Spatial specificity of functional magnetic resonance imaging (fMRI) signals to sub-millimeter functional architecture remains controversial. To investigate this issue, high-resolution fMRI in response to visual stimulus was obtained in isoflurane-anesthetized cats at 9.4 T using conventional gradient-echo (GE) and spin-echo (SE) techniques; blood oxygenation-level dependent (BOLD) and cerebral blood volume (CBV)-weighted data were acquired without and with injection of 10 mg Fe/kg monocrystalline iron oxide nanoparticles (MION), respectively. Studies after MION injection at two SE times show that the T2′ contribution to SE fMRI is minimal. GE and SE BOLD changes were spread across the cortical layers. GE and SE CBV-weighted fMRI responses peaked at the middle cortical layer, which has the highest experimentally-determined microvascular volume; full-width at half-maximum was <1.0 mm. Parenchymal sensitivity of GE CBV-weighted fMRI was ∼3 times higher than that of SE CBV-weighted fMRI and ∼1.5 times higher than that of BOLD fMRI. It is well known that GE CBV-weighted fMRI detects a volume change in vessels of all sizes, while SE CBV-weighted fMRI is heavily weighted toward microvascular changes. Peak CBV change of 10% at the middle of the cortex in GE measurements was 1.8 times higher than that in SE measurements, indicating that CBV changes occur predominantly for vasculature connecting the intracortical vessels and capillaries. Our data supports the notion of laminar-dependent CBV regulation at a sub-millimeter scale.
Bibliographical noteFunding Information:
Supported by NIH (EB03375, EB03324, EB02013, NS44589, RR08079) and McKnight Foundation. We thank Drs. Hiro Fukuda, Kazuto Masamoto, Tao Jin, and Toshihiro Hayashi for insightful discussion and helpful suggestions.
Copyright 2008 Elsevier B.V., All rights reserved.
- Cortical layers
- Hemodynamic response