Optical monitoring and detection of spinal cord ischemia

Postoperative Spinal Cord Ischemia Monitoring: A Review of Techniques Available After Endovascular Aortic Repair

BACKGROUND

Spinal cord ischemia is one of the complications that can occur after open and endovascular thoracoabdominal aortic repair. This occurs despite various perioperative approaches, including distal aortic perfusion, hybrid procedures with extra anatomical bypasses, motor-evoked potential, and cerebrospinal fluid drainage. The inability to recognize spinal ischemia in a timely manner remains a devastating complication after thoracoabdominal aortic repair.

OBJECTIVES

This review aims to look at novel technologies that are designed for continuous monitoring to detect early changes that signal the development of spinal cord ischemia and to discuss their benefits and limitations.

METHODS

We conducted a systematic review of the technologies available for continuous monitoring in the intensive care unit (ICU) for early detection of spinal cord ischemia. Studies were eligible for inclusion if they used different technologies for monitoring spinal ischemia during the postoperative period. All articles that were not available in English were excluded. To ensure that all relevant articles were included, no other significant restrictions were imposed.

RESULTS

We identified 59 studies from outset to December 2022 to be included in our study. New techniques have been studied as potentially useful monitoring tools that could provide simple and effective monitoring of the spinal cord. These include Near-infrared spectroscopy, Contrast-enhanced ultrasound, Magnetic Resonance Imaging, fiber optic monitoring of the spinal cord, and CSF biomarkers.

CONCLUSION

Despite the development of new techniques to monitor for postoperative spinal cord ischemia, their use remains limited. We recommend more future research to ensure rapid intervention for our patients.

Intraoperative Neurophysiological Monitoring Alerts During Three-Column Osteotomy: Incidence and Risk Factors

PRUPOSE

To analyze the incidence and risk factors of intraoperative neurophysiological monitoring (IONM) alerts in patients undergoing three-column osteotomy.

METHODS

A total of 551 patients (340 males and 211 females) with an average age of 31.9 years undergoing posterior 3-column osteotomy were retrospectively reviewed. The coronal Cobb angle of main curve and sagittal global kyphosis were measured on preoperative standing whole spinal x-rays. The Frankel scores at preoperation, postoperation, and the last follow-up were recorded and applied for assessment of neurologic status. Surgical procedures and other factors associated with IONM alerts were analyzed.

RESULTS

A total of 98 (17.8%) IONM alerts were reported during surgery, including 82 somatosensory evoked potential alerts and 91 motor evoked potential alerts. Positive wake-up test was revealed in 57 patients (10.3%) even after prompt managements, and new neurologic deficits were observed in 50 patients (9.1%) at immediate postoperation. Of the 50 patients with new neurologic deficits at postoperation, the Frankel scores were A in 5 patients, B in 4, C in 9, and D in 32. The χ 2 test showed that patients with congenital deformities, global kyphosis >90°, vertebral column resection procedure, cervicothoracic/thoracic osteotomy, blood loss >3,000 mL, and preoperative neurologic deficit were at a higher risk of IONM alerts.

CONCLUSIONS

The incidence of IONM alerts in patients undergoing 3-column osteotomy was 17.8%. Congenital deformities, global kyphosis >90°, vertebral column resection, cervicothoracic/thoracic osteotomy, blood loss >3,000 mL, and preoperative neurologic deficit indicated high risk of IONM alerts.

The neurovascular unit in healthy and injured spinal cord

The neurovascular unit (NVU) reflects the close temporal and spatial link between neurons and blood vessels. However, the understanding of the NVU in the spinal cord is far from clear and largely based on generalized knowledge obtained from the brain. Herein, we review the present knowledge of the NVU and highlight candidate approaches to investigate the NVU, particularly focusing on the spinal cord. Several unique features maintain the highly regulated microenvironment in the NVU. Autoregulation and neurovascular coupling ensure regional blood flow meets the metabolic demand according to the blood supply or local neural activation. The blood-central nervous system barrier partitions the circulating blood from neural parenchyma and facilitates the selective exchange of substances. Furthermore, we discuss spinal cord injury (SCI) as a common injury from the perspective of NVU dysfunction. Hopefully, this review will help expand the understanding of the NVU in the spinal cord and inspire new insights into SCI.

