This invention pertains specifically to locating a blood vessel of a patient for cannulation, and guiding a cannula for subsequent insertion into the blood vessel, for purposes of adding substances (e.g. medicine), removing substances (e.g. blood samples), monitoring the patient (e.g. blood pressure), or the like. 
The procedure of positively locating a blood vessel for venipuncture is difficult under certain conditions, including where the patient is in hypotension, where the blood vessel is not close to the skin due to patient obesity or other factors, where the blood vessel tends to displace during cannulation due to factors such as thickening of the wall of the blood vessel, or the like. 
When locating a blood vessel for cannulation, it is desirable to accurately sense the location of the blood vessel through noninvasive means.  Due to normal curvature along the length of the blood vessel, it may be difficult to accurately place a cannula if only one point on the blood vessel is located.  It is thus desirable to locate the blood vessel, account for orientation of the blood vessel, and insert the cannula adjacent the location point or points to reduce the possibility of missing the blood vessel. 
Once the blood vessel has been located, the cannula must be aligned along the length of the blood vessel at an angle suitable for insertion into the blood vessel, and in an imaginary plane generally passing through the blood vessel.  If the angle is too great, the cannula may penetrate entirely through the blood vessel, including through the far wall of the blood vessel.  If the angle with the blood vessel is too small, penetration may be difficult, or may fail.  If the length of the cannula is outside the recited imaginary plane, the cannula may pass sideways out of the blood vessel whereby suitable cannulation is also frustrated. 
It is an object of this invention to provide a blood vessel cannulation device which enables effectively sensing a blood vessel at a first location, and correspondingly enables effectively inserting a cannula adjacent the first location, in alignment along the blood vessel, and thus into the blood vessel. 
It is another object of this invention to provide a blood vessel cannulation device which enables sensing the blood vessel, accounts for the orientation of the blood vessel by aligning the sensor location point or points with the blood vessel, and facilitates effective insertion of a cannula into the blood vessel at a suitable angle adjacent the sensed location. 
It is a further object of this invention to provide a blood vessel cannulation device which enables sensing the location of a blood vessel through the use of multiple sensors with distinguishable outputs to allow for rapid and accurate location of the blood vessel, and accurate alignment of the sensors and cannula with the blood vessel.