Flow measurement by using the signal decrease of moving scatterers in spatially encoded Fourier domain optical coherence tomography

The depth reflectivity profile of Fourier domain optical coherence tomography (FD-OCT) is estimated from the inverse Fourier transform of the spectral interference signals (interferograms). As a result, the axial resolution is fundamentally limited by the coherence length of the light source. We demonstrate that using the autoregressive spectral estimation technique instead of the inverse Fourier transform, to analyze the spectral interferograms can improve the axial resolution. We name this method spectral estimation OCT (SE-OCT). SE-OCT breaks the coherence length limitation and improves the axial resolution by a factor of up to 4.7 compared with FD-OCT. Furthermore, SE-OCT provides complete sidelobe suppression in the depth point-spread function, further improving the image quality. We demonstrate that these technical advances enables clear identification of corneal endothelium anatomical details ex vivo that cannot be identified using the corresponding FD-OCT. Given that SE-OCT can be implemented in the FD-OCT devices without any hardware changes, the new capabilities provided by SE-OCT are likely to offer immediate improvements to the diagnosis and management of diseases based on OCT imaging.

Optical design of a spectrometer for all-reflective optics based line scan Fourier domain optical coherence tomography (FD-OCT) imaging has been reported in this work for high-speed scanning. FD-OCT imaging data acquisition offers significantly improved imaging speed in the depth direction compared to conventional time domain optical coherence tomography (TD-OCT). On the other hand, line focused scanning improve imaging speed in the transverse direction compare to commonly used flying spot scanning. Combination of FD-OCT acquisition and line focused scanner can give higher imaging speed. Spectrometer is a critical submodule in FD-OCT system. Apart from the spectrometer optical resolution, and detector array resolution, the chromatic aberration should be considered to design a high-quality FD-OCT imaging spectrometer. The proposed imaging spectrometer consists of a planer reflective grating, off-axis parabolic cylindrical mirror and a CCD array detector. Mirror focusing reduces the chromatic aberration because of its insensitivity to the wavelength of the laser beam, therefore the spectrometer image quality enhanced by the reflective optics focusing. Spot profile fall-off characteristic was analyzed by using ZEMAX optical design software.

A digital frequency ramping method (DFRM) is proposed to improve the signal-to-noise ratio (SNR) of Doppler flow imaging in Fourier-domain optical coherence tomography (FDOCT). To examine the efficacy of DFRM for enhancing flow detection, computer simulation and tissue phantom study were conducted for phase noise reduction and flow quantification. In addition, the utility of this technique was validated in our in vivo clinical bladder imaging with endoscopic FDOCT. The Doppler flow images reconstructed by DFRM were compared with the counterparts by traditional Doppler FDOCT. The results demonstrate that DFRM enables real-time Doppler FDOCT imaging at significantly enhanced sensitivity without hardware modification, thus rendering it uniquely suitable for endoscopic subsurface blood flow imaging and diagnosis.

Blood flow measurement with spectrometer-based Fourier domain optical coherence tomography (FD OCT) is limited by the motion-induced signal fading and the resulting reduction of flow sensitivity. In this study, a combination of the established Doppler OCT and the numerically simulated signal damping due to obliquely moved scatterers is used to estimate the systolic blood flow velocities in the in vivo mouse model at which the standard Doppler OCT does not work any longer.

Measuring oxygen delivery in brain tissue is important for identifying the pathophysiological changes associated with brain injury and various diseases such as cancer, stroke, and Alzheimer's disease. We have developed a multi-modal imaging system for minimally invasive measurement of cerebral oxygenation and blood flow in small animals with high spatial resolution. The system allows for simultaneous measurement of blood flow using Fourier-domain optical coherence tomography, and oxygen partial pressure (pO₂) using either confocal or multiphoton phosphorescence lifetime imaging with exogenous porphyrin-based dyes sensitive to dissolved oxygen. Here we present the changes in pO₂ and blood flow in superficial cortical vessels of Sprague Dawley rats in response to conditions such as hypoxia, hyperoxia, and functional stimulation. pO₂ measurements display considerable heterogeneity over distances that cannot be resolved with more widely used oxygen-monitoring techniques such as BOLD-fMRI. Large increases in blood flow are observed in response to functional stimulation and hypoxia. Our system allows for quantification of cerebral metabolic rate of oxygen (CMRO2) with high spatial resolution, providing a better understanding of metabolic dynamics during functional stimulation and under various neuropathologies. Ultimately, better insight into the underlying mechanisms of neuropathologies will facilitate the development of improved therapeutic strategies to minimize damage to brain tissue.

