An optically based rigid-body six-degrees of freedom (DOF) measurement system optimized for prospective (real-time) motion correction in magnetic resonance imaging (MRI) applications is described. By optimizing system capabilities to the specific applications requirements, the six-DOF measurement is accomplished using a single camera and simple three-disc fiducial at rates of $50\mathrm{Hz}$ . The algorithm utilizes successive approximation to solve the three point pose problem for angles close to the origin. Convergence to submicroradian levels occurs within 20 iterations for angles in an approximate half- radian ( $\mathrm{29}\mathrm{°}$ ) arc centered on the origin. The overall absolute accuracy of the instrument is $10–100\mathrm{\mu m}$ for translational and $<100\mathrm{\mu rad}$ ( $0.005°$ ) for rotational motions. Results for head nodding and speech tasks are presented for subjects in the MR scanner, and the instrument results are compared to standard prospective acquisition correction.