Cortical layer-dependent arterial blood volume changes: Improved spatial specificity relative to BOLD fMRI

Abstract

The spatial specificity of functional hemodynamic responses was examined by simultaneous mapping of BOLD changes and quantitative changes in cerebral arterial blood volume (ΔCBV_a) across the cortical depth in cats (n = 7) during 40-s visual stimulation. Studies were performed at 9.4 T using the recently developed, non-invasive magnetization transfer (MT)-varied gradient-echo (GE) fMRI technique to separate signals from MT-independent arterial blood and MT-dependent tissue. The highest conventional BOLD signal changes occurred at the cortical surface, where large pial veins exist, whereas the highest CBV_a changes occurred in the middle of the cortex, where T₁-weighted images show a hyperintense layer. In the middle cortical region, the average BOLD change (echo time = 20 ms) was 1.16 ± 0.45% during stimulation and − 0.59 ± 0.31% during the post-stimulus period, while the average ΔCBV_a was 0.33 ± 0.02 ml/100 g during stimulation and −0.08 ± 0.12 ml/100 g post-stimulus (post-stimulus ΔCBV_a is not statistically significant). Time-dependencies of the ΔCBV_a cortical profiles are similar to total CBV responses previously measured during visual stimulation in cats with a susceptibility contrast agent indicating, that blood volume changes mostly originate from arterial vessels. Our findings demonstrate the value of non-invasive and quantitative ΔCBV_a measurement in high-resolution MT-varied GE fMRI studies, where spatial specificity is better localized to sites of neural activity as compared with conventional GE BOLD changes.

Introduction

Most functional MRI studies have been performed using conventional blood oxygenation level-dependent (BOLD) methodology (Ogawa et al., 1990) with a spatial resolution of several millimeters. One crucial question is whether the area of activity indicated by BOLD fMRI fully corresponds with the actual site of neural activation. To examine the specificity of fMRI signals, the cortical layer model can be used. The cerebral cortex consists of six cellular layers between the pial surface and the underlying white matter, with the layers running parallel to the pial surface. The middle cortical layer in sensory areas, layer IV, contains granular cells, and has a direct input from the thalamus by thalamo-cortical projections. Moreover, layer IV has the highest density of capillary mesh and synapses, and during sensory stimulation it has the highest metabolic rate and blood flow change (Woolsey et al., 1996).

The commonly used gradient-echo (GE) BOLD technique produces signals sensitive to deoxyhemoglobin changes, resulting in signal changes heavily weighted by baseline venous blood volumes. Therefore, the highest GE BOLD signal changes often appear in areas with large veins, which can be far away from neural activation sites. For example, the highest conventional GE BOLD signal changes in cats (Zhao et al., 2006, Zhao et al., 2004), monkeys (Goense and Logothetis, 2006) and humans (Ress et al., 2007, Truong and Song, 2009) occur at the surface of the visual cortex, not in the middle cortical layer. To reduce these large vessel contributions to fMRI, the spin-echo (SE) BOLD technique at high magnetic fields has been proposed; this approach suppresses the extravascular contribution of large vessels and minimizes the intravascular venous signals by setting the echo time > T₂ of blood (Duong et al., 2003, Kim and Ugurbil, 2003, Lee et al., 1999, Yacoub et al., 2003, Zhao et al., 2004). In the cortical layer model, the highest SE BOLD signals are found within the cortex, not at the surface of the cortex (Zhao et al., 2004), indicating that SE BOLD improves the spatial specificity to sites of neural activity. Since BOLD signals are related to a mismatch between cerebral blood flow (CBF) and oxygen consumption changes, its spatial response should be broader than that of CBF. In fact, the profiles of SE BOLD signals across the cortex show that the signal difference between different cortical layers is quite small (Zhao et al., 2006). Thus, it is important to use different fMRI methodologies, each relying on a single physiological parameter, to examine the intrinsic spatial specificity of hemodynamic responses and to interpret BOLD data.

Single physiological parameter-based fMRI includes arterial spin labeling (ASL) for measuring CBF, and the susceptibility contrast agent method for detecting total cerebral blood volume (CBV_t) change. The highest CBF and CBV_t responses occur in the middle of the cortex and their profiles across the cortex are sharper than those from SE BOLD (Jin and Kim, 2008a, Zhao et al., 2006), indicating that CBF and CBV_t responses possess greater specificity for neuronal active sites. However, CBF-based fMRI provides poor sensitivity and temporal resolution, and CBV_t-based fMRI requires contrast agent injection. Based on our total and arterial CBV studies in the rat somatosensory cortex, the increased change in CBV during neural activation originates mainly from arterial rather than venous blood volume changes (Kim et al., 2007). This finding suggests that cerebral arterial blood volume (CBV_a) changes may specifically occur in the middle cortical layer, the site of neural activation, in a manner similar to what occurs with total CBV changes; validation of this possibility requires further study.

In this study, we applied the magnetization transfer (MT)-varied fMRI technique (Kim et al., 2008) to simultaneously measure stimulus-induced CBV_a changes (units of ml/100 g) and BOLD responses across the visual cortical layers in cats in an effort to evaluate the spatial specificity of functional CBV_a changes (ΔCBV_a). CBV_a represents blood volume within arterial vessels of all sizes, and includes a portion of capillaries before water exchange occurs. For ΔCBV_a measurements, the MT-varied GE fMRI method is technically simple and provides a higher temporal resolution as compared with both the MOdulation of TIssue and VEssel (MOTIVE) technique with MT-varied ASL (Kim and Kim, 2005) and the Look-Locker echo planar imaging (EPI) technique with ASL (Brookes et al., 2007). We selected the cat visual cortex model to compare data previously obtained in our laboratory by other fMRI techniques. The stimulus-induced R₂⁎ changes at varied MT level and ΔCBV_a were measured during stimulation and post-stimulation periods. We found that time-dependencies of CBV_a changes across the cortex are similar to our previous CBV_t findings, and that MT-varied GE fMRI technique is useful for obtaining quantitative CBV_a changes non-invasively with high resolution.