Fig. 2. Intravascular and extravascular contributions to functional R₁ρ change in the cat visual cortex.

A slightly bright band (indicated by black arrows) within the gray matter (outlined in green) in high-resolution T₁-weighted EPI image (A) indicates cortical layer IV (Kim and Kim, 2011). Pixels along the white band (yellow) were chosen for quantitative analyses of cat's functional studies (B). All active pixels were overlaid in color on their respective baseline images (C-F), and the vertical grayscale bar indicates the baseline R₁ρ values (for E and F). Functional percentage signal change maps were obtained by T₂-weighted (TSL = 0, C) and T₁ρ-weighted fMRI (TSL = 50 ms and ω₁ = ~500 Hz, D) without MION during visual stimulation. Higher changes were observed in T₁ρ-weighted fMRI, indicating an increase in T₁ρ (decrease in R₁ρ). To determine the blood contribution, R₁ρ change maps obtained without (E) and with (F) 5 mg/kg MION were compared. An increase in R₁ρ at the surface of the cortex is due to a volume fraction change between tissue and CSF, while a decrease in R₁ρ is located at the middle of the cortex. (G) At the middle cortical ROI (yellow pixels in B), the functional R₂ change (ΔR₂) is nearly suppressed with MION injection, whereas ΔR₁ρ only decreases slightly.