Coupled air lasing gain and Mie scattering loss: an aerosol effect in filament-induced plasma spectroscopy.

Femtosecond laser filament-induced plasma spectroscopy (FIPS) demonstrates great potential in remote sensing for identifying atmospheric pollutant molecules. Due to the widespread aerosols in the atmosphere, remote detection based on FIPS would be affected by both the excitation and the propagation of fingerprint fluorescence, which still remain elusive. Here the physical model of filament-induced aerosol fluorescence is established to reveal the combined effect of Mie scattering and amplification spontaneous emission, which is subsequently proven by experimental results, the dependence of the backward fluorescence on the interaction length between filaments and aerosols. These findings provide an insight into the complicated aerosol effect in the overall physical process of FIPS including propagation, excitation, and emission, paving the way to its practical application in atmospheric remote sensing.

Validity of evoked potential as biomarker for predicting early neural function changes after thoracic spinal decompression surgery in patients with neurological deficits.

OBJECTIVE: To evaluate the validity of intraoperative evoked potential (EP) including motor evoked potential (MEP) and somatosensory evoked potentials (SEP) as a biomarker for predicting neural function changes after thoracic spinal decompression (TSD) surgery. METHOD: A consecutive series of 336 TSD surgeries were reviewed between 2010 and 2021 from four spine center. All patients with TSD were divided into 3 groups according to different intraoperative EP results: group 1, EP alerts; group 2, no obvious EP deterioration; group 3, EP improvement compared with baselines. The lower limb Japanese Orthopedic Association (JOA) scores (as well as early and long-term JOA recovery rate) were utilized to quantitatively assess pre- and postoperative neural function change. RESULTS: Among the 3 subgroups according to the different EP changes, the early JOA recovery rate (RR%) in the EP improvement group was significantly better than the other two groups (51.3 ± 58.6* vs. 27.5 ± 31.2 and 33.3 ± 43.1; p < 0.01) after 3-month follow-up. The mean MEP and SEP amplitude were from 116 ± 57 µV to 347 ± 71 µV (p < 0.01) and from 1.86 ± 0.24 µV to 2.65 ± 0.29 µV (p < 0.01) between spinal cord pre-decompression and post-decompression. Moreover, multivariate logistic regression analysis revealed that risk factors of EP improvement were duration of symptom (p < 0.001, OR 10.9) and Preop. neurologic deficit degree (p = 0.013, OR 7.46). CONCLUSION: The intraoperative EP can predict postoperative neural function changes as a biomarker during TSD. Patient with EP improvement probably has better prognosis for early neural function recovery. The duration of symptom and preoperative neurologic deficit degree may be related to intraoperative EP improvement.

A critical event frequent lead to reversible spinal cord injury during vertebral column resection surgery.

OBJECTIVE: To report a "critical phase" (between osteotomy completion and correction beginning) that will frequently lead to the reversible intraoperative neurophysiological monitoring (IOM) change during posterior vertebral column resection (PVCR) surgery. METHODS: The study sample consisted of 120 patients with severe spine deformity who underwent PVCR and deformity correction surgeries. Those patients were recruited consecutively from 2010 to 2018 January in our spine center. The detailed IOM data (the amplitude of MEP & SEP) and its corresponding surgical points were collected prospectively. Early and long-term postoperative neurologic outcomes were assessed for the following functions: motor, sensory, and pain at immediate postoperative and 1-year post-operation in this cases series. RESULTS: A total of 105 (105/120) patients presented varying degrees of IOM reduction in the critical phase; the mean IOM amplitude retention vs rescue rate was 27% ± 11.2 versus 58% ± 16.9, P < 0.01 (MEP) & 34% ± 8.3 versus 66% ± 12.4 P < 0.01 (SEP). Patients with postoperative spinal deficits often had a significantly longer IOM-alerting duration than the patients without (p < 0.01, Mann-Whitney U-test), and IOM-alerting duration greater than 39.5 min was identified as an independent predictor of the risk of postoperative spinal deficits. CONCLUSIONS: The reversible IOM events probably often appear in the critical phase during PVCR surgery. The new postoperative spinal deficits are possible for patients without satisfied IOM recovery or alerting duration greater than 39.5 min. Timely and suitable surgical interventions are useful for rescuing the IOM alerts.

