Microsoft Word - ISMRM2009-001790.DOC

Introduction The use of fast field-cycling (FFC) with MRI has increased over recent years. The ability to change the external magnetic field (B0) during MR data acquisition affords access to new contrast mechanisms. In this work, we have applied the FFC technique to magnetisation transfer contrast (MTC) imaging experiments. A continuous-wave off-resonance irradiation pulse (or MT pulse) is widely employed in MTC imaging [1] to saturate semisolid protons, without directly affecting mobile protons. In order to achieve this conventionally, a constant RF magnetic field (B1) is required with varying RF offset frequency. However, B1 tends to decrease significantly with increasing offset frequency, particularly at low field, due to the limited bandwidth of the RF transmit system, in turn requiring B1 correction [2]. Moreover, the range of offset frequencies available may be limited. Here, we demonstrate an alternative off-resonance MTC method using the FFC technique which makes it possible to counter these complications. Essentially, the MT pulse is applied at constant frequency, but B0 is altered (using FFC) in order to achieve offresonance irradiation. The key technical feature of this new method is to switch the external magnetic field between levels (B0 to B0e) for the duration of the MT irradiation, then – crucially – back to B0 for signal acquisition. Therefore it provides the same off-resonance effect as the conventional MTC technique, but without the necessity for B1 correction, and with effectively no limitation on the resonance offset (actually the field offset).