Kohji Ohbayashi

Fourier domain optical coherence tomography using optical demultiplexers imaging at 60,000,000 lines/s.

We describe high-speed Fourier domain optical coherence tomography (OCT) using optical demultiplexers (ODs) for spectral dispersion. The OD enables separation of a narrow spectral band of 14 GHz (0.11 nm) from a broadband incident light at 256 different frequencies in 25.0 GHz intervals centered at 192.2 THz (1559.8 nm). OCT imaging of 60,000,000 axial scans per second was achieved through parallel signal acquisition using 256 balanced photoreceivers to simultaneously detect all the output signals from the ODs in a Fourier domain OCT system. OCT imaging at a 16 kHz frame rate, 1100 A-lines per frame, 3 mm depth range, and 23 microm resolution was demonstrated using a resonant scanner for lateral scanning.

Spectral domain optical coherence tomography of multi-MHz A-scan rates at 1310 nm range and real-time 4D-display up to 41 volumes/second

An ultrafast frequency domain optical coherence tomography system was developed at A-scan rates between 2.5 and 10 MHz, a B-scan rate of 4 or 8 kHz, and volume-rates between 12 and 41 volumes/second. In the case of the worst duty ratio of 10%, the averaged A-scan rate was 1 MHz. Two optical demultiplexers at a center wavelength of 1310 nm were used for linear-k spectral dispersion and simultaneous differential signal detection at 320 wavelengths. The depth-range, sensitivity, sensitivity roll-off by 6 dB, and axial resolution were 4 mm, 97 dB, 6 mm, and 23 μm, respectively. Using FPGAs for FFT and a GPU for volume rendering, a real-time 4D display was demonstrated at a rate up to 41 volumes/second for an image size of 256 (axial) × 128 × 128 (lateral) voxels.

Full-range imaging of eye accommodation by high-speed long-depth range optical frequency domain imaging

We describe a high-speed long-depth range optical frequency domain imaging (OFDI) system employing a long-coherence length tunable source and demonstrate dynamic full-range imaging of the anterior segment of the eye including from the cornea surface to the posterior capsule of the crystalline lens with a depth range of 12 mm without removing complex conjugate image ambiguity. The tunable source spanned from 1260 to 1360 nm with an average output power of 15.8 mW. The fast A-scan rate of 20,000 per second provided dynamic OFDI and dependence of the whole anterior segment change on time following abrupt relaxation from the accommodated to the relaxed status, which was measured for a healthy eye and that with an intraocular lens.

Optical imaging of hard and soft dental tissues using discretely swept optical frequency domain reflectometry optical coherence tomography at wavelengths from 1560 to 1600 nm.

We have been developing a Mach-Zehnder type of optical frequency domain reflectometry optical coherence tomography (OFDR-OCT) that uses discretely swept superstructure-grating distributed Bragg- reflector (SSG-DBR) lasers developed for telecommunication fields and has a 12-mm-depth range. We report images obtained with L-band (1560 to 1600 nm) and C-band (1529 to 1568 nm) SSG-DBR sources at 0.5 micros/step, which is 20 times faster than the scanning speed used to obtain the images we reported previously. Despite the faster scanning, we obtain good OCT images of both hard and soft dental tissues in vitro and in vivo.

140-nm Quasi-Continuous Fast Sweep Using SSG-DBR Lasers

We demonstrate a single-mode and fast wavelength swept light source by employing superstructure-grating distributed Bragg reflector (SSG-DBR) lasers for use in optical frequency-domain reflectometry optical coherence tomography. A noteworthy feature is that our laser employs a thermal drift compensator (TDC). By utilizing the TDC, we achieved a fast wavelength sweep without any unexpected mode hopping. We newly designed and fabricated SSG-DBR lasers covering the following four wavelength bands: 1496-1529, 1529-1564, 1564-1601, and 1601-1639 nm. We achieved a stable operation for the whole wavelength range. A wide wavelength tuning of 143 nm was achieved within 1.4 ms. A single-mode, wide, and fast wavelength sweep was demonstrated.

