The integration of Fourier-domain detection strategies in optical coherence tomography has opened the possibility of high-resolution, cross-sectional imaging over large fields of view in biological tissues [1
]. Optical frequency domain imaging (OFDI), which is based on frequency domain ranging techniques, has been applied through narrow diameter catheters and endoscopes [5
] for imaging the coronary arteries [6
] and distal esophagus [10
] in patients. In intracoronary OFDI, a chirped beam probes the radial profile of the vessel being imaged. The probe collects the scattered light and transmits it to the proximal end of the catheter, where the scattered light interferes with a reference beam to produce a spectral fringe pattern that mimics the radial profile of the vessel wall. OFDI side-imaging probes consist of single mode fiber and distal optics with a microlens and a prism, or a polished ball lens fused at the fiber tip. The axial resolution, which is about 10 µm, depends on the wavelength sweep range of the imaging beam and the transverse resolution is determined by the spot size. As most probes incorporate small lenses for focusing the imaging beam, the transverse resolution is on the order of 10 to 20 µm.
In order to create a two- or three-dimensional map of vessels, the fiber probe is inserted into a vessel through a transparent stationary outer sheath and rotated with an automated pullback as shown in
. Here, a rotary junction motor spins the fiber in its sheath at a rate of about 100 revolutions per second, while a linear stepper motor pulls the fiber back within the sheath at speeds of 5 to 20 mm/s [6
]. Assembling cross-sectional images acquired during the pullback movement typically assumes that the measured imaging data are spaced at equal intervals along the helical track. Although the motor controls the velocity of the fiber probe relative to its protective sheath, the sheath itself can move significantly within the coronary artery during the cardiac cycle, resulting in image distortion (
). Although distortions arising from displacements of the sheath that are perpendicular to its axis can be partially compensated through simple surface-aligning algorithms [5
], these routines artificially straighten the vessel and, as a result, data processed in this manner lose the true, natural contour of the coronary wall. Accurately correcting longitudinal displacements based on image characteristics alone is even more challenging.
Intracoronary OFDI method and image data displacement due to vessel motion.
Longitudinal reconstruction of a 3D intracoronary OFDI data set obtained from a swine coronary artery in vivo. Yellow arrows mark locations of significant motion artifact caused by cardiac motion.
In this paper, we demonstrate a novel motion tracking system using a heterodyne Doppler interferometer to compensate for image distortion in intracoronary OFDI. This method uses dual, wavelength division multiplexed (WDM) monochromatic beams to track the relative radial and longitudinal vessel motion. Motion of the vessel is acquired by a frequency shift of the backscattered light caused by the Doppler effect. The re-registration of scans is performed with the Doppler frequency information, showing the improvement in endoscopic images.