Determining 3D kinematics of the hip using video fluoroscopy : guidelines for balancing radiation dose and registration accuracy

Background: Video fluoroscopy is a technique currently used to retrieve the in vivo three-dimensional kinematics of human joints during activities of daily living. Minimization of the radiation dose absorbed by the subject during the measurement is a priority and has not been thoroughly addressed so far. This issue is critical for the motion analysis of the hip joint, because of the proximity of the gonads. The aims of this study were to determine the x-ray voltage and the irradiation angle that minimize the effective dose and to achieve the best compromise between delivered dose and accuracy in motion retrieval. Methods: Effective dose for a fluoroscopic study of the hip was estimated by means of Monte Carlo simulations and dosimetry measurements. Accuracy in pose retrieval for the different viewing angles was evaluated by registration of simulated radiographs of a hip prosthesis during a prescribed virtual motion. Results: Absorbed dose can be minimized to about one-sixth of the maximum estimated values by irradiating at the optimal angle of 45° from the posterior side and by operating at 80 kV. At this angle, accuracy in retrieval of internal-external rotation is poorer compared with the other viewing angles. Conclusion: The irradiation angle that minimizes the delivered dose does not necessarily correspond to the optimal angle for the accuracy in pose retrieval, for all rotations. For some applications, single-plane fluoroscopy may be a valid lower dose alternative to the dual-plane methods, despite their better accuracy.