aOCT, an adaptation of conventional OCT, generates macroscopic cross-sectional views of hollow organs rather than microscopic subsurface images by increasing the axial scanning distance from < 3 mm (from the probe head) to approximately 36 mm, allowing the entire inner perimeter of large airways to be viewed. The aOCT system uses emission from a broadband light source, with a mean wavelength of 1,310 nm, coupled into a single-mode optical fiber. This light is then split in two, with half directed to a structure of interest (eg, a bronchial airway), and the other half (the reference beam) to an optical delay line (Fig 2). This delay line contains a galvanometer mirror, which constantly varies the scanning distance such that delay line lengths up to 36 mm can be achieved. The reflected and backscattered light from these two paths is recombined at the photodetectors to produce an interference signal that corresponds to matched optical lengths and from which accurate probe-to-tissue distance can be derived. Our 1.8-m optical fiber is sheathed by a biplex stainless steel torque coil that protects the fiber as it rotates. The fiber terminates in a microoptical probe (diameter, 1.3 mm) that collimates the beam and deflects it at 90° to the probe (ie, approximately perpendicular to the airway wall). A plastic catheter (outer diameter, 2.2 mm) surrounds the entire fiber and coil, and is capped with a silicon plug (Fig 3). While the surrounding catheter remains stationary, the fiberoptic probe within rotates at 2.5 Hz to build up a rotational scan of the airway lumen. This enables cross-sectional images of structures such as the central airways to be obtained and displayed in a fashion similar to axial CT scan slices. Probe rotation is controlled mechanically by a motorized translation stage, which can also advance and retract the probe within the stationary catheter. Continuous translation during rotation enables 3D images to be built up. The aOCT unit scans hollow organs with diameters up to 72 mm and has been shown14–16 to be suitable for quantitative imaging of the human upper airway during wakefulness and sleep.