Filling the dead-time gap in zero echo time MRI: Principles compared.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biophysics

Background:

• Short coherence lifetimes (T2 or T2*) in tissues pose challenges for efficient MRI.
• Zero Echo Time (ZTE) techniques, including algebraic ZTE, PETRA, and WASPI, are used for rapid MRI of such tissues.
• A key challenge for ZTE methods is reconstructing central k-space data missed during the dead time after radiofrequency excitation.

Purpose of the Study:

• To directly compare the performance of algebraic ZTE, PETRA, and WASPI.
• To specifically evaluate their behavior with ultra-short-lived spins (ultra-short T2*).

Main Methods:

• Acquisition and reconstruction strategies were aligned for a direct comparison of the three ZTE techniques.
• Image quality and performance with short T2* were assessed using point spread functions, 3D simulations, and phantom/bone samples.
• Samples with both short (<1 ms) and ultra-short (<100 μs) T2* values were utilized.

Main Results:

• Algebraic ZTE is effective for smaller k-space gaps (up to ~3 Nyquist dwells).
• PETRA demonstrates robust imaging with minimal image quality compromise, even at larger k-space gaps.
• WASPI may be susceptible to artifacts when imaging species with ultra-short T2*.

Conclusions:

• All techniques can produce artifact-free short T2* MRI when k-space gaps are small (<4 dwells) and T2* is significantly longer than the dead time.
• When these conditions are not met, PETRA generally offers superior image quality compared to WASPI.
• Careful consideration of technique limitations is necessary for optimal short T2* MRI, particularly with challenging samples.