INTRODUCTION

The portable Halbach-array-based MRI knee scanner with less homogeneous B₀ field¹ has built-in gradient which reduces the hardware complexity by reducing the number of gradient coils and amplifiers. However, the system requires to have RF coils with wider bandwidth to work with the B₀ inhomogeneity, and high sensitivity to compensate a low B₀ field strength_. Solenoid coils with cylindrical profile² or tapered profile^3,4 were designed to achieve high B₁ sensitivity and improved homogeneity. However, traditional solenoid RF coils are narrow band in nature which necessitates the use of resistor in the tuning / matching circuit to increase the bandwidth although additional loss is introduced³. There is a need to explore RF coil designs with widened bandwidth without much compromise in sensitivity and homogeneity.

In the literature, single-turn crossed elliptical RF coil was proposed to provide better access for lower neck imaging⁵. However, multi-turn crossed ellipses and its potential to increase the bandwidth have not been explored earlier since such need has only emerged with the recent developments in portable MRI. Hence, in this work, we propose a crossed-elliptical RF volume coil with a winding pattern consisting of multiple crossed elliptical turns symmetrically distributed in the azimuthal direction, to achieve an increased bandwidth with improved B₁ homogeneity and without much compromise of field intensity within a cylindrical volume of a diameter and a length of both 95mm and being suitable to be used in the portable MRI knee scanners. It can be scaled to other body-part dedicated scanners.

METHODS

The proposed coil has N crossed elliptical loops that are symmetrically distributed in the azimuthal direction with an angular pitch (p_ϕ) as shown in Fig.1. The working frequency is 2.84MHz for the Halbach array that has an average field of 67mT¹. The ratio of the coil length (L) and the diameter (D) was set to unity to maintain symmetry in the structure, L=D=105mm to fit in the Halbach array, and for a cylindrical field of view (FoV) with a diameter and a length of both 95mm, $$$\varnothing$$$95L95mm. In this design, N=4, and thus p_ϕ is 45^o. The diameter of the wire is 1mm. The proposed RF volume coil was simulated using frequency domain solver in CST microwave design studio (2021) and Sim4Life⁶. For comparison, solenoid coils with a comparable fractional bandwidth (FBW) (N=6), or a comparable B₁^avg (N=7), and with the same L and D were simulated.

RESULTS AND DISCUSSION

The design parameters, and the simulated performances (FBW, Bandwidth(BW), Q, B₁^avg, inhomogeneity(IH), inductance(L) and resistance(R) ) at 2.84MHz($$$f_0$$$) are tabulated in Fig.2(a) and (b), respectively. Fig.2(c) shows the S11s of the coils under consideration. IH is obtained from root mean square deviation (σ) relative to B₁^avg (σ/B₁^avg×100) within the defined FoV. As shown in Fig.2(b), when a solenoid has a comparable BW (N=6) as that of the proposed coil, B₁^avg decreases and B₁-IH is worsen within the considered 3D FoV. Whereas, at N=7 when the solenoid has a comparable B₁^avg, the BW is reduced to 0.43% (15.7% reduction compared to the proposed coil) and meanwhile the B₁-IH stays high. The comparison to the solenoids with the same L and D shows that the proposed coil has widened BW while keeping the field strength high and the B₁-IH low.The simulated B₁ distributions of the considered coils at 2.84MHz on the xy-planes (z=0,10,20,30mm) and the xz-plane (y=0) within the FoV are shown in Fig.3. Comparing the field distribution of the proposed coil across the different xy-planes at row 1, B₁^avg is spread uniformly with only a difference of 1.8µT between z = 0 and 30mm whereas both solenoid coils show a reduction of about 6.0µT in B₁^avg between the two xy-planes. Comparing the field distribution in the xz-plane (y=0), the proposed coil shows about 5-6% improved homogeneity compared to the two solenoids.
Fig. 4 shows the simulated B₁ distributions at the two cutoff frequencies $$$(f_0±BW/2)$$$ of the proposed coil ($$$f_L$$$=2.833MHz_, $$$f_U$$$=2.847MHz) and $$$f_0$$$, on the central xy- and xz-plane, for the three coils under comparison. As shown in Fig. 4, the proposed coil shows a reduction of about 5.7µT when moving from the center to the sides of the band and a stable IH of 6-7%. The six-turn solenoid shows a lower B₁^avg, and a higher B₁^avg reduction crossing the band while the IH is at 12% moving along the z-direction. For the seven-turn solenoid, at all frequencies, B₁^avg on the xy-plane is higher than those of the proposed coil; at 2.84MHz, B₁^avg on the xz-plane is comparable to those of the proposed coil, and becomes smaller at $$$f_L$$$ and $$$f_U$$$. Moreover, comparing these two solenoids, the seven-turn one has a higher B₁^avg reduction of 8.6µT at the same band span, and the IH is similarly at 12% along the z-direction as the six-turn one. Based on the observations, the proposed coil shows higher and more uniform B₁ across $$$f_L$$$ and $$$f_U$$$ in the FoV compared to the two solenoids.

CONCLUSION

The proposed crossed-elliptical volume coil shows a widened BW and a good balance in B₁ field strength and homogeneity in the FoV for knee scans ($$$\varnothing$$$95L95mm). It is promising for the portable MRI scanners that have less homogeneous B₀.

Acknowledgements

No acknowledgement found.