Stress Relaxation of Porcine Septal Cartilage During Nd:YAG (λ=1.32 μm) Laser Irradiation: Mechanical, Optical, and Thermal Responses

Laser-assisted cartilage reshaping is mediated by thermally induced stress relaxation, and may be used to alter cartilage morphology for reconstructive surgical procedures in the upper airway and face without carving, morselizing, or suturing. Internal stress σ(t), integrated backscattered light intensity I(t) from a He-Ne probe laser (λ=632.8 nm), and radiometric surface temperature Sc(t) were measured during the reshaping of porcine nasal septal cartilage using a pulsed Nd:YAG laser (λ=1.32 μm). Internal stress and integrated backscattered light intensity were observed to increase, plateau, and then decrease in similar ways during laser irradiation. The plateau region occurred when the cartilage front surface temperature approached 65 °C. I(t) was utilized in a feedback control procedure to reshape cartilage specimens from a flat to a curved geometry. Immediately following laser irradiation, the tissues were rehydrated in normal saline for 15 min while wrapped around a small dowel. A stable shape change was retained for 21 days while the specimens were stored in normal saline at 5 °C. The backscattered light intensity signal mirrors underlying changes in internal stress, and further rate of change or slope of I(t) is nearly zero when the surface temperature reaches about 65 °C. Measurements of I(t) (or, equivalently, the fractional change in integrated backscattered light intensity ΔI(t)/I0) may be used to control the process of laser-assisted cartilage reshaping and minimize nonspecific thermal injury due to uncontrolled heating. © 1998 Society of Photo-Optical Instrumentation Engineers.

Damping of the wrist joint during voluntary movement

Damping characteristics of the musculoskeletal system were investigated during rapid voluntary wrist flexion movements. Oscillations about the final position were induced by introducing a load with the characteristics of negative damping, which artificially reduced the damping of the wrist. Subjects responded to increases in the negatively damped load by stronger cocontraction of wrist flexor and extensor muscles during the stabilization phase of the movement. However, their ability to counteract the effects of the negatively damped load diminished as the negative damping increased. Consequently, the number and frequency of oscillations increased. The oscillations were accompanied by phase-locked muscle activity superimposed on underlying tonic muscle activation. The wrist stiffness and damping coefficient increased with the increased cocontraction that accompanied more negatively damped loads, although changes in the damping coefficient were less systematic than the stiffness. Analysis of successive half-cycles of the oscillation revealed that the wrist stiffness and damping coefficient increased, despite decreasing muscle activation, as oscillation amplitude and velocity declined. This indicates that the inverse dependence of the damping coefficient on oscillation velocity contributes significantly to damping of joint motion. It is suggested that this property helps to offset a negative contribution to damping from the stretch reflex.

Signal attenuation and localization in optical coherence tomography studied by Monte Carlo simulation

A Monte Carlo model has been developed for optical coherence tomography (OCT). A geometrical optics implementation of the OCT probe with low-coherence interferometric detection was combined with three-dimensional stochastic Monte Carlo modelling of photon propagation in the homogeneous sample medium. Optical properties of the sample were selected to simulate intralipid and blood, representing moderately (g = 0.7) and highly (g = 0.99) anisotropic scattering respectively. For shallow optical depths in simulated intralipid (<3 scattering mean free path (mfp) units), the number of detected backscattered photons followed the extinction-single-backscatter model, and OCT was found to detect only minimally scattered photons. Within this depth range the backscatter positions of detected photons corresponded well with the nominal focus position of the probe. For propagation to deeper positions in intralipid, localization of backscattering was quickly lost due to detection of stray photons, and the number of detected photons remained constant with increasing depth in the non-absorbing medium. For strongly forward-directed scattering in simulated blood, the number of detected photons approached the extinction-single-backscatter model only for very shallow depths (<2 mfp units). However, backscattering positions for detected photons correlated well with the nominal focus position of the probe even for optical depths greater than 40 mfp units.

Infrared imaging of in vivo microvasculature following pulsed laser irradiation

Infrared emission images of the chick chorioallantoic membrane (CAM) microvasculature following pulsed laser irradiation were recorded using a high speed infrared focal plane array camera. A three-dimensional tomographic reconstruction algorithm was applied to compute the initial space-dependent temperature increase in discrete CAM blood vessels caused by light absorption. The proposed method may provide consistent estimates of the physical dimensions of subsurface blood vessels and may be useful in understanding a variety of biomedical engineering problems involving laser-tissue interaction. © 1998 Society of Photo-Optical Instrumentation Engineers.

Accuracy and noise in optical Doppler tomography studied by Monte Carlo simulation

A Monte Carlo model has been developed for optical Doppler tomography (ODT) within the framework of a model for optical coherence tomography (OCT). A phantom situation represented by blood flowing in a horizontal 100 microm diameter vessel placed at 250 microm axial depth in 2% intralipid solution was implemented for the Monte Carlo simulation, and a similar configuration used for experimental ODT measurements in the laboratory. Simulated depth profiles through the centre of the vessel of average Doppler frequency demonstrated an accuracy of 3-4% deviation in frequency values and position localization of flow borders, compared with true values. Stochastic Doppler frequency noise was experimentally observed as a shadowing in regions underneath the vessel and also seen in simulated Doppler frequency depth profiles. By Monte Carlo simulation, this Doppler noise was shown to represent a nearly constant level over an investigated 100 microm interval of depth underneath the vessel. The noise level was essentially independent of the numerical aperture of the detector and angle between the flow velocity and the direction of observation, as long as this angle was larger than 60 degrees. Since this angle determines the magnitude of the Doppler frequency for backscattering from the flow region, this means that the signal-to-noise ratio between Doppler signal from the flow region to Doppler noise from regions underneath the flow is improved by decreasing the angle between the flow direction and direction of observation. Doppler noise values from Monte Carlo simulations were compared with values from statistical analysis.

