This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: ULTRASOUND, Subject name: Radiology
Optimal Doppler angle should be 
Select and explain. 
A. 90 degrees, B. 45-60 degrees, C. 60-80 degrees, D. 80-110 degrees
B: 45-60 degrees
Answer: B: 45-60 degrees 
Explanation: Ideal Doppler Angle for color doppler is 0 degrees ,Most blood vessels course parallel to the skin and zero Doppler angle is seldom obtainable. In addition, no Doppler shift is obtained at 90 degrees since the cosine of 90 degrees is 0. Thus, the optimal Doppler angle lies between 45 and 60 degrees.