Evaluation of k-Space Acquisition Strategies in Nonuniform B0 (NuBo) MRI.

PURPOSE: To evaluate k-space acquisition strategies for magnetic resonance imaging (MRI) in a nonuniform B0 (NuBo) field-cycling system, focusing on image quality, scan-time efficiency, robustness to B0 inhomogeneity, and RF coil bandwidth constraints.

METHODS

Three acquisition strategies were compared: single-shot turbo spin echo (SS-TSE), multi-echo spin echo (MESE) with repeated phase encoding, and spin echo single-point imaging (SE-SPI) without applied readout (RO) gradients. Experiments were performed on an open, low-field, field-cycling MRI system using a nonuniform electromagnet. Imaging performance was assessed using a phantom and biological samples under matched total scan-time conditions. RF coils with wide- and narrow-bandwidth designs were evaluated to study sensitivity trade-offs. Retrospective k-space undersampling and compressed sensing reconstruction were applied to investigate potential scan-time reduction for all acquisition strategies.

RESULTS

SE-SPI consistently achieved larger full width at half maximum (FWHM) values than SS-TSE and MESE, indicating improved edge definition. Edge sharpness was higher along the RO (PE1) direction but comparable along the PE (PE2) direction. SS-TSE provided the fastest k-space acquisition and the highest central k-space SNR but exhibited increased blurring and bandwidth-related signal loss. MESE improved edge definition over SS-TSE while requiring additional polarization cycles. SE-SPI also showed greater robustness to compressed sensing undersampling.

CONCLUSION

SE-SPI provides a robust acquisition strategy for MRI in nonuniform B0 field-cycling systems by improving image fidelity, maintaining uniform signal intensity, enabling efficient use of high-Q RF coils, and supporting compressed sensing acceleration.