This report presents a state-of-the-art multimodality imaging device that combines multi-spectral optoacoustic microscopy (OAM) and optical coherence tomography (OCT) to chart absorbers in live tadpoles (Xenopus laevis) accurately. The OAM channel captures maps of five internal contrast agents: melanin, hemoglobin, collagen, glucose, and lipids. A novel method was developed to achieve this by assuming that each voxel in the 3D-OAM image exhibits a single chromophore contributing to the optoacoustic signal. The device is powered by a single optical source (SuperK Compact, NKT Photonics) that operates across an ultra-wide spectral range of 450 to 2400 nm. The set-up was optimized by minimizing optical aberrations and attenuation on optical components to stimulate the sample effectively. Using optical pulses of 2 ns duration and a repetition rate of 20 kHz, the device imaged tadpoles in their embryonic stage at multiple wavelengths, using narrow spectral windows of 25 nm bandwidth within the broad spectrum of the supercontinuum source at a time. In addition, an ultra-high-resolution OCT imaging channel operating at 1300 nm (spectral bandwidth 180 nm) was created and incorporated into the device. The OCT channel, also powered by a commercial supercontinuum source (SuperK EXTREME EXR9, NKT Photonics), was used for guidance purposes and to help determine the location of the chromophores.
Diagnosing skin cancer, such as basal cell carcinoma, requires a biopsy, which is a time-consuming and expensive process. Therefore, it is essential to explore alternative diagnostic methods that are both more efficient and effective. We developed a handheld optical coherence microscopy (OCM) imaging device to achieve high-resolution optical biopsies in real-time. The instrument uses a variable focus liquid lens that allows fast shifting of the focus inside the sample, resulting in high-resolution lateral images throughout an extended axial imaging range. Our instrument can produce images with an axial resolution of approximately 5 μm, currently limited by the light source employed, and better than 2 μm transversal resolution images. The acquisition, data processing, and display of the 3D volumes are performed in real time, primarily enabled by the Master-Slave approach employed to produce the optical biopsies. The acquisition rate of the current camera used in the spectrometer is limited to 70 kHz. Our benchmarking shows that the real-time operation of the instrument can be sustained even at over 250 kHz solely by utilizing the computing power of the CPU, with no need to employ graphic cards or FPGAs. The instrument’s capability is showcased through images featuring various samples, such as an IR card and skin.
In this report, a novel calibration method is introduced, which can be used in camera-based Optical Coherence Tomography (OCT) instruments employing several spectrometers. To ensure that all spectrometers are calibrated, i.e. they sense the same spectral range and the distribution of the optical frequencies across the pixels of the cameras is the same, a hybrid method was used involving (i) a hardware procedure for an initial estimation of the edges of the spectra and (ii) a numerical Monte-Carlo based technique. The utility of such a procedure is demonstrated in an OCT system using a Balance-Detection (BD) scheme. The OCT system employs a single transmission diffraction grating and is driven by a supercontinuum source operating in the visible spectral range. Spectral alignment is paramount in producing high-sensitivity images free of artefacts. To ensure correct calibration, and speed up the calibration procedure, the Master-Slave (MS) technique of generating axial reflectivity profiles is employed. Preliminary results show an improvement of the signal of ~ 3dB and a mitigation of the background noise of over 5 dB.
In conventional optoacoustic microscopy, nanosecond pulse duration lasers are employed. When a laser delivering shorter pulse durations is used, it is expected that, from a theoretical point of view, broader, higher-frequency acoustic waves to be generated, therefore a better axial resolution of the instrument. In the present report, this advantage, offered by a picosecond duration pulse laser, to experimentally demonstrate that the axial resolution of an optoacoustic microscopy instrument can be enhanced was exploited. In comparison to a 2 ns pulse duration, an improvement in the axial resolution of ~50% is demonstrated by using excitations with pulses of duration ⪅100 ps. Details of an optoacoustic microscopy instrument, operating at 532 nm, capable to provide high-resolution axial and lateral optoacoustic images, are also presented. The capabilities of the instrument are demonstrated by in-vivo images of Xenopus laevis brain with a similar ~ 3.8 μm lateral resolution throughout the whole axial imaging range.
