Phase-encode order and its effect on contrast and artifact in single-shot RARE sequences

Area of Science:

• Magnetic Resonance Imaging
• Biomedical Engineering
• Physics

Background:

• Single-shot rapid acquisition relaxation enhanced (RARE) sequences offer fast imaging capabilities.
• Tissue contrast in MRI is influenced by the order of phase-encoding steps.
• Imaging short T2 species presents a challenge in conventional MRI.

Purpose of the Study:

• To investigate the impact of echo ordering in single-shot RARE sequences on tissue contrast.
• To establish the relationship between zero phase (ZP) echo timing and transverse relaxation times (T2).
• To characterize and predict banding artifacts in ultrafast echo planar imaging.

Main Methods:

• Utilized a 128-echo single-shot RARE sequence with variable phase-encode ordering.
• Performed qualitative imaging of human brain and quantitative analysis using NiCl2 phantoms.
• Employed a 128-echo Carr-Purcell-Meiboom-Gill (CPMG) sequence for T2 quantification.
• Experimentally demonstrated and simulated banding artifacts in a rectangular phantom.

Main Results:

• Manipulating echo order significantly alters tissue contrast in single-shot RARE imaging.
• The timing of the ZP echo correlates with the effective echo time (TE) and T2 values.
• Banding artifacts were observed and their dependence on sequence parameters and T2 was quantified.
• Simulations accurately predicted artifact characteristics and tissue overlap.

Conclusions:

• Optimized echo ordering in RARE sequences enables imaging of short T2 tissues.
• The ZP echo timing serves as a reliable indicator of T2 in this imaging modality.
• Understanding banding artifacts is crucial for accurate interpretation of ultrafast echo planar images.