Tunable broadband terahertz polarizer using graphene-metal hybrid metasurface.

We demonstrate an electrically tunable polarizer for terahertz (THz) frequency electromagnetic waves formed from a hybrid graphene-metal metasurface. Broadband (>3 THz) polarization-dependent modulation of THz transmission is demonstrated as a function of the graphene conductivity for various wire grid geometries, each tuned by gating using an overlaid ion gel. We show a strong enhancement of modulation (up to ∼17 times) compared to graphene wire grids in the frequency range of 0.2-2.5 THz upon introduction of the metallic elements. Theoretical calculations, considering both plasmonic coupling and Drude absorption, are in good agreement with our experimental findings.

Increasing the sensitivity of terahertz split ring resonator metamaterials for dielectric sensing by localized substrate etching.

We demonstrate a significant enhancement in the sensitivity of split ring resonator terahertz metamaterial dielectric sensors by the introduction of etched trenches into their inductive-capacitive gap area, both through finite element simulations and in experiments performed using terahertz time-domain spectroscopy. The enhanced sensitivity is demonstrated by observation of an increased frequency shift in response to overlaid dielectric material of thicknesses up to 18 µm deposited on to the sensor surface. We show that sensitivity to the dielectric is enhanced by a factor of up to ∼2.7 times by the incorporation of locally etched trenches with a depth of ∼3.4 µm, for example, and discuss the effect of the etching on the electrical properties of the sensors. Our experimental findings are in good agreement with simulations of the sensors obtained using finite element methods.

Archie Brain: celebrating 30 years of development in laryngeal mask airways.

The practice of anaesthesia was revolutionised by the ideas of Archie Brain. The routine use of a facemask to manage the airway was not a hands-free technique, despite the development of various harnesses, and made adequate record-keeping difficult. The tracheal tube was associated with some morbidity, which some felt was unsuitable for day surgery. Brain developed an airway management device that was less stressful to the patient than tracheal intubation, and was, however, as safe as using a facemask and airway. Brain also hoped his device would function for cases where mask ventilation was particularly difficult and thus give anaesthetists a safer alternative to a complex intubation, especially in emergency scenarios.

Anesthetic Management for Cesarean Delivery in a Patient with a Gigantic Intracranial Tumor.

A 31-year-old G5P1 patient with unremarkable past medical history at 29 weeks of gestation was diagnosed with a gigantic left frontotemporal brain mass. Initial clinical management as an inpatient achieved an improvement in the symptoms. The patient and surgical team agreed to schedule a cesarean delivery at 32 weeks of gestation if no neurological deterioration was observed. Intraoperative course with general endotracheal anesthesia and bilateral transversus abdominis plane block was uneventful and promoted efficient postoperative pain control. Seven days after delivery, the patient underwent craniotomy for brain tumor resection. This report describes the anesthetic management of a patient with an intracranial tumor during pregnancy.

Optic nerve head microvasculature of the rabbit eye.

Vascular luminal castings of rabbit eyes were microdissected and studied with scanning electron microscopy to elucidate the three-dimensional angioarchitecture of the optic nerve head. Using sequential microdissection, an incomplete arterial circle was identified as terminal branches of two to three short posterior ciliary arteries around the optic nerve head. Several recurrent branches from the arterial circle form a pial arterial network. This pial system supplies the optic nerve head microvasculature and receives numerous venules from them. The only large vessel to enter the optic nerve is a central retinal artery that has few branches within the optic nerve and provides several branches at the surface of the optic disc. Moderately numerous vessels connect the retinal and ciliary vascular layers within the optic nerve head. Few arterioles to the optic nerve head arise from the choroid; however, there are a small number of capillary and numerous venous connections between them. These results indicate that the principal blood supply of the rabbit optic nerve head is derived from the short posterior ciliary arteries by the arterial circle. The retinal arteries contribute to the surface vasculature of the optic nerve head. The pial system also plays a significant role in both supply and drainage of the rabbit optic nerve head.

An endothelin-1 induced model of optic nerve ischemia in the rabbit.