Detection of 1.4 µg/m3 Na+ in aerosol at a 30 m distance using 1 kHz femtosecond laser filamentation in air.

An optimized remote material detection scheme based on the laser filament-induced plasma spectroscopy and light detection and ranging (FIPS-LIDAR) is proposed in this work. The elemental composition and concentration of aerosol are measured by FIPS-LIDAR. By focusing the femtosecond laser with a large aperture (Φ41 cm) concave mirror and coaxial fluorescence collection scheme, the remote detection of aerosol in air at µg/m3 level has been realized at a distance of 30 m. The limit of detection for Na+ in aerosol droplets is 8 ppm (3 µg/m3 in air), which is the lowest detection limit that has been reported using millijoule femtosecond laser pulse (4.4 mJ). Furthermore, using spectral preprocessing and optimization of the proposed significance of peak (SOP) algorithm, feature peak signals are extracted from weak signals and the limit of detection can be further decreased to 1.4 µg/m3.

Sub-ppb NaCl aerosol detection at a distance of 30 meters by femtosecond laser induced plasma spectroscopy.

In this work, sub-ppb aerosol detection is achieved by femtosecond laser filament with a single pulse energy of 4 mJ at a distance of 30 m. A concave mirror with an open aperture of 41.4 cm is employed in an off-axis optical system to focus the femtosecond laser beam and collect the fluorescence of NaCl aerosol. The simulation and experimental results show that the astigmatism can be greatly reduced when femtosecond laser beam is incident non-symmetrically on the concave mirror. Compared with the case that femtosecond laser strikes at the center of the concave mirror, the intensity of acoustic signal emitted from the optical filament is increased by 69.5 times, and the detection of limit of sodium element in aerosol is reduced by 86%, which is down to 0.32 ppb. The improved excitation scheme in this work utilizes the nonsymmetrical beam spot on the concave mirror to compensate the non-symmetry induced by the off-axis setup, reducing the astigmatism of the focusing laser beam and decreasing the sodium chloride aerosol's detection of limit.

Suppressing turbulence-induced laser beam wandering by using an axicon.

Laser beam wandering induced by air turbulence could be crucial for applications such as free-space optical communications, LIDAR, and remote air lasing. In this study, the influence of air turbulence on the transverse wandering of a Gauss laser beam with and without an axicon as the focusing optical component has been investigated. It has been demonstrated that the turbulence-induced beam wandering could be dramatically suppressed by using an axicon. Further, the corresponding mechanism has been discussed. This work would bring great benefits to many practical applications relying on a stable laser beam in a turbulent atmosphere.

A Simulation of Air Lasing Seeded by an External Wave in a Femtosecond Laser Filament.

Air lasers induced by femtosecond laser filaments play an important role in remote sensing applications. Few studies have been dedicated to the spatial distribution of external-seeded air laser radiation in the laser filament based on the numerical simulation method, which can pave the way to understanding the mechanism of the external-seeded air lasing process during filamentation. In this study, numerical simulations of the propagation of an air laser seeded by an external plane wave with a wavelength of 391 nm during femtosecond laser filamentation were performed. The results indicated that the air laser's beam intensity distribution varies from a ring pattern to a donut pattern when the filament length and nitrogen ion density are raised as a result of the defocusing and lasing effects of the filament plasma. Here, the ring pattern is formed by several thin rings, while the donut pattern refers to a notably thicker, ring-like structure. In addition, it has been demonstrated that the air laser's beam power would increase exponentially versus the filament length and the nitrogen ion density. The knowledge about the angular distribution of air lasing could be important for optimizing the detection geometry of the LIDAR setup, including the view angle and the size of the collecting optical component.

