Qualitative and quantitative hard-tissue MRI with portable Halbach scanners

TL;DR

This paper demonstrates the feasibility of in vivo imaging and quantitative relaxation mapping of soft and hard tissues using a low-cost, portable MRI scanner with zero echo time imaging, enabling detailed musculoskeletal imaging in affordable systems.

Contribution

It introduces a comprehensive framework for artifact-free ZTE imaging at low field, including calibration, field mapping, and model-based reconstruction, enabling quantitative imaging of hard tissues in portable MRI.

Findings

Produced 3D images of ligaments, tendons, cartilage, and bone within 15 minutes.

Enabled first in vivo T1 measurements of hard tissues at low magnetic field.

Validated the approach against standard MRI sequences.

Abstract

Purpose: 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 affected by strong field inhomogeneities. Methods: A complete framework for artifact-free ZTE imaging at low field was developed, including: (i) RF pulse pre/counteremphasis 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. Results: The optimized PETRA sequence produced 3D images of knees and ankles within clinically compatible times (< 15 min), revealing hard tissues such as ligaments, tendons, cartilage, and bone that are invisible in spin-echo sequences. The extended SPDS method enabled accurate field mapping, while the VFA approach provided the first in vivo T1 measurements of hard tissues at B0 < 0.1 T. Conclusions: 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.