Optical coherence tomography.

Optical coherence tomography (OCT) is a non-contact method for imaging the topological and internal microstructure of samples in three dimensions. OCT can be configured as a conventional microscope, as an ophthalmic scanner, or using endoscopes and small diameter catheters for accessing internal biological organs. In this Primer, we describe the principles underpinning the different instrument configurations that are tailored to distinct imaging applications and explain the origin of signal, based on light scattering and propagation. Although OCT has been used for imaging inanimate objects, we focus our discussion on biological and medical imaging. We examine the signal processing methods and algorithms that make OCT exquisitely sensitive to reflections as weak as just a few photons and that reveal functional information in addition to structure. Image processing, display and interpretation, which are all critical for effective biomedical imaging, are discussed in the context of specific applications. Finally, we consider image artifacts and limitations that commonly arise and reflect on future advances and opportunities.

Single shot, time-resolved measurement of the coherence properties of OCT swept source lasers.

A novel, time-resolved interferometric technique is presented that allows the reconstruction of the complex electric field output of a swept source laser in a single-shot measurement. The power of the technique is demonstrated by examining a short cavity swept source designed for optical coherence tomography (OCT) applications with a spectral width of over 100 nm. The novel analysis allows a time-resolved real-time characterization of the roll-off, optical spectrum, linewidth, and coherence properties of a dynamic, rapidly swept laser source.

Dynamics of a short cavity swept source OCT laser.

We investigate the behaviour of a short cavity swept source laser with an intra cavity swept filter both experimentally and theoretically. We characterise the behaviour of the device with real-time intensity measurements using a fast digital oscilloscope, showing several distinct regimes, most notably regions of mode-hopping, frequency sliding mode-locking and chaos. A delay differential equation model is proposed that shows close agreement with the experimental results. The model is also used to determine important quantities such as the minimum and maximum sweep speeds for the mode-locking regime. It is also shown that by varying the filter width the maximum sweep speed can be increased but at a cost of increasing the instantaneous linewidth. The consequent impacts on optical coherence tomography applications are analysed.

Projection OCT fundus imaging for visualising outer retinal pathology in non-exudative age-related macular degeneration.

AIMS: To demonstrate ultrahigh-resolution, three-dimensional optical coherence tomography (3D-OCT) and projection OCT fundus imaging for enhanced visualisation of outer retinal pathology in non-exudative age-related macular degeneration (AMD). METHODS: A high-speed, 3.5 mum resolution OCT prototype instrument was developed for the ophthalmic clinic. Eighty-three patients with non-exudative AMD were imaged. Projection OCT fundus images were generated from 3D-OCT data by selectively summing different retinal depth levels. Results were compared with standard ophthalmic examination, including fundus photography and fluorescein angiography, when indicated. RESULTS: Projection OCT fundus imaging enhanced the visualisation of outer retinal pathology in non-exudative AMD. Different types of drusen exhibited distinct features in projection OCT images. Photoreceptor disruption was indicated by loss of the photoreceptor inner/outer segment (IS/OS) boundary and external limiting membrane (ELM). RPE atrophy can be assessed using choroid-level projection OCT images. CONCLUSIONS: Projection OCT fundus imaging facilities rapid interpretation of large 3D-OCT data sets. Projection OCT enhances contrast and visualises outer retinal pathology not visible with standard fundus imaging or OCT fundus imaging. Projection OCT fundus images enable registration with standard ophthalmic diagnostics and cross-sectional OCT images. Outer retinal alterations can be assessed and drusen morphology, photoreceptor impairment and pigmentary abnormalities identified.

Correlation of spectral optical coherence tomography with fluorescein and indocyanine green angiography in multiple evanescent white dot syndrome.

AIMS: To determine the spatial location of lesions in multiple evanescent white dot syndrome (MEWDS) with the aid of spectral optical coherence topography (SOCT), fluorescein angiography (FA) and indocyanine green angiography (ICGA). METHODS: A novel method of three-dimensional SOCT data analysis called reflectivity maps was introduced. The reflectivity maps display the distribution of a back-reflected intensity taken only from individual retinal layers located at specific distance from the reference plane. Reflectivity maps of the inner retina, the junction between photoreceptor inner and outer segments (IS/OS), retinal pigment epithelium and choroid of the patient with MEWDS were created and correlated with FA and ICGA. RESULTS: During the acute stage of MEWDS, the reflectivity map of the IS/OS junction displayed areas of reduced reflectivity that showed a strong positive correlation with hypofluorescent ICGA spots and a weaker but positive correlation with hyperfluorescent FA dots. SOCT examination did not reveal any pathological changes involving either any other retinal layers or the inner choroid. CONCLUSION: Disseminated disruptions of the IS/OS junction seen on SOCT cross-sectional images in the acute stage of MEWDS form the pattern of spots that can be correlated with those revealed by ICGA. This suggests that hypofluorescent ICGA spots indicate alternations in the retinal pigment epithelium-photoreceptor complex and do not represent inflammatory choroidal lesions.

