By Karl Stupic
At low magnetic field strengths, the correspondingly low frequencies, present challenges that are largely ignorable at high fields and high frequencies. Because of the low frequency required for radiofrequency (RF) excitation, the quality factor (Q) of the coil is incredibly important as rising and falling edges of the RF pulse will be distorted. These distortions impact the overall validity of measurements particularly in sequences where pulse shape, length, amplitude, and phase stability are important such as relaxation. Recent work by M. Martin et al. has demonstrated the impact on relaxation measurements by various coil Q, however signal-to-noise ratio (SNR) penalties are taken by reducing the Q to improve RF pulse fidelity. As SNR already suffers at low field strengths, further decreases in SNR can become challenging as signal averaging to regain loses consumes time.
Presented here is an actively controlled RF coil circuit with two different Q circuits. This circuit design allows for the use of a low Q circuit for excitation, therefore maintaining the desired RF fidelity, and a high Q circuit for detection to maintain a higher SNR. Typical Q-switching has been done to reduce ring down time in RF coils or to actively detune a coil in separated transmit / receive coil systems. Our goal in this work has been to maintain the highest SNR possible while ensuring RF fidelity for valid, quantitative measurements.