductor that is 8 cm in diameter with a width of 6 mm.

Figure 2

(a) Average E/B₁ ratio (normalized E field units: V/m) of a plane parallel to the coil face versus distance from the 8 cm diameter coil face w/ phantom. The knee one centimeter from the center of the coil is due to the fact that the phantom is one centimeter from the center of the coil. (b) SAR versus distance from coil. Curves included for surface coil, surface coil w/ shielding with 5 cm between conductor and shielding, and microstrip with 5 mm thick Teflon insulator.

Figure 3

Colormaps indicating the magnitudes of the E/B₁ ratio across a plane with a phantom (FOV: 18 cm × 18 cm) parallel to and at a distance 2 cm (a-c), 5 cm (d-f), and 8 cm (g-i) away from the center of the coil for the surface coil, shielded coil with 5 cm distance between conductor and shielding, and 5 cm thick microstrip respectively.

Figure 4

Colormaps indicating the magnitudes of the SAR across a plane with a phantom (FOV: 18 cm × 18 cm) parallel to and at a distance 2 cm (a-c), 5 cm (d-f), and 8 cm (g-i) away from the center of the coil for the surface coil, shielded coil with 5 cm distance between conductor and shielding, and 5 cm thick microstrip respectively.

Figure 5

Average signal-to-noise ratio (SNR) versus distance from the coil with a loaded phantom. Curves included for surface coil, shielded coil with 5 cm distance between conductor and shielding, and 5 cm thick microstrip.