Abstract
BackgroundThe specificity of gradient echo (GE)-BOLD laminar fMRI activation profiles is degraded by intracortical veins that drain blood from lower to upper cortical layers, propagating activation signal in the same direction. This work describes an approach to obtain layer specific profiles by deconvolving the measured profiles with a physiological Point Spread Function (PSF). New methodIt is shown that the PSF can be characterised by a TE-dependent peak to tail (p2t) value that is independent of cortical depth and can be estimated by simulation. An experimental estimation of individual p2t values and the sensitivity of the deconvolved profiles to variations in p2t is obtained using laminar data measured with a multi-echo 3D-FLASH sequence. These profiles are echo time dependent, but the underlying neuronal response is the same, allowing a data-based estimation of the PSF. ResultsThe deconvolved profiles are highly similar to the gold-standard obtained from extremely high resolution 3D-EPI data, for a range of p2t values of 5–9, which covers both the empirically determined value (6.8) and the value obtained by simulation (6.3). -Comparison with Existing Method(s) Corrected profiles show a flatter shape across the cortex and a high level of similarity with the gold-standard, defined as a subset of profiles that are unaffected by intracortical veins. ConclusionsWe conclude that deconvolution is a robust approach for removing the effect of signal propagation through intracortical veins. This makes it possible to obtain profiles with high laminar specificity while benefitting from the higher efficiency of GE-BOLD sequences.
Highlights
Within the iso-cortex the type and density of brain cells varies following a laminar pattern
Corrected profiles show a flatter shape across the cortex and a high level of similarity with the goldstandard, defined as a subset of profiles that are unaffected by intracortical veins
Equation 1 where yi is the blood oxygenation level dependent (BOLD) response measured at bin i, peaki is the peak value of the Point Spread Function (PSF) measured at bin (BOLD response directly related to the local neuronal response), spreadi,k is the spread of the PSF
Summary
Within the iso-cortex the type and density of brain cells varies following a laminar pattern. The most widely used method in fMRI measures the Gradient Echo (GE) based blood oxygenation level dependent (BOLD) signal. In layer-specific fMRI the spatial specificity of the measured BOLD profile is degraded by the contribution from emerging or intracortical veins. These are the veins that are oriented perpendicular to the pial surface and drain blood uni-directionally from lower to upper layers. The specificity of gradient echo (GE)-BOLD laminar fMRI activation profiles is degraded by intracortical veins that drain blood from lower to upper cortical layers, propagating activation signal in the same direction. This work describes an approach to obtain layer specific profiles by deconvolving the measured profiles with a physiological Point Spread Function (PSF)
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