Quantitative assessment of blood flow, blood volume and blood oxygenation effects in functional magnetic resonance imaging

Abstract

The ability to measure the effects of local alterations in blood flow, blood volume and oxygenation by nuclear magnetic resonance has stimulated a surge of activity in functional MRI of many organs, particularly in its application to cognitive neuroscience. However, the exact description of these effects in terms of the interrelations between the MRI signal changes and the basic physiological parameters has remained an elusive goal. We here present this fundamental theory for spin-echo signal changes in perfused tissue and validate it in vivo in the cat brain by using the physiological alteration of hypoxic hypoxia. These experiments show that high-resolution absolute blood volume images can be obtained by using hemoglobin as a natural intravascular contrast agent. The theory also correctly predicts the magnitude of spin-echo MRI signal intensity changes on brain activation and thereby provides a sound physiological basis for these types of studies.

Similarities and Differences in Arterial Responses to Hypercapnia and Visual Stimulation

“…The opposite scenario is also theoretically possible in which an artifactually diminished arterial compartment could result if vasodilatation exceeds the CBF increase, leading to decreased blood velocity. As an estimate of the potential error, if the arteriolar fraction of the microvasculature is B20% (van Zijl et al, 1998;Uludag et al, 2009) and if the whole microvascular volume is B2% to 2.5% of a voxel (Uludag et al, 2009), then the arteriolar volume is B0.4% to 0.5% of a voxel. This would be the rough maximum amount of artifactual aBV enlargement, if we were to assume the extreme case of the entire arteriolar contribution being shunted from the microvascular compartment to the arteriolar compartment.…”

Water-Diffusion Slowdown in the Human Visual Cortex on Visual Stimulation Precedes Vascular Responses

“…The BOLD fMRI signal originates from the variations in vascular deoxy-Hb content (Ogawa et al, 1993;van Zijl et al, 1998), thus reflecting a complex balance between metabolism, rCBV, and rCBF variations. Differences can be observed between the SE-BOLD and GE-BOLD signals, because of a differential sensitivity to the vessel size (Boxerman et al, 1995), the tissue, and the intravascular components.…”

“…(Different results have been reported by Jin and Kim (2008), but the discrepancy could be well explained by the different parameters used in this study, especially lower b values and much shorter diffusion times.) By contrast, the indirect T2* t component is expected to share the temporal profile of the deoxyhemoglobin vascular content (BOLD) time course (Boxerman et al, 1995;van Zijl et al, 1998). This indirect vascular effect on T2* t could also easily explain the increase in the diffusion-sensitized signal, observed during vascular challenges, such as hypercapnia (Miller et al, 2007), which is known to decrease the vascular deoxy-Hb content.…”

“…The extent, dynamics, and underlying mechanisms of this neurovascular coupling are not yet fully understood (Iadecola and Nedergaard, 2007;Magistretti and Pellerin, 1999), and the BOLD signal depends on several parameters, so its biophysical link with neuronal activation is not straightforward (Buxton et al, 1998(Buxton et al, , 2004Malonek et al, 1997;Sirotin and Das, 2009;van Zijl et al, 1998). Although the coupling between neuronal activation, cell metabolism, and hemodynamic changes has been shown for BOLD fMRI (Logothetis et al, 2001), it may fail in some pathologic conditions (Lehericy et al, 2002).…”

How accurate is magnetic resonance imaging of brain function?

“…faithful to the borders of the perfusion increase can be obtained with Hahn spin-echo (HSE) BOLD at high but not low magnetic fields. HSE fMRI responds to apparent changes in T 2 (as opposed to T p 2 ) originating from the diffusion of water in the presence of magnetic-field gradients generated in the extravascular space around microvasculature [36,37], as well as from the exchange of water into and out of red blood cells in the blood itself [38][39][40]. The former provides spatial specificity of ,100 mm because capillaries are separated on average by 25 mm [41].…”