PURPOSE: To evaluate blood flow reduction and topographic optic nerve changes after the local administration of endothelin-1 in vivo, delivered to the perineural region of the anterior optic nerve in the rabbit. METHODS: Endothelin-1 (five rabbits) in a dosage of 0.1 microgram/day or balanced salt solution (two rabbits) was delivered to the perineural region of the anterior optic nerve with osmotically driven minipumps. Optic nerve blood flow was determined by the colored microspheres technique after 14 days of local endothelin-1 or balanced salt solution administration to the microvasculature of the optic nerve. In addition, optic nerve blood flow was determined in two rabbits that had no minipump implants. The morphologic changes induced by reduction of blood flow were assessed in five additional rabbits implanted with osmotically driven minipumps containing endothelin-1 (0.1 microgram/day). These rabbits were observed for 8 weeks, and the morphologic optic nerve changes were monitored with a confocal scanning laser ophthalmoscope. RESULTS: Independent of intraocular pressure, endothelin-1 induced a decrease in blood flow of approximately 38% in the experimental eye, compared to the decrease induced by balanced salt solution or to the decrease in rabbits without minipumps (analysis of covariance, P = 0.0092). Multivariate statistical analysis showed a significant change in topometric parameters (cup area, cup depth, rim volume) obtained with a confocal scanning laser ophthalmoscope, indicating an increase in optic nerve cupping and a decrease of the perineural rim volume in the experimental eyes (P = 0.017). CONCLUSIONS: The current results suggest that morphologic optic nerve alterations can be induced experimentally in the rabbit model after ischemia produced by the local administration of endothelin-1 to the perineural region of the anterior optic nerve.

Sources of variability of topometric data with a scanning laser ophthalmoscope.

OBJECTIVE: To determine in a two-part study whether misalignment between the patient and the laser scanner is a major source of variability with the Heidelberg Retina Tomograph (Heidelberg [Germany] Engineering). METHODS: Three topographic images of the right optic nerve were acquired with the Heidelberg Retina Tomograph in eight patients with glaucoma. The correlations between average cup volume, variability of cup volume estimates, and direction of imaging were evaluated. Furthermore, the correlations between average rim volume, variability of rim volume estimates, and direction of imaging were evaluated. Next, the optic nerve cup volume and rim volume estimates of a rabbit's left eye were compared between three series of five topographic images acquired from three slightly different directions. RESULTS: Average cup volume, variability of cup volume estimates, and variability in the direction of imaging correlated significantly among the patients with glaucoma (multiple R2 = .95; P < .001). Average rim volume, variability of rim volume estimates, variability in the direction of imaging, and variability in the mean height of the contour line also correlated significantly (multiple R2 = .88; P = .03). In the rabbit eye, the cup volume and the rim volume differed significantly among the three image series (analysis of variance, P < .001 and P = .04, respectively). CONCLUSION: Misalignment between the patient and the laser scanner may account for significant variability with the Heidelberg Retina Tomograph.

Blood product use in cardiac revascularization: comparison of on- and off-pump techniques.

BACKGROUND: Cardiac revascularization on a beating heart avoids the side effects of cardiopulmonary bypass (eg, neurologic injury, hemodilution, and coagulopathy). We examined perioperative bleeding and use of blood products during coronary artery bypass grafting using either on-pump or off-pump techniques. METHOD: The charts of 126 patients who had coronary artery bypass grafting were reviewed. Data from 66 patients revascularized off pump and 60 patients with cardiopulmonary bypass (on pump) were analyzed using unpaired Student's t test. RESULTS: Average age was 62.5 years in either group. More patients received heparin preoperatively in the off-pump group that resulted in mild elevation of preoperative partial thromboplastin time and activated clotting time (40.4 +/- 2.9 seconds and 150.1 +/- 5.3 seconds, respectively). However, the off-pump group had less perioperative (intraoperative or postoperative) bleeding (2312 +/- 212 mL versus 3251 +/- 155 mL, p < 0.05) and required fewer blood products compared with the on-pump group. Hemoglobin and platelets decreased more in the conventional on-pump group. CONCLUSIONS: Avoiding cardiopulmonary bypass decreases perioperative bleeding and, consequently, reduces the use of blood products after coronary artery bypass grafting, which might result in fewer transfusion-related complications.

Ciliary vasoconstriction after topical adrenergic drugs.

