Optical systems and methods to track the position of a needle in subsurface structures, such as tissues or organs, and co-register the information with ultrasound are described herein. An optical fiber in a needle catheter is used to transmit light inside of the structure. The light is intensity modulated at sufficiently high frequencies such that the time of arrival of the light can be used to determine the distance of the needle from an optical detector at the tissue surface. The position of the needle can be tracked by combining data obtained using different modulation frequencies and/or wavelengths of light. By using multiple detectors at different positions, the location of the needle in 3D space can be triangulated using light, and the data can be integrated with ultrasound to obtain the anatomical structure.
