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Introduction: FMRI is based on changes in the local volume and the oxygenation of blood caused by brain activation. Being able to observe both oxygenation and the activation itself during fMRI could help to investigate the formation of the BOLD effect and its temporal and spatial distribution.
Previously, we have presented a completely integrated setup for concurrent fMRI and intrinsic optical imaging (IOI) inside the MRI magnet [1]. We now have extended this technique to also enable direct observation of the activation by including fluorescence based imaging of the intracellular calcium concentration.
Methods: The rats are placed into a fully integrated in-bore system based on a magnetic field proof camera and a tandem lens system to directly observe the brain surface. Light of four alternating wavelengths illuminates the brain, three used for IOI, while 492 nm light is used to image intracellular calcium using a fluorescent probe. For this, a viral vector encoding GCaMP is injected into the brain 4 to 6 weeks before the experiment distributed in 6-7 injections over the entire forepaw region of the somatosensory cortex. A bandpass filter in the illumination path and a longpass filter in front of the camera allow to observe the fluorescence signals without blocking the IOI wavelengths. Forepaw stimulation with concurrent high-resolution fMRI and optical imaging is performed in a 14.1 T magnet.
Results/Discussion: Four to six weeks after virus injection, GCaMP is expressed almost uniformly over the entire observed region, as shown by ex-vivo fluorescence microscopy of the isolated brain (Fig. 1).
Veins at the brain surface can be identified clearly in both anatomical MR images and optical images and are used for accurate colocalization of both modalities. The optical images with three different wavelengths were processed with the differential pathlength approach [2,3] to obtain quantitative values for the activation-induced changes in the concentrations of oxygenated (HbO), deoxygenated (HbR) and total (HbT) hemoglobin. The flourescence data acquired at 492 nm showed a sharp increase during the stimulation period. fMRI data was processed with SPM to calculate t-maps. Spatial distributions of all five measured parameters and a timecourse over the activated region for a 20 s stimulation are shown in Fig. 2.
Conclusions: While sequential, separate imaging of the single modalities is possible for many task-based, qualitative experiments, resting state fMRI or quantitative measurements require simultaneous acquisition of all parameters needed. The technique presented here will be used to investigate the formation of the BOLD effect to be able to better interpret the results of fMRI studies.