Rabbits underwent the single-dose or long-term therapeutic administration of the adrenergic drugs phenylephrine hydrochloride, timolol maleate, and betaxolol hydrochloride. After a single dose, all three drugs caused substantial, localized constriction in the arterioles that supply the ciliary processes but did not affect the downstream bore of the same vessels. After seven weeks of a daily dose, tolerance reduced the response to betaxolol to insignificant levels and that to phenylephrine substantially, whereas timolol maleate continued to produce identical levels of vasoconstriction to those seen with single-dose administration. In addition to the consequent lowering of perfusion of the ciliary processes and presumptive impact on aqueous humor production, vasoconstriction also reinforces concerns about impaired vascular perfusion of eyes undergoing long-term ocular therapy.

Microvasculature of the human optic nerve.

PURPOSE: Methyl methacrylate vascular corrosion casting techniques were used to examine the normal anterior optic nerve microvasculature in 18 human eye bank eyes. METHODS: Selective cannulation of the central retinal artery, the short posterior ciliary arteries, or both, allowed the methyl methacrylate to be injected into the anterior optic nerve circulation. Preflushing with tissue plasminogen activator greatly enhanced the filling of the fine microvasculature by dissolving the intraluminal clots. RESULTS: The superficial nerve fiber layer of the optic nerve received its primary blood supply from the central retinal artery. In 11 of 13 eyes injected with methyl methacrylate through the short posterior ciliary arteries, there was a perineural, circular arterial anastomosis (circle of Zinn-Haller) at the scleral level. Branches from this circle penetrated the optic nerve to supply the prelaminar and laminar regions and the peripapillary choroid. In the two eyes without this arterial circle, direct branches from the short posterior ciliary arteries supplied the anterior optic nerve. The venous drainage of the anterior optic nerve was almost entirely through the central retinal vein and its tributaries. CONCLUSIONS: This study demonstrates that the main arterial vascular supply to the anterior optic nerve is from the short posterior ciliary arteries. The contribution of the peripapillary choroid to the anterior optic nerve is minimal in comparison to the direct contribution from the short posterior ciliary arteries.

Optic nerve vasomotor effects of topical beta-adrenergic antagonists in rabbits.

PURPOSE: To examine the anterior optic nerve vasomotor effects of nonselective and relatively beta-1-selective beta-adrenergic antagonists in rabbits, because different influences on optic nerve blood flow with these medications have been suggested. METHODS: After topical therapy for 30 days with either timolol maleate 0.5% (six rabbits), betaxolol hydrochloride 0.5% (six rabbits), or placebo (two rabbits), the microvasculature of the optic nerve was examined with an intraluminal microvascular corrosion casting technique. The investigators were masked to both the medication group and the treated eye. The constriction, in percent of the downstream vessel caliber, was measured at the vascular branching point of arterioles supplying the anterior optic nerve. An average constriction was calculated and compared between the medication groups and between the treated and the contralateral, untreated eyes. RESULTS: Constriction values from a total of 218 arterioles supplying the anterior optic nerve were obtained for the 14 rabbits. The means of the average constriction on the treated side were comparable between the groups treated with timolol maleate, betaxolol hydrochloride, and placebo (one-way analysis of variance, P = .64), as well as between the treated and untreated eyes (two-tailed t-test for paired variables, P = .68 for timolol maleate and P = .42 for betaxolol hydrochloride). The statistical power to find a difference of 5% or more average constriction was at least 90%. CONCLUSIONS: Both relatively selective and nonselective beta-adrenergic antagonists produce no observable optic nerve vasomotor effects in the rabbit eye.

The influence of ocular pulsatility on scanning laser Doppler flowmetry.