Retinal imaging by spectral optical coherence tomography.

PURPOSE: To demonstrate applicability of high speed spectral optical coherence tomography (SOCT) method for imaging retinal pathologies in clinical conditions. METHODS: SOCT was performed in 67 eyes with different macular diseases. Examinations were carried out with the prototype SOCT instrument constructed in the Institute of Physics, Nicolaus Copernicus University, Torun, Poland. A broadband superluminescent diode was used as a light source. RESULTS: The disturbances of retinal layer structure concerning mainly outer segments of photoreceptors were observed in case of central serous chorioretinopathy and choroidal neovascularization in age-related macular degeneration. Large drusen were often related to significant changes of outer nuclear layer thickness and reflectivity. CONCLUSIONS: SOCT detects small disturbances of the retinal structure and helps to precisely determine layers involved in different pathologies.

In vivo measurement of retinal physiology with high-speed ultrahigh-resolution optical coherence tomography.

Noninvasive in vivo functional optical imaging of the intact retina is demonstrated by using high-speed, ultrahigh-resolution optical coherence tomography (OCT). Imaging was performed with 2.8 microm resolution at a rate of 24,000 axial scans per second. A white-light stimulus was applied to the dark-adapted rat retina, and the average reflectivities from different intraretinal layers were monitored as a function of time. A 10%-15% increase in the average amplitude reflectance of the photoreceptor outer segments was observed in response to the stimulus. The spatial distribution of the change in the OCT signal is consistent with an increase in backscatter from the photoreceptor outer segments. To our knowledge, this is the first in vivo demonstration of OCT functional imaging in the intact retina.

Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography.

We demonstrate a new technique for frequency-swept laser operation--Fourier domain mode locking (FDML)--and its application for swept-source optical coherence tomography (OCT) imaging. FDML is analogous to active laser mode locking for short pulse generation, except that the spectrum rather than the amplitude of the light field is modulated. High-speed, narrowband optical frequency sweeps are generated with a repetition period equal to the fundamental or a harmonic of cavity round-trip time. An FDML laser is constructed using a long fiber ring cavity, a semiconductor optical amplifier, and a tunable fiber Fabry-Perot filter. Effective sweep rates of up to 290 kHz are demonstrated with a 105 nm tuning range at 1300 nm center wavelength. The average output power is 3mW directly from the laser and 20 mW after post-amplification. Using the FDML laser for swept-source OCT, sensitivities of 108 dB are achieved and dynamic linewidths are narrow enough to enable imaging over a 7 mm depth with only a 7.5 dB decrease in sensitivity. We demonstrate swept-source OCT imaging with acquisition rates of up to 232,000 axial scans per second. This corresponds to 906 frames/second with 256 transverse pixel images, and 3.5 volumes/second with a 256x128x256 voxel element 3-DOCT data set. The FDML laser is ideal for swept-source OCT imaging, thus enabling high imaging speeds and large imaging depths.

Amplified, frequency swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles.

We demonstrate a high-speed, frequency swept, 1300 nm laser source for frequency domain reflectometry and OCT with Fourier domain/swept-source detection. The laser uses a fiber coupled, semiconductor amplifier and a tunable fiber Fabry-Perot filter. We present scaling principles which predict the maximum frequency sweep speed and trade offs in output power, noise and instantaneous linewidth performance. The use of an amplification stage for increasing output power and for spectral shaping is discussed in detail. The laser generates ~45 mW instantaneous peak power at 20 kHz sweep rates with a tuning range of ~120 nm full width. In frequency domain reflectometry and OCT applications the frequency swept laser achieves 108 dB sensitivity and ~10 mum axial resolution in tissue. We also present a fast algorithm for real time calibration of the fringe signal to equally spaced sampling in frequency for high speed OCT image preview.

Three-dimensional and C-mode OCT imaging with a compact, frequency swept laser source at 1300 nm.

