Advanced template matching method for estimation of intervertebral kinematics of lumbar spine

Diagnosis of low back pain and other degenerative spinal pathologies can be extremely difficult and, so far, there are not accepted standards. In general, such pathologies are associated with alteration of mechanical properties of spine and, in particular, with the instability of spinal motion. Intervertebral kinematics can be a valuable, objective method to assess the functionality of spinal segments. Fluoroscopic imaging system can provide continuous screening of lumbar tracts during patient's motion, with an acceptable low X-ray dose. Estimation of intervertebral kinematics relies on accurate recognition of vertebrae positions throughout the fluoroscopic sequence: specific vertebrae features are identified and tracked either by manual selection or by automated methods. This study presents a new method of vertebra tracking, based on image template matching of the contour of the vertebral body for an accurate intervertebral kinematics analysis. An image gradient operator was utilized to obtain the vertebral contours; it operates after an edge-preserving smoothing filter designed to reduce low dose X-ray image noise. Once a template is defined for each vertebra, this is used to determine the best vertebral location in each image throughout the fluoroscopic sequence. Accuracy of the proposed method was tested using images of a calibration model. Average error achieved for the intervertebral angle is of the order of 0.4° and approximately 2mm for the intervertebral centre of rotation. Five fluoroscopic lumbar sequences of healthy volunteers undergoing passive flexion-extension motion were processed. The intervertebral kinematics was compared with other methods (automated and manual) by an estimation of measurement error. Results showed that the current method provides a better representation of the evolution over time of kinematic parameters. In particular, root mean square differences between the current method and a manual selection procedure performed by an experienced and trained clinician resulted 1.3° for the intervertebral angles and 0.9 mm for the intervertebral trajectory. The proposed method provides an effective, automated and objective technique for estimation of intervertebral kinematics of lumbar spine.