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Results for “Thermal Diffusion”

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At least 451 records · Page 25Linked to original sources

PubMed1983

Time limits of reversible cortical ischemia.

The duration and degree of reversible cortical ischemia was evaluated by monitoring the direct cortical response (DCR) and cortical blood flow as determined by a thermal diffusion flow probe (CBFp) in a small segment of cortex. Global ischemia was induced in 90 lightly anesthetized cats during recording. The thresholds for attenuation and abolition of the DCR were 23.7 +/- 6.9 (SD) and 8.7 +/- 3.4 ml/100 g/minute, respectively. With the abrupt drop of CBFp to 5 ml/100 g/minute or below, the DCR was obliterated in 2 to 6 minutes. Recovery of the DCR was likely to occur if it had been absent for only 10 minutes, but after 40 minutes without a DCR no animal recovered DCR, even though the animals were monitored for 6 hours. Correlation of CBFp with time of ischemia was done by analyzing the CBFp curve minute by minute and computing the mean CBFp during the ischemic insult. This was then compared to recovery of DCR. By plotting time (T) in minutes, CBFp in ml/100 g/minute, and recovery of DCR, T was identified for levels of CBFp when DCR would be unlikely to recover. The time limits of cortical ischemia are described for blood flows from 0 to 20 ml/100 g/minute. The physiological recovery of the cerebral cortex from ischemia is a function of the depth and duration of the ischemic episode.

Animals↗

PubMed2002

Measurement of the thermal coefficients of rewritable phase-change optical recording media.

We describe a method to estimate the heat capacity of the substrate, the dielectric layer, and the phase-change layer of phase-change optical recording media as well as the thermal conductivity of the phase-change layer in its crystalline state. Measurements were carried out on spinning disks with the beam of light focused and locked onto the groove track. The method relies on the identification of the solid-to-liquid phase transition that occurs in the phase-change layer and takes advantage of the dependence of thermal diffusion on track velocity and irradiation time.

Journal Article↗

PubMed2002

Estimation of thermal coefficients of magneto-optical media.

Previously we described a method for estimating the thermal conductivity of magneto-optic recording media. The method relies on identifying the laser power that brings the maximum temperature of the TbFeCo layer to as high as the Curie temperature. We extensively use a similar method to estimate the heat capacity of a dielectric layer, a TbFeCo layer, and an aluminum alloy layer of magneto-optic recording media. Measurements are conducted on static disks with a beam of light focused on a TbFeCo layer. The method has the advantage of thermal diffusion depending on a multilayer structure and irradiation time.

Journal Article↗

PubMed2002

Photothermal deflection studies of GaAs epitaxial layers.

Photothermal beam deflection studies were carried out with GaAs epitaxial double layers grown on semi-insulating GaAs substrates. The impurity densities in thin epitaxial layers were found to influence the effective thermal diffusivity of the entire structure.

Journal Article↗

PubMed2003

Optimum pulse duration and radiant exposure for vascular laser therapy of dark port-wine skin: a theoretical study.

Laser therapy for cutaneous hypervascular malformations such as port-wine stain birthmarks is currently not feasible for dark-skinned individuals. We study the effects of pulse duration, radiant exposure, and cryogen spray cooling (CSC) on the thermal response of skin, using a Monte Carlo based optical-thermal model. Thermal injury to the epidermis decreases with increasing pulse duration during irradiation at a constant radiant exposure; however, maintaining vascular injury requires that the radiant exposure also increase. At short pulse durations, only a minimal increase in radiant exposure is necessary for a therapeutic effect to be achieved because thermal diffusion from the vessels is minimal. However, at longer pulse durations the radiant exposure must be greatly increased. There exists an optimum pulse duration at which minimal damage to the epidermis and significant injury within the targeted vasculature occur. For example, the model predicts optimum pulse durations of approximately 1.5, 6, and 20 ms for vessel diameters of 40, 80, and 120 microm, respectively. Optimization of laser pulse duration and radiant exposure in combination with CSC may offer a means to treat cutaneous lesions in dark-skinned individuals.

Blood Vessels↗

PubMed2004

Amorphization induced by subnanosecond laser pulses in phase-change optical recording media.

We have investigated the dynamics of amorphization induced in phase-change optical recording media by focused laser pulses of subnanosecond duration. We initiated localized amorphism by using a focused laser beam to melt the phase-change material and completed the change by rapid cooling by means of thermal diffusion. These studies were conducted by use of real-time reflectivity measurements with a pump-and-probe technique in which both pump and probe pulses had a duration of approximately 510 ps. Our transient-reflectivity measurements indicate that the process that leads to amorphism has three distinct stages, namely, rapid melting, solidification, and slow relaxation.

