Wireless, customizable coaxially shielded coils for magnetic resonance imaging

Abstract

Anatomy-specific radio frequency receive coil arrays routinely adopted in magnetic resonance imaging (MRI) for signal acquisition are commonly burdened by their bulky, fixed, and rigid configurations, which may impose patient discomfort, bothersome positioning, and suboptimal sensitivity in certain situations. Herein, leveraging coaxial cables' inherent flexibility and electric field confining property, we present wireless, ultralightweight, coaxially shielded, passive detuning MRI coils achieving a signal-to-noise ratio comparable to or surpassing that of commercially available cutting-edge receive coil arrays with the potential for improved patient comfort, ease of implementation, and substantially reduced costs. The proposed coils demonstrate versatility by functioning both independently in form-fitting configurations, closely adapting to relatively small anatomical sites, and collectively by inductively coupling together as metamaterials, allowing for extension of the field of view of their coverage to encompass larger anatomical regions without compromising coil sensitivity. The wireless, coaxially shielded MRI coils reported herein pave the way toward next-generation MRI coils.

Fig. 1.. Application scenarios of the coaxially shielded wireless coils in MRI systems.

(A to D) Form-fitting coils working independently for imaging finger (A), wrist (B), ankle (C), and knee (D). (E and F) Coaxial coil arrays (metamaterials) functioning collectively for imaging limb (E) and spine (F).

Fig. 2.. EM characterizations of a CCR.

(A) Configuration of a CCR. (B) Reflection spectrum of a CCR. (C) Electric current profiles along inner and outer conductors’ surfaces at resonance mode. E-field, electric field; H-field, magnetic field. (D) Electric currents’ distribution. a.u., arbitrary units. (E) Magnetic field patterns on the coronal, sagittal, and axial cutting planes of a CCR at its resonance mode.

Fig. 3.. CCRs with frequency tunability, size customization, and self-adaptivity.

(A) Illustration of a CCR with tuning sleeve. (B) Electric field patterns under different tuning angles. (C) Reflection spectrum under different tuning angles. (D) Configuration of a CCR with multiple gaps. (E) Electric current profiles of a CCR with multiple gaps. (F) Resonance frequency variations of CCRs with either multiple gaps or a single gap as the diameter increases. (G) Configuration of a CCR loaded with a pair of diodes. (H) Nonlinear reflection spectrum. (I) Reflection coefficient of a CCR at its resonating state as a function of excitation power strength.

Fig. 4.. Form-fitting coils derived from the reconfigurations of a CCR.

(A to D) Configuration (A), reflection spectrum (B), and magnetic field patterns [(C) and (D)] of a CCR with three pairs of gaps. (E to H) The configuration (E), magnetic field pattern (F), and application scenarios [(G) and (H)] of the form-fitting coil for spine imaging. (I to L) The configuration (I), magnetic field pattern (J), and application scenarios [(K) and (L)] of the form-fitting coil for small extremities imaging. (M to O) The configuration (M), magnetic field pattern (N), and application scenario (O) of the form-fitting coil for finger imaging.

Fig. 5.. Metamaterial-enabled CCR arrays.

(A to C) The configuration (A), electric current profiles (B), and magnetic field pattern (C) of a two-turn CCR with diameter of 100 mm. (D to H) The configuration (D), reflection spectrum (E), magnetic field pattern (F), and application scenarios [(G) and (H)] of the H-metamaterial. Inset of (E): Electric current directions in each unit cell of the H-metamaterial at three resonance modes. (I to L) The configuration (I), reflection spectrum (J), magnetic field pattern (K), and application scenario (L) of the V-metamaterial. Inset of (J): Electric current directions in each unit cell of the V-metamaterial at three resonance modes.

Fig. 6.. MRI validations with phantoms for form-fitting coaxial coils.

(A and B) Experimental setups for the coaxial spine coil (A) and the Philips FlexCoverage posterior coil (B). (C to E) SNR images captured by the BC only (C), the BC combined with the coaxial spine coil (D), and the FlexCoverage posterior coil (E). (F) SNR enhancement ratio by spine coils. (G and H) Experimental setups for the coaxial extremity coil (G) and the Philips dStream small extremity coil (H). (I to K) SNR images captured by the BC only (I), the BC combined with the coaxial extremity coil (J), and the dStream extremity coil (K). (L) SNR enhancement ratio by extremity coils. (M and N) Experimental setups for the coaxial finger coil (M) and the Philips dStream HandWrist 16ch coil (N). (O to Q) SNR images captured by the BC only (O), the BC combined with the coaxial finger coil (P), and the dStream HandWrist 16ch coil (Q). (R) SNR enhancement ratio by finger coils.

Fig. 7.. MRI validations with phantoms for metamaterials enabled coil arrays.

(A and B) Experimental setups for the H-metamaterial (A) and the V-metamaterial (B) enabled coaxial coil arrays. (C and D) SNR images correspond to experimental setups illustrated in (A) and (B). (E) Comparisons of SNR enhancement ratios.

Fig. 8.. Comparisons of image quality captured by coaxial coils and their counterpart surface coil arrays.

(A) Comparisons of images captured by the BC only, the BC enhanced by the coaxial spine coil, and the FlexCoverage posterior coil. (B) Comparisons between the coaxial extremity coil and the dStream small extremity 16ch coil. (C) Comparisons between the coaxial finger coil and the dStream HandWrist 16ch coil. (D) Images captured by the H-metamaterial and the V-metamaterial enabled coil arrays.