Ultrasound Backscattered Tensor Imaging of the Brain: An ex vivo feasibility study

In neurosurgeries, brain shift and tumor removal may render the preoperative MRI diffusion tensor scan irrelevant. There is a need for real time and accurate mapping of the brain white matter fibers. Towards solving this problem, for the first time, we demonstrate the feasibility of ultrasound backscatter tensor imaging (BTI) in locating and measuring the corpus callosum (CC), the largest white matter fibers, in an ex vivo formalin-fixed rat brain. BTI analyzes the coherence in the backscatter signal at different transducer-to-fiber orientations to estimate regions of high anisotropy and thus the presence of fibers. We collected ultrasound radio frequency signals for 180 degrees with a step-size of 10 degrees. At each step, we used focused ultrasound beams to scan the central axis of the rat transverse plane. We then calculated and mapped the coherence factors (CF) to infer the size and location of the CC at two locations of high and low anisotropy, respectively. Lastly, we compared our results in the high anisotropy plane to a rat brain MRI Diffusion Tensor Imaging (DTI) atlas. The CC thickness in the measured plane was 0.87 mm (atlas) vs. 1.0 +/- 0.3 mm (CF map), while the distance to the rat brain medial dorsal surface was at 1.53 mm (atlas) vs. 1.7 +/- 0.3 mm (CF map). This is an ongoing study with limitations in the axial and lateral resolution, speed of acquisition, and signal to noise ratio. To our best knowledge, this is a first study in demonstrating the potential of BTI in detecting the corpus callosum with promising results to warrant further efforts towards clinical translation.