Doppler echocardiogaphy is based on the Doppler effect, which was described by the Austrian phy$icist Christian Doppler in 1842. The Doppler effect states that sound frequency increases as the sound source moves towards the observer and decreased as the source moves away. In the circulatory system, the moving target is the red blood cells (rbc). When an ultrasound beam witb known frequency (Fo) is transmitted to the heart or great vessels, it is reflecteid by rbc's. The frequency of reflected ultrasound waves (Fr) increases when rbc'd are moving towards the source of ultrasound and vice versa. The change id frequency between transmitted and reflected sound is seemed the frequenc/y shiR (AF) or Doppler shiR (Fr-Fo). If cos 0 is 0 degree [i.e., the ultrasound beam is parallel with the direction of blood flow), the maximal frequency shiR is measured because the cosine of 0 degree is 1. This explains the proper alignment of ultrasound beam or cursor with the flow of blobd to achieve accurate information.