Adrian Bradu

Adaptive optics enhanced simultaneous en-face optical coherence tomography and scanning laser ophthalmoscopy.

A novel combination of adaptive optics with a simultaneous en-face OCT/SLO system for high resolution imaging of the in-vivo human retina is presented. Pairs of retinal images are shown and performance of the system is evaluated with and without dynamic wavefront correction. The adaptive optics closed loop system operates at a frame rate of 9 Hz and incorporates a Shack-Hartmann wavefront sensor based on a highly sensitive Andor camera and a 37 actuator OKO membrane deformable mirror to correct for ocular aberrations. The system produces C-scan pairs of images at a frame rate of 2 Hz. The correction of aberrations produced by the adaptive optics closed-loop system increased the signal-to-noise ratio in images obtained from volunteer eyes by up to 6 dB in the OCT channel and up to 9 dB in the SLO channel. A slight improvement in the lateral resolution was also obtained, from 6.5 mum before to 5 mum after closing the adaptive optics loop.

Quality assessment of dental treatments using en-face optical coherence tomography

The present study evaluates the potential of en-face optical coherence tomography (OCT) as a possible noninvasive high resolution method for supplying necessary information on the material defects of dental prostheses and microleakage at prosthetic interfaces. Teeth are also imaged after several treatment methods to asses material defects and microleakage at the tooth-filling interface, and the presence or absence of apical microleakage, as well as to evaluate the quality of bracket bonding on dental hard tissue. C-scan and B-scan OCT images as well as confocal images are acquired from a large range of samples. Gaps between the dental interfaces and material defects are clearly exposed.

Real-time resampling in Fourier domain optical coherence tomography using a graphics processing unit

Fourier domain optical coherence tomography (FD-OCT) requires either a linear-in-wavenumber spectrometer or a computationally heavy software algorithm to recalibrate the acquired optical signal from wavelength to wavenumber. The first method is sensitive to the position of the prism in the spectrometer, while the second method drastically slows down the system speed when it is implemented on a serially oriented central processing unit. We implement the full resampling process on a commercial graphics processing unit (GPU), distributing the necessary calculations to many stream processors that operate in parallel. A comparison between several recalibration methods is made in terms of performance and image quality. The GPU is also used to accelerate the fast Fourier transform (FFT) and to remove the background noise, thereby achieving full GPU-based signal processing without the need for extra resampling hardware. A display rate of 25 framessec is achieved for processed images (1,024 x 1,024 pixels) using a line-scan charge-coupled device (CCD) camera operating at 25.6 kHz.

Master–slave interferometry for parallel spectral domain interferometry sensing and versatile 3D optical coherence tomography

Conventional spectral domain interferometry (SDI) methods suffer from the need of data linearization. When applied to optical coherence tomography (OCT), conventional SDI methods are limited in their 3D capability, as they cannot deliver direct en-face cuts. Here we introduce a novel SDI method, which eliminates these disadvantages. We denote this method as Master - Slave Interferometry (MSI), because a signal is acquired by a slave interferometer for an optical path difference (OPD) value determined by a master interferometer. The MSI method radically changes the main building block of an SDI sensor and of a spectral domain OCT set-up. The serially provided signal in conventional technology is replaced by multiple signals, a signal for each OPD point in the object investigated. This opens novel avenues in parallel sensing and in parallelization of signal processing in 3D-OCT, with applications in high- resolution medical imaging and microscopy investigation of biosamples. Eliminating the need of linearization leads to lower cost OCT systems and opens potential avenues in increasing the speed of production of en-face OCT images in comparison with conventional SDI.

Dual optical coherence tomography/fluorescence microscopy for monitoring of Drosophila melanogaster larval heart

This article demonstrates a combined instrument of two imaging modalities to acquire information on cardiac function in larval Drosophila melanogaster: optical coherence tomography (OCT) and laser scanning fluorescence microscopy (LSFM). For this purpose, a dedicated imaging instrument able to sequentially provide cross-sectional OCT and C-scan LSFM images has been developed. With this dual-imaging system, the heart can be easily located and visualized within the specimen and the change of the heart shape in a cardiac cycle can be monitored.

Design and testing of prototype handheld scanning probes for optical coherence tomography.

Three simple and low-cost configurations of handheld scanning probes for optical coherence tomography have been developed. Their design and testing for dentistry applications are presented. The first two configurations were built exclusively from available off-the-shelf optomechanical components, which, to the best of our knowledge, are the first designs of this type. The third configuration includes these components in an optimized and ergonomic probe. All the designs are presented in detail to allow for their duplication in any laboratory with a minimum effort, for applications that range from educational to high-end clinical investigations. Requirements that have to be fulfilled to achieve configurations which are reliable, ergonomic—for clinical environments, and easy to build are presented. While a range of applications is possible for the prototypes developed, in this study the handheld probes are tested ex vivo with a spectral domain optical coherence tomography system built in-house, for dental constructs. A previous testing with a swept source optical coherence tomography system has also been performed both in vivo and ex vivo for ear, nose, and throat—in a medical environment. The applications use the capability of optical coherence tomography to achieve real-time, high-resolution, non-contact, and non-destructive interferometric investigations with micrometer resolutions and millimeter penetration depth inside the sample. In this study, testing the quality of the material of one of the most used types of dental prosthesis, metalo-ceramic is thus demonstrated.

