We present a new optics design for the OCT balloon imaging catheter. Different from the reported elegant approach where a single lens was formed at the end of a fiber (e.g., by thermal melting) and the engineering conditions were empirically determined for each desired set of resolution and working distance,¹⁰ the new design utilized an imaging beam mode-field reducer and compound lenses, with the optical parameters well predicted by ray tracing and the engineering process greatly simplified. The OCT balloon catheter consisted of an 18-mm-diam transparent balloon (when inflated) with a 1.4-mm inner lumen [Fig. 1(a)] and a 1.3-mm-diam OCT imaging catheter of a 39-µm focused spot size at an ~9-mm working distance, situated within the inner lumen of the balloon [Figs. 1(a) and 1(b)]. The basic schematic of the new distal end optics design for the imaging catheter is shown in Fig. 1(c). The design utilized a microcompound lens made of a glass rod, and two gradient-index (GRIN) rod lenses (uncoated). The glass rod and the first GRIN rod lens formed a spot size reducer, which tightly focused the beam from the single-mode fiber (SMF) down to a size smaller than the mode field diameter of the SMF. This first focused spot was then relayed to the sample by the second GRIN rod lens with a lager diameter (e.g., 1.0 mm). The beam would diverge at a much faster rate after the first GRIN rod lens due to the reduced spot size, and thus a larger numerical aperture of ~0.5 was used for the second GRIN rod lens.

To facilitate the engineering process, the glass rod and the first GRIN rod lens were chosen to have the same diameter as an SMF (i.e., 125 µm), permitting these components to be robustly connected one after another using thermal fusion. For a 0.3-pitch 125-µm GRIN rod lens (the first GRIN lens), the length of the glass rod (~0.3 mm) that connected the SMF and the first GRIN lens must be carefully selected to achieve the smallest focused spot size at the exit surface of the first GRIN lens. This spot size was estimated to be 3.5 µm by ray tracing, representing a significant reduction from the original SMF core size (i.e., 9.3 µm). The final spot size and working distance were controlled by the spacer gap between the first and second GRIN rod lenses [see Fig. 1(c)]. This gap (~150 µm) was fine tuned during the catheter assembling process so that the desired working distance was achieved (~9 mm). The small gap between the two GRIN rod lenses permitted use of optical cement to glue together the two lenses of different diameters (125 µm and 1 mm, respectively), which is similar to the procedure used in engineering a traditional OCT imaging catheter of a single lens and a short working distance.⁹ When used in series, these components at the distal end of the catheter formed a microcompound lens that allowed the catheter to achieve our desired working distance, while maintaining a reasonably small focused spot size, a small overall diameter, and a very simple, robust engineering process. The microprism (with the hypotenuse coated with aluminum) was the last optical component that was glued onto the second GRIN lens to redirect the beam 90 deg toward the tissue. The entire distal end optics of the catheter was then encased in a metal guard for protection. A hollow metal coil of a 0.9-mm outer diameter was used to house the SMF that ran from the proximal to the distal end of the imaging catheter and to transfer the torque for circular imaging. The measured parameters of the catheter were a working distance of 9.6 mm and a spot size of 39 µm, very close to the target values predicted by ray tracing (i.e., 9.0-mm working distance and 34.5-µm focused spot size). The outer diameter of the imaging catheter including the protection guard was approximately 1.3 mm, which fitted easily into the inner lumen of the balloon. The proximal end of the catheter was connected to a fiber optic rotary joint [not shown in Fig. 1(c)] which functioned as the catheter scanning mechanism. The rotary joint was then placed on a computer-controlled, hand-held linear translation stage with micrometer precision. A 3-D spiral beam scanning pattern was created by pulling back the rotary joint (along with the imaging catheter within a stationary balloon) while the joint was rotating, which is similar to the pullback operation of an intravascular ultrasound (IVUS) catheter.¹¹

Preliminary real-time OCT imaging with the balloon catheter was experimentally demonstrated using a swept source OCT system (SSOCT) [Fig. 2(a)] where a frequency swept light source was employed with a center wavelength of 1310 nm, a full width half maximum (FWHM) bandwidth of ~106nm, and a sweep rate of 6 kHz (corresponding to 12,000 axial scans/sec). The light from the source was amplified to a total power of ~25 mW using a semiconductor optical amplifier (SOA).¹² A portion (~5%) of the source light from the SOA was sent to a stand-alone interferometer that had an optical path length offset between its two arms roughly equal to the designed imaging depth (i.e., ~2.4 mm) to produce optical comb signals for triggering data acquisition. The remaining light was coupled to the OCT setup, which utilized a balanced detection scheme. The incident power at the tissue was approximately 4 mW. The optical spectrum after the SOA is shown in Fig. 2(b). Since the SOA was operating at its saturation current, the powers of forward and backward frequency sweeps were approximately equal. At 6-kHz sweep rate, full-circumferential images were acquired (by the balloon imaging catheter), processed, displayed, and saved in real time at 4 frames/sec with each frame consisting of 2000 × 3000 pixels (axial × circumferential). The measured resolution of the imaging system was 12 µm (axial) and 39 µm (circumferential), with a detection sensitivity of ~110 dB (at a 4-mW incident power on the sample).

In vivo full-circumferential 3-D imaging of pig esophagus was conducted with the balloon imaging catheter along with the SSOCT system. The animal was sedated and an overtube was inserted into the upper esophagus. The deflated balloon was passed through the overtube and into the esophagus. An endoscope was then passed through the overtube behind the balloon to monitor the balloon position. A 2-cm-long segment of the esophagus was imaged using the spiral scanning pattern (rotation plus pullback) with 20-µm spacing between adjacent circumferential frames. The entire imaging took ~4 min. A real-time 3-D spiral imaging movie is shown in Video 1. A representative 2-D cross sectional image (snapshot) is displayed in Fig. 3(a). The image clearly shows that the balloon catheter, in conjunction with the SSOCT system, is capable of delineating the normal layered structures of the esophagus in vivo, including the epithelium, lamina propria, muscularis mucosae, submucosae, and muscularis propria. The reconstructed 3-D image from the series circumferential images with a 20-µm pitch is shown in Fig. 3(b). These initial results demonstrate the ability of the as-designed OCT balloon imaging catheter to generate full-circumferential cross sectional images over a significant longitudinal length of the esophagus with a high resolution.