text: Doppler analysis is the variation in frequency caused by motion toward or away from the observer. The classic example is the train whistle which sounds lower when it's moving away from the observer in the station or higher when it's moving toward the observer in the station. An observer next to the train, there will be no change (higher or lower) in the tone of the whistle. In fact, when the train comes by the station, the whistle tone when the train is next to you, is the tone an observer in the train (or when the train is just sitting still) hears. \n\nDoppler ultrasound for blood flow is much the same. The sensor is different (reflected high frequency sound) and the moving thing is different (fluid in tubes, ie, blood in arteries and veins), but the principle is the same. \n\nThere are of course, obvious differences. No one can stand right next to an artery and listen to the blood 'whistle' change tone up or down, and the moving thing (blood) is continuous, not a single thing like a locomotive, or even several of them.\n\nSo the ultrasound people have to do a slightly off line Doppler because they can't get right in line with the blood flow. It veers all over the place anyway as the arteries go from hither to thither. Look up one of those skeletal visible arteries human bodies and trace one of the arteries, say that big one on the inside of the thigh (it's the femoral artery). Not many rail straight stretches there.\n\nWhat is done instead is to look from the side at the artery, and calculate flow velocity from that measurement. More complex than the train whistle example, but it saves cutting into legs or whatever to get a sensor into 'in line' position. So what angle can be used?\n\nWell, neither 0 or 180 degrees are possible. Would have to dig in to get into position. And 90 degrees would be useless since it would look at blood flow from the side -- no relative motion toward or away to measure. Obviously, something in between (0 - 90 or 90 - 180 depending on your perspective) is the only practical choice. The actual path of the blood vessel being looked at (with respect to the skin surface where the ultrasound head will be) will affect the possible angles of the ultrasonic beam bounced off all that moving blood, but the favored angle you mention is probably dictated by the ultrasound machine (hardware or software) which may have limits of one sort or another. So for one machine the best angles may have to be between 45-60  degrees while another might range from 45-70 degrees. \n\nYou might want to check the manufacturer's web site, which might have a specification sheet or a user manual which would address the issue. A friendly physician or PA might be able to illuminate the issue, as might an ultrasound technician.