Standard high-speed Fourier domain optical coherence tomography (FD-OCT) and a modified version of FD-OCT; a line-field FD-OCT (LF-FDOCT) are demonstrated. LF-FDOCT is using the principle of FD-OCT for its depth resolution and a one-dimensional imaging optics for its one-dimensional lateral resolution. A mechanical C-scan drived by a galvano scanner is introduced into the FD-FDOCT, which shows a cross sectional OCT image without any mechanical scanning. The improved version of FD-OCT visualizes the three-dimensional structure of a sample with only one-dimensional scanning. Both standard FD-OCT and LF-FDOCT are applied to dermatological applications and visualize the inner structure of an in vivo human fingerpad.

The measurement of ocular blood flow is important in studying the pathophysiology and treatment of several leading causes of blindness. A pilot study was performed to evaluate the total retinal blood flow in glaucoma patient using Fourier domain optical coherence tomography. For normal people, the measured total retinal flow was between 40.8 and 60.2 μl/minute. We found that eyes with glaucoma had decreased retinal blood flow and average flow veocity, while the venous cross sectional areas were essentially the same as normal. The decrease in blood flow was highly correlated with the severity of visual field loss.

Optical coherence tomography (OCT) is a new imaging modality that is being actively used in a variety of medical applications. Currently, most of the OCT systems operate in the time domain, which requires scanning the optical path length in the reference arm in order to obtain the in-depth profile, i.e. A scan. This however limits the system scanning speed. To avoid the axial scanning and therefore improve the system scanning speed, a novel OCT system is recently proposed by a number of groups that operates in the frequency domain, i.e. the spectral OCT. In this paper, we report the spectral OCT system being constructed at Tianjin University. The system has a dynamic range at 78dB and is capable of scanning speed at 12 seconds per image, largely limited by the bottleneck of data transferring from the CCD camera currently employed to the computer. The SOCT imaging results obtained from the animal tissues (cornea from an intact porcine eye) <i>in vitro</i> will be presented.

High speed complex full-range Fourier domain optical coherence tomography (FD-OCT) is demonstrated. This FD-OCT requires only a single A-scan for each single transversal position for full-range Fourier domain optical coherence tomography. The Fourier transform method is applied along the direction of the B-scan to reconstruct complex spectra, and the complex spectra compose a full-range OCT image.

Optical coherence tomography (OCT) is a non-invasive imaging method widely used in ophthalmology. Recent developments have produced OCT devices for imaging the skin. The purpose of this study was to investigate Fourier Domain OCT morphological features of periocular basal cell carcinoma (BCC) in correlation with conventional histopathology. Consecutive patients with periocular nodular BCC were prospectively examined with VivoSight OCT (Michelson Ltd) prior to surgical excision. OCT slice mode images were analysed using criteria defined for conventional and HD-OCT; the images were correlated to haematoxylin and eosin stained histology sections. A total of 15 patients with periocular BCC were recruited. Three categories of BCC morphological features were identified from slice mode OCT images: (1) Epidermal changes included epidermal thinning (15/15; 100%), ulceration and crusting (5/15, 33.3%) and decreased density of hair follicles (8/15; 53.3%); (2) Intralesional features included hyporeflective nodules (15/15; 100%), hyper-reflective edges (15/15; 100%) and hyporeflective central necrosis (3/15; 20%) (3) Perilesional features included hyporeflective borders (11/15; 73%), hypereflective margins (15/15; 100%) and dilated blood vessels (5/15; 33%). This study demonstrated that Fourier Domain OCT imaging offers additional information in the identification of morphological features of nodular BCC compared to conventional OCT diagnostic criteria. VivoSight produced fast, non-invasive imaging of skin lesions in the periocular region and high correlation with histology. Further studies are necessary to investigate OCT features of different histological subtypes of BCC.