PURPOSE: To determine the effect of the cardiac cycle on scanning laser Doppler flowmeter measurements of retinal capillary blood flow in rhesus monkeys and humans. METHODS: Multiple scanning laser Doppler flowmetry images of rhesus monkey and human retinal capillary blood flow over a range of heart rates were obtained. Average flow values were determined for the 64 scan lines that compose the two-dimensional flow map. Cutaneous blood flow was measured simultaneously with a laser Doppler flowmeter. The temporal relationships between retinal capillary blood flow, peripheral arterial pulse, and cutaneous blood flow were determined. In addition, human retinal capillary blood flow in a 10 x 10-pixel area during different phases of the cardiac cycle was compared. RESULTS: Regular oscillations in human and rhesus monkey retinal capillary blood flow are evident as alternating bright and dark horizontal bands in scanning laser Doppler flowmetry images. These fluctuations are temporally correlated with cutaneous blood flow. Linear regression of actual vs predicted heart rate based on peaks in retinal capillary flow yielded r = 0.999 in a rhesus monkey and 0.938 in a human. Retinal capillary blood flow in a 10 x 10-pixel area fluctuated as much as 50% depending on the phase of the cardiac cycle. CONCLUSIONS: The alternating bright and dark banding pattern observed in scanning laser Doppler flowmetry scans of retinal capillary blood flow is related to the cardiac pulse. The errors introduced by pulse-related fluctuations in retinal capillary blood flow are significant and must be minimized or corrected for accurate and reproducible measurements of ocular hemodynamics.

Trabecular repopulation by anterior trabecular meshwork cells after laser trabeculoplasty.

To study further the transient increase in trabecular cell division within the first two days after laser trabeculoplasty in human corneoscleral explant organ cultures, we used a pulse-chase protocol in which immediately after laser treatment 3H-thymidine was added to the culture medium for 48 hours (the pulse period). Fresh medium without radiolabel was then added for variable times (the chase period) before termination of the experiment. Autoradiography was used to follow changes in the regional distribution of the cells that divided during the pulse period and had 3H-thymidine-labeled DNA. Laser-treated explants, evaluated after a pulse with no chase, showed a fourfold increase in cell division (P less than .001) over nontreated controls. Nearly 60% of this cell division was localized to the anterior, nonfiltering region of the trabecular meshwork where it inserts into the cornea beneath Schwalbe's line. Trabecular cell division in other regions of the meshwork was not increased over controls at this time. After seven or 14 days of chase without radiolabel, the regional distribution of radiolabeled cells changed in laser-treated explants but not in controls. By 14 days, only 26% of the labeled cells remained in this anterior insert region, while 60% were found in the region of the burn sites. Macroautoradiography of whole explants corroborated these observations. Our data support the hypothesis that laser trabeculoplasty causes early cell division by a population of cells in the anterior meshwork; these new cells then migrate and repopulate the burn sites over the next few weeks.

Interpleural analgesia for the treatment of severe cancer pain in terminally ill patients.

Interpleural analgesia was used to alleviate acute, severe exacerbations of chronic pain unrelieved by pharmacologic therapy in ten terminally ill cancer patients. Pain from metastatic disease to the neck, arms, chest, brachial plexus, thorax, or abdomen was effectively eliminated between 7 hr and 40 days in nine patients, who died with minimal or no pain. The technique was performed primarily using bupivacaine. No side effects were detected. Interpleural analgesia appears to be effective in rapidly controlling acute exacerbations of cancer pain in terminally ill patients. Moreover, it may also be a suitable therapy for moribund patients when used as a continuous-infusion technique.

Optic nerve vasomotor effects of topical apraclonidine hydrochloride.

AIMS: To examine, in vivo, the anterior optic nerve vasomotor effects of chronic apraclonidine hydrochloride in rabbits. METHODS: After local treatment in one randomly chosen eye with apraclonidine hydrochloride 0.5% over 21 days, the microvasculature of the optic nerve was examined in five rabbits using an intraluminal microvascular corrosion casting technique. The investigators were masked as to which eye was treated. The vasoconstriction near the branching point of arterioles supplying the optic nerve was calculated as a percentage of the downstream vessel calibre. An average constriction was calculated and compared between the treated and the contralateral, untreated, eyes by means of a two tailed t test for paired variables. Constriction values of a total of 72 arterioles supplying the optic nerve were obtained for the five rabbits. RESULTS: The average constriction in the treated and the control eyes was comparable (p = 0.96). CONCLUSION: Chronic administration of apraclonidine hydrochloride 0.5% produces no observable optic nerve vasomotor effects in the rabbit eye.

Experimental validation of predicted temperature rises in tissue-mimicking materials.

