In vivo ¹H₂O T₂^† measurement in the human occipital lobe at 4T and 7T by Carr-Purcell MRI: Detection of microscopic susceptibility contrast

A high-resolution spin-echo imaging method is presented (called CP-LASER) which exploits the spin refocusing capability of an adiabatic Carr-Purcell (CP) pulse sequence to measure apparent ¹H₂O transverse relaxation (T₂^†) and generate contrast based on microscopic tissue susceptibility. High-resolution CP-LASER images of the human occipital lobe were acquired at four different echo times from six subjects at 4T and eight subjects at 7T to investigate the effect of magnetic field strength (B₀) and the CP interpulse time (τ_cp) on T₂^†. Susceptibility contrast was identified and T₂^† was quantified for long τ_cp (> 10 ms) and short τ_cp (7 ms at 4T and 6 ms at 7T) in gray matter, white matter, and cerebral spinal fluid. The ¹H₂O relaxation rate constants (1/T₂^†) of gray and white matter each increased approximately linearly with field strength and T₂^† was inversely related to τ_cp. The average T₂^† value of gray matter was 19% and 9% smaller than that of white matter at 4T and 7T, respectively. These results are consistent with higher levels of compartmentalized ferritin and increased blood volume in gray matter compared to white matter in this region of the brain.