Lossless Compressed Sensing of Photon Counts for Fast Diffuse Correlation Spectroscopy

Diffuse Correlation Spectroscopy (DCS), a noninvasive optical technique, measures deep tissue blood flow using avalanche photon counting modules and data acquisition devices such as FPGAs or correlator boards. Conventional DCS instruments use in-processor counter modules that consume 32 bits/channel which is inefficient for low-photon budget situations prevalent in diffuse optics. Scaling these photon counters for large-scale imaging applications is difficult due to bandwidth and processing time considerations. Here, we introduce a new, lossless compressed sensing approach for fast and efficient detection of photon counts. The compressed DCS method uses an array of binary-coded-decimal counters to record photon counts from 8 channels simultaneously as a single 32-bit number. We validate the compressed DCS approach by comparisons with conventional DCS in experiments on tissue simulating phantoms and in-vivo arm cuff occlusion. Lossless compressed DCS was implemented with 87.5% compression efficiency. In tissue simulating phantoms, it was able to accurately estimate a tissue blood flow index, with no statistically significant difference compared to conventional DCS. Compressed DCS also recorded blood flow in vivo, in human forearm, with signal-to-noise ratio and dynamic range comparable to conventional DCS. Lossless 87.5% efficient compressed sensing counting of photon counts meets and exceeds benchmarks set by conventional DCS systems, offering a low-cost alternative for fast (~100 Hz) deep tissue blood flow measurement with optics.

Lawrence K, Tachtsidis I, Tong Y, Torricelli A, Urner T, Wabnitz H, Wolf M, Wolf U, Xu S, Yang C, Yodh AG, Yücel MA, Zhou W. Optical imaging and spectroscopy for the study of the human brain: status report

This report is the second part of a comprehensive two-part series aimed at reviewing an extensive and diverse toolkit of novel methods to explore brain health and function. While the first report focused on neurophotonic tools mostly applicable to animal studies, here, we highlight optical spectroscopy and imaging methods relevant to noninvasive human brain studies. We outline current state-of-the-art technologies and software advances, explore the most recent impact of these technologies on neuroscience and clinical applications, identify the areas where innovation is needed, and provide an outlook for the future directions.

Advances in Neuroimaging and Monitoring to Defend Cerebral Perfusion in Noncardiac Surgery

Noncardiac surgery conveys a substantial risk of secondary organ dysfunction and injury. Neurocognitive dysfunction and covert stroke are emerging as major forms of perioperative organ dysfunction, but a better understanding of perioperative neurobiology is required to identify effective treatment strategies. The likelihood and severity of perioperative brain injury may be increased by intraoperative hemodynamic dysfunction, tissue hypoperfusion, and a failure to recognize complications early in their development. Advances in neuroimaging and monitoring techniques, including optical, sonographic, and magnetic resonance, have progressed beyond structural imaging and now enable noninvasive assessment of cerebral perfusion, vascular reserve, metabolism, and neurologic function at the bedside. Translation of these imaging methods into the perioperative setting has highlighted several potential avenues to optimize tissue perfusion and deliver neuroprotection. This review introduces the methods, metrics, and evidence underlying emerging optical and magnetic resonance neuroimaging methods and discusses their potential experimental and clinical utility in the setting of noncardiac surgery.

Fast diffuse correlation spectroscopy with a low-cost, fiber-less embedded diode laser

Diffuse correlation spectroscopy (DCS), a popular optical technique for fast noninvasive measurement of blood flow, is commonly implemented using expensive fiber-coupled long coherence length laser systems. Here, we report the development of a portable and fiber-less approach that can be used as a low-cost alternative to illuminate tissue in DCS instruments. We validate the accuracy and noise characteristics of the fiber-less DCS laser source, by comparisons against traditional DCS light sources, with experiments on controlled tissue-simulating phantoms and in humans.

