Final answer:
The vascular probe of a Doppler device should be positioned at an angle of 45 degrees for optimal measurement of blood flow velocity and detection of Doppler shift.
Step-by-step explanation:
When using a Doppler device to measure blood flow, the vascular probe should be placed at an angle that is neither perpendicular nor in line with the blood flow. The recommended angle is typically around 45 to 60 degrees. However, the ideal angle is 45 degrees, as it is a compromise between parallel and perpendicular that provides accurate velocity measurements while ensuring the Doppler shift is detectable and optimally measurable. Placing the probe at a 90 degrees angle would reflect no Doppler shift, while a 0-degree angle might not be practical as it requires the probe to be aligned completely parallel to the blood flow.