Imaging PPG for In Vivo Human Tissue Perfusion Assessment during Surgery

Low-cost camera-based assessment of venous hemodynamics in the lower limbs: a study on young healthy volunteers

This paper presents a non-contact and cost-effective method to assess venous hemodynamics along the lower limbs using photoplethysmography imaging (PPGI). Seventeen healthy volunteers performed the venous muscle pump test, inducing venous blood volume changes in their lower legs, which were recorded using a webcam. PPGI signals were extracted from three regions along the lower leg. Key parameters derived from a physiological model were evaluated and analyzed statistically: perfusion amplitude, ejection time constant, and peripheral venous flow index. The method demonstrated robust estimation of physiologically explainable parameters, and the potential to improve venous function diagnostics with high spatial resolution.

Exploiting Dual-Wavelength Depolarization of Skin-Tissues for Camera-Based Perfusion Monitoring

Perfusion index (PI), the ratio between variable pulsatile (AC) and non-pulsatile (DC) components in a photoplethysmographic (PPG) signal, is an indirect and non-invasive measure of peripheral perfusion. PI has been widely used in assessing sympathetic block success, and monitoring hemodynamics in anesthesia and intensive care. Based on the principle of dual-wavelength depolarization (DWD) of skin tissues, we propose to investigate its opportunity in quantifying the skin perfusion contactlessly. The proposed method exploits the characteristic changes in chromaticity caused by skin depolarization and chromophore absorption. The experimental results of DWD, obtained with the post occlusive reactive hyperemia test and the local cooling and heating test, were compared to the PI values obtained from the patient monitor and photoplethysmography imaging (PPGI). The comparison demonstrated the feasibility of using DWD for PI measurement. Clinical trials conducted in the anesthesia recovery room and operating theatre further showed that DWD is potentially a new metric for camera-based non-contact skin perfusion monitoring during clinical operations, such as the guidance in anesthetic surgery.

Camera-on-tip endoscope for in vivo cardiovascular diagnostics and surgical guidance

Cardiovascular imaging with camera-on-tip endoscopes has the potential to provide physiologically relevant data on the tissue state and device placement that can improve clinical outcomes. In this work, we review the unmet clinical need for image-based in vivo cardiovascular diagnostics and guidance for minimally invasive procedures. We present a 7 Fr camera-on-tip endoscope with fibre-coupled multispectral illumination that includes methods for imaging in a blood-filled field of view (FOV). We demonstrate that the endoscope can be navigated from the femoral artery to cardiac regions such as the left atrium and left ventricle in a porcine model, where in vivo images of the cardiac walls are recorded. We further show that physiologically relevant parameters such as heart rate and respiration can be extracted from the images and that changes to tissue state can be inferred from the imaging data. Finally, a methodology for merging the imaging data with diffuse reflection spectroscopy (DRS) recorded through the optical fibre is outlined.

Imaging Photoplethysmography (iPPG) in Head and Neck Reconstructive Surgery: A Novel Technique for Noninvasive Flap Perfusion Monitoring

BACKGROUND

Evaluate imaging photoplethysmography (iPPG) as a novel noninvasive technique to assess flap perfusion in head and neck free flap reconstructive (FFR) surgeries.

METHODS

Intraoperative iPPG was performed in 17 patients undergoing FFR surgery. Imaging consisted of a 30-s video from which perfusion maps were extracted, providing detailed information about blood flow and pulsatility in the flap microvasculature. During each procedure, iPPG acquisitions were acquired representing distinct perfusion conditions of the flap (fully perfused/ischemic/reperfused). When possible, postoperative measurements were performed to assess flap recovery during the critical time period (3 days) and long-term follow-up (30 days).

RESULTS

Perfusion maps, displaying iPPG amplitude and delay times, correlated strongly (p < 0.001) with the perfusion status of the tissue. One case of postoperative thrombosis, leading to flap failure, was identified with iPPG. After surgical revision in this case, flap perfusion was restored and confirmed by iPPG. Postoperative follow-up imaging allowed for objective visualization of flap recovery short term (3 days) and up to 30 days after the surgical procedure.

CONCLUSIONS

This study shows that iPPG is suitable for objective and noninvasive assessment of flap perfusion in head and neck FFR surgery. In addition, postoperative monitoring shows potential for assessing flap perfusion in patients with increased risk of postoperative complications.