Ultrasound in Traumatic Spinal Cord Injury: A Wide-Open Field

Traumatic spinal cord injury (SCI) is a common and devastating condition. In the absence of effective validated therapies, there is an urgent need for novel methods to achieve injury stabilization, regeneration, and functional restoration in SCI patients. Ultrasound is a versatile platform technology that can provide a foundation for viable diagnostic and therapeutic interventions in SCI. In particular, real-time perfusion and inflammatory biomarker monitoring, focal pharmaceutical delivery, and neuromodulation are capabilities that can be harnessed to advance our knowledge of SCI pathophysiology and to develop novel management and treatment options. Our review suggests that studies that evaluate the benefits and risks of ultrasound in SCI are severely lacking and our understanding of the technology's potential impact remains poorly understood. Although the complex anatomy and physiology of the spine and the spinal cord remain significant challenges, continued technological advances will help the field overcome the current barriers and bring ultrasound to the forefront of SCI research and development.

Minimally invasive intrathecal spinal cord imaging with optical coherence tomography

SIGNIFICANCE

Imaging of the spinal cord is challenging due to the surrounding bony anatomy, physiologic motion, and the small diameter of the spinal cord. This precludes the use of non-invasive imaging techniques in assessing structural changes related to trauma and evaluating residual function.

AIM

The purpose of our research was to apply endovascular technology and techniques and construct a preclinical animal model of intrathecal spinal cord imaging using optical coherence tomography (OCT).

APPROACH

Five animals (2 Yorkshire Swine and 3 New Zealand Rabbits) were utilized. Intrathecal access was gained using a 16-guage Tuohy, and an OCT catheter was advanced under roadmap technique into the cervical canal. The OCT catheter has a motorized pullback, and a total length of 54 mm of the spinal canal is imaged.

RESULTS

Image acquisition was successful for all animals. There were no instances of difficult catheter navigation, enabling OCT imaging rostrally to C2. The thecal sac provided excellent thoroughfare for the OCT catheter. The clear cerebrospinal fluid also provided an excellent medium for image acquisition, with no detectable artifact from the contents of the cerebrospinal fluid. The anatomical space of the spinal canal could be readily appreciated including: dural lining of the thecal sac, epidural veins, pial lining of the spinal cord, arachnoid bands, dentate ligaments, and nerve rootlets/roots.

CONCLUSION

Minimally invasive intrathecal imaging using endovascular OCT was feasible in this preclinical animal study. The repurposing of an endovascular device for spinal imaging comes with limitations, and a spine-specific device is necessary.

Whole body measurements using near-infrared spectroscopy in a rat spinal cord contusion injury model

BACKGROUND

Spinal cord injuries cause great damage to the central nervous system as well as the peripheral vasculature. While treatments for spinal cord injury typically focus on the spine itself, improvements in the function of the peripheral vasculature after spinal cord injury have shown to improve overall neurological recovery.

OBJECTIVE

This study focused on the use of near-infrared spectroscopy (NIRS) as a mode to monitor cerebral and peripheral vascular condition non-invasively during the recovery process.

DESIGN

Animal research study.

METHODS

Rats underwent spinal contusion or sham injury and relative concentrations of de-/oxyhemoglobin (Δ[HbO]/Δ[Hb]) over time were measured over the cerebral, spinal, and pedal regions via NIRS. Correlational relationships across the body were determined. Rats received 1 NIRS measurement before injury and 3 after injury: 4, 7, and 14 days post.

RESULTS

Correlational relationships between signals across the body, between animals with and without spinal cord injury, indicate that NIRS was able to detect patterns of vascular change in the spine and the periphery occurring secondary to spinal cord injury and evolving during subsequent recovery. Additionally, NIRS determined an overall correlational decrease within the central nervous system, between spinal and cerebral measurements.

CONCLUSION

NIRS was able to closely reflect physiologic changes in the rat during recovery, demonstrating a promising method to monitor whole body hemodynamics after spinal cord injury.

Ischemic spinal cord injury - experimental evidence and evolution of protective measures

BACKGROUND

Paraplegia remains one of the most devastating complications of descending and thoracoabdominal aortic repair. The aim of this review is to outline the current state of art in the rapidly developing field of spinal cord injury (SCI) research.

METHODS

A review of PubMed and Web of Science databases was performed using the following terms and their combinations: spinal cord, injury, ischemia, ischemia-reperfusion, ischemic spinal cord injury, paraplegia, paraparesis. Articles published before July 2019 were screened and included if considered relevant.