Assessment of the sealant/tooth interface using optical coherence tomography

Sealant materials are typically employed in dentistry in order to prevent the development of cavities on the teeth. They prevent bacterial adhesion to enamel, thus arresting the development of demineralization and of caries. In this study, the critical zone of the interface between different sealant materials and the results of the dental work for the teeth processed were investigated ex vivo using swept source (SS) optical coherence tomography (OCT). Optical inspection and X-ray investigation revealed no defects, while SS-OCT proved capable to asses exactly the position, the nature, and the dimensions of each type of these defects. Specifically, different failures were targeted into the structure of pit and fissure sealants, including bubbles, internal cracks, structural defects of sealant material, and structural defects of enamel, with uncovered sealant material and enamel/sealant interface (marginal integrity and marginal adaptation of dental sealant). The investigation of the possible types of defects that may appear into this dental interface was thus accomplished – for the dental practitioner.

Dual instrument for in vivo and ex vivo OCT imaging in an ENT department

A dual instrument is assembled to investigate the usefulness of optical coherence tomography (OCT) imaging in an ear, nose and throat (ENT) department. Instrument 1 is dedicated to in vivo laryngeal investigation, based on an endoscope probe head assembled by compounding a miniature transversal flying spot scanning probe with a commercial fiber bundle endoscope. This dual probe head is used to implement a dual channel nasolaryngeal endoscopy-OCT system. The two probe heads are used to provide simultaneously OCT cross section images and en face fiber bundle endoscopic images. Instrument 2 is dedicated to either in vivo imaging of accessible surface skin and mucosal lesions of the scalp, face, neck and oral cavity or ex vivo imaging of the same excised tissues, based on a single OCT channel. This uses a better interface optics in a hand held probe. The two instruments share sequentially, the swept source at 1300 nm, the photo-detector unit and the imaging PC. An aiming red laser is permanently connected to the two instruments. This projects visible light collinearly with the 1300 nm beam and allows pixel correspondence between the en face endoscopy image and the cross section OCT image in Instrument 1, as well as surface guidance in Instrument 2 for the operator. The dual channel instrument was initially tested on phantom models and then on patients with suspect laryngeal lesions in a busy ENT practice. This feasibility study demonstrates the OCT potential of the dual imaging instrument as a useful tool in the testing and translation of OCT technology from the lab to the clinic. Instrument 1 is under investigation as a possible endoscopic screening tool for early laryngeal cancer. Larger size and better quality cross-section OCT images produced by Instrument 2 provide a reference base for comparison and continuing research on imaging freshly excised tissue, as well as in vivo interrogation of more superficial skin and mucosal lesions in the head and neck patient.

Extra long imaging range swept source optical coherence tomography using re-circulation loops

One of the main drawbacks of the swept source optical coherence tomography (SS-OCT) is its limited axial range. A novel interferometer configuration is proposed, equipped in each arm with an adjustable path length ring. By compensating the losses in the rings using semiconductor optical amplifiers, multiple paths A-scans can be obtained which when combined axially, can lead to an extremely long overall axial range. The effect of the re-circulation loops is equivalent with extending the coherence length of the swept source. In this way, the axial imaging range in SS-OCT can be pushed well beyond the limit imposed by the coherence length of the laser, to exceed in principle many centimeters.

Imaging the eye fundus with real-time en-face spectral domain optical coherence tomography

Real-time display of processed en-face spectral domain optical coherence tomography (SD-OCT) images is important for diagnosis. However, due to many steps of data processing requirements, such as Fast Fourier transformation (FFT), data re-sampling, spectral shaping, apodization, zero padding, followed by software cut of the 3D volume acquired to produce an en-face slice, conventional high-speed SD-OCT cannot render an en-face OCT image in real time. Recently we demonstrated a Master/Slave (MS)-OCT method that is highly parallelizable, as it provides reflectivity values of points at depth within an A-scan in parallel. This allows direct production of en-face images. In addition, the MS-OCT method does not require data linearization, which further simplifies the processing. The computation in our previous paper was however time consuming. In this paper we present an optimized algorithm that can be used to provide en-face MS-OCT images much quicker. Using such an algorithm we demonstrate around 10 times faster production of sets of en-face OCT images than previously obtained as well as simultaneous real-time display of up to 4 en-face OCT images of 200 × 200 pixels(2) from the fovea and the optic nerve of a volunteer. We also demonstrate 3D and B-scan OCT images obtained from sets of MS-OCT C-scans, i.e. with no FFT and no intermediate step of generation of A-scans.