Abstract
Quantitative BOLD (qBOLD) is a technique for mapping oxygen extraction fraction (OEF) and deoxygenated blood volume (DBV) in the human brain. Recent measurements using an asymmetric spin echo (ASE) based qBOLD approach produced estimates of DBV which were systematically higher than measurements from other techniques. In this study, we investigate two hypotheses for the origin of this DBV overestimation using simulations and consider the implications for experimental measurements. Investigations were performed by combining Monte Carlo simulations of extravascular signal with an analytical model of the intravascular signal. Hypothesis 1DBV overestimation is due to the presence of intravascular signal which is not accounted for in the analysis model. Intravascular signal was found to have a weak effect on qBOLD parameter estimates. Hypothesis 2DBV overestimation is due to the effects of diffusion which are not accounted for in the analysis model. The effect of diffusion on the extravascular signal was found to result in a vessel radius dependent variation in qBOLD parameter estimates. In particular, DBV overestimation peaks for vessels with radii from 20 to 30 μm and is OEF dependent. This results in the systematic underestimation of OEF. ImplicationsThe impact on experimental qBOLD measurements was investigated by simulating a more physiologically realistic distribution of vessel sizes with a small number of discrete radii. Overestimation of DBV consistent with previous experiments was observed, which was also found to be OEF dependent. This results in the progressive underestimation of the measured OEF. Furthermore, the relationship between the measured OEF and the true OEF was found to be dependent on echo time and spin echo displacement time.The results of this study demonstrate the limitations of current ASE based qBOLD measurements and provide a foundation for the optimisation of future acquisition approaches.
Highlights
The quantitative BOLD technique is a relaxometry based approach for mapping oxygen extraction fraction (OEF) and deoxygenated blood volume (DBV) in the human brain (He and Yablonskiy, 2007)
Estimates of the apparent OEF increase monotonically with vessel radius reaching the value predicted by the static dephasing regime (SDR) quantitative BOLD (qBOLD) model as the vessel radius approaches 1000 μm (Fig. 4c,f)
An increase in the value of the diffusion coefficient would result in a linear translation to the right along the x-axis for data plotted against such a log vessel radius
Summary
The quantitative BOLD (qBOLD) technique is a relaxometry based approach for mapping oxygen extraction fraction (OEF) and deoxygenated blood volume (DBV) in the human brain (He and Yablonskiy, 2007). QBOLD data can be acquired using the Asymmetric Spin Echo (ASE) pulse sequence, which provides a direct measurement of the reversible relaxation rate, R20, and eliminates the need to remove R2-weighting from the acquired signal (required by GESSE) (An and Lin, 2003; Stone and Blockley, 2017). It is unclear whether a similar diffusion effect is present in ASE data. Estimates of DBV made using this ASE based acquisition are systematically higher than those reported for GESSE based measurements (He and Yablonskiy, 2007), suggesting that different effects may be at play
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