A sensor delivery device for monitoring a fluid pressure within a vascular structure of a patient, the sensor delivery device comprising: an elongated tubular sheath sized for sliding along a guidewire, the sheath having an outer surface; a sensor assembly comprising: a fiber optic sensor; a housing surrounding the fiber optic sensor, the housing comprising a proximal end having a proximal aperture, a distal end, an inner surface adjoined to the outer surface of the sheath, and an outer surface between the proximal end and the distal end, the outer surface spaced from the inner surface and defining a distal aperture along a side or top of the housing, the outer surface of the housing being opposite the inner surface of the housing; a first cavity between the distal end of the sensor and the distal aperture of the housing, the distal aperture being in fluid communication with the first cavity; a filler extending distally outside of the housing and extending at least distally from the distal end of the housing, the filler having an outer surface that tapers inward toward the outer surface of the sheath as the filler extends distally; a second cavity in the filler adjoining the distal aperture of the housing, the second cavity comprising an opening at the outer surface of the filler, so that pressure is transmitted through the opening of the second cavity, then through the second cavity, the distal aperture of the housing, the first cavity, and to the sensor, and an optical fiber extending along the outer surface of the sheath, passing through the proximal aperture of the housing and adjoined to the fiber optic sensor at a distal end of the optical fiber. an elongated tubular sheath sized for sliding along a guidewire, the sheath having an outer surface; a sensor assembly comprising: a fiber optic sensor; a housing surrounding the fiber optic sensor, the housing comprising a proximal end having a proximal aperture, a distal end, an inner surface adjoined to the outer surface of the sheath, and an outer surface between the proximal end and the distal end, the outer surface spaced from the inner surface and defining a distal aperture along a side or top of the housing, the outer surface of the housing being opposite the inner surface of the housing; a first cavity between the distal end of the sensor and the distal aperture of the housing, the distal aperture being in fluid communication with the first cavity; a filler extending distally outside of the housing and extending at least distally from the distal end of the housing, the filler having an outer surface that tapers inward toward the outer surface of the sheath as the filler extends distally; a second cavity in the filler adjoining the distal aperture of the housing, the second cavity comprising an opening at the outer surface of the filler, so that pressure is transmitted through the opening of the second cavity, then through the second cavity, the distal aperture of the housing, the first cavity, and to the sensor, and an optical fiber extending along the outer surface of the sheath, passing through the proximal aperture of the housing and adjoined to the fiber optic sensor at a distal end of the optical fiber. a fiber optic sensor; a housing surrounding the fiber optic sensor, the housing comprising a proximal end having a proximal aperture, a distal end, an inner surface adjoined to the outer surface of the sheath, and an outer surface between the proximal end and the distal end, the outer surface spaced from the inner surface and defining a distal aperture along a side or top of the housing, the outer surface of the housing being opposite the inner surface of the housing; a first cavity between the distal end of the sensor and the distal aperture of the housing, the distal aperture being in fluid communication with the first cavity; a filler extending distally outside of the housing and extending at least distally from the distal end of the housing, the filler having an outer surface that tapers inward toward the outer surface of the sheath as the filler extends distally; a second cavity in the filler adjoining the distal aperture of the housing, the second cavity comprising an opening at the outer surface of the filler, so that pressure is transmitted through the opening of the second cavity, then through the second cavity, the distal aperture of the housing, the first cavity, and to the sensor, and an optical fiber extending along the outer surface of the sheath, passing through the proximal aperture of the housing and adjoined to the fiber optic sensor at a distal end of the optical fiber.