Journal Article↗

PubMed2003

Spectral depth profiling of arbitrary surfaces by thermal emission decay-Fourier transform infrared spectroscopy.

We report a new spectroscopic technique that combines step-scanning Fourier transform infrared spectroscopy with opto-thermal transient emission radiometry (OTTER) in order to provide near-surface depth-resolved spectra in the range 700-1800 cm(-1). It works nondestructively, without contact, with samples of arbitrary shape and size, without requiring prior preparation. The depth of surface probed depends on the thermal diffusivity of the sample; for organic materials it is approximately 10 microm. With homogeneous samples, absolute absorption coefficients can be measured. With two-layered samples, the technique proved able to distinguish between the spectral properties of the top layer and the substrate and to estimate the thickness of the top layer. We present a theoretical analysis with the main design features of the instrumentation and software, together with studies of homogeneous and layered samples, to validate the methods and illustrate the potential of the technique for practical applications.

Adhesives↗

PubMed2005

Blind source separation of photoacoustic depth profiles into independent components.

Step-scan photoacoustic spectroscopy is a powerful tool to nondestructively retrieve depth related information from a sample. Through digital signal processing a series of spectra with effectively different modulation frequencies, probing different thermal diffusion lengths within a sample, can be collected simultaneously. For layered samples spectra of the constituent layers can then be obtained by calculating spectra at specific phase angles from the in-phase and quadrature data through phase projection. However, without prior knowledge of the spectra of the constituent layers, this approach can be difficult. In this report we present an alternate possibility for evaluating step scan photoacoustic data, namely independent component analysis (ICA), which allows for ''blind separation'' of the mixed photoacoustic spectra without prior knowledge of the constituent spectra. Phase projection and ICA are applied to photoacoustic data acquired from a multilayer sample in an attempt to isolate the spectra of the constituent layers. The results for the two methods are comparable, with ICA offering the advantage that no prior information about the pure spectra of the sample layers is needed.

Acoustics↗

PubMed2003

Estimation of temperature transients for biomass pretreatment in tubular batch reactors and impact on xylan hydrolysis kinetics.

A combined heat transfer/kinetic model was developed to quantify temperature variations in small tubular batch reactors and estimate the effect of deviations from isothermal operation on the kinetics of biomass pretreatment. Assuming that heat transfer was dominated by conduction in the radial direction, a classic parabolic time-dependent partial differential equation was applied to describe the temperature in the system and dedimensionalized to provide a single solution for application to all situations. A dimensionless expression for the reaction kinetics for xylan hydrolysis was then developed, and a single parameter expressed as the dimensionless ratio of the first-order rate constant times the tube radius squared divided by the thermal diffusivity was found to control the reaction rate. Three different characterizations of the deviation between the concentration profile predicted for isothermal xylan hydrolysis and that based on the transient temperature were directly related to this dimensionless rate constant parameter for both catalyzed and uncatalyzed hydrolysis kinetics. These results were then used to project the relationship between deviations in yield from isothermal results and the tube radius and reaction time.

Biomass↗

PubMed2005

Effect of bacterial meningitis complicating severe head trauma upon brain microdialysis and cerebral perfusion.

INTRODUCTION: This article describes changes in regional cerebral perfusion and brain metabolism in a case of bacterial meningitis complicating severe traumatic brain injury. CASE REPORT: As part of clinical monitoring of patients with severe head injury, cerebral microdialysis was performed and extracellular concentration of glucose, lactate, glutamate, and pyruvate was determined. A thermal diffusion probe was used for bedside monitoring of cerebral blood flow. Acinetobacter meningitis complicated the clinical course on the seventh post-admission day and dramatically altered the neurochemistry. Microdialysate analysis showed glucose under the detection limit, lactate at moderately high levels, and a marked increase in glutamate and pyruvate levels. A reduction of cerebral perfusion was detected in the early phase of meningitis, probably secondary to vascular complications related to the inflammatory process. DISCUSSION: This case describes an emerging area of study and practice in patients with brain injury. It demonstrates how cerebral perfusion monitoring and study of brain metabolism can provide an early detection of secondary events that complicate severe head injury and can contribute to a better understanding of the complex pathogenetic mechanisms responsible for neuronal damage.

Acinetobacter Infections↗

PubMed1975

[Free-flow electrophoresis. I. Theoretical and experimental investigations of the influence of mechanical and electrokinetic variables on the efficiency of the method].

