No Increase of the Blood Oxygenation Level-Dependent Functional Magnetic Resonance Imaging Signal with Higher Field Strength: Implications for Brain Activation Studies

Abstract

Experimental data up to 7.0 T show that the blood oxygenation level-dependent (BOLD) signal of functional magnetic resonance imaging (fMRI) increases with higher magnetic field strength. Although several studies at 11.7 T report higher BOLD signal compared with studies at 7.0 T, no direct comparison at these two field strengths has been performed under the exact same conditions. It therefore remains unclear whether the expected increase of BOLD effect with field strength will still continue to hold for fields Ͼ7.0 T. To examine this issue, we compared the BOLD activation signal at 7.0 and 11.7 T with the two common sequences, spin-echo (SE) and gradient-echo (GE) echo planar imaging (EPI). We chose the physiologically well controlled rat model of electrical forepaw stimulation under medetomidine sedation. While a linear to superlinear increase in activation with field strengths up to 7.0 T was reported in the literature, we observed no significant activation difference between 7.0 and 11.7 T with either SE or GE. Discussing the results in light of the four-component model of the BOLD signal, we showed that at high field only two extravascular contributions remain relevant, while both intravascular components vanish. Constancy of the BOLD effect is discussed due to motional narrowing, i.e., susceptibility gradients become so strong that phase variance of diffusing spins decreases and therefore the BOLD signal also decreases. This finding will be of high significance for the planning of future human and animal fMRI studies at high fields and their quantitative analysis.

A mini-cap for simultaneous EEG and fMRI recording in rodents

“…3B shows the histogram of T2* value obtained from the segmented rat brain including both the gray and white matter. The distributions of T2* [conditions: (1) 26.0 ± 10.1, (2) 25.2 ± 10.1, (3); 25.9 ± 9.9] were similar with those reported in a recent study (Seehafer et al, 2010). Using the one-sample JarqueBera test (Jarque and Bera, 1987), we confirmed that the distributions of the T2* in the segmented brain were Gaussians (i.e.…”

Direct imaging of macrovascular and microvascular contributions to BOLD fMRI in layers IV–V of the rat whisker–barrel cortex

“…Therefore, in most macro vessel voxels, the water contribution from vessels is small, ranging from ~10% to ~50%. In addition, short T2* of blood at 11.7T magnetic field strength decreases the intravascular contribution further (Keilholz et al, 2006; Seehafer et al, 2010). Thus, the extravascular effects from macrovasculature are likely to dominate the BOLD signal detected by the gradient echo EPI sequence.…”

Improving the use of principal component analysis to reduce physiological noise and motion artifacts to increase the sensitivity of task-based fMRI

“…With increasing field strength, physiological noise limits the achievable image signal-to-noise ratio (SNR) (Kruger et al, 2001), but not the BOLD contrast (Gati et al, 1997). However, other factors may limit BOLD contrast above 7 T (Seehafer et al, 2010). …”