Cortical layer-dependent BOLD and CBV responses measured by spin-echo and gradient-echo fMRI: Insights into hemodynamic regulation Fuqiang Zhao, a Ping Wang, a Kristy Hendrich, a Kamil Ugurbil, b and Seong-Gi Kim a, * a Department of Neurobiology, University of Pittsburgh, Pittsburgh, PA 15203, USA b Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN 55455, USA Received 28 July 2005; revised 6 November 2005; accepted 10 November 2005 Available online 18 January 2006 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), respec- tively. Studies after MION injection at two SE times show that the T 2 V 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 microvascu- lar 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. D 2005 Elsevier Inc. All rights reserved. Keywords: fMRI; Hemodynamic response; BOLD; CBV; Cortical layers; Microvessels Introduction Functional magnetic resonance imaging (fMRI) techniques have been a method of choice for visualizing neural activity in humans. To date, most fMRI studies have been performed using conventional blood oxygenation level-dependent (BOLD) metho- dology (Ogawa et al., 1990) with a spatial resolution of several millimeters. However, it is not clear whether neural activity- induced vascular responses are specific to sub-millimeter func- tional structures. To obtain high-resolution fMRI, the imaging signals should be relatively specific to parenchyma with reduced sensitivity to large vessels. This can be achieved by various data acquisition approaches, including contrast agent administration (Kennan et al., 1998; Mandeville et al., 1998, 2001; van Bruggen et al., 1998) and/or spin-echo (SE) data acquisition at high magnetic fields (Ogawa et al., 1993; Boxerman et al., 1995; Lee et al., 1999, 2002; Kim and Ogawa, 2002). Injection of monocrystalline iron oxide nanoparticles (MION) as an intravas- cular contrast agent induces a susceptibility effect in and around the vasculature and MION therefore behaves as a plasma blood volume tracer (Kennan et al., 1998; Mandeville et al., 1998, 2001; van Bruggen et al., 1998). This cerebral blood volume (CBV)- weighted fMRI approach has been used to improve the spatial localization of fMRI signals, where the highest gradient-echo (GE) CBV change was observed at the middle of the rat somatosensory cortex, while the highest GE BOLD signal was observed at the surface of the cortex (Mandeville and Marota, 1999; Lu et al., 2004). In our laboratory, a similar result was seen in the cat visual cortex at 4.7 T (Harel et al., 2002b). To characterize the susceptibility effects on relaxation of GE and SE tissue signals, a number of analytical biophysical models have been developed (Ogawa et al., 1993; Kennan et al., 1994; Weisskoff et al., 1994; Yablonskiy and Haacke, 1994; Kiselev and Posse, 1999; Jensen and Chandra, 2000), and also Monte Carlo simulations have been performed (Ogawa et al., 1993; Boxerman et al., 1995). It should be noted that the intravascular BOLD signal 1053-8119/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2005.11.013 * Corresponding author. 3025 East Carson Street, Pittsburgh, PA 15203, USA. Fax: +1 412 383 6799. E-mail address: kimsg@pitt.edu (S.-G. Kim). Available online on ScienceDirect (www.sciencedirect.com). www.elsevier.com/locate/ynimg NeuroImage 30 (2006) 1149 – 1160