[Show abstract][Hide abstract] ABSTRACT: Little is known regarding the changes in blood oxygen tension (P(O2)) with changes in brain function. This work aimed to measure the blood P(O2) in surface arteries and veins as well as tissue with evoked somato-sensory stimulation in the anesthetized rat. Electrical stimulation of the forepaw induced average increases in blood flow of 44% as well as increases in the tissue P(O2) of 28%. More importantly, increases in P(O2) throughout pial arteries (resting diameters=59 to 129 microm) and pial veins (resting diameters=62 to 361 microm) were observed. The largest increases in vascular P(O2) were observed in the small veins (from 33 to 40 mm Hg) and small arteries (from 78 to 88 mm Hg). The changes in oxygen saturation (S(O2)) were calculated and the largest increases were observed in small veins (Delta=+11%) while its increase in small arteries was small (Delta=+4%). The average diameter of arterial vessels was observed to increase by 4 to 6% while that of veins was not observed to change with evoked stimulation. These findings show that the increases in arterial P(O2) contribute to the hyper-oxygenation of tissue and, mostly likely, also to the signal changes in hemoglobin-based functional imaging methods (e.g. BOLD fMRI).
Journal of cerebral blood flow and metabolism: official journal of the International Society of Cerebral Blood Flow and Metabolism 10/2009; 30(2):428-39. · 5.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Stimulation-induced changes in transverse relaxation rates can provide important insight into underlying physiological changes in blood oxygenation level-dependent (BOLD) contrast. It is often assumed that BOLD fractional signal change (DeltaS/S) is linearly dependent on echo time (TE). This relationship was evaluated at 9.4 T during visual stimulation in cats with gradient-echo (GE) and spin-echo (SE) echo-planar imaging (EPI). The TE dependence of GE DeltaS/S is close to linear in both the parenchyma and large vessel area at the cortical surface for TEs of 6-20 ms. However, this dependence is nonlinear for SE studies in the TE range of 16-70 ms unless a diffusion-weighting of b = 200 s/mm(2) is applied. This behavior is not caused by inflow effects, T(2)* decay during data acquisition in SE-EPI, or extravascular spin density changes. Our results are explained by a two-compartment model in which the extravascular contribution to DeltaS/S vs. TE is linear, while the intravascular contribution can be nonlinear depending on the magnetic field strength and TE. At 9.4 T, the large-vessel IV signal can be minimized by using long TE and/or moderate diffusion weighting. Thus, stimulation-induced relaxation rate changes should be carefully determined, and their physiological meanings should be interpreted with caution.
Magnetic Resonance in Medicine 07/2006; 55(6):1281-90. · 3.40 Impact Factor