Accuracy of subsurface temperature distributions computed from pulsed photothermal radiometry

Pulsed photothermal radiometry (PPTR) is a non-contact method for determining the temperature increase in subsurface chromophore layers immediately following pulsed laser irradiation. In this paper the inherent limitations of PPTR are identified. A time record of infrared emission from a test material due to laser heating of a subsurface chromophore layer is calculated and used as input data for a non-negatively constrained conjugate gradient algorithm. Position and magnitude of temperature increase in a model chromophore layer immediately following pulsed laser irradiation are computed. Differences between simulated and computed temperature increase are reported as a function of thickness, depth and signal-to-noise ratio (SNR). The average depth of the chromophore layer and integral of temperature increase in the test material are accurately predicted by the algorithm. When the thickness/depth ratio is less than 25%, the computed peak temperature increase is always significantly less than the true value. Moreover, the computed thickness of the chromophore layer is much larger than the true value. The accuracy of the computed subsurface temperature distribution is investigated with the singular value decomposition of the kernel matrix. The relatively small number of right singular vectors that may be used (8% of the rank of the kernel matrix) to represent the simulated temperature increase in the test material limits the accuracy of PPTR. We show that relative error between simulated and computed temperature increase is essentially constant for a particular thickness/depth ratio.

A comparative study of human skin thermal response to sapphire contact and cryogen spray cooling

Surface cooling, in conjunction with various thermally mediated therapeutic procedures, can provide a means to protect superficial tissues from injury while achieving destruction of deeper targeted structures. We have investigated the thermal response of in-vivo human skin to: 1) contact cooling with a sapphire window (6-12 degrees C); and 2) spray cooling with a freon substitute cryogen [tetrafluoroethane; boiling point approximately -26 degrees C at 1 atmospheric pressure (atm)]. Measurements utilizing infrared radiometry show surface temperature reductions from 30 degrees C to 14-19 degrees C are obtained within approximately 1 s in response to sapphire contact cooling. Surface temperature reductions to values between 5 degrees C and -9 degrees C are obtained in response to 20-100-ms cryogen spurts. Computational results, based on fitting the measured radiometric surface temperature to estimate heat transfer parameters, show: 1) temperature reductions remain localized to approximately 200 microns of superficial tissue; and 2) values of heat flux and total energy removed per unit skin surface area at least doubled when using cryogen spray cooling.

Quantization of human motions and learning of accurate movements

This paper presents a mathematical model for the learning of accurate human arm movements. Its main features are that the movement is the superposition of smooth submovements, the intrinsic deviation of arm movements is considered, visual and kinesthetic feed-back are integrated in the motion control, and the movement duration and accuracy are optimized with practice. This model is consistent with the jerky arm movements of infants, and may explain how the adult motion behavior emerges from the infant behavior. Comparison with measurements of adult movements shows that the kinematics of accurate movements are well predicted by the model.

Optical absorption of blood depends on temperature during a 0.5 ms laser pulse at 586 nm

Optical properties are important parameters in port wine stain laser treatment models. In this study we investigated whether changes in blood optical properties occur during a 0.5 ms laser pulse. Blood from three volunteers was irradiated in vitro with laser pulses (radiant exposure 2-12 J cm-2, wavelength 586 nm, pulse length 0.5 ms). Reflection and transmission coefficients, measured using double integrating spheres, decreased slightly during the first part of the pulse. At 2.9 J cm-2 radiant exposure, the reflectance increased, independent of total radiant exposure of the pulse. This was caused by blood coagulation. A second sudden increase in reflection and a significant increase in transmission occurred near 6.3 J cm-2 and was accompanied by a "popping" sound, indicating rapid expansion of bubbles due to blood vaporization. A multilayered model of blood was used to fit calculated transmission coefficient curves to the measurements and determine temperature-dependent optical blood absorption. Heat diffusion was shown to be of minor importance. A 2.5-fold increase in absorption for temperatures increasing from 20 to 100 degrees C, accurately describes transmission coefficients measured up to 2.9 J cm-2.

Optical Doppler tomography: imaging in vivo blood flow dynamics following pharmacological intervention and photodynamic therapy

A noninvasive optical technique has been developed for imaging in vivo blood flow dynamics and vessel structure with high spatial resolution. The technique is based on optical Doppler tomography, which combines Doppler velocimetry with optical coherence tomography to measure blood flow velocity at discrete spatial locations in turbid biological tissue. Applications of this technique for monitoring changes in blood flow dynamics and vessel structure following pharmacological intervention and photodynamic therapy are demonstrated.

Three-dimensional reconstruction of port wine stain vascular anatomy from serial histological sections

Port wine stains (PWSs) treated with a flashlamp-pumped pulsed dye laser show a variability in clinical response that is incompletely understood. To identify any vascular structure that might adversely affect treatment response, we obtained a three-dimensional reconstruction of the vascular anatomy of a non-responsive, light-purple superficial PWS on the forearm. The reconstructed PWS consisted of multiple clusters of small diameter (10-50 microns) blood vessels. We propose that this and similar structures, which have not been identified in the literature, have limited the efficacy of laser therapy. Further study is required to clarify the role of vessel clusters for laser treatment of PWSs, and the corresponding dosimetry necessary to clear non-responsive lesions. We expect that three-dimensional reconstruction of PWS vascular anatomy will provide the basis for (i) accurate PWS classification, (ii) guidance for selection of more effective laser dosimetry, and (iii) a standard against which to assess non-invasive diagnostic imaging techniques.

