In vivo transcranial imaging of blood perfusion in rat brain using contrast-enhanced ultrasound

We have investigated the feasibility of in vivo high resolution imaging of blood perfusion in the rat brain tissue using a dual-mode ultrasound array (DMUA). The DMUA is capable of generating therapeutic transcranial focused ultrasound (tFUS) beams as well as forming images of the target in synthetic-aperture (SA) mode. Low mechanical index (MI) SA frames are used to estimate the dynamic changes in perfusion, which could be used for guidance, monitoring, or damage assessment. We have previously demonstrated the advantages of the post-beamforming third-order Volterra filter in imaging nonlinear echo components due to UCA oscillations using diagnostic probes. In this study, we decompose the beamformed transcranial DMUA SA data into its linear, quadratic, and cubic components and evaluate a temporal perfusion index (TPI) to characterize perfusion. The data collection used a 3.2-MHz, concave (40-mm ROC) DMUA operating in SA mode at a frame rate of approximately 23 fps and in the low MI setting (about 0.05 to 0.11). The experiment was performed on healthy rats (250-300 grams). Targestar-P UCA (60 μL in 140 μL of sterile saline) was injected through the animal tail vein. Images from the linear, quadratic, and cubic components were formed as LB-mode, QB-mode and CB-mode, respectively. TPI image frames were computed from each image component after limiting the dynamic range appropriately. The results clearly show separation of TPI values and demonstrate the feasibility of high-resolution estimation of blood perfusion in rat brain transcranially.