An apparatus for assessing the severity of stenosis in a blood vessel includes an elongate body including a distal portion and a centering assembly. The centering assembly is actuatable to selectively center the elongate body in the vessel. A pressure sensor is disposed adjacent the centering assembly and is configured to detect fluid pressure in the vessel. A processing system receives the measured pressure from the pressure sensor, receives data representing the cross-sectional area of the vessel, receives data representing the size of the distal portion, calculates a offset correlation based on the size of the distal portion and based on the size of the vessel, and calculates a fractional flow reserve (FFR) for the vessel as an index of stenosis severity taking into account the offset correlation and the measured fluid pressure from the pressure sensor.