Noninvasive imaging of in vivo blood flow velocity using optical Doppler tomography

We report the development of an optical technique for noninvasive imaging of in vivo blood flow dynamics and tissue structures with high spatial resolution (2-10 microm) in biological systems. The technique is based on optical Doppler tomography (ODT), which combines Doppler velocimetry with optical coherence tomography to measure blood flow velocity at discrete spatial locations. The exceptionally high resolution of ODT permits noninvasive in vivo imaging of both blood microcirculation and tissue structures surrounding the vessel, which has significance for biomedical research and clinical applications. Tomographic imaging of in vivo blood flow velocity in the chick chorioallantoic membrane and in rodent skin is demonstrated.

Two-dimensional birefringence imaging in biological tissue by polarization-sensitive optical coherence tomography

Using a low-coherence Michelson interferometer, we measure two-dimensional images of optical birefringence in bovine tendon as a function of depth. Polarization-sensitive detection of the signal formed by interference of backscattered light from the sample and a mirror in the reference arm give the optical phase delay between light that is propagating along the fast and slow axes of the birefringent tendon. Images showing the change in birefringence in response to laser irradiation are presented. The technique permits rapid noncontact investigation of tissue structural properties through two-dimensional imaging of birefringence.

Non-invasive determination of port wine stain anatomy and physiology for optimal laser treatment strategies

The treatment of port wine stains (PWSs) using a flashlamp-pumped pulsed dye laser is often performed using virtually identical irradiation parameters. Although encouraging clinical results have been reported, we propose that lasers will only reach their full potential provided treatment parameters match individual PWS anatomy and physiology. The purpose of this paper is to review the progress made on the technical development and clinical implementation of (i) infrared tomography (IRT), optical reflectance spectroscopy (ORS) and optical low-coherence reflectometry (OLCR) to obtain in vivo diagnostic data on individual PWS anatomy and physiology and (ii) models of light and heat propagation, predicting irreversible vascular injury in human skin, to select optimal laser wavelength, pulse duration, spot size and radiant exposure for complete PWS blanching in the fewest possible treatment sessions. Although non-invasive optical sensing techniques may provide significant diagnostic data, development of a realistic model will require a better understanding of relevant mechanisms for irreversible vascular injury.

Nd:YAG laser irradiation in conjunction with cryogen spray cooling induces deep and spatially selective photocoagulation in animal models

Successful laser treatment of haemangiomas requires selective photocoagulation of subsurface targeted blood vessels without thermal damage to the overlying epidermis. We present an in vivo experimental procedure, using a chicken comb animal model, and an infrared feedback system to deliver repetitive cryogen spurts (of the order of milliseconds) during continuous Nd:YAG laser irradiation. Gross and histologic observations show deep-tissue photocoagulation is achieved, while superficial structures are protected from thermal injury due to cryogen spray cooling. Experimental observation of epidermis protection in chicken comb animal models suggests selective photocoagulation of subsurface targeted blood vessels for successful treatment of haemangiomas can be achieved by repetitive applications of a cryogen spurt during continuous Nd:YAG laser irradiation.

Wavelengths for port wine stain laser treatment: influence of vessel radius and skin anatomy

Recent Monte Carlo computations in realistic port wine stain (PWS) models containing numerous uniformly distributed vessels suggest equal depth of vascular injury at wavelengths of 577 and 585 nm. This finding contradicts clinical experience and previous theory. From a skin model containing normal and PWS vessels in separate dermal layers, we estimate analytically the average volumetric heat production in the deepest targeted PWS vessel. The fluence rate distribution is approximated by Beer's law, which depends upon the tissue's effective attenuation coefficient, and includes a homogeneous fractional volumetric blood concentration corrected for finite-size blood vessels. The model predicts 585-587 nm wavelengths are optimal in adult PWSs containing at least one layer of small-radius blood vessels. In superficial PWSs, typically in young children with small-radius vessels, 577-580 nm wavelengths are optimal. Wavelength-independent results similar to those from Monte Carlo models are valid in single-layered PWSs of large-radius vessels. In conclusion, the volumetric heat production in the deepest targeted PWS blood vessel can be maximized on an individual patient basis. However, absorption of 585-587 nm wavelengths is sufficiently high in superficial lesions, so we hypothesize that these wavelengths may be considered adequate for the treatment of any PWS.

Optical Doppler tomographic imaging of fluid flow velocity in highly scattering media

An optical Doppler tomography (ODT) system that permits imaging of fluid flow velocity in highly scattering media is described. ODT combines Doppler velocimetry with the high spatial resolution of low-coherence optical interferometry to measure fluid flow velocity at discrete spatial locations. Tomographic imaging of particle flow velocity within a circular conduit submerged 1 mm below the surface in a highly scattering phantom of Intralipid is demonstrated.

Measurement of fluid-flow-velocity profile in turbid media by the use of optical Doppler tomography

Optical Doppler tomography is demonstrated to be a simple, accurate, and noncontact method for measuring the fluid velocity of laminar flow in small-diameter (~0.5-mm) ducts. Studies are described that utilize circular (square) plastic (glass) ducts infused with a moving suspension of polymer microspheres in air and buried in an optically turbid medium. The measurement of Doppler-shifted frequencies of backscattered light from moving microspheres is used to construct a high-resolution spatial profile of fluid-flow velocity in the ducts.

