Optical Coherence Tomography (OCT) is a cross-sectional optical imaging technique [1
] that can be used to obtain images of living human tissues with an axial resolution of several microns. OCT is routinely used in the field of ophthalmology [2
] and multiple medical applications for this technology have been proposed [4
] in other multiple scattering tissues. Recent research has demonstrated that OCT is capable of identifying the features of plaques that cause sudden cardiac death [9
] and dysplasia in Barrett's esophagus [11
]. Unfortunately, application of OCT in the fields of cardiology, gastroenterology, and ophthalmology is limited by its inability to obtain data when the surface height of the tissue varies by more than the total reference arm scan length or coherence gating range. This limitation of current OCT systems prevents imaging of many clinically relevant sites [11
Current OCT systems have a fixed coherence gate range [13
], typically ~3 mm, over which image information may be obtained. This range is selected as a compromise that allows imaging of tissue with a penetration of 1-1.5 mm while allowing for small (~1 mm) variations in distance between the probe and tissue surface. For tissues such as the proximal coronary arteries with a luminal diameter as large as 5 mm, a scanning range of 3 mm is not sufficient. The range requirement is severe for imaging coronary arteries since in many cases, the catheter rests against one side of the vessel wall, as it is shown in . As a result, the arterial wall opposite the catheter is not within the coherence gate range, preventing full visualization of one side of the arterial wall. For a large artery (), large portions of the vessel wall may be outside the gate range. Tissue height variations can be even greater in the gastrointestinal (GI) tract. As a result, circumferential scanning devices for the GI tract require deinsufflation [14
] in order to image the mucosal surface, which can preclude imaging of significant portions of the GI mucosa.
OCT images of coronary arteries obtained in vivo; A- Artery with the catheter resting against the vessel wall; B-Large artery with a significant portion of the image outside of the coherence range. Tick marks, 500 μm.
One solution that could accommodate large surface height variations would be to increase the coherence gating range of the interferometer. However, a larger range would stress the delay scanning mechanism and decrease the system sensitivity due to an increase in the interferometric signal bandwidth [13
In this paper, we present a simple but very efficient method, which we term adaptive ranging (AR), that enables an increased OCT scan range. This method adaptively detects the surface of the sample and adjusts the reference delay-scanning offset so that the surface data is always at the top of the image. Since this feedback control corrects for surface height variation, and tissue penetration depths do not typically exceed ~1.5 mm, the scan range for a single axial scan may be reduced to 1.5 mm, resulting in a commensurate increase in sensitivity. AR technology operates in real-time and does not require major hardware modifications to conventional OCT systems.