RESULTS

The review focuses on the topic of SCI and the developments concerning methods of monitoring, diagnostics and prevention of SCI.

CONCLUSIONS

Translation of novel technologies from bench to bedside and into everyday clinical practice is challenging, however each of the developing areas hold great promise in SCI prevention.

Perspective on optical imaging for functional assessment in musculoskeletal extremity trauma surgery

SIGNIFICANCE

Extremity injury represents the leading cause of trauma hospitalizations among adults under the age of 65 years, and long-term impairments are often substantial. Restoring function depends, in large part, on bone and soft tissue healing. Thus, decisions around treatment strategy are based on assessment of the healing potential of injured bone and/or soft tissue. However, at the present, this assessment is based on subjective clinical clues and/or cadaveric studies without any objective measure. Optical imaging is an ideal method to solve several of these issues.

AIM

The aim is to highlight the current challenges in assessing bone and tissue perfusion/viability and the potentially high impact applications for optical imaging in orthopaedic surgery.

APPROACH

The prospective will review the current challenges faced by the orthopaedic surgeon and briefly discuss optical imaging tools that have been published. With this in mind, it will suggest key research areas that could be evolved to help make surgical assessments more objective and quantitative.

RESULTS

Orthopaedic surgical procedures should benefit from incorporation of methods to measure functional blood perfusion or tissue metabolism. The types of measurements though can vary in the depth of tissue sampled, with some being quite superficial and others sensing several millimeters into the tissue. Most of these intrasurgical imaging tools represent an ideal way to improve surgical treatment of orthopaedic injuries due to their inherent point-of-care use and their compatibility with real-time management.

CONCLUSION

While there are several optical measurements to directly measure bone function, the choice of tools can determine also the signal strength and depth of sampling. For orthopaedic surgery, real-time data regarding bone and tissue perfusion should lead to more effective patient-specific management of common orthopaedic conditions, requiring deeper penetrance commonly seen with indocyanine green imaging. This will lower morbidity and result in decreased variability associated with how these conditions are managed.

Multi-Site Optical Monitoring of Spinal Cord Ischemia during Spine Distraction

Optimal surgical management of spine trauma will restore blood flow to the ischemic spinal cord. However, spine stabilization may also further exacerbate injury by inducing ischemia. Current electrophysiological technology is not capable of detecting acute changes in spinal cord blood flow or localizing ischemia. Further, alerts are delayed and unreliable. We developed an epidural optical device capable of directly measuring and immediately detecting changes in spinal cord blood flow using diffuse correlation spectroscopy (DCS). Herein we test the hypothesis that our device can continuously monitor blood flow during spine distraction. Additionally, we demonstrate the ability of our device to monitor multiple sites along the spinal cord and axially resolve changes in spinal cord blood flow. DCS-measured blood flow in the spinal cord was monitored at up to three spatial locations (cranial to, at, and caudal to the distraction site) during surgical distraction in a sheep model. Distraction was halted at 50% of baseline blood flow at the distraction site. We were able to monitor blood flow with DCS in multiple regions of the spinal cord simultaneously at ∼1 Hz. The distraction site had a greater decrement in flow than sites cranial to the injury (median -40 vs. -7%,). This pilot study demonstrated high temporal resolution and the capacity to axially resolve changes in spinal cord blood flow at and remote from the site of distraction. These early results suggest that this technology may assist in the surgical management of spine trauma and in corrective surgery of the spine.

Spinal cord autoregulation using near-infrared spectroscopy under normal, hypovolemic, and post-fluid resuscitation conditions in a swine model: a comparison with cerebral autoregulation

Background

Few studies have investigated spinal cord autoregulation using near-infrared spectroscopy (NIRS). Here, we assessed spinal cord autoregulation under normal, hypovolemic, and post-fluid resuscitation conditions compared with cerebral autoregulation.