Doppler grid surface scanning applications for pulmonary subsurface parenchymal perfusion assessment

Subsurface perfusion to lung parenchyma underlying the pleura is difficult to assess in live ventilated animals. The purpose of this study was to assess applicability of a newly developed laser Doppler grid scanning imaging technology that measures perfusion of pleural subsurface lung regions in intact normal and abnormal animal lungs. Eighty-six Doppler grid perfusion measurements were performed in five New Zealand White Rabbits (3-5 kg); four with unilateral bullous lung disease, one normal control. Left upper lobe lung surface was exposed to 10 1-sec spot Nd:YAG exposures (70 W/cm2). One week following laser exposure, all rabbits underwent sequential bilateral open thoracotomy. Unaffected left lower lobes in these animals and all four lobes of a previously untreated rabbit were used as controls. Pleural subsurface perfusion measurements were recorded over a contiguous 900-pixel square surface grid using quantitative noncontact laser Doppler imaging during open thoracotomy procedures. Scans were obtained in a normal volume ventilation mode, at 30 cm of inspiratory hold airway pressure, and postinflation. A perfusion-pressure response curve was obtained in normal lung at 10-, 20-, and 30-cm static airway pressure. Post mortem measurements were used as 0 flow controls. Normal lung tissue was found to have relatively high pleural subsurface perfusion (1362 +/- 328 corrected units on a scale of 0-4095). Areas of atelectasis had decreased perfusion (659 +/- 512 U., 48.4 +/- 12.5% compared to normal lung, p < 0.02), but returned to normal levels after inflation of the lung (1253 +/- 363 U., p = 0.21 compared to normal). Pleural subsurface perfusion decreased uniformly and progressively as lung inflation pressure increased (p < 0.0001). Perfusion increased immediately to supranormal values following release of high inspiratory inflation pressure holds (1603 +/- 626 U., 117 +/- 18% compared to normal lung, p = 0.03). Bullae had markedly decreased perfusion (541 +/- 68 U.) that was not further reduced by increased inflation pressures. Noncontact laser Doppler grid perfusion imaging appears to provide a new tool for measuring pleural subsurface perfusion over a large area of lung surface in clinical experimental settings. Results are rapid, reproducible, and consistent. Sampling errors inherent in current point sampling Doppler flow techniques are reduced by the multiple contiguous measurements. We have used this technique to demonstrate inspiratory pressure-related reduction in pleural subsurface perfusion in normal lung, reversible decreased perfusion in atelectatic regions, and reduced perfusion in bullous and laser-treated lung regions.

Spatially selective photocoagulation of biological tissues: feasibility study utilizing cryogen spray cooling

Successful laser treatment of selected dermatoses such as hemangiomas requires thermally induced damage to blood vessels while protecting the epidermis. We present and test a procedure in a rabbit liver tissue model that utilizes cryogen spray cooling during continuous Nd:YAG laser irradiation to induce deep photocoagulation necrosis while protecting superficial tissues from thermal injury. Gross and histologic observations are consistent with calculated thicknesses of protected and photocoagulated tissues and demonstrate the feasibility of inducing spatially selective photocoagulation when cryogen spray cooling is used in conjunction with laser irradiation. This procedure may be useful in the thermal treatment of some pathological conditions for which it is desired that deep photocoagulation be induced while protecting superficial tissues.

Depth determination of chromophores in human skin by pulsed photothermal radiometry

We report on the application of pulsed photothermal radiometry (PPTR) to determine the depth of in-vitro and in-vivo subsurface chromophores in biological materials. Measurements provided by PPTR in combination with a nonnegative constrained conjugate-gradient algorithm are used to determine the initial temperature distribution in a biological material immediately following pulsed laser irradiation. Within the experimental error, chromophore depths (50-450 µm) in 55 in-vitro collagen phantoms determined by PPTR and optical low-coherence reflectometry are equivalent. The depths of port-wine-stain blood vessels determined by PPTR correlate very well with their locations found by computer-assisted microscopic observation of histologic sections. The mean blood-vessel depth deduced from PPTR and histologic observation is statistically indistinguishable (p > 0.94).

Dynamic heat capacity changes of laser-irradiated type I collagen films

BACKGROUND AND OBJECTIVE

A common assumption made in the thermal response of biological materials due to laser irradiation is the constancy of the specific heat capacity at constant pressure, CP. In this investigation, CP of pure hydrated Type I collagen films is measured in time during laser irradiation.

STUDY DESIGN/MATERIALS AND METHODS

A Nd:YAG laser scanning calorimeter is developed and used to test the constant heat capacity assumption by monitoring transient, laser-induced thermal transitions in the collagen films.

RESULTS

Results of preliminary studies on the irreversible, laser induced thermal denaturation of collagen with heating rates of up to 110 K/sec show a broad CP transition that can attain large values (20 J/g K).

CONCLUSION

The magnitude of the CP change that occurs in response to laser irradiation shows that the assumption of a constant CP when modeling heat transport in tissues is not always valid.

