Simultaneous measurement of BOLD-fMRI and optical imaging of intrinsic signals for determination of oxygenation and CBV at high spatial and temporal resolution

Introduction To be able to better understand the formation and the dynamics of the BOLD effect, simultaneous measurements of the different components contributing to BOLD are required. We combined optical imaging of intrinsic signals, which is able to quantitatively measure oxygenation and CBV changes at high spatial and temporal resolution, with ultra high field fMRI. A fully magnetic field compatible optical imaging setup for combined optical and MR imaging of rats was developed, with the goal of improving our understanding of the BOLD effect and contributing to a better analysis of fMRI studies. Methods Optical imaging of intrinsic signals takes advantage of the different absorption coefficients of oxygenated and deoxygenated hemoglobin. To obtain optical imaging and fMRI data with similar quality and resolution as in separate experiments, a tailor-made, magnetic field proof high-sensitivity camera was combined with commercial optical components. The skull of anaesthetized rats was thinned to translucency and illuminated by light with four different wavelengths for quantitative determination of oxygenation and CBV. Brain activation was applied under electrical forepaw stimulation with varying parameters and observed with fMRI and OI (50 ms per image, four interleaved wavelengths). The optical images were used to determine oxygenation and CBV using published pathlength data (1). Results/Discussion OI and fMRI images with excellent quality were acquired. Venous structures could be identified clearly in both modalities and were used to overlay the different parameter images. Fig. 2 shows timecourses for varying stimulation durations between 1 s and 5 s. BOLD, oxygenation and CBV timecourses agree well. Concurrent MRI and OI can potentially help to improve our understanding of the BOLD effect and the hemodynamic processes that contribute to the fMRI signal. A previous approach used an endoscope to get the light out of the magnet to a conventional camera (2). By using a specially designed camera, which combines high sensitivity with full compatibility with the magnetic field, and high-performance optical components, we are able to obtain images from inside the magnet without losses in quality compared to those acquired in standard OI experiments. The combination with ultra-high field MRI ensures also a high fMRI image quality and spatial and temporal resolution. Conclusion Combining intrinsic optical imaging with fMRI can help to disentangle the different factors contributing to the BOLD effect and yield important information to enhance both OI and fMRI experiments. Furthermore, the developed setup can be combined with other optical techniques to measure further neuroscientific parameters at the same time, like Ca, speckle-flow or voltage sensitive die imaging