An apparatus is provided for cannulation of blood vessels. The apparatus comprises a sensor assembly including two linear transducer arrays oriented perpendicular to each other to form a “T” shape to provide ultrasound images of at least one blood vessel in a portion of a patient's body in two perpendicular planes. The sensor assembly includes a housing having a graphically marked surface to facilitate orientation of the sensor assembly on the patient and guidance of a needle toward a desired target vessel during the cannulation procedure. The housing is configured to engage a frame element having a pair of wings extending laterally therefrom, the wings each bearing a magnet to cooperate with a reference location element secured to a patient's skin and bearing magnetically responsive shims thereon to place, align and secure the sensor assembly to the patient's skin at a desired location.