Separating Surface Reflectance from Volume Reflectance in Medical Hyperspectral Imaging

Hyperspectral imaging has shown great promise for diagnostic applications, particularly in cancer surgery. However, non-bulk tissue-related spectral variations complicate the data analysis. Common techniques, such as standard normal variate normalization, often lead to a loss of amplitude and scattering information. This study investigates a novel approach to address these spectral variations in hyperspectral images of optical phantoms and excised human breast tissue. Our method separates surface and volume reflectance, hypothesizing that spectral variability arises from significant variations in surface reflectance across pixels. An illumination setup was developed to measure samples with a hyperspectral camera from different axial positions but with identical zenith angles. This configuration, combined with a novel data analysis approach, allows for the estimation and separation of surface reflectance for each direction and volume reflectance across all directions. Validated with optical phantoms, our method achieved an 83% reduction in spectral variability. Its functionality was further demonstrated in excised human breast tissue. Our method effectively addresses variations caused by surface reflectance or glare while conserving surface reflectance information, which may enhance sample analysis and evaluation. It benefits samples with unknown refractive index spectra and can be easily adapted and applied across a wide range of fields where hyperspectral imaging is used.

Deep learning and remote photoplethysmography powered advancements in contactless physiological measurement

In recent decades, there has been ongoing development in the application of computer vision (CV) in the medical field. As conventional contact-based physiological measurement techniques often restrict a patient's mobility in the clinical environment, the ability to achieve continuous, comfortable and convenient monitoring is thus a topic of interest to researchers. One type of CV application is remote imaging photoplethysmography (rPPG), which can predict vital signs using a video or image. While contactless physiological measurement techniques have an excellent application prospect, the lack of uniformity or standardization of contactless vital monitoring methods limits their application in remote healthcare/telehealth settings. Several methods have been developed to improve this limitation and solve the heterogeneity of video signals caused by movement, lighting, and equipment. The fundamental algorithms include traditional algorithms with optimization and developing deep learning (DL) algorithms. This article aims to provide an in-depth review of current Artificial Intelligence (AI) methods using CV and DL in contactless physiological measurement and a comprehensive summary of the latest development of contactless measurement techniques for skin perfusion, respiratory rate, blood oxygen saturation, heart rate, heart rate variability, and blood pressure.

Alternative intraoperative optical imaging modalities for fluorescence angiography in gastrointestinal surgery: spectral imaging and imaging photoplethysmography

INTRODUCTION

Intraoperative near-infrared fluorescence angiography with indocyanine green (ICG-FA) is a well-established modality in gastrointestinal surgery. Its main drawback is the application of a fluorescent agent with possible side effects for patients. The goal of this review paper is the presentation of alternative, non-invasive optical imaging methods and their comparison with ICG-FA.

MATERIAL AND METHODS

The principles of ICG-FA, spectral imaging, imaging photoplethysmography (iPPG), and their applications in gastrointestinal surgery are described based on selected published works.

RESULTS

The main applications of the three modalities are the evaluation of tissue perfusion, the identification of risk structures, and tissue segmentation or classification. While the ICG-FA images are mainly evaluated visually, leading to subjective interpretations, quantitative physiological parameters and tissue segmentation are provided in spectral imaging and iPPG. The combination of ICG-FA and spectral imaging is a promising method.

CONCLUSIONS

Non-invasive spectral imaging and iPPG have shown promising results in gastrointestinal surgery. They can overcome the main drawbacks of ICG-FA, i.e. the use of contrast agents, the lack of quantitative analysis, repeatability, and a difficult standardization of the acquisition. Further technical improvements and clinical evaluations are necessary to establish them in daily clinical routine.

Laser Speckle Contrast Imaging-based diagnosis of severe mesenteric traction syndrome: Hemodynamics and prostacyclin - A prospective cohort study

BRIEF ABSTRACT

Today, the diagnosis and grading of mesenteric traction syndrome relies on a subjective assessment of facial flushing. However, this method has several limitations. In this study, Laser Speckle Contrast Imaging and a predefined cut-off value are assessed and validated for the objective identification of severe mesenteric traction syndrome.

