High-resolution in vivo kinematic tracking with customized injectable fluorescent nanoparticles

Markerless keypoint trackers, algorithms trained to identify specific points on an animal, have transformed neuroscience and enabled movement quantification without the attachment of surface markers. However, while these approaches represent a major advancement, they have yet to achieve the precision of motion capture in humans and have not been benchmarked against ground-truth datasets in common model organisms. Moreover, the extent to which they can be used to reliably track movement kinematics remains unclear. Here, we describe a tracking method that uses near-infrared quantum dots as injectable markers. We demonstrate that our tags can be injected beneath the skin or into joints and imaged for months in freely moving mice noninvasively using standard cameras. Last, we create a large ground-truth dataset for training the next generation of markerless systems. By using injectable landmarks in the joints, this method brings us closer to understanding how the brain controls skeletal movements.