Optical coherence tomography (OCT) is a noninvasive imaging modality generating cross sectional and volumetric images of translucent samples. In Fourier domain OCT (FD OCT), the depth profile is calculated by a fast Fourier transformation of the interference spectrum, providing speed and SNR advantage and thus making FD OCT well suitable in biomedical applications. The interference spectrum can be acquired spectrally resolved in spectral domain OCT or time-resolved in optical frequency domain imaging (OFDI). Since OCT images still suffer from motion artifacts, especially under in vivo conditions, increased depth scan rates are required. Therefor, the principle of Fourier domain mode locking has been presented by R. Huber et al. circumventing the speed limitations of conventional FD OCT systems. In FDML lasers, a long single mode fiber is inserted in the ring resonator of the laser resulting in an optical round trip time of a few microseconds. Sweeping the wavelength synchronously by a tunable Fabry-Perot filter can provide wavelength sweeps with repetition rates up to a few MHz used for OFDI. Imaging of subpleural lung tissue for investigation of lung dynamics and its elastic properties is a further biomedical application demanding high-speed OCT imaging techniques. For the first time, the visualization of subpleural alveolar structures of a rabbit lung is presented by the use of an FDML-based OCT system enabling repetition rates of 49.5 kHz and 122.6 kHz, respectively.

We describe a high-speed Fourier domain optical coherence tomography (OCT) using optical de-multiplexers for spectral dispersion of interferograms. The optical de-multiplexer enables to separate 256 narrow spectral bands from a broadband incident light in 25.0 GHz frequency interval centered at 192.2 THz (1559.8 nm) and allows simultaneous detection of all the bands at the speed of DAQ. Using the optical de-multiplexers into a Fourier domain OCT system as spectral analyzers, OCT imaging of 60,000,000 axial scans per second has been achieved. Using a resonant scanner for lateral scan, 16 kHz frame rate, 1400 A-lines per frame, 3 mm depth range, 23 micron meter resolution OCT imaging has been demonstrated.

An improved Fourier domain Optical Coherence Tomography (FdOCT) technique is proposed as a new kind of ophthalmic OCT, which enables non-invasive imaging of the retina, the iris and the lens in vivo without an axial mechanical scan of the reference mirror. The FdOCT tomograms of various parts of human eye in vivo, to our knowledge, are the first obtained to date. The detailed images of the human eye are reconstructed from spectral data by the differential method. The tomograms are free of the parasitic autocorrelation terms.

Fixed-point arithmetic is used to provide faster and smaller implementations in many digital signal processing applications, including medical imaging, at the expense of decreased accuracy. In particular, when a Fast Fourier Transform (FFT)-Inverse Fast Fourier Transform (IFFT) pair are required as part of the calculation, the error introduced into the calculations can be significant. For some applications, such as Fourier Domain Optical Coherence Tomography (FD-OCT), this degradation is unacceptable. Our study shows that using a conventional fixed-point FFT-IFFT pair, such as Xilinx's FFT core, can produce an average 6-bit error for a 1024-point FFT using 12-bit input data in a 32-bit arithmetic system. The majority of the error is caused by quantization effects, particularly on the phase information of input signal. For this reason, in phase sensitive applications such as FD-OCT, the error dominates the fixed-point calculation: 78% in 16-bit and 51% in 32-bit systems. This work presents a parameterized 32 to 4096-point integer FFT implementation for FPGAs that uses a Split-Radix algorithm to reduce the number of multiplies and improve latency. Integer FFTs are perfectly reconstructible, with zero reconstruction error. Here, we specifically analyze a 1024-point Integer Split-Radix FFT (Int-SRFFT) and IFFT pair that perfectly reconstructs the original 12-bit input data using 22-bit arithmetic, compared to the average 6-bit error for a 1024-point FFT using 32-bit fixed-point arithmetic. The pipelined architecture of this design has a latency of 29.06us for a 1024-point FFT, and a throughput of more than 34 thousand 1024-point FFTs/second for a 22-bit datapath at an operating frequency of 274MHz. Although our Int-SRFFT is perfectly reconstructible, compared to Xilinx's fixed-point FFT, it has ~6% more flipflops, ~63% more LUTs, 5.3x more BRAMs, and a ~44% increase in latency. However, compared to a previous fixed-point SRFFT design on an FPGA, our throughput is 15.5x greater.

In vivo measurement of blood flow in the retina has been made possible with the advent of Fourier domain optical coherence tomography (OCT). Doppler OCT has seen many advances in recent years in algorithms used for quantifying blood flow. We compare the relative retinal blood flow estimates as measured by the standard phase-resolved (PR) algorithm and the more recent moving-scatterer-sensitive (MSS) algorithm as a function of vessel size. We find that the PR-to-MSS flow ratio significantly decreases with decreasing vessel diameter. We also develop a simulation to approximate the scattering from blood cells in tissue and compare the relative blood flow estimates. The flow ratio measured with simulation closely matches that found in vivo. Our simulation predicts that whereas PR underestimates the flow, MSS overestimates it. Our simulation may help to correct for algorithm bias in in vivo retinal flow estimates.