Design and construction of an actively frequency-switchable RF coil for field-dependent Magnetisation Transfer Contrast MRI with fast field-cycling

Magnetisation Transfer Contrast (MTC) is an important MR contrast-generating mechanism to characterise the MR-invisible macromolecular protons using an off-resonance pre-saturation RF irradiation pulse (or MT pulse). MTC MRI is normally implemented at a fixed magnetic field; however, it may be useful to evaluate changes of the MT effect as a function of external magnetic field strength (B0). In order to conduct field-dependent MTC experiments with a single MR system, two techniques are crucially needed. B0 should be able to be switched between levels during irradiation of the MT pulse. At the same time, the resonance frequency of the RF coil (f0) should also be able to be shifted to the corresponding value. Switching B0 is attained by the fast field-cycling technique, while in order to switch f0, a specially designed multi-tunable RF coil is required. Here, we designed and constructed an actively frequency-switchable RF coil for frequencies at and below 2.5 MHz. The design employed PIN diodes, and enabled switching f0 between five different values, with excellent impedance matching (approximately −37 dB S11 reflection) and Q-factor of about 100 at each configuration.