An Extended Theory and Method for Double-Tuned Conformal Birdcage Coil in MRI

• Published in: IEEE transactions on instrumentation and measurement Vol. 73; pp. 1 - 11
• Main Authors: Xie, Junyao, Guan, Jinglong, Yao, Kaiwen, Liu, Min, You, Xueqiu, Sun, Huijun, Chen, Zhong
• Format: Journal Article
• Language: English
• Published: New York IEEE 2024; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Coils; Conductors; conformal birdcage coil; Deformation; H/¹⁹F double-tuned; high sensitivity; Inductance; joint tuning method; Magnetic resonance imaging; magnetic resonance imaging (MRI); Numerical analysis; Radio frequency; Runways; Sensitivity; Signal to noise ratio; Tuning
• ISSN: 0018-9456; 1557-9662
• DOI: 10.1109/TIM.2023.3342226

A better fit to the detection area has always been one of the magnetic resonance imaging (MRI) coil development goals. For many application scenarios and organism detection sites, conformal birdcage coil (CBC) has a better attachment and higher filling factor than conventional circular birdcage coil, improving the MRI image signal-to-noise ratio effectively. However, the structural asymmetry of CBC causes the original resonant mode of the circular birdcage coil to split, with a bigger frequency difference as the deformation increases, affecting the strength and uniformity of the corresponding radio frequency (RF) fields significantly in an indistinct way. In this article, we present an extended-expression form of noncircular birdcage coils, which is more general than the existing research results. Based on the extended expression, we propose a more conformal, runway-shaped CBC structure that can achieve higher sensitivity. We also introduce a new design method for <inline-formula> <tex-math notation="LaTeX">^{1}\text{H}/^{19}\text{F} </tex-math></inline-formula> double-tuned CBC, which utilizes mode splitting generated by deformation without inserting additional tuning networks. This method employs numerical analysis and simulation to design the coil structure and tuning, allowing both 1 H and 19 F resonant modes to achieve optimized RF field performance simultaneously. To verify the method, a runway-shaped <inline-formula> <tex-math notation="LaTeX">^{1}\text{H}/^{19}\text{F} </tex-math></inline-formula> double-tuned CBC with specific parameters applicable to a 7-T small-bore MRI system is developed. The effectiveness and applicability of our proposed theories, structures, and methodologies have been fully validated through homogeneous phantom and subject MRI experiments conducted with this coil. Compared to commercial probes with coils of similar physical dimensions, CBC probes exhibit good usability, conformability, higher sensitivity, and better uniformity for flat tissue structures.