Sources of phase changes in BOLD and CBV‐weighted fMRI

Abstract

Phase changes in blood oxygenation-level dependent (BOLD) fMRI have been observed in humans; however, their exact origin has not yet been fully elucidated. To investigate this issue, we acquired gradient-echo (GE) BOLD and cerebral blood volume (CBV)-weighted fMRI data in anesthetized cats during visual stimulation at 4.7T and 9.4T, before and after injection of a superparamagnetic contrast agent (monocrystalline iron oxide nanoparticles, MION), respectively. In BOLD fMRI, large positive changes in both magnitude and phase were observed predominantly in the cortical surface area, where the large draining veins reside. In CBV-weighted fMRI, large negative changes in both magnitude and phase were detected mainly in the middle cortical area, where the greatest CBV change takes place. Additionally, the phase change was temporally correlated with the magnitude response and was linearly dependent on the echo time (TE), which cannot be explained by the intravascular (IV) contribution and functional temperature change. Phase changes with the opposite polarity were also observed in the regions around the dominant phase changes. These phase Functional magnetic resonance imaging (fMRI) based on blood oxygenation level-dependent (BOLD) (1-3) signal has become a routine tool for studying the brain, particularly in human subjects. As an alternative to BOLD, cerebral blood volume (CBV)-weighted fMRI can be performed with the use of superparamagnetic exogenous intravascular (IV) contrast agents, such as monocrystalline iron oxide nanoparticles (MION), in animal models (4 -7). The physical source of both the BOLD and the CBV-weighted fMRI signals is the change in field inhomogeneity generated by the magnetic susceptibility effect in blood due to paramagnetic deoxyhemoglobin or exogenous contrast agents (1). During brain activation, changes in deoxyhemoglobin and blood volume modulate the field inhomogeneity inside and outside blood vessels and thus the spins' coherence, which in turn alters the magnitude of the MRI signal.The susceptibility effects of blood vessels can be approximately modeled as infinite cylinders (8,9). The resonance frequency of a water proton is shifted from the Larmor frequency due to the susceptibility difference between blood and tissue water. For extravascular (EV) water, the magnetic field change (⌬B) is a sum of magnetic field changes induced by all blood vessels that exhibit magnetic perturbations, and can be written as:where ⌬ is the susceptibility difference between IV blood and surrounding tissue, R is the vessel radius, r is the distance between the point of interest and the center of the vessel in the plane through the point of interest and perpendicular to the vessel, is the angle between the main magnetic field B 0 and the vessel orientation, and is the angle between position vector r in this plane and the projection of B 0 in the plane. When the frequency shift induced by ⌬B is integrated around vessels (i.e., ranges from 0°to 360°), the average frequency shift for all the protons in an imagin...

Adaptive independent vector analysis for multi-subject complex-valued fMRI data

“…As CNR increased, ̅ showed a nearly linear increase. This conforms to existing findings about fMRI phase characteristics (Menon, 2002;Zhao et al, 2007;Tomasi and Caparelli, 2007;Hagberg et al, 2008;Yu et al, 2015). Fig.…”

Biophysical mechanisms of phase contrast in gradient echo MRI

“…This has been discussed previously (5,17,18), and here we only provide some useful relationships describing this effect. Assuming that the blood vessels are not arranged in any specific ''orientationally preferred'' fashion in both GM and WM, the Lorentz sphere concept can be applied and the frequency shift, ⌬f d , of the extravascular MR signal due to the presence of deoxyhemoglobin in the blood can be evaluated as:…”

Functional quantitative susceptibility mapping (fQSM)

“…At high spatial resolution, the phase of the fMRI time-series has been used to identify the dominant non-local BOLD effects due to large veins and to remove their contribution from the conventional magnitude BOLD statistical maps (Menon, 2002). Recently, a novel biophysical model for phase changes in BOLD fMRI based on the Lorentz-sphere approach was proposed (Zhao et al, 2007).…”