Selective in vivo bone imaging with long-T₂ suppressed PETRA MRI

Purpose: To design and evaluate an optimized PETRA (point-wise encoding time reduction with radial acquisition) sequence with long-T₂ suppression at 3 Tesla. Methods: An adiabatic inversion recovery-based scheme was used to null the long-T₂ signal. To minimize scan time, the signal was sampled multiple times after each inversion with variable excitation flip angles designed to yield constant short-T₂ signal amplitude. The excitation pulses were phase-modulated, allowing for increased flip angle and higher signal-to-noise ratio (SNR). A fast, noniterative image reconstruction algorithm was designed to minimize image artifacts due to nonuniform excitation profile. Results: Phase-modulated pulse excitation, along with the noniterative reconstruction algorithm, allows the use of larger radiofrequency pulse flip angles, resulting in effective suppression of long-T₂ protons and improved image SNR without causing image artifacts. Midtibia images representative of collagen-bound water yielded SNR of 15 at 1-mm isotropic resolution in 6.5 minutes with a standard extremity coil. Further, the technology is shown to be suited for generating multi-angle projection images of bone akin to X-ray images displaying subtle anatomic detail. Conclusion: Optimized long-T₂ suppressed PETRA allows imaging of bone matrix water unencumbered by long-T₂ soft tissue and pore water protons, opening up new possibilities for anatomic bone imaging at isotropic resolution and quantification in clinically practical scan times. Magn Reson Med 77:989–997, 2017.