Qualitative and quantitative hard-tissue MRI with portable Halbach scanners

Abstract To demonstrate the feasibility of performing in-vivo imaging and quantitative relaxation mapping of soft and hard tissues using a low-cost, portable MRI scanner, and to establish the methodological foundations for zero echo time (ZTE) imaging in systems subject to strong field inhomogeneities. A complete framework for artifact-mitigated ZTE imaging at low field was developed, including: (i) RF pulse pre/counter-emphasis calibration to minimize ring-down and electronics switching time; (ii) an extension of a recent single-point double-shot (SPDS) protocol for simultaneous B₀ and B₁ mapping; and (iii) a model-based reconstruction incorporating these field maps into the encoding matrix. ZTE imaging and variable flip angle (VFA) T₁ mapping were performed on phantoms and in-vivo human knees and ankles, and benchmarked against standard RARE and STIR acquisitions. The optimized PETRA sequence produced 3D images of knees and ankles in <15 min, revealing hard tissues such as ligaments, tendons, cartilage, and bone, usually not visible with standard sequences. The extended SPDS method was used for B₀ mapping, while the VFA approach provided the first in-vivo T₁ measurements of hard tissues at B₀<0. 1 T. The proposed framework broadens the range of pulse sequences feasible in portable low-field MRI and demonstrates the potential of ZTE for quantitative and structural imaging of musculoskeletal tissues in affordable Halbach-based systems.