We demonstrate high resolution, three-dimensional OCT imaging with a high speed, frequency swept 1300 nm laser source. A new external cavity semiconductor laser design, optimized for application to swept source OCT, is discussed. The design of the laser enables adjustment of an internal spectral filter to change the filter bandwidth and provides a robust bulk optics design. The laser generates ~30 mW instantaneous peak power at an effective 16 kHz sweep rate with a tuning range of ~133 nm full width. In frequency domain reflectometry and OCT applications, 109 dB sensitivity and ~10 microm axial resolution in tissue can be achieved with the swept laser. The high imaging speeds enable three-dimensional OCT imaging, including zone focusing or C-mode imaging and image fusion to acquire large depth of field data sets with high resolution. In addition, three-dimensional OCT data provides coherence gated en face images similar to optical coherence microscopy (OCM) and also enables the generation of images similar to confocal microscopy by summing signals in the axial direction. High speed, three-dimensional OCT imaging can provide comprehensive data which combines the advantages of optical coherence tomography and microscopy in a single system.

Full range complex spectral optical coherence tomography technique in eye imaging.

We demonstrate a new implementation of complex spectral optical coherence tomography (OCT) in biomedical imaging. By reconstruction of both amplitude and phase we are able to use the negative and positive optical path differences to get images of objects of considerable thickness. An accompanying reduction of coherent noise improves the quality of the images. The property of the complex spectral OCT that permits the measurement range to be increased and permits the simultaneous use of phase and amplitude in spectral systems was not described previously. To show the potential of this technique we measured an anterior chamber of a porcine eye in vitro.

Spectral measurement of absorption by spectroscopic frequency-domain optical coherence tomography.

A new method of measurement that essentially combines Fourier-domain optical coherence tomography with spectroscopy is introduced. By use of a windowed Fourier transform it is possible to obtain, in addition to the object structure, spectroscopic information such as the absorption properties of materials. The feasibility of this new method for performing depth-resolved spectroscopy is demonstrated with a glass filter plate. The results are compared with theoretically calculated spectra by use of the well-known spectral characteristics of the light source and the filter plate.

[Audiological diagnosis in Military Medical Academy in Warsaw]. / Diagnostyka narzadu sluchu w Pracowni Audiologicznej CSK WAM.

Basis clinical proceeders in hearing loss diagnosis and the instruments used in the Department of Otolaryngology of the Military Medical Academy in Warsaw were presented.

[A case of objective tinnitus]. / Przypadek obiektywnych szumów usznych.

The authors presented a case of objective tinnitus in patient with glomus tumor suspicion. Tinnitus epidemiology, classification, causes and diagnostic procedure were described. Any organic alterations, which would cause tinnitus, did not find.

[Diagnostic criteria of occupational hearing damage according to current regulations]. / Kryteria zawodowego uszkodzenia sluchu w swietle obowiazujacych przepisów.

Problems connected with the hearing damage occur also among soldiers during military service. We discuss Polish current obligatory regulations describing types, and permissible noise values in labour environment together with requirements for performing measurements, and compare them with such regulations in other countries. We consider general principles for hearing protection currently in force in the army, and practical problems connected with them.

[Post-traumatic deafness in the materials of the ENT department CCH MMA in 1989-1994]. / Gluchoty pourazowe w materiale Kliniki Otolaryngologicznej CSK WAM w latach 1989-1994.

In the period of 1989-1994 in our ENT Department 22 patients with injury deafness were treated. In 8 patients the cause of the deafness was acoustic trauma and in 14 patients mechanical injury. In 6 patients we recognized temporal bone fracture. The authors compared their results with results of other authors published in period of 1963-1968. There was statistically significant increase in the quantity of mechanical injury deafness.

[Diagnostic difficulties in neck tumors]. / Trudnosci diagnostyczne w przypadku guzów szyi.

Neck tumors classification and accessory investigations helpful in diagnosis were presented. An attention was paid to the fact, that not seldom provisional diagnosis appeared to be wrong. Due to exact examination necessity of surgical treatment in some cases was excluded. As a proof of this three clinical cases were demonstrated.

[A case of hemorrhagic inflammation of an external acoustic duct]. / Przypadek krwotocznego zapalenia przewodów sluchowych zewnetrznych.

We report a case a twenty-year-old male with purpura of his limbs, and acute, diffused inflammation of the skin of the external acoustic duct. Complaints occur in the course of upper respiratory tract infection treated with antibiotic. Administered treatment resulted in quick subsidence of general symptoms with prolonged healing of external acoustic duct requiring surgical debridement (incision, and evaluation of bloody contents).