Abstract: A sensing apparatus for performing sensing in a blood vessel of a biological subject includes a guidewire including a circumferential window positioned proximate a distal end, the guidewire being configured to be positioned in the blood vessel of the biological subject, an optical fiber extending substantially from a proximal end of the guidewire to a position proximate the window and a substantially conical reflector positioned proximate the window and aligned with an axis of the optical fiber. The conical reflector is configured to reflect stimulating radiation emitted from the optical fiber so that the stimulating radiation passes substantially radially through the window to thereby expose the blood vessel to the stimulating radiation and reflect sample radiation emitted from within the blood vessel so that the sample radiation is transmitted to the optical fiber.