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A new non-invasive method is presented that allows visualization of changes in the myocardium subsequent to depolarization based on high speed, real time ultrasound imaging. The Duke Phased Array scanner, T5, was used to acquire images. T5 features 512 independent transmit channels and 1024 independent receive channels with up to 32 to 1 parallel processing, with image rates up to 600 frames/sec. For this study, T5 was programmed to create 2D images at a 12 cm range using a 96 element, 3.5 MHz 1D phased array at the rate of 290 frames/sec for an 80° field of view. High speed images of the long axis of the LV were obtained parasternally and continually stored. By positioning the transducer or by shifting the images in playback, efforts were made to align the septum parallel to the transducer face, since data was analyzed in Cartesian coordinates. Images from one cardiac cycle were selected for analysis. Sequential images were subtracted throughout the cardiac cycle to produce real time high speed difference images. Resultant difference images showed changes associated with either mechanical or positional changes as bright targets. A lack of such changes resulted in no image information, i.e. a blank screen. Brightness changes during ventricular contraction were very evident, but changes were also observed during the QRS complex when there was no apparent motion of the myocardium or septum. 10 normal volunteers and 2 left bundle branch block (LBBB) patients were scanned and their EKG simultaneously recorded. All difference images showed a propagating brightness wave traveling in the septum from base to apex during the QRS complex. To quantify this phenomenon, brightness of the difference images was measured at different known spatial locations along the septum as a function of time as quantized by the frame rate. This brightness wave was first observed about 42 msec after the Q wave and traveled at an average velocity of 2.1 ± 0.4 m/sec along the septum in all 12 subjects. Duration of this wave was 17 ± 4 msec for normal volunteers and 20.8 and 22.2 msec for the two LBBB patients. We believe that this brightness wave indicates the initial mechanical changes in the septum following depolarization and this method is a first step toward non-invasive assessment of myocardial activation.