Giant enhancement of acoustic and fluorescence emission from an off-axis reflective femtosecond laser filamentation system.

Femtosecond laser filamentation propagating tens of meters to several kilometers with high intensity in the atmosphere has been demonstrated as a powerful tool for remote sensing. In contrast to the refractive systems, the reflective optical systems possess a variety of advantages including broad bandwidth, large aperture, light weight and low energy loss. However, astigmatic aberration is inevitably introduced by off-axis reflective mirrors. It can greatly affect the filament quality, which is critical for exciting and detecting the fluorescence of target molecules. Here we elaborately design a free-form phase plate to correct the astigmatism in off-axis reflective optical systems. It is demonstrated that the free-form surface exhibits excellent performance, significantly reducing the astigmatic difference from 44 cm to 4 cm and increasing the maximum acoustic intensity by a factor of 53. In addition, extremely strong nitrogen fluorescence spectra have been detected. These results indicate that the free-form phase plate can effectively compensate for astigmatic aberration in off-axis reflective system, providing a guiding significance for the optimal control of filamentation and remote sensing.

Interdigitated photoconductive antenna-based two-color femtosecond laser filamentation THz time-domain spectral detection.

Two-color femtosecond (fs) laser filamentation in the gas medium is an effective way to generate broadband and high intensity terahertz (THz) pulse. The interdigitated photoconductive antenna (iPCA) has the advantages of both broadband detection and high signal-to-noise ratio (SNR), which is a very effective way to detect the THz pulse produced by two-color fs laser filamentation. The THz signal from two-color fs laser filamentation is comprehensively characterized by the iPCA, which achieves high SNR, high sensitivity, and polarization detection. This work provides a new idea for high power broadband THz coherent detection.

Effect of laser repetition rate on the fluorescence characteristic of a long-distance femtosecond laser filament.

In this paper, the effect of the laser repetition rate on the long-distance femtosecond laser filament in air is investigated by measuring the fluorescence characteristic of the filament. A femtosecond laser filament emits fluorescence due to the thermodynamical relaxation of the plasma channel. Experimental results show that as the repetition rate of femtosecond laser increases, the fluorescence of the filament induced by a single laser pulse weakens, and the position of the filament moves away from the focusing lens. These phenomena may be attributed to the slow hydrodynamical recovery process of air after being excited by a femtosecond laser filament, whose characteristic time is on the millisecond time scale and comparable to the inter-pulse duration of the femtosecond laser pulse train. This finding suggests that at a high laser repetition rate, to generate an intense laser filament, the femtosecond laser beam should scan across the air to eliminate the adverse effect of slow air relaxation, which is beneficial to laser filament remote sensing.

Utilization of intraoperative neuromonitoring during the Woodward procedure for treatment of Sprengel deformity.

BACKGROUND: Brachial plexus injury (BPI) leading to palsy of the upper extremities is the most serious complication of the Woodward procedure for treatment of Sprengel deformity. Intraoperative neuromonitoring (IONM) is widely used for detecting emerging spinal cord or peripheral nerve injury during spinal and shoulder surgery. However, to date, its utilization in pediatric patients with Sprengel deformity is limited. Furthermore, it remains unclear whether IONM can help prevent BPI during surgery. The purpose of the current study was to assess the feasibility and effectiveness of IONM for early identification and prevention of nerve injury during the Woodward procedure. METHODS: We retrospectively reviewed the records of patients who underwent the Woodward procedure for Sprengel deformity at our institution between January 2017 and January 2020. IONM, including somatosensory evoked potentials (SEP) and motor evoked potentials (MEPs), was performed in all patients. Detailed IONM data were collected and analyzed. Preoperative and postoperative cosmetic appearance (according to the Cavendish classification), shoulder joint abduction function, and radiologic evaluation of the scapula were reviewed. Surgical complications were recorded. RESULTS: Forty-six patients (19 girls, 27 boys) were included (mean age, 5.1 ± 2.1 years). Both SEP and MEP (amplitude of the abductor pollicis) were successfully performed (100%). MEP alerts occurred in 3 patients (6.5%). After scapula position adjustment, signals recovered in 2 patients and remained unchanged in 1 patient-this patient exhibited postoperative motor deficits that resolved completely by 4 months recovery. The SEP amplitudes decreased in all 3 patients but did not reach the warning criteria. Forty patients were classified as grade III and 6 as grade IV in the Cavendish classification, whereas 35 patients were classified as grade II and 11 as grade III in the Rigault scale. The preoperative Cavendish grade was III (III, IV) and the postoperative Cavendish grade was I (I, II) (χ2 = 88.098, P < .001). The preoperative Rigault grade was II (II, III) and the postoperative Rigault grade was I (I, II) (χ2 = 62.133, P < .001). The mean arc of shoulder joint abduction improved from 99° ± 8° to 167° ± 7° (t = -45.871, P < .001) after surgery. Except for temporary motor deficits detected in 1 patient, no other postoperative complications were observed through the time of final follow-up. CONCLUSION: IONM during the Woodward procedure for Sprengel deformity is feasible and effective in detecting intraoperative neurologic changes and may be effective in preventing BPI associated with surgery.