Imaging laser heated subsurface chromophores in biological materials: determination of lateral physical dimensions

We describe a non-contact method using infrared radiometry to determine lateral physical dimensions of laser heated subsurface chromophores in biological materials. An imaging equation is derived that relates measured radiometric temperature change to the reduced two-dimensional temperature increase of laser heated chromophores. From measured images of radiometric temperature change, the lateral physical dimensions of chromophores positioned in an in vitro model of human skin are determined by deconvolution of the derived imaging equation using a non-negative constrained conjugate gradient algorithm. Conditions for optimum spatial resolution are found by analysis of a derived radiometric transfer function and correspond to superficial chromophores and/or weak infrared absorption in a laser irradiated biological material. Analysis indicates that if the infrared attenuation coefficient is sufficiently small (i.e., less than 10mm-1), infrared radiometry in combination with a deconvolution algorithm allows estimation of lateral physical dimensions of laser heated subsurface chromophores in human skin.

Dynamic epidermal cooling in conjunction with laser-induced photothermolysis of port wine stain blood vessels

When a cryogen spurt is applied to the skin surface for an appropriately short period of time (on the order of tens of milliseconds), the spatial distribution of cooling remains localized in the normal overlying epidermis, while leaving the temperature of the deeper port wine stain (PWS) blood vessels unchanged. Furthermore, cooling continues after pulsed laser exposure as cryogen remaining on the surface evaporates and removes heat deposited by light absorption in epidermal melanin. An additional advantage of dynamic cooling is a reduction in the level of pain and discomfort associated with flashlamp-pumped pulsed dye laser therapy of PWS. Preliminary clinical studies and supporting theoretical calculations demonstrate the feasibility of selective epidermal cooling while achieving photothermolysis of blood vessels during pulsed laser treatment of PWS.

Inability to activate muscles maximally during cocontraction and the effect on joint stiffness

In order to determine the maximum joint stiffness that could be produced by cocontraction of wrist flexor and extensor muscles, experiments were conducted in which healthy human subjects stabilized a wrist manipulandum that was made mechanically unstable by using positive position feedback to create a load with the characteristics of a negative spring. To determine a subject's limit of stability, the negative stiffness of the manipulandum was increased by increments until the subject could no longer reliably stabilize the manipulandum in a 1 degree target window. Static wrist stiffness was measured by applying a 3 degree rampand-hold displacement of the manipulandum, which stretched the wrist flexor muscles. As the load stiffness was made more and more negative, subjects responded by increasing the level of cocontraction of flexor and extensor muscles to increase the stiffness of the wrist. The stiffness measured at a subject's limit of stability was taken as the maximum stiffness that the subject could achieve by cocontraction of wrist flexor and extensor muscles. In almost all cases, this value was as large or larger than that measured when the subject was asked to cocontract maximally to stiffen the wrist in the absence of any load. Static wrist stiffness was also measured when subjects reciprocally activated flexor or extensor muscles to hold the manipulandum in the target window against a load generated by a stretched spring. We found a strong linear correlation between wrist stiffness and flexor torque over the range of torques used in this study (20-80% maximal voluntary contraction). The maximum stiffness achieved by cocontraction of wrist flexor and extensor muscles was less than 50% of the maximum value predicted from the joint stiffness measured during matched reciprocal activation of flexor and extensor muscles. EMG recorded from either wrist flexor or extensor muscles during maximal cocontraction confirmed that this reduced stiffness was due to lower levels of activation during cocontraction of flexor and extensor muscles than during reciprocal contraction.

A theoretical study of the thermal response of skin to cryogen spray cooling and pulsed laser irradiation: implications for treatment of port wine stain birthmarks

The successful treatment of port wine stain (PWS) patients undergoing laser therapy is based on selective thermal coagulation of blood vessels without damaging the normal overlying epidermis. Cryogen spray cooling of skin may offer an effective method for minimizing epidermal thermal injury. Inasmuch as the density of melanosomes and depth of PWS blood vessels can vary considerably, an optimum cooling strategy is required on an individual patient basis. We present a theoretical study of the thermal response of various pigmented PWS lesions to spray cooling in conjunction with flashlamp-pumped pulsed dye laser irradiation (585 nm). Results of our model indicate that precooling of skin using tetrafluoroethane as the cryogen spray is sufficient to eliminate epidermal thermal injury when using incident fluences less than 10 J cm-2 and 8 J cm-2 on patients with intermediate and high epidermal melanin content, respectively. Cryogens that have lower boiling points than tetrafluoroethane may allow successful treatment when using fluences equal to or greater than those indicated.

Depth profiling of laser-heated chromophores in biological tissues by pulsed photothermal radiometry

A solution method is proposed to the inverse problem of determining the unknown initial temperature distribution in a laser-exposed test material from measurements provided by infrared radiometry. A Fredholm integral equation of the first kind is derived that relates the temporal evolution of the infrared signal amplitude to the unknown initial temperature distribution in the exposed test material. The singular-value decomposition is used to demonstrate the severely ill-posed nature of the derived inverse problem. Three inversion methods are used to estimate solutions for the initial temperature distribution. A nonnegatively constrained conjugate-gradient algorithm using early termination is found superior to unconstrained inversion methods and is applied to image the depth of laser-heated chromophores in human skin.

