Measurement of Musculoskeletal Motion Using Vector Tissue Doppler Imaging

Abstract

The goal of this project is to develop, characterize and validate vector tissue Doppler imaging (vTDI) to measure dynamic musculoskeletal motion. We have developed a vector tissue Doppler imaging system using a clinical ultrasound scanner with a research interface. This system estimates motion in two or more independent directions using multiple electronicallycontrolled transmitters and receivers oriented in different directions. The vector tissue Doppler method combines the multiple velocity estimates producing a single velocity vector with magnitude and direction. We characterized this system in vitro by changing four parameters, namely, beam steering angle, depth of transmit focus, angle of velocity vector and the depth of the scatterer relative to the beam overlap region. Our results show that changing these parameters have minimal effect on the velocity and angle estimates, and robust velocity vector estimates can be obtained under a variety of conditions. The mean velocity error was less than 6% of the maximum detectable velocity. We then performed some preliminary in vivo experiments to measure the velocity of the rectus femoris muscle group during a tendon tap in normal volunteers. Our goal was to investigate whether the muscle elongation velocities during a brisk tendon tap fall within the normal range of velocities that are expected due to rapid stretch of limb segments. We found that the equivalent velocities elicited during standard patellar tendon jerk test are within the range of velocities (3.26 rad s-1 to 8.23 rad s-1) encountered in typical everyday activities, but the angular accelerations substantially exceeded the accelerations encountered in everyday activities (191.8 rad s-2 to 4038.6 rad s-2). Our study provides the experimental evidence to support the non- physiological character of the tendon taps which is used during standard neurological tests. We also investigated the feasibility of using vector tissue Doppler velocity estimates as a reliable clinical outcome measure in children with cerebral palsy (CP) and who have foot drop, or inadequate ankle dorsiflexion during the swing phase of gait. We measured the tibialis anterior tendon contraction velocities during ankle dorsiflexion. Our preliminary results from this study show that tendon velocities estimated using vTDI have a strong linear correlation with the joint angular velocity estimated using a conventional 3D motion capture system. We observed a peak tendon velocity of 5.42±1.01 cm/s during dorsiflexion and a peak velocity of 8.02±2.33 cm/s during the passive relaxation phase of movement. Our preliminary studies demonstrate that vector Doppler may be used as clinical outcome measures and for studying efficiency of movement control. In the future, vector tissue Doppler imaging (vTDI) may be used to better understand gait disorders in patients suffering from cerebral palsy, spinal cord and brain trauma and other neuromuscular disorders.