Off-resonance magnetisation transfer contrast (MTC) MRI using fast field-cycling (FFC)

Magnetisation transfer contrast (MTC) is an important MR contrast generating mechanism to characterise the undetectable bound protons indirectly using the decreased signal intensity of the observable free protons. MTC imaging typically employs a range of off-resonance RF pre-saturation pulse with maintaining the RF magnetic field (B1) at a specified value. However, this presents a technical difficulty, particularly at low field, because the larger offset frequencies tend to be outside the bandwidth of the RF transmit system, causing B1 to vary with the frequency offset. Here, we demonstrate a novel off-resonance irradiation method using fast field-cycling which allows switching of the external magnetic field between several chosen strengths, while holding constant the RF frequency and B1 level. This permits one to avoid the problem of B1 variation as a function of frequency offset. The results obtained by this new technique are in excellent agreement with those obtained by the conventional technique.