Application of optical flow algorithms to laser speckle imaging

Mitigation of Motion Artifacts in Handheld Laser Speckle Contrast Imaging Illustrated on Psoriasis Lesions

BACKGROUND

Handheld laser speckle contrast imaging (LSCI) is crucial in clinical settings, but motion artifacts (MA) can compromise perfusion image reliability. Current prevention and suppression methods are often impractical or complex. Machine vision techniques, promising in medical imaging, could improve signal quality, but their use in suppressing MA is still unexplored.

OBJECTIVE

We propose an innovative method based on linear regression for MA correction (MAC) in LSCI and validate it in vivo.

METHODS

We performed paired handheld and mounted LSCI measurements on 14 subjects with psoriasis using the previously validated handheld perfusion imager (HAPI). By marking lesion boundaries for clinical purposes, the HAPI used a monochromatic camera for both speckle imaging and motion detection, simplifying hardware requirements. Accurate estimation of relative displacements between the test object and LSCI probe allowed us to apply MAC to the perfusion images.

RESULTS

Local perfusion values correlated with applied speed were used to calculate and correct MA. The difference between mean perfusion in handheld and mounted modes after MAC significantly decreased (median error 14.2 perfusion units (p.u.) on lesions before correction (p 0.0005) and 0.5 p.u. after correction (p=0.2)).

CONCLUSION

The findings provide evidence for robust handheld LSCI and validate the MA technique in psoriasis case. Of the two causes of MA-on-surface speeds and wavefront tilt-we address the former and correct mean perfusion, assuming constant temporal perfusion at each location.

SIGNIFICANCE

We describe a practical, non-contact, marker-free technique for reliable handheld perfusion imaging, supporting further clinical translation in plastic surgery and burns.

Oxygenation heterogeneity facilitates spatiotemporal flow pattern visualization inside human blood vessels using photoacoustic computed tomography

Hemodynamics can be explored through various biomedical imaging techniques. However, observing transient spatiotemporal variations in the saturation of oxygen (sO2) within human blood vessels proves challenging with conventional methods. In this study, we employed photoacoustic computed tomography (PACT) to reconstruct the evolving spatiotemporal patterns in a human vein. Through analysis of the multi-wavelength photoacoustic (PA) spectrum, we illustrated the dynamic distribution within blood vessels. Additionally, we computationally rendered the dynamic process of venous blood flowing into the major vein and entering a branching vessel. Notably, we successfully recovered, in real time, the parabolic wavefront profile of laminar flow inside a deep vein in vivo-a first-time achievement. While the study is preliminary, the demonstrated capability of dynamic sO2 imaging holds promise for new applications in biology and medicine.

Applications of Laser Speckle Contrast Imaging Technology in Dermatology

Laser speckle contrast imaging or laser speckle imaging (LSI) is a noninvasive imaging technology that can detect areas of dynamic perfusion or vascular flow. Thus, LSI has shown increasing diagnostic utility in various pathologies and has been employed for intraoperative, postoperative, and long-term monitoring in many medical specialties. Recently, LSI has gained traction in clinical dermatology because it can be effective in the assessment of pathologies that are associated with increased perfusion and hypervascularity compared with that of normal tissue. To date, LSI has been found to be highly accurate in monitoring skin graft reperfusion, determining the severity of burns, evaluating neurosurgical revascularization, assessing persistent perfusion in capillary malformations after laser therapy, and differentiating malignant and benign skin lesions. LSI affords the advantage of noninvasively assessing lesions before more invasive methods of diagnosis, such as tissue biopsy, while remaining inexpensive and exhibiting no adverse events to date. However, potential obstacles to its clinical use include tissue movement artifact, primarily qualitative data, and unclear impact on clinical practice given the lack of superiority data compared with the current standard-of-care diagnostic methods. In this review, we discuss the clinical applications of LSI in dermatology for use in the diagnosis and monitoring of vascular, neoplastic, and inflammatory skin conditions.

Optical Flow-Based Full-Field Quantitative Blood-Flow Velocimetry Using Temporal Direction Filtering and Peak Interpolation

The quantitative measurement of the microvascular blood-flow velocity is critical to the early diagnosis of microvascular dysfunction, yet there are several challenges with the current quantitative flow velocity imaging techniques for the microvasculature. Optical flow analysis allows for the quantitative imaging of the blood-flow velocity with a high spatial resolution, using the variation in pixel brightness between consecutive frames to trace the motion of red blood cells. However, the traditional optical flow algorithm usually suffers from strong noise from the background tissue, and a significant underestimation of the blood-flow speed in blood vessels, due to the errors in detecting the feature points in optical images. Here, we propose a temporal direction filtering and peak interpolation optical flow method (TPIOF) to suppress the background noise, and improve the accuracy of the blood-flow velocity estimation. In vitro phantom experiments and in vivo animal experiments were performed to validate the improvements in our new method.

