Laminar Signal Change of BOLD and VASO in Human Visual Cortex at 7T
Atena Akbari1, Saskia Bollmann1, Markus Barth1
1The University of Queensland, Brisbane, Queensland
Sub-millimeter resolution functional magnetic resonance imaging (fMRI) has enabled spatial mapping of the brain response in a layer-dependent manner . While the blood-oxygen-level-dependent (BOLD) contrast is most used for fMRI due to its high sensitivity, it has limited spatial specificity. This limitation has been the main motivation for developing the non-BOLD contrast mechanisms. Vascular-space-occupancy (VASO)  fMRI is based on nulling the blood signal and reflects blood volume changes during activation, which has shown superior spatial specificity compare to BOLD . The majority of laminar-fMRI studies were focused on motor cortex [4, 5] with only a few examining VASO in human visual cortex. V1 seems more challenging for VASO , e.g. as it is very thin and highly convoluted. In this study, we examined the signal changes of BOLD and VASO signals in the human visual cortex using sub-millimeter resolution.
Five volunteers participated in our study after having given written informed consent. Data were acquired using SS-SI VASO sequence  on a MAGNETOM 7T whole-body scanner (Siemens Healthcare, Erlangen, Germany) with a 32-channel head coil (Nova Medical, Wilmington, US). The human visual (V1) cortex was imaged and gradient-echo (GRE) BOLD and VASO images were obtained in an interleaved fashion with the following parameters: in-plane resolution=0.8 mm, slice thickness=1 mm, TR/TI/TE = 2277/650/25 ms, GRAPPA = 2, 3D EPI readout, number of slices = 22, number of measurement = 550. A black and white random noise pattern was used as the visual stimulation paradigm in 18 blocks of 30 s length in three runs. Anatomical images were obtained with an isotropic resolution of 0.75 x 0.75 x 0.75 mm3 using MP2RAGE. BOLD and VASO images of each run were realigned and motion corrected using SPM 12 (Wellcome Trust Centre, University College London, UK). Nulled images (VASO) were normalized by the non-nulled ones (BOLD) to correct for the introduced BOLD effect . A General Linear Model was applied and voxels with a significant level of p<0.05 (uncorrected) are visualized in Figure 1. EPI T1-weighted images were used for distinguishing the CSF/GM and GM/WM borders in V1. Laminar analysis was conducted using a 3D grid of layers with the open software package LAYNII .
Fig. 1 depicts the activation map in V1 in a participant. As expected, the BOLD signal covers a larger area as it is dominated by the blood oxygenation change in surface vessels, while CBV signal change is more confined to grey matter areas. The laminar profiles show clear differences, with BOLD signal change increasing linearly towards the GM/CSF border (weighted towards the site of large veins) and a peak of the VASO signal change within GM, indicating a higher spatial specificity of the VASO measurement compared to BOLD (Fig. 1C).
In conclusion, we showed that laminar signal changes of BOLD and VASO can be obtained in human primary visual cortex with sub-millimeter resolution (0.8 mm). Results indicate that the spatial specificity of VASO signal is superior compared to BOLD, and the applicability of laminar-fMRI VASO for resolving the cortical-depth dependent neuronal activity.
Brain Stimulation Methods:
Non-Invasive Stimulation Methods Other
BOLD fMRI 2
Non-BOLD fMRI 1
Modeling and Analysis Methods:
fMRI CONTRAST MECHANISMS
Other - Vascular space occupancy (VASO)
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2. Lu, H., et al., Functional magnetic resonance imaging based on changes in vascular space occupancy. Magnetic Resonance in Medicine, 2003. 50(2): p. 263-274.
3. Huber, L., Mapping Human Brain Activity by Functional Magnetic Resonance Imaging of Blood Volume. 2014.
4. Guidi, M., et al., Lamina-dependent calibrated BOLD response in human primary motor cortex. Neuroimage, 2016. 141: p. 250-261.
5. Huber, L., et al., High-resolution CBV-fMRI allows mapping of laminar activity and connectivity of cortical input and output in human M1. Neuron, 2017. 96(6): p. 1253-1263. e7.
7. Huber, L., et al., Slab‐selective, BOLD‐corrected VASO at 7 Tesla provides measures of cerebral blood volume reactivity with high signal‐to‐noise ratio. Magnetic resonance in medicine, 2014. 72(1): p. 137-148.