CEREBRAL BLOOD VOLUME MAPPING IN HUMANS AT 7T WITH HYPEROXIC CONTRAST

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Increasing the inspired fraction of oxygen (FiO2>0.21) has been shown in numerous studies to produce positive contrast enhancement in T2*-weighted images based on the blood oxygenation-dependent (BOLD) effect. Recently, it has been shown that cerebral blood volume (CBV) can be accurately calculated by measuring signal changes in EPI at 3T during short epochs (<6min) of mild hyperoxia. Typically, images acquired in this fashion have low spatial resolution (approximately 4x4x6mm) because of the need for: (a) high contrast-to-noise ratio (CNR), since tissue signal changes are typically small, and (b) high temporal resolution to average the signal fluctuations inherent to heavily T2*-weighted EPI. Ultra-high field strengths (>7T) can enhance the CNR of this experiment due to the nonlinear increase in BOLD contrast with field strength, potentially allowing for significantly increased spatial resolution. However, performing a high- resolution CBV experiment using EPI is difficult at 7T for a number of reasons. Geometric distortions and signal dropout due to B0 inhomogeneity near air-tissue interfaces are substantial problems at 7T. Even more significantly, quantifying CBV with this method requires that the BOLD contrast be primarily intravascular. Since venous blood at 7T has a T2*<10ms, a short echo time (TE<9 ms) and readout length are required that are less than standard EPI sequences can produce even at high bandwidth and with partial-Fourier (PF) encoding. Furthermore, acquiring thin slices using a 2D approach is difficult to do due to slice profile imperfections. To address these problems, we are investigating the use of high resolution (1x1x2mm) steady-state acquisition partial-Fourier encoded segmented 3D EPI for hyperoxia-based CBV maps at 7T.