Sensing with Femtosecond Laser Filamentation.

Femtosecond laser filamentation is a unique nonlinear optical phenomenon when high-power ultrafast laser propagation in all transparent optical media. During filamentation in the atmosphere, the ultrastrong field of 1013-1014 W/cm2 with a large distance ranging from meter to kilometers can effectively ionize, break, and excite the molecules and fragments, resulting in characteristic fingerprint emissions, which provide a great opportunity for investigating strong-field molecules interaction in complicated environments, especially remote sensing. Additionally, the ultrastrong intensity inside the filament can damage almost all the detectors and ignite various intricate higher order nonlinear optical effects. These extreme physical conditions and complicated phenomena make the sensing and controlling of filamentation challenging. This paper mainly focuses on recent research advances in sensing with femtosecond laser filamentation, including fundamental physics, sensing and manipulating methods, typical filament-based sensing techniques and application scenarios, opportunities, and challenges toward the filament-based remote sensing under different complicated conditions.

Practice guidelines for the supervising professional: intraoperative neurophysiological monitoring.

The American Society of Neurophysiological Monitoring (ASNM) was founded in 1989 as the American Society of Evoked Potential Monitoring. From the beginning, the Society has been made up of physicians, doctoral degree holders, Technologists, and all those interested in furthering the profession. The Society changed its name to the ASNM and held its first Annual Meeting in 1990. It remains the largest worldwide organization dedicated solely to the scientifically-based advancement of intraoperative neurophysiology. The primary goal of the ASNM is to assure the quality of patient care during procedures monitoring the nervous system. This goal is accomplished primarily through programs in education, advocacy of basic and clinical research, and publication of guidelines, among other endeavors. The ASNM is committed to the development of medically sound and clinically relevant guidelines for the performance of intraoperative neurophysiology. Guidelines are formulated based on exhaustive literature review, recruitment of expert opinion, and broad consensus among ASNM membership. Input is likewise sought from sister societies and related constituencies. Adherence to a literature-based, formalized process characterizes the construction of all ASNM guidelines. The guidelines covering the Professional Practice of intraoperative neurophysiological monitoring were initially published January 24th, 2013, and subsequently that document has undergone review and revision to accommodate broad inter- and intra-societal feedback. This current version of the ASNM Professional Practice Guideline was fully approved for publication according to ASNM bylaws on February 22nd, 2018, and thus overwrites and supersedes the initial guideline.

Effects of Dexmedetomidine on motor- and somatosensory-evoked potentials in patients with thoracic spinal cord tumor: a randomized controlled trial.