Methods

Ten pigs (36.1 ± 1.1 kg) were anesthetized with 2.5% isoflurane, before phenylephrine administration at 0.5, 1, 2, and 5 μg kg⁻¹ min⁻¹ in a stepwise fashion at 10-min intervals (baseline), followed by similar administration of sodium nitroprusside (SNP). Hypovolemia was induced by a 600-ml bleed (25% estimated total blood volume). Only phenylephrine was readministered (same protocol). Hypovolemia was reversed by infusing 600 ml hydroxyethyl starch, before readministering phenylephrine and SNP. The relationships between mean arterial pressure (MAP) and cerebral, thoracic, and lumbar spinal cord tissue oxygenation indices (TOIs) were evaluated.

Results

Thoracic and lumbar spinal cord TOIs were approximately 15% and 10% lower, respectively, than the cerebral TOI at similar MAPs. The average relationship between MAP and each TOI showed an autoregulatory pattern, but negative correlations were observed in the cerebral TOI during phenylephrine infusion. A 600-ml bleed lowered each relationship < 5% and subsequent fluid resuscitation did not change the relationship. Individual oxygenation responses to blood pressure indicated that the spinal cord is more pressure-passive than the cerebrum. Paradoxical responses (an inverse relationship of tissue oxygenation to MAP) were observed particularly in cerebrum during phenylephrine infusion and were rare in the spinal cord.

Conclusions

Spinal cord autoregulation is less robust than cerebral autoregulation and more pressure-dependent. Similar to cerebral oxygenation, spinal cord oxygenation is volume-tolerant but is more sensitive to hypotension.

Optics Based Label-Free Techniques and Applications in Brain Monitoring

Functional near-infrared spectroscopy (fNIRS) has been utilized already around three decades for monitoring the brain, in particular, oxygenation changes in the cerebral cortex. In addition, other optical techniques are currently developed for in vivo imaging and in the near future can be potentially used more in human brain research. This paper reviews the most common label-free optical technologies exploited in brain monitoring and their current and potential clinical applications. Label-free tissue monitoring techniques do not require the addition of dyes or molecular contrast agents. The following optical techniques are considered: fNIRS, diffuse correlations spectroscopy (DCS), photoacoustic imaging (PAI) and optical coherence tomography (OCT). Furthermore, wearable optical brain monitoring with the most common applications is discussed.

Contactless treatment for scoliosis by electromagnetically controlled shape-memory alloy rods: a preliminary study in rabbits

PURPOSE

To evaluate the safety and efficacy of a system aiming to correct scoliosis called "electromagnetically controlled shape-memory alloy rods" (EC-SMAR) used in a rabbit model.

METHODS

We heat-treated shape-memory alloy (SMA) rods to achieve a transition temperature between 34 and 47 °C and a C-shape austenite phase. We then developed a water-cooled generator capable of generating an alternating magnetic field (100 kHz) for induction heating. We next studied the efficacy of this system in vitro and determined some parameters prior to proceeding with animal experiments. We then employed a rabbit model, in which we fixed a straight rod along the spinous processes intraoperatively, and conducted induction heating postoperatively every 4 days for 1 month, while performing periodic X-ray assessments.

RESULTS

Significant kyphotic deformations with Cobb angles of about 45° (p < 0.01) were created in five rabbits, and no complications occurred throughout the experiment. The rabbits are still very much alive and do not show any signs of discomfort.

CONCLUSIONS

This is the first system that can modulate spinal deformation in a gradual, contactless, noninvasive manner through electromagnetic induction heating applied to SMA alloy rods. Although this study dealt with healthy spines, it provides promising evidence that this device also has the capacity to correct human kyphosis and even scoliosis in the future. These slides can be retrieved under Electronic Supplementary Material.