In the present paper, various types of band-broadening effects in free-flow electrophoresis were investigated. They resulted from the velocity profiles of the liquid curtain and electroosmosis, temperature gradient, thermal diffusion and sample inlet geometry. An analytical free-flow electrophoresis apparatus permitted easy observation of these parameters. The experiments showed that the influence of the temperature gradient was negligible, whereas the effect of the velocity profiles on band broadening was higher than theoretically expected. In preparative work this is of utmost importance, since overlap of bands is not desirable. In this case, the zeta potential of the walls can be adjusted to that of the material to be separated, resulting in a considerable reduction of band broadening. Various approaches are indicated. Further attention was given to the influence of the relaxation effect on the separation. Conditions are shown where particles are separated either according to their surface charge density or to their size. The practical relevance of the results is discussed.

Electrophoresis↗

PubMed1989

Regional cortical blood flow during extra-intracranial bypass surgery in young patients with moyamoya disease.

Regional cortical blood flow (rCBF) was measured with a thermal diffusion flow probe in 14 cortical regions of 12 young patients with moyamoya disease before and after completion of superficial temporal-middle cerebral artery anastomosis. The prebypass rCBF value was low in most patients, especially in the frontal regions. On temporary occlusion of the cortical artery during the surgical procedure, no significant drop in rCBF occurred. rCBF increased immediately after anastomosis in all but one region and continued to increase for 5-10 minutes in four of the eight regions measured. In the other four regions, rCBF declined gradually after the initial increase, and the final increase was slight. The average increase in rCBF was significant at 1-2 minutes and at 5-10 minutes after anastomosis. However, the latter increase did not bring the flow into the normal range. It may be that the initial increase in postbypass rCBF is determined solely by the pressure gradient between the donor and recipient arteries, and that subsequent rCBF is controlled by other factors within the brain. In one patient who underwent double anastomoses to the frontal and temporal lobes, neither anastomosis increased rCBF in the non-corresponding lobe to recipient artery. This suggests that there is no direct connection between supra- and infra-Sylvian arteries and supports the concept of nonuniform epicerebral microcirculation in moyamoya disease.

Adolescent↗

PubMed1993

Efficacy of superficial temporal to middle cerebral artery anastomosis against hemodynamic stroke.

The effect of superficial temporal artery-middle cerebral artery anastomosis on regional cerebral blood flow (rCBF) was measured on 31 patients with ischemic cerebrovascular diseases. rCBF was measured on the precentral and superior temporal gyri during surgery using the thermal diffusion technique. The mean rCBF was significantly lower than controls before anastomosis, but increased significantly after. Severer clinical symptoms were associated with lower rCBF. The rCBF increased significantly in patients with transient ischemic attack, reversible ischemic neurological deficit, and minor and major completed stroke. All 31 patients demonstrated dysautoregulation, both before and after anastomosis. However, the rCBF had increased above the ischemic threshold during hypotension, achieving a reserve capacity to prevent hemodynamic stroke.

Adult↗

PubMed1979

The influence of pneumatization of mastoid bone on caloric nystagmus response. A clinical study and a mathematical model.

Pursuing the problem whether and why the pneumatization of mastoid bone has any influence on caloric nystagmus, we examined 15 large or extensively (L.P.) and 15 poorly (P.P.) pneumatized subjects. Following water irrigation (44 degrees and 30 degrees C, 30 sec) we checked the parameters maximum SPV, max. frequency, latency, duration and time of max. response by ENG recordings. Both groups differed significantly for all parameters except duration (p less than 0.001). On comparing these findings with an earlier proposed model (Bock & Bromm, 1977) we could show a good correspondence for the P.P. group by doubling the parameter 'thermal diffusivity of mastoid bone'. An even better correspondence could be found by diminishing the parameter 'coefficient of temperature changes by perfusion'. The theoretical and practical implications and conclusions of our findings are discussed.

Caloric Tests↗

PubMed1989

Nonhyperemic blood flow restoration and brain edema in experimental focal cerebral ischemia.

The effect of suppression of postischemic reactive hyperemia on the blood-brain barrier (BBB) and ischemic brain edema after temporary focal cerebral ischemia was studied in cats under ketamine and alpha-chloralose anesthesia. Regional cerebral blood flow (rCBF) was measured by a thermal diffusion method and a hydrogen clearance method. The animals were separated into three groups. In Group A, the left middle cerebral artery (MCA) was occluded for 6 hours. In Group B, the MCA was occluded for 3 hours and then reperfused for 3 hours; postischemic hyperemia was suppressed to the preischemic level by regulating the degree of MCA constriction. In Group C, the MCA was occluded for 3 hours and reperfused for 3 hours without suppressing the postischemic reactive hyperemia. The brain was removed and cut coronally at the site of rCBF measurement. The degree of ischemic edema was assessed by gravimetry in samples taken from the coronal section and correlated with the degree of BBB disruption at the corresponding sites, evaluated by densitometric determination of Evans blue discoloration. The findings showed that 1) ischemic edema was significantly exacerbated by postischemic hyperemia during reperfusion in parallel with the degree of BBB opening to serum proteins, and 2) suppression of postischemic hyperemia significantly reduced the exacerbation of ischemic edema and BBB opening. These findings indicate that blood flow may be restored without significant exacerbation of postischemic edema by the suppression of postischemic hyperemia in focal cerebral ischemia.