Conventional optoacoustic microscopy (OAM) instruments have at their core a nanosecond pulse duration laser. If lasers with a shorter pulse duration are used, broader, higher frequency ultrasound waves are expected to be generated and as a result, the axial resolution of the instrument is improved. Here, we exploit the advantage offered by a picosecond duration pulse laser to enhance the axial resolution of an OAM instrument. In comparison to an instrument equipped with a 2-ns pulse duration laser, an improvement in the axial resolution of 50% is experimentally demonstrated by using excitation pulses of only 85 ps. To illustrate the capability of the instrument to generate high-quality optoacoustic images, en-face, in-vivo images of the brain of Xenopus laevis tadpole are presented with a lateral resolution of 3.8 μm throughout the entire axial imaging range.
Robust, non-destructive testing imaging instruments, capable to provide valuable information from within the body of materials is important for both quality control and the development of new materials, for industrial and medical applications. Conventional non-destructive testing (NDT) methods, such as radiographic or ultrasound-based techniques, allow for deep axial range imaging, however, they are either using non-safe radiation or/and exhibit low imaging resolutions. The speed at which the standard NDT methods deliver images is also limited. The development of photoacoustic (PA) and optical coherence tomography (OCT) applications in the field of NDT have grown exponentially over the past years, offering faster, higher resolution images. Both techniques, PA and OCT bring a plethora of benefits to the current methods. However, a multitude of challenges still needs to be addressed to truly make either of them the technique of choice for NDT applications. In this manuscript, a short overview of the challenges that these two imaging techniques are facing when used for NDT applications is presented. Illustrative high-resolution images, produced by a dual PA/OCT imaging instrument developed within the Applied Optics Group at the University of Kent are presented. These images demonstrate unique capabilities for NDT applications.
Dental shade determination and seamless integration of restorative work in the oral cavity are challenging and important tasks in the everyday clinical dental practice. The aim of this in-vitro study was to evaluate comparatively the capability of software-based color analysis of mobile phone photography, with the spectrophotometric and visual methods for dental shade determination. Visual shade determination of the incisal, middle, and cervical thirds parts of ten extracted human teeth was performed using the Vita Classical and Vita 3D Master stock shade guides. Shade determination of the thirds of each tooth was performed using the Vita Easyshade spectrophotometer. Subsequently, photographs of each tooth were captured using a mobile phone camera. Color charts were produced using an in-house image processing technique, and the tooth color captured by the mobile phone photography was matched to the shade guides. The results show that the camera-based method had better agreement with the spectrophotometric and visual methods when the Vita Classical shade guide was employed. Software-based color analysis of mobile phone photography should be further explored for its use as an affordable potential tool for increasing objectivity and accuracy in dental shade determination.
Optical Coherence Tomography instruments can provide images of ultrahigh, depth dependent axial resolution. Here we demonstrate an instrument employing visible light, enhanced by the Master-Slave technique, which deliver nearly constant axial resolution over the whole imaging range.
Supercontinuum (SC) lasers are of high interest for applications like multispectral photoacoustic imaging (MSPAI), where the wide optical bandwidth of the SC laser system facilitates functional investigations on top of the structural information of various endogenous agents inside the human body. The current work addresses a promising attempt at devising high pulse energy SC laser source using telecom-range diode laser systems and few meters of standard single-mode fibers for various MSPAI applications in near-infrared (NIR) and extended-NIR wavelength regions.
We report on further progress made on enhancing the capabilities of a multi-imaging modality instrument capable of producing high resolution images of biological tissues. At the core of the instrument is a supercontinuum (SC) source. Two SC sources commercialized by NKT Photonics were employed for our experiments: SuperK COMPACT and SuperK Extreme (EXR9). Using these two sources, we assembled an instrument capable to simultaneously provide in real-time cross-section high-resolution Optical Coherence Tomography (OCT) and Photo-acoustic (PA) images in various spectral ranges. Currently, the OCT channel is operating in the IR range around 1300 nm to allow better penetration into the tissue using either the COMPACT or the EXR9. The measured optical power on the sample is in both cases above 9.5 mW. An in-house spectrometer equipped with a sensitive InGaAs camera capable of operating at 47 kHz and sampling data over a spectral range from 1205 to 1395 nm was developed. A constant axial resolution provided by the instrument in the OCT channel over a range of 1.5 mm was experimentally measured (4.96 μm), matching the theoretical prediction. The spectral range 500-800 nm was used for PA channel. The COMPACT, used in the PA channel, can select the central wavelength and the spectral bandwidth of operations. Typically, the optical energy per pulse on the sample is superior to 60 nJ when a bandwidth superior to 50 nm is employed. This make the instrument usable for PA imaging of tissues.
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