Diffuse Correlation Spectroscopy Analysis Implemented on a Field Programmable Gate Array

Diffusive correlation spectroscopy (DCS) is an emerging optical technique that measures blood perfusion in deep tissue. In a DCS measurement, temporal changes in the interference pattern of light, which has passed through tissue, are quantified by an autocorrelation function. This autocorrelation function is further parameterized through a non-linear curve fit to a solution to the diffusion equation for coherence transport. The computational load for this non-linear curve fitting is a barrier for deployment of DCS for clinical use, where real-time results, as well as instrument size and simplicity, are important considerations. We have mitigated this computational bottleneck through development of a hardware analyzer for DCS. This analyzer implements the DCS curving fitting algorithm on digital logic circuit using Field Programmable Gate Array (FPGA) technology. The FPGA analyzer is more efficient than a typical software analysis solution. The analyzer module can be easily duplicated for processing multiple channels of DCS data in real-time. We have demonstrated the utility of this analyzer in pre-clinical large animal studies of spinal cord ischemia. In combination with previously described FPGA implementations of auto-correlators, this hardware analyzer can provide a complete device-on-a-chip solution for DCS signal processing. Such a component will enable new DCS applications demanding mobility and real-time processing.

Monitoring spinal cord hemodynamics and tissue oxygenation: a review of the literature with special focus on the near-infrared spectroscopy technique

STUDY DESIGN

Review.

OBJECTIVES

Clinical studies have shown that the hemodynamic management of patients following acute spinal cord injury (SCI) is an important aspect of their treatment for maintaining spinal cord (SC) perfusion and minimizing ischemic secondary injury to the SC. While this highlights the importance of ensuring adequate perfusion and oxygenation to the injured cord, a method for the real-time monitoring of these hemodynamic measures within the SC is lacking. The purpose of this review is to discuss current and potential methods for SC hemodynamic monitoring with special focus on applications using near-infrared spectroscopy (NIRS).

METHODS

A literature search using the PubMed database. All peer-reviewed articles on NIRS monitoring of SC published from inception to May 2019 were reviewed.

RESULTS

Among 125 papers related to SC hemodynamics monitoring, 26 focused on direct/indirect NIRS monitoring of the SC.

DISCUSSION

Current options for continuous, non-invasive, and real-time monitoring of SC hemodynamics are challenging and limited in scope. As a relatively new technique, NIRS has been successfully used for monitoring human cerebral hemodynamics, and has shown promising results in intraoperative assessment of SC hemodynamics in both human and animal models. Although utilizing NIRS to monitor the SC has been validated, applying NIRS clinically following SCI requires further development and investigation.

CONCLUSIONS

NIRS is a promising non-invasive technique with the potential to provide real-time monitoring of relevant parameters in the SC. Currently, in its first developmental stages, further clinical and experimental studies are mandatory to ensure the validity and safety of NIRS techniques.

Risk Factor Analysis of Change in Intraoperative Neurophysiologic Monitoring During Cervical Open Door Laminoplasty

OBJECTIVE

The aim of this study is to determine the risk factors affecting intraoperative neurophysiologic monitoring (IONM) changes, when such changes take place, and clinical outcomes associated with IONM change during cervical open door laminoplasty (COL) for cervical compressive myelopathy.

METHODS

Between 2010 and 2015, 79 patients who underwent COL with IONM recording were studied. Changes in motor evoked potentials or somatosensory evoked potentials over an alarm criterion were defined as IONM change. Patients with IONM change were assigned to the alarm group, and the others were classified as the control group. Baseline data and radiographic measurements were compared between the 2 groups. Radiologic parameters including maximal compression level (MCL), area and diameter of the spinal canal and ventral compressive lesion, stenosis grade, and occupying ratio of area (ORA) and length at the MCL were measured with magnetic resonance imaging.

RESULTS

Thirteen patients were assigned to the alarm group and 66 patients were assigned to the control group. Multivariate analysis identified ORA at the MCL (odds ratio, 1.520; 95% confidence interval, 1.192-1.37; P = 0.001) as an independent risk factor for IONM change. Immediately after decompression, the IONM change occurred. One of 4 patients who did not fully recover from the IONM change had postoperative motor deficits.

CONCLUSIONS

IONM change during COL occurred immediately after decompression, and a risk factor of IONM change was ORA at the MCL. If the IONM change was not fully recovered, a new motor deficit occurred after COL.

