Ultra-low-field magnetization transfer imaging at 0.055T with low specific absorption rate

Purpose: To demonstrate magnetization transfer (MT) effects with low specific absorption rate (SAR) on ultra-low-field (ULF) MRI. Methods: MT imaging was implemented by using sinc-modulated RF pulse train (SPT) modules to provide bilateral off-resonance irradiation. They were incorporated into 3D gradient echo (GRE) and fast spin echo (FSE) protocols on a shielding-free 0.055T head scanner. MT effects were first verified using phantoms. Brain MT imaging was conducted in both healthy subjects and patients. Results: MT effects were clearly observed in phantoms using six SPT modules with total flip angle 3600 degrees at central primary saturation bands of approximate offset +/- 786Hz, even in the presence of large relative B-0 inhomogeneity. For brain, strong MT effects were observed in gray matter, white matter, and muscle in 3D GRE and FSE imaging using six and sixteen SPT modules with total flip angle 3600 degrees and 9600 degrees, respectively. Fat, cerebrospinal fluid, and blood exhibited relatively weak MT effects. MT preparation enhanced tissue contrasts in T2-weighted and FLAIR-like images, and improved brain lesion delineation. The estimated MT SAR was 0.0024 and 0.0008W/kg for two protocols, respectively, which is far below the US Food and Drug Administration (FDA) limit of 3.0W/kg. Conclusion: Robust MT effects can be readily obtained at ULF with extremely low SAR, despite poor relative B-0 homogeneity in ppm. This unique advantage enables flexible MT pulse design and implementation on low-cost ULF MRI platforms to achieve strong MT effects in brain and beyond, potentially augmenting their clinical utility in the future.