Animals↗

PubMed2001

Continuous monitoring of regional cerebral blood flow during temporary arterial occlusion in aneurysm surgery.

OBJECT: Temporary arterial occlusion (TAO) during aneurysm surgery carries the risk of ischemic sequelae. Because monitoring of regional cerebral blood flow (rCBF) may limit neurological damage, the authors evaluated a novel thermal diffusion (TD) microprobe for use in the continuous and quantitative assessment of rCBF during TAO. METHODS: Following subcortical implantation of the device at a depth of 20 mm in the middle cerebral artery or anterior cerebral artery territory, rCBF was continuously monitored by TD microprobe (TD-rCBF) throughout surgery in 20 patients harboring anterior circulation aneurysms; 46 occlusive episodes were recorded. Postoperative radiographic evidence of new infarction was used as the threshold for failure of occlusion tolerance. The mean subcortical TD-rCBF decreased from 27.8+/-8.4 ml/100 g/min at baseline to 13.7+/-11.1 ml/100 g/min (p < 0.0001) during TAO. The TD microprobe showed an immediate exponential decline of TD-rCBF on clip placement. On average, 50% of the total decrease was reached after 12 seconds, thus rapidly indicating the severity of hypoperfusion. Following clip removal, TD-rCBF returned to baseline levels after an average interval of 32 seconds, and subsequently demonstrated a transient hyperperfusion to 41.4+/-18.3 ml/l 00 g/min (p < 0.001). The occurrence of postoperative infarction (15%) and the extent of postischemic hyperperfusion correlated with the depth of occlusion-induced ischemia. CONCLUSIONS: The new TD microprobe provides a sensitive, continuous, and real-time assessment of intraoperative rCBF during TAO. Occlusion-induced ischemia is reliably detected within the 1st minute after clip application. In the future, this may enable the surgeon to alter the surgical strategy early after TAO to prevent ischemic brain injury.

Adult↗

PubMed2003

Nicardipine augments local myocardial perfusion after coronary artery reperfusion in dogs.

Nicardipine is a potent coronary and systemic vasodilator without depression of ventricular function. We investigated the changes in local myocardial perfusion (LMP) according to the nicardipine administration after coronary reperfusion in a beating canine model. A Doppler probe was placed around the left anterior descending coronary artery (LAD) and thermal diffusion microprobe was implanted in the myocardium perfused by the exposed LAD. To define the nicardipine effects, we compared the two groups (control group, n=7 vs nicardipine group, n=7). In nicardipine group, 5 microgram/kg/min nicardipine was infused continuously. After the release of the LAD occlusion, LAD blood flow were increased compared to the baseline of both groups. However, there was no difference between groups in the LAD blood flow. The LMP after LAD reperfusion did not recover to the baseline level until 30 min after LAD reperfusion in control group (74%, 52% and 70% at 10, 20 and 30 min after LAD reperfusion, respectively). In nicardipine group, however, the LMP recovered to the baseline level at 20 min (99%), and increased more than the baseline level at 30 min (141%) after LAD reperfusion. Our findings suggest that the nicardipine augments the LMP following the release of a coronary occlusion.

Animals↗

PubMedDate not supplied

Laser thermal keratoplasty using a continuous wave diode laser.

BACKGROUND: Laser thermal keratoplasty is currently performed with a pulsed Ho:YAG laser at 2.07 microns wavelength. Long-term stability depends critically on the coagulation depth of each cone and thus on emission wavelength (absorption in corneal tissue) and focusing, all contributing to controlled stable collagen shrinkage. To achieve this, a temperature range of 65 degrees to 90 degrees C is needed. A continuous wave laser source meets the coagulation requirements more effectively by avoiding tissue cooling by thermal diffusion as well as the peak temperatures of pulsed lasers, which counteracts the intended central corneal steepening. METHODS: A continuous wave diode laser was developed, emitting at 1.885 microns with a maximal energy output of 450 mW. In a contact focusing application, the absorption depth in water as a function of wavelength was measured. Using laser parameters, comparable to those used for a pulsed Ho:YAG laser in contact mode, coagulation spots in human cornea were applied for the continuous wave diode laser. RESULTS: The macroscopic and microscopic effects of the diode laser coagulation on corneas in vitro and in situ were comparable to those of the Ho:YAG laser, if a comparable amount of total energy per spot was applied. CONCLUSION: Due to better optimized laser-collagen interaction, higher corrections and more stable clinical refractive effects appear achievable using the continuous wave diode laser.

Cadaver↗