Increasingly, it is recognised that diagnostic ultrasound is capable of causing temperature rises sufficient to damage tissue. Predictions of this heating are often based on simplified models of both the in vivo conditions and the relevant physical effects. Few measurements have been made to verify these predictions, however, particularly for the tightly focused beams often employed in diagnosis. Furthermore, non-linear effects in both the acoustic field and the surrounding medium have largely been ignored in calculations. To provide an alternative way to estimate the heating, NPL has developed a measurement system to determine directly the temperature rise in tissue-mimicking materials. If necessary, the measurement results can be processed to model the effects of blood perfusion. The temperature is measured using thin-film thermocouples, which have essentially no interaction with the ultrasound or thermal fields. Measurements were made on transducers operating in the frequency range 2-10 MHz and with focal beam-widths from 1 to 3 mm. The measurements agree with theoretical predictions that use either the measured beam-profile or a simplified (Gaussian) model. The results are also compared with the standard soft-tissue models developed by the National Council on Radiation Protection and Measurements (NCRP) and jointly between the American Institute of Ultrasound in Medicine and the National Electrical Manufacturers Association (AIUM/NEMA). On average, the predictions of the NCRP formula are 15% higher than the measurements, confirming its validity as a worst-case model. The predictions based on the AIUM/NEMA formula, however, are typically 30% lower than the measured values.

Comparison of two theoretical models for predicting non-linear propagation in medical ultrasound fields.

Non-linear propagation models are required to predict the fields from medical ultrasonic equipment, particularly diagnostic devices and lithotripters. This need arises because of the requirement to know the safety and effectiveness of these instruments. Several theoretical models have been developed to take account of non-linear propagation as well as diffraction, focusing and attenuation, but little work has been done to validate them. This paper compares two theoretical models with each other and with measurements in the field of a 3.5 MHz focused transducer. One model uses an approximation based on modelling the beam profile with a Gaussian function, whereas the other utilises a full three-dimensional finite difference method, using a uniform transducer excitation function. Comparisons are made in the time and frequency domain at the focus for four different source levels and in general the results agree to within about 10%. However, an important conclusion is that the finite amplitude field of a real transducer may differ significantly from that of an ideal piston source, particularly for the weakly focused beams used in diagnostic ultrasound.

Finite amplitude distortion of the pulsed fields used in diagnostic ultrasound.

A simple theoretical model of the non-linear propagation of pulsed focused acoustic beams is described. It enables the distortion of the peak cycle of the pulse to be calculated from a few experimentally measured parameters. The model is discussed, and justified for application to the fields from medical ultrasonic diagnostic equipment. It is particularly relevant for specifying the degree of distortion present, as might be required by future written standards for diagnostic equipment performance. Preliminary experimental verification of the model is reported.

Prediction of in situ exposure to ultrasound: an improved method.

The acoustic output of medical ultrasonic equipment is usually measured in water but to determine the safety (or performance) of a machine it is necessary to know the in situ acoustic pressure levels inside a patient. At present, when estimates of these levels are made, a linear propagation model is used although in practice the propagation is nonlinear. This paper shows that such a model can lead to large errors (80% or more in pressure) and in particular that in situ predictions of the peak-negative acoustic pressure are too low. To describe the field in water and in tissue it is necessary to take account of diffraction, attenuation, and dispersion as well as nonlinear propagation. It is difficult to use acoustic output levels in water to predict in situ values because the amplitude in water approaches a limit, an effect known as saturation. Nevertheless, a method of making such predictions is presented in this paper and is validated by comparison with experiment. The method is relatively time-consuming to implement and has not yet been applied to pulsed fields, so there is a need for more complete and simpler methods.

Perfusion corrections for ultrasonic heating in nonperfused media.

The linear bio-heat transfer equation proposed by Pennes is widely used to predict temperature rise in perfused media. In this article, a mathematical relationship between the predicted temperature increase under perfused and nonperfused conditions is derived based on an analysis of the Pennes equation. The perfused temperature at time t and position r, Tperf(r,t) can be calculated from the unperfused temperature history Tunperf(r,t) and the time constant for perfusion, tau: [formula: see text] When the full nonperfused temperature history is not available, a simpler approximate method of estimating the perfused temperature is also suggested, requiring only a knowledge of the time constant for perfusion. Results are given showing the effects of perfusion after different exposure times for a range of beamwidths and perfusion time constants.