1. An external, non-invasive method for monitoring passage of at least one food bolus through an esophagus whereby to evaluate esophageal function of a patient, comprising the steps: (a) non-invasively placing proximate an outside surface of the patient at least one ultrasonic transducer; (b) acquiring, from said ultrasonic transducer, ultrasonic image data of said at least one bolus at a plurality of temporal intervals as said at least one bolus passes through the esophagus after being swallowed by the patient; (c) tracking said at least one bolus in said image data to identify esophagus and bolus image data corresponding to said bolus and to said esophagus in said image data by; (1) determining global motion of the esophagus in the image data acquired in step (b): (2) aligning image data in consecutive image data by correcting for the global motion determined in step (c)(1); and (3) identifying esophagus image data by first identifying a position of the bolus image in successive images corresponding to the bolus traversing the esophagus; and (d) determining at least one esophagus function parameter from the said bolus and esophagus image data.