Fourier domain multispectral multiple scattering low coherence interferometry

Coaxial Bright and Dark Field Optical Coherence Tomography

To improve the signal collection efficiency of Optical Coherence Tomography (OCT) for biomedical applications. A novel coaxial optical design was implemented, utilizing a wavefront-division beam splitter in the sample arm with a 45-degree rod mirror. This design allowed for the simultaneous collection of bright and dark field signals. The bright field signal was detected within its circular aperture in a manner similar to standard OCT, while the dark field signal passed through an annular-shaped aperture and was collected by the same spectrometer via a fiber array. This new configuration improved the signal collection efficiency by ∼3 dB for typical biological tissues. Dark-field OCT images were found to provide higher resolution, contrast and distinct information compared to standard bright-field OCT. By compounding bright and dark field images, speckle noise was suppressed by ∼ √2 . These advantages were validated using Teflon phantoms, chicken breast ex vivo, and human skin in vivo. This new OCT configuration significantly enhances signal collection efficiency and image quality, offering great potential for improving OCT technology with better depth, contrast, resolution, speckles, and signal-to-noise ratio. We believe that the bright and dark field signals will enable more comprehensive tissue characterization with the angled scattered light. This advancement will greatly promote the OCT technology in various clinical and biomedical research applications.

Speckle-resolved optical coherence tomography for mesoscopic imaging within scattering media

Light scattering poses a challenge for imaging deep in scattering media as the ballistic light exponentially attenuates with depth. In contrast to the ballistic light, the multiply scattered light penetrates deeper and also contains information about the sample. One technique to image deeper is to selectively detect only a subset of the multiply scattered light, namely the 'snake' photons, which are predominantly forward scattered and retain more direct information than the more strongly scattered light. In this work, we develop a technique, termed speckle-resolved optical coherence tomography (srOCT), for efficiently detecting these 'snake' photons to enable imaging deeper in scattering media. The system couples spatio-angular filtering with speckle-resolved interferometric detection to preferentially and efficiently detect the weakly scattered 'snake' photons. With our proof-of-concept system, we demonstrate depth-resolved imaging beyond the ballistic limit, up to a depth of 90 round-trip MFPs in a scattering phantom and a depth of 4.5 mm of chicken tissue at 0.4 mm axial and lateral resolution.

Deep imaging with 1.3 µm dual-axis optical coherence tomography and an enhanced depth of focus

For many clinical applications, such as dermatology, optical coherence tomography (OCT) suffers from limited penetration depth due primarily to the highly scattering nature of biological tissues. Here, we present a novel implementation of dual-axis optical coherence tomography (DA-OCT) that offers improved depth penetration in skin imaging at 1.3 µm compared to conventional OCT. Several unique aspects of DA-OCT are examined here, including the requirements for scattering properties to realize the improvement and the limited depth of focus (DOF) inherent to the technique. To overcome this limitation, our approach uses a tunable lens to coordinate focal plane selection with image acquisition to create an enhanced DOF for DA-OCT. This improvement in penetration depth is quantified experimentally against conventional on-axis OCT using tissue phantoms and mouse skin. The results presented here suggest the potential use of DA-OCT in situations where a high degree of scattering limits depth penetration in OCT imaging.

Origin of improved depth penetration in dual-axis optical coherence tomography: a Monte Carlo study

Recent studies have demonstrated that extended imaging depth can be achieved using dual-axis optical coherence tomography (DA-OCT). By illuminating and collecting at an oblique angle, multiple forward scattered photons from large probing depths are preferentially detected. However, the mechanism behind the enhancement of imaging depth needs further illumination. Here, the signal of a DA-OCT system is studied using a Monte Carlo simulation. We modeled light transport in tissue and recorded the spatial and angular distribution of photons exiting the tissue surface. Results indicate that the spatial separation and offset angle created by the non-telecentric scanning configuration promote the collection of more deeply propagating photons than conventional on-axis OCT.

