Ultralow-field and spin-locking relaxation dispersion in postmortem pig brain

Area of Science:

• Magnetic Resonance Imaging
• Biophysics
• Neuroimaging

Background:

• High-field Magnetic Resonance Imaging (MRI) is limited by tissue heating during spin locking.
• Understanding tissue-specific relaxation differences is crucial for optimizing MRI techniques.
• Ultralow field MRI (ULF-MRI) offers an alternative approach with potentially different contrast mechanisms.

Purpose of the Study:

• To quantitatively compare relaxation dispersion between ultralow field (ULF) and high field (7 Tesla) MRI in postmortem pig brain.
• To investigate tissue-specific differences in gray and white matter.
• To assess the potential advantages of ULF-MRI for in vivo human brain imaging.

Main Methods:

• Postmortem pig brain gray and white matter specimens were analyzed.
• T1 relaxation was measured at ultralow fields (58.7–235.0 µT) using ULF-MRI with SQUID detection.
• T1ρ relaxation was measured at 7 Tesla (5.0–235.0 µT) using a commercial MRI scanner.

Main Results:

• T1ρ at 7T was 50–100% longer than T1ULF at matched field strengths.
• T1ULF dispersion was nearly linear, while T1ρ7T dispersion was highly nonlinear.
• A subtle elbow in T1ULF dispersion at ~140 µT may indicate macromolecular local fields, suggesting different dominant relaxation mechanisms.

Conclusions:

• ULF-MRI may offer unique quantitative benefits for human brain imaging.
• ULF-MRI avoids the severe heating limitations associated with high-field spin locking.
• ULF-MRI exhibits fundamentally different sensitivity to neural tissue's macromolecular structure compared to high-field MRI.