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-T-2 suppression at 3 Tesla. Methods: An adiabatic inversion recovery-based scheme was used to null the long-T-2 signal. To minimize scan time, the signal was sampled multiple times after each inversion with variable excitation flip angles designed to yield constant short-T-2 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-T-2 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-T-2 suppressed PETRA allows imaging of bone matrix water unencumbered by long-T-2 soft tissue and pore water protons, opening up new possibilities for anatomic bone imaging at isotropic resolution and quantification in clinically practical scan times. (C) 2016 International Society for Magnetic Resonance in Medicine.