Application of laser speckle blood perfusion imaging in the evaluation of erectile function in rats

Intracavernosal pressure measurement is the gold standard for evaluating erectile function in experimental animals, but it has the shortcoming of being invasive. This study aimed to explore the application of laser speckle perfusion imaging in evaluating erectile function in rats. Sixty Sprague Dawley rats were randomly divided into the sham operation and model groups (n = 30 each). A rat model of neuroinjury erectile dysfunction was established by surgically damaging the bilateral cavernous nerves in the model group. Simulated surgery was performed in the sham operation group; the nerves were not damaged. Erectile function was evaluated by comparing the changes in intracavernosal pressure and blood flow fluctuations when the cavernous nerve was stimulated using the same voltage parameters. Intracavernosal pressure in the model group was significantly lower than that in the other group when using 2.5 V. No significant difference was found in cavernous blood flow fluctuation between the two groups when using 0.5 V. Cavernous blood flow fluctuation in the model group after 2.5 V, 5 V and 7.5 V stimulations was significantly lower than that in the sham operation group. Evaluating erectile function in rats is feasible by measuring the cavernous blood flow using laser speckle perfusion imaging.

Sub-Rayleigh second-order correlation imaging using spatially distributive colored noise speckle patterns

We present a novel method, to our knowledge, to synthesize non-trivial speckle patterns that can enable sub-Rayleigh second-order correlation imaging. The speckle patterns acquire a unique anti-correlation in the spatial intensity fluctuation by introducing the blue noise distribution on spatial Fourier power spectrum to the input light fields through amplitude modulation. Illuminating objects with the blue noise speckle patterns can lead to a sub-diffraction limit imaging system with a resolution more than three times higher than first-order imaging, which is comparable to the resolving power of ninth order correlation imaging with thermal light. Our method opens a new route towards non-trivial speckle pattern generation by tailoring amplitudes in spatial Fourier power spectrum of the input light fields and provides a versatile scheme for constructing sub-Rayleigh imaging and microscopy systems without invoking complicated higher-order correlations.

Laser speckle contrast imaging system using nanosecond pulse laser source

SIGNIFICANCE

Nanosecond-pulsed laser has proven to be used to obtain the velocity of blood using the speckle contrast method. Without the scanning time, it has potential for achieving fast two-dimensional blood flow images in a photoacoustic imaging system with the same pulsed laser.

AIM

Our study aimed to evaluate the qualities of regional cerebral blood flow (rCBF) obtained in a laser speckle contrast imaging (LSCI) system using continuous wave (cw) and nanosecond pulse laser sources.

APPROACH

First, a LSCI system consisting of a cw laser with a wavelength of 632.8 nm and a cw laser/nanosecond pulse laser with a wavelength of 532 nm was developed. This system was used to obtain rCBF images of mouse in vivo with two different laser sources.

RESULTS

Continuous wave lasers (532 and 632.8 nm) show different imaging characteristics for rCBF imaging. The rCBF images obtained using 532-nm nanosecond pulse laser showed higher resolution than those using 532-nm cw laser. There was no significant difference in the results using nanosecond pulse laser among various pulse widths or repetition rates.

CONCLUSIONS

It is proved that a nanosecond pulse laser could be used for LSCI.

Research on Moving Target Tracking Based on FDRIG Optical Flow

Aiming at the problem of moving target recognition, a moving target tracking model based on FDRIG optical flow is proposed. First, the optical flow equation was analyzed from the theory of optical flow. Then, with the energy functional minimization, the FDRIG optical flow technique was proposed. Taking a road section of a university campus as an experimental section, 30 vehicle motion sequence images were considered as objects to form a vehicle motion sequence image with a complex background. The proposed FDRIG optical flow was used to calculate the vehicle motion optical flow field by the Halcon software. Comparable with the classic Horn and Schunck (HS) and Lucas and Kande (LK) optical flow algorithm, the monitoring results proved that the FDRIG optical flow was highly precise and fast when tracking a moving target. The Ettlinger Tor traffic scene was then taken as the second experimental object; FDRIG optical flow was used to analyze vehicle motion. The superior performance of the FDRIG optical flow was further verified. The whole research work shows that FDRIG optical flow has good performance and speed in tracking moving targets and can be used to monitor complex target motion information in real-time.

Characterization of ageing resistant transparent nanocrystalline yttria-stabilized zirconia implants

The "Window to the Brain" is a transparent cranial implant under development, based on nanocrystalline yttria-stabilized zirconia (nc-YSZ) transparent ceramic material. Previous work has demonstrated the feasibility of this material to facilitate brain imaging over time, but the long-term stability of the material over decades in the body is unknown. In this study, the low-temperature degradation (LTD) of nc-YSZ of 3, 6, and 8 mol % yttria is compared before and after accelerated ageing treatments following ISO standards for assessing the ageing resistance of zirconia ceramics. After 100 hr of accelerated ageing (equivalent to many decades of ageing in the body), the samples do not show any signs of phase transformation to monoclinic by X-ray diffraction and micro-Raman spectroscopy. Moreover, the mechanical hardness of the samples did not decrease, and changes in optical transmittance from 500 to 1000 nm due to ageing treatments was minimal (below 3% for all samples), and unlikely to be due to phase transformation of surface crystals to monoclinic. These results indicate the nc-YSZ has excellent ageing resistance and can withstand long-term implantation conditions without exhibiting LTD.