Mouse BOLD fMRI at ultrahigh field detects somatosensory networks including thalamic nuclei

Forepaw somatosensory stimulation induces neural activities in relay thalamic nuclei, the primary somatosensory cortex of forelimb (S1FL), and the secondary somatosensory cortex (S2). However, rodent fMRI studies of somatosensory stimulation have commonly reported BOLD changes only in S1FL, which may be due to side effects of anesthetics and/or the low sensitivity in the thalamus. Thus, we have obtained mouse BOLD fMRI under newly-adopted ketamine-xylazine anesthesia. High-resolution BOLD fMRI obtained with same imaging parameters at 9.4T versus 15.2T shows the improvement of functional detectability by ≥ 2 times at 15.2T due to higher signal intensity and larger BOLD response. The fMRI responses at 15.2T were robustly observed at well-known somatosensory networks including thalamus. Second, echo-time-dependent BOLD signals are dominant based on multi-echo fMRI data. A ratio of BOLD responses in S1FL to thalamus is ∼2, which is not related to different baseline T₂^∗ or different cerebral blood volume. Third, group-averaged 15.2T BOLD maps show activities in S1FL, S2, motor cortices, and thalamic nuclei, which agree well with neural tracing network data from the Allen Institute, demonstrating that fMRI detects entire somatosensory networks. Our data suggest that ultrahigh field fMRI provides a unique window into understanding functional networks in normal and transgenic mouse models noninvasively.