EMG analysis of harmaline-induced tremor in normal and three strains of mutant mice with Purkinje cell degeneration and the role of the inferior olive

1. The effects of intraperitoneal injections of 10 mg/kg harmaline were tested in normal mice and three strains of cerebellar mutant mice with Purkinje cell degeneration. Ten normal (wild-type) mice (+/+), as well as five lurcher (lc/+), six nervous (nr/nr), and eight Purkinje cell degeneration (pcd/pcd) mutants were implanted with chronic electromyogram (EMG) electrodes in the hamstring and quadriceps muscle groups of the right hindlimb. 2. EMGs were recorded in each of the mice during spontaneous activity before and after intraperitoneal injections of 0.3 ml harmaline (10 mg/kg). Spectral analysis was used to quantify the amplitude and frequency of tremor found in the EMGs after harmaline administration. Normal mice responded to harmaline with strong, continuous 11- to 14-Hz tremor. Mutants from the pcd/pcd strain also reacted with continuous tremor, but of lower amplitude and frequency. In contrast, nr/nr mutants exhibited intermittent paroxysmal tremor lasting for only a few seconds, and lc/+ mutants showed no evidence of tremor whatsoever. 3. In order to detect covert tremor that was possibly not revealed by focal intramuscular EMG recordings, several mutant and normal mice were also tested on a suspended platform to which an accelerometer was attached. The results confirmed the findings from EMG recordings. 4. An incidental observation made during the course of this study was that harmaline tremor disappeared from the normal mouse during swimming and reappeared when the animal was withdrawn from the water. 5. Although Purkinje cells appeared to increase both the depth of modulation and the frequency of tremor, the inhibitory action of the cerebellar cortex does not seem to be essential for the generation of tremor. 6. Parasagittal cerebellar sections of the normal, wild-type mice and the three strains of cerebellar mutant mice of various ages were stained with cresyl violet and examined for Purkinje cell degeneration. Purkinje cell degeneration was found to be complete in the pcd/pcd and lc/+ strains. Although an initial examination of parasagittal sections of the nr/nr strain failed to find any surviving Purkinje cells, further examination of sections cut in the coronal plane revealed small clusters of Purkinje cells in the vermal area of the posterior lobe. 7. The retrograde transport of wheat-germ-agglutinin-conjugated horseradish peroxidase (WGA-HRP) pressure-injected into the cerebellar cortex was used to study the olivocerebellar projections in the wild-type mice and the three strains of cerebellar mutant mice.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of fluid flow velocity by optical Doppler tomography

The spatial profiles of fluid flow velocity in transparent glass and turbid collagen conduits are measured by optical Doppler tomography (ODT). The flow velocity at a discrete user-specified spatial location in the conduit is determined by measurement of the Doppler shift of backscattered light from microspheres suspended in the flowing fluid. Experimental data and theoretical calculations are in excellent agreement. ODT is an accurate method for the characterization of high-resolution fluid flow velocity.

Dynamic epidermal cooling during pulsed laser treatment of port-wine stain. A new methodology with preliminary clinical evaluation

BACKGROUND AND DESIGN

The clinical objective in the treatment of a patient with port-wine stain (PWS) undergoing laser therapy is to maximize thermal damage to the PWS, while at the same time minimizing nonspecific injury to the normal overlying epidermis. With dynamic cooling, the epidermis can be cooled selectively. When a cryogen spurt is applied to the skin surface for an appropriately short period of time (on the order of tens of milliseconds), the cooling remains localized in the epidermis, while leaving the temperature of the deeper PWS vessels unchanged.

RESULTS

Comparative measurements obtained by a fast infrared imaging detector demonstrated that the surface temperature prior to laser exposure could be reduced by as much as 40 degrees C using the dynamic cooling technique. No skin surface textural changes were noted on PWS test sites cooled with a 20- to 80-millisecond cryogen spurt after flashlamp-pumped pulsed dye laser (FLPPDL) exposure (lambda = 585 nm; tau p = 450 microseconds) at the maximum light dosage possible (10 J/cm2). In contrast, epidermal necrosis occurred on the uncooled sites after such exposure. Six months after laser exposure, clinically significant blanching on the cooled sites indicates laser photothermolysis of PWS blood vessels did occur.

CONCLUSIONS

Our preliminary experiments demonstrate the feasibility of selectively cooling the normal overlying epidermis without affecting the temperature of the deeper PWS vessels. Furthermore, protection of the epidermis from thermal injury, produced by melanin light absorption at clinically relevant wavelengths, can be achieved effectively. An additional advantage of dynamic epidermal cooling is reduction of patient discomfort associated with FLPPDL therapy. Further studies are under way to determine an optimum strategy for applying this dynamic cooling technique during pulsed laser treatment of patients with PWS and others with selected dermatoses (dermal melanocytic lesions and tattoos).

Characterization of human scalp hairs by optical low-coherence reflectometry

Optical low-coherence reflectometry is used to investigate the internal structure and optical properties of human scalp hair. Regardless of hair color, the refractive index of the cortical region remains within the range of 1.56-1.59. The amplitude of the backscattered infrared light coupled into different-colored hair confirms the relative melanin content. Discontinuities in the refractive index permit identification of distinct structural layers within the hair shaft.

Selective cooling of biological tissues: application for thermally mediated therapeutic procedures

The ability to control the degree and spatial distribution of cooling in biological tissues during a thermally mediated therapeutic procedure would be useful for several biomedical applications of lasers. We present a theory based on the solution of the heat conduction equation that demonstrates the feasibility of selectively cooling biological tissues. Model predictions are compared with infrared thermal measurements of in vivo human skin in response to cooling by a cryogen spurt. The presence of a boundary layer, undergoing a liquid-vapour phase transition, is associated with a relatively large thermal convection coefficient (approximately 40 kW m-2 K-1), which gives rise to the observed surface temperature reductions (30-40 degrees C). The degree and the spatial-temporal distribution of cooling are shown to be directly related to the cryogen spurt duration.