Dual-axis optical coherence tomography for deep tissue imaging

We have developed dual-axis optical coherence tomography (DA-OCT) which enables deep tissue imaging by using a novel off-axis illumination/detection configuration. DA-OCT offers a 100-fold speed increase compared with its predecessor, multispectral multiple-scattering low coherence interferometry (ms2/LCI), by using a new beam scanning mechanism based on a microelectro-mechanical system (MEMS) mirror. The data acquisition scheme was altered to take advantage of this scanning speed, producing tomographic images at a rate of 4 frames (B-scans) per second. DA-OCT differs from ms2/LCI in that the dual axes intersect at a shallower depth (∼1  mm). This difference, coupled with the faster scanning speed, shifts the detection priority from multiply scattered to ballistic light. The utility of this approach was demonstrated by imaging both ex vivo porcine ear skin and in vivo rat skin from a McFarlane flap model. The enhanced penetration depth provided by the DA-OCT system will be beneficial to various clinical applications in dermatology and surgery.

Deep imaging of absorption and scattering features by multispectral multiple scattering low coherence interferometry

We have developed frequency domain multispectral multiple scattering low coherence interferometry (ms2/LCI) for deep imaging of absorption and scattering contrast. Using tissue-mimicking phantoms that match the full scattering phase function of human dermal tissue, we demonstrate that ms2/LCI can provide a signal/noise ratio (SNR) improvement of 15.4 dB over conventional OCT at an imaging depth of 1 mm. The enhanced SNR and penetration depth provided by ms2/LCI could be leveraged for a variety of clinical applications including the assessment of burn injuries where current clinical classification of severity only provides limited accuracy. The utility of the approach was demonstrated by imaging a tissue phantom simulating a partial-thickness burn revealing good spectroscopic contrast between healthy and injured tissue regions deep below the sample surface. Finally, healthy rat skin was imaged in vivo with both a commercial OCT instrument and our custom ms2/LCI system. The results demonstrate that ms2/LCI is capable of obtaining spectroscopic information far beyond the penetration depth provided by conventional OCT.

Evaluation of burn severity in vivo in a mouse model using spectroscopic optical coherence tomography

Clinical management of burn injuries depends upon an accurate assessment of the depth of the wound. Current diagnostic methods rely primarily on subjective visual inspection, which can produce variable results. In this study, spectroscopic optical coherence tomography was used to objectively evaluate burn injuries in vivo in a mouse model. Significant spectral differences were observed and correlated with the depth of the injury as determined by histopathology. The relevance of these results to clinical burn management in human tissues is discussed.

Noise characterization of supercontinuum sources for low-coherence interferometry applications

We examine the noise properties of supercontinuum light sources when used in low-coherence interferometry applications. The first application is a multiple-scattering low-coherence interferometry (ms2/LCI) system, where high power and long image acquisition times are required to image deep into tissue. For this system, we compare the noise characteristics of two supercontinuum sources from different suppliers. Both sources have long-term drift that limits the amount of time over which signal averaging is advantageous for reducing noise. The second application is a high-resolution optical coherence tomography system, where broadband light is needed for high axial resolution. For this system, we compare the noise performance of the two supercontinuum sources and a light source based on four superluminescent diodes (SLD) using imaging contrast as a comparative metric. We find that the NKT SuperK has superior noise performance compared with the Fianium SC-450-4, but neither meets the performance of the SLD.

In vivo analysis of burns in a mouse model using spectroscopic optical coherence tomography

Spectroscopic analysis of biological tissues can provide insight into changes in structure and function due to disease or injury. Depth-resolved spectroscopic measurements can be implemented for tissue imaging using optical coherence tomography (OCT). Here, spectroscopic OCT is applied to in vivo measurement of burn injury in a mouse model. Data processing and analysis methods are compared for their accuracy. Overall accuracy in classifying burned tissue was found to be as high as 91%, producing an area under the curve of a receiver operating characteristic curve of 0.97. The origins of the spectral changes are identified by correlation with histopathology.