Design and construction of an actively frequency-switchable RF coil for fast field-cycling magnetisation transfer contrast MRI

Introduction Magnetisation transfer contrast (MTC) is an important MR contrast generating mechanism to characterise the immobile macromolecular protons using an off-resonance pre-saturation RF pulse (or MT pulse). As the MT effect occurs by MT pulse irradiation, the condition of the MT pulse is the main source to produce MTC. MTC is normally implemented at a fixed magnetic field; however, it may provide valuable information to evaluate changes of the MT effect as a function of main magnetic field strength (B0) [1,2]. In order to conduct field-dependent MTC experiments, we need techniques which enable both B0 and the resonance frequency of the RF coil (f0) to shift simultaneously during only MT pulse irradiation and return to the original condition during the MR signal acquisition. Switching of B0 is achieved by the so-called fast field-cycling (FFC) technique [3,4], while switching of f0 requires a special multi-tunable RF coil. Previously, this type of coil was built with double (or triple) coils, tank circuits or varactor diodes [5-7]. Their switchable frequencies were, however, limited to two (or three) values or the range of switchable frequency was limited (particularly at low field). In this work, we designed and constructed an actively frequency-switchable RF coil. The design employs PIN diodes, and enables switching the coil’s resonance frequency between five levels without limitation of the switchable frequency range.