Functional imaging of the rat brain with micro-ultrasound

Linear array based micro-ultrasound provides 40150um resolution over a significant depth of field at frames rates as high as 1000 fps. Current imaging modalities for investigating in vivo brain function are challenged to provide this combination of imaging parameters. The present experiment was carried out to investigate the potential of micro-ultrasound in neuroimaging of rodents in vivo. Adult male Sprague-Dawley rats were anesthetized with isoflurane. To enable high frequency ultrasound imaging of the brain, stereotaxic surgery was done to prepare a small cranial window. Non-linear Ultrasound imaging (amplitude modulation) was performed using high frequency linear array (Vevo2100, VisualSonics), equipped with a 20 MHz center frequency probe. The probe was positioned appropriately to reveal 3 regions of interest, the forelimb representation in the primary somatosensory cortex (S1FL), primary motor cortex (Ml), and thalamus (Th). Ultrasound contrast agent (Micromarker, VisualSonics) at 40uL/min and 2 ? 10⁹ microbubbles/mL was infused through the tail vein. When signal intensity reached a steady state, a contrast disruption pulse was delivered to assess brain reperfusion during electrical stimulation to the forelimb and 10% CO₂ inhalation. Average signal intensity from S1FL, Ml, and Th regions were acquired and the slope and plateau values of the reperfusion curves were calculated. Slope and plateau are indices of flow and total blood volume in the regions of interest respectively. An increase in both the initial slope and plateau of the reperfusion curve were observed in the S1FL and Th, while only an increase in plateau was observed in Ml during the electrical stimulation to the forepaw (P<0.05). Only a significant increase in slope was observed in S1FL during the CO₂ inhalation (P<0.05). These data demonstrate the potential micro-ultrasound for functional brain imaging in animal models. The imaging parameters tradeoff affo- rded by nonlinear contrast Microultrasound with depth penetration sufficient to enclose the entire cerebrum provides a unique window into the study of cerebral hemodynamics, both in the cortex and in the deep grey matter.