Differential vascular response to laser photothermolysis

Individual blood vessels in the chick choriallantoic membrane were selectively coagulated through photothermolysis, using pulsed laser irradiation at 585 nm. Pulse durations were chosen to be 0.45 ms and 10 ms, which correspond to the thermal relaxation times in blood vessels of 30 microns and 150 microns diameter, respectively. The short pulses, at a light fluence F = 3 Jcm-2, caused permanent occlusion of vessels of 40 microns diameter or less, whereas larger caliber vessels (60-120 microns) required F = 4-5 Jcm-2. The long-duration pulses, at F = 7 Jcm-2, caused coagulation of the larger diameter vessels; the small-caliber vessels and capillaries showed resistance to photothermolysis and required multiple exposures to achieve coagulation. The fluence versus diameter (F versus d) relationship for coagulation was calculated for the two pulse durations. The energy deposited in a cylindrical absorber of diameter d by an optical field, incident perpendicular to the vessel, was expressed analytically and compared with the energy required to coagulate a blood vessel of the same lumen dimeter. When thermal diffusion is incorporated into the model, our findings can be accounted for quantitatively. This information will be of use for improving the laser treatment of port wine stains and other vasculopathies. A surprising observation was that arterioles were damaged at lower incident energy densities than venules having the same lumen diameter, despite the fact that absorbance in oxygen-rich and oxygen-poor blood is the same at 585 nm.

The effects of skinfold thickness on the selectivity of surface EMG

We investigated the effects of skinfold thickness and electrode orientation on the ability to record selectively from a localized region of a muscle using arrays of surface electrodes. EMG activity elicited by electrical stimulation and by voluntary contraction of the biceps muscle was recorded from subjects with skinfold thicknesses ranging from 2 mm to 21 mm. The selectivity of the surface electrodes increased as the skinfold thickness decreased; action potentials were more rapidly attenuated and underwent less low-pass filtering. As a result, the EMG recorded during a voluntary contraction at one site became less highly correlated with that recorded at neighboring sites as skinfold thickness decreased. We were able to determine the axis of action potential propagation (muscle fiber direction) through comparison of the amplitude and delay of cross-correlation peaks from pairs of colinear electrodes oriented at angles to one another, although the thicker the skinfold the lower the resolution. It was clear that the ability to localize EMG signal sources deteriorated as the amount of subcutaneous fat between the surface recording site and the active muscle fibers increased.

Compensation for mechanically unstable loading in voluntary wrist movement

In order to study the roles of muscle mechanics and reflex feedback in stabilizing movement, experiments were conducted in which healthy human subjects performed targeted wrist movements under conditions where the damping of the wrist was reduced with a load having the property of negative viscosity. If the movement speed and negative viscosity. If the movement speed and negative viscosity were sufficiently high, the wrist oscillated for several hundred milliseconds about the final target position. Subjects increased the activation of both wrist flexor and extensor muscles to increase joint stiffness to damp the oscillations. With practice, both the tendency to oscillate and the level of muscle activation were reduced. A small bias torque in either direction, added to the negative viscosity, enhanced the oscillations as well as the amount of flexor and extensor muscle activation during the stabilization phase of fast movements. The tendency for the wrist to oscillate was also seen during slow movements where the oscillations were superimposed upon the voluntary movement. We suggest that this reduction in mechanical stability is primarily of reflex origin. As wrist stiffness increases, the natural frequency of the wrist also increases, which in turn produces an increase in the phase lag of the torque generated by the myotatic reflex with respect to wrist angular velocity, effectively reducing damping. The oscillation frequency was often close to a critical frequency for stability at which torque, due to the myotatic reflex, lagged angular velocity by 180 degrees (6-7.5 Hz). Nevertheless, subjects were able to damp these oscillations, probably because the torque due to intrinsic muscle stiffness (in phase with position and hence lagging velocity by only 90 degrees) dominated the torque contribution of the myotatic reflex. Increasing stiffness with declining oscillation amplitude may also have contributed significantly to damping.

Pulsed photothermal radiometry of port-wine-stain lesions

Pulsed photothermal radiometry is used to map the heat deposition in human skin after a short laser pulse. It uses an IR (HgCdTe) detector for a rapid noncontact measurement of the skin surface temperature based on the blackbody emission in the 8-12-microm spectrum. The heat deposited by the laser pulse in the superficial epidermis causes an immediate temperature jump, and the heat deposited in basal epidermal melanin and deep port wine stains diffuses to the surface before detection. The time course of the surface temperature T(z = 0, t), indicates the initial spatial distribution of heat, T(z, t = 0), deposited by the laser.

Dependence of elbow viscoelastic behavior on speed and loading in voluntary movements

The objective of this study was to characterize the mechanical behavior of the elbow joint during voluntary movement, for different speeds of movement and under different loading conditions. Torque pulses of 50 ms duration were applied at movement onset and at peak velocity on random trials. The displacement away from the unperturbed trajectory was used as an indicator of the relative compliance of the elbow under different conditions. We found that both the displacement and the time taken to return to the unperturbed trajectory decreased if the overall speed of the movement increased or if a viscous resistive load was added, implying lower joint compliance. It was possible to account qualitatively for differences in mechanical behavior from observed muscle activation patterns.