BACKGROUND: We hypothesized that the addition of dexmedetomidine in a clinically relevant dose to propofol-remifentanil anesthesia regimen does not exert an adverse effect on motor-evoked potentials (MEP) and somatosensory-evoked potentials (SSEP) in adult patients undergoing thoracic spinal cord tumor resection. METHODS: Seventy-one adult patients were randomized into three groups. Propofol group (n = 25): propofol-remifentanil regimenand the dosage was adjusted to maintain the bispectral index (BIS) between 40 and 50. DP adjusted group (n = 23): Dexmedetomidine (0.5 µg/kg loading dose infused over 10 min followed by a constant infusion of 0.5 µg/kg/h) was added to the propofol-remifentanil regimen and propofol was adjusted to maintain BIS between 40 and 50. DP unadjusted group (n = 23): Dexmedetomidine (administer as DP adjusted group) was added to the propofol-remifentanil regimen and propofol was not adjusted. All patients received MEP, SSEP and BIS monitoring. RESULTS: There were no significant changes in the amplitude and latency of MEP and SSEP among different groups (P > 0.05). The estimated propofol plasma concentration in DP adjusted group (2.7 ± 0.3 µg/ml) was significantly lower than in propofol group (3.1 ± 0.2 µg/ml) and DP unadjusted group (3.1 ± 0.2 µg/ml) (P = 0.000). BIS in DP unadjusted group (35 ± 5) was significantly lower than in propofol group (44 ± 3) (P = 0.000). CONCLUSIONS: The addition of dexmedetomidine to propofol-remifentanil regimen does not exert an adverse effect on MEP and SSEP monitoring in adult patients undergoing thoracic spinal cord tumor resection. TRIAL REGISTRATION: The study was registered with the Chinese Clinical Trial Registry on January 31st, 2014. The reference number was ChiCTR-TRC-14004229.

Simple method to enhance terahertz radiation from femtosecond laser filament array with a step phase plate.

In this work, we experimentally demonstrate a 200% enhancement of terahertz (THz) wave amplitude generated by femtosecond laser filamentation in air. The experimental setup simply uses a semicircular phase plate to generate two parallel filaments. Temporally overlapped THz pulses from two filaments coherently add up, giving rise to significant enhancement of the THz pulse amplitude. It has been foreseen that further enhancement would be achieved if the design of phase plates could be optimized to generate a filament array. This simple method makes full use of the laser energy and could potentially open a new approach to remotely enhance the THz emission in air.

Safety and efficacy of motor mapping utilizing short pulse train direct cortical stimulation.

BACKGROUND/AIMS: A major goal of intracranial surgery is to maximize resection while minimizing neurological morbidity, particularly motor dysfunction. Direct cortical stimulation (DCS) is a common intraoperative adjunct used to identify functional motor cortex. In this study, we report on the safety/efficacy of short pulse train DCS (direct cortical stimulation motor-evoked potential, dcMEP) for motor mapping and monitoring during intracranial surgery. METHODS: A retrospective analysis of 29 patients undergoing elective craniotomy for lesions near the motor cortex was performed. dcMEP mapping (40-120 V, 500-1,000 Hz, 5-9 pulses/s, 1- to 3-ms interstimulus interval, monopolar, 50-µs pulse width) was performed either alone (n = 29) or in addition to standard DCS (n = 6). Outcome measures were positive MEPs and the presence of seizures during stimulation. dcMEP-based continuous corticospinal tract (CST) monitoring was also performed. Changes in stimulation threshold and new postoperative neurological deficits were recorded. RESULTS: dcMEP mapping success was 96% and was not affected by preoperative motor status. Intraoperative seizure rates for dcMEP were 3% and were not related to preoperative seizure status. CST monitoring success rate was 96%, and changes in stimulation threshold were predictive of new permanent motor deficits. CONCLUSIONS: dcMEP is an effective method for mapping motor function and may prove useful for continuous CST monitoring.

Transcranial MEP threshold voltages and current densities simulated with finite element modelling.

