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.