2J-5 Ultrafast Ultrasonic Imaging of In Vivo Muscle Contraction

Although numerous techniques are commonly used to study muscular or neuromuscular diseases, in vivo muscle contraction remains a difficult event to image in real time. On one hand, time accurate techniques such as for example, mechanomyography, electromyography, or acceleromyography; allow only the assessment of global parameters of musculo-tendinous complex. These estimated parameters are classically the force developed by a group of muscles fibers during contraction, the compound action potential or the acceleration of a muscle. On the other hand, mapping of local muscle structure parameters during contraction with a submillimetric resolution can be achieved by magnetic resonance imaging or ultrasound Doppler tissue imaging. However, these imaging modalities are not fast enough (less than hundreds of frames per second) to follow transient phenomena. Here, a new way of imaging the motion of an in vivo contracting muscle is proposed. The principle is to use an ultrafast ultrasound scanner to follow with a submillimetric resolution the axial motion of the muscle tissue in a two dimensional (2D) plane. This imaging technique designed at the "Laboratoire Ondes et Acoustique" for transient elastography, gives 2D radio frequency images at a few thousands Hertz (up to 5000 framesmiddots^-1) and leads to both local and transient displacements of the muscle in vivo. Displacements as low as one micrometer are computed from the one dimensional (1D) cross-correlation between consecutive images. From these displacements, both local and transient informations on the muscle contraction are extracted such as the contraction time, the relaxation time or the mean velocity of the propagation of the contraction along a muscle fiber. During experiments the ultrafast scanner driving a 128 elements 8 MHz probe triggers a homemade electrostimulation device linked to a pair of electrodes placed on the motor point of the biceps brachii and at the elbow junction. In vivo studies of- the contraction of a muscle are reported here by these first transient experiments achieved on the biceps brachii. The velocity distribution of the displacements induced in the tissue during contraction is measured as well as the main temporal features of the contraction