A system for locating a blood vessel in a patient and for penetrating the blood vessel, comprising: a sensor wire having: a hollow, elongate rigid tube including a lumen extending from a proximal portion to a distal portion, the tube having a longitudinal axis and being positionable at different angles relative to the blood vessel; an ultrasonic sensor coupled to the distal portion, the sensor configured to transmit and receive ultrasound waves distally along the longitudinal axis to detect Doppler shift; and at least one communication line extending from the sensor to a communication connection assembly positioned adjacent the proximal portion, wherein the rigid tube is configured to maintain the ultrasonic sensor and communication connection assembly in substantial alignment with the longitudinal axis during use; a processor configured to: acquire Doppler ultrasound data from a blood vessel through the sensor wire; analyze the data to determine the presence or absence and direction of blood flow in front of the sensor wire; characterized in that the processor is further configured to: output to a visual display the average Doppler shift associated with different angles and positions of the sensor wire with respect to the blood vessel of the patient; and indicate to a user an appropriate approach angle for penetration of the blood vessel before any actual penetration of tissue. a sensor wire having: a hollow, elongate rigid tube including a lumen extending from a proximal portion to a distal portion, the tube having a longitudinal axis and being positionable at different angles relative to the blood vessel; an ultrasonic sensor coupled to the distal portion, the sensor configured to transmit and receive ultrasound waves distally along the longitudinal axis to detect Doppler shift; and at least one communication line extending from the sensor to a communication connection assembly positioned adjacent the proximal portion, wherein the rigid tube is configured to maintain the ultrasonic sensor and communication connection assembly in substantial alignment with the longitudinal axis during use; a hollow, elongate rigid tube including a lumen extending from a proximal portion to a distal portion, the tube having a longitudinal axis and being positionable at different angles relative to the blood vessel; an ultrasonic sensor coupled to the distal portion, the sensor configured to transmit and receive ultrasound waves distally along the longitudinal axis to detect Doppler shift; and at least one communication line extending from the sensor to a communication connection assembly positioned adjacent the proximal portion, wherein the rigid tube is configured to maintain the ultrasonic sensor and communication connection assembly in substantial alignment with the longitudinal axis during use; a processor configured to: acquire Doppler ultrasound data from a blood vessel through the sensor wire; analyze the data to determine the presence or absence and direction of blood flow in front of the sensor wire; characterized in that the processor is further configured to: output to a visual display the average Doppler shift associated with different angles and positions of the sensor wire with respect to the blood vessel of the patient; and indicate to a user an appropriate approach angle for penetration of the blood vessel before any actual penetration of tissue. acquire Doppler ultrasound data from a blood vessel through the sensor wire; analyze the data to determine the presence or absence and direction of blood flow in front of the sensor wire; characterized in that the processor is further configured to: output to a visual display the average Doppler shift associated with different angles and positions of the sensor wire with respect to the blood vessel of the patient; and indicate to a user an appropriate approach angle for penetration of the blood vessel before any actual penetration of tissue. output to a visual display the average Doppler shift associated with different angles and positions of the sensor wire with respect to the blood vessel of the patient; and indicate to a user an appropriate approach angle for penetration of the blood vessel before any actual penetration of tissue.