A model for the generation of movements requiring endpoint precision

A model is proposed in which movement accuracy is regulated by means of corrective actions taken at discrete intervals throughout the course of a movement. A movement, as represented by its tangential velocity profile, cna be decomposed into a series of one or more submovements. Each submovement consists of a prototype velocity profile which can be scaled in magnitude and duration. For planar two-joint movements, we demonstrate that these submovements can be mathematically represented either in terms of velocity profiles or in terms of the underlying joint torque profiles. In either case, the submovements superimpose linearly to produce the composite movement. The model provides a very good fit to tangential velocity profiles recorded from human subjects during three-dimensional arm movements with constraints on accuracy and speed. The model assumes that when a submovement is present, its onset is associated with a change in the direction of the hand path and/or a zero crossing or inflection in at least one of the components of the velocity vector. The model is consistent with a strategy in which precision is achieved by periodic discrete actions which redirect the moving arm in order to bring the hand closer to the target. Since submovements were also observed in slow movements where accuracy constraints had been relaxed, we hypothesize that the strategy of superimposing a series of submovements to make one composite movement may be a general one. We suggest that it would be particularly appropriate for the process of learning a new motor skill.

Reciprocal and coactivation commands for fast wrist movements

According to the equilibrium-point hypothesis, movements are produced by means of displacement of the invariant torque/angle characteristic (IC) of the joint and change in its slope. Displacement is produced via the central reciprocal (R) command while the coactivation (C) command specifies the slope of the IC. Neurophysiologically, the R command is associated with reciprocal changes in the membrane potentials of agonist and antagonist motoneurons while the C command is associated with their simultaneous depolarisation. These commands were investigated in single joint wrist movements by perturbation methods. Subjects normally made free flexion movements to a target at 30 degrees but on random trials they were either opposed by a spring-like load or assisted by a load. The former was generated using negative linear position feedback; the latter using positive position feedback to a torque motor. Subjects were instructed not to correct errors arising from perturbations. Both peak velocity and EMG patterns were strongly affected by load conditions. Subjects undershot or overshot the target when opposing or assisting loads were presented, respectively. However, after removing the load (700 ms later), the target position was regained indicating that the IC was stable despite the perturbation. In two other experiments, subjects initially trained to reach the target with opposing or assisting loads, while on random trials, the load was not presented. Depending on training conditions, the subject shifted the IC by different amounts. The slope of the IC varied independently of the magnitude of its positional shift. We conclude that R and C commands can be specified independently.(ABSTRACT TRUNCATED AT 250 WORDS)

Wrist muscle activation patterns and stiffness associated with stable and unstable mechanical loads

The objectives of this study were to examine the effects of load mechanical characteristics and agonist-antagonist muscle cocontraction, on the stretch reflex response of wrist flexor muscles, and to measure the associated wrist stiffness. Subjects were required to maintain a constant wrist angle while operating against flexor loads with different stability characteristics (constant, elastic or unstable). We measured the stretch reflex responses and joint stiffness by applying step displacements of 3 degrees and 10 degrees. Subjects used very little cocontraction of wrist flexor and extensor muscles when the load was constant or elastic, but increased cocontraction dramatically when the load was unstable, in order to increase the wrist stiffness. Although the magnitude of stretch reflex responses also increased with cocontraction, this simply reflected the level of tonic flexor muscle activity. We found no evidence to suggest that phasic stretch reflexes contributed significantly to the joint stiffness in this task. Clear differences in flexor muscle synergy were observed in the presence and absence of cocontraction, particularly when comparing the FCR and FCU muscles.

Cutaneous afferent activity in the median nerve during grasping in the primate

Neural activity was recorded from the median nerve of a monkey during grasping and lifting, using a chronically implanted cuff electrode. At the onset of lifting, there was an initial dynamic response during which the intensity of the neural signal increased rapidly. This neural response attained its peak value well before the displacement, the load force or the grip force. The time course and peak of the rectified, integrated neurogram were best correlated with the rate of change of grip force. The neural activity declined exponentially to a steady value following the initial peak. During steady holding the mean amplitude of the neurogram was best correlated with the mean grip force. At the end of the holding phase there was a short burst of neural activity as the monkey relaxed the grip force and released the object. During some blocks of trials pulse perturbations were applied to the object. When the monkey did not increase the grip force in advance of the perturbation, the perturbation produced a relatively large displacement of the object and a burst of neural activity whose onset coincided with the onset of displacement. When the monkey anticipated the perturbation by increasing the grip force during the holding period preceding the perturbation, the perturbation produced a relatively small displacement and relatively little increase in neural activity.

Temperature dependence of the Verdet constant in several diamagnetic glasses

Measured temperature dependences of the Verdet constants of SiO(2), SF-57, and BK-7 are approximately 10(-4)/K within 3-20% of Becquerel formula estimates.

The effect of accuracy constraints on three-dimensional movement kinematics

The kinematics of three-dimensional arm movements were recorded during a task in which subjects were required to place a peg in a hole. The accuracy constraint was varied by using holes of different diameters. If the diameter of the hole was large relative to the diameter of the peg, the tangential velocity profile of hand trajectories was relatively symmetric and bell-shaped, but it became increasingly asymmetric as the diameter of the hole was reduced. Peak tangential velocity decreased, overall movement duration increased and the proportion of the movement spent in deceleration increased systematically. The shape of the accelerative phase of the velocity profile showed little dependence on hole diameter, but the decelerative phase became increasingly irregular as the hole diameter was reduced. This irregularity was attributed to submovements corresponding to small changes in the direction of the hand path. On the other hand, deliberately slowing the movement in the absence of a strict accuracy constraint induced a change in the velocity profile which produced irregularity in both the accelerative and decelerative phases of the movement. The results of our experiments are consistent with the idea that movements requiring extreme accuracy and other slow movements are composed of a series of submovements. In the case of movements requiring accuracy these submovements may represent corrective actions that are taken throughout the course of the movement.