RF Head Coil Design with Improved RF Magnetic Near-Fields Uniformity for Magnetic Resonance Imaging (MRI) Systems

Abstract

Higher magnetic field strength in magnetic resonance imaging (MRI) systems offers higher signal-to-noise ratio (SNR), contrast, and spatial resolution in MR images. However, the wavelength in ultra-high fields (7 tesla and beyond) becomes shorter than the human body at the Larmor frequency with increasing static magnetic field (B₀) of MRI system. At short wavelengths, interference effect appears resulting in non- uniformity of the RF magnetic near-field (B₁) over the subject and MR images may have spatially anomalous contrast. The B₁ near-field generated by the transverse electromagnetic (TEM) RF coil's microstrip line element has a maximum near the center of its length and falls off towards both ends. In this study, a double trapezoidal shaped microstrip transmission line element is proposed to obtain uniform B₁ field distribution by gradual impedance variation. Two multi-channel RF head coils with uniform and trapezoidal shape elements were built and tested with a phantom at 7T MRI scanner for comparison. The simulation and experimental results show stronger and more uniform B₁⁺ near-field with the trapezoidal shape.

Keywords: B1 field uniformity; Magnetic Resonance Imaging (MRI); RF coil; Transverse electromagnetic (TEM) resonator.

Fig. 1

A structure illustration of a general Magnetic Resonance Imaging (MRI) scanner.

Fig. 2

A schematic diagram of TEM RF coil element and a sketch of the axial RF near-field along the line length.

Fig. 3

Sketch of generated RF magnetic near-field (B₁) distributions of a TEM coil element along the length of a MTL (in YZ plane): (a) a balanced condition, (b) (c) unbalanced conditions, and (d) a balanced and improved uniformity condition.

Fig. 4

Conductor shapes of MTL coil element: (a) uniform shape, (b) (c) stepped impedance shape, and (d) trapezoidal shape.

Fig. 5

Simulation results of surface current density (J_surf) distributions: (a) the uniform MTL coil element, (b) (c) stepped impedance coil element, (d) the proposed double trapezoidal shape coil element and (e) the narrow version of (d).

Fig. 6

(a) a simulation setup of a single element with a phantom and simulation results of cross-section magnetic field (H-field) distributions along the conductor line (YZ plane) according to the conductor shapes (top); (b) the uniform microstrip transmission line coil element, (c) the stepped impedance coil element, (d) the proposed double trapezoidal shape coil element and (e) the narrow version of (d).

Fig. 7

Multi-channel volume coil simulation setup and results with a phantom model: (a) RF coil with the double trapezoidal shape, (b) the cross-section of the magnetic (H) field of (a) in YZ plane, (c) RF coil with the uniform shape, and (d) the cross-section of the magnetic (H) field of (c) in YZ plane.

Fig. 8

(a) the uniform and (b) the proposed double trapezoidal 8 channel RF head coils and individual elements (zoomed).

Fig. 9

MRI experimental setup at 7T in CMRR (Center for Magnetic Resonance Research at University of Minnesota).

Fig. 10

Experimental B₁⁺ map results (μT/sqrt(W)) inside a phantom along the length of the RF coil element (YZ plane) and the sampled points (1 to 6) with; (a) the uniform single element, (b) the proposed double trapezoidal single element, (c) the subtraction from (b) to (a), and (d) the B₁⁺ field improvement as a percentage.

Fig. 11

Experimental B₁⁺ map results (μT/sqrt(W)) inside a cylindrical phantom with a XY plane perpendicular to the length of the element and the sampled points (1 to 6). The central slices of (a) the trapezoidal and (b) uniform single coil element, and end region slices of (c) the trapezoidal and (d) uniform single coil element.

Fig. 12

Experimental 8-channel RF coil B₁⁺ map slices (μT/sqrt(W)) inside a cylindrical phantom along XY plane. The center slices of (a) an 8-channel RF coil with the proposed double trapezoidal elements and (c) the uniform shape elements, and end region slices of (b) with trapezoidal and (d) uniform elements.