TECHNICAL CONSIDERATIONS ON THE VALIDITY OF BLOOD OXYGENATION LEVEL-DEPENDENT-BASED MR ASSESSMENT OF VASCULAR DEOXYGENATION
Nuria M. Hirsch, Vivien Tóth, Annette Förschler, Hendrik Kooijman, Claus Zimmer, Christine Preibisch. (09-05-2014).Wiley Online Library, 2014, DOI 10.1002/nbm.3131
Research Area B
A blood oxygenation level-dependent (BOLD)-based apparent relative oxygen extraction fraction (rOEF) as a semi-quantitative marker of vascular deoxygenation has recently been introduced in clinical studies of patients with glioma and stroke, yielding promising results. These rOEF measurements are based on independent quantiﬁcation of the transverse relaxation times T 2 and T 2 * and relative cerebral blood volume (rCBV). Simulations demonstrate that small errors in any of the underlying measures may result in a large deviation of the calculated rOEF. Therefore, we investigated the validity of such measurements. For this, we evaluated the quantitative measurements of T 2 and T 2 * at 3 T in a gel phantom, in healthy subjects and in healthy tissue of patients with brain tumors. We calculated rOEF maps covering large portions of the brain from T 2 , T 2 * and rCBV [routinely measured in patients using dynamic susceptibility contrast (DSC)], and obtained rOEF values of 0.63 ± 0.16 and 0.90 ± 0.21 in healthy-appearing gray matter (GM) and white matter (WM), respectively; values of about 0.4 are usually reported. Quantitative T 2 mapping using the fast, clinically feasible, multi-echo gradient spin echo (GRASE) approach yields signiﬁcantly higher values than much slower multiple single spin echo (SE) experiments. Although T 2 * mapping is reliable in magnetically homogeneous tissues, uncorrectable macroscopic background gradients and other effects (e.g. iron deposition) shorten T 2 *. Cerebral blood volume (CBV) measurement using DSC and normalization to WM yields robust estimates of rCBV in healthy-appearing brain tissue; absolute quantiﬁcation of the venous fraction of CBV, however, is difﬁcult to achieve. Our study demonstrates that quantitative measurements of rOEF are currently biased by inherent difﬁculties in T 2 and CBV quanti ﬁ cation, but also by inadequacies of the underlying model. We argue, however, that standardized, reproducible measurements of apparent T 2 , T 2 * and rCBV may still allow the estimation of a meaningful apparent rOEF, which requires further validation in clinical studies.