16 March 2011 Multiresolution voxel decomposition of complex-valued BOLD signals reveals phasor turbulence
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Abstract
High spatial resolution functional MRI (fMRI) technology continues to enable smaller voxel sizes, providing details about neuronal activity in terms of spatial localization and specificity. The BOLD contrast mechanism can be examined by numerical simulation. By representing a complex timeseries with a dynamic phasor in a polar coordinate system, a complex-valued BOLD signal manifests as an inward spiral: the radial distance represents the signal amplitude and the polar angle the signal phase angle. For normal fMRI settings (millimeter resolution, 3T main field, and 30ms relaxation time), the BOLD phasors usually evolve in a form of inward spiraling. Under some extreme parameter settings (high resolution, high field, or long relaxation time), the phasors may become turbulent (i.e. exhibit disordered spiraling). In this paper, we will report on a BOLD phasor turbulence phenomenon resulting from coarse-to-fine multiresolution voxel decomposition. In our implementation, a vasculature-laden voxel (320×320×320 micron3) is decomposed into a sequence of subvoxels (160×160 ×160, 80×80×80, 40×40×40, 20×20×20 micron3). Our simulations reveal the following phenomena: 1) Ultrahigh spatial resolution (several tens of microns, e.g. 20micron) BOLD fMRI may cause signal turbulence, primarily occurring at vessel boundaries; 2) The intravascular signal is prone to turbulence but its contribution to the voxel signal is greatly suppressed by the blood volume fraction; 3) There is no signal turbulence under small angle condition; 4) For millimeter-resolution fMRI with small relaxation time (< 30ms), signal turbulence is unlikely to occur. We explain the high-resolution BOLD signal turbulence from the perspective of the emergence of BOLD field irregularity.
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Zikuan Chen, Zikuan Chen, Zeyuan Chen, Zeyuan Chen, Vince Calhoun, Vince Calhoun, } "Multiresolution voxel decomposition of complex-valued BOLD signals reveals phasor turbulence", Proc. SPIE 7961, Medical Imaging 2011: Physics of Medical Imaging, 79613Y (16 March 2011); doi: 10.1117/12.877295; https://doi.org/10.1117/12.877295
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