Blood oxygenation level‐dependent functional MRI signal turbulence caused by ultrahigh spatial resolution: numerical simulation and theoretical explanation. (28th August 2012)

• Record Type: Journal Article
• Authors: Chen, Zikuan; Chen, Zeyuan; Calhoun, Vince
• Is Part Of: NMR in biomedicine. Volume 26:Number 3(2013:Mar.)
• Page Start: 248
• Page End: 264
• Publication Date: 2012-08-28
• Subjects: BOLD fMRI -- BOLD signal simulation -- intravoxel dephasing -- multiresolution -- phasor turbulence -- fieldmap histogram -- intravascular and extravascular partition -- voxel symmetry; Nuclear magnetic resonance -- Periodicals; Magnetic Resonance Spectroscopy -- Periodicals; 574
• DOI: 10.1002/nbm.2842
• Languages: English
• ISSNs: 0952-3480

Abstract

Abstract : High‐spatial‐resolution functional MRI (fMRI) can enhance image contrast and improve spatial specificity for brain activity mapping. As the voxel size is reduced, an irregular magnetic fieldmap will emerge as a result of less local averaging, and will lead to abnormal fMRI signal evolution with respect to the image acquisition TE. In this article, we report this signal turbulence phenomenon observed in simulations of ultrahigh‐spatial‐resolution blood oxygenation level‐dependent (BOLD) fMRI (voxel size of less than 50 × 50 × 50 µm 3 ). We present a four‐level coarse‐to‐fine multiresolution BOLD fMRI signal simulation. Based on the statistical histogram of an intravoxel fieldmap, we reformulate the intravoxel dephasing summation (a form of Riemann sum) into a new formula that is a discrete Fourier transformation of the intravoxel fieldmap histogram (a form of Lebesgue sum). We interpret the BOLD signal formation by relating its magnitude (phase) to the even (odd) symmetry of the fieldmap histogram. Based on multiresolution BOLD signal simulation, we find that the signal turbulence mainly emerges at the vessel boundary, and that there are only a few voxels (less than 10%) in an ultrahigh‐resolution image that reveal turbulence in the form of sparse point noise. Our simulation also shows that, for typical human brain imaging of the cerebral cortex with millimeter resolution, TE < 30 ms and B 0 = 3 T, we are unlikely to observe BOLD signal turbulence. Overall, the main causes of voxel signal turbulence include a high spatial resolution, high field, long TE and large vessel. Copyright © 2012 John Wiley & Sons, Ltd. Abstract : As spatial resolution increases from millimeters to several tens of micrometers, the intravoxel dephasing signals reveal turbulent evolution (nonspiraling and rapid looping), manifesting as sparse image noise. By reformulating the intravoxel dephasing summation into a discrete Fourier transform of the intravoxel field histogram, we find the relationships between the complex signal and the field symmetry. The factors for signal turbulence consist of high spatial resolution, high field, long TE and voxel location at the vascular boundary.