The development of cuffless blood pressure monitoring devices is attracting attention. The method of measuring vessel diameter from ultrasound A-mode waveforms is expected to be applied to daily blood pressure measurement due to the simplicity of the device. On the other hand, there is a problem in that the received intensity of the reflected wave from the blood vessel wall decreases due to displacement caused by body movement, resulting in incorrect measurement of the vessel diameter. In this paper, we examine how displacement of the positional relationship between the sensor and the blood vessel affects the measurement of blood vessel diameter and present a solution to the sensitivity reduction caused by angular displacement in the direction of the vessel axis by sending ultrasonic waves radially in the direction of the vessel axis. As a verification, a prototype sensor with an acoustic lens was fabricated. Evaluation of the sensor using a silicone resin tube model of a blood vessel showed that it maintained received intensity of about 50%of the strongest received intensity over a range of ±10°and the calculated blood vessel diameter was accurate to±0.05mm over a range of ±10°. Since this technology does not require a complex system, it is expected to be applied to devices that daily blood pressure measurement without the patient’s consciousness.
