Exploiting magnetic resonance angiography imaging improves model estimation of BOLD signal

Abstract

The change of BOLD signal relies heavily upon the resting blood volume fraction ([Formula: see text]) associated with regional vasculature. However, existing hemodynamic data assimilation studies pretermit such concern. They simply assign the value in a physiologically plausible range to get over ill-conditioning of the assimilation problem and fail to explore actual [Formula: see text]. Such performance might lead to unreliable model estimation. In this work, we present the first exploration of the influence of [Formula: see text] on fMRI data assimilation, where actual [Formula: see text] within a given cortical area was calibrated by an MR angiography experiment and then was augmented into the assimilation scheme. We have investigated the impact of [Formula: see text] on single-region data assimilation and multi-region data assimilation (dynamic cause modeling, DCM) in a classical flashing checkerboard experiment. Results show that the employment of an assumed [Formula: see text] in fMRI data assimilation is only suitable for fMRI signal reconstruction and activation detection grounded on this signal, and not suitable for estimation of unobserved states and effective connectivity study. We thereby argue that introducing physically realistic [Formula: see text] in the assimilation process may provide more reliable estimation of physiological information, which contributes to a better understanding of the underlying hemodynamic processes. Such an effort is valuable and should be well appreciated.

Figure 1. Schematic illustration of the hemodynamic Balloon model.

Figure 2. The vasculature of one subject.

Figure 3. Regions of interest.

Because BOLD contrast is highly weighted by venous blood content, activation areas often overlap with large vein regions. Two regions of interest (ROIs) were selected from visual cortex according to activation detection (warm color) and vascular information (cool color). The spatial resolution of venography map was downsample to identical with that of fMRI image. (: Primary visual cortex; : Visual area ).

Figure 4. Estimated BOLD signal (Left), and reconstructed physiological states (Right) from the greatest activated locus in primary visual cortex (V).

For comparative purpose, model estimation was also performed with a typical assumed . Real value is .

Figure 5. Results of a two-node DCM analysis applied to the flashing checkerboard experiment.

The coupling parameters calculated with actual are shown alongside the corresponding connections. The values in brackets are parameters estimated with assumed . in visual area V, in V and assumed in two areas. and represent external inputs into the system; and are the hemodynamic observations and arrows indicate connections.