Numerical Compensation of Dispersion Mismatch in Discretely Swept Optical-Frequency-Domain-Reflectometry Optical Coherence Tomography

We propose a numerical method to compensate dispersion mismatch in optical coherence tomography (OCT) based on optical-frequency-domain reflectyometry (OFDR). Dispersion mismatch causes phase distortion in interferograms depending on wavelength and results in a loss of resolution. We introduce a method to perform the numerical compensation directly to experimental interferograms in real space. Experimental tests are performed for the case of the Mach–Zehnder type OFDR-OCT using a super-structured-grating distributed-Bragg-reflector (SSG-DBR) laser, in which the wavenumber of the optical source is swept discretely by equal interval. It is shown that the method is effective to improve images for layered transparency sheets. We also apply the method to OFDR-OCT imaging of a human nail.

The impact of real-time 3d imaging by ultra-high speed optical coherence tomography in urothelial carcinoma

BackgroundOptical coherence tomography (OCT) has become a promising diagnostic tool in many medical fields. In particular, noninvasive real-time optical biopsy of internal organs is one of the most attractive applications of OCT, enabling in situ diagnosis of carcinoma at an early stage. We used an ultra-high speed OCT system for real-time three-dimensional (3D) imaging of three excised specimens of advanced urothelial carcinoma (UC) and investigated the association of the images with results from histopathological examination.Case presentationsCase 1 was a 69-year-old man underwent radical cystectomy for muscle-invasive UC (pT2). Case 2 was a 53-year-old man underwent laparoscopic nephroureterectomy and partial cystectomy for left ureter carcinoma (pT2) and case 3 was a 77-year-old woman underwent radical cystectomy for advanced bladder carcinoma (pT3b). Real-time 3D OCT images of normal bladder wall and ureter showed three layers, including the urothelium, lamina propria, and muscularis layer. In contrast, normal structure was not seen in the muscle-invasive UC area or the scar tissue area.ConclusionsThis study highlighted a new diagnostic method with potential application for UC diagnosis. We will investigate more cases in the future and expect improvement in the diagnosing efficiency of carcinoma in situ or organ-confined muscle-invasive cancer by cystoscopy or ureteroscopy with ultra-high speed OCT.

Tissue imaging by OFDR-OCT using an SSG-DBR laser

Optical coherence tomography (OCT) system based on optical frequency-domain reflectometry (OFDR) has been developed using a superstructure-grating distributed Bragg reflector (SSG-DBR) laser, which can tune the wavelength from 1533 to 1574 nm stepwise with tuning speed of 10micro s per 0.1 nm step. Theoretical expressions of OCT imaging by the discretely swept OFDR-OCT system are described. OFDR-OCT images are demonstrated for a few biological tissues; an extracted canine, human skin, human nail, and anterior segment of enucleated porcine eye.

Tuning of successively scanned two monolithic Vernier-tuned lasers and selective data sampling in optical comb swept source optical coherence tomography

Monolithic Vernier tuned super-structure grating distributed Bragg reflector (SSG-DBR) lasers are expected to become one of the most promising sources for swept source optical coherence tomography (SS-OCT) with a long coherence length, reduced sensitivity roll-off, and potential capability for a very fast A-scan rate. However, previous implementations of the lasers suffer from four main problems: 1) frequencies deviate from the targeted values when scanned, 2) large amounts of noise appear associated with abrupt changes in injection currents, 3) optically aliased noise appears due to a long coherence length, and 4) the narrow wavelength coverage of a single chip limits resolution. We have developed a method of dynamical frequency tuning, a method of selective data sampling to eliminate current switching noise, an interferometer to reduce aliased noise, and an excess-noise-free connection of two serially scanned lasers to enhance resolution to solve these problems. An optical frequency comb SS-OCT system was achieved with a sensitivity of 124 dB and a dynamic range of 55-72 dB that depended on the depth at an A-scan rate of 3.1 kHz with a resolution of 15 μm by discretely scanning two SSG-DBR lasers, i.e., L-band (1.560-1.599 μm) and UL-band (1.598-1.640 μm). A few OCT images with excellent image penetration depth were obtained.

A method of improving scanning speed and resolution of OFDR-OCT using multiple SSG-DBR lasers simultaneously

The superstructured-grating distributed-Bragg-reflector laser is a small (shorter than 1 mm in length) and relatively cheap swept source for optical-frequency-domain- reflectometry optical coherence tomography (OFDR-OCT), which practically enables use of multiple sources in a single OCT system. Simultaneous scanning of multiple sources over different wavelength regions and at different wavelength values in the same wavelength region enable improvement of the resolution and scanning speed, respectively. Those improvements have been demonstrated using C-band and L-band SSG-DBR sources.