Final answer:
The most significant Doppler shift for measuring peak velocity during a carotid duplex ultrasound is observed at scanning angles of 0 or 180 degrees because the ultrasound beam is then parallel to the blood flow.
Step-by-step explanation:
The most significant Doppler shift during a carotid duplex ultrasound is achieved when the scanning angle is closest to zero degrees relative to the direction of blood flow. This occurs at angles of either 0 or 180 degrees. When the ultrasound beam is parallel to the direction of blood flow, the observed frequency change — or Doppler shift — is greatest, allowing for the most accurate measurement of peak velocity.
If, for example, there is a constriction in an artery, blood must move faster through the constriction to maintain the same flow rate. This change in velocity will be most pronounced and measurable by Doppler ultrasound when the incident angle of the ultrasound beam is either directly aligned with the flow or directly opposite to it (0 or 180 degrees). In contrast, at a 90-degree angle, there would be no Doppler shift, as the component of velocity along the beam direction is zero.