OBJECTIVE: The aim of this study was to compare stimulation thresholds and current densities in the brain for transcranial motor evoked potentials (tcMEPs) from the hands and feet with linked quadripolar (LQP), M3-M4 and C1-C2 electrode montages. METHODS: Twenty-five patients underwent cerebral vascular surgery with tcMEP monitoring. tcMEP voltage thresholds were compared between LQP (C1, M3, C2, M4), C1-C2, and M3-M4 montages. In a finite element model (FEM), hand, arm, and leg regions of interest (ROIs) on the cortical motor homunculus were segmented. Current densities in these ROIs at tcMEP thresholds were compared across tcMEP electrode montages. RESULTS: LQP tcMEP thresholds were 61.5 volts for hands and 95.2 volts for feet. Thresholds were higher for M3-M4 (hands, 89.4 V; feet, 141.3 V) and C1-C2 (hands: 137.3 V; feet: 194.7 V). Total current at threshold voltage was greater for LQP (hands, 210.9 mA; feet, 311.3 mA) compared to M3-M4 (hands, 166.8 mA; feet, 256.6 mA), but similar to C1-C2 (hands, 246.7 mA; feet, 341.1 mA). In FEM simulations, current density and local current density topography in the hand ROI at threshold were very similar for LQP, M3-M4 and C1-C2. CONCLUSIONS: TcMEP voltage thresholds were least for LQP, and lesser for M3-M4 compared to C1-C2. In FEM simulations, resistance to current to hand ROI was ordered the same (LQP < M3-M4 < C1-C2). The local distribution of current density in motor cortex with tcMEP was mainly determined by cortical geometry. SIGNIFICANCE: Current densities and resistance to current simulated with FEM may explain threshold requirements for tcMEP electrode montages.

Anomalous transmission and Anderson localization for alternating propagated and evanescent waves at the deep-subwavelength scale.

Deep-subwavelength features have a minimal impact on wave transport in all dielectric systems; thus the homogenization approach was commonly adopted. Recently, the breakdown of effective medium theory (EMT) for the incident wave near the total reflection (TR) angle was demonstrated in a deep-subwavelength dielectric multilayer. Additionally, anomalous transmission was reported at angles exceeding the TR angle when introducing disorder and was attributed to Anderson localization. Here we firstly demonstrated that the alleged anomalous transmission also occurs in the disorder-free case, illustrating that attributing anomalous transmission to Anderson localization deserves a more in-depth study. To clarify the underlying physics of this asserted anomalous transmission, Anderson localization and broken EMT, the incident angle dependent reflectivity and modes for ordered and disordered deep-subwavelength multilayers were investigated systematically. Actually, the EMT is still convincing and the anomalous transmission is reasonable after a simple correction. However, the anomalous transmission is more accessible and the permittivity correction is more imperative in the disordered system due to the Anderson localization. These findings can be expanded to other wave systems such as acoustic waves and matter waves, providing insight into EMT and deepening our understanding of the intriguing transport phenomena in deep subwavelength systems.

Intraoperative neuromonitoring during surgery for lumbar stenosis.

The indications for neuromonitoring during lumbar stenosis surgery are defined by the risks associated with patient positioning, the approach, decompression of neural elements, deformity correction, and instrument implantation. The routine use of EMG and SEP alone during lumbar stenosis surgery is no longer supported by the literature. Lateral approach neuromonitoring with EMG only is also suspect. Lumbar stenosis patients often present with multiple co-morbidities which put them at risk during routine pre-surgical positioning. Frequently encountered morbid obesity and/or diabetes mellitus may play a role in monitorable and preventable brachial plexopathy after "superman" positioning or femoral neuropathy from groin pressure after prone positioning, for example. Deformity correction in lumbar stenosis surgery often demands advanced implementation of multiple neuromonitoring modalities: EMG, SEP, and MEP. Because the bulbocavernosus reflex detects the function of the conus medullaris and sacral somato afferent/efferent fibers of the cauda equina, it may also be recorded. The recommendation to record pedicle screw thresholds has become more nuanced as surgeon dependence on 3D imaging, navigation, and robotics has increased. Neuromonitoring in lumbar stenosis surgery has been subject mainly to uncontrolled case series; prospective cohort trials are also needed.