BACKGROUND

Severe mesenteric traction syndrome (MTS) is associated with increased postoperative morbidity. The diagnosis is based on an assessment of the developed facial flushing. Today this is performed subjectively, as no objective method exists. One possible objective method is Laser Speckle Contrast Imaging (LSCI), which has been used to show significantly higher facial skin blood flow in patients developing severe MTS. Using these data, a cut-off value has been identified. This study aimed to validate our predefined LSCI cut-off value for identifying severe MTS.

METHODS

A prospective cohort study was performed on patients planned for open esophagectomy or pancreatic surgery from March 2021 to April 2022. All patients underwent continuous measurement of forehead skin blood flow using LSCI during the first hour of surgery. Using the predefined cut-off value, the severity of MTS was graded. In addition, blood samples for prostacyclin (PGI₂) analysis and hemodynamics were collected at predefined time points to validate the cut-off value.

MAIN RESULTS

Sixty patients were included in the study. Using our predefined LSCI cut-off value, 21 (35 %) patients were identified as developing severe MTS. These patients were found to have higher concentrations of 6-Keto-PGF_aα (p = 0.002), lower SVR (p < 0.001), lower MAP (p = 0.004), and higher CO (p < 0.001) 15 min into surgery, as compared with patients not developing severe MTS.

CONCLUSION

This study validated our LSCI cut-off value for the objective identification of severe MTS patients as this group developed increased concentrations of PGI₂ and more pronounced hemodynamic alterations compared with patients not developing severe MTS.

Random matrix-based laser speckle contrast imaging enables quasi-3D blood flow imaging in laparoscopic surgery

Laser speckle contrast imaging (LSCI) provides full-field and label-free imaging of blood flow and tissue perfusion. It has emerged in the clinical environment, including the surgical microscope and endoscope. Although traditional LSCI has been improved in resolution and SNR, there are still challenges in clinical translations. In this study, we applied a random matrix description for the statistical separation of single and multiple scattering components in LSCI using a dual-sensor laparoscopy. Both in-vitro tissue phantom and in-vivo rat experiments were performed to test the new laparoscopy in the laboratory environment. This random matrix-based LSCI (rmLSCI) provides the blood flow and tissue perfusion in superficial and deeper tissue respectively, which is particularly useful in intraoperative laparoscopic surgery. The new laparoscopy provides the rmLSCI contrast images and white light video monitoring simultaneously. Pre-clinical swine experiment was also performed to demonstrate the quasi-3D reconstruction of the rmLSCI method. The quasi-3D ability of the rmLSCI method shows more potential in other clinical diagnostics and therapies using gastroscopy, colonoscopy, surgical microscope, etc.

Soft wireless sternal patch to detect systemic vasoconstriction using photoplethysmography

Vasoconstriction is a crucial physiological process that serves as the body's primary blood pressure regulation mechanism and a key marker of numerous harmful health conditions. The ability to detect vasoconstriction in real time would be crucial for detecting blood pressure, identifying sympathetic arousals, characterizing patient wellbeing, detecting sickle cell anemia attacks early, and identifying complications caused by hypertension medications. However, vasoconstriction manifests weakly in traditional photoplethysmogram (PPG) measurement locations, like the finger, toe, and ear. Here, we report a wireless, fully integrated, soft sternal patch to capture PPG signals from the sternum, an anatomical region that exhibits a robust vasoconstrictive response. With healthy controls, the device is highly capable of detecting vasoconstriction induced endogenously and exogenously. Furthermore, in overnight trials with patients with sleep apnea, the device shows a high agreement (r² = 0.74) in vasoconstriction detection with a commercial system, demonstrating its potential use in portable, continuous, long-term vasoconstriction monitoring.

Imaging Photoplethysmography for Noninvasive Anastomotic Perfusion Assessment in Intestinal Surgery

INTRODUCTION

Anastomotic leakage after gastrointestinal surgery has a high impact on patient's quality of life and its origin is associated with inadequate perfusion. Imaging photoplethysmography (iPPG) is a noninvasive imaging technique that measures blood-volume changes in the microvascular tissue bed and detects changes in tissue perfusion.

MATERIALS AND METHODS

Intraoperative iPPG imaging was performed in 29 patients undergoing an open segment resection of the small intestine or colon. During each surgery, imaging was performed on fully perfused (true positives) and ischemic intestines (true negatives) and the anastomosis (unknowns). Imaging consisted of a 30-s video from which perfusion maps were extracted, providing detailed information about blood flow within the intestine microvasculature. To detect the predictive capabilities of iPPG, true positive and true negative perfusion conditions were used to develop two different perfusion classification methods.

