Devices and systems for determining fractional flow reserve. In at least one embodiment of a device for determining fractional flow reserve of a fluid within a luminal organ, the device comprises an elongated body sized and shaped to fit within a luminal organ, and at least two sensors positioned along the elongated body a predetermined distance from one another, wherein the device is operable to detect a first fluid with a first parameter having a first value using at least one of the at least two sensors when the device is positioned within the luminal organ, and wherein the device is further operable to detect a second fluid having a second parameter, wherein the second parameter of the second fluid has a second value different from the first value, upon introduction of the second fluid within the luminal organ at or near the at least two sensors, and wherein the device is further operable to determine fractional flow reserve when the device is positioned within the luminal organ at or near a stenosis, wherein the fractional flow reserve is based upon a flow velocity obtained by the device, a mean aortic pressure within the luminal organ, and at least one cross-sectional area at or near the stenosis.
