The described endoscope-compatible double-balloon EOCT catheter allows imaging of the esophageal mucosa with and without direct balloon-tissue contact, which will allow us to study the influence of pressure on detection of dysplasia in BE. The two imaging schemes generate volumetric images of the esophagus rapidly without exchanging balloons. Deployment of the catheter through the endoscope is important because the two-scheme protocol is only possible with endoscopic visualization. Previous EOCT catheters, without balloons, were commonly designed for deployment through the endoscope [3
]. Endoscope-guided deployment of an EOCT catheter bears advantages such as visual guidance and placement of the catheter, documentation of the procedure, and correlation of OCT and endoscopic views of the same tissue sites. It will also facilitate biopsy-correlation studies and, importantly, minimize the time added to the procedure when EOCT is employed in the clinic. Endoscopic deployment has not been demonstrated in previous reports of balloon-EOCT imaging [22
]. A limitation of the current double-balloon EOCT system is that it is not designed to image arbitrarily long segments of BE. After investigating the consequences of balloon-tissue contact in BE surveillance, it will be necessary to design a probe capable of imaging long segments with optimum contact and pressure. In the demonstration presented here, the balloons were inflated to a larger diameter during double-balloon imaging than single-balloon imaging because the tissue within the gap has a smaller diameter than that supported by the balloons. Alternatively, if we find that it is important to maintain the balloons at a fixed diameter for a study, a shorter working-distance probe can be rapidly exchanged while the balloons remain in place in the esophagus.
The instrument reported here represents the first demonstration of a spectral-domain OCT system for gastrointestinal endoscopy. The usability of our SD-OCT system and image quality are comparable to previously demonstrated endoscopic swept light source OCT (SS-OCT) [21
]. While SD-OCT is the conventional technology in retinal OCT imaging (which is the dominant clinical application of OCT imaging) [33
], SS-OCT, also known as optical frequency domain imaging (OFDI), has been employed for all previous implementations of Fourier-domain OCT for endoscopy. There are several reasons for using SS-OCT. First, because of commercially available components, SD-OCT is more readily implemented than SS-OCT in the 830 nm range commonly used for retinal imaging, while at 1300 nm, commonly used in endoscopy, high quality components have been more readily available and more cost effective for SS-OCT than for SD-OCT. Rapidly scanning tunable lasers for SS-OCT were developed before fast line-scan cameras were available for SD-OCT at 1300 nm [21
]. Furthermore, balloon-based EOCT requires a relatively long axial imaging range, to accommodate the axial position of the tissue which varies with radial position and moves due to peristalsis. SS-OCT more readily accomplishes long axial range because it generally benefits from less fall-off [34
], can more easily resolve complex-conjugate ambiguity [35
], and is not limited by a fixed number of spectral samples, as is SD-OCT. The endoscopic SD-OCT system reported here makes use of an InGaAs line-scan camera with 1024 pixels and a readout rate of 47,000 lines per second. This enabled SD-OCT that is not only fast enough for clinical pull-back procedures, but also has sufficient axial range (4.2 mm) to accommodate the variance of tissue position that we experienced in swine esophagus in vivo
. InGaAs line-scan cameras appropriate for SD-OCT with more pixels and faster read-out will become available in the near future, further improving the feasibility of SD-OCT for endoscopic applications. The fall-off was improved by use of a linear-in-wavenumber spectrometer [32
], and the probe optics was corrected to create a nearly Gaussian beam, so that high quality images are readily obtained. From these results and observations, we conclude that SD-OCT is a feasible alternative to SS-OCT for endoscopic imaging. We employed the SD-OCT configuration so that in the future we can incorporate our previously reported ultra-broadband light source into the EOCT system for improvement of the axial resolution (~5 μm) [38
]. Ultrahigh resolution OCT reduces speckle size and enables visualization of finer morphological features in esophageal images, as previously reported in a time-domain configuration [17
], and may potentially benefit BE diagnosis. However, for a fixed-sized detector array, accommodating a broader bandwidth would trade-off with a shorter axial imaging range.
Some preliminary observations of the effects of balloon-tissue contact are apparent in and . The natural mucosal surface topology is apparent in the double-balloon image, whereas the mucosal surface was compressed and smoothed by the balloon in the single-balloon image. The balloon-flattened surface resulted in more stable illumination and therefore more uniform image brightness. However, unpublished data has shown that surface topology may be useful for detecting dysplasia in BE [40
]. Detailed structure in the muscularis mucosa, especially blood vessels, is more clearly observed in the double-balloon images. The imaging depth in the single-balloon images is greater, with multiple layers of muscularis propria visible, which is consistent with previous observations [30
]. While we expected the double-balloon imaging scheme to result in higher variability of the axial position of the tissue as a function of radial position, the difference we observed was small, as seen in . However, the movement of the tissue due to peristalsis was greater with double-balloon imaging compared to single-balloon imaging. These observations suggest the need to further investigate the advantages and disadvantages of balloon-tissue contact and pressure, how tissue features in BE are altered, and how the changes affect detection of dysplasia in BE.