RESULTS

iPPG-derived perfusion parameters were highly correlated with perfusion-perfused or ischemic-in intestinal tissues. A perfusion confidence map distinguished perfused and ischemic intestinal tissues with 96% sensitivity and 86% specificity. Anastomosis images were scored as adequately perfused in 86% of cases and 14% inconclusive. The cubic-Support Vector Machine achieved 90.9% accuracy and an area under the curve of 96%. No anastomosis-related postoperative complications were encountered in this study.

CONCLUSIONS

This study shows that noninvasive intraoperative iPPG is suitable for the objective assessment of small intestine and colon anastomotic perfusion. In addition, two perfusion classification methods were developed, providing the first step in an intestinal perfusion prediction model.

Continuous intraoperative perfusion monitoring of free microvascular anastomosed fasciocutaneous flaps using remote photoplethysmography

Flap loss through limited perfusion remains a major complication in reconstructive surgery. Continuous monitoring of perfusion will facilitate early detection of insufficient perfusion. Remote or imaging photoplethysmography (rPPG/iPPG) as a non-contact, non-ionizing, and non-invasive monitoring technique provides objective and reproducible information on physiological parameters. The aim of this study is to establish rPPG for intra- and postoperative monitoring of flap perfusion in patients undergoing reconstruction with free fasciocutaneous flaps (FFCF). We developed a monitoring algorithm for flap perfusion, which was evaluated in 15 patients. For 14 patients, ischemia of the FFCF in the forearm and successful reperfusion of the implanted FFCF was quantified based on the local signal. One FFCF showed no perfusion after reperfusion and devitalized in the course. Intraoperative monitoring of perfusion with rPPG provides objective and reproducible results. Therefore, rPPG is a promising technology for standard flap perfusion monitoring on low costs without the need for additional monitoring devices.

Contactless SpO2 with an RGB camera: experimental proof of calibrated SpO2

Camera-based blood oxygen saturation (SpO2) monitoring allows reliable measurements without touching the skin and is therefore very attractive when there is a risk of cross-infection, in case of fragile skin, and/or to improve the clinical workflow. Despite promising results, productization of the technology is hampered by the unavailability of adequate hardware, especially a camera, which can capture the optimal wavelengths for SpO2 measurements in the red near-infrared region. A regular color (RGB) camera is attractive because of its availability, but also poses several risks and challenges which affect the accuracy of the measurement. To mitigate the most important risks, we propose to add low-cost commercial off-the-shelf (COTS) components to the setup. We executed two studies with this setup: one at a hypoxia lab with SpO2 values in the range 70 - 100% with the purpose to determine the calibration model, and the other study on volunteers to investigate the accuracy for different spot-check scenarios. The proposed processing pipeline includes face tracking and a robust method to estimate the ratio of relative amplitudes of the photoplethysmographic waveforms. Results show that the error is smaller than 4 percent points for realistic screening scenarios where the subject is seated, either with or without head support and/or ambient light.

Intraoperative visualization and quantitative assessment of tissue perfusion by imaging photoplethysmography: comparison with ICG fluorescence angiography

Intraoperative monitoring of tissue perfusion is of great importance for optimizing surgery and reducing postoperative complications. To date, there is no standard procedure for assessing blood circulation in routine clinical practice. Over the past decade, indocyanine green (ICG) fluorescence angiography is most commonly used for intraoperative perfusion evaluation. Imaging photoplethysmography (iPPG) potentially enables contactless assessment of the blood supply to organs. However, no strong evidence of this potential has been provided so far. Here we report results of a comparative assessment of tissue perfusion obtained using custom-made iPPG and commercial ICG-fluorescence systems during eight different gastrointestinal surgeries. Both systems allow mapping the blood-supply distribution over organs. It was demonstrated for the first time that the quantitative assessment of blood perfusion by iPPG is in good agreement with that obtained by ICG-fluorescence imaging in all surgical cases under study. iPPG can become an objective quantitative monitoring system for tissue perfusion in the operating room due to its simplicity, low cost and no need for any agent injections.