Laser safety in fiber-optic monitoring of spinal cord hemodynamics: a preclinical evaluation

The prevention and treatment of spinal cord injury are focused upon the maintenance of spinal cord blood flow, yet no technology exists to monitor spinal cord ischemia. We recently demonstrated continuous monitoring of spinal cord ischemia with diffuse correlation and optical spectroscopies using an optical probe. Prior to clinical translation of this technology, it is critically important to demonstrate the safety profile of spinal cord exposure to the required light. To our knowledge, this is the first report of in situ safety testing of such a monitor. We expose the spinal cord to laser light utilizing a custom fiber-optic epidural probe in a survival surgery model (11 adult Dorset sheep). We compare the tissue illumination from our instrument with the American National Standards Institute maximum permissible exposures. We experimentally evaluate neurological and pathological outcomes of the irradiated sheep associated with prolonged exposure to the laser source and evaluate heating in ex vivo spinal cord samples. Spinal cord tissue was exposed to light levels at ∼18  ×   the maximum permissible exposure for the eye and ∼  (  1  /  3  )    ×   for the skin. Multidisciplinary testing revealed no functional neurological sequelae, histopathologic evidence of laser-related injury to the spinal cord, or significant temperature changes in ex vivo samples. Low tissue irradiance and the lack of neurological, pathological, and temperature changes upon prolonged exposure to the laser source offer evidence that spinal cord tissues can be monitored safely with near-infrared optical probes placed within the epidural space.

Validation of diffuse correlation spectroscopy sensitivity to nicotinamide-induced blood flow elevation in the murine hindlimb using the fluorescent microsphere technique

Nicotinamide has been shown to affect blood flow in both tumor and normal tissues, including skeletal muscle. Intraperitoneal injection of nicotinamide was used as a simple intervention to test the sensitivity of noninvasive diffuse correlation spectroscopy (DCS) to changes in blood flow in the murine left quadriceps femoris skeletal muscle. DCS was then compared with the gold-standard fluorescent microsphere (FM) technique for validation. The nicotinamide dose-response experiment showed that relative blood flow measured by DCS increased following treatment with 500- and 1000-mg  /  kg nicotinamide. The DCS and FM technique comparison showed that blood flow index measured by DCS was correlated with FM counts quantified by image analysis. The results of this study show that DCS is sensitive to nicotinamide-induced blood flow elevation in the murine left quadriceps femoris. Additionally, the results of the comparison were consistent with similar studies in higher-order animal models, suggesting that mouse models can be effectively employed to investigate the utility of DCS for various blood flow measurement applications.

Changes in transcranial electrical motor-evoked potentials during the early and reversible stage of permanent spinal cord ischemia predict spinal cord injury in a rabbit animal model

The present study examined changes in the transcranial electrical motor-evoked potentials (TceMEP) waveform to predict neurological deficits and histopathological changes during the early and reversible stage of different levels of permanent spinal cord ischemic injury in a rabbit animal model. A total of 24 New Zealand rabbits were randomly divided into four groups of 6 rabbits each. Group 1 underwent a ligation of the lumbar artery at three levels (L1-L3), group 2 underwent a ligation of the lumbar artery at four levels (L1-L4) and group 3 underwent a ligation of the lumbar artery at five levels (L1-L5). The sham group contained 6 rabbits and did not receive ligation. TceMEP was recorded within 5 min of ligation and, 2 days later, motor function was assessed and the spinal cords were removed for histological examination. Following spinal cord injury, the relationship between variations in the TceMEP waveform and motor function and pathological damage was analyzed. It was observed that the amplitude of TceMEP began to decrease within 1 min of lumbar artery ligation and that the amplitude stabilized within 5 min. These amplitude changes that occurred within 5 min of different levels of permanent spinal cord ischemic injury were positively related to changes in motor function following recovery from anesthesia and 2 days after ligation. The Pearson correlation coefficient was 0.980 and 0.923 for these two time points, respectively (P<0.001). In addition, the amplitude changes were positively related to pathological damage, with a Pearson correlation coefficient of 0.945 (P<0.001). The results of the present study suggested that amplitude changes in TceMEP are particularly sensitive to ischemia. Ischemia may be detected within 1 min and the amplitude changes begin to stabilize within 5 min following ligation of the lumbar artery. The use of intraoperative monitoring of TceMEP allows for the detection of spinal cord ischemic injury with no time delay, which may allow for protective measures to be taken to prevent the occurrence of irreversible spinal cord injury.

Non-invasive diffuse correlation tomography reveals spatial and temporal blood flow differences in murine bone grafting approaches

Longitudinal blood flow during murine bone graft healing was monitored non-invasively using diffuse correlation tomography. The system utilized spatially dense data from a scanning set-up, non-linear reconstruction, and micro-CT anatomical information. Weekly in vivo measurements were performed. Blood flow changes in autografts, which heal successfully, were localized to graft regions and consistent across mice. Poor healing allografts showed heterogeneous blood flow elevation and high inter-subject variabilities. Allografts with tissue-engineered periosteum showed responses intermediate to both autografts and allografts, consistent with healing observed. These findings suggest that spatiotemporal blood flow changes can be utilized to differentiate the degree of bone graft healing.

Fiber-optic Monitoring of Spinal Cord Hemodynamics in Experimental Aortic Occlusion

BACKGROUND

Spinal cord ischemia occurs frequently during thoracic aneurysm repair. Current methods based on electrophysiology techniques to detect ischemia are indirect, non-specific, and temporally slow. In this article, the authors report the testing of a spinal cord blood flow and oxygenation monitor, based on diffuse correlation and optical spectroscopies, during aortic occlusion in a sheep model.

METHODS

Testing was carried out in 16 Dorset sheep. Sensitivity in detecting spinal cord blood flow and oxygenation changes during aortic occlusion, pharmacologically induced hypotension and hypertension, and physiologically induced hypoxia/hypercarbia was assessed. Accuracy of the diffuse correlation spectroscopy measurements was determined via comparison with microsphere blood flow measurements. Precision was assessed through repeated measurements in response to pharmacologic interventions.

RESULTS

The fiber-optic probe can be placed percutaneously and is capable of continuously measuring spinal cord blood flow and oxygenation preoperatively, intraoperatively, and postoperatively. The device is sensitive to spinal cord blood flow and oxygenation changes associated with aortic occlusion, immediately detecting a decrease in blood flow (-65 ± 32%; n = 32) and blood oxygenation (-17 ± 13%, n = 11) in 100% of trials. Comparison of spinal cord blood flow measurements by the device with microsphere measurements led to a correlation of R = 0.49, P < 0.01, and the within-sheep coefficient of variation was 9.69%. Finally, diffuse correlation spectroscopy is temporally more sensitive to ischemic interventions than motor-evoked potentials.

CONCLUSION

The first-generation spinal fiber-optic monitoring device offers a novel and potentially important step forward in the monitoring of spinal cord ischemia.

Characterization of blood flow in the mouse dorsal spinal venous system before and after dorsal spinal vein occlusion

The availability of transgenic strains has made the laboratory mouse a popular model for the study of healthy and diseased state spinal cord (SC). Essential to identifying physiologic and pathologic events is an understanding of the microvascular network and flow patterns of the SC. Using 2-photon excited fluorescence (2PEF) microscopy we performed in vivo measurements of blood flow in the lower thoracic portion of the mouse dorsal spinal vein (dSV) and in the first upstream branches supplying it, denoted as dorsal ascending venules (dAVs). We found that the dSV had large radiculomedullary veins (RMVs) exiting the SC, and that flow in the dSV between pairs of RMVs was bidirectional. Volumetric flow increased in each direction away from the point of bifurcation. Flow in the upstream dAVs varied with diameter in a manner consistent with a constant distal pressure source. By performing ex vivo 2PEF microscopy of fluorescent-gel perfused tissue, we created a 3-D map of the dorsal spinal vasculature. From these data, we constructed a simple model that predicted changes in the flow of upstream branches after occlusion of the dSV in different locations. Using an atraumatic model of dSV occlusion, we confirmed the predictions of this model in vivo.

Diffuse correlation spectroscopy for measurement of cerebral blood flow: future prospects

Diffuse correlation spectroscopy (DCS) is an emerging optical modality used to measure cortical cerebral blood flow. This outlook presents a brief overview of the technology, summarizing the advantages and limitations of the method, and describing its recent applications to animal, adult, and infant cohorts. At last, the paper highlights future applications where DCS may play a pivotal role individualizing patient management and enhancing our understanding of neurovascular coupling, activation, and brain development.