Magnetomotive Optical Coherence Elastography for Measuring Biomechanical Properties of Tissue using Magnetic Nanoparticles

Biomechanical properties of tissue are indicative of health and disease, and the ability to readily measure them is instrumental for the diagnosis of early-stage changes. We present a new method for measuring elastic properties of tissue-like phantoms, which employs Fe$_3$O$_4$ nanoparticles as contrast agents in a technique called magnetomotive optical coherence elastography (MMOCE). PDMS-based samples similar to soft biological tissue (0.5-12 kPa) were prepared, with nanoparticles embedded within their volume. The magnetic nanoparticles are displaced upon probing with an external magnetic field, engaging the sample in axial motion. M-mode MMOCE phase data was acquired concomitantly at a rate of 29 kHz, allowing for the tracking of scatterers in the sample with a displacement sensitivity of 11 nm. The scaterers in the samples underwent underdamped oscillations when the magnetic field was applied step-wise. We extracted the damping constants and the natural frequencies of oscillation (30-200 Hz) from the time-resolved displacement traces. A microindentation apparatus was used to measure the Young's moduli of the samples for validation and calibration with the MMOCE measurements. This novel real-time non-invasive technique affords the potential for \textit{in vivo} studies.