8-Channel RF head coil of MRI with automatic tuning and matching

The Radio Frequency (RF) coil is an essential part of hardware in Magnetic Resonance Imaging (MRI) systems and microstrip transmission line (TEM) RF coils have been widely used for high-field applications to excite and receive the nuclear magnetic resonance signals. These coils are typically terminated by capacitors. On one end two variable capacitors, the matching capacitor (Cm) and tuning capacitor (Ct), and on the other end a fixed value capacitor (Cf) form a capacitively tuned, matched, and foreshortened half-wave resonator. These resonant coil elements have narrow bandwidth due to their high quality factors (Qs). High transmit power efficiency and receive Signal-to-Noise Ratio (SNR) depend on a well-tuned and matched coil element. Conversely, the variable body loading of these coil elements can adversely impact both tuning and matching, and therefore power efficiency and SNR of transmit/receive signals. Loading effects are problematic and a manual tuning is a time-consuming adjustment. It, however, is the only method to avoid loading problems at present. This study demonstrates the automatic frequency tuning and impedance matching technique for the optimal coil efficiency. An 8-channel RF head coil has been successfully built and tested with a fully automatic tuning and matching function at 7tesla (T). It offers real-time fast operation (less than 550ms per channel) and accurate frequency tuning and impedance matching (less than -20dB in the reflected coefficient, S₁₁, at the Larmor frequency) resulting in the high power efficiency (4% ~ 21% improvement for each channel).

I. INTRODUCTION

Ultra-high magnetic fields (7T and higher) MRI systems are advantageous because they increase SNR which can be used to improve the spatial resolution in MR images. The SNR linearly correlates with the field strength. So the traditional clinical 1.5T magnets are being replaced by 3T and research magnets of 7T, 9.4T and 11.74T are being used for both animals and the human [1]–[4]. The higher field strengths linearly increase the frequency of the nuclear precession by the Larmor frequency. This can complicate the design of RF coils at increased frequencies, such as 297MHz for 7T, 